In this article we will discuss about:- 1. Introduction to Casting Process 2. Aims in Making a Casting 3. Types 4. Patterns 5. Design 6. Technical Aspects of Making Sound Castings 7. Inspection 8. Defects.
Introduction to Casting Process:
Casting process is based on the property of a liquid to take up the shape of vessel containing it. Molten metal poured into a mould flows into the corners and fills all the voids. When metal solidifies, it takes the shape of mould but not exactly the same because solid being denser there is reduction of volume (2 to 9%). For obtaining correct dimensions, provision needs to be made for shrinkage of metal. Hollow components can be produced by inserting a core into the cavity in the mould.
Casting is one of the most versatile form of mechanical process for producing components; because there is no limit to the size, shape and intricacy of the articles that can be produced by casting. It offers one of the cheapest methods and gives high strength and rigidity even to intricate parts, which are difficult to produce by other methods of manufacturing.
One of the most attractive feature of casting is its ability to form any shape in one operation. This results in saving labour costs and avoids any problem which might arise in joining sub-assemblies together. It is often possible to produce accurate castings in metals which are difficult to machine to size. In castings, a high degree of reproducibility is possible. Once the shape of the mould has been established and alloy composition fixed, control of melting and pouring conditions ensures that all castings are identical.
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Due to cost of pattern, casting becomes attractive over other methods of fabrication, when quantities to be produced are adequate. Due to the above facts, cast parts are gaining importance day-by-day. Large savings in weight, can be achieved as the metal can be placed exactly, where it is required. Castings possess the complete range of mechanical and physical properties.
Castings can be produced in large numbers rapidly at low cost. Castings can also be made in wide range of dimensional tolerances and surface finish. Casting is not always the best method of the various production techniques, but possibilities of casting methods are of much significance and should not be overlooked for new or revised designs.
Although, all the metals can be cast, but iron is mostly used, because of its fluidity, small shrinkage and the ease with which its properties can be controlled. The type of moulding material used has an important influence on the ease of making the mould and its cost, the permanency of mould, the speed of production, the rate of cooling of molten metal, surface roughness, dimensional tolerances and the mechanical strength of the casting.
Principle of casting consists of introducing the molten metal into a cavity and mould of the desired shape and allowing it to solidify. When it is removed from the mould, the casting is of same shape but slightly smaller, due to contraction of metal. The molten metal passes through four stages (viz. liquid stage, mushy stage, plastic stage and solid stage) till the solidification takes place.
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The metal within solidification range is at mushy stage. Contraction of metal begins immediately this range is over and the metal comes into plastic stage.
The mould into which metal is poured is made of some heat resisting material. Sand is most often used, as it can be easily packed to any shape and is somewhat porous, and resists high temperatures. Moreover, silica sand is low in cost, has long life, and is available in wide range of grain sizes and shapes. Pure silica sand is not suitable for moulding, since it lacks binding qualities.
The binding qualities can be obtained by adding 8 to 15% of clay. Some natural moulding sands such as Nangloi sand are adequately bonded with clay, when quarried need little alteration to make them suitable for use. Molasses (extract of sugar cane) is also often used as binding material, when quick mouldings arc required and moulds need not be dried. Clay, when added to the sand and damped by water forms a mixture which becomes cohesive and is easily shaped into moulds.
The size of sand grains will depend on the type of work to be done. For small and intricate castings the use of fine sand is desirable so that all the details of the mould will be brought out sharply. As the size of the casting increases, the sand particles likewise should be coarser to permit the ready escape of gases generated in the mould.
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Sharp, irregular-shaped grains are usually preferred because of their ability to interlock and add strength to mould.
The main advantages of sand processes are the wide choice of alloys, and size and design of casting they allow. The limitations are the difficulties of meeting exacting requirements like dimensional accuracy, surface quality and internal soundness.
Inherent weaknesses in castings result from certain characteristics (like shrinkage, segregation, gas porosity and low hot strength) associated with solidification process. Poor moulding and foundry practices can result into several casting defects, which may render the castings to be rejected.
Aims in Making a Casting:
There are basically two reasons for making a casting:
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(i) Liquid metal must be solidified before further processing is possible.
