UNIT 2 MOULDING MATERIALS

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1 UNIT 2 MOULDING MATERIALS Moulding Materials Structure 2.1 Introduction Objectives 2.2 Moulding Sands Properties of a Good Moulding Sand Principal Ingredients of Moulding Sands Other Additives to Moulding Sands Specifications and Testing of Moulding Sand Moulds with Different Types of Sand Other Sand-based Moulding Methods 2.3 Materials for Core Making 2.4 Shell Moulding Material 2.5 Investment Moulding Material 2.6 Moulds for Other Casting Processes 2.7 Summary 2.8 Key Words 2.9 Answers to SAQs 2.1 INTRODUCTION In casting process the molten metal is poured int o a mould cavity. Therefore suitability of a casting operation depends on the selection of an appropriate moulding process and mould material. Suitability of a moulding material depends upon the type of material being poured, number of castings being made, the type of casting, quality requirement by the customer and finally on the mould and core making equipment owned by the foundry. Objectives After reading this unit you, should be able to learn about the main features of various moulding materials used in the foundry industry, and about the applicability of these moulding materials in various casting processes. Moulds can be made of number of moulding materials like sand, metal, waxes etc. Even the properties of the above mentioned few basic materials and other moulding materials can be altered by adding different types of additives. In the sections to follow we shall discuss about the important characteristics of moulding materials. 2.2 MOULDING SANDS Foundry sand consists primarily of clean, uniformly sized, high-quality silica sand or lake sand that is bonded to form moulds for ferrous (iron and steel) and non-ferrous (copper, aluminum, brass) metal castings. Although these sands are clean prior to use. The automotive industry and its parts suppliers ar e the major generators of foundry sand. 25

2 Principle of Metal Casting The most common casting process used in the foundry industry is the sand cast system. Virtually all sand cast moulds for ferrous castings are of the green sand type. Green sand consists of high-quality silica sand, about 10 percent bentonite clay (as the binder), 2 to 5 percent water and about 5 percent sea coal (a carbonaceous mould additive to improve casting finish). The type of metal being cast determines which additive and what gradation of sand is used. The green sand used in the process constitutes 90 percent of the moulding materials used. In addition to green sand moulds, chemically bonded sand cast systems are also used. These systems involve the use of one or more organic binders (usually proprietary) in conjunction with catalysts and different hardening/setting procedures. Foundry sand makes up about 97 percent of this mixture. Chemically bonded systems are most often used for cores (used to produce cavities that are not practical to produce by normal moulding operations) and for moulds for non-ferrous castings. The sand used for green sand moulding must fulfill a number of requirements : (i) (ii) It must pack tightly around the pattern, which means that it must have flowability. It should be capable of being deformed slightly without cracking, so that the pattern can be withdrawn. In other words, it must exhibit plastic deformation. (iii) It must have sufficient strength to strip from the pattern and support its own weight without deforming, and to withstand the pressure of molten metal when the mould is cast. It must therefore have green strength. (iv) It must be permeable, so that gases and steam can escape from the mould during casting. (v) It must have dry strength, to prevent erosion of the mould surface by liquid metal during pouring as the surface of the mould cavity dries out. (vi) It must have refractoriness, to withstand the high temperature involved in pouring without melting or fusing to the casting. (vii) With the exception of refractoriness, all of these requirements are dependent on the amount of active clay present and on the water content of the mixture. For the abovesaid qualities, moulding sand must have some of the properties. It is not essential for the sand to have all these properties at the same time rather it depends on the application for which sand is going to be used. According to the application these properties can be obtained at the same time economics can be worked out Properties of a Good Moulding Sand To provide satisfactory and uniform results, the sand used to make moulds must be carefully prepared. Ordinary silica sands are compounded with additives to meet following four requirements which are essential to a moulding sand. (i) (ii) Refractoriness : It is defined as the ability to withstand high temperatures. Therefore higher the pouring temperature higher will be the required refractoriness. For lower pouring temperatures even the lower value will work. Refractoriness is provided by the basic nature of the sand. Cohesiveness (also referred to as bond) : It is defined as the ability to retain a given shape. Cohesiveness of sand is ascertained by amount of bonding materials present such as clay in presence of moisture. (iii) Permeability : It is the ability to permit gases to escape through it. The rate in millimeter per minute at which air will pass through the sand under a standard condition of pressure is used as index of permeability. It is dependent on the size of the sand particles, the amount and type of clay or 26

