Manufacturing 12

The counter-gravity processes have a number of distinct advantages. Because the molten metal is withdrawn from below the surface of its ladle, it is generally free of slag and dross and has a very low level of inclusions. The vacuum or low-pressure filling allows the metal to flow with little turbulence, further enhancing metal quality. The reduction in metallic inclusions improves machinability and enables mechanical properties to approach or equal those of wrought material. Since the gating system does not need to control turbulence, simpler gating systems can be used, reducing the amount of metal that does not become product. In the countergravity process, between 60% and 95% of the withdrawn metal becomes cast product, compared to a 15% to 50% level for gravity-poured castings. The pressure differential enables metal to flow into thinner sections, and lower "pouring" temperatures can be used, resulting in improved grain structure and better surface finish. EVAPORATIVE PATTERN (FULL-MOLD AND LOST-FOAM) CASTING Several limitations are common to most of the casting processes that have been presented. Some form of pattern is usually required, and this pattern may be costly to design and fabricate. Pattern costs may be hard to justify, especially when the number of identical castings is rather small or the part is extremely complex. In addition, reuseable patterns must be withdrawn from the mold, and this withdrawal often requires some form of design modification or compromise, division into multiple pieces, or special molding procedures. Investment casting overcomes the withdrawal limitations through the use of patterns that can be removed by melting and vaporization. Unfortunately, investment casting has its own set of limitations, including a large number of individual operations and the need to remove the investment material from the finished casting. In the evaporative pattern processes, the pattern is made of expanded polystyrene (EPS), or expanded polymethylmethacrylate (EPMMA), and remains in the mold. During the pour, the heat of the molten metal melts and burns the polystyrene, and the metal fills the space that was previously occupied by the pattern. When small quantities are required, patterns can be cut by hand or machined from pieces of foamed polystyrene (a material similar to that used in Styrofoam drinking cups).This material is extremely light in weight and can be cut by a number of methods, including ones as simple as an electrically heated wire. Preformed material in the form of a pouring basin, sprue, runner segments, and risers can be attached with hot-melt glue to form a complete gating and pattern assembly. Small products can be assembled into clusters or trees, similar to investment casting. When producing larger quantities of identical parts, a metal mold or die is generally used to mass-produce the evaporative patterns. Hard beads of polystyrene are first preexpanded and stabilized. The preexpanded beads are then injected into a heated metal die or mold, usually made from aluminum. A steam cycle causes them to further expand, fill the die, and fuse, after which they are cooled in the mold.The resulting pattern, a replica of the product to be cast, consists of about 2.5% polymer and 97.5% air. Pattern dies can be quite complex, and large quantities of patterns can be accurately and rapidly produced. When size or complexity is great, or geometry prevents easy removal, the pattern can be divided into multiple segments, or slices, which are then assembled by hot-melt gluing. The ideal glue should be strong, fast setting, and produce a minimum amount of gas when it decomposes or combusts. After a polystyrene gating system is attached to the polystyrene pattern, there are several options for the completion of the mold. In the full-mold process, shown schematically in Figure 12-31, green sand or some type of chemically bonded (no-bake) sand is compacted around the pattern and gating system, taking care not to crush or distort it. The mold is then poured like a conventional sand-mold casting. In the lost-foam process, depicted schematically in Figure 12-32, the polystyrene assembly is first dipped into a water-based ceramic that wets both external and internal surfaces and forms a thin refractory coating.The coating must be thin enough and sufficiently

veloped. Separate pieces are joined to a primary pattern segment by beveled grooves or pins (Figure 12-7). After molding, the primary segment of the pattern is withdrawn. The hole that is created then permits the remaining segments to be sequentially extracted. Loose-piece patterns are expensive.They require careful maintenance, slow the molding process, and increase molding costs.They do, however, enable the sand casting of complex shapes that would otherwise require the full-mold, lost-foam, or investment processes. SANDS AND SAND CONDITIONING The sand used to make molds must be carefully prepared if it is to provide satisfactory and uniform results. Ordinary silica (SiO2), zircon, olivine, or chromite sands are compounded with additives to meet four requirements: 1. Refractoriness: the ability to withstand high temperatures without melting, fracture, or deterioration 2. Cohesiveness (also referred to as bond): the ability to retain a given shape when packed into a mold 3. Permeability: the ability of mold cavity, mold, and core gases to escape through the sand 4. Collapsibility: the ability to accommodate metal shrinkage after solidification and provide for easy removal of the casting through mold disintegration (shakeout) Refractoriness is provided by the basic nature of the sand. Cohesiveness, bond, or strength is obtained by coating the sand grains with clays, such as bentonite, kaolinite, or illite, that become cohesive when moistened. Collapsibility is sometimes enhanced by adding cereals or other organic materials, such as cellulose, that burn out when they come in contact with the hot metal.The combustion of these materials reduces both the volume and strength of the restraining sand. Permeability is a function of the size of the sand particles, the amount and type of clay or bonding agent, the moisture content, and the compacting pressure. Good molding sand always represents a compromise between competing factors. The size of the sand particles, the amount of bonding agent (such as clay), the moisture content, and the organic additives are all selected to obtain an acceptable compromise among the four basic requirements.The overall composition must be carefully controlled to ensure satisfactory and consistent results. Since molding material is often reclaimed and recycled, the temperature of the mold during pouring and solidification is also important. If organic materials have been incorporated into the mix to provide collapsibility, a portion will burn during the pour.Adjustments will be necessary, and ultimately some or all of the mold material may have to be discarded and replaced with new. A typical green-sand mixture contains about 88% silica sand, 9% clay, and 3% water. To achieve good molding, it is important for each grain of sand to be coated uniformly with the proper amount of additive agents. This is achieved by putting the ingredients through a muller, a device that kneads, rolls, and stirs the sand. Figure 12-8 shows both a continuous and batch-type muller, with each producing the desired mixing

All molding material must have sufficient strength to retain the integrity of the mold cavity while the mold is being handled between molding and pouring. The mold material must also withstand the erosion of the liquid metal as it flows into the mold and the pressures induced by a column of molten metal. The compressive strength of the sand (also referred to as green compressive strength) is a measure of the mold strength at this stage of processing. It is determined by removing the rammed specimen from the compacting tube and placing it in a mechanical testing device. A compressive load is then applied until the specimen breaks, which usually occurs in the range of 10 to 30 psi (0.07 to 0.2 MPa). If there is too little moisture in the sand, the grains will be poorly bonded and strength will be poor. If there is excess moisture, the extra water acts as a lubricant and strength is again poor. In between, there is a condition of maximum strength with an optimum water content that will vary with the content of other materials in the mix. A similar optimum also applies to permeability, since unwetted clay blocks vent passages, as does excess water. Sand coated with a uniform thin film of moist clay provides the best molding properties. A ratio of one part water to three parts clay (by weight) is often a good starting point. The hardness of compacted sand can give additional insight into the strength and permeability characteristics of a mold. Hardness can be determined by the resistance of the sand to the penetration of a 0.2-in. (5.08-mm)-diameter spring-loaded steel ball. A typical test instrument is shown in Figure 12-10. Compactibility is determined by sifting loose sand into a steel cylinder, leveling off the column, striking it three times with the standard weight (as in making a standard rammed specimen), and then measuring the final height.The percent compactibility is the change in height divided by the original height, times 100%. This value can often be correlated with the moisture content of the sand, where a compactibility of around 45% indicates a proper level of moisture. A low compactibility is usually associated with too little moisture. SAND PROPERTIES AND SAND-RELATED DEFECTS The characteristics of the sand granules themselves can be very influential in determining the properties of foundry molding material.Round grains give good permeability and minimize the amount of clay required because of their low surface area.Angular sands give better green strength because of the mechanical interlocking of the grains.Large grains provide good permeability and better resistance to high-temperature melting and expansion, while fine-grained sands produce a better surface finish on the final casting. Uniform-size sands give good permeability, while a distribution of sizes enhances surface finish. Silica sand is cheap and lightweight, but when hot metal is poured into a silica sand mold, the sand becomes hot, and at or about 585°C (1085°F) it undergoes a phase transformation that is accompanied by a substantial expansion in volume. Because sand is a poor thermal conductor, only the sand that is adjacent to the mold cavity becomes hot and expands. The remaining material stays fairly cool, does not expand, and often provides a high degree of mechanical restraint. Because of this uneven heating, the sand at the surface of the mold cavity may buckle or fold. Castings with large, flat surfaces are more prone to sand expansion defects since a considerable amount of expansion must occur in a single direction. Sand expansion defects can be minimized in a number of ways. Certain particle geometries permit the sand grains to slide over one another, thereby relieving the expansion stresses. Excess clay can be added to absorb the expansion, or volatile additives, such as cellulose, can be added to the mix.When the casting is poured, the cellulose burns, creating voids that can accommodate the sand expansion. Another alternative is the use of olivine or zircon sand in place of silica. Since these sands do not undergo phase transformations upon heating, their expansion is only about one-half that of silica sand. Unfortunately, these sands are much more expensive and heavier in weight than the more commonly used silica. Trapped or evolved gas can create gas-related voids or blows in finished castings. The most common causes are low sand permeability (often associated with angular, fine, or wide-size distribution sands, fine sand additives, and overcompaction) and large FIGURE 12-10 Sand mold hardness tester. (Courtesy of Dietert Foundry Testing Equipment Inc., Detroit, MI) dega-c12_283-312-hr 1/9/07 3:52 PM Page 289 290 CHAPTER 12 Expendable-Mold Casting Processes amounts of evolved gas due to high mold-material moisture or excessive amounts of volatiles. If adjustments to the mold composition are not sufficient to eliminate the voids, vent passages may have to be cut into the mold, a procedure that may add significantly to the mold-making cost. Molten metal can also penetrate between the sand grains, causing the mold material to become embedded in the surface of the casting. This defect, known as penetration, can be the result of high pouring temperatures (excess fluidity), high metal pressure (possibly due to excessive cope height or pouring from too high an elevation above the mold), or the use of high-permeability sands with coarse, uniform particles. Fine-grained materials, such as silica flour, can be blended in to fill the voids, but this reduces permeability and increases the likelihood of both gas and expansion defects. Hot tears or cracks can form in castings made from metals or alloys with large amounts of solidification shrinkage. As the metal contracts during solidification and cooling to room temperature, it may find itself restrained by a strong mold or core.Tensile stresses can develop while the metal is still partially liquid or fully solidified but still hot and weak. If these stresses become great enough, the casting will crack. Hot tears are often attributed to poor mold collapsibility. Additives, such as cellulose, can be used to improve the collapsibility of sand molds. Table 12-1 summarizes the many desirable properties of a sand-based molding material. THE MAKING OF SAND MOLDS When only a few castings are to be made, hand ramming is often the preferred method of packing sand to make a sand mold. Hand ramming, however, is slow, labor intensive, and usually results in nonuniform compaction. For normal production, sand molds are generally made using specially designed molding machines.The various methods differ in the type of flask, the way the sand is packed within the flask, whether mechanical assistance is provided to turn or handle the mold, and whether a flask is even required. In all cases, however, the molding machines greatly reduce the labor and required skill, and also lead to castings with good dimensional accuracy and consistency. Molding usually begins with a pattern, like the match-plate pattern discussed earlier, and a flask. The flasks may be straight-walled containers with guide pins or removable jackets, and they are generally constructed of lightweight aluminum or magnesium. Figure 12-11 shows a snap flask, so named because it is designed to snap open for easy removal after the mold material has been packed in place. The mixed sand (mold material) can be packed in the flask by one or more basic techniques. A sand slinger uses a rotating impeller to fling or throw sand against the

EXPENDABLE GRAPHITE MOLDS For metals such as titanium, which tend to react with many of the more common mold materials, powdered graphite can be combined with additives, such as cement, starch, and water, and compacted around a pattern.After "setting," the pattern is removed and the mold is fired at 1000°C (1800°F) to consolidate the graphite. The casting is poured, and the mold is broken to remove the product. RUBBER-MOLD CASTING Artificial elastomers can also be compounded in liquid form and poured over a pattern to produce a semirigid mold. These molds are sufficiently flexible to permit stripping from an intricate shape or patterns with reverse-taper surfaces. Unfortunately, rubber molds are generally limited to small castings and low-melting-point materials. The wax patterns used in investment casting are often made by rubber-mold casting, as are small quantities of finished parts made from plastics or metals that can be poured at temperatures below 250°C (500°F). ■ 12.5 EXPENDABLE-MOLD PROCESSES USING SINGLE-USE PATTERNS INVESTMENT CASTING Investment casting is actually a very old process—used in ancient China and Egypt and more recently performed by dentists and jewelers for a number of years. It was not until the end of World War II, however, that it attained a significant degree of industrial importance. Products such as rocket components and jet engine turbine blades required the fabrication of high-precision complex shapes from high-melting-point metals that are not easily machined. Investment casting offers almost unlimited freedom in both the complexity of shapes and the types of materials that can be cast, and millions of investment castings are now produced each year. Investment casting uses the same type of molding aggregate as the ceramic molding process and typically involves the following sequential steps: 1. Produce a master pattern—a modified replica of the desired product made from metal, wood, plastic, or some other easily worked material. 2. From the master pattern, produce a master die. This can be made from low-meltingpoint metal, steel, or possibly even wood. If a low-melting-point metal is used, the die may be cast directly from the master pattern. Rubber molds can also be made directly from the master pattern. Steel dies are often machined directly, eliminating the need for step 1. 3. Produce wax patterns. Patterns are made by pouring molten wax into the master die, or injecting it under pressure (injection molding), and allowing it to harden. Release agents, such as silicone sprays, are used to assist in pattern removal. Plastic and frozen mercury are alternate pattern materials. The polystyrene plastic may be preferred for producing thin and complex surfaces, where its higher strength and greater durability are desired. Frozen mercury is seldom used because of its cost, handling probFIGURE 12-27 Group of intricate cutters produced by ceramic mold casting. (Courtesy of Avnet Shaw Division of Avnet, Inc., Phoenix, AZ) dega-c12_283-312-hr 1/9/07 3:52 PM Page 304 SECTION 12.5 Expendable-Mold Processes Using Single-Use Patterns 305 lems, and toxicity. If cores are required, they can generally be made from soluble wax or ceramic. The soluble wax cores are dissolved out of the patterns prior to further processing, while the ceramic cores remain and are not removed until after solidification of the metal casting. 4. Assemble the wax patterns onto a common wax sprue. Using heated tools and melted wax, a number of wax patterns can be attached to a central sprue and runner system to create a pattern cluster, or a tree. If the product is sufficiently complex that its pattern could not be withdrawn from a single master die, the pattern may be made in pieces and assembled prior to attachment. 5. Coat the cluster or tree with a thin layer of investment material.This step is usually accomplished by dipping into a watery slurry of finely ground refractory material.A thin but very smooth layer of investment material is deposited onto the wax pattern, ensuring a smooth surface and good detail in the final product. 6. Form additional investment around the coated cluster.After the initial layer has dried, the cluster can be redipped, but this time the wet ceramic is coated with a layer of sand or coarse refractory, a process called stuccoing. After drying, the process is repeated until the investment coating has the desired thickness (typically 5 to 15 mm or to inch with up to eight layers). As an alternative, the single-dipped cluster can be placed upside down in a flask and liquid investment material poured around it. The flask is then vibrated to remove entrapped air and ensure that the investment material now surrounds all surfaces of the cluster. 7. Allow the investment to fully harden. 8. Remove the wax pattern from the mold by melting or dissolving. Molds or trees are generally placed upside down in an oven where the wax can melt and run out, and any residue subsequently vaporizes.This step is the most distinctive feature of the process because it enables a complex pattern to be removed from a single-piece mold. Extremely complex shapes can be readily cast. (Note: In the early years of the process, only small parts were cast, and when the molds were placed in the oven, the molten wax was absorbed into the porous investment. Because the wax "disappeared," the process was called the lost-wax process, and the name is still used.) 9. Heat the mold in preparation for pouring. Heating to 550° to 1100°C (1000° to 2000°F) ensures complete removal of the mold wax, cures the mold to give added strength, and allows the molten metal to retain its heat and flow more readily into all of the thin sections and details. Mold heating also gives better dimensional control because the mold and the metal can shrink together during cooling. 10. Pour the molten metal. While gravity pouring is the simplest, other methods may be used to ensure complete filling of the mold.When complex, thin sections are involved, mold filling may be assisted by positive air pressure, evacuation of the air from the mold, or some form of centrifugal process. 11. Remove the solidified casting from the mold. After solidification, techniques such as mechanical chipping or vibration, high-pressure water jet, or sand blasting are used to break the mold and remove the mold material from the metal casting. Figure 12-28 depicts the investment procedure, where the investment material fills the entire flask, and Figure 12-29 shows the shell-investment method. Table 12-6 summarizes the features of investment casting. Compared to other methods of casting, investment casting is a complex process and tends to be rather expensive. However, its unique advantages can often justify its use, and many of the steps can be easily automated. Extremely complex shapes can be cast as a single piece.Thin sections,down to 0.40 mm (0.015 in.),can be produced.Excellent dimensional precision can be achieved in combination with very smooth as-cast surfaces.Machining can often be completely eliminated or greatly reduced.When machining is required,allowances of as little as 0.4 to 1 mm (0.015 to 0.040 in.) are usually ample. These capabilities are especially attractive when making products from the high-melting-temperature, difficultto-machine metals that cannot be cast with plaster- or metal-mold processes.