(ii) To produce finished or semi-finished articles (in the sense that these require machining, etc.). The various reasons under this second category, which are of more importance, can be
(a) Casting is often the cheapest and most direct way of producing a shape with certain desired mechanical properties. Desired mechanical properties can be attained by operations like suitable control of alloy composition, grain structure and heat treatment.
(b) Casting is best suited where components are desired in low quantities as high cost of mechanical working processes like rolling, forging, extrusion etc. requiring heavy equipment can be justified only when components in large quantities are required.
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(c) Certain metals and alloys such as highly creep resistant metal-based alloys for gas turbines can’t be worked mechanically and can be cast only.
(d) Intricate shapes having internal openings and complex sectional variations can be produced quickly and economically by casting since liquid metals can flow into any form, whereas tooling and machine costs in mechanical working, would be too high to produce them.
(e) Heavy equipment like machine beds, ship’s propellers, etc. can be cast easily in the required size rather than fabricating them by joining several smaller pieces.
(f) Casting is best suited for composite components requiring different properties in different sections. These are made by incorporating prefabricated inserts in a casting, some examples are: steel screw threads in zinc die castings, aluminium conductors into slots in iron armatures for electric motors, wear resistant skins onto shock resistant components etc.
The suitability of the casting operation for the material to be cast depends on the thermal properties such as melting temperature, thermal conductivity and coefficient of linear expansion of both the material to be cast and the mould material, the solubility of and the chemical reaction between the metal and the mould material, and the effect of the surrounding environment at different temperatures on the material being cast.
Types of Casting of Metals:
(i) Chilled Casting:
The mould used for chilled casting is either of a metal having high melting point or the mould may be given a lining of such a metal. This results in rapid conduction of heat from the surface of the casting with the result that the casting will have harder outer surface and softer inner core. Chilled castings are very useful in making railway carriage wheels.
The degree of graphitisation is influenced by the presence of silicon which tends to cause the cementite to break down during cooling to form ferrite and graphite. Since graphitisation requires time, the effect of silicon can be offset by rapid cooling or chilling of metal. Thick sections are faced with chills to conduct away heat. Chills are also used to produce local surface hardness.
(ii) Centrifugal Casting:
The casting produced by pouring the metal in speedily rotating mould is called Centrifugal casting. The crystallisation of the molten metal takes place from the farthest end unlike the normal casting where the crystallisation takes place from sides which are exposed to cooling. Therefore, centrifugal castings are uniform and strong. Even thin castings symmetrical about the axis of a cylinder can be obtained from this method of casting.
(iii) Malleable Cast Iron:
It can be obtained by annealing the castings. The cast product is packed in an oxidising material such as iron ore or in an inert material such as ground fireclay. The pack is put into an oven and is heated to a temperature of about 870°C. It is kept at that temperature for about two days and is then allowed to cool at the rate of 5 to 10 degrees per hour.
Iron ore acting as an oxidising agent reacts with carbon and carbon dioxide escapes. The annealed cast product is free from carbon. If the cast product is packed in an inert material, slow cooling will separate out the temper carbon. Malleable cast irons are used for complicated structures.
(iv) Inoculated Cast Iron:
The molten pig iron before casting is inoculated with soluble silicon compounds like calcium silicide. The silicon added in this way has got a better effect. Pearlitic cast iron having microstructure of small flakes of graphite set in pearlite is formed in this way.
Patterns for Casting:
A pattern is defined as a model of a casting, constructed in such a way that it can be used for forming an impression (mould) in moulding sand.
The first step in making a casting is to prepare a model, known as a pattern, which differs in a number of respects from the resulting casting. These differences, known as pattern allowances, compensate for metal shrinkage, provide sufficient metal for machining the surfaces and facilitate moulding.
Most patterns are made of wood because of its cheapness, ease of availability, lightness, ease of obtaining smooth surface and preserving surface by applying coating of shellac, ability to be worked on easily; ease of jointing, ease of fabricating to numerous shapes. However, wood is easily affected by moisture, its shape changes by change in moisture, it wears out quickly by sand abrasion, it may warp during improper storing, it cannot stand rough usage.
Wood used for pattern making should be properly dried and it should be straight grained and free from knots and free from excessive sap wood. The chief Indian varieties of timber used for making patterns are—Burmateak, pine wood and Mahogany. Sometimes sal, deodar, shisham, walnut and apple are also used. Where durability and strength are required, patterns are made from metal—usually aluminium alloy, brass, or magnesium alloy. For mass production, steel or cast iron pattern may be preferred.
A wooden master pattern is first made from which the metal pattern is cast. The pattern is made oversize to allow for the normal shrinkage of metal during cooling. Since each material has a different shrinkage rate, the pattern maker uses a special scale for making his measurements. Some patterns are made of plaster, plastic compound, wax etc.
Pattern Construction Details:
(i) Fillets:
A fillet is a concave connecting surface for the rounding out of a corner at two intersecting planes. In all castings sharp corners should be avoided. Rounded corners and fillets assist material in moulding, since there is less tendency for the sand to break out, when the pattern is withdrawn.
The metal flows into the mould more easily, and there is less danger of sand washing into the mould. The appearance of casting is improved and it is generally stronger, having fewer internal or shrinkage strains.
A casting in a mould cools on the outside first. As the cooling progresses to centre, the grains of metal arrange themselves normal to surface in a dendritic structure. In patterns, that have sharp corners, there is a tendency for the metal at the corners to open up because of shrinkage.
Fillets are made of wood, leather, or wax.
(ii) Section Thickness:
So far as service and design factors permit, all sections should be as uniform as possible. When light section must be adjacent to heavy sections, the transition should be as gradual as possible since abrupt changes in thickness always result in strains, which are likely to cause cracks.
(iii) Sanding and Shellacking the Patterns:
For getting good finish, patterns are made smooth with abrasive paper. After sanding, shellac is applied to obtain hard and smooth surface coating on pattern.
It is important to note that the degree of finish that can be obtained on the casting depends largely on the smoothness of pattern.
Materials Used for Patterns:
i. Wood:
Although patterns are made from a variety of woods, white pine is a favourite choice because it is straight grained, light and easy to work, and has little tendency to warp. When a more durable wood is necessary for fragile patterns or hard use, mahogany is preferred. Other woods suitable for pattern making are cherry, beech, poplar, bass wood and maple, the last being especially desirable for work on the lathe.
Before using a wood for pattern making, it must be dried and seasoned. Kiln dying, which requires only a short time, reduces the moisture to a minimum, drives off volatile matter, and is mostly employed.
Since wood is susceptible to moisture, it tends to warp and wears out quickly due to sand abrasion. Metal patterns are preferred where durability is the consideration.
ii. Metal:
Many of the patterns used in production work are made of metal because of its ability to withstand hard use. Furthermore, metal patterns do not change their shape when subjected to moist conditions and require a minimum of maintenance work to keep them in operating condition.
Aluminium is the best of all the metals because it is easy to work, light in weight, resistant to corrosion, easy to cast. Cast iron is cheap, easy to file and fit, strong, gives a good smooth mould surface with sharp edges and is resistant to abrasive action of sand.
But it is heavy, easily broken and has hard edges difficult to be machined. Brass is strong, tough, does not rust, takes better surface finish than cast iron, withstands sand abrasion better. Brass patterns are, however, heavier and costly.
iii. Polystyrene:
This is known as a consumable pattern because the heat of the molten metal vaporises the pattern so that, it leaves the mould in the form of gas. The plastic is light, easily sculptured with a sharp knife, and the pattern can be made of several parts cemented together. Special care is required in ramming the mould as this material is fragile. It is best suited for cases, where it is difficult or impossible to draw the pattern.
Machines Used in Pattern Making:
1. Circular Saw:
It is the most widely used, very accurate and versatile machine. It is used for ripping, cross cutting, mitering, bevelling, and grooving lumber. It consists of a flat surface or table upon which the work rests while being cut; a circular cutting blade; a cut-off guide; and a ripping fence.
Tilting mechanisms on circular saws, either by tilting of table or by tilting saw arbor, provide for angular cuts. These mechanisms are of two basic designs. Both types of tilting arrangements are capable of tilting to a 45° angle.
2. Band Saw:
This machine is ideally suited for making curved or irregular cuts in wood. It is designed to cut wood by means of an endless metal saw band that travels over the rims of two or more rotating wheels.
For curves of small radii, it is necessary to employ narrow saw blades, which will not bind, when the work is being turned. Angular cuts are obtained by tilting the saw table.
3. Jig or Scroll Saw:
It is used for making intricate irregular cuts on small work. It can be used conveniently for internal cuts by threading the blade through a hole bored into the work.
4. Jointer:
The jointer planes the wood by the action of revolving cutter head. The board is hand fed against the revolving cutter head, which planes fine chips from the wood. An adjustable fence guides the board at a pre-determined angle to the surface of the table. This fence may be adjusted to a right angle, with the table or may be inclined for bevel and angular cuts.
5. Wood-Turning Lathe:
Patterns for cylindrical castings are usually made by the pattern maker on the wood- turning lathe.
The wood stock in a wood-turning lathe rotates and is shaped by the cutting action of turning tool held against the rotating work.
Long and slender work is generally used for disk-shaped pieces.
6. Abrasive-Disc Machine or Disc Sander:
It is used for shaping or finishing flat surfaces on small pieces of wood and sanding the draft on small patterns. It consists of an electric motor with a metal disc mounted on its shaft, on which an abrasive disc is cemented. An adjustable work table is located below the centre of the face of the disc for supporting the work.
7. Abrasive-Belt Machine:
It makes use of an endless abrasive belt which travels over two drums. Cloth- back abrasive belts are available in several grits for sanding or polishing wood or metal. This machine is especially useful in shaping patterns or parts of patterns that would normally require the use of hand tools.
The work is supported by an adjustable work-table that may be tilted at a predetermined angle.
8. Drill Press:
In addition to drilling and boring holes, this machine is used for mortising, shaping, and routing with the special attachments available.
9. Wood Shaper or Wood Moulder:
In this machine, a cutter head carrying a cutter rotates about a vertical axis. It has a horizontal table similar to a jointer. Wooden piece is fed by hand along the table against the cutter and guided by an adjustable fence. A number of shapes can be imparted to the surface of the wood according to the profile of teeth on the cutter.
10. Mortiser:
It is used to facilitate the cutting of mortise and tenon. Chisel mortising machine consists of a revolving spindle carrying an auger bit at the bottom end. Auger bit rotates at a higher speed inside a square hollow chisel having four sides of equal width.
As hollow chisel is forced into the wood, the auger bit bores the hole, which is cut square by the sharp end of the chisel. Thus, auger bit and chisel work together—performing boring of a square hole. By shifting the work on the table, the mortise of the desired length with square ends and square bottom is produced.
Design of Castings:
The following points should be remembered while designing any casting:
(1) Parting lines should be in one plane wherever it is possible.
(2) Sections should be uniform, and if any change in section thickness is desired, it should be made as gradual as possible.
(3) Sharp corners should be avoided as they retard the metal flow or cause eddies and trap air.
(4) Section junction should be joined by fillets, especially where they are subjected to service stresses, because the fillets reduce stress concentration and give a crystalline structure which results in stronger castings.
(5) Ribbing is desirable on thin sections of considerable area especially in case of flats. It not only gives stiffness and minimises war-page but also provides feeder channels for larger cross-sectional area.
(6) Ribs or beads may be used to strengthen thin sections, where trimming flash is required.
(7) Bosses and other sections must be so designed that they can be fed with molten metal from the gate. Liberal fillets should be allowed where bosses join the supporting structure.
(8) Both internal and external under-cuts should be avoided wherever possible.
(9) The use of intersecting cores should be avoided as far as possible.
Predesign Considerations in Design of Castings:
A good design engineer should consider the following points:
(i) Design should be such that the pattern can be adopted for economical moulding by standard methods.
(ii) Gates, risers and chills should be positioned properly to ensure soundness.
(iii) Section size and shape should not be such as to cause undue stresses in the mould and consequent tearing and cracking.
(iv) Directional solidification be established and controlled.
(v) If casting is of complicated shape, it can be broken down into component parts and the separate castings welded together. For instance, bosses and cumbersome projections instead of casting integral can be welded to make foundry operations easier, cheaper and more dependable.
(vi) To prevent tearing, small tie bars are used in critical areas. To reduce stress slightly, straight members like spokes to wheels may be curved.
(vii) Some thin sections of castings cool and contract much faster than heavier members, and if such members are attached rigidly, high stresses will develop. If members are curved, the small sections will tear to relieve the stresses or deform (wrap) plastically without tearing.