3 bonding agent, the moisture content, and the compacting pressure. The mould must be enough porous to permit the gases to escape and avoid defects due to entrapped gases. (iv) Collapsibility : It is the ability of disintegration of the cohesive mould as a result of metal shrinkage. Collapsibility is generally obtained by adding organic material, such as cellulose, cereals etc., that burn out when these are exposed to hot metal. The combustion reduces both the volume and strength of the restraining sand. Green Strength : Property which tells strength and plasticity of the sand once the water has been mixed to it. Dry Strength : Once the metal is poured inside the cavity, sand adjacent to the hot metal losses its water. Dry strength is the strength of dry sand to resist erosion and pressure of the molten metal. Thermal Stability : It is the property of the sand to remain stable dimensionally under high temperature or heating condition. If the mould surface due to lack of thermal stability crack, buckle or flake off, it will lead to defective casting. Resusability : It is preferred to use the moulding sand which can be reused for the number of operations. Good moulding sand always represents a compromise between conflicting factors. To obtain an acceptable compromise of the four basic requirements the size of the sand particles, the amount of bonding agent (such as clay), the moisture content, and the percentage of organic matter are all selected. The composition is carefully controlled to assure satisfactory and consistent results. (A typical green-sand mixture contains about 85% silica sand, 9% clay, and 3% water and 3% organic additives.) Since moulding material is often reclaimed and recycled, the organic material has to be added again as a portion of it will burn during the pour. Some of the mould material may have to be discarded and replaced with new. It is also important for each grain of the sand to be coated uniformly with the additive agents. This is achieved by putting the ingredients through a muller, a device that kneads, rolls, and stirs the sand. All such devices along with testing equipment are discussed in Appendix-A to this unit Principal Ingredients of Moulding Sands The principal ingredients of moulding sands are : (i) (ii) Silica sand grains, Clay (bond), (iii) Moisture, and (iv) Organic additives. Silica Sand Grains Silica sand grains impart refractoriness, chemical resistivity, and permeability to the sand. They are specified according to their average size and shape. The finer grains would lead to more intimate contact and lower the permeability. However, fine grains tend to fortify the mould and lessen its tendency to get distorted. The shapes of the grain may vary from round to angular. The grains are classified according to their shape as below : (i) (ii) Rounded Grains Subangular Grains (iii) Angular Grains (iv) Compounded Grains Moulding Materials 27

4 28 Principle of Metal Casting Clay Moisture In practice, sand grains contain mixed grain shapes. A sub-angular -to-rounded grain mixture would be the best combination. Clay imparts the necessary tensile strength to the moulding sand so that after ramming, the mould does not lose its shape. However, as the quantity of the clay is increased, the permeability of the mould is reduced. Clay is defined by the American Foundrymen s Society (A.F.S.), as those particles of sand (under 20 microns in diameter) that fail to settle at a rate of 25 mm per minute, when suspended in water. Clay consists of two ingredients: fine silt and true clay. Fine silt is a sort of foreign matter of mineral deposit and has no bonding power. True clay supplies the necessary bond. Under high magnification, true clay is found to be made up of extremely minute aggregates of crystalline particles, called clay minerals. These clay minerals are further composed of flake-shaped particles, about 2 microns in diameter, which are seen to lie flat on one another. Clay acquires its bonding action only in the presence of the requisite amount of moisture. When water is added to clay, it penetrates the mixture and forms a microfilm which coats the surface of each flake. The molecules of water forming this film are not in the original fluid state but in a fixed and definite position. As more water is added, the thickness of the film increases up to a certain limit after which the excess water remains in the fluid state. The thickness of this water film varies with the clay mineral. The bonding quality of clay depends on the maximum thickness of water film it can maintain. When sand is rammed in a mould, the sand grains are forced together. The clay coating on each grain acts in such a way that it not only locks the grains in position but also makes them retain that position. If the water added is the exact quantity required to form the film, the bonding action is best. If the water is in excess, strength is reduced and the mould gets weakened. Thus, moisture content is one of the most important parameters affecting mould and core characteristics and consequently, the quality of the sand produced Other Additives to Moulding Sands Additives are mixed during sand preparation according to the requirement of molten metal and base san d to obtain specific characteristics in the sand. This helps in improving certain property of the base sand like high temperature plasticity, metal penetration property, surface finish etc. The commonly used additives are given below. Iron Oxide Molasses Cold Dust It is used for both moulding and core making sand to improve high temperature plasticity and deep metal penetration and hot strength. In core sand it prevents cracking of cores. Good quality iron oxide should have iron oxide content not less than 93% and iron content not less than 65%. It is commonly used in moulding of iron castings. It is obtained as a by-product during sugar refining. It is added to achieve high dry strength and collapsibility. This also increases green strength. However, due to the high hygroscopicity of the mix prepared with molasses, its use is not much favoured for good quality casting. It is used in green strength and dry strength moulding for protecting mould surfaces against the action of molten metal and improving surfac e finish of cast iron castings. This also reduces expansion and metal penetration. When the molten metal comes in contact with mould surfaces containing cold dust, a gaseous envelope is formed which resists the fusion of sand to metal.

5 Sodium Silicate This is most commonly used binder in air setting or self hardening process. Many processes make use of sodium silicate as a binder along with a solid or gaseous hardener like CO 2. Fibrous Materials Dextrin Used to improve collapsibility, prevent scabbing and expansion defects. The commonly used materials are wood flour, straw, asbestos, sawdust, dried glass and manure. It is a binder which increases air setting strength, toughness and collapsibility and prevents sand from quickly drying. During pouring it gasif ies producing voids between sand grains and allowing their expansion without distortion. Dextrin is commonly used with core sand to increase dry strength and as a binder for mould and core washes. Sulphite Lye It is a by-product of cellulose industry and is used for imparting better dry strength, hot strength and collapsibility to moulds. Its use is more favoured in the production of large and heavy iron castings Specifications and Testing of Moulding Sand Moulding sand is specified in terms of the size and shape of the silica grains it contains, the clay content, and the moisture content. These are determined as follows : Maintaining consistent sand quality is of little concern to the casting designer, but it is a significant matter to the foundry worker, who is expected to deliver consistent, highquality products. Standard tests and procedures have been developed to evaluate grain size, moisture content, clay content, and compactability, as well as mould hardness, permeability, and strength. These test procedures have been explained in Appendix-A of this block in detail. Grain size is determined by shaking a known amount of clean, dry sand downward through a set of 11 standard sieves of decreasing mesh size. After shaking for 15 minutes, the amount remaining on each sieve is weighed, and the weights are converted into an AFS (American Foundrymen s Society) grain fineness number. Moisture content is usually determined by a special device that measures the electrical conductivity of a small sample of sand that is compressed between two prongs. Another method is to measure the weight lost from a 50-g sample after it has been subjected to a temperature of about 230ºF (110ºC) for sufficient time to drive off all the water. Clay content can be determined by washing the clay from a 50-g sample of moulding sand in water that contains sufficient sodium hydroxide to make it alkaline. Several cycles of agitation and washing may be required to fully remove the clay. The remaining sand is then dried and weighed to determine the amount of clay in the original sample. Permeability and strength tests are conducted on a standard rammed specimen. A sufficient amount of sand is placed into a 2-inch-diameter steel tube so that after a 14-1b weight is dropped three times from a height of 2 inch, the final height of the specimen is within 1/32 of 2 inch. Permeability is a measure of how easily gases can pass through the narrow voids between the sand grains. Air in the mould before pouring (plus the steam that is produced when the hot metal contacts the moisture in the sand) must be allowed to escape, rather than be trapped in the casting as porosity or blow holes. During the permeability test, the sample tube containing the rammed specimen is placed on a device and subjected to an air pressure of 10g/cm 2. By means of either a flow rate determination or measurement of the pressure between the orifice and the sand, an AFS permeability number is determined. Most test devices are calibrated to provide a direct readout of the permeability number. Moulding Materials 29

6 Principle of Metal Casting The compressive strength of the sand is determined by removing the rammed specimen from the tube and placing it in a mechanical testing device. A compressive load is then applied until the specimen breaks, usually in the range of 10 to 30 psi (0.07 to 0.2 MPa). When there is too little moisture in the sand, the grains are poorly bonded and strength is poor. When there is excess moisture, the extra water acts as a lubricant and strength is again poor. Thus there is a maximum strength and an optimum water content will vary with the content of other materials in the mix. A similar optimum also applies to permeability. Sand coated with a uniform thin film of moist clay provides the best moulding properties. A ratio of 1 part water to 3 parts clay (by weight) is often a good starting point. The hardness of the compacted sand can provide a quick indication of mould strength and give additional insight into the strength-permeability characteristics. It can be measured by an instrument, which determines the resistance of the sand to penetration by a 0.2-inch (5.08-mm)-diameter spring-loaded steel ball. Compactability is determined by sifting sand into a steel cylinder, leveling off the column, striking it three times with a standard weight (as in the permeability test), and then measuring the final height. The percent compactability is the change in height divided by the original height, times 100%. The value can often be correlated with the moisture content of the sand, with a compactability around 45% indicating a proper level of moisture. A low compactability