In the V-process or vacuum molding, a vacuum performs the role of the sand binder. Figure 12-20 depicts the production sequence, which begins by draping a thin sheet of heat-softened plastic over a special vented pattern. A vacuum is applied within the pattern, drawing the sheet tight to its surface.A special vacuum flask is then placed over the pattern; the flask is filled with vibrated dry, unbonded sand; a sprue and pouring cup are formed; and a second sheet of plastic is placed over the mold. A vacuum is then drawn on the flask itself, compacting the sand to provide the necessary strength and hardness.The pattern vacuum is released, and the pattern is then withdrawn.The other segment of the two-part cope-and-drag mold is made in a similar fashion, and the mold halves are assembled to produce a plastic-lined cavity. The mold is then poured with a

PATTERNS AND PATTERN MATERIALS The first step in making a sand casting is the design and construction of a pattern. This is a duplicate of the part to be cast, modified in accordance with the requirements of the casting process, the metal being cast, and the particular molding technique that is being used. Selection of the pattern material is determined by the number of castings to be made, the size and shape of the casting, the desired dimensional precision, and the molding process. Wood patterns are relatively easy to make and are frequently used when small quantities of castings are required. Wood, however, is not very dimensionally stable. It may warp or swell with changes in humidity, and it tends to wear with repeated use. Metal patterns are more expensive but are more stable and durable. Hard plastics, such as urethanes, offer another alternative and are often preferred with processes that use strong, organically bonded sands that tend to stick to other pattern materials. In the full-mold and lost-foam processes, expanded polystyrene (EPS) is used, and investment casting uses patterns made from wax. In the latter processes, both the pattern and the mold are single-use, each being destroyed when a casting is produced. TYPES OF PATTERNS Many types of patterns are used in the foundry industry, with selection being based on the number of duplicate castings required and the complexity of the part. One-piece or solid patterns, such as the one shown in Figure 12-2, are the simplest and often the least expensive type.They are essentially a duplicate of the part to be cast, modified only by the various allowances discussed in Chapter 11 and by the possible addition of core prints. One-piece patterns are relatively cheap to construct, but the subsequent molding process is usually slow. As a result, they are generally used when the shape is relatively simple and the number of duplicate castings is rather small. If the one-piece pattern is simple in shape and contains a flat surface, it can be placed directly on a follow board.The entire mold cavity will be created in one segment of the mold, with the follow board forming the parting surface. If the parting plane is to be more centrally located, special follow boards are produced with inset cavities that position the one-piece pattern at the correct depth for the parting line. Figure 12-3 illustrates this technique, where the follow board again forms the parting surface. Split patterns are used when moderate quantities of a casting are desired. The pattern is divided into two segments along what will become the parting plane of the mold. The bottom segment of the pattern is positioned in the drag portion of a flask, and the bottom segment of the mold is produced. This portion of the flask is then inverted, and the upper segment of the pattern and flask are attached.Tapered pins in the cope half of the pattern align with holes in the drag segment to assure proper positioning. Mold material is then packed around the full pattern to form the upper segment (cope) of the mold.The two segments of the flask are separated, and the pattern pieces are removed to produce the mold cavity. Sprues and runners are cut, and the mold is then reassembled, ready for

PLASTER MOLD CASTING In plaster molding the mold material is plaster of paris (also known as calcium sulfate or gypsum), combined with various additives to improve green strength, dry strength, permeability, and castability. Talc or magnesium oxide can be added to prevent cracking and reduce the setting time. Lime or cement helps to reduce expansion during baking. Glass fibers can be added to improve strength, and sand can be used as a filler. The mold material is first mixed with water, and the creamy slurry is then poured over a metal pattern (wood patterns tend to warp or swell) and allowed to set. Hydration of the plaster produces a hard mold that can be easily stripped from the pattern. (Note: Flexible rubber patterns can be used when complex angular surfaces or reentrant angles are required.The plaster is strong enough to retain its shape during pattern removal.) The plaster mold is then baked to remove excess water, assembled, and poured. With metal patterns and plaster mold material, surface finish and dimensional accuracy are both excellent. Cooling is slow, since the plaster has low heat capacity and low thermal conductivity. The poured metal stays hot and can flow into thin sections and replicate fine detail, which can often reduce machining cost. Unfortunately, plaster casting is limited to the lower-melting-temperature nonferrous alloys (such as aluminum, copper, magnesium, and zinc). At the high temperatures of ferrous metal casting, the plaster would first undergo a phase transformation and then melt, and the water of hydration can cause the mold to explode. Table 12-4 summarizes the features of plaster mold casting.