(viii) For sound castings, temperature gradients in solidifying castings must be properly controlled. Heavy sections should not be fed through light sections. Designer should attempt to limit junctions to as few as possible. If limiting the number of junctions is not possible, it is advisable to cast in segments and join them together later by other fastening methods.
(ix) If possible sections should be designed to taper towards riser. It must be remembered that the casting soundness obtained by natural solidification is proportional to the degree of tapering allowed.
(x) Since tendency of a casting to warp or tear is proportional to its length and complexity, it is advisable to separate casting into parts and fasten them together by other methods.
(xi) Since it is very difficult to obtain horizontal flat surfaces at top because of centre line shrinkage and collection of slag, dross, and other impurities lighter than metal on the upper flat surfaces, it is advisable to have curved surfaces on top and incorporate flat surfaces on vertical sides.
Technical Aspects of Making Sound Castings:
(1) Tapers must be incorporated in all sections in order to facilitate removal of pattern from moulds, or separate solidified castings from permanent moulds. Taper also induces directional solidification.
(2) All corners should be rounded and re-entrant sections must be avoided as far as possible in order to permit free flow of liquid metal. Generous fillets should be incorporated.
(3) Pattern maker’s shrinkage allowance should be provided to allow for shrinkage of metal from freezing point to room temperature. Thus, depending upon metal, all dimensions are scaled up between 2-5%.
(4) Cores must be used to form cavities and re-entrant sections.
(5) Shape of component must be carefully examined to determine the location of suitable parting line (position where the opposite halves of the mould meet and where it is broken open to remove pattern or casting). It is not necessary that parting line be a flat plane, it can be stepped also.
(6) It must be ensured that part of the shape gets buried or becomes keyed into the mould cavity so as not to damage the mould or casting upon removal.
(7) The problems of pattern making should be kept in mind. The number and complexity of cores needed should be minimised.
(8) Minimum thickness of cast parts for ease of metal flow should be as under:
(9) Very often, by making and studying the model of castings, important casting decisions can be taken. Following this approach, areas of potential shrinkage and tearing can be predicted, the best location of gates and risers determined, casting repairs can be eliminated by incorporating suitable design changes, money saving changes in pattern construction can be spotted before making working patterns.
Fettling or Cleaning of Castings:
Castings require cleaning and trimming before passing them on for machining, or other operations. Fettling is not a precision operation like machining. Its purpose is to remove all unnecessary metal, and the production of a reasonably smooth finish.
For light castings the fettling is limited to removing runners and risers and cleaning off any adhering sand. In heavier and more intricate castings, more work is necessary to remove cores, flash metal at joints and surface imperfections.
Generally, higher the metal melting point, the greater the surface roughness of the castings. Thus the heavier castings in grey and malleable cast iron and cast steel call for the most work in fettling.
Removal of Sand:
After fettling, any sand adhering to the mould and cores is removed. Method of sand removal is dependent on the size of the casting and the intricacy of the casting.
A simple casting can be easily cleaned by rapping with a hammer followed by wire brushing or shot blasting. If design is more intricate, it takes longer to remove cores and wire reinforcements. Although a wide range of castings are made in green sand moulds, cores are more usually dried.
Dried sand, if artificially bonded, does not break easily. On the other hand in such cases casting surface requires little cleaning once the sand is removed. Cleaning by picks and bars alone becomes very laborious in case of intricate castings. Pneumatic picks are, therefore, used to some extent. But these are heavy and considerable dust is created by their exhaust.
Water sprays in conjunction with pneumatic pick eliminate the dust problem.
Modern automatic units for smaller castings rotate or tumble the castings under the grit blast. Fig. 3.78 illustrates one such unit where abrasive is mechanically blasted into castings which are being tumbled on an apron conveyor. Sand or shots could be hurled on the surface of casting by air, water or mechanically by rapidly rotating paddles.
Wet Process of Cleaning (Water Blast Cleaning):
This process eliminates the dust problem. In the simplest process, a gun, easily handled by one man, is used which projects a fine stream of water and sand against the casting at a high pressure.