correlates with too little moisture Moulds with Different Types of Sand Sands are classified as natural or synthetic type based on the presence of clay bonding material. Naturally bonded sand contains clay. Synthetic sand is composed of various base sands with bonding agents added to produce desired moulding characteristics. Silica is the major base sand with some use being made of zircon, olivine and chromite. Green-Sand, Dry-Sand, and Skin-Dried Moulds In green-sand moulding, the mould material is composed of sand with a binder of clay, water, and additives. Tooling costs are low, and the entire process is quite inexpensive. Almost any metal can be cast, and there are few limits on the size, shape, weight, and complexity of the products. Design limitations are usually related to the rough surface finish, poor dimensional accuracy, and the need for subsequent machining. Still other problems can be attributed to the low strength of the mould material and the moisture that is present in the binder. Because some intricate large parts are difficult to cast to required size and dimensions by green sand techniques, dry sand is often used as the mould material for casting such parts. The dry sand mix includes a base sand that is coarser than that used in green sand moulding to facilitate natural venting and mould drying. Pitch is added as a carbon material along with the cereal, molasses, dextrine, glutrin and resin. These additives thermoset at the dryin g temperature of 149 to 316ºC (300 to 600ºF) to produce high strength and rigid mould walls. Dry sand moulding consists of the green sand modified by baking the mould at 204 to 316ºC (400 to 600ºF). Moulds generally are dried in large mould ovens; alternatively, large heaters may be used. Large or medium-size castings of complex configurations (such as frames, engine cylinders, large gears, and housings) are often made by dry sand techniques. Both ferrous and non-ferrous metals are cast in this type of mould. Dry sand moulding is more expensive than green sand, however, it has the advantages of producing castings with increased strength, more exact dimensions, and smoother finishes. These dry-sand moulds are not very popular, however, because of the long tim e required for drying, the added cost of that operation, and the availability of practical alternatives. An attractive compromise is to produce a skin-dried mould, drying only the sand that is adjacent to the mould cavity. Torches are often used to 30

7 perform the drying, and the water is usually removed to a depth of about one-half inch. Moulds used for the casting of steel are almost always skin -dried, because the pouring temperatures are significantly higher than those for cast iron. These moulds may also be given a high-silica treatment prior to drying to increase the refractoriness of the surface, or the more-stable zircon sand can be used as a facing. Additional binders, such as molasses, lin-seed oil, or corn flour, may be added to the facing sand to provide additional strength to the skin -dried segment. Sodium Silicate-CO 2 Moulding Material Moulds (and cores) can also be made from a sand that receives its strength from the addition of 3 to 4% sodium silicate, a liquid inorganic binder that is also known as water glass. The sand can be mixed with the liquid sodium silicate in a standard muller and can be packed into flasks by any of the methods. It remains soft and mouldable until it is exposed to a flow of CO2 gas, after which it hardens in a matter of sec onds by the reaction. Na 2 SiO 3 + CO 2 Na 2 CO 3 + SiO 2 (colloidal) The CO 2 gas is nontoxic and odourless, and no heating is required to drive the reaction. The hardened sands, however, have poor collapsibility, making shakeout and core removal difficult. Unlike most other sands, the heating that occurs as a result of the pour makes the mould even stronger (a phenomenon similar to the firing of a ceramic material). In addition, care must be taken to prevent the carbon dioxide in the air from hardening the sand before the mould-making process is complete. A modification of the CO2 process can be used when certain portions of a mould require higher strength, better accuracy, thinner sections, or deeper draws than can be achieved with ordinary moulding sand. Sand mixed with sodium silicate is packed around a metal pattern to a depth of about 1 inch, followed by regular moulding sand as a backing material. After the mould is fully rammed, CO 2 is introduced through vents in the metal pattern. This hardens the adjacent sand, and the pattern can now be withdrawn with less possibility of damaging the mould. No-Bake, Air-Set or Chemically Bonded Sands An alternative to the sodium silicate process involves the use of organic resin binders that cure by chemical reactions that occur at room temperature. Two or more binder components are mixed with the sand just prior to the moulding operation, and the curing reactions begin immediately. Because the mix is workable for only a short period of time, the moulds (or cores ) must be made in a reasonably rapid fashion. After a few minutes to a few hours (depending on the specific binder and curing agent), the sands harden enough to be removed from the pattern