The Antioch process is a variation of plaster mold casting where the mold material is comprised of 50% plaster and 50% sand, mixed with water. An autoclave process is used to prepare the molds, which offer improved permeability and reduced solidification time. The addition of a foaming agent to a plaster-water mix can add fine air bubbles that increase the material volume by 50-100%.The resulting molds have much improved permeability compared to the conventional process. CERAMIC MOLD CASTING Ceramic mold casting (summarized in Table 12-5) is similar to plaster mold casting, except that the mold is now made from a ceramic material that can withstand the highermelting-temperature metals. Much like the plaster process, ceramic molding can produce thin sections, fine detail, and smooth surfaces, thereby eliminating a considerable amount of finish machining. These advantages, however, must be weighed against the greater cost of the mold material. For large molds, the ceramic can be used to produce a facing around the pattern, which is then backed up by a less expensive material such as reuseable fireclay. One of the most popular of the ceramic molding techniques is the Shaw process. A reusable pattern is positioned inside a slightly tapered flask, and a slurry-like mixture of refractory aggregate, hydrolyzed ethyl silicate, alcohol, and a gelling agent is poured on top. This mixture sets to a rubbery state that permits removal of both the pattern and the flask. The mold surface is then ignited with a torch. Most of the volatiles are consumed during the "burn-off," and a three-dimensional network of microscopic cracks (microcrazing) forms in the ceramic. The gaps are small enough to prevent metal penetration but large enough to provide venting of air and gas (permeability) and to accommodate both the thermal expansion of the ceramic particles during the pour and the subsequent shrinkage of the solidified metal.A baking operation then removes all of the remaining volatiles, making the mold hard and rigid. Ceramic molds are often preheated prior to pouring to ensure proper filling and to control the solidification characteristics of the metal.

The molds used for the casting of large steel parts are almost always skin dried, because the pouring temperatures for steel are significantly higher than those for cast iron. These molds may also be given a high-silica wash prior to drying to increase the refractoriness of the surface, or the more thermally stable zircon sand may be used as a facing. Additional binders, such as molasses, linseed oil, or corn flour, can be added to the facing sand to enhance the strength of the skin-dried segment. SODIUM SILICATE-CO2 MOLDING Molds (and cores) can also be made from sand that receives its strength from the addition of 3% to 6% sodium silicate, an inorganic liquid binder, commonly 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 discussed previously in this chapter. It remains soft and moldable until it is exposed to a flow of CO2 gas. It then hardens in a matter of seconds by the reaction: The CO2 gas is nontoxic, nonflammable, and odorless, and no heating is required to initiate or drive the reaction. The sands achieve a tensile strength of about 40 psi (0.3 MPa) after five seconds of CO2 gassing, with strength increasing to 100-200 psi (0.7-1.4 MPa) after 24 hours of aging. The hardened sands, however, have extremely poor collapsibility, making shakeout and core removal quite difficult. Unlike most other sands, the heating that occurs as a result of the pour actually serves to make the mold stronger (a phenomenon similar to the firing of a ceramic material). Additives that will burn out during the pour are frequently used to enhance the collapsibility of sodium Na2SiO3 + CO2 : Na2CO3 + SiO2 1colloidal2 dega-c12_283-312-hr 1/9/07 3:52 PM Page 294 SECTION 12.2 Sand Casting 295 silicate molds. Care must also be taken to prevent the carbon dioxide in the air from hardening the premixed sand before the mold-making process is complete. A modification of this process can be used when certain portions of a mold require better accuracy, thinner sections, or deeper draws than can be achieved with ordinary molding sand. Sand mixed with sodium silicate is packed around a special metal pattern to a thickness of about 1 in., followed by regular molding sand as a backing material. After the sand is fully compacted, CO2 is introduced through vents in the metal pattern. The adjacent sand is further hardened, and the pattern can be withdrawn with less possibility of damage to the mold. NO-BAKE, AIR-SET, OR CHEMICALLY BONDED SANDS An alternative to the sodium silicate-CO2 process involves room-temperature chemical reactions that can occur between organic or inorganic resin binders and liquid curing agents or catalysts. The two or more components are mixed with sand just prior to the molding operation, and the curing reactions begin immediately.The molds (or cores) are then made in a reasonably rapid fashion, since the mix remains workable for only a short period of time.After a few minutes to a few hours at room temperature (depending on the specific binder and curing agent), the sands harden sufficiently to permit removal from the pattern without concern for distortion.After time for additional curing and the possible application of a refractory coating, the molds are then ready for pour. No-bake molding can be used with virtually all engineering metals over a wide range of product sizes and weights. Since the time for mold curing slows production, no-bake molding is generally limited to low to medium-production quantities. The cost of no-bake molding is about 20-30% greater than green-sand molding, so no-bake is generally used where offsetting savings can be achieved. Products can also be designed with thinner sections, deeper draws, and smaller draft, and the rigid molds enable high dimensional precision, along with good surface finish. Since no-bake sand can be compacted by only light vibrations, patterns can often be made from wood, plastic, fiberglass, or even Styrofoam, thereby reducing pattern cost. A wide variety of no-bake sand systems are available, with selection being based on the metal being poured, the cure time desired, the complexity and thickness of the casting, and possible desire for sand reclamation. Like the molds produced by the sodium silicate process, no-bake offers good hot strength and high resistance to mold-related casting defects. In contrast to the sodium silicate material, however, the no-bake molds decompose readily after the metal has been poured, providing excellent shakeout characteristics. Permeability must be good, since the heat causes the resins to decompose to hydrogen, water vapor, carbon oxides, and various hydrocarbons—all gases that must be vented. Air-set molding and chemically bonded sands are other terms that have been used to describe the no-bake process. SHELL MOLDING Another popular sand casting process is shell molding, the basic steps of which are described below and illustrated in Figure 12-18. 1. The individual grains of fine silica sand are first precoated with a thin layer of thermosetting phenolic resin and heat-sensitive liquid catalyst. This material is then dumped, blown, or shot onto a metal pattern (usually some form of cast iron) that has been preheated to a temperature between 230° and 315°C (450° and 600°F). During a period of sustained contact, heat from the pattern partially cures (polymerizes and crosslinks) a layer of material. This forms a strong, solid-bonded region adjacent to the pattern. The actual thickness of cured material depends on the pattern temperature and the time of contact but typically ranges between 10 and 20 mm (0.4 to 0.8 in.). 2. The pattern and sand mixture are then inverted, allowing the excess (uncured) sand to drop free. Only the layer of partially cured material remains adhered to the pattern. 3. The pattern with adhering shell is then placed in an oven, where additional heating completes the curing process.

The versatility of metal casting is made possible by a number of distinctly different processes, each with its own set of characteristic advantages and benefits. Selection of the best process requires a familiarization with the various options and capabilities as well as an understanding of the needs of the specific product. Some factors to be considered include the desired dimensional precision and surface quality, the number of castings to be produced, the type of pattern and core box that will be needed, the cost of making the required mold or die, and restrictions imposed by the selected material. As we begin to survey the various casting processes, it is helpful to have some form of process classification. One approach focuses on the molds and patterns and utilizes the following three categories: 1. Single-use molds with multiple-use patterns 2. Single-use molds with single-use patterns 3. Multiple-use molds Categories 1 and 2 are often combined under the more general heading expendable-mold casting processes, and these processes will be presented in this chapter. Sand, plaster, ceramics, or other refractory materials are combined with binders to form the mold. Those processes where a mold can be used multiple times will be presented in Chapter 13. The multiple-use molds are usually made from metal. Since the casting processes are primarily used to produce metal products, the emphasis of the casting chapters will be on metal casting. The metals most frequently cast are iron, steel, stainless steel, aluminum alloys, brass, bronze and other copper alloys, magnesium alloys, certain zinc alloys, and nickel-based superalloys. Among these, cast iron and aluminum are the most common, primarily because of their low cost, good fluidity, adaptability to a variety of processes, and the wide range of product properties that are available. The processes used to fabricate products from polymers, ceramics (including glass), and composites, including casting processes, will be discussed in Chapter 15. ■ 12.2 SAND CASTING Sand casting is by far the most common and possibly the most versatile of the casting processes, accounting for over 90% of all metal castings. Granular refractory material (such as silica, zircon, olivine, or chromite sand) is mixed with small amounts of other dega-c12_283-312-hr 1/9/07 3:52 PM Page 283 materials, such as clay and water, and is then packed around a pattern that has the shape of the desired casting. Because the grains can pack into thin sections and can be economically used in large quantities, products spanning a wide range of sizes and detail can be made by this method. If the pattern is to be removed before pouring, the mold is usually made in two or more segments.An opening called a sprue hole is cut from the top of the mold through the sand and connected to a system of channels called runners.The molten metal is poured down the sprue hole, flows through the runners, and enters the mold cavity through one or more openings, called gates. Gravity flow is the most common means of introducing the metal into the mold.The metal is allowed to solidify, and the mold is then broken to permit removal of the finished casting. Because the mold is destroyed in product removal, a new mold must be made for each casting. Figure 12-1 shows the essential

To function properly, casting cores must have the following characteristics: 1. Sufficient strength before hardening if they will be handled in the "green" condition. 2. Sufficient hardness and strength after hardening to withstand handling and the forces of the casting process. As metal fills the mold, most cores want to "float." The cores must be strong enough to resist the induced stresses, and the supports must be sufficient to hold them in place. Flowing metal can also cause surface erosion. Compressive strength should be between 100 and 500 psi (0.7 to 3.5 MPa). 3. A smooth surface. 4. Minimum generation of gases when heated by the pour. 5. Adequate permeability to permit the escape of gas. Since cores are largely surrounded by molten metal, the gases must escape through the core. 6. Adequate refractoriness. Being surrounded by hot metal, cores can become quite a bit hotter than the adjacent mold material.They should not melt or adhere to the casting. 7. Collapsibility.After pouring, the cores must be weak enough to permit the casting to shrink as it cools, thereby preventing cracking. In addition, the cores must be easily removed from the interior of the finished product via shakeout. Various techniques have been developed to enhance the natural properties of cores and core materials. Additional strength can be imparted by the addition of internal wires or rods. Collapsibility can be enhanced by producing hollow cores or by placing a material such as straw in the center. Hollow cores may be used to provide for the escape of trapped or evolved gases.Vent holes can be formed by pushing small wires into the core, and coke or cinders are sometimes placed in the center of large cores to enhance venting. Since the gases must be expelled from the casting, and the core material itself must be removed to produce the desired hole or cavity, the cores must be connected to the outer surfaces of the mold cavity. Recesses at these connection points, known as core prints, are used to support the cores and hold them in proper position during mold filling. The dry-sand cores in Figures 12-22c and 12-22d are supported by core prints. If the cores do not pass completely through the casting, where they can be supported on both ends, a single core print may not be able to provide sufficient support. Additional measures may also be necessary to support the weight of large cores or keep lighter ones from becoming buoyant as the molten metal fills the cavity. Small metal supports, called chaplets, can be placed between cores and the surfaces of a mold cavity, as illustrated in Figure 12-24. Because the chaplets are positioned within the mold cavity, they become an integral part of the finished casting. Chaplets should therefore be of the same, or at least comparable, composition as the material being poured. They should be large enough that they do not completely melt and permit the core to move, but small enough that their surface melts and fuses with the

pouring basin may also be hand cut, or it may be shaped by a protruding segment on the squeeze board. The gates and runners are usually included on the pattern. The pattern board is removed and the segments of the mold are reassembled ready for pour. Heavy metal weights are often placed on top of the molds to prevent the cope section from rising and "floating" when the hydrostatic pressure of the molten metal presses upward.The weights are left in place until solidification is complete, and they are then moved to other molds. For mass-production molding, a number of automatic mold-making methods have been developed.These include automatic match-plate molding, automatic cope-and-drag molding, and methods that produce some form of stacked segments. Figure 12-15 shows the production sequence for one of the variations of automatic match-plate molding, where the sand is introduced into the cope-and-drag mold segments from the side and then vertically compressed.The two-part cope-and-drag mold is produced in one station, with a single pattern and one machine squeeze cycle. The compressed blocks are extracted from the molding machine and are poured in a flaskless condition. Figure 12-16 depicts the vertically parted flaskless molding process, where the pattern has been rotated into a vertical position and the cope-and-drag impressions are now incorporated into opposing sides of a compaction machine. Molding sand is deposited between the patterns and squeezed with a horizontal motion.The patterns are withdrawn, cores are set, and the mold block is then joined to those that were previously molded. Since each block contains both a right-hand cavity and a left-hand cavity, an entire mold is made with each cycle of the machine. (Note: Previous techniques required two separate molding operations to produce the individual cope and drag segments of a two-part mold.) A vertical gating system is usually included on one side of the pattern, and the

`pouring basin may also be hand cut, or it may be shaped by a protruding segment on the squeeze board. The gates and runners are usually included on the pattern. The pattern board is removed and the segments of the mold are reassembled ready for pour. Heavy metal weights are often placed on top of the molds to prevent the cope section from rising and "floating" when the hydrostatic pressure of the molten metal presses upward.The weights are left in place until solidification is complete, and they are then moved to other molds. For mass-production molding, a number of automatic mold-making methods have been developed.These include automatic match-plate molding, automatic cope-and-drag molding, and methods that produce some form of stacked segments. Figure 12-15 shows the production sequence for one of the variations of automatic match-plate molding, where the sand is introduced into the cope-and-drag mold segments from the side and then vertically compressed.The two-part cope-and-drag mold is produced in one station, with a single pattern and one machine squeeze cycle. The compressed blocks are extracted from the molding machine and are poured in a flaskless condition. Figure 12-16 depicts the vertically parted flaskless molding process, where the pattern has been rotated into a vertical position and the cope-and-drag impressions are now incorporated into opposing sides of a compaction machine. Molding sand is deposited between the patterns and squeezed with a horizontal motion.The patterns are withdrawn, cores are set, and the mold block is then joined to those that were previously molded. Since each block contains both a right-hand cavity and a left-hand cavity, an entire mold is made with each cycle of the machine. (Note: Previous techniques required two separate molding operations to produce the individual cope and drag segments of a two-part mold.) A vertical gating system is usually included on one side of the pattern, and the

and solidification tends to proceed in a directional manner back toward the gate. For many castings, risers are not required. Metal yield (product weight versus the weight of poured metal) tends to be rather high. For these and other reasons, evaporative-pattern casting has grown rapidly in popularity and use.Table 12-7 summarizes the process and its capabilities. ■ 12.6 SHAKEOUT, CLEANING, AND FINISHING In each of the casting processes presented in this chapter, the final step involves separating the castings from the molds and mold material. Shakeout operations are designed to separate the molds and sand from the flasks (i.e., containers), separate the castings from the molding sand, and separate or remove the cores from the castings. Punchout machines can be used to force the entire contents of a flask (both molding sand and casting) from the container.Vibratory machines, which can operate on either the entire flasks or the extracted contents, are available in a range of styles, sizes, and vibratory frequencies. Rotary separators remove the sand from castings by placing the mold contents inside a slow-turning, large-diameter, rotating drum. The tumbling action breaks the gates and runners from the castings, crushes lumps of sand, and extracts the cores. Because of possible damage to lightweight or thin-sectioned castings, rotary tumbling is usually restricted to cast iron, steel, and brass castings of reasonable thickness. Processes such as blast cleaning can be used to remove adhering sand, oxide scale, and parting-line burrs. Compressed air or centrifugal force is used to propel abrasive particles against the surfaces of the casting. The propelled media can be metal shot (usually iron or steel), fine aluminum oxide, glass beads, or naturally occurring quartz or silica. The blasting action may be combined with some form of tumbling or robotic manipulation to expose the various surfaces.Additional finishing operations may include grinding, trimming, or various forms of machining. ■ 12.7 SUMMARY Liquids have the characteristic property that they assume the shape of their container. A number of processes have been developed to create shaped containers and then utilize liquid fluidity and subsequent solidification to produce desired shapes. Each process has its unique set of capabilities, advantages, and limitations, and the selection of the best method for a given application requires an understanding of all possible options.This chapter has presented processes that produce castings with a single-use (expendable) mold.The following chapter will supplement this knowledge with a survey of multiple-use mold processe

assembled molds are usually poured individually. If a common horizontal runner is used to connect multiple mold segments, the method is known as the H-process. Since metal cools as it travels through long runners, the individual cavities of the H-process often fill with different-temperature metal. To assure product uniformity, most producers reject the H-process, preferring to pour their vertically parted molds individually. In stack molding, sections containing a cope impression on the bottom and a drag impression on the top are piled vertically on top of one another. Metal is poured down a common vertical sprue, which is connected to horizontal gating systems at each of the parting planes. For molds that are too large to be made either by hand ramming or by one of the previously discussed molding processes, large flasks can be placed directly on the foundry floor.Various types of mechanical aids, such as a sand slinger, can then be used to add and pack the sand. Pneumatic rammers can provide additional tamping. Even larger molds can be constructed in sunken pits. Because of the size, complexity, and need for strength, pit molds are often constructed by assembling smaller sections of baked or dried sand.Added binders may be required to provide the strength required for these large molds. GREEN-SAND, DRY-SAND, AND SKIN-DRIED MOLDS Green-sand casting (where the term green implies that the mold material has not been fired or cured) is the most widely used process for casting both ferrous and nonferrous metals. The mold material is composed of sand blended with clay, water, and additives, and the molds fill by gravity feed.Tooling costs are low, and the entire process is one of the least expensive of the casting methods. Almost any metal can be cast, and there are few limits on the size, shape, weight, and complexity of the products. Over the years, green-sand casting has evolved from a manually intensive operation to a mechanized and automated system capable of producing over 300 molds per hour. As a result, it can be economically applied to both small and large production runs. Design limitations are usually related to the rough surface finish and poor dimensional accuracy—and the resulting need for finish machining. Still other problems can be attributed to the low strength of the mold material and the moisture that is present in the clay-and-water binder.Table 12-2 provides a process summary for green-sand casting, and Figure 12-17 shows a variety of parts that have been produced in aluminum. Some of the problems associated with the green-sand process can be reduced if we heat the mold to a temperature between 150° and 300°C (300° to 575°F) and bake it until most of the moisture is driven off.This drying strengthens the mold and reduces the volume of gas generated when the hot metal enters the cavity. Dry-sand molds are very durable and may be stored for a relatively long period of time.They are not very popular, however, because of the long time required for drying, the added cost of that operation, and the availability of alternative processes.An attractive compromise may be the production of a skin-dried mold, drying only the sand that is adjacent to the mold cavity.Torches are often used to perform the drying, and the water is usually removed to a depth of about 13 mm ( inch).

hile most investment castings are less than 10 cm (4 in.) in size and weigh less than kg (1 lb), castings up to 1 m (36 in.) and 35 kg (80 lb) have been produced. Products ranging from stainless steel or titanium golf club heads to superalloy turbine blades have become quite routine. Figure 12-30 shows some typical investment castings. One should note that a high degree of shape complexity is a common characteristic of investment cast products. The high cost of dies to make the wax patterns has traditionally limited investment casting to large production quantities. Recent advances in rapid prototyping, however, now enable the production of wax-like patterns directly from CAD data. The absence of part-specific tooling now enables the economical casting of one-of-a-kind or small-quantity products using the investment methods. The majority of investment castings now fall within the range of 100 to 10,000 pieces per year. COUNTER-GRAVITY INVESTMENT CASTING Counter-gravity investment casting turns the pouring process upside down. In one variation of the process, a ceramic shell mold is placed in an open-bottom chamber with the sprue end down. The open end of the sprue is lowered into a pool of molten metal, and the bottom of the chamber is set against a seal. A vacuum is then induced within the chamber.As the air is withdrawn, the vacuum draws metal up through the central sprue and into the mold. The castings are allowed to solidify, the vacuum is released, and any unsolidified metal flows back into the melt. In another variation, a low-pressure inert gas is used to push the molten metal upward into the mold. This approach is discussed in more detail and is also illustrated in the section on low-pressure permanent-mold casting in Chapter 13.

metal being cast. Since chaplets are one more source of possible defects and may become a location of weakness in the finished casting, efforts are generally made to minimize their use. Additional sections of mold material can also be used to produce castings with reentrant angles. Figure 12-25 depicts a round pulley with a recessed groove around its perimeter. By using a third segment of flask, called a cheek, and adding a second parting plane, the entire mold can be made by conventional green-sand molding around withdrawable patterns. While additional molding operations are required, this may be an attractive approach for small production runs. If we want to produce a large number of identical pulleys, rapid machine molding of a simple green-sand mold might be preferred. As shown in Figure 12-26, the pattern would be modified to include a seat for an inserted ring-shaped core. Molding time is reduced at the expense of a core box and a separate core-making operation

of the mold, but they are also known as green-sand cores. Green-sand cores have a relatively low strength. If the protrusions are long or narrow, it might be difficult to withdraw the pattern without breaking them, or they might not have enough strength to even support their own weight. For long cores, a considerable amount of machining may still be required to remove the draft that must be provided on the pattern. In addition, green-sand cores are not an option for more complex shapes, where it might be impossible to withdraw the pattern. Dry-sand cores can overcome some of the cited difficulties.These cores are produced separate from the remainder of the mold and are then inserted into core prints that hold them in position. The sketches in Figures 12-22c and 12-22d show dry-sand cores in the vertical and horizontal positions. Dry-sand cores can be made in a number of ways. In each, the sand, mixed with some form of binder, is packed into a wood or metal core box that contains a cavity of the desired shape. A dump-core box such as the one shown in Figure 12-23 offers the simplest approach. Sand is packed into