For heavier equipment the gun operated mechanically from outside the chamber is used. The gun is mounted in a carriage which can be moved the entire length of the room vertically as may be required. The gun swivels in the carriage in a cone of more than 90°. All the motions are controlled by two levers outside the room actuated hydraulically and electrically. This process cleans the surfaces of the casting rapidly and thoroughly.
Sand Blasting and Shot Blasting:
It consists of projecting, as high velocity, a stream of fine abrasive against the casting to produce a thorough cleaning of the surface.
In the sand blasting the abrasive used is sand of fairly large grain size. In sand blasting, the sand (silica) being in granular form, is comparatively soft and it rapidly breaks into smaller size on impact with a hard metallic surface. Thus the life of the abrasive is less. Further the silica dust can lead to tubercular disease.
In shot blasting, round, or angular, chilled iron shots are used in place of sand resulting in better cleaning and longer life. The selection of round or angular shot depends on the finish desired and the material to be treated.
The round shot is generally restricted to the shot blasting of non-ferrous castings. Chopped steel wire and steel grit are claimed to be economical because of their long life. The abrasives may be propelled either by pneumatic means or airless.
Pneumatic-Type Shot Blasting Machine:
A stream of shot falls into a chamber through which compressed air at pressure of 1.5 to 2 kg/cm2 for grey iron and non-ferrous castings, and upto 6 kg/cm2 for cast steel is passed. The shot and dislodged material then fall on perforated floor of the cleaning machine. Sand and shots are then separated by a screen and suction dust extraction system.
After removal of adhering sand or refractory, gates and risers are removed by chipping, flogging, shearing, sawing, abrasive wheel cutting and flame cutting.
After removing gates and risers, castings are given one or more finishing operations like polishing, brushing and buffing. The purpose is to smooth the gate and riser areas of the casting, remove any excess metal remaining on the casting and improve appearance.
Inspection of Casting:
Castings are inspected thoroughly to find the presence of internal flaws as well as external defects.
Various methods of inspecting castings are as follows:
1. Visual Examination:
It consists of examining the surface of the casting with the unaided eye, a magnifying lens or a low-power microscope: checking of thicknesses by using calipers, gauges, templates, etc. It requires great skill on the part of the inspector the locate the defects.
2. Sound or Percussion Tests:
It consists of suitably supporting the part and swings it freely and then tapping it with hammer to set up vibrations and produce a certain characteristic tone.
The sound produced is listened electronically and compared with that of a sound casting.
3. Pressure Tests:
These are employed to locate the leaks by subjecting casting to a pressure of one and a half times to two times the working pressure. A pressure gauge indicates presence of leak.
For testing pneumatically, the casting is immersed in a tank carrying water and air pressure is applied. It there is a leak, air bubbles will be noticed.
4. Magnetic Particle Inspection:
Magnetic particle inspection is used for iron or steel and their alloys possessing magnetic properties. The casting is first magnetised. Iron filings are then sprinkled which align themselves in the direction of the lines of force. The defect is noticed as the iron filings jumble around the defect.
5. Fluorescent Powder Inspection:
Fluorescent powder inspection can be used on all materials to detect surface defects. Castings are immersed in a warm suspension of fluorescent powder in penetrating oil. The suspension penetrates minute cracks and pores. Excess suspension is washed away in warm water, and the casting is dusted with a drying powder. This draws some of the suspension from the cracks to the surface, where if fluoresces and is readily visible under ultraviolet light.
6. Radiographic or X-Ray Examination:
Radiant energy from an X-ray tube or gamma-ray source is passed through the section of the casting, and intensity of emergent rays recorded on a film held on the opposite surface. Defects in the form of voids or cracks are recorded as blackened areas on the film.
7. Ultrasonic Testing of Castings:
It is based on the principle of reflection of high frequency sound waves. If the section is homogeneous and free from defect, the wave is reflected back after travelling through the whole of the section. If casting contains some defect, the wave is reflected from the surface of the defect and returns in a shorter period of time. An oscillograph is used to detect the lengths of time.
Defects in Castings due to Improper Moulding and Remedial Actions:
Many defects occur in castings if moulding sand is not of proper composition and if not properly prepared. Any carelessness in preparation of mould also results into casting defects. Various defects that occur due to faulty sand conditions and their remedial measures are tabulated in Table 3.6.
