EXPENDABLE-MOLD CASTING PROCESSES Chapter 14 EXPENDABLE-MOLD CASTING PROCESSES

14.2 SANDCASTING Sand casting, by far the most popular of the casting processes, uses ordinary sand as the primary mold material. The sand grains are mixed with small amounts of other materials, such as clay and water, to improve moldability and cohesive strength, and are then packed around a pattern that has the shape of the desired casting. If the pattern must be removed before pouring, the mold is usually made in two or more pieces.

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, metal being cast, and particular molding technique that is being used. The pattern material is determined primarily by the number of castings to be made but is also influenced by the size and shape of the casting, the desired dimensional precision, and the molding process.

Patterns and Pattern Materials 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 dimensionally stable and more 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.

Types of Patterns Since many types of patterns are used in the foundry industry, selection is usually 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 14-2, are the simplest and often the least expensive type to make. This type of pattern is essentially a duplicate of the part to be cast, modified only by the various allowances discussed in Chapter 13 and they are relatively cheap to construct, but the molding process is usually slow. One-piece patterns, therefore, are generally used when the shape is relatively simple and the number of duplicate castings is rather small.

Types of Patterns

Types of Patterns Split Patterns are used when moderate quantities of duplicate castