Foundry Technology
Manufacturing ?
Manufacturing ?
Manufacturing in its broadest sense is the process of converting raw materials in to useful products
It includes i) Design of the product ii) Selection of raw materials and iii)The sequence of processes through which the product will be manufactured
Types Of Manufacturing Processes ?
Manufacturing Processes Manufacturing Processes Casting Machining F & W tech TMRP FT NTM
Fabrication Powder Forming Metal forming by joining metallurgy F & W tech
Casting ?
Casting
Casting The production of shaped articles by pouring molten metal into moulds Steps in casting seem simple: 1. 2. 3.
Melt the metal Pour it into a mold Let it freeze
Process in which molten metal flows by gravity or other force into a mold where it solidifies in the shape of the mold cavity
It involves a series of operations i) Pattern making ii) Mould making iii) Melting iv) Pouring v) Cleaning Similar to that of making ice cubes in a refrigerator
Foundry technology
Foundry is the place where metal is melted and casting are produced. Casting is one of the oldest manufacturing process and even today is the first step in manufacturing most of the products Foundry engineering deals with the processes of making castings in moulds formed in either sand or some other material The art of foundry is ancient
History
One of the man’s first operations with metal was melting the ore and pouring it into suitable moulds The earliest use of the metals was mostly for the purpose of knives, arrow points, coins and tools The casting process is said to have been practiced in early historic times by the craftsmen of Greek and Roman civilizations Copper and bronze were common in ancient times, but evidence indicates that iron also had been discovered and developed in the period around 2000 BC Around 500 BC the metals began to be used for statues of gods and goddesses Subsequently, a still greater application of metals figured in armoury, guns and war materials
Since then , the whole art of metal founding has emerged as an exact science Today we have a variety of moulding processes and melting equipment and a host of metals and their alloys And though the techniques and methods of production have changed considerably, the basic principles still remain almost the same Castings have several characteristics that clearly define their role in modern equipment used for transportation, communication, power, agriculture, construction and in industry
Major routes (Engg Components )
Engineering structures may be produced from metal in the cast and the final shape may be developed by many different combinations of manufacturing process In certain cases the best results can be obtained by using composite construction Here particular advantages of separate process can be combined to achieve the most effective overall design Cast welded assemblies offers a good example of composite construction
Advantages of casting
Design advantages : i) Size: castings may weigh as much as 200 tons or be as small as a wire of 0.5mm diameter ii) Complexity: The most simple or complex curved surfaces, inside or outside , and complicated shapes, which would otherwise be very difficult or impossible to machine, forge or fabricate, can usually be cast iii) Weight saving: As the metal can be placed exactly where it is required , large saving in weight is achieved iv) Production of prototypes: The casting process is ideally suited to the production of models or prototypes required for creating new designs
Material composition : All types of engineering alloys can be cast using appropriate foundry techniques Low cast: Casting is usually found to be the cheapest method of metal shaping Dimensional accuracy: Castings can be made to fairly close dimensional tolerances by choosing the proper type of moulding and casting process Versatility in production: Metal casting is adaptable to all types of production
Foundry industry
The scope of the foundry industry encompasses a major segment of our national economy It has been described as an 8.5 billion dollar industry, employing directly and indirectly 475000people Castings are used in transportation, communication, construction, agriculture, power generators , in aerospace and atomic energy applications Because of their wide spread use, castings are produced almost everywhere that manufacturing occurs
Importance of foundry
Casting can produce complex shapes ( internal or external), parts with hallow sections or internal cavities and parts having irregular curved surfaces It can also make parts which are difficult to produce by any other methods It offers high production rate with improved material properties, produces good surface finish and ensures close dimensional tolerances Important casting material include cast iron, copper alloys, zinc, aluminium, nickel and magnesium
Big parts: engine blocks and heads for automotive vehicles, wood burning stoves, machine frames, railway wheels, pipes, church bells, big statues, and pump housings Small parts: dental crowns, jewelry, small statues, and frying pans All varieties of metals can be cast, ferrous and nonferrous There are many casting processes available today and the selection of the best method to produce a particular part depends on several basic factors, such as cost, size, finish, production rate, tolerance, section-thickness. Physical-mechanical properties, intricacy or design, machinability and weldability
Basics of foundry (Revision)
Pattern, Pattern material, Different patterns, Pattern making Mould making, moulding sand, sand ingredients Core making, core sand, sand ingredients Moulding methods Gating system Special casting methods Melting furnaces Defects in casting
The Mold in Casting
Contains cavity whose geometry determines part shape Actual size and shape of cavity must be slightly oversized to allow for shrinkage of metal during solidification and cooling Molds are made of a variety of materials, including sand, plaster, ceramic, and metal
Two forms of mold: (a) open mold, simply a container in the shape of the desired part; and (b) closed mold, in which the mold geometry is more complex and requires a gating system (passageway) leading into the cavity
Basic operations
Pattern making (replica of object) Core making (Hollow shapes) Moulding Melting Pouring Cleaning & inspection
Sections of foundry Metal
Sand additives
Sand
Sand mixing & preparation Melting Handling molten metal
Moulding
Pouring Melting Section
Shaking out Finishing Heat treatment Inspection & Testing
Moulding Section
General layout of foundry Foundry store
Melting Furnaces Grinding
Sand Moulds ready for pouring Muller Inspection Bench
Core making
Syllabus Unit-I
Foundry Metallurgy: Oxidation of liquid metals . Gas dissolution in liquid metals. Methods of degassing. Fluidity. Factors affecting fluidity. Fluidity tests. Hot tearing. Shrinkage of liquid metals (notes)
Casting Design: Introduction to casting design (FT by Peter Beeley) Design for minimum casting stresses. Design for directional solidification. Design for metal flow. Safety factors. Design for low pattern cost and model making as an aid in design
Syllabus Contn…. Unit-II Pattern & core box design: i) Types of patterns( single piece, split, match) ii) Pattern allowances( shrinkage, machining, draft) iii) Pattern materials ( wood, metal, plastic) iv) Preparations of patterns v) Master pattern (pattern made prior to the manufacture of actual metal pattern) its design and use vi) Cores vii) Types of cores( one piece open side, split core) viii) Core prints xi) Use of CAD/CAM for pattern manufacturing (Any basic foundry books)
Unit-III Solidification of castings: Crystallization and development of cast structure i) Nucleation ( First microscopic metal crystallite) ii) Growth ( more metal crystallite) iii) Dendritic growth ( trunk like structure of a tree) Structure of castings. Significance and practical control of cast structure. Grain shape and orientation . Grain size. Refinement and modification of cast structure Concept of progressive & directional solidification ( Foundry Tech by O P Khanna) Feeding of castings: i) Feeding characteristics of alloys (Beeley) ii) Geometry influence on solidification iii) Methods of feeding casting iv) Gating technique v) Casting temp & pouring speed vi) Use of padding , chill, insulators( for directional solidification) Unit-IV Foundry mechanization: Need for modernization & mechanization, Moulding and core making. Melting ,pouring , shake out equipment and fettling, dust and fume control Material handling equipments for sand moulds, cores, molten metal and castings Reclamation of sands plant layout for foundries (P L Jain)
Reference books
Foundry technology by Beeley P. R
Principles of foundry technology by P.L.Jain
Foundry Technology by O. P. Khanna
Principles of metal casting by Heine & Loper
Casting design
Casting design
The purpose of design is to achieve functional performance of structures at minimum cost ( gear used in lathe and sugar cane machines ) Design of a component as casting requires close coordination between i) the mechanical engineer making a functional design from the various stress calculations & ii) foundry engineer to modify the design to suit foundry process of manufacture for optimum performance and cost
Initial considerations in design
The final shape may be developed by many different combinations of manufacturing process Nature of shaping process exercises a characteristic influence upon metallurgical structure and properties The cost factor often predominates in process selection The most important of these is the quantity requirement as it affects production and tooling costs Is the design permit economical manufacture by standard methods Can gates, risers, and chills be positioned properly to ensure soundness
Considerations
Are the section size and configuration such as to cause undue stresses in the mold and consequent tearing or cracking Can directional solidification be established and controlled Should the casting be broken down into component parts and the separate castings welded together Is it possible to delete certain members such as feet, bosses and weld them to the casting later to make foundry operations easier, cheaper and more durable
Principle methods of shaping metals
Fig gives a general picture of the basic operations and alternative paths to the production of a finished engineering structure Each shaping operation offers compelling advantages within a limited field But direct competition exists over a wide range of products In certain cases the best results can be obtained by using composite construction Here particular advantages of separate processes can be combined to achieve the most effective overall design Cast welded assemblies offers a good example
An adequate knowledge of casting processes is highly essential for the persons engaged in casting design
In case of complicated castings, the designer should discuss cast part design with foundry superintendent, sales engineer and pattern maker so that the end results are economical and satisfactory
Casting design: a problem
Casting design poses two problems: One for the Engineer, the other for the Foundryman 1. The Engineer must know : “How to design a casting so that it will actually have the requisite strength and functional properties”
2. The Foundryman must be able to: “ To make the casting so that it has the strength and functional properties the engineer intended”
General Design Rules
Rule 1: Consult a foundryman Rule 2: Construct a model Rule 3: Design for casting soundness Rule 4: Avoid sharp angles and corners Rule 5: Reduce number of adjoining sections Rule 6: Design for uniformity of section Rule 7: Proportions dimensions of inner walls Rule 8: Avoid abrupt section changes Rule 9: Fillet all sharp angles Rule 10: Design ribs and brackets for maximum effectiveness
Rule 1: Consult a Foundryman
It is all too common to design to suit the engineering department but not the foundry, and the result may be failure or disappointment So consultation between designer and foundryman will permit consideration of the foundry problems that are likely to be encountered and will promote the making of a sound casting The time and cost of manufacture can also be considered at this preliminary stage of casting design
Rule 2: Construct a small model or visualize the casting in the mould
Create a three-dimensional drawing or construct a small model This procedure permits study of how metal will enter the mold, how solidification proceeds and shows what parts have to be fed to assure casting soundness In today’s world, this is easily done with CAD and three dimensional isometrics A model to scale or full size in the form of a pattern that can be used later will help the designer to see how cores must be designed and placed or omitted It will help the foundryman to decide how to mold the casting, detect casting weakness, where to place gates and risers, and answer other questions affecting casting soundness, cost and delivery
Rule3:Design for casting soundness
Most metals and alloys shrink when they solidify Therefore, design so that all members of the parts increase in dimensions progressively to one or more suitable locations where feeder heads can be placed to offset liquid shrinkage The fig shows correct and incorrect methods of design Those assure soundness of section
Rule 4: Avoid sharp angles and corners
Solidification of molten metal always proceeds from the mold face, forming unbalanced crystal grains that penetrate into the mass at right angles to the plane of cooling surface A simple section presents uniform cooling and greatest freedom from mechanical weakness
When two or more sections conjoin, mechanical weakness is induced at the junction and free cooling is interrupted, creating a hot spot
In designing adjoining sections, avoid acute angles Replace all sharp angles with radii and minimize heat and stress concentration Fig 1,2 and 3 illustrate poor designs that result in local structural weakness Fig 4 and 5 show recommended design which assures improved strength and solidify Fig 6 shows a common defect involving a T section The improved design as shown in fig 7 removed the hot spot and stress concentration
Rule 5: Bring the minimum number of sections together
A well designed casting brings the minimum number of sections together and avoids acute angles
Rule 6: Design for uniformity of sections
Design all sections as nearly uniform in thickness as possible Design on left caused defects shown. Correct design shown on right
Failing this, all heavy sections should be accessible for feeding
The hydraulic coupling shown in fig was originally designed with a 2” core through the centre This gave excessive metal and caused local porosity Redesigning with sections of reasonable uniformity of thickness corrected the difficulties, reduces the weight of the casting and lowered the cost of manufacture
Rule 7:Proportions dimensions of inner walls
Inner sections of castings, resulting from complex cores, cool much slower that outer sections and cause variations is strength properties A good rule is to reduce inner sections to 9/10 of the thickness of the outer wall Avoid rapid sections changes and sharp angles
The inside diameter of cylinder and bushings should exceed the wall thickness of casting
When inside diameter of cylinder is less than the wall thickness of the casting, it is better to cast solid Holes can be produced by cheaper and safer methods than by coring
Rule 8: Avoid abrupt sections changes
The difference in the relative thickness of adjoining sections should be minimum and not exceed a ratio of 2:1
Rule 9: Fillet all sharp angles
Fillets have three functional purposes: 1) To reduce stress concentration in the casting in service 2) To eliminate cracks, tears and draws at re-entry angles 3) To make corners more moldable and to eliminate hot spots
In the case of “V” or “Y” sections and other angular forms, always design so as to allow a generous radius to avoid localization of heat
Rule 10: Design ribs and brackets for maximum effectiveness
Design of castings
The purpose of design is to achieve functional performance of structures at minimum cost An adequate knowledge of casting processes is highly essential for the persons engaged in casting design In case of complicated castings, the designer should discuss cast part design with foundry superintendent, sales engineer and pattern maker so that the end results are economical and satisfactory
Considerations
Are the section size and configuration such as to cause undue stresses in the mold and consequent tearing or cracking Can directional solidification be established and controlled Should the casting be broken down into component parts and the separate castings welded together (composite construction) Is it possible to delete certain members such as feet, bosses and weld them to the casting later to make foundry operations easier, cheaper and more durable
Important design considerations
Preferably a model should be made for the complicated castings and checked whether is it possible to obtain sound metal in all critical areas of the casting Based up on experience certain design considerations have been formed for good casting design and they are Design for minimum casting stresses Design for directional solidification Design for metal flow Cast weld design Design for minimum costing Functional design
Design for minimum casting stresses
Because of high pouring temperature, ferrous castings are particularly susceptible to external cracking or tearing in the mold These defects may be caused by influences external to the castings or by inherent characteristics The only external influence is the effect of the mold, which may restrain normal contraction and cause cracks or tears Stresses arising from inherent conditions are those due to composition of the metal . Metal composition influences tearing tendencies in at least three ways 1) by the inherent strength 2)by the existence and extent of solid transformations and 3) by the presence of impurities
1. 2.
3.
In order to ensure a) maximum disposal of stresses, and b) minimum stress concentration The following design rules must be applied to mold castings External corners should be rounded with radii that are 10 to 20% of the section thickness In joining sections of unequal size, the radius plays an important role. As shown in fig – The best way to change section thickness when necessary is to do it gradually Replace sharp angles and corners with suitable radii
When mold cavity is filled with molten metal, the inside corner of the casting is heated to higher temperature than other surfaces of the mold cavity. Due to this overheating, the metal at inside corners solidifies more slowly and this delay in solidification may cause shrinkage defects at this location
Better
Good
Since stress concentration develops in any sharp inside corner during cooling, if theses stresses exceed the strength of the metal, a heat check or hot tear may form 4 .Use tubular and reinforced C sections rather than standard I, H and channel sections to obtain improved load bearing capabilities under complex loads 5. Simplify , streamline and stagger complex sections to obtain improved stiffness If the casting is large, and of the complex shape, it may prove desirable to cast two or more sections and weld the parts together
Design for directional solidification
As the molten metal cools in the mould and solidifies, it shrinks in volume The contraction of the metal or volumetric shrinkage takes place in three stages Liquid contraction Solidification contraction Solid contraction
Liquid contraction: liquid contraction occurs when the molten metal cools from the temperature at which it is poured to the temperature at which solidification commences Solidification contraction : solidification contraction takes place during the time the metal changes from the liquid state to the solid Solid contraction : solid contraction takes place when the solidified metal cools from solidification temperature to room temperature
Since all the parts of the casting do not cool at the same rate due to varying section thickness and differing rates of heat loss to adjoining mould walls So some parts tend to solidify more quickly than the others Thus cause voids and cavities in certain regions of the casting These voids must be filled up with the liquid metal from the portion of the casting that is still liquid For getting sound casting without voids, the solidification should start from the thinnest and farthest part and progressively move towards the risers which should be last to solidify
If the solidification takes place in this manner, the casting will be sound with neither voids nor internal shrinkage . This process is known as directional solidification
Directional solidification
If solidification occurs in a desired direction or occurring in a particular direction then it is called directional solidification Controlling the solidification of metal after it has entered the mould cavity is the prime factor for achieving directional solidification Thus, for the castings to be sound free from voids and shrinkage defects, the solidification should be such that it starts at the thinner section, continuous to the thick section
Reasons for directional solidification
Different rates of cooling in diff rates
Complicated geometry
In extremely complicated geometry , it is very difficult to determine the relative rate with which the metal solidifies in various parts Due to different rates of cooling in different parts, stresses will be set up in the casting which may give rise to cracks in it This can be avoided by directional extraction of heat from the liquid metal So that solidification is proceeds from one end to the other
Advantages of directional solidification 1.
2.
3.
4.
Solidification of the metal will be uniform It produces sound casting, free from voids and shrinkage defects It produces uniform microstructures. Thus giving desirable properties Since here riser is the last part to solidify all the impurities given out by the first solidifying metal will be collected in the riser
Design for directional solidification
It involves the designing such that the parts most distant from the available liquid metal will solidify first leading to successive feeding of the contracting metal by the still liquid metal until the heaviest and last to freeze section is reached The risers are attached to the casting in such a location that they supply hot metal to the shrinkage casting until it is completely solidified
Design for directional solidification
1.
To get the complete idea of directional solidification we taken T junction as example The mass of metal in the region A is more than that in region B, C, and D In a T junction the region A being larger mass of metal than B, C and D, under normal cooling conditions , it is a hot spot , is last to solidify When solidification starts at A, sections B,C and D are already solidifies. Hence for shrinkage, liquid metal is not available and shrinkage void is developed at A Various corrective measures to eliminate hot spot in a section are shown in fig Bring the minimum number of sections together
2
3
4
Heavy sections should not be fed through light sections because light sections will solidify first, will not be able to supply liquid metal to heavy sections while they solidify and hence will cause shrinkage defects in the heavy sections Riser should be placed suitably to ensure soundness of the casting . Sections should be taper towards riser Horizontal flat surfaces should be avoided : Castings with flat surfaces are difficult to make because i) It is hard to prevent centre-line shrinkage ii) Slag,dross and other impurities lighter than metal tend to collect on upper flat surfaces It is best to have curved surfaces or design flat surfaces either vertical or inclined to the horizontal
5 Parting line placement i) Avoid placing parting line at machining areas ii) Keep parting line as even as possible so that pattern and mold making is simplified iii) Place gate and riser at the parting line, as far as possible, directly on the heavy section of the casting iv) Locate heavier sections at or near the parting line where they can be easily fed by a riser
Methods of achieving directional solidification Controlling the solidification of metal after it has entered the mould cavity is the prime factor for achieving directional solidification Directional solidification can be achieved by
1.Design and positioning of risers 2.Padding 3.Using exothermic materials 4.Use of Chills 5.Use of insulating material 6.Use of electric arc over the riser 7.Pouring rates and temperature
1.Design and positioning of risers
In a casting the gating and risering system should be designed such that it favours directional solidification In the gating design, locating the runners and risers at the proper position is a very important factor The process of directional solidification is greatly influenced by the following factors Feeding distance Riser shape and size Riser location Types of risers Riserless design
Feeding distance
Solidification will be good if the maximum distance L (feeding) is not greater than 4.5 times the minimum thickness T If L exceeds 4.5 times T. it is better to use more than one riser for the casting Thus it is necessary to know the feeding distance to design the number and location of risers
Riser shape and size
The most efficient shape a riser can assume is that which will lose a minimum of heat and thereby keep the metal in a molten state as long as possible This condition can be met when the riser is spherical in shape so that its surface area is a minimum As it is difficult to make spherical riser next best shape is the cylinder As regards the height of the riser, it must be tall enough to ensure that any pipe formed in it does not penetrate casting The ratio of height to diameter usually varies from 1:1 to 1.5:1
Riser location
The location of the riser should be chosen keeping in view the metal to be cast and the feasibility of directional solidification The riser may be located either at the top of the casting or at the side Top risering is extensively used for light metals as it enables the benefit of metallostatic pressure in the riser The number of risers has to be more than one so as to derive its most effective use In such cases, their spacing should be carefully arranged so as to minimize the shrinkage
Types of risers
Risers may be classified as open and blind risers In the open riser, the upper surface is open to the atmosphere and the riser is usually placed on the top of the casting or at the parting plane The open riser rarely extends downwards in to the drag Derives feeding pressure from the atmosphere and from the force of gravity on the metal contained in the riser The blind riser, on the other hand, is surrounded by moulding sand on all sides and is in the form of a rounded cavity in the mould placed at the side or top of the casting It may be placed in the cope or drag
Since this riser is closed from all sides, atmospheric pressure is completely shut out The pressure due to the force of gravity is also reduced due to the formation of vacuum within its body In some of the improved designs, a permeable dry sand core, fitted at the top of the blind riser
Riserless design
As the use of risers on casting decreases the yield, efforts should always be made to reduce the size of risers to the minimum There are instances when risers are completely eliminated through proper mould design, right selection of moulding materials and following correct moulding and casting techniques
2.Padding
It may not possible to overcome completely the effects of shrinkage in the central thin sections of the casting Padding may be the solution for this For this thinner sections should be tapered towards heavier section to achieve directional solidification This tapering of thinner section towards thicker section is known as padding This will require extra material If padding is not provided central line shrinkage or porosity will result in thinner section After solidification the extra padding material can be removed by machining
3.Use of exothermic materials Exothermic materials are the mixtures of the oxide of the metal to be cast and aluminium metal in powder form Which produces large amounts of heat when come in contact with hot metal Molten metal in the riser is heated in order to aid directional solidification Riser metal remains molten until the whole mould cavity is solidified This is done by adding exothermic material i) Either at the surface of the molten metal in the riser (after pouring) ii) Or to the sand in the riser walls
These comes in contact with molten metal undergo exothermic reactions and release heat
4.Use of chills
Directional solidification is induced by chilling the metal at those points where it is necessary that solidification should begin Chills are normally metal inserts which are placed at appropriate locations in the mould to speed up the solidification of a particular portion of the casting Chills extracts heat from the casting at a fast rate Use of chills becomes necessary when it is not possible to locate a riser on the casting Both external & internal chills can be used for this purpose
External chills
External chills are normally metal inserts of steel, cast iron or copper. Chills increase the rate of solidification by faster heat extraction External chills are placed in the mould walls that they come in direct contact with the molten metal in the mould cavity These do not form the integral part of the casting after solidification External chills are not consumed unlike internal chills and hence are reusable External chills are coated with red lead or moulding sand. Further external chills are two types direct & indirect
Internal chills
Internal chills are also metal objects which form the integral part of the casting after solidification Internal chills should be made of same composition as that of the metal to be cast Internal chills are extended in the mould Surface of internal chills must be free from moisture, grease and any impurities otherwise they produce defects
5.Use of insulating material
Risers can be made more efficient by employing artificial means to keep the top of the riser from freezing This is done by the use of certain insulating materials which are placed around the riser The defect may occur at the riser which will be the last portion to solidify Powdered graphite, charcoal, oat husks and refractory powders are used as insulating materials These when added on top of the riser, provide a shield such that the riser metal does not get solidified due to atmosphere contact
6.Use of electric arc over the riser
The necessary electric circuit is formed between the riser and the electrode After completely filling of the mould, an arc is initiated in the riser metal of the casting This will keep the molten metal in the riser hotter for a longer duration of time Thus casting shrinkage is taken care of
7.Pouring rates and temperature
Even pouring rates and temperature can be controlled to achieve directional solidification If the pouring rates are low, the metal gets cooled faster by the time it reaches the farthest point, and end up in improper solidification On the other hand, very high pouring rates may lead to problems like mold erosion, dross inclusion etc Moderate pouring rates help to retain high temperature even at the farthest end of the mould and promote directional solidification
Similarly, pouring temperature should be sufficient to keep the molten metal hot enough to reach the last part of the mould Too high temperatures will have problems of oxidation and cause mould related defects However there should be a good combination of pouring time and temperature and to be decided based on design of risering system, the size of the mould cavity and characteristics of the metal being cast
Design for metal flow
Minimum section thickness which permits metal to flow and fill the complete casting This decides the minimum section thickness of the casting that can be cast The minimum section thickness is a function of metal composition , fluidity of the metal, and temperature of the metal Pure metals will give maximum fluid flow and very thin sections can be cast Whereas , alloys have low fluid flow due to the coexistence of solid and liquid phase Higher metal temperature will promote higher fluidity in the metal
Such problems as gas contamination, inclusion of dross or slag, and aspiration of gas are factors that must be recognized when designing A little reflection will show that these problems are connected with the major problem of having the metal enter the mold in quite and uniform manner In other words , these are problems concerned with fluid flow, and laws governing fluids can be studied to improve design First of all, it should be recognized that liquids flow either in a streamlined laminar fashion or in a turbulent manner Turbulent flow creates such problems as inclusion of dross or slag, aspiration of air into the metal, erosion of the mold walls and roughening of the casting surface
The flow of liquid in a mold is also governed by other variables Too thin a casting section and too far (distance) will develop defects So approximate section thickness recommended for sand casting of diff metals is given below
Cast weld design (Multiple construction)
Since long and complex cast parts tend to warp or tear , it is highly desirable to separate the casting into parts and then weld different parts together Cast weld fabrication technique is more economical than all cast or all welded construction At this stage the possibility of using multiple construction and other supplementary techniques arises
1 Cast –fabricated construction ( multiple construction) 2 Inserts 3 Bimetal casting techniques 4 Surface treatment of castings
Cast –fabricated construction
Even casting is frequently in competition with other methods of forming, a wide field exists within which two or more techniques best features are exploited A single casting may be broken down in to smaller units to be subsequently fabricated by welding, bolting or other mechanical means Theses assembly techniques can be used either for economic reasons or to simplify the technical problems in the production of larger units The use of cast nodes to simplify fabrication and extend design capability is seen in the car body space frame (aluminium alloy extrusions are interconnected by spot welds)
Here weight saving and reduced tooling and production costs were combined with increased torsional stiffness Large flanges on cast pipework or long projections on compact forms are features which can usefully be produced separately and subsequently welded to the main body of the casting Apart from simplification, structural division enables the best material to be utilized for each section of the design . It is thus possible to incorporate wear resisting alloys at critical points where their particular properties can be exploited The above emphases on the use of castings in multiple construction needs to be seen as one useful approach to design
Supplementary techniques
Apart from multiple construction, several supplementary techniques are available to extend the design potential of cast products They include the local introduction of special properties by means of inserts, bimetal casting and surface treatment Such techniques may be employed either as a means of lowering product cost or to achieve otherwise unobtainable combinations of properties
Inserts
Metal inserts, of different composition from that of the casting, may be incorporated permanently in the structure by placing in the mould before pouring Inserts are widely used in die castings as local reinforcements for the relatively soft parent alloys For example steel or bronze bushes can be included to provide bearing surfaces Machinable inserts can be embodied in sand castings of wear resistance alloys (which are difficult to machine, enabling bores, keyways and similar features to be cut after casting) Fig illustrates a series of crane wheels in austenitic manganese steel, provided with cast-in carbon steel bushes to carry the shafts (page No393)
Bimetal casting technique
True bimetal casting of tubular form are produced by sequential pouring of separate alloys in centrifugal casting Alloys of widely different properties can be combined in a single structure by successive pouring of two alloys under closely controlled conditions Alloy combinations are restricted by the relative melting temperatures and the danger of excessive remelting of the outer by the inner layer But under suitable conditions a metallurgical bond is achieved without this difficulty A example is the spinning of cast iron friction linings into steel casings for the production of aircraft brake drum (page no 395) It is a process whereby two alloys are poured and centrifugally cast into one metallurgically bonded brake drum. The outer rim of the drum where the gear teeth are cut is composed of tin bronze. The inner portion where the shaft is to be keyed or splined is manganese bronze
Surface treatment of castings
Many processes are used to develop special properties over the whole or part of the casting surface They are adopted either for enhanced appearance and corrosion resistance or to improve wear or fatigue resistance These can be achieved by surface hardening , by generation of residual compressive stresses in the surface layers, shot peening, carburising and nitriding Polishing also contributes to fatigue resistance One modern development in this is laser treatment
Laser treatment
In this electrical energy is converted to a high intensity coherent light beam Which produces a remote local heating effect on contact with the metal surface Rapid heating of a thin layer is followed by self quenching The effect can be used to achieve martensitic transformations and hard cases on cast irons and steels of suitable composition
The surface treatments extend the effective range of application of the parent alloy The choice must again be made on both technical and economic grounds, including suitability of the treatment to the bulk material Thus a decision must often be made between a cheaper material with a surface coating and a more extensive alloy with intrinsic surface properties required
Design for minimum costing
A good casting design will incorporate saving wherever possible The following economic factors may be considered to incorporate saving i) If possible, modify the casting design in order to - simplify pattern construction, - Withdraw the pattern from the sand mold easily ii) Reduce the number of operations required to make mold and cores and to assemble them for making a casting iii) Place the parting line properly iv) If it is difficult to produce a one piece casting, the cast weld construction may prove to be a good alternative
v) The economics involved in cored holes versus drilled holes should be studied on an individual basis for each casting. Large holes are generally produced with the help of cores vi) Good casting design takes in to consideration as how castings will held for machining and other subsequent operations vii) A good casting design aims at reducing the weight of the casting especially when it is a component of aircraft, spacecraft or an automobile car
Economic Casting
The casting should be so designed that it can be most economically produced Economy is one of the prime aspects of all designs. Many times, more importance given to economy may reduce the functional requirements of a casting Thus , it is always necessary to strike a balance, and select an optimum design meeting economic as well as functional design For a given casting job, there is usually one casting process that will produce the best result and be least expensive The designer must select this optimum process from among the methods available
The optimum process may be defined as the one that will give the lowest total cost of the part produced (not the lowest cost of casting alone) Importance should be given to the following aspects which have a direct impact on the economics of the productThe total cost of a part includes I) Tooling cost II) Rough casting costs, and III) Machining and finishing costs
I) Tooling costs
Proper types of tooling must be designed and used according to the requirements of quality, quantity, and delivery period Tooling in casting process includes patterns, cores, core/mould boxes etc Priority must be given to design of tools which are reusable and useful for other operations These costs are divided over the number of parts produced The casting design should try to reduce the tooling costs, to achieve economy In case of short production runs making pattern equipment is cheap compared with form tools
II) Rough casting cost
These include 1) Metal costs 2) Direct foundry costs 1)Metal costs: This is the most important of the cost factors. It can be reduced. These costs can be minimized by a) Designing for minimum section size b) Close dimensional tolerances c) Simplifying the moulding and casting procedures d) Minimizing wastes
a) Designing for minimum section size
The minimum section thickness which may be cast for several alloys depends on the metal to be cast Such information is generally provided in metal data hand books Example: the minimum section thickness for al-alloys in sand castings ranges from 3 to 5 mm, for copper alloys in sand casting it is about 3mm, for grey and white iron it is about 3mm, for steels it is about 5mm The minimum section thickness in die castings differs from that in sand castings and usually ranges from 1.75 to 2.5mm for various metal alloys Which are all can be had from the standard data hand book
b) Close dimensional tolerances
The dimensional accuracy is an important consideration in the design of castings Usually the casting require machining allowances for machining, shrinkage allowances to compensate metal shrinkage, and drafts to facilitate easy withdrawal of patterns In green sand moulding the dimensions obtained on the casting depends on shrinkage, mold hardness, mold permeability, alignment of flasks, temperature effects, stability of moulding sand etc On the other hand , in die casting the controlling factors are less compared to sand casting and hence close dimensional tolerances can be obtained
c) Simplifying the moulding and casting procedures It is possible to design a casting to simplify the foundry operations of molding, coring and cleaning Many changes in the casting design which make molding easier and economical are to be considered Some important factors considered in simplification of foundry practice are i) Pattern drawing ii) Elimination of coring iii) Casting simplification iv) Shape & size
i) Pattern drawing
Creating complex casting shapes makes pattern drawing & moulding difficult and such designs are to be avoided or modified Undercuts or protruding bosses, flanges etc ( which lie above or below the parting line) require the use of cores or loose pieces Such situations can be eliminated with a better design Two such examples, in which undercuts and protruding bosses are eliminated so that pattern drawing becomes easier are illustrated in fig
Deep pockets, especially when they are thin sections, are difficult to draw in green sand molding Such castings may be moulded in drag with sufficient draft, which makes pattern drawing easier, and sand does not cling on to the pattern Such an improved design making pattern drawing easier If the casting is designed with a minimum of vertical surfaces and good taper on the side walls and ribs, then draft is not needed as an allowance on the pattern fig
ii) Elimination of coring
Moulding with cores is generally both expensive and time consuming, as compared to green sand moulding without cores The original design may involve the use of core, but redesign with the elimination or reduction in coring in the mould is beneficial Fig illustrates one such redesign in which coring is eliminated In the original design undercuts are required to provide clearance for other components in the assembly In the improved design these undercuts are eliminated along with the need for cores
Another benefit of elimination of coring is the reduction in cleaning operations In a mould the errors in core assemblies are additive Hence the elimination of coring improves the overall dimensional accuracy of the mould Also, elimination of coring eliminates the core shifting problem In case cores cannot be eliminated in the design, then the cores must securely anchored with prints and chaplets to prevent shifting or raising and related casting defects
iii) Casting simplification
The important means of reducing molding and coring problems is the simplification of casting itself For simplification of the process, sometimes a one piece casting can be made in two or more pieces, and then assembled by welding or by bolting For example, the casting shown in figure, is difficult to cast in a single piece . But it can be cast into four pieces, and then assembled them into a single component, by welding or some other means
iv) Shape and size
Compactness in design is another method of simplification of the foundry practice The compact design permits more castings per mold and less mold sand is needed per casting This reduces labour and material cost Compactness refers to the elimination of unnecessary extensions attached to the main body of the casting , which need not be cast with the main body of the casting
Rough casting cost contn…..
2)Direct foundry costs: These can be reduced by simplifying the procedures of moulding, core making, cleaning, etc as far as possible Any changes in the casting design that would make moulding, core making, and fettling easier and less costly without affecting the design and functional requirements of the casting should be considered
III) Machining and finishing costs Most castings require machining and finishing before they are used as a product Machining and finishing time spent on a casting can be saved in many cases if appropriate tolerances are specified and unduly high machining allowances are not provided on the pattern These costs can be kept to a minimum by designing the casting to Close dimensional tolerances Minimum wastage Defect free solidification Use of cores
Clear understanding between the designer and foundryman can help in reducing unnecessary machining work
It is important to note that the overall cost of casting varies with factors other than basic material cost So the selling prices based solely upon casting weight can be fundamentally unsound The true cost depends not only upon weight of metal but depends upon the individual design, quality and quantity requirements up on manufacturing technique and costs The foundry costs for an individual casting are not simply related to the weight or volume of cast metal but rather to the complexity of the design.
Functional design
The most important consideration in casting design is how functional it will be Function which a casting is supposed to perform is more important than its cost, ease of manufacture or appearance Sometimes there will be only one design, which will meet the service requirements and any modification would lead to unsatisfactory service of the designed component In such cases even if the design is not favorable for foundry practices, the foundry man may have to produce the casting as it is, by using all the technical abilities available to him In certain cases, the designer may alter the structural characteristics of the part, so as to produce a better and more economical casting . However, this should not affect the functional requirements of the product
Pattern and core box design
Pattern
The main tooling for sand casting is the pattern that is used to create the mold cavity. The pattern is a full size model of the part that makes an impression in the sand mold However, some internal surfaces may not be included in the pattern, as they will be created by separate cores.
The quality of the casting produced depends largely on the allowances provided, material of the pattern, its design and construction The costs of the pattern and the related equipment are reflected in the cost of the casting
Pattern design
A good pattern may produce a sound casting but a bad pattern will always result in a poor casting Therefore, in order to construct a good pattern, the factors listed below must be taken into consideration while designing a pattern 1. A pattern should be accurate as regards its dimensions and possess very good surface finish 2. Considering the following factors a proper material for making the pattern should be selected
The number of castings to be produced ( Metal – quantity is large) The desired dimensional accuracy and surface finish required for the casting Nature of molding process( sand casting, permanent mold, shell, investment) Method of molding ( hand or machine) Shape, complexity and size of the casting Type of molding materials ( sand . metallic) The chances of repeat orders
3. A pattern should carry all those allowances which are essential to simplify the molding operations and impart accurate dimensions to the final casting Shrinkage or contraction allowances Machining or finish allowances Draft or taper allowances Distortion or camber allowances Shake or rapping allowances
4. In case of split patterns, the parting surface should be such that the maximum portion of the pattern remains in the drag. Offset parting is beneficial
5. All sharp edges and corners should be rounded It helps removing the pattern from the moulding box Permits a smooth flow of the metal into the mold cavity Minimizes casting stresses and strain and produces good castings
6. The type of pattern should be selected after giving due considerations to all factors Match plate patterns are preferred for machine molding whereas both match plate and gated are recommended for small parts on mass scale
7. Suitably designed and located runners and gates and attached to the patterns reduce the work of the moulder 8. Core prints provided with the pattern should be of optimum size and suitably located 9. All those patterns of the castings for whom repeat orders are expected should be coated with preservatives, suitably marked
Preparation before pattern making The preparatory work includes decision about
1
Considerations as regards the value of allowances to be used 2 The type and form of material to be used 3 The type of pattern to suit the method of moulding to be adopted 4 The provision of core Constructional details, including the provision of loose pieces, core prints etc The method of gating and feeding to be followed
Pattern allowances
“Pattern allowances” is a vital feature in pattern design as it affects the dimensional characteristics Thus, when the pattern is produced, certain allowances must be given on the sizes specified in the finished component drawing so that a casting with particular specifications can be produced The selection of correct allowances greatly helps to reduce machining costs and avoid rejections The allowances usually considered on patterns are now detailed
Pattern allowances A pattern is always larger in size as compared to the final casting It carries certain allowances due to metallurgical and mechanical reasons Metallurgical – Shrinkage Mechanical -- Machining, draft, shake, distortion
Trial and Error method
It is very difficult to predict exact allowances to be given by any mathematical rule or process. Trial and error is best to achieve adjustment of pattern dimensions to obtain desired sizes. The selection of correct allowances greatly helps to reduce machining cost and avoid rejections.