pattern. The slinger is manipulated to progressively deposit compacted sand into the mold. Sand slinging is a common method of achieving uniform sand compaction when making large molds and large castings. In a method known as jolting, a flask is positioned over a pattern, filled with sand, and the pattern, flask, and sand are then lifted and dropped several times, as shown in Figure 12-12. The weight and kinetic energy of the sand produces optimum packing at the bottom of the mass, that is, 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 an air-operated squeeze head, a flexible diaphragm, or small, individually activated squeeze heads to compact the sand. The squeezing motion provides firm packing adjacent to the squeeze head, with density diminishing as you move farther into the mold. Figure 12-13 illustrates the squeezing process, and Figure 12-14 compares the density achieved by squeezing with a flat plate and squeezing with a flexible diaphragm. In match-plate molding, a combination of jolting and squeezing is often used to produce a more uniform density throughout the mold.The match-plate pattern is positioned between the cope and drag sections of a flask, and the assembly is placed drag side up on the molding machine. A parting compound is sprinkled on the pattern, and the drag section of the flask is filled with mixed sand. The entire assembly is then jolted a specified number of times to pack the sand around the drag side of the pattern. A squeeze head is then swung into place, and pressure is applied to complete the drag portion of the mold. The entire flask is then inverted, and a squeezing operation is performed to compact loose sand in the cope segment. (Note: Jolting here might cause the already-compacted sand to break free of the inverted drag section of the pattern!) Since the drag segment sees both jolting and squeezing, while the cope is only squeezed, the pattern side with the greatest detail is generally placed in the drag. If the cope and drag segments of a mold are made on separate machines (using separate cope-and-drag patterns), the combination of jolting and squeezing can be performed on each segment of the mold. The sprue hole is most often cut by hand, with this operation being performed before removal of the pattern to prevent loose sand from falling into the mold cavity.The

permeable to permit the escape of the molten and gaseous pattern material, but rigid enough to prevent mold collapse during pouring. After the coating dries, the pattern assembly is suspended in a one-piece flask and surrounded by fine unbonded sand.Vibration ensures that the sand compacts around the pattern and fills all cavities and passages. During the pour, molten metal melts, vaporizes, and replaces the expanded polystyrene, while the coating isolates the metal from the loose, unbonded sand.After the casting has cooled and solidified, the loose sand is then dumped from the flask, freeing the casting and attached gating system.The backup sand can then be reused, provided the coating residue is removed and the organic condensates are periodically burned off. Figure 12-33 shows the series of operations used in producing a rather complex lost-foam casting. The full-mold and lost-foam processes can produce castings of any size in both ferrous and nonferrous metals. Since the pattern need not be withdrawn, no draft is required in the design. Complex patterns can be produced to make shapes that would ordinarily require multiple cores, loose-piece patterns, or extensive finish machining. Multicomponent assemblies can often be replaced by a single casting. Because of the high precision and smooth surface finish, machining and finishing operations can often be reduced or totally eliminated. Fragile or complex-geometry cores are no longer required, and the absence of parting lines eliminates the need to remove associated lines or fins on the metal casting. As the molten metal progresses through the pattern, it loses heat due to the melting and volatilizing of the foam.As a result, the material farthest from the gate is the coolest

pour. Figure 12-4 shows a split pattern that also contains several core prints (lighter color). Match-plate patterns, like the one shown in Figure 12-5, further simplify the process and can be coupled with modern molding machines to produce large quantities of duplicate molds. The cope and drag segments of a split pattern are permanently fastened to opposite sides of a wood or metal match plate.The match plate is positioned between the upper and lower flask segments. Mold material is then packed on both sides of the match plate to form the cope and drag segments of a two-part mold. The mold sections are then separated and the match-plate pattern is removed.The pins and guide holes ensure that the cavities in the cope and drag will be in proper alignment upon reassembly. The necessary gates, runners, and risers are usually incorporated on the match plate as well. This guarantees that these features will be uniform and of the proper size in each mold, thereby reducing the possibility of defects. Figure 12-5 further illustrates the common practice of including more than one pattern on a single match plate. When large quantities of identical parts are to be produced, or when the casting is quite large, it may be desirable to have the cope and drag halves of split patterns attached to separate pattern boards.These cope-and-drag patterns enable independent molding of the cope and drag segments of a mold. Large molds can be handled more easily in separate segments, and small molds can be made at a faster rate if a machine is only producing one segment. Figure 12-6 shows the mating pieces of a typical cope-and-drag pattern. When the geometry of the product is such that a one-piece or two-piece pattern could not be removed from the molding sand, a loose-piece pattern can sometimes be de

the cavity and scraped level with the top surface (which acts like the parting line in a traditional mold). A wood or metal plate is then placed over the top of the box, and the box is inverted and lifted, leaving the molded sand resting on the plate.After baking or hardening, the core segments are assembled with hot-melt glue or some other bonding agent. Rough spots along the parting line are removed with files or sanding belts, and the final core may be given a thin coating to provide a smoother surface or greater resistance to heat. Graphite, silica, or mica can be sprayed or brushed onto the surface. Single-piece cores can be made in a split-core box. Two halves of a core box are clamped together, with an opening in one or both ends through which sand is introduced and rammed.After the sand is compacted, the halves of the box are separated to permit removal of the core. Cores with a uniform cross section can be formed by a coreextruding machine and cut to the desired length as the product emerges.The individual cores are then placed in core supports for subsequent hardening. More complex cores can be made in core-blowing machines that use separating dies and receive the sand in a manner similar to injection molding or die casting. Cores are frequently the most fragile part of a mold assembly.To provide the necessary strength, the various core-making processes utilize a number of special binders. In the core-oil process, sand is blended with about 1% vegetable or synthetic oil, along with 2-4% water and about 1% cereal or clay to help develop green strength (i.e., to help retain the shape prior to curing).The wet sand is blown or rammed into a relatively simple core box at room temperature.The fragile uncured cores are then gently transferred to flat plates or special supports and placed in convection ovens at 200° to 260°C (400° to 500°F) for curing. The heat causes the binder to cross-link or polymerize, producing a strong organic bond between the grains of sand. While the process is simple and the materials are inexpensive, the dimensional accuracy of the resultant cores is often difficult to maintain. In the hot-box method, sand blended with a liquid thermosetting binder and catalyst is packed into a core box that has been heated to around 230°C (450°F).When the sand is heated, the initial stages of curing begin within 10 to 30 seconds. After this brief period, the core can be removed from the pattern and will hold its shape during subsequent handling. For some materials, the cure completes through an exothermic curing reaction. For others, further baking is required to complete the process. In the above methods, cores must be handled in an uncured or partially cured state, and breakage or distortion is not uncommon. Processes that produce finished cores while still in the core box and do not require heating operations would appear to offer distinct advantages. In the cold-box process, binder-coated sand is first blown into a room-temperature core box, which can now be made from wood, metal, or even plastic. The box is sealed, and a gas or vaporized catalyst is then passed through the permeable sand to polymerize the resin. In a variation of the process, hollow cores are produced by introducing small amounts of curing gas through holes in the core-box pattern, with the uncured sand in the center being dumped and reused. Unfortunately, the required gases tend to be either toxic (an amine gas) or odorous (SO2), making special handling of both incoming and exhaust gas a process requirement. Room-temperature cores can also be made with the air-set or no-bake sands.These systems eliminate the gassing operation of the cold-box process through the use of a reactive organic resin and a curing catalyst. As discussed previously, there is only a brief period of time to form the core once the components have been mixed. Shell molding is another core-making alternative, producing hollow cores with excellent strength and permeability. Selecting the actual method of core production is usually based on a number of considerations, including production quantity, production rate, required precision, required surface finish, and the metal being poured. Certain metals may be sensitive to gases that are emitted from the cores when they come into contact with the hot metal. Other materials with low pouring temperatures may not break down the binder sufficiently to provide collapsibility and easy removal from the final casting.