and are ready to pour. Various no-bake sand systems are available, with selection being based on the metal being poured and the specific sand performance characteristics that are desired. Each system is based on organic resin binders, curing agents or catalysts, and various additives and modifiers. Like the sodium silicate moulds, no-bake offers high dimensional accuracy, good hot strength, and high resistance to mouldrelated casting defects. Patterns can incorporate thinner sections and deeper draws. In contrast to the sodium silicate material, however, the no-bake moulds decompose readily after the metal has solidified, providing excellent shakeout characteristics Other Sand-based Moulding Methods Over the years, various processes have been proposed to overcome some of the limitations of the more traditional metho ds. While few have become commercially significant, several are included here to illustrate the nature of these efforts. Moulding Materials 31

8 Principle of Metal Casting In one method, known as the V-process or vacuum moulding, a vacuum is used in place of a sand binder. A vacuum flask is then placed over the pattern, the flask is filled with sand, a sprue and pouring cup are formed, and a second sheet of plastic is placed over the mould. A second vacuum is then drawn on the flask itself, compacting the sand and providing the necessary strength and hardness. The pattern vacuum is released, the pattern is withdrawn, and the mould halves are assembled. The mould is poured while maintaining a vacuum in both the cope and drag segments of the flask. Advantages of the vacuum process include the total absence of moisture-related defects. Since no binder is used, binder cost is eliminated and the sand is completely reusable. No fumes (binders burning up) are generated during the pouring operation. Shakeout characteristics are exceptional; the mould virtually collapses when the vacuum is released. Unfortunately, the process is relatively slow because of the additional steps and the time required to pull a sufficient vacuum. In another process, sand with a small amount of clay and quite a bit of water is first packed around a pattern. The pattern is removed and liquid nitrogen is sprayed onto the mould surface. The ice that forms becomes the binder, and the mould is then poured while it is in its frozen condition. As with the V-process, binder cost is low and shakeout is excellent. 2.3 MATERIALS FOR CORE MAKING Casting processes are unique in their ability to incorporate internal cavities or reentrant sections with relative ease. To produce these features, however, it is often necessary to use cores as part of the mould. While these cores constitute an added cost, they do much to expand the capabilities of the process, and good design practice can often facilitate and simplify their use. For cores natural sand containing small percentages of clay can be used but synthetic sand is preferred. Green sand cores in dried condition can also be used, with the bonding agents like linseed oil and cereals. The basic advantage of organic binders as compared to clay is that they break down under the effect of the heat and can easily be removed from the castings at shakeout. It is extremely important that proper baking times and temperatures be established for the various binders and for variation in core size. A properly baked core does not produce harmful gases, has adequate strength and collapse at the right time after metal is poured around it. Recently, plastics have started to supplement linseed oil as core binder. Urea formaldehyde and phenol formaldehyde are the two most widely used. Urea formaldehyde breaks down at very low temperature as compared to phenol or linseed oil, so it is used in the low melting metals. A still more recent development in core making is sodium silicate-co 2 moulding, which has already been discussed in the earlier sections of this unit. SAQ 1 (a) What are the basic requirements of a proper moulding sand? (b) Explain how green strength and permeability are affected by : (c) (d) (i) (ii) Grain shape of the sand Grain size of the sand (iii) Grain size distribution Distinguish between pore water and free water. Explain their effects on the green strength of the sand. Name the various additives used in moulding sand and explain how they affect its properties. 32

9 (e) (f) What are the basic requirements of a core sand? In what respect does it differ from the moulding sand? Give the mechanism of hardening in carbon-dioxide moulding and explain the factors on which it depends. Moulding Materials 2.4 SHELL MOULDING MATERIAL Many moulds are now being made by the shell-moulding process, which offers better surface finish than can be obtained with ordinary sand moulding, better dimensional accuracy, and a higher production rate with reduced labour requirements. In many cases, the process can be completely mechanized and adapted for mass production. The full shell moulding process has been described in the next unit. The sand used for shell moulding consists of a mixture of the following ingredients : (i) (ii) Dry sand grains, AFS fineness 60 to 140 distributed over 4 to 5 screens. Synthetic resin binder, 3 to 10 per cent by weight. Resins which may be used are the phenolformaldehydes, urea formaldehydes, alkyds, and polyesters. The resin must be a thermosetting plastic, and is used as a powder in dry mixtures. It may also be applied as a liquid and then dried on the sand grains. For moulding, the mixture must be dry and free flowing. The shell is cured in two stages. When the sand mixture drops onto a pattern heated to about 350 to 700 o F, the plastic partially thermosets and builds up a coherent sand shell next to the pattern. The thickness of this shell is about ¼ to ¾ inch and is dependent on the pattern temperature, dwell time on the pattern, and the sand mixture. The shell, still on the pattern, can then be cured by heating it to 450 to 650 o F for 3 to 1 min. Stripping the shell from the mould presents a problem since the shell is very strong and grips the mould tightly. A mould release agent, or parting agent, is used to obtain clean stripping when the ejector pins push the shell off the pattern. Silicone parting solutions, sprayed on the pattern, have been found satisfactory. Shell moulding is probably used more for making cores than moulds. A variant of the process, known as the hot-box process, employs a heated core box. The moulding mixture again contains 1.5 to 4.0 per cent resin of the furane or furfuraldehyde type. Heat from the core box causes the catalysts to start an exothermic polymerization process. As the sand temperature rises, the resin polymerizes and the mass hardness. Moulds are made by assembling the hot-box cores. 2.5 INVESTMENT MOULDING MATERIAL The expandable pattern materials used with the investment casting process are wax and foam polystyrene. The latter is a thermoplastic material that vaporizes at molten metal temperatures. Both of these materials are said to be expandable, or consumable, because the pattern is used up, or lost, when forming the mould cavity where the part is shaped. For these materials, the same basic properties are required as with any pattern material. They are low ash content, good fluidity to reproduce detail, low contraction and expansion characteristics, stability, reproducibility, and compatibility with other process materials. Additionally, pattern materials also should be easy to join and assemble, nontoxic, economical, and easily available. Of these two main pattern materials, waxes are the most widely used. Numerous wax formulas, both natural and synthetic, have been developed to meet the requirements of the investment casting industry. Thus, inexpensive waxes with a variety of properties are 33

10 34 Principle of Metal Casting available for use. One of the prime consideration in selecting wax is that it be removable by heat without leaving a residue in the mould cavity. Ash, soot, or a waste left in the investment could contaminate the final casting. Complete burn-out of the wax should occur between 427 to 538º C (800 to 1000º F). The ability of wax to retain the intended shape throughout the pattern forming and investing procedures is another important material property requirement. A wax that is too soft is likely to distort under the weight of the investment material. Several waxes with varying degrees of strength may be used in the production of one casting. For example, in the areas of intricate detail of the part, it may be necessary to use wax that is soft and pliable. Sometimes, a viscous wax is desirable for dipping, pouring, or brushing. In general, overall wax economy is important, because much of the wax is lost in the burn-out and usually not available for reuse Type of Wax The following categories of pattern wax are basic to the investment casting process: Natural waxes, the most important of which are beeswax, carnauba, ceresin, and condelilla vegetable waxes. Natural waxes normally are added to other waxes and resins in order to obtain the desired pattern wax properties. Mineral waxes, of which Mounta in wax, extracted from brown coal, is the most important. It is sometimes used unmodified, but has an inherent strength disadvantage. Petroleum waxes, including paraffin and microcrystalline waxes, are often used for investment patterns. These may be used in combination because their properties tend to be complementary. Waxes may be deficient as patterns in certain applications. Therefore, additives are used to improve their properties. Most pattern waxes, therefore, are blends of synthetic, animal, and veg etable waxes with various resin and plastic additives. Polystyrene Polystyrene plastics in the form of foam are used to produce some patterns for solid mould applications and are also used in the form of foam, for patterns used in the evaporative casting process. The evaporative process involves the use of expandable polystyrene beads (particles of polystyrene plastic containing 5 to 8 % of a volatile liquid expanding agent). The beads are pre-expanded by steam, vacuum, or hot air to a predetermined density. The pattern dies are made of cast aluminum and machined to finish dimensions. Mould cavity and core walls contain small holes or vents, through which steam can enter the mould cavity. Expanded beads are blown into the mould, completely filling the cavity. Steam heats the expanded beads, causing them to further expand, thereby filling the void areas between the spherical beads and fusing them together. Both the mould and pattern are then water cooled, after which the polystyrene pattern is ejected. Foam pattern is now ready for assembly with pouring cup, sprue, runner, and ingate. The polystyrene pattern and gating system is assembled with a contact cement. The pattern assembly is coated by dipping it into permeable refractory mix. Fillers and Binders The investment materials used to produce a slurry for coating the pattern are generally separated into two categories. Fillers are the materials used to hold the refractory particles together as a fluid-like slurry. Binders provide the basic mould refractory material. Slurries are prepared by adding the refractory powder or flour filler to a binder liquid and then using sufficient agitation to break up agglomerates and to thoroughly wet and disperse the powder. Mixing is continued until the viscosity of the slurry reaches the proper level for coating the pattern. Continued stirring is required in production to keep the powder from settling out of suspension. Either rotating tanks or propeller mixers are used for this purpose.

11 As a general rule, ethyl silicate is not used as a binder for the first and second coats of the ceramic shell process, because the drying rate is too fast and shell surface cracking results from the stress caused by thermal expansion of the refractory. When mixing the slurries, the refractory flour is added to the binder to ensure good dispersion of solids. To obtain good coverage of the patterns, a wetting agent should be used for the first ceramic shell coat. Moulding Materials 2.6 MOULDS FOR OTHER CASTING PROCESSES Centrifugal Casting Both expandable and permanent moulds are used for centrifugal casting. Expandable Moulds Materials Expandable moulds may be of green sand, dry sand, CO 2 etc. Expandable moulds are recommended when chilling tendency is to be taken care of. Green sand is generally used in ex pandable moulds. This needs special bonding material to impart strength. Special purpose resins and phenolic binders are generally used as binders. Dry sand moulds hardened with CO 2 are also used sometimes. Mould washes are used with expandable moulds to prevent erosion of the mould by molten metal. Permanent Moulds SAQ 2 Permanent moulds are used when large quantity of identical shape is the requirement. Metallic moulds facilitate faster cooling rates and thus refine grain structure. The common materials for permanent moulds are steel, copper, graphite etc. Steel moulds facilitate specific solidification condition and are sensitive to thermal shocks. To avoid thermal shocks these moulds are sprayed with Zirconium or aluminum based sprays. Copper moulds are specifically used because of their higher thermal conductivity. Use of copper should be limited because of high cost and in arriving at correct dimension. Graphite moulds are increasingly becoming popular because of their low cost, excellent thermal conductivity and resistance to thermal shocks. However, these could be maintained well below the ovulation temperature of graphite. (a) (b) (c) Why do you understand by expandable materials? Why wax is used as one of the investment moulding material? How the type of wax used for pattern making is decided? 2.7 SUMMARY Success of castings depend to a great extent on the right selection of the moulding/casting materials. Therefore, it is necessary to be very careful while selecting a moulding material. To achieve this objective, it is necessary to study the properties and behaviour of the moulding materials under different working conditions. In the present unit, different types of casting materials have been discussed. Special emphasis has been given to different types of sand, as it is one of the most widely used 35

12 Principle of Metal Casting casting material. Other materials used for different casting processes like shell moulding, permanent moulding and investment casting processes etc. have also been described. Therefore, after studying this unit, it is easy to select right material for different types of casting processes as it gives fairly good idea about the properties of different casting materials. 2.8 ANSWERS TO SAQs Refer the preceding text and the books given under Further Reading. 36

13 Moulding Materials The Making of Sand Moulds Hand ramming may be the preferred method of mould making when only a few castings are to be made from any given design, and some small foundries still make their moulds by this method. In most cases, however, sand moulds are made by specially designed moulding machines. The various methods differ in the type of flask required, the way sand is packed within the flask, whether mechanical assistance is provided to turn or handle the mould, and whether a flask is even required. In all cases, however, the moulding machines greatly reduce he labor and skill required, and lead to castings with better dimensional accuracy and consistency. Moulding usually begins with a pattern, such as the match-plate pattern discussed earlier, and a flask. The flasks may be either straight-walled containers with guide pins or removable jackets, and they