Types of pattern allowances
Shrinkage or contraction allowances (Metallurgical)
Machining or finish allowances
Draft allowances or taper allowances
Distortion or camber allowances
Shaking or rapping allowances
(Mechanical)
Shrinkage allowances It refers to the reduction in volume caused when metal loses temp in solid state Almost all cast metals shrink volumetrically after solidification So casting is made oversize by an amount equal to that of shrinkage or contraction.
Shrinkage allowances Contn… Different metals shrink at different rates because shrinkage is the property of the cast metal or alloy Metal shrinkage depends up on
The cast metal or alloy
Casting dimensions
Pouring temperature of the metal( cast iron poured at high temp will shrink more than that poured at lower temp) Moulding conditions ( Mould material & moulding method)
Shrinkage allowances for cast metals The value of shrinkage as obtained from table is only a guideline Actual contraction Depends on several factors such as i) Composition of metal and impurities and constituent present ii) Method of moulding used, mould design, resistance offered by the mould to shrinkage iii) Pouring temperature iv) Intricacy of the casting
Machining allowances It provides for sufficient excess metal on all cast surfaces that require machining Machining allowance is the extra material added to certain parts of the casting to enable their finishing or machining to the required size The amount that is to be added depends on the size & shape of casting
Reasons for machining allowances
Castings get oxidized in the mould- scales It is intended to remove surfaces roughness and other imperfections from the castings It is required to achieve exact casting dimensions
Providing too large machining allowance -heavier casting- rise in production cost Providing too small machining allowances- difficulty in machining- rejection
So allowances should be optimum
How much? Depends on
Nature of metal( Ferrous or non ferrous)
Size and shape of the casting
The type of m/c operations (grinding removes much lesser than turning)
Mould process employed ( metal mould need little m/c )
The degree of surface finish required
The following schematic table gives machining allowances recommended for different cast metals.
Machining allowances
Draft allowances It is given to all surfaces perpendicular to the parting line The draft allowance is given so that the pattern can be easily removed from moulding material
A
B Draft
Surfaces of the pattern are given a slight taper in a direction parallel to which it is being withdrawn
Draft allowances contn…. It is easy to draw the pattern having taper allowances out of the mould without damaging the mould walls Draft allowance is imparted on internal as well as external surfaces
Draft allowances contn… The amount of draft depends on i) The length of the vertical side of the pattern to be extracted ii) The intricacy of the pattern iii) The method of moulding Height of wall( mm) < 50 51-100 101-200 201-300 301-500 501-800
External 0.8 1.2 1.8 2.5 3.5 5
Taper (mm) Internal 1.8 2.5 3.5 5 8 12
The draft varies for hand and machine moulding – more draft for hand compared to m/c moulding
Distortion allowances Some times casting get distorted during cooling due to i) Typical shape ( irregular shape) ii) All its parts do not shrink uniformly iii) If arms are having unequal thickness If the casting has the form It will tend
SO Initially make taper
Gets parallel
The distortion may occur due to internal stress and unequal heat transfer
Distortion allowances Contn… Distortion can be practically eliminated by providing an distortion allowances The amount of distortion that may occur is extremely difficult to work out because it depends on several parameters
The usual approach followed is trial and error- producing a trial casting - measured – camber is provided in the reverse direction
Reasons
The distortion in casting may occur due to internal stresses in casting Which in turn may be caused on account of unequal cooling rates of different sections of the casting Unequal heat transfer rates
Remedies for distortion
Modification of casting design in consultation with the pattern maker and the foundryman so as to avoid abrupt changes in sections Improving foundry practice and reducing casting strain by selecting metal that will be subject to minimum contraction Providing sufficient machining allowances to cover the distortion effect Controlling pouring temperature Straightening distorted casting ( cold pressing for small and medium sized castings and hot pressing by heating under load to 400-500ºc for large castings )
Shaking allowances This is provided for easy removal of pattern
A pattern is shaken or rapped by striking the same with a wooden piece from side to side Pattern is loosened a little in the mould cavity Rapping enlarges the mould cavity which results in bigger sized casting Negative allowance is provided on the pattern Pattern dimensions are kept smaller in order to compensate the enlargement of the mould cavity due to rapping Normally provided to large size casting It is negligible in case of small castings
In small and medium size castings, this allowances may be ignored But for large sized castings or where high precision is desired, this allowances should be considered Its value is decided by experience or by trial as no guidelines can be made for this allowances
Pattern Materials
Pattern materials Materials used to prepare pattern Factors which affect the materials
The number of castings to be produced ( metal for large no’s ) The desired dimensional accuracy and surface finish required for the castings ( Metal & Plastic)
Method of moulding ( Hand or machine)
Nature of moulding process
Shape , complexity and size of the casting
Type of moulding materials
The chances of repeat orders
Properties of pattern materials
Easily worked, shaped & joined
Light in weight for easy handling
Strong, hard and durable
Resistance to wear, corrosion, abrasion & chemical action
Available at low cost
It can be repaired
Different materials Wide variety of pattern materials are available some of important them are
Wood
Metal
Plastic
Plaster
Wax
Different materials have their own advantages, limitations and the field of applications
Wood-Large in size & small in number- Less wt & less life Metal-Small in size & large in number- Heavy & higher life Plastic- for high dimenstional accuracy- surface finish Plaster- Small intricate shapes- correct water & additives Wax- Very high dimensional accuracy
Wood Wood is the most commonly used material for patterns as it satisfies many of the requirements
Inexpensive Easily available in large quantities It can be easily shaped or worked and joined to form any complex shape Light in weight Can be repaired easily Easy to obtain good surface finish Wooden patterns can be preserved for quite long times with the help of suitable wood preservatives ( shellac)
Wood, like all leaving matter, is composed of cells resembling long thick tubes with tapered ends The cell wall consists of cellulose fibres, aligned parallel to the axis of the cells, and bonded together by a complex amorphous material called lignin Wood contains 50-60% cellulose and 20-35% lignin Smaller amounts of other carbohydrates are also present
Properties of wood
Wood to be used for making patterns should be free from knots and other common defects. It should be straight grained It should be properly seasoned in order to make it:
Stable in dimensions Stronger and lighter Resistant to decay Take preservative, paint, or polish
Drawbacks
They absorb moisture and change shape
They cannot withstand rough handling
They are week as compared to metal patterns
Wooden patterns may shrink and swell
They are week wear resistant
If the castings are large in size and small in number wooden patterns are preferred
Seasoning of wood Natural method Artificial methods Natural method :i) Here wood is stacked suitably in open spaces and subjected to air Draying for a period of time extending up to one full cycle of weather conditions ii) Timber is immersed in river water- moisture is removed then air draying for short time Artificial method: i) Kiln method: The timber is stacked in draying kiln and subjected to fast air draying by allowing hot air to pass through the kiln chamber ii) Electrical method: Fairly large sections of timber are exposed to a high frequency electric field such that the moisture content is brought down to the desired level within few minutes iii) Chemical method: Wood is kept immersed in suitable salt solutions and then exposed to air for drying
Common woods
White pine
Teak
Mahogany
Deodar
Shisham
Besides the woods Plywood boards are also used- only for flat type pattern
Common woods Contn… i) White pine: Though week , is often favored for its extreme lightness, stability, ease of working and ability to take good finish ii) Teak : Teak is a hard and strong unaffected by fungus and easily available in the country iii) Mahogany: It is a hard , strong and very durable type of wood with negligible shrinkage or swelling after seasoning iv) Deodar: Is a soft variety, slightly harder than pine but easily machinable and takes good polish
Metal Metallic patterns are used where repetitive production of castings is required in large quantities Many of the patterns used in production work are made of metal because of its Ability to withstand hard use
Unlike wooden patterns they do not absorb moisture, they retain their shape They are more stronger and accurate They posses life much longer than wooden patterns They have accurate dimensional tolerances They are far stable under different environments It is easy to obtain smooth surface finish
Drawbacks
Expensive as compared to wood patterns
Are not easily repaired
Patterns may get rusted
They are heavier than wooden patterns
They cannot be machined easily as wooden ones
If the castings are large in number metal patterns are preferred
Common metals
Aluminium
Steel
Cast iron
Brass
Comparative evaluation of metals Factors
Aluminium Steel
Cast iron
Brass
Availability Castability Machinability Surface finish Lending to modification Weight Brittleness Tendency to oxidation Requiring machining cost
Good Less difficult Very good Very good Good Very light Low No Not much Medium
Good Good Good Good Good Very heavy High Yes Yes Low
Good Good Very good Very good Very good Heavy Low No Not much High
Good Difficult Good Good Good Very heavy Low Yes Less Low
Plastic Both thermosetting and thermoplastic materials are used for pattern work
It gives fine surface finish It is more durable A plastic pattern does not involve any appreciable change in its size & shape It is light weight Good strength High wear& corrosion resistance Pattern does not stick to mould material Patterns are easy to make Good resistance to chemical attacks
Drawbacks
Pattern itself is costly Plastic patterns are fragile and thus light sections may need metal reinforcements May not work well when subject to conditions of severe shock as in machine moulding
Thermosetting plastic material is used for long lasting and durable patterns (Epoxy & polyester resin)
Thermoplastic material is used for short runs or piece work (Polystyrene)
Comparative evaluation of plastics Epoxy resins Very popular because of their
Easily castable nature
High strength to weight ratio
Polyester resins As a cheaper substitute for epoxy resins
The patterns produced are rigid
Patterns are strong
Durable
Setting time is very less
Production rates can be much faster
Low cost of working Good resistance to wear and abrasion Complete immunity from the action of moisture
The cost is less than half that of epoxy resin
Plaster Plaster of Paris when mixed with a correct quantity of water sets in a given time and forms a hard mass having high compressive strength
It can be easily worked by using wood working tools
Intricate shape can be cast without any difficulty
It has high compressive strength
High surface finish can be achieved
Free from corrosion
By careful design and use of hardening materials, strength and hardness of plaster can be enhanced
Drawbacks
It cannot withstand severe shock
It can be used only to make small casting patterns
It cannot withstand more than 300 kg/Sq mm
Wax
Wax patterns are excellent for the investment casting It is mainly used where high accuracy is necessary . The material generally used are blends of several types of waxes and other additives
Provides very good surface finish
They impart high accuracy
Free from corrosion
No oxidation
Pattern is not removed Made in a water cooled two piece metal mould or a die
Drawbacks
It cannot withstand high compressive ramming
It cannot withstand severe shock
Waxes used are paraffin , shellac, bees and microcrystalline wax
The normal practice of forming wax pattern is to inject liquid or semiliquid wax into a split die Solid injection also is used to avoid shrinkage and for better strength , however , much higher injection pressure are required for solid injection If wax is poured into the die in a liquid state under gravity, it will shrink much more and the size of the pattern will be affected As the patterns are generally small in size, a number of patterns are welded to a common runner or sprue through in-gates The whole assembly of patterns complete with runner, sprue etc Which is often termed a ‘tree’ , is then invested to prepare the investment mould
Patterns
Pattern
The main tooling for sand casting is the pattern that is used to create the mold cavity. The pattern is a full size model of the part that makes an impression in the sand mold However, some internal surfaces may not be included in the pattern, as they will be created by separate cores.
Choice of Patterns The type of pattern selected for particular casting will depend up on The size & shape of the casting Simple- single piece Complex- split pattern More complex- loose piece Small sizes- Gated The number of castings required small no- single piece more no- Gated pattern Method of moulding employed Hand moulding –single piece M/C moulding – Cope & drag Problems associated with the moulding operations such as withdrawing the pattern from the mould Easy- Single difficult- Split more difficult –loose
Types of patterns
Single piece or one piece or solid pattern- simple, large , no withdrawal problem Split Patterns – i) Two piece ii) Multi piece – Difficulty in withdrawal Cope and drag patterns ( separate plates) ( High mechansied) Match plate pattern ( Split pattern on either side of common plate) Sweep pattern ( Surface of revolutionary contour) Wooden board Follow board pattern ( Fragile and thinn) Gated pattern ( mass prodn, Rapid moulding, gate and runner are part of pattern) Loose piece pattern ( complex shapes)
Single pattern
The one piece or single pattern is the most inexpensive of all pattern Generally made of wood, it is best suited for limited production Moulding with such a pattern requires manual operation It is inexpensive and used for large castings of simple shape With these patterns , the moulder has to cut his own runners, feeding gates and risers The mould cavity of this pattern will be either in the drag or entirely in the cope
Split pattern i) Two piece
Many patterns cant be made in a single piece because of the difficulties encountered in removing them from the mould To eliminate this difficulty some patterns are made in two parts so that half of the pattern will rest in the lower part of the mould and half in the upper part The split in the pattern occurs at the parting line of the mould The two parts of the patterns are alligned with dowel pins
ii) Multi piece
Sometimes a pattern is constructed in three or more parts for complicated castings
Cope & drag pattern
Cope and drag plates consists of the cope and drag parts of the pattern mounted on separate plates The cope and drag halves of the mould may thus be made separately by workers on diff moulding machines The two moulds are prepared separately using separate machines and assembled Separate cope and drag plate are more costly, but this type of pattern equipment is usually necessary in high speed mechanized or automated moulding
Match plate pattern
Large quantity production of small castings require match plate patterns The cope and drag portions of the pattern are mounted on opposite sides of a wood or metal plate conforming to the parting line Match plate patterns are also integrally cast in which cast pattern and plate are cast as one piece in sand or plaster mould A number of different sized and shaped patterns may be mounted on one match plate
Sweep pattern
It is used when the casting has a surface of revolution contour such as cyclinderical , bell shape etc The sweep pattern consists of a wooden board having a shape corresponding to the shape of desired casting Sweep patterns avoids the necessity of making a full, large circular and costly three dimensional pattern
Follow board pattern
A follow board is a wooden board and is used for supporting a pattern which is very thin and fragile and which may collapse under the pressure when the sand above the pattern is being rammed
Follow board supports the weak pattern
Follow board holds the pattern till the moulding is over
Gated pattern
Gated patterns are used when parts to be caste are of simple shape and produced in large quantities Here number of castings are produced in a single multicavity mould by joining group of patterns The gates and runners for the molten metal are formed by the connecting parts between the individual patterns Generally patterns are made from metals to remain stronger so that they can be reused to prepare many moulds The time spent by the moulder in cutting gates & runner is eliminated
Loose piece pattern
Certain patterns cannot be withdrawn once they are embedded in the moulding sand Such patterns are usually made with one or more loose piece for facilitate their removal form the mould box
Core & Core prints
Core
Core is defined as that portion of the mould which forms the hollow interior of the casting or hole through the casting Core means mass of dry sand which is prepared separately, baked in an oven and then placed in the mould Cores are required to create the recesses, undercuts and interior cavities that are often a part of casting Cores are also used on the outside of the casting to form features such as lettering on the side of a casting or deep external pockets
Functions of cores
For hallow castings, cores provide the means of forming the main internal cavities Cores may provide external undercuts features Cores may be inserted to achieve deep recesses in the castings Cores may be used to form the gating system of large size molds
Characteristics of cores
The core should have sufficient strength and hardness in both green and dry states so that it may be able to support its own weight and withstand force of the molten metal High permeability to let the mold gases escape through the mold walls The core should be able to withstand the high temperatures of the molten metal Should have smooth surfaces to ensure a smooth casting The core sand should produce a minimum amount of gas when it comes in contact with molten metal After the metal solidifies , the core should disintegrate and collapse, otherwise difficulty may be experienced in removing it from the casting
Steps in making cores
Core sand preparation
Making the cores
Baking the cores
Finishing the cores
Setting the cores
Core sand preparation The characteristics required by core are imparted to core sands by selecting proper type and amount of binders, additives and by suitably baking the cores Core sand ingredients are i) Granular refractories ii) Core binders iii) Water iv) Special additives i) Granular refractories: a) Clean, pure and dry silica sand b) Zircon c) Olivine d) Carbon ii) Core binders: used to holds sand grains together and gives strength to cores a) Organic binders : Core oil, Cereal, Wood product binders b) Inorganic binders : Fire clay, Bentonite, Silica flour c) Other binders : Portland cement, sodium silicate iii)Water: In a core sand mix, water may vary between 2.5 to 7%.binders and additives work only in the presence of moisture iv) Special additives: Facing material, cushion material
Making the cores
Small cores can be made manually in hand rammed core boxes. Cores on mass scale are rapidly produced on a variety of core making machine i) Jolt machine (Flask raised and dropped) ii) Sand slinger ( throughing a stream of sand downwards) iii) Core extrusion machine iv) Core blower (The core sand is forced with a high velocity)
Baking the cores
Ones the cores have been prepared manually or on a machine they are placed on core plates and sent for baking. Core baking develops the properties of the organic binders Baking
Core ovens Continuous type Batch type Drawer type ( Small sized) Rack (All size)
Dielectric bakers (heating) (An ac applied bet pair of flat plate electrode)
Core ovens
Core ovens can be used for baking cores for both limited and mass production Core ovens are heated by fuel like coal, coke, oil etc They are further classified as i) Batch type ii) Continuous type
Batch type
They bake cores in batches
They can handle wide variety of cores of small and medium sizes
Here cores are loaded on trolley. The trolley is pushed inside the oven and the oven is switched on
Continuous type
Preferred for a high production of small and medium size cores
Cores move slowly on a conveyor
The loading and unloading is continuous
Dielectric bakers
These bakes in a small fraction of the time The core to be heated dielectrically is placed between the two electrodes and a high current is passed through it Cores get baked uniformly from centre to surface Top electrode is an aluminium plate whereas the conveying belt made up of steel acts as the lower electrode
Core making process
There are mainly three methods used to strengthen the core i)By baking ( Base sand+binder+additives+water) ii) By gassing ( Base sand+sodium silicate binder+additives)+ CO2 iii) By exposing to atm air ( Base sand +binder+ hardner+ catalyst)
Finishing cores
Baked cores are finished before they can be set in the mould Core finishing consists of i) Cleaning ii) Sizing ( Dimensional accurate) iii) Core assembly (Joining more than two)
Setting the cores Core setting means placing cores in the mould
Types of cores The cores used in foundries are classified according to their shape and Position in the mould Horizontal core
Vertical core
Hanging or cover core
Balanced core
Drop or stop off core
Ram up core
Horizontal core It is the most common type and it is positioned horizontally in the mould A horizontal core may have any shape circular or some other section depending up on the shape of the cavity required in the casting
Mould Core
The ends of the core rest in the seats provided by the core prints on the pattern
Vertical core The core, when required to be placed along a vertical axis in the mould is known as vertical core The upper end of the core is forced in the cope and the lower end in the drag The two ends of a vertical core are supported in core seats in the cope & drag
Horizontal & Vertical cores are used in foundry wok more frequently than any others
Hanging or cover core If the core hangs from the cope and does not have any support at the bottom in the drag , it is referred to as hanging core
A hanging core provided with a hole through which molten metal reaches the mould cavity
Hole
Core Mould
Balanced core A balanced core is one which is supported and balanced from its one end only
When the casting has opening only on one side, then a balanced core is used Core print should be sufficiently large to support the weight of the core which extends in to the mould cavity and it should be able to withstand the force of buoyancy of the molten metal which surrounds it Chaplets are used to support the core in the mould cavity
Drop or stop off core A stop off core is used when a hole , recess or cavity required in a Casting is not in line with the parting surface, rather it is above or Below the parting line of the casting
Core
A stop off core is employed to make a cavity which cannot be made with other types of cores
Mould
Ram up core Sometimes the core is set in the mould with the pattern before ramming it, such a core is known as ram up core
It is used a) when cored details is located in an inaccessible position b) For both interior & exterior portions of a casting
Mould Core
Core prints The core print is added projection on the pattern, which forms a seat in the mould for resting the core during the pouring operation. 1)The holes recesses etc of various sizes and shapes in the castings can be obtained without any difficulty by using cores. 2)To support the core in the mould cavity, an impression in the form of a recesses is made in the mould cavity with the help of a projection 3)The projection on the pattern is referred to as core prints The size & shape of the core print should be adequate so that the weight of the core can be supported during the casting operation
Types of core prints
Horizontal core print
Vertical core print
Cover core print
Balanced core print
Wing core print
Horizontal core print This type is provided when the core is laid horizontally in the mould
Core print
Core print
One core print is kept at either end of the core. The pattern is in two halves and the core is placed at the parting plane, such as required to obtain the central hole or bore of a pulley or a pipe
Vertical core print This is another common type of core print, in which the core stands Cope print vertically in the mould
Drag print
The two core prints , one for the top support and other or bottom Support are called cope and drag prints respectively. There are also occasions when the cope print is not provided and the core rests only in the bottom print
Cover core print Also known as hanging core-print, this type is provided when the entire Pattern is rammed in drag and the core is suspended from the cope side Without allowing it to have support at the bottom As there is no bottom print, the top print has to be made passive
The size of the core print will depend on i) The volume of core to be suspended ii) Wheather chaplets are used iii) The type & strength of sand mix
Balanced core print When the shape of the casting is such that it is not possible to support the core from the both sides, a balanced print is required.