through the use of rotating blades that lift, fluff, and redistribute the material and wheels that compress and squeeze.After mixing, the sand is often discharged through an aerator, which fluffs it for further handling. SAND TESTING If a foundry is to produce high-quality products, it is important that it maintain a consistent quality in its molding sand.The sand itself can be characterized by grain size, grain shape, surface smoothness, density, and contaminants. Blended molding sand can be characterized by moisture content, clay content, and compactability. Key properties of compacted sand or finished molds include mold hardness,permeability, and strength. Standard tests and procedures have been developed to evaluate many of these properties. Grain size can be determined by shaking a known amount of clean, dry sand downward through a set of 11 standard screens or sieves of decreasing mesh size.After being shaken for 15 minutes, the amount of material remaining on each sieve is weighed, and these weights are used to compute an AFS (American Foundry Society) grain fineness number. Moisture content can be determined by a special device that measures the electrical conductivity of a small sample of compressed sand.A more direct method is to measure the weight lost by a 50-gram sample after it has been subjected to a temperature of about 110°C (230°F) for sufficient time to drive off all the water. Clay content is determined by washing the clay from a 50-gram sample of molding sand, using 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 removed from the original sample. Permeability and strength tests are conducted on compacted sands, using a standard rammed specimen. An amount of sand is first placed into a 2-inch-diameter steel tube.A 14-pound weight is then dropped on it three times from a height of 2 inches, and the height of the resulting specimen must be within inch of a targeted 2-inch height. Permeability is a measure of how easily gases can pass through the narrow voids between the sand grains.Air in the mold before pouring, plus the steam that is produced when the hot metal contacts the moisture in the sand along with various combustion gases, must all be allowed to escape, rather than prevent mold filling or be trapped in the casting as porosity or blowholes. During the permeability test, shown schematically in Figure 12-9, a sample tube containing the standard rammed specimen is subjected to an air pressure of 10 g/cm2 . By means of either a flow rate determination or a measurement of the steady-state pressure between the orifice and the sand specimen, an AFS permeability number1 can be computed. Most test devices are now calibrated to provide a direct readout of this numbe

4. The hardened shell, with tensile strength between 350 and 450 psi (2.4-3.1 MPa), is then stripped from the pattern. 5. Two or more shells are then clamped or glued together with a thermoset adhesive to produce a mold, which may be poured immediately or stored almost indefinitely. 6. To provide extra support during the pour, shell molds are often placed in a pouring jacket and surrounded with metal shot, sand, or gravel. Because the shell is formed and partially cured around a metal pattern, the process offers excellent dimensional accuracy.Tolerances of 0.08 to 0.13 mm (0.003 to 0.005 in.) are quite common. Shell-mold sand is typically finer than ordinary foundry sand and, in combination with the plastic resin, produces a very smooth casting surface. Cleaning, machining, and other finishing costs can be significantly reduced, and the mold process offers an excellent level of product consistency. Figure 12-19 shows a set of metal patterns, the two shells before clamping, and the resulting shell-mold casting. Machines for making shell molds vary from simple ones for small operations to large, completely automated devices for mass production.The cost of a metal pattern is often rather high, and its design must include the gate and runner system, since these cannot be cut after molding. Large amounts of expensive binder are required, but the amount of material actually used to form a thin shell is not that great. High productivity, low labor costs, smooth surfaces, and a level of precision that reduces the amount of subsequent machining all combine to make the process economical for even moderate quantities.The thin shell provides for the easy escape of gases that evolve during the pour, and the volume of evolved gas is rather low because of the absence of moisture in the mold material. When the shell becomes hot, some of the resin binder burns out, providing excellent collapsibility and shakeout characteristics. In addition, both the molding sand and completed shells can be stored for indefinite periods of time. Table 12-3 summarizes the features of shell molding. OTHER SAND-BASED MOLDING METHODS Over the years, a variety of processes have been proposed to overcome some of the limitations or difficulties of the more traditional methods. While few have become commercially significant, several are included here to illustrate the nature of these efforts.

v4. The hardened shell, with tensile strength between 350 and 450 psi (2.4-3.1 MPa), is then stripped from the pattern. 5. Two or more shells are then clamped or glued together with a thermoset adhesive to produce a mold, which may be poured immediately or stored almost indefinitely. 6. To provide extra support during the pour, shell molds are often placed in a pouring jacket and surrounded with metal shot, sand, or gravel. Because the shell is formed and partially cured around a metal pattern, the process offers excellent dimensional accuracy.Tolerances of 0.08 to 0.13 mm (0.003 to 0.005 in.) are quite common. Shell-mold sand is typically finer than ordinary foundry sand and, in combination with the plastic resin, produces a very smooth casting surface. Cleaning, machining, and other finishing costs can be significantly reduced, and the mold process offers an excellent level of product consistency. Figure 12-19 shows a set of metal patterns, the two shells before clamping, and the resulting shell-mold casting. Machines for making shell molds vary from simple ones for small operations to large, completely automated devices for mass production.The cost of a metal pattern is often rather high, and its design must include the gate and runner system, since these cannot be cut after molding. Large amounts of expensive binder are required, but the amount of material actually used to form a thin shell is not that great. High productivity, low labor costs, smooth surfaces, and a level of precision that reduces the amount of subsequent machining all combine to make the process economical for even moderate quantities.The thin shell provides for the easy escape of gases that evolve during the pour, and the volume of evolved gas is rather low because of the absence of moisture in the mold material. When the shell becomes hot, some of the resin binder burns out, providing excellent collapsibility and shakeout characteristics. In addition, both the molding sand and completed shells can be stored for indefinite periods of time. Table 12-3 summarizes the features of shell molding. OTHER SAND-BASED MOLDING METHODS Over the years, a variety of processes have been proposed to overcome some of the limitations or difficulties of the more traditional methods. While few have become commercially significant, several are included here to illustrate the nature of these efforts.

vacuum of 300-600 torr being maintained in both the cope and drag segments of the flask. During the pour, the thin plastic film melts and vaporizes and is replaced immediately by metal, allowing the vacuum to continue holding the sand in shape until the casting has cooled and solidified. When the vacuum is released, the sand reverts to its loose, unbonded state and falls away from the casting. With the vacuum serving as the binder, there is a total absence of moisture-related defects; binder cost is eliminated; and the loose, dry sand is completely and directly reusable. With no clay, water, or other binder to impair permeability, finer sands can be used, resulting in better surface finish in the resulting castings.With no burning binders, there are no fumes generated during the pouring operation. Shakeout characteristics are exceptional, since the mold 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. The V-process is used primarily for the production of prototype, frequently modified, or low- to medium-volume parts (more than 10 but less than 15,000). In the Eff-set process, wet sand with just enough clay to prevent mold collapse is packed around a pattern.The pattern is removed, and the surface of the mold is sprayed with liquid nitrogen. The ice that forms serves as the binder, and the molten metal is poured into the mold while the surface is in its frozen condition.This process offers low binder cost and excellent shakeout but is not being used in a commercial operation. ■ 12.3 CORES AND CORE MAKING Casting processes are unique in their ability to easily incorporate complex internal cavities or reentrant sections. To produce these features, however, it is often necessary to use cores as part of the mold. Figure 12-21 shows an example of a product that makes extensive use of cores to produce the various cylinders, cooling passages, and other internal features. While cores constitute an added cost, they significantly expand the capabilities of the process. Cores can often be used to improve casting design and optimize processes. Consider the simple belt pulley shown schematically in Figure 12-22. Various methods of fabrication are suggested in the four sketches, beginning with the casting of a solid form and the subsequent machining of the through-hole for the drive shaft. A large volume of metal would have to be removed by a secondary machining process. A more economical approach would be to make the pulley with a cast-in hole. In Figure 12-22b each half of the pattern includes a tapered hole, which fills with the same green sand being used for the remainder of the mold.These protruding sections are an integral part

Manufacturing 12

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