are generally constructed of aluminum or magnesium. Figure shows a snap flask that can open slightly to permit withdrawal after the mould packing is complete. The sand is generally packed in the flask by one or more basic techniques. In a method known as jolting, sand is placed on top of the pattern, and the pattern, flask, and sand are then lifted and dropped several times, as shown in Figure The kinetic energy of the sand produces optimum packing around the pattern. Jolting machines can be used on the first half of a match-plate pattern or on both halves of a cope-and-drag operation. Squeezing machines use either an air -operated squeeze head, a flexible diaphragm, or small individually activated squeeze heads to compact the sand. Squeezing provides firm packing near the squeeze head, but the density diminishes as you move farther into the mould. High-pressure machines with a flexible diaphragm, commonly called Taccone machines, can produce a more uniform density around all parts of an irregular pattern. Figure illustrate the squeezing process, and Figure 14-4 compares squeezing with a flat plate and squeezing with a flexible diaphragm. A combination of jolting and squeezing is often used to produce a more uniform density throughout the mould. Here a match-plate pattern is positioned between the cope and drag sections of a flask, and the assembly is placed upside down on the moulding machine. A parting compound is sprinkled on the pattern, and the top section of the flask is filled with sand. The entire assembly is then jolted a specified number of times to pack the sand around the pattern. A squeeze head is then swung into place, and pressure is applied to complete the upper portion of the mould. The flask can be inverted and operations repeated on the cope half, or the cope and drag can be made on separate machines using cope-and-drag patterns. Unless the moulds are very small, the moulding machines usually provide mechanical assistance for inverting the heavy moulds. While some patterns include the sprue hole, it may also be cut by hand. (Hand cutting is performed before removal of the pattern, so loose sand does not fall into the mould cav ities.) The pouring basin may also be hand cut, or it may be formed by a protruding shape on the squeeze board. The gates and runners are usually included on the pattern. Because of the cost and time required for hand labor, the growing trend is to design patterns that will minimize the amount of hand working. After the mould is completed, the tapered flask may be removed to prevent possible damage to the flask during the pour. A slip jacket, an inexpensive metal band, may be positioned around the mould to hold the sand in place. Heavy metal weights are often place on top of the moulds to prevent the sections from separating as the hydrostatic pressure of the molten metal presses upward on the cope. This slip jackets and added weights are needed only during pouring and for a few minutes afterward, 37

14 Principle of Metal Casting while solidification occurs. They can then be removed and place on other moulds, thereby reducing the amount of equipment needed in the operation. For mass-production moulding, a number of automatic mould -making devices have been developed. These include automatic match-plate machines, automatic copeand-drag machines, and machines that produce some form of stacked segments. Figure depicts a vertically parted flaskless moulding machine, where the cope-and-drag patterns are incorporated into opposing sides of the mould. Sand is deposited between the patterns and squeezed with a horizontal motion. The patterns are withdrawn, cores are set, and the mould block is joined to those that were previously moulded. Since each block contains the right-hand cavity of one mould and the left-hand cavity of another, an entire mould is made with each cycle of the machine. (Previous techniques required two separate moulding operations to produce the individual cope and drag segments of a mould.) A vertical gating system is usually included on the pattern. Vertically parted moulds can be poured individually, or a common runner and sprue system can be used to connect a number of mould segments. The latter method is known as the H-process. In stack moulding, sections containing both a cope and a drag impression are piled vertically on top of one another. Metal is poured down a common sprue, which is connected to a gating system at each of the parting planes. For moulds that are too large to be made by either hand ramming or with one of the many types of moulding machines, large flasks can be place on the foundry floor. Various types of mechanical aids are then used to add and pack the sand. One such device, a sand slinger, uses impeller blades to fling the sand into the flask at reasonably high velocity. If this is done skillfully, large moulds can be made with uniform compaction throughout. Additional tamping can be done with a pneumatic rammer. Extremely large moulds can be made in sunken pits. Because of the size, the complexity, and the need for strength, pit moulds are frequently assembled using smaller sections of baked or dried sand. Added binders may be required to provide the necessary strength. 38