The core and core print are then so designed that the part of the core in the mould cavity balances and the other part rests in the core seat
Wing core print When a hole or recess is required to be cored above or below the Parting line, a wing core print
In the arrangement, as the core is over hanging from one side the core print has to be large and generous taper is required to be provided on the vertical walls of the core
Core print
Mould
Core print
The core print is added projection on the pattern, which forms a seat in the mould for resting the core during the pouring operation. The size & shape of the core print should be adequate so that the weight of the core can be supported during the casting operation
Core print design
In order to support the core in the mould cavity, an impression in the form of a recess or seat is made in the mould with the help of a projection suitably placed on the pattern This projection provided on the pattern , which will produce a corresponding cavity in the mould to support the core called core print To ensure the desired accuracy of production of the casting, the core must be adequately supported in the position and be rigidly clamped
Requirements of the core
Once it is assembled in place and the mould closed, the core should not move or shift its position or get displaced Withstanding the buoyancy force or side way thrust of the molten metal To fulfill these requirements, the design of the core prints must be prepared
Major considerations in core print design
The print must balance the body. So that the core stays in place during mold assembly The print must withstand the buoyancy force of the metal and not get crushed The print must not shift during mould filling The print should minimize the deflection of the core The print should minimize the heat transfer from the core to the mould The print should allow internal gases generated in the core to escape to the mould
Theoretical consideration
Buoyancy of the molten metal is the main factor which may cause the core to rise or shift. So the force with which the core is held in the core print must be larger than the force of the buoyancy Since the core print holding force must be larger than the buoyancy force , the difference between buoyancy force and core print holding force, known as unsupported load, must be negative
So unsupported load = Core buoyancy force - Core print holding force Where core buoyancy force = (density of cast metal – density of core sand) X core volume Core print holding force = Total core print surface area X Compressive strength of mould sand The unsupported load so obtained should be zero or negative
Results verification
The unsupported load so obtained should be zero or negative to ensure stability of the core in the mould
If the result is a positive value, the proportions of the core print should be altered and the unsupported load again worked out to ascertain that its value is negative or zero
Dimensions of core print from tables The following ratios of length to diameter of the core print can be used for guidance D
For core supported on both sides
2D
Core length (mm)
Types of mould
< 50
Length of the core print (mm) for diameters 0-50
51-100
101-160
161-250
251-400
green
20
25
30
35
-
51-100
green
30
35
40
45
50
101-200
green
35
40
45
60
90/75
201-400
Green/dry
40
50
70/60
110/65
180/90
401-700
Green/dry
-
80/60
125/75
190/90
-/110
701-1200
Green/dry
-
-
135/90
220/110
-/130
Dimensions of core print from tables d For cores supported vertically Height of core (mm)
Length of bottom core print for diameters of core d <50
51-100
101-160
161-250
251-400
<50
30
30
30
30
30
51-100
30
30
40
40
50
101-200
40
50
50
60
60
201-400
50
60
60
70
70
401-700
60
70
70
80
80
701-1200
-
90
100
100
110
h
Core boxes
Like patterns core boxes also may be made of either wood or metal Wood is the more popular material because it is easy to glue and joint, freely available and cheap Metal core boxes, however, are preferred where better quality is desired and cores are to be mass produced The metals used for making core boxes are cast iron, steel, brass, bronze and aluminum alloy The cavity in the core box must be of the exact shape and size of the core required
Types of core boxes The shape of the core determines the type of core box to be used Half core box
Slab core box
Split core box
Strickle core box
Gang core box
Loose piece core box
Half core box When the shape of the core required is such that it can be prepared in identical halves, a half core box should be used
The halves are produced, one after the other, with the help of this core box and after they are dried, they are pasted together to form a complete core
Slab core box
If the core produced by the core box does not require any pasting and is complete in itself, the box designed is referred to as a slab core box
It makes full core at a time. It is employed for making rectangular, square, trapezoidal cores
Split core box
Split box
When W the core box is in two parts and a complete Core h results from a single ramming, the box Isecalled a split core box n
For alignment of the two parts, dowel pins are fixed in one part and corresponding holes are made in the other
core
For preparing the core, the two parts of the core box are held together by a clamp, and the core sand rammed from one side. The core is then taken out by separating the two parts
Strickle core box This is used when the core is required to have an irregular surface, which cannot be easily rammed by other methods. In this case, the desired irregular shape is achieved by striking off the core sand from the top of the core box with a piece of wood called a strickle board
The strickle is cut to correspond exactly to the contour of the required core
The strickle board is also ideal for large-sized work, such as for a pipe bend of, say 600 mm diameter or more, for which a regular core box, if prepared, would be too expensive
Gang core box
It contains a number of core cavities so that More than one cores can be rammed at one time
Pattern layout
The quality & cost of the patterns produced is largely affected by the layout A well designed layout can enable efficient and economical production Before the pattern is taken up for manufacture, the layout must be produced Layout is full scale drawing in either orthogonal or isometric projection , with complete details of the pattern including allowances, core prints, parting planes, loose pieces etc
Layout should have
The type of construction to be followed Joints used Proportions of wooden pieces and all other relevant details including size, shape and location of gates, risers etc are depicted on this layout which can be drawn on a sheet of plywood Lines are drawn with a scriber so that they are permanent Dimensions are accurately transferred on to the pattern by using dividers
Once the layout is prepared and checked by inspector it becomes easy for the pattern maker to make the pattern
Pattern drawings Pattern drawings are made by technical cell considering
Machining allowances Core print sizes and shapes Contraction allowances Draft angles Machining pads and fillets
The pattern maker must make his own full scale layout, This helps him in understanding the drawings, fixing the dimensions, construction method, machining sequence required for his pattern
Each and every information regarding manufacturing process is contained in the drawing itself - LAYOUT
Steps involved in pattern layout
Get the working drawing of the part for which the pattern is to be made Study the same carefully and thoroughly Make two views of the part drawing on a drawing sheet using shrink rule Add machining allowances as per the requirements Depending up on the method of moulding provide the draft allowance Depending up on the size and shape of the casting other pattern allowances may also be added Layout made by shrink rule
Machining allowances Draft allowances
Steps in pattern preparation
1 2
3
Study the pattern layout carefully and establish a) location of parting surface b) Number of parts in which the pattern will be made Using the various hand tools and pattern making machine fabricate the different parts of the pattern Join the different parts by glue and nails Inspect the pattern as regards the alignment of different portions of the pattern and its dimensional accuracy Fill the wax in all the fillets in order to remove sharp corners For smoothening and finishing purposes sand the pattern and give a shellac coating Impart suitable colors to the pattern for identification purpose
Master pattern
Most metal patterns are manufactured by the casting process Simple (regular shape) and small sized patterns can be produced by machining Occasionally patterns are also produced by the method of joining together by screwing, bolting or welding For all cast metal patterns, another pattern is required to produce the casting
Master pattern
The pattern made prior to the manufacture of actual metal pattern is called master pattern The master pattern is to be so designed and manufactured that it will produce the finished metal patterns of correct size
Manufacture of master pattern The pattern maker should consider the following points before making a master pattern
The design of the pattern is made depending up on the material in which the pattern is to be made Pattern maker should have the complete layout of the pattern and core box Sufficient ribbing with proper metal thickness should be given inside the hallow portion of the pattern for obtaining strength and rigidity The pattern maker should decide the m/c process of his patterns while making the master pattern After obtaining the castings it is very essential to check it for any deformity. These defects can be checked by simple visual observation with the help of straight edge
Allowances to be considered Allowances on the master pattern should be such that the metal patterns can produce castings of the desired size.
The contraction allowances on the master pattern dimensions would include the contraction of the casting and the contraction of the metal pattern Machining is required to be done on the casting as well as on the metal pattern. The machining allowances provided on both are added, and the sum of these all the allowances provided on the master pattern
Materials used to make master pattern
Wood: wood is the most commonly used material for making master patterns. Good quality wood like teak or deodar dully seasoned, which is strong and dimensionally stable is used Metal: Metals such as aluminium, brass or bronze are also used to make master patterns. These are made by machining from previously cast blocks or standard cast products
Computers in industry
Computers have now become an tool in every walk of life and for all sorts of applications, may it be administrative, operational, technological, trading, sports , communication or even domestic fields From industrial point of view, they have been in use in the administrative areas of finance, accounting, personnel records, wage and salaries Now computers have been introduced for various technological applications such as drawing and design, process selection, tooling design, component design, machine design, production planning and control etc
Use of CAD/CAM for pattern making Like in any other industry, computers have found a place in foundry also. Computers come in handy for simplifying various foundry operations
At the present juncture when the demand on the foundry industry to produce castings in large quantities of superior metallurgical quality and to close dimensional accuracy is fast increasing from sophisticated users like i) Automobile ii) Aeronautical iii) Electrical iv) Power plant v) Machine tool industries So computers use has been increased in foundry
Computers used in foundry
Casting design and development for optimum quality, weight and cost Carrying out and maintaining pattern design history , life, usage data & repair data Sand control for obtaining desired sand & mould characteristics Methodology and laying down correct technology of pattern and other foundry tooling design, mould design, die design and gating design Controlling heat treatments cycles Process control
Computers used in foundry contnn…
Deciding chemical composition to obtain desired properties, charge control melting & melt control Quality evaluation, quality records, inspection & testing Maintaining data base for the entire data about production quantities. Sand testing, production schedules, melt cycles etc Records about wages and salaries to staff and workers. Labor inputs Maintaining data about daily, weekly and monthly production and productivity Production planning and control, such as preparing technology sheets for every item and up dating it from time to time , job cards, tool tickets, inspection cards, scheduling charts and production control charts
Casting design and simulation
Casting design involves converting the part design to the tooling design, showing orientation in the mould, parting line, application of draft and allowances , gating and feeding systems, core boxes Simulation includes mould filling and casting solidification, useful for optimizing the design of gating and risering systems Casting model is the main input for simulation The purpose of simulation is to model the underlying physics so that important process variables can be identified and controlled If the process of filling and solidifying a mould cavity is accurately modelled, shrinkage cavities and other casting defects can be predicted
Software packages for foundry use
Magma soft : powerful simulation tools for the optimization of castings and foundry processes Cast CAE: developed as a tool for mould filling and solidification simulation AFS solid 2000: Is being serviced and marketed by the American Foundrymens society : the software combines thermal and volumetric calculations NovaFlow and NovaSolid: the main feature include gravity and flow consideration during mould filling and solidification proCAST: using FEM been adopted for thermal calculation database, micromodelling
CAD/CAM in foundry
The first step is computer aided design. In which a solid geometric model of the component is created on a computer, often directly in 3D. From this model, 2D drawings are produced, plotted and sent to the foundry They use sophisticated solid modelling software to create the component details with allowances Numerical control programs are generated These pgms can be loaded to CNC After the part design is finalized, it is sent to a computer aided manufacturing programmer to plan the tooling on a CNC machine
Rapid prototyping
Initially intended for creating prototypes of complex shaped products to verify their form, fit and to some extent , their function Based on philosophy of converting 3D CAD model of the part into a series of 2D cross sectional layers stacked on top of one another (fig) Defined as process that automatically create physical prototype from 3D CAD model in a short period of time Suppose if u design a CAD model the same part comes out of printer Magic printer If u design a cylindrical bearing –gives the print command – the part comes out of printer
Basic elements of RP
You should have a PC- create a solid model in CAD Then from this model CAD software gives a dot STL file This STL file is sent to a physical machine- 3D printerwhich makes model based on CAD information
Basics of printer
Here parts are build by adding layer by layer The concept used is if u give a CAD model . Which is sliced in to different layers
Pattern layout
The quality & cost of the patterns produced is largely affected by the layout A well designed layout can enable efficient and economical production Before the pattern is taken up for manufacture, the layout must be produced Layout is full scale drawing in either orthogonal or isometric projection , with complete details of the pattern including allowances, core prints, parting planes, loose pieces etc
Layout should have
The type of construction to be followed Joints used Proportions of wooden pieces and all other relevant details including size, shape and location of gates, risers etc are depicted on this layout which can be drawn on a sheet of plywood Lines are drawn with a scriber so that they are permanent Dimensions are accurately transferred on to the pattern by using dividers
Once the layout is prepared and checked by inspector it becomes easy for the pattern maker to make the pattern
Pattern drawings Pattern drawings are made by technical cell considering
Machining allowances Core print sizes and shapes Contraction allowances Draft angles Machining pads and fillets
The pattern maker must make his own full scale layout, This helps him in understanding the drawings, fixing the dimensions, construction method, machining sequence required for his pattern
Each and every information regarding manufacturing process is contained in the drawing itself - LAYOUT
Steps involved in pattern layout
Get the working drawing of the part for which the pattern is to be made Study the same carefully and thoroughly Make two views of the part drawing on a drawing sheet using shrink rule Add machining allowances as per the requirements Depending up on the method of moulding provide the draft allowance Depending up on the size and shape of the casting other pattern allowances may also be added Layout made by shrink rule
Machining allowances Draft allowances
Steps in pattern preparation
1 2
3
Study the pattern layout carefully and establish a) location of parting surface b) Number of parts in which the pattern will be made Using the various hand tools and pattern making machine fabricate the different parts of the pattern Join the different parts by glue and nails Inspect the pattern as regards the alignment of different portions of the pattern and its dimensional accuracy Fill the wax in all the fillets in order to remove sharp corners For smoothening and finishing purposes sand the pattern and give a shellac coating Impart suitable colors to the pattern for identification purpose
Master pattern
Most metal patterns are manufactured by the casting process Simple (regular shape) and small sized patterns can be produced by machining Occasionally patterns are also produced by the method of joining together by screwing, bolting or welding For all cast metal patterns, another pattern is required to produce the casting
Master pattern
The pattern made prior to the manufacture of actual metal pattern is called master pattern The master pattern is to be so designed and manufactured that it will produce the finished metal patterns of correct size
Metal pattern
Master pattern Mould cavity by master pattern
Manufacture of master pattern The pattern maker should consider the following points before making a master pattern
The design of the pattern is made depending up on the material in which the pattern is to be made Pattern maker should have the complete layout of the pattern and core box If required, at the parting line, a collar type metal strip is provided in master pattern which will help in mounting the pattern on match plate Sufficient ribbing with proper metal thickness should be given inside the hallow portion of the pattern for obtaining strength and rigidity The pattern maker should decide the m/c process of his patterns while making the master pattern After obtaining the castings it is very essential to check it for any deformity. These defects can be checked by simple visual observation with the help of straight edge
Allowances to be considered Allowances on the master pattern should be such that the metal patterns can produce castings of the desired size.
The contraction allowances on the master pattern dimensions would include the contraction of the casting and the contraction of the metal pattern Machining is required to be done on the casting as well as on the metal pattern. The machining allowances provided on both are added, and the sum of these all the allowances provided on the master pattern
Shrinkage & machining
Master pattern
Shrinkage & machining
Metal pattern
casting
Materials used to make master pattern
Wood: wood is the most commonly used material for making master patterns. Good quality wood like teak or deodar dully seasoned, which is strong and dimensionally stable is used Plywood: alternatively impregnated and compressed wood or plastic wood can also be used Metal: occasionally metals such as aluminium, brass or bronze are also used to make master patterns. These are made by machining from previously cast blocks or standard rolled products
Computers in industry
Computers have now become an tool in every walk of life and for all sorts of applications, may it be administrative, operational, technological, trading, sports , communication or even domestic fields From industrial point of view, they have been in use in the administrative areas of finance, accounting, personnel records, wage and salaries Now computers have been introduced for various technological applications such as drawing and design, process selection, tooling design, component design, machine design, production planning and control etc
Use of CAD/CAM for pattern making Like in any other industry, computers have found a place in foundry also. Computers come in handy for simplifying various foundry operations
At the present juncture when the demand on the foundry industry to produce castings in large quantities of superior metallurgical quality and to close dimensional accuracy is fast increasing from sophisticated users like i) Automobile ii) Aeronautical iii) Electrical iv) Power plant v) Machine tool industries So computers use has been increased in foundry
Computers used in foundry
Casting design and development for optimum quality, weight and cost Carrying out and maintaining pattern design history , life, usage data & repair data Sand control for obtaining desired sand & mould characteristics Methodology and laying down correct technology of pattern and other foundry tooling design, mould design, die design and gating design Controlling heat treatments cycles Process control
Computers used in foundry contnn…
Deciding chemical composition to obtain desired properties, charge control melting & melt control Quality evaluation, quality records, inspection & testing Maintaining data base for the entire data about production quantities. Sand testing, production schedules, melt cycles etc Records about wages and salaries to staff and workers. Labor inputs Maintaining data about daily, weekly and monthly production and productivity Production planning and control, such as preparing technology sheets for every item and up dating it from time to time , job cards, tool tickets, inspection cards, scheduling charts and production control charts
Casting design and simulation
Casting design involves converting the part design to the tooling design, showing orientation in the mould, parting line, application of draft and allowances , gating and feeding systems, core boxes, pattern plates Simulation includes mould filling and casting solidification, useful for optimizing the design of gating and risering systems Casting model is the main input for simulation The purpose of simulation is to model the underlying physics so that important process variables can be identified and controlled If the process of filling and solidifying a mould cavity is accurately modelled, shrinkage cavities and other casting defects can be predicted
Software packages for foundry use
Magma soft : powerful simulation tools for the optimization of castings and foundry processes Cast CAE: developed as a tool for mould filling and solidification simulation AFS solid 2000: Is being serviced and marketed by the American Foundrymens society : the software combines thermal and volumetric calculations NovaFlow and NovaSolid: the main feature include gravity and flow consideration during mould filling and solidification proCAST: using FEM been adopted for thermal calculation database, micromodelling
CAD/CAM in foundry
The first step is computer aided design. In which a solid geometric model of the component is created on a computer, often directly in 3D. From this model, 2D drawings are produced, plotted and sent to the foundry They use sophisticated solid modelling software to create the component details with allowances Numerical control programs are generated These pgms can be loaded to CNC After the part design is finalized, it is sent to a computer aided manufacturing programmer to plan the tooling on a CNC machine
Rapid prototyping
Initially intended for creating prototypes of complex shaped products to verify their form, fit and to some extent , their function Based on philosophy of converting 3D CAD model of the part into a series of 2D cross sectional layers stacked on top of one another (fig) Defined as process that automatically create physical prototype from 3D CAD model in a short period of time Suppose if u design a CAD model the same part comes out of printer Magic printer If u design a cylindrical bearing –gives the print command – the part comes out of printer
Basic elements of RP
You should have a PC- create a solid model in CAD Then from this model CAD software gives a dot STL file This STL file is sent to a physical machine- 3D printerwhich makes model based on CAD information
Basics of printer
Here parts are build by adding layer by layer The concept used is if u give a CAD model . Which is sliced in to different layers
Solidification
Introduction
Of all the engineering phases involved in manufacturing a quality casting, the most imported is the mechanism by which metal freeze The techniques for melting and handling the metal and preparing the mould are fairly well understood, and are subject to constant, positive control in any good shop But as soon as the metal fills the mould, control ends, and solidification proceeds according to the whims of nature Even today the many phenomena that influence solidification are not completely understood
Solidification of castings
The mechanism of solidification of casting and its control for obtaining sound castings is the most important problem of foundry men The proper understanding of the solidification mechanism is essential for preventing defects due to shrinkage of the metal As soon as the molten metal is poured in a sand mold, the process of solidification starts During solidification, cast form develops cohesion (attaching atoms together) and acquires structural (form) characteristics
Solidification process
It is a process involving the phase transformation of liquid metal to a solid state. This casting process results in removal of heat from the hot liquid metal resulting in conversion of liquid to solid Solidification process is also referred to as freezing process During freezing of molten metal in the mould, the latent heat of fusion of the metal has to be transferred to the surrounding atm through the mould to achieve transformation of liquid to solid metal The solidified metal will take the shape of the mould
Definition
Solidification may be defined in simple terms as the conversion of liquid metal at a high temperature to solid metal at a lower temperature Solidification involves the agglomeration (a collection) of atoms & their growth
Solidification mechanism
A through understanding of the mechanism of solidification , the rate of heat loss from the material to the mould etc is essential to predict how the casting will solidify and thus avoid casting defects like gas porosity, hot tears etc Since solidification requires energy to produce a crystalline structure some super cooling is required before the liquid metal starts to solidify It is provided by the walls of the mould which provides sites around which crystals can grow initially and subsequently by the solidified particles and the metal itself
Solidification mechanism contn..
So the crystals starts to grow from the mould walls and the process continues as more heat is lost, with crystal growing inwards until the whole of the metal has solidified The crystal near the mold walls are small and equiaxed on further solidification, crystal grow with their axes perpendicular to the mould and these are columnar in shape* The reason for columnar crystal growth in direction perpendicular to mould wall will be obvious from curves shown below
Reasons for columnar crystal Just after pouring
With lapse of time Gradient dec is max Gradient dec is min
Temp
Distance from mould walls
Shows distribution of temperature along the distance from the mold face at the time of pouring of metal and subsequently as time lapses It will be seen that temperature gradient is decreasing being maximum at mold face and thus short grains near face and columnar further away from face of mold
Solidification of pure metals
Pure metals melt and solidify at a single temperature which may be termed as melting point or freezing point Above freezing point the metal is in liquid state and below freezing point, it is in solid state In case of pure metals there are two methods of cooling i) Equilibrium cooling ii) Rapid cooling
Consider pure liquid metal poured into a mould cavity allowed to solidify As soon as the molten metal comes in contact with mould surface, the liquid metal experiences sudden cooling which is referred to as chilling action. This results in the formation of thin layer of solid metal along the moulds walls- due to chilling – grains are very small As the liquid metal cools further as a result of continuous heat extraction from the mold media these grains will grow As the heat transfer takes place from the mould surface to the atm , the interface will move at a constant speed towards the centre of the casting
Equilibrium cooling If a number of temp measurements are taken at different times while the pure metal cooled under equilibrium conditions from the molten state till it solidify
Liquid metal cools from A to B Form B to C, the melt liberates latent heat of fusion (hold occurs, temp remains constant), metal cools quite quickly to its freezing pt (here actual nuclei are formed- beginning of solidification) The liquid metal starts solidifying at B and it is partly liquid and partly solid at any point between B & C and at C the metal is purely solid (growth of nuclei will take place slowly & continuously) From C to D the solid metal cools and tend to room temperature point c is refereed to as end of solidification- entire liquid converted
Rapid cooling If a number of temp measurements are taken at different times while the pure metal cooled under rapid conditions from the molten state till it solidify
Nucleation of solid does not start at point B, but it does so at B’ i.e after the liquid metal has super cooled by amount δT. This phenomenon is known as super cooling Liquid metal cools from A to B From B to B’ supercooling is taken place ie below its freezing pt From B’ to C solidification takes place From C to D, the solidified metal cools to room temperature
Super cooling behavior is due to the fact that the cooling rate is faster than the rate of nucleation
Solidification of alloys
In an alloy, the solidification initially occurs at the metal mould surface like in the pure metal. However the presence of other metal at the centre also causes nucleation leading to grain formation & growth From A to B, the alloy is in liquid state Solidification starts at B and completes at point C Unlike pure metals, solidification occurs throught the temp range ( from TB to TC) solidification takes place along the temp line Heat liberates gradually from B to C and it tends to increase the time required for the solidification From C to D the solid metal cools and tends to reach the room temp
Crystallization from cast melt
Crystallization from a metal melt involves the successive stages of nucleation and growth.
The location & relative rates of these two phenomena within the liquid determine the final structure of the solid
Mechanism of solidification
The mechanism of solidification is to be studied in detail to understand the process of solidification The solidification process consists of two stages. They are i) Nucleation of minute crystals ii) Growth of these crystals in to grains
Nucleation
Nucleation is the beginning of a phase transformation The process of formation of first stable fine particles is called nucleation The extraction of heat is more rapid near the mold walls than at any other portion The first microscopic crystallites called nuclei form here
Nucleation Nucleation may involve
The assembly of proper kinds of atoms by diffusion (spreading) Formation of critical sized particles ( nuclei) of the new (solid) phase
Types of nucleation Basically two types of nucleation process has been identified they are i) Homogeneous nucleation ii) Heterogeneous nucleation
Homogeneous nucleation
The formation of nuclei in liquid metal in a solidification process, without the aid of foreign particles ( impurities) is termed as homogeneous or self nucleation This type of nucleation is observed when solids are formed within its own melt without any aid of any foreign particles Homogeneous nucleation occurs in perfectly homogeneous materials as in pure metals
As the molten metal starts cooling below liquids temperature atoms tend to agglomerate( together) and form embryos When these particles (embryos) reaches a critical size , nuclei are formed which will grow further Particles having size larger than critical size will sustain and help nucleation process
Heterogeneous nucleation
The formation of nuclei in liquid metal in a solidification process with the assistance of foreign particles (impurities) is called heterogeneous nucleation Generally foreign particles will be present in the melt which alter the liquid solid interface energy sufficient to assist in nucleation thereby reducing the amount of super cooling required to effect nucleation In heterogeneous nucleation the initial growth interface is provided by a foreign particle included or formed in the melt Once the initial nuclei are established, two possibilities exists for further crystallization, more solid may be deposited up on the first nuclei or fresh nucleation of the same
Growth
Minute nuclei or the crystallites formed during the process, grows by atoms adding on to the nuclei. This growth proceeds with release of high energy at the crystal melt interface as they posses high thermal energy With further decrease in temperature, the nuclei grow rapidly along certain crystallographic direction These crystals grow and they tend to form structure resembling a tree. This structure is often termed as dendrite * Dendrite in 2D & 3D
Dendritic growth proceeds from each crystal nucleus until they meet each other from the neighboring nuclei The remaining liquid in between the dendrite crystallizes in to the orientation of the strongest dendrite The process of solidification ends when the last film of liquid at the grain boundry has solidified At the end of the solidification process, only grains remains in the microstructure with no indication of dendritic growth
Grain structure of cast metals
When a hot molten metal is poured in to a cold mold, the initial rate of heat extraction is very high . The liquid metal near the mould wall surfaces is often cooled below its freezing point ( under cooled) and many fine equiaxed dendrites form in this narrow surface zone, termed chill zone After the initial thermal shock, those dendrite oriented most favorable grow perpendicular to the mold wall to form elongated or columnar dendrites Pure metals, or metals that freeze with a very narrow mushy zone may have only these two types of grains chill and columnar
Grain structure of cast metals A pure metal solidifies at a constant temperature equal to its freezing point (same as melting point)
Cooling curve for a pure metal during casting
Grain structure of cast metals
Due to chilling action of mold wall, a thin skin of solid metal is formed at the interface immediately after pouring Skin thickness increases to form a shell around the molten metal as solidification progresses Rate of freezing depends on heat transfer into mold, as well as thermal properties of the metal
Characteristic grain structure in a casting of a pure metal, showing randomly oriented grains of small size near the mold wall, and large columnar grains oriented toward the center of the casting
Grain structure of cast metals Most alloys freeze over a temperature range rather than at a single temperature
Cooling curve for a pure alloy casting
Grain structure of cast metals
Metals that freeze with a wider mushy zone tend to nucleate new grains during late stages of solidification These new grains are equiaxed, and the region where they appear is termed the equiaxed zone Figure shows the cross section of a simple casting of an alloy such as medium –carbon steel which may show all three types of grain structure
Characteristic grain structure in an alloy casting, showing segregation of alloying components in center of casting
Cast structures Schematic illustration of three cast structures solidified in a square mold: (a) pure metals; (b) solid solution alloys; and © structure obtained by using nucleating agents. Source: G. W. Form, J. F. Wallace, and A. Cibula
Solidification contn…
The refinement and modification of cast structures
Three factors have been to be significant in determining the metallographic structure of castings . Those are nucleation, growth behavior and crystal multiplication The most common aim in structure control is the pursuit of refinement through one or some of theses possibilities The first effect of most refining treatments is the reduction or elimination of the columnar zone, which is followed by a progressive increase in the number of equiaxed grains
Practical measures for size control and modification
Variation of cooling rate
Chemical treatment of the liquid metal
Manipulation of melt superheat
Agitation during freezing
Variation of cooling rate
The refining effect of increased cooling rate applies both to primary grain size and to substructure In the latter case the effect is upon the growth processes rather than nucleation The combined effects of increased cooling rate upon both grain size and substructure in cast iron can be summarized in the following diagram Increased cooling rate
Increased under cooling Large no of grains
Increased growth rate Branching Finer graphite within grains
Chemical treatment of the liquid metal
The decrease in grain size brought about by increasing normal alloy content – is comparatively small Highly effective grain refinement can be accomplished by inoculation Inoculation – the addition to the melt of small amounts of substances designed to promote nucleation In certain cases the function may be to modify the growth rather than the nucleation process When the homogeneous nucleation occurs the cast structure will have different grain structures and hence different properties across the section of the casting
From strength point of view uniform and fine grains have enhanced mechanical properties and hence it is desired to obtain fine grains in the castings This necessitates the artificial means of inducing nucleation throughout the melt This process is referred to as inoculation Inoculation is the process of inducing artificial nucleation of grains throughout the melt Inoculant is introduced during tapping of the molten metal or during transfer of metal from one type of ladle to another Although most chemical treatments involve additions to the melt, either within the furnace or during pouring
Manipulation of melt superheat
A further influence upon nucleation may be exercised by superheating the metal to a higher temperature than that used for pouring The influence of superheat is held to be associated with the behavior of heterogeneous nuclei during thermal cycling The most usual effect is to produce coarsening of the grain structure
Agitation during freezing
It has long been realized that nucleation could be brought about by physical disturbances of under cooled liquid , for example stirring or gas evolution This may be attributed to the widespread distribution of nuclei originally produced at the surface The use of high energy ultrasonic treatment during solidification as a means of structural refinement Other practical possibilities include electromagnetic stirring with the aid of induction coils
Grain refinement & modification
When the molten metal has completely solidified, the microstructure show grains, grain boundaries, inclusions etc The properties of the solidified metal or casting is greatly influenced by the grain size It is possible to predict the mechanical properties of the casting by knowing the grain size Grain size is controlled by the solidification variables like solidification time, cooling rate, temperature etc Grain size can also be controlled by treating the molten metal with certain agents before casting This is referred to as grain refinement
Grain refinement
It is a process of refining the grain structure in the metal or alloys to ensure both uniform grain in thick and thin section It is process involving controlling the grain size to a low value It is well established that smaller the grain size higher is the strength properties and lower is the ductility values Larger grain size reduces strength but enhances the ductility values In order to reduce the grain size a number of agents are used The agents used for the purpose are referred to as grain refiners Grain refiners provide large numbers of small grains in the structure Grain refiners are available in the elemental form or as compounds
Mechanism of grain refinement
By adding agents in the elemental or combined form, a number of nuclei are formed in the molten metal These promote simultaneous growth of nuclei resulting in large number of smaller sized grains when solidification is completed Agents will be normally in the solid form especially used as compounds These refiners will form a solid phase when added to the melt and gets dispersed uniformly throughout the melt and forms centers for the formation of nuclei
Refiners used
Aluminium , titanium and boron are used as grain refiners in all aluminium alloys Carbon and zirconium are used as refiners in magnesium alloys Silicon, aluminium, titanium are used as refiners in steel
Steps involved in refinement
Molten metal is taken out of the furnace in a ladle after checking the temperature During melting a covering flux is used to remove the slag and other impurities The temperature of molten metal is checked with a thermocouple and at the correct temperature degassing is carried out using inert gas The slag is skimmed off from the surface of the metal Now small amount of grain refiners in the form of granules is added to the bottom of the crucible and genteelly stirred After few seconds of the treatment, the molten metal is slowly poured in to the mould cavity
Flow chart for refinement Molten metal Check temperature using thermocouple Degassing using Inert gas Slag skimming Addition of grain refiners (granules) Gentle stirring Pouring into mould cavity
Effect of grain size on mechanical properties
Grain size influences the behavior of the metal YS
UTS Elongation
properties
Grain size
YS- Yield strength UTS- ultimate tensile strength As the grain size increases, yield strength and ultimate strength decreases Whereas ductility property is improved
Modification
Modification is a process of changing the shape of the second phase elements Certain solid agents are used for this Percentage of these elements are very small and the effect is remarkable Modification is carried out extensively in aluminium silicone alloys
Inoculation
A wide variety of inoculants are presently used in the foundry Most of these have been developed only in recent years An inoculant is an addition made to a melt, usually late in the melting operation, which alter the solidification structure of the cast metal Some common inoculation treatments include i) Grain refinement of aluminium and magnesium alloys ii) Graphitization for gray cast iron iii) Use of magnesium or cerium to produce ductile iron
Grain refining of aluminium and magnesium alloys
In grain refining aluminium alloys, less than 0.2 percent titanium or 0.02 percent boron is sufficient to reduce the cast grain size of the alloy from as much as 0.10inch in diameter to as little as 0.005inch in diameter Magnesium – aluminium alloys are grain refined by small additions of carbon to the melt Magnesium alloys which do not contain aluminium are usually grain refined with small additions of zirconium
Graphitization for gray cast iron
A slight different type of inoculation treatment is the graphitizing inoculation of gray cast iron Addition of small quantities of materials to molten iron which will bring about remarkable enhanced properties in the iron without appreciably changing the composition of the iron This alters the solidification process, increases the degree of nucleation of the iron, prevents chill formation and avoid the undesirable inter-dendritic graphite structure Common graphitizers employed are ferrosilicon, nickel-silicon, silicon-manganese-zirconium These are added late in the melting operation
Use of magnesium or cerium to produce ductile iron
A most important and very new inoculation treatment is that of adding small quantities of magnesium (or cerium) to cast iron to produce ductile iron The addition of as little as 0.04 percent residual magnesium – tremendous improvement in mechanical properties of the alloys
A metallurgic ally similar phenomenon to the magnesium treatment of ductile iron is the sodium inoculation of aluminium – silicon alloys
Concept of Progressive & directional solidification
Introduction
During solidification, the complete casting parts do not cool at the same rate, due to i) the varying sections ii)differing rates of heat transfer to mould walls Some parts tend to solidify more quickly than others This sort of uneven solidification causes voids and cavities due to shrinkage Thus for the casting to be sound it must be free from voids and shrinkage defects
Types of solidification
Solidification of metal or alloy can take place in two ways depending on the solidification variables
Progressive solidification
Directional solidification
Progressive solidification
In a casting there is a tendency for the metal to solidify from outside to the interior This condition of having a partially solid and partially liquid zone growing from outside to inward is called progressive solidification Here heat extraction occurs more or less at the same rate in all direction Solidification takes place equally in all direction solidification occurs in almost at the same time in X & Y direction fig 1
When the mould cavity is filled with molten metal, the metal adjacent to the walls of the moulds cools and solidifies first This results in a shell (cast) of solid metal, with the centre of the section remaining liquid While there is a zone between the liquid interior and solid exterior wherein the metal is in a semisolid or mushy state The solidification then proceeds inwards towards the centre of the section This solidification is called as progressive solidification Solid
Liquid Mushy
When metals such as low carbon steels, solidify in a mould cavity they do so by forming a solid skin which progresses inward until the entire casting is solid As the casting solidifies , a pipe is formed, primarily from solidification shrinkage If the walls of the mould are parallel, an elongated pipe may spread nearly to the bottom fig 2
Directional solidification
If solidification occurs in a desired direction or occurring in a particular direction then it is called directional solidification Controlling the solidification of metal after it has entered the mould cavity is the prime factor for achieving directional solidification Thus, for the castings to be sound free from voids and shrinkage defects, the solidification should be such that it starts at the thinner section, continuous to the thick section fig3
Reasons for directional solidification
Different rates of cooling in diff rates
Complicated geometry
In extremely complicated geometry , it is very difficult to determine the relative rate with which the metal solidifies in various parts Due to different rates of cooling in different parts, stresses will be set up in the casting which may give rise to cracks in it This can be avoided by directional extraction of heat from the liquid metal So that solidification is proceeds from one end to the other
Design for directional solidification
It involves the designing such that the parts most distant from the available liquid metal will solidify first leading to successive feeding of the contracting metal by the still liquid metal until the heaviest and last to freeze section is reached The risers are attached to the casting in such a location that they supply hot metal to the shrinkage casting until it is completely solidified
Advantages of directional solidification 1.
2.
3.
4.
Solidification of the metal will be uniform It produces sound casting, free from voids and shrinkage defects It produces uniform microstructures. Thus giving desirable properties Since here riser is the last part to solidify all the impurities given out by the first solidifying metal will be collected in the riser
Methods of achieving directional solidification Controlling the solidification of metal after it has entered the mould cavity is the prime factor for achieving directional solidification Directional solidification can be achieved by
Proper riser location Padding Using exothermic materials Chills Use of insulating material Use of electric arc over the riser
Proper riser location
Proper riser location is made to prevent improper solidification of molten metal otherwise that leads to formation of shrinkage cavity, porosity, surface depression as shown below*
Fig 4
Padding
It may not possible to overcome completely the effects of shrinkage in the central thin sections of the casting Padding may be the solution for this For this thinner sections should be tapered towards heavier section to achieve directional solidification This tapering of thinner section towards thicker section is known as padding This will require extra material If padding is not provided central line shrinkage or porosity will result in thinner section After solidification the extra padding material can be removed by machining Fig 5
Use of exothermic materials
Fig 6
Exothermic materials are the mixtures of the oxide of the metal to be cast and aluminium metal in powder form Which produces large amounts of heat when come in contact with hot metal Molten metal in the riser is heated in order to aid directional solidification Riser metal remains molten until the whole mould cavity is solidified This is done by adding exothermic material i) Either at the surface of the molten metal in the riser (after pouring) ii) Or to the sand in the riser walls
These comes in contact with molten metal undergo exothermic reactions and release heat
Use of chills
Directional solidification is induced by chilling the metal at those points where it is necessary that solidification should begin Chills are normally metal inserts which are placed at appropriate locations in the mould to speed up the solidification of a particular portion of the casting Chills extracts heat from the casting at a fast rate Use of chills becomes necessary when it is not possible to locate a riser on the casting Both external & internal chills can be used for this purpose Fig 7
External chills
External chills are normally metal inserts of steel, cast iron or copper. Chills increase the rate of solidification by faster heat extraction External chills are placed in the mould walls that they come in direct contact with the molten metal in the mould cavity These do not form the integral part of the casting after solidification External chills are not consumed unlike internal chills and hence are reusable External chills are coated with red lead or moulding sand. Further external chills are two types direct & indirect Fig 8
Internal chills
Internal chills are also metal objects which form the integral part of the casting after solidification Internal chills should be made of same composition as that of the metal to be cast Internal chills are extended in the mould Surface of internal chills must be free from moisture, grease and any impurities otherwise they produce defects
Use of insulating material
Risers can be made more efficient by employing artificial means to keep the top of the riser from freezing This is done by the use of certain insulating materials which are placed around the riser The defect may occur at the riser which will be the last portion to solidify
Fig 9
Use of electric arc over the riser
The necessary electric circuit is formed between the riser and the electrode After completely filling of the mould, an arc is initiated in the riser metal of the casting This will keep the molten metal in the riser hotter for a longer duration of time Thus casting shrinkage is taken care of Fig 10
The refinement and modification of cast structures
Three factors have been to be significant in determining the metallographic structure of castings . Those are nucleation, growth behavior and crystal multiplication The most common aim in structure control is the pursuit of refinement through one or some of theses possibilities The first effect of most refining treatments is the reduction or elimination of the columnar zone, which is followed by a progressive increase in the number of equiaxed grains
The significance and practical control of cast The principal factors governing the final metallographic structure of a casting are
Constitution and thermal properties of the alloy Casting design and dimensions Thermal properties of the mould Final casting temperature Conditions for heterogeneous nucleation Subsequent heat treatment Structure of a casting is a function of alloy composition and casting geometry
Practical measures for size control and modification
Variation of cooling rate
Chemical treatment of the liquid metal
Manipulation of melt superheat
Agitation during freezing
Variation of cooling rate
The refining effect of increased cooling rate applies both to primary grain size and to substructure In the latter case the effect is upon the growth processes rather than nucleation The combined effects of increased cooling rate upon both grain size and substructure in cast iron can be summarized in the following diagram Increased cooling rate
Increased under cooling Large no of grains
Increased growth rate Branching Finer graphite within grains
Chemical treatment of the liquid metal
The decrease in grain size brought about by increasing normal alloy content – is comparatively small Highly effective grain refinement can be accomplished by inoculation Inoculation – the addition to the melt of small amounts of substances designed to promote nucleation In certain cases the function may be to modify the growth rather than the nucleation process When the homogeneous nucleation occurs the cast structure will have different grain structures and hence different properties across the section of the casting
From strength point of view uniform and fine grains have enhanced mechanical properties and hence it is desired to obtain fine grains in the castings This necessitates the artificial means of inducing nucleation throughout the melt This process is referred to as inoculation Inoculation is the process of inducing artificial nucleation of grains throughout the melt Inoculant is introduced during tapping of the molten metal or during transfer of metal from one type of ladle to another Although most chemical treatments involve additions to the melt, either within the furnace or during pouring
Manipulation of melt superheat
A further influence upon nucleation may be exercised by superheating the metal to a higher temperature than that used for pouring The influence of superheat is held to be associated with the behavior of heterogeneous nuclei during thermal cycling The most usual effect is to produce coarsening of the grain structure
Agitation during freezing
It has long been realized that nucleation could be brought about by physical disturbances of under cooled liquid , for example stirring or gas evolution This may be attributed to the widespread distribution of nuclei originally produced at the surface The use of high energy ultrasonic treatment during solidification as a means of structural refinement Other practical possibilities include electromagnetic stirring with the aid of induction coils
Grain refinement
Usually metals & alloys solidify with coarse columnar grain structure under normal casting conditions
Characteristic grain structure in a casting of a pure metal
Characteristic grain structure in an alloy casting
Grain refinement & modification
When the molten metal has completely solidified, the microstructure show grains, grain boundaries, inclusions etc The properties of the solidified metal or casting is greatly influenced by the grain size It is possible to predict the mechanical properties of the casting by knowing the grain size Grain size is controlled by the solidification variables like solidification time, cooling rate, temperature etc Grain size can also be controlled by treating the molten metal with certain agents before casting This is referred to as grain refinement
Grain refinement
It is a process of refining the grain structure in the metal or alloys to ensure both uniform grain in thick and thin section It is process involving controlling the grain size to a low value It is well established that smaller the grain size higher is the strength properties and lower is the ductility values Larger grain size reduces strength but enhances the ductility values In order to reduce the grain size a number of agents are used The agents used for the purpose are referred to as grain refiners Grain refiners provide large numbers of small grains in the structure Grain refiners are available in the elemental form or as compounds
Mechanism of grain refinement
By adding agents in the elemental or combined form, a number of nuclei are formed in the molten metal These promote simultaneous growth of nuclei resulting in large number of smaller sized grains when solidification is completed Agents will be normally in the solid form especially used as compounds These refiners will form a solid phase when added to the melt and gets dispersed uniformly throughout the melt and forms centers for the formation of nuclei
Refiners used
Aluminium , titanium and boron are used as grain refiners in all aluminium alloys Carbon and zirconium are used as refiners in magnesium alloys Silicon, aluminium, titanium are used as refiners in steel
Steps involved in refinement
Molten metal is taken out of the furnace in a ladle after checking the temperature During melting a covering flux is used to remove the slag and other impurities The temperature of molten metal is checked with a thermocouple and at the correct temperature degassing is carried out using inert gas The slag is skimmed off from the surface of the metal Now small amount of grain refiners in the form of granules is added to the bottom of the crucible and genteelly stirred After few seconds of the treatment, the molten metal is slowly poured in to the mould cavity
Flow chart for refinement Molten metal Check temperature using thermocouple Degassing using Inert gas Slag skimming Addition of grain refiners (granules) Gentle stirring Pouring into mould cavity
Effect of grain size on mechanical properties
Grain size influences the behavior of the metal YS
UTS Elongation
properties
Grain size
YS- Yield strength UTS- ultimate tensile strength As the grain size increases, yield strength and ultimate strength decreases Whereas ductility property is improved
Modification
Modification is a process of changing the shape of the second phase elements Certain solid agents are used for this Percentage of these elements are very small and the effect is remarkable Modification is carried out extensively in aluminium silicone alloys
Inoculation
A wide variety of inoculants are presently used in the foundry Most of these have been developed only in recent years An inoculant is an addition made to a melt, usually late in the melting operation, which alter the solidification structure of the cast metal Some common inoculation treatments include i) Grain refinement of aluminium and magnesium alloys ii) Graphitization for gray cast iron iii) Use of magnesium or cerium to produce ductile iron
Grain refining of aluminium and magnesium alloys
In grain refining aluminium alloys, less than 0.2 percent titanium or 0.02 percent boron is sufficient to reduce the cast grain size of the alloy from as much as 0.10inch in diameter to as little as 0.005inch in diameter Magnesium – aluminium alloys are grain refined by small additions of carbon to the melt Magnesium alloys which do not contain aluminium are usually grain refined with small additions of zirconium
Graphitization for gray cast iron
A slight different type of inoculation treatment is the graphitizing inoculation of gray cast iron Addition of small quantities of materials to molten iron which will bring about remarkable enhanced properties in the iron without appreciably changing the composition of the iron This alters the solidification process, increases the degree of nucleation of the iron, prevents chill formation and avoid the undesirable inter-dendritic graphite structure Common graphitizers employed are ferrosilicon, nickel-silicon, silicon-manganese-zirconium These are added late in the melting operation
Use of magnesium or cerium to produce ductile iron
A most important and very new inoculation treatment is that of adding small quantities of magnesium (or cerium) to cast iron to produce ductile iron The addition of as little as 0.04 percent residual magnesium – tremendous improvement in mechanical properties of the alloys
It is possible to develop fine equiaxed grains by grain refinement Grain refinement is the method of formation of fine equiaxed solidification structure throught the material The add of grain refines reduces any tendency for columnar grains to form Three factors have been shown to be significant in determining the metallographic structure of castings – nucleation, growth behaviour, and crystal multiplication The most common aim in structure control is the pursuit of refinement through one or some of theses possibilities The first effect of most refining treatments is the reduction or elimination of the columnar zone , which is followed by a progressive increase in the number of equiaxed grains
Grain refinement leads to
High toughness
High yield strength
Improved machinability
Good surface finish
Resistance to hot tearing
Various other desirable properties
MODERNIZATION & MECHANIZATION OF FOUNDRY
Need for modernization & mechanization
An average person visualizes a foundry as a dark, dirty place In general foundry is a place where lots of smoke, heat etc will invariably be there It is a common feeling that the generally atmosphere and working conditions in a foundry are quite hazardous This picture is true to some extent Thus there is a need to modernize the foundry and to improve the working conditions in a foundry
Need for modernization & mechanization contn…
Over the last few years lot of attention has been paid to modernize the entire foundry operations This results to increased production, improved working conditions in the shop with an eye to ensuring a safe , healthy and happy life for the workers The areas in which such measures are possible include
Changing over to better & newer foundry equipments Employing newer, better & more economical moulding, melting and casting techniques Removing conditions which make a foundry dirty, dusty and smoke filled
Results of modernization & mechanization
Improving quality of the castings
Increasing production
Reducing production cost
Increasing safety to the workers
Making working conditions pleasant and less tiring
Building up morale of the workers
Mechanization
Mechanization means the utilization of machinery for work previously done by hand Machinery may be used for
Preparing sand, moulds and cores Transporting the sand and moulds Transporting and pouring the molten metal
The extent to which mechanization can be adopted in a foundry varies according to the quality and type of production For small orders as well as for the production of large sized castings, mechanization is both uneconomical and unpractical
Advantages
Increased production from a given foundry floor space Casting posses closer dimensional tolerances, improved surface finish and higher accuracy Both time and labour are saved since heavy lifting and other laborious operations are carried out mechanically More hygienic and healthy working conditions in foundries Minimized casting defects Production of sound castings Faster rate of production Reduced production costs
Disadvantages
Uneconomical for jobbing foundries producing castings in small quantities and not expecting repetitive orders Increased chances of unemployment Substitution of machines in place of men causes a gradual disappearance of the art of hand moulding
Elements of mechanization Mechanized sand preparation units Moulding & core making equipments Melting, pouring & shakeout units Dust & fume control equipments Material handling equipments and conveyor systems
Mechanized foundry Mould assembly furnace
Casting station
Core insertation
Moulding machines
Knockout station
Cooling trays
Overhead hoppers
Cooling tunnel
conveyors
Sand conveyor Sand mixing plant
Sand reconditioning Plant
Mechanized foundry contnn….. Castings are knocked out of the moulds on a vibratory grid at knockout station Sand passes down in to a hopper through the grid and the castings vibrate off in to the cooling trays The sand on a conveyor passes through the reconditioning chamber by an overhead belt conveyor in to hoppers situated above the molding machine After the molds and cores have been assembled, again on a conveyor, they are carried to casting station Molds filled with molten metal are cooled as they pass through cooling tunnel and eventually reach knockout station and a new foundry cycle starts
Area for mechanization Mechanization has a distinct impact on areas concerned with Preparation & control of sand Moulding & core making Melting, pouring & shakeout operation Material handling Dust & fume control
Sand reclamation and sand preparation Sand obtained after knockout operation is very uneven because
It may contain red-hot lumps, steamy dump lumps, some completely unaltered sand as well as lumps of core sand and finally divided dry sand from the breakdown of core It may also contains rods, wires or springs used to strengthen the cores Molten metal spilled on top of molds sets solid and become mixed with moulding sand during shake out operation
Flow sheet of sand Additives
New sand Sand mixer Aerator Molding m/c hopper
Cores Molten metal
Reclaimed Sand Storage Screening Magnetic separator
Mould Knockout station Casting Finishing
Lump Breakers separator Core lumps
Equipments used in sand preparation Magnetic separator Gyratory riddle Rotary sand kicker Sand muller Screenerator or sand aerator Sand reclamation units
Magnetic separator
The moulding sand, which comes from the shakeout station, is made free from all iron particles & foreign matter before reusing it. Magnetic separator separates the iron pieces, wire nails, iron shots etc from the sand Magnetic separator consists of a magnetized pulley over which rolls a flat rubber or canvas belt The pulley may be either of permanent magnet type or electromagnetic type carrying DC magnetic coils Hopper
Metal particles Sand of non magnetic
Gyratory riddle
After the iron pieces are separated the sand is usually passed through a screen or riddle for removing the pieces of dry sand cores, hard lumps of sand and other refuse Gyratory riddle can screen the sand at much higher speed as compared to hand riddles The riddles are operated by compressed air or electric motors. Compressed air : Consists of an air cylinder , a reciprocating piston and a screen fitted to the end of the piston. The reciprocating action of the piston moving through a very short stroke length causes screening action of the riddle Electric motors: the action is due to gyratory motion. The screen is supported from a frame to which is also attached an electric motor. An unbalanced wheel is attached to the motor shaft. As the shaft rotates, it causes the wheel to move out of balance – wobbling motin
Rotary sand kicker
A definite improvement over hand mixing
Straddles long heaps of sand, previously moistened with water
Mixes the mass with vigorously rotating blades as the machine moves along the roll.
Sand muller
Mulling is the process of kneading and working sand for the purpose of distributing the ingredients in to a homogeneous mixture The function of muller in a foundry is to condition the moulding sand for making it fit for reuse Generally consists of a cylindrical pan in which two heavy rollers roll in a circular path Two ploughs are also carried with the rollers Ploughs scrap the sand from bottom of the pan and bring it in front of the rollers
Sand reclamation units Sand
obtained after shake out operation is very uneven because
It may contain red hot lumps, steamy damp lumps, some completely unaltered sand as well as lumps of cores sand and finely divided dry sand from the breakdown of cores It may also contain rods, wires or springs used to strengthen the cores Molten metal spilled on top of molds sets solid and becomes mixed with molding sand during shake out operation There is build up of clay and organic coating on sand grains
Sand reclamation
Refers to the treatment of used moulding sand so that it regains its original condition and can be reused again and again with minimum addition of new sand
It is the process of restoring used foundry sand to as nearly its original condition as possible
Moulding & core making
The basic requirements of a mechanized foundry is suitable machines for moulding & core making These machines are desired for performing most of the work previously done by hand, They help in i) Making mould faster ii) Producing castings to a greater degree of accuracy & uniformity iii) Making use of semiskilled workmen iv) Lowering the production cost
Machine moulding
Production of uniform castings by hand moulding from loose patterns requires considerable moulding skill i) To make the joint ii) To draw the pattern correctly without enlarging the mould cavity and iii) To cut the ingates and risers When large quantities of castings are required, moulding machines are employed
Moulding process
Moulding process may be classified according to weather the mould is prepared by hand tools or with the aid of some moulding machines as i) Hand moulding ii) Machine moulding
Hand moulding is generally found to be economical when the castings are required in a small number When similar castings are required in large quantities, hand moulding being more time taking and laborious, become expensive . Machine moulding is generally employed at such places The production of a mould entails two main operations 1) Ramming the sand 2) Removing the pattern
Advantages There are factors other than quick production are
Uniform density of mould hardness, resulting in a higher standardization of casting finish All runners, ingates and risers can be moulded . The correct size of gating is very important, and, obviously, difficulty to produce manually Correct shape and size of the finished casting. The enlargement of the mould size is eliminated Less skilled worker can do the job. Whereas in hand moulding greater skill is required Machining allowances may be reduced
Classification of moulding machines
Broadly classified in to two categories: i) Hand operated moulding machines ii) Power operated moulding machines i)Hand operated moulding machines: one or more operations like ramming, pattern drawing etc are performed by the machine which is manually operated either by a hand lever or pedal control. Theses machines do not make use of any external power Types: i) Plain stripper type pattern draw machine ii) Pin-lift or push-off type pattern draw or straight draw m/c iii) Roll-over type pattern draw machine
Plain stripper type pattern draw machine Consists of a stationery frame or table which supports the flask, a movable pattern plate on the top of which the pattern is screwed Arrangement for moving the pattern plate consists of a rack and pinion or an arm lever Moulding box Pattern
Cavity
Pattern plate For ramming pattern plate is kept in raised position During ramming
Pattern plate is lowered
Pattern withdrawn
Pin-lift or push-off type pattern draw or straight draw machine
In this arrangement, pattern plate along with the pattern remains stationary while the moulding box complete with the sand mould is raised, being supported on four pins passing through the pattern plate Pattern plate & pattern remains stationary The flask is placed around the pattern The flask is filled with sand, hand rammed The flask is pushed upwards away from the pattern Light flasks may be lifted with the help of a lever and heavy ones by an air operated ram
Roll-over type pattern draw machine sand
Trunnion
Stationary
pattern
After filling the sand and ramming by hand, the whole assembly of the pattern plate , pattern and moulding box is rolled over and brought in to horizontal position The table along with mould lowered, leaving the pattern and the pattern plate in the inverted position
Power operated moulding machines
The power-operated moulding machines make use of hydraulic or pneumatic action to perform various operations during the moulding process Such as Raising or lowering the table for pattern withdrawal Ramming the sand by squeeze, jolt or combined squeeze& jolt actions Rolling over the moulding boxes, etc Manual labour and fatigue are reduced and the production rate of moulds is increased
Types
Squeeze machines
Jolt machines
Jolt-squeeze machines
Sand slinger
Squeeze machines squeeze
In the squeeze method, the box is filled with the moulding sand, and the sand is squeezed Sand against a pressure board pneumatically or Pattern hydraulically until the mould attains desired density
Limitations : i)By squeezing, the sand is packed more densely at the top where the squeeze head presses against the sand and the density decreases uniformly with the depth ii) The squeeze method is therefore restricted to moulds not more than 150mm in depth
Jolt machines In the jolting method, the box is first filled with the moulding sand and then the table supporting the box is mechanically raised and dropped in succession
Table
Due to the sudden change in inertia at the end of each fall, the sand gets packed and rammed. This action of raising and dropping the table is called jolting The jolting load exerted during moulding varies from 200kg to 1000kg according to the size of the machine
Packing of the sand is caused by work done by the K.E of the falling sand Drawback: The sand is rammed hardest at the parting plane and around the pattern and remains less dense in the top layers
Jolt-squeeze machines
For overcoming the drawbacks of both squeeze and jolt principle of ramming and to obtain uniform density and hardness in all portions of the mould, a combination of squeeze and jolt actions is generally employed Jolting action is used for consolidating the sand on the face of the pattern. It is followed by squeezing action for imparting the desired density in the upper portion of the mould The jolt-squeeze moulding machine is so constructed that both squeeze and jolt actions can be obtained one after the other The table is attached to a cylindrical piston, called the jolt piston, which is raised and dropped in the jolt cyclinder by the action of compressed air Followed by squeezing
Sand slinger Impeller head In the slinging operation, the consolidation and ramming of sand is achieved by means of impact with the pattern. Basically the sand slinging machine throwing a stream of sand downward, through a slinging head, on to the pattern at high velocity Due to rapid ejection, the sand particles settle down instantly and get rammed
The design of the sand slinger incorporates a high speed rotary impeller, pipes. band conveyor, bucket elevator, and a ejecting head attached to a swivelling arm The ejecting head moved all over the moulding box – attain uniform density of sand
Core making machines
Core making operation is performed by machines similar to those used for making moulds Suitability of a particular type of machine is influenced by the factors i) Quantity to be produced ii) Type of core iii) Size of core iv) Intricacy and complexity of design
Operations required to produce a core
Place sand in core box and ram firmly and evenly Place core plate in position on top of core Turnover core box and plate Vibrate Pass core to drying unit
Ramming of the core sand can be done by
Hand ramming of the sand into the core box blowing of sand into core box jolting of the sand in the core box vibrating and compression by squeezing sand into core box
Core making machines
Jolt machine
Sand slinger
Core extrusion machine or continuous core making machine
Core blowing machine
Shell core machine
Jolt machine
Core sand can be jolt-rammed in a dump type core box on a jolting machine In jolting machine, the core box filled with sand is mounted on a jolt table which is raised to a certain height and then dropped under its own weight. Sand gets rammed due to sudden fall
Sand slinger
Sand slinger is used for filling and ramming large core boxes Sand slinger makes use of a high speed rotating sand impeller which through the sand in the core box at high velocities The impact action of the sand stream is responsible for compacting the core
Core extrusion machine spiral conveyor die
sand
hopper
power or hand driven simple cores of regular & uniform cross section can be extruded easily
core of square, round, hexagonal are produced rapidly on a core machine
Core blowing machine compressed air
Its for small and medium sized cores
A core blowing machine makes use of compressed air to blow the sand into the core box at high velocity
sand
The sand carrying compressed air is responsible for filling the core box and ramming the core
The air incoming with the sand is exhausted through vents provided in the core box core box with vent
Core box are made up of aluminium
Shell core machine shell core
Shell cores can be made manually or machines The core box is heated to a temperature of the order of 400 to 600°F A sand mixed with about 2 to 5% thermosetting resin of phenolic type is either dumped or blown into the preheated metal core box The resin is allowed to melt to the specified thickness The resin gets cured The excess sand is dumped or removed The hardened core is extracted from the core box
Melting, pouring & shakeout units
Melting, pouring & shakeout units Melting and pouring operations in a mechanized foundry involve
Melting furnaces
Mechanisms for material and fuel transport
Mechanical devices for charging fuel and raw materials
Mechanically operated ladles
Melting furnaces
Before pouring in to the mould, the metal to be cast should be in the molten state This is done by melting the metal in a furnace Blast furnace performs basic melting (of ore) operation A foundry furnace remelts the metal to be cast Different furnaces are employed for remelting ferrous and nonferrous materials Heat in a remelting furnace is created by, combustion of diesel, electric arc, electric resistance etc A furnace contains a high temperature zone or region surrounded by a refractory wall structure which withstands high temperatures and being insulating minimizes heat losses to the surroundings So metal to be remelted is placed in the high temperature region of the furnaces
Choice or selection of remelting furnaces
Initial cost of the furnace Fuel cost Kind of metal or alloy to be melted Melting and pouring temperature of the metal to be cast Quantity of the metal to be melted Method of pouring desired Cost of furnace repair and maintenance Chances of metal to absorb impurities during melting Quality of the finished product desired Speed of melting the alloys Cost of operation and other production requirements Degree of cleanliness or pollution
Furnaces for melting Furnaces
Gray cast iron
steel
Non ferrous metals
Cupola
Open hearth
Crucible
Air furnace Rotary furnaces
Electric furnace Arc furnace Induction
Pot furnaces
Electric arc furnace.
converter
Reverberatory Rotary Induction Electric arc
Furnaces for Casting Processes • Furnaces most commonly used in foundries: − Cupolas − Direct fuel-fired furnaces − Crucible furnaces − Electric-arc furnaces − Induction furnaces
Cupolas Vertical cylindrical furnace equipped with tapping spout near base • Used only for cast irons, and although other furnaces are also used, largest tonnage of cast iron is melted in cupolas • The "charge," consisting of iron, coke, flux, and possible alloying elements, is loaded through a charging door located less than halfway up height of cupola
Direct Fuel-Fired Furnaces
Small open-hearth in which charge is heated by natural gas fuel burners located on side of furnace Furnace roof assists heating action by reflecting flame down against charge At bottom of hearth is a tap hole to release molten metal Generally used for nonferrous metals such as copper-base alloys and aluminum
Crucible Furnaces Metal is melted without direct contact with burning fuel mixture Sometimes called indirect fuel-fired furnaces Container (crucible) is made of refractory material or high-temperature steel alloy Used for nonferrous metals such as bronze, brass, and alloys of zinc and aluminum Three types used in foundries: (a) lift-out type, (b) stationary, (c) tilting
Crucible Furnaces contn….
Three types of crucible furnaces: (a) lift-out crucible, (b) stationary pot, from which molten metal must be ladled, and (c) tilting-pot furnace
Electric-Arc Furnaces
Charge is melted by heat generated from an electric arc High power consumption, but electric-arc furnaces can be designed for high melting capacity Used primarily for melting steel
Induction Furnaces
Uses alternating current passing through a coil to develop magnetic field in metal Induced current causes rapid heating and melting Electromagnetic force field also causes mixing action in liquid metal Since metal does not contact heating elements, the environment can be closely controlled, which results in molten metals of high quality and purity Melting steel, cast iron, and aluminum alloys are common applications in foundry work
Pouring equipments Pouring ladles Ladles:Ladles are used to carry molten metal from the furnace to the molding boxes a) Large reservoir or holding ladle: Molten metal from the furnace is generally tapped in to a large receiving ladle, from where the same is distributed to smaller ladles for pouring in to the moulds. Holding ladles store molten metal temporarily Holding ladles have steel shell usually lined with firebricks A holding ladle can receive and supply the molten metal simultaneously
Pouring equipments contn…
b) Crane or monorail ladles
These are again large ladles but of course smaller in size as compared to holding ladles such ladle make use of crane or monorail in order to reach the place where molds have been kept ready for pouring A typical ladle of this kind could contain 350kg of molten Metal and one person could do the pouring operation.
C) Lip pouring ladle: Its action is just like that of pouring water out of a jug.Since slag comes on the top of molten metal, it is difficult to pour clean metal with the help of a lip pouring ladle. So skimming bar is used to remove the slag A lip pouring ladle may be conveyed by a crane and tilted by a hand wheel or lever A lip pouring ladle can be used to handle almost all Casting metals and alloys such as Al, Mg, brass,bronze cast iron and steel
D) Teapot ladle: a teapot ladle resembles ordinary teapot except that pouring spot is inside the body of the ladle Since the molten metal existing near the bottom of the ladle gets poured into the mould, it is clean and free from slag A teapot ladle pours the molten metal with less momentum as compared to that of bottom pouring ladle Teapot ladles are preferred for pouring steel.
E) Bottom pour ladle: It has a tap hole at the bottom suitably stoppered by a refractory covered vertical rod having a graphite plug A bottom pour ladle helps pouring clean and slagfree metal from below the surface of the ladle A bottom pour ladle is preferred for pouring steel
Mold shake out
After the molds have been filled with molten metal and cooled down, they are knocked out to separate castings from the sand. For shake out , small sized molds are dropped on an inclined grating – the sand mold breaks, sand loosens and passes down through the grating into a hopper, whereas the molding boxes and castings remain on the top of grating. Heavy molds are shaken by placing them on a beam structure hinged at one end and free to raise and drop its other end through a small height by a motor driven cam. Alternate raising and dropping of the other end of the beam structure results in breaking the sand mold
Mold shake out
For small & medium sized molds shake out units are used for shake out Where they are shaken loosen and break up sand portions of the mold Molding box & casting Shake out unit
Hopper
Magnetic pulley
Belt conveyor
Bin for iron particles
Mold shake out
The shake out unit consists of a perforated plate or a grid which is attached to vibrating frame and can be vibrated or shaken Molding box and castings remain on the top of the shake out unit and are removed mechanically Whereas sand falls through the shake out unit in to a hopper located under it and is carried by belt conveyor to a magnetic pulley to remove undesired pieces of ferrous metals such as nails, wires etc The sand, then, falls on to another conveyor belt and is taken to a sand reconditioning plant
Material handling equipments
Material handling equipments are the heart of a mechanized foundry for its effective, efficient and successful operation In large scale production foundries, material handling equipments help in lowering the production cost. As flow of material is accelerated – man hours are effectively saved Different materials and equipments of a foundry which need to be transported or handled are i) Sands, both molding and core sands ii) Molding boxes both empty and rammed
Sands
Sands are handled and transported from one place to another with the help of following material handling equipments in mechanized foundry i) From knock out unit to magnetic separator- Apron conveyor ii) From magnetic separator to riddles for screening-Apron iii) From screens to sand reconditioning unit-Belt conveyor iv) From sand store to sand mixing plant-Belt, crane, industrial trucks v) From sand reconditioning plant to sand mixing – Belt conveyor
Apron conveyor: these make use of over-lapping steel plates, hinged at the ends in such a way that when assembled, they serve as a belt - It can be used for transporting materials that ar too hot to be carried by a belt- less speed- maintenances cost is more Belt conveyor: used for transporting sand in horizontal and vertical direction – it consists of an endless belt , two pulleys, rollers or idlers for carrying the loaded belt and returning the empty belt – the head pulley is used for driving the belt – connected to motor Endless belt
Head pulley
Cranes :An overhead travelling crane may be more convenient for carrying the sand for small foundries
Industrial trucks: here forks are attached to a column on the truck. Forks can be lifted to the desired height and material can be dropped in to certain distances
Empty molding boxes
From knockout station to cooling trays- Apron conveyor From cooling trays to molding machine- roller conveyor From molding machine to casting station- Belt or roller conveyor
Roller conveyor
Material handling equipments and conveyor systems
Introduction
Material handling equipment is an invaluable asset in the rapid and economic production of castings on a large scale The foundry which is the receiving centre of huge quantities of diverse materials, requires to be suitably equipped to ensure efficient handling and treatment of items, such as Sand Moulds and cores Molten metal and Castings
1)Handling sands
Sands are required to be conveyed from one part of the production foundry to another for various purposes Sand is taken from shake out station to the riddle for screening From the screen to the magnetic separator From the magnetic separator to the reconditioning plant From storage to the mixing or conditioning plant Reconditioned sand has to be sent top distribution mains and then to work stations All this traveling to and fro is conducted on a sand conveyor
The wide range of sand conveyors covers specified areas of work in the foundry shop Belt conveyor Bucket elevator Apron conveyor Mono-rail conveyor Crane
Belt conveyor
The belt conveyor is commonly used for transferring sand from one place to another in travel that requires a horizontal or an inclined direction of movement It consist of endless belt, two pulleys ( head and tail) Rollers or idlers for carrying the loaded belt and returning the empty belt Belt tightening mechanism and belt cleaners The belts are available in various sizes and strengths and are made of cotton plies with synthetic rubber Since such material cannot withstand more than about 150ºC, the belt conveyor can be used only when the temperature of the sand is within the limit Where hot sand is to be transported, a metal conveyor is used
The head pulley is used for driving the belt and is connected to an electric motor through reduction gear To prevent sand from spilling and to keep it in the centre , the idlers which support the belt are arranged in three pieces Each set of idlers has three rollers , one horizontal in the centre and two slightly inclined on the sides These idlers should not be spaced very far apart , otherwise the belt will slag too much under load When the sand conveyor is used for inclined travel, the angle of inclined should not exceed 15º and 25º for tempered sand to keep it from rolling backwards
Bucket elevator
When the sand is to be conveyed vertically upwards, a bucket elevator is ideal There are two pulleys, one at the top and the other at the bottom which carry an endless belt The belt carries a number of buckets all around and the whole assembly is enclosed in a steel casing which has two openings One at the bottom for feeding and the other at the top for discharge The bucket elevator is generally preferred to the inclined belt conveyor owing to a saving in space and cost
Apron conveyor
The apron conveyor’s overlapping steel plates, hinged at the ends , serve, when assembled, the same purpose as a belt The advantages of an apron conveyor is that it can be used for transporting materials that are too hot to be carried by a belt The drawbacks are that it cannot be used at high speeds, there are chances of sand spillage and leakage through the plates The cost of maintenance as well as the initial cost are higher
Mono rail conveyor
The mono-rail conveyor is frequently used for carrying sand and other items The sand is filled in bucket or containers of the drop bottom type and transported from one place to another on an overhead mono rail The conveyor may be either manually or electrically operated
Crane
For small foundries engaged in jobbing work, an overhead traveling crane may be more convenient for carrying sand The sand is filled in a bottom discharge type of bucket which is transferred with the help of a crane
2)Handling moulds
In a production foundry, moulds may be conveyed from the mould production section to the storage where they remain stationery fro pouring The moulds when cool are conveyed to a shake out station And after shake out operation, the emptied flasks, moulding boards etc are returned to the mould production area All these transport operations in a mechanized foundry are conducted on a suitable conveyor system Roller conveyor Pallet conveyor Overhead conveyor
Roller conveyor
The roller conveyor has two beams fixed on trestles of suitable height and supporting laterally arranged rollers above It may be either of the gravity or the power driven type Gravity conveyors are those in which no power is used to drive the rollers and move the moulds Here moulds need to be pushed by the operator to cause them to move on the rollers The beams may be fixed at a slight incline to facilitate the moulds movement by the force of gravity In the case of power driven conveyors some of the rollers at fixed intervals along the length are driven by electric motor This type conveyor is expensive
Pallet conveyor
The pallet or car type conveyor is the most efficient means of moving moulds in large scale foundries From the mould production area to the pouring department and after allowing adequate time for cooling to the shake out station It has pallets made of cast iron or steel plates mounted on wheels which can roll along a narrow gauge track
Overhead conveyor
Sometimes an overhead conveyor of the mono rail type is also employed for transporting moulds in small foundries The completed moulds are placed on the platform of the conveyor and the platform is carried to the pouring area by an overhead mono rail After the casting has solidified the same carrier is moved to the shake out station
3)Handling molten metal
For pouring molten metal into moulds two systems are commonly followed The molten metal is transferred in ladles to this area with the help of a traveling crane and is poured into the stored moulds Where the moulds are constantly moving , the metal is brought on a mono rail conveyor and poured into the moving moulds
4) Handling castings
After the casting are removed from the moulds at the shake out station , they are transported on a suitable conveyor to the cleaning and fettling section The range of conveyors includes Plate band conveyor Roller conveyor Overhead conveyor
Plate band conveyor
A plate band conveyor, is used in many foundries for carrying castings to the fettling or inspection sections It is excellent for both horizontal and inclined travels and for short as well as long distances The plates are joined together and mounted on a continuous chain moving on power driven sprockets
Sand reclamation
Sand reclamation
Refers to the treatment of used moulding sand so that it regains its original condition and can be reused again and again with minimum addition of new sand
It is the process of restoring used foundry sand to as nearly its original condition as possible
When sand has to recondition
Sand reclamation is desirable only when the cost of operation does not greatly exceed the cost of new sand i.e it must not exceed the cost of sand plus freight charges Sand reclamation is desirable only when sand is used in relatively large quantities Whenever there is a problem of disposal of sand
Advantages
Properly reclaimed sand is equivalent in performance to new sand in all respects Problem of finding places to dump huge quantities of used sand is solved Expensive disposal of used sand is eliminated Even though the base cost of sand is relatively low, freight costs are extremely high Storage of reclaimed sand is automatic. Thus the trouble some task of unloading cars of sand is obviated Sand (by reclamation) is available at constant moisture content and of consistent quality particularly grain size and size distribution
Necessity of reclamation *
The sand grains at and near the mould metal interface undergo physical and chemical changes due to intense heat This results in drying , burning, and baking of the materials surrounding the sand grains in to a hard tightly adhering coating The coating is due to repetitive usage of sand leads to coating called non silica fraction which consists of silica flour, spent and free clay- fine
Reclamation process Essentially, the reclamation process consists of
Crushing of sand lumps
Removal of bond from the grain surfaces
Reclamation may be done by mechanical, chemical or thermal means
Types of reclamation
The methods of doing reclamation are i) Dry reclamation ii) Wet reclamation iii) Thermal reclamation iv) Combined wet(or dry) reclamation plus thermal reclamation
Dry reclamation
Dry reclamation removes fines such as silica flour, spent , free clay and iron oxide particles Usually iron and steel particles are removed by magnetic separator and lumps are crushed and screened If a pneumatic scrubber is included , then coatings on the sand grains are removed
procedure
Iron and steel particles are first removed by magnetic separation and the used sand is fed to a scalping screen Lumps that are left on screen may be discarded or crushed and rescreened
Dry reclamation contn…
The sand that passes through the screen enters the pneumatic sand scrubber unit where the grains are repeatedly picked up and rubbed against each other to scrub off the coatings The sand grains then travel to an outlet while fines & dust are separated from the air As it is dry process it will not remove all clay coating
It does not restore the sand to its initial quality since some organic and carbonaceous materials may cling to individual sand grains
Wet reclamation
Similar to dry reclamation, its purpose is to remove fines and foreign materials and to clean the individual sand grains of coatings Complete removal of clay coatings can be obtained in well designed wet reclamation units Used sand is first passed through a magnetic separator and screens, as in dry reclamation It is then mixed with water to form a slurry, passed to a primary classifier to remove fines and to a wet storage tank The sand water slurry next enters one or more sand scrubbers which rub the sand grains against one another to remove the clay
Dry reclamation contn..
The sand scrubber is simply a tank with a large propeller rotates at high velocity in the sand water slurry, creating the rubbing action Next the slurry moves to a secondary classifier to remove fines created in the cleaning operation Finally the water is removed from the sand by a filter and draying unit.
It does not restore the sand to its initial quality since some organic and carbonaceous materials may cling to individual sand grains
Thermal reclamation
Comprises heating used sand to 1200- 1500 ºF in furnaces which are usually of the multiple hearth type This treatment removes carbonaceous and organic materials effectively ( by burning ) Even in these specialized cases thermal reclamation should be combined with screens and classifiers to assure proper grain size control of the used sand
Combining wet & thermal reclamation
The benefits of each are realized and it is possible to reclaim foundry sand to their original quality Fines, adherent clay, organic and carbonaceous materials are all removed The sand is restored and the castings made from such reclaimed sand are as good as those made in new sand The used sand, after passing through the wet portion of the system is classified, dewatered and then enters a rotary hearth furnace for the thermal treatment at 1200- 1500ºF
Dust & fume controls in foundries
Pollutants in a foundry
Foundries are among the industrial plants causing environment pollution, producing substantial quantities of air pollutants Sand preparation, handling, molding, core making, shake out and fettling operations causes much solid (particulate) dust emission of silica powder, binders etc which are highly injurious to human Melting furnaces, particularly those using coke or coal as fuel, generate smoke, fumes, gases like oxides of carbon and nitrogen Hot liquid metal poured in to molds generates gases and vapours from binders and coatings of mold & cores
Major pollutants Work area
Pollutant
Emission concentration g/cubic meter
Pattern shop
Sawdust, wood chips
Heavy
Sand preparation
Dust and fines powder materials
100-175 75-150
Moulding & core making
Sand Fines Binder dust vapours
50-100 100-175 75-150 Light
Mould drying and ladle
CO, SO2
Light
Cupola
SO2 CO Unburnt hydrocarbons , smoke Metallic oxides Coke dust
Light Heavy Heavy Moderate 100-175
Electric arc furnace
Dust , CO, SO2 oxides Nitrogen cyanide, fluoride
Moderate Light
Electric induction
Dust, oxides, smoke
Light
Pouring & mould cooling
CO Binders fumes
Light Moderate
Knock out
Sand, fines and dust
200-350
Emission controlling
With rising awareness of the hazards of environmental pollution over the entire world. Indian foundries are also gearing up for suitable control measures The basic means of controlling the emission of pollutants are
Changing the production process Supplying adequate make up air Proper aeration & ventilation of the shop Reduction of pollutants at source by taking appropriate control measures Dispersion & dilution of pollutants in the air space
Emissions during melting operations
The emissions from melting units need special attention as they may carry large quantities of constituents harmful to ecology and the environment The particulate emissions from a cupola may be of three types
Metallic oxides Silicon and calcium oxides, resulting from lining erosion, embedded moulding or core sands Combustible materials, which include coke particles or coke dust, vaporized or partly burnt oil or grease
Particulate emission from cupola The actual amount of each constituent may depend on the type of raw material used, the lining quality and its condition and operating conditions The particle sizes of these emissions vary from 1 micron to 500 micron Particle size Percent obtained (wt) Source >50 μ 40-80 Mechanical abrasion of refractor & charge 5 to 50 μ 20-40 Undissolved coke from upper portion of charge column <5 μ 5-20 Oxides of melting
Gaseous emissions from cupola
The gaseous emissions from cupola are composed of CO2, CO, N2, smaller amounts of SO2 & H2 Gases carried in the exhaust like CO2, N2 and H2 are harmless and need not to be collected The rate of evaluation of CO, which is due to incomplete combustion of carbon, is about 680 to 800 m3/h and the total exhaust gas volume is 4500 to 5400 m3/h in case of 5 ton capacity cold blast cupola These volume increases almost in direct proportion to the capacity Capacity of cupola t/hr
Amount of gas exhausted m3/h
CO m3/h
5
4500-5400
680-800
18-22
10
9000-10000
1350-1500
18-20
15
12000-14000
1800-2100
16-18
20
17000-20000
2500-3000
17-20
Dust kg/ton of metal
Emissions from electric furnaces
Compared to cupola furnaces, emissions from electric furnaces are small Arc furnaces emit large amount of pollutants, such as dust, smoke, co, oxides of nitrogen and sulphur along with process gases The actual amount of these constituents depends on
Nature of the raw materials used The manufacturing process and Exhaust system for the gases
The amount of dust exhausted from arc furnaces varies from 7 to 9kg/tonne of steel The volume of gases evolved varies from 350 to 450 m3/tonne of steel
Emissions from induction furnaces
Emissions from induction furnaces are relatively much smaller than arc furnace emissions They are originating from charge materials Which may consists of rust, dust, dirt, paint or grease and those originating from chemical reactions
Emissions in other areas of foundry Moulding & core making section
The pollutant generated in the moulding and core making section contains mainly dust However the amount of dust generated is much less as compared to other departments Harmful substances such as carbon monoxide and sulphur dioxide are evolved while drying moulds & cores They depends on the type of fuel and the quantity of the same consumed during drying moulds and cores
Emissions in other areas of foundry Sand preparation, knockout and sand handling a)
Amount of dust generated in sand mixers: The moulding and core sands in foundries are prepared in sand mixers of various designs. The ingredients mixed are also different at different times with edge runner mill of capacity up to 1.5 tonnes/batch, the dust in the air will be approximately 7.5 g/m3 . The particle size distribution in the dust is as follows
Dia of particles μm <5 content of fraction % 0
5-10 10-20 12 1.9
20-40 10
40-60 1.4
60 74.7
From sand mixers of centrifugal type with capacity up to 120 t/h, the dust in the air would be approximately 4 g/m3. The particle size distribution of dust is as follows Dia of particles μm <5 5-10 10-20 20-40 40-60 60 Content of fraction % 4.7 6 20 23.3 16 30 b) Generation of dust in knock out section: In steel foundries, knocking out of the castings out of mould boxes is most detrimental to health because of the evolution of harmful vapours, gases and dust containing approx 68% of particles of size 0-2 μm in diameter and 32% between 2-10 μm in diameter
c) Amount of gases evolved when pouring moulds: The amount of CO evolved when pouring molten cast iron depends on the mass of the castings and the time of cooling in the moulds they are usually 2-7 g/m3 of particle of 1.5 -32 μm in diameter d) Dust during cutting of gating & risering: A large amount of dust is generated when runner and riser of cast steel is cut. Its concentration in the air reaches 6-8g/m3 and in the immediate vicinity of work points up to 20g/m3 The particle size distribution of the dust formed when cutting cast steel is Dia of particles μm <2 2-5 5-10 >10 Content of fraction% 89-92 4.9-8.2 1.7-1.8 1.3-1.1
e) Heat treatment furnaces: The main pollutants in gases from heat treatment furnaces are carbon monoxide and sulphur dioxide. The gases are normally discharged into the atm through high stack which will facilitate dispersion of sulphur dioxide in to the upper atm and will reduce its ground level concentration f) Pattern shop: in pattern shop, where wood working machineries are installed, a local exhaust system to capture the saw dust, chips etc should be provided
Dust & fume control
It is of importance that the air polluted by foundry work be cleansed to maintain hygienic working conditions The atmosphere in the pattern shop is charged with fine particles of sawdust Dust and sand particles are exhausted when sand is mixed and prepared during moulding, and shake-out and fettling operations Fumes are produced during melting , melt transfer, and pouring operations It is essential to devise a system for collecting all the dust and fumes so produced and disposing them so that they do not pollute the atmosphere in the foundry When a foundry layout is planned, provision should be made for dust and fume control
Materials requiring to be separated may be classified into two broad categories I) Particulate matter: here particles are either solid such as dust, fume, smoke and fly ash or liquids such as mist and fog II) Gaseous matter: where the contaminant may be either gas over the entire range of atmospheric and process temperatures and pressures or liquid at lower temperature, and gas at the temperature and pressure of its release into atmosphere
Equipment for dust control
Filter : The filter serves to remove particulate matter from gas or air streams by retaining it on the porous structure through which the gas flows The porous structure is usually a woven or felted fabric. The filter must be periodically cleaned or replaced Filters are commonly employed in pattern shops on various woodworking machines, such as band saw, circular saw, sanding machines They are also used on cupola collection system in conjunction with other equipments, such as after-burners, gas coolers Sand reclamation plants also use filter for separating fines from sand grains
Equipment for dust control
Cyclone: This works on the principle of centrifugal separation. A vortex motion of the particulate matter is created within the collector This motion provides the centrifugal force which propels the particles against the conical section of the collector and forces it out of the chamber at the apex. The cyclone are used in sand preparation plants for separating sand particles from air In cleaning the cupola exhaust In moulding shops and shake out stations
Equipment for dust control
Mechanical collectors: these devices include settling chambers, water arrangement, fan arrangement Which collect particulate matter by gravity or centrifugal force As their efficiency of collection is generally rated low , they are used as precleaning devices
They also function in combination with filters
Cupola exhaust systems often make use of mechanical collectors
Equipment for dust control
Scrubber: The scrubber is employed primarily for removing gases and vapour phase contaminants from the carrier gas, though it can also remove particulate matter A liquid, usually water, is introduced into the collector and it either dissolves or chemically reacts with the contaminant collected Scrubbers are ideal for cleaning the exhausts of cupola and arc furnaces.
Methods for fume control
Apart from the use of methods and materials we have following measures , namely:
General ventilation of the workspace
Local extraction in the immediate region of the work
Local extraction at the point of dust creation
General ventilation
General ventilation has been commonly sought through upward movement of air, brought about by high level extraction fans Here airflow being created by extraction close to the points of dust formation In this case local exhaust is used as an aid to general shop ventilation
Local extraction in the immediate region of the work
Local exhaust systems are based on two different concepts Extraction may be localized to the work region, either by the use of hoods and booths, positioned in the path of the main dust stream Exhausted fettling benches, designed to maintain a positive air current past the work and away from the operator
Local extraction at the point of dust creation
In the second type of local system extraction is more intensely localized at the machine or tool , using integral suction attachments providing closely confined high velocity air streams Such systems are available for each type of fettling equipment
Combination devices
Some devices combine features of the both equipments so dust and fumes are controlled most economically and with a minimum pressure drop There are cyclones in which liquid is sprayed, and scrubbers in which cyclonic action is used Packed bed filters, operated wet, and packed bed scrubbers are similar to each other The equipment designed to separate particulate matter is called a filter The same when designed to separate gaseous contaminants is called a scrubber
Plant layout for foundries
Plant layout
Plant layout involves arranging the physical plant facilities in such a way that which gives maximum efficiency in the combination of men, materials and machines
Plant layout is defined as a floor place for arranging the desired machinery and equipment in one best place, to permit the quickest flow of material at the lowest cost
Foundry has buyers market
As foundry industry has moving from a seller’s to a buyer’s market, it characterized by stiff competition and reduced profits Since profits are lower, it is logical that a plant with lower production costs is better One of the most effective ways to cut down production costs is to eliminate all non productive plant activities A good layout is one that provides for full utilization of available equipment for production, material handling devices and manpower
Plant site location Some of the necessities for the location of a foundry are
Availability of high tension power supply substation, especially when electricity is needed for melting Available of large area of industrial land due to need for storing large quantities of materials like melting scrap, molding sand etc Suitable distance from normally populated urban areas due to pollution, dust, fumes, heat and smoke generated from the foundry Availability of area for dumping waste materials like burnt sand, slag, used refractories Availability of water, roads and other basic infrastructure facilities Sufficient auxiliary space for future expansion
Necessities of plant layout
Operations in metal casting involve relatively larger handling and movement of materials compared to other production process Approximately 4 tonnes of moulding sand and additives, 2 tonnes of charge materials, refractoreies and many other bulk items like mold -boxes, patterns etc are needed for every tonne of casting produced Many auxiliary services like pattern shop, maintenances, sand and refractory preparation, scrap segregation etc are needed along with the basic operations of molding, melting and finishing, which complicate material handling and flow
Advantages of a good layout
Some of the advantages to be gained from good plant layout in a foundry are
Improvement in the manufacturing process
Improved quality control
Improved materials handling
Minimum equipment investment
Effective use of available area
Improved utilization of labour
Improved employee morale
Advantages contn…
Improvement in the manufacturing process: This may be due to i) Elimination or reduction of delays through improved arrangements or better work balance between machines or operators ii) Smoother material flow in the process iii) Improved control by incorporating methods for identifying and inspecting goods in process Improved quality control: A good layout incorporates the quality considerations in a manner that ensures maximum control and minimum cost Improved materials handling: Materials handling is improved by proper location of equipment, reduced handling distances, and closer coordination of the entire handling activity
Advantages contn…
Minimum equipment investment: planned machine balance and location leads to minimum equipment investment Effective use of available area: well planned layout offers an opportunity to place equipment and services in such a manner that the most effective coordination is possible. Locating equipment and services such that they can perform multiple functions Improved utilization of labour: proper plant layout allows the design of individual operations, the process, the flow, and material handling in such a manner that each worker can effectively apply his activities to the best overall plant effort
Advantages contn….
Improved employee morale: A layout that provides for employee convenience and comport inevitably boosts employee morale A design that incorporates such items as correct lighting, proper cooling and ventilation, noise and vibration control, sufficient and convenient rest rooms and lunch facilities leads to efficient job performance and to reduced idle time
Steps in planning a foundry layout For any new foundry in order to realize good performance systematic procedure for planning a layout must be followed. The various steps are
Analyse the product to be manufactured: This includes the types of castings to be produced: their range of size and weights and composition, annual tonnage and the maximum piece weight. Determine the process required to manufacture the product: The sequence of operation for producing the products must be properly prepared or developed. This helps in logical locations of various equipments like moulding, melting, inspection etc
Steps in planning a foundry layout
Prepare layout planning charts: These include a) The flow process showing all operations, moves, storages and inspection in sequence b) Standard time for each operation obtained from time studies c) Machine selection d) Machine balance e) Manpower requirements Determine work stations: The requirements of machine, operator, materials, and service areas must be considered. This is accomplished by using man-machine and/or operation charts Analyse storage area requirements: The size and location of storage area should be studied in relation to the production activity. This includes i) Storage of raw materials ii) in process storage and iii) finished goods storage
Steps in planning a foundry layout
Establish minimum aisle width: Clearance around the various pieces of machinery and between departments should be determined before starting the layout Establishment office requirements: These will depend upon the scope of operational activities. The exact requirements of space should be worked out and provided in the layout Consider personal facilities and services: Allow for such items as first aid, lunch and refreshment centers, lockers, rest rooms, and parking Provide for future expansion: This may include sufficient provision for the addition of new product lines or for increased demand for the existing products
Layout of foundry
Foundry layout shows the plan view of the various sections of the foundry It indicates the activities of the foundry like material movement, direction of flow, details of material storage, location of moulding, melting, cleaning, inspection etc It also indicates the size of the foundry It indicates the nature of foundry such as mechanised, unmechanised etc A good layout is one which maximises the production for the given shop floor area which uses least material transportation for a given ton of casting produced
Types of foundry layout
The foundry layouts can be generally classified in two ways I) Based on nature of foundry Jobbing foundry Captive foundry Mechanised foundry
II) Based on the total tonnage of castings produced
Small sized foundry Medium sized foundry Large sized foundry
Jobbing foundry
Produces small quantity of castings as and when orders are placed by different customers. Here castings are produced against job orders The job items keep on changing every now and then Not much stress focused on the severe quality standards Simple & general type of castings are produced Normally hand moulding is employed Invariably green sand moulding is used Production may not be there for all the shifts in a day The layout will be small and simple Most of the activities are carried out by hand only
Layout of Jobbing foundry
Captive foundry
This type of foundry caters to the need of its own The foundry is normally a part of parent organization and will supply the castings against its requirements No external orders are entertained except the parent organization Orders will be repetitive in nature and the production is continuous Machines are used to make the molds and core, for cleaning the castings, transporting the casted parts Electric furnaces like arc furnace or induction furnace used in general Productivity is high
Layout of captive foundry
Mechanised foundry
Generally it is a big foundry with well laid out plan A number of machines are used in the shop floor Sand mixing is done by machine Moulding and core making is done by machines Moulds are transported by conveyors and overhead cranes are used for material transportation The foundry normally accepts external orders in bulk quantities The orders will be repetitive in nature A well maintained patternshop, sandplant, melting shop, moulding section etc are present Machines are used for most of the activities and the human skill is kept to a minimum Certain times robots are also used at various levels in the shop floor
Layout of mechanised foundry
Layout of small sized foundry (below 600 tonns/year)
Produces small sized castings up to few hundred kilograms single piece
Layout of medium foundry
Produces medium sized castings upto 1000kilos single piece
Layout of large foundry Produces heavy castings weighing several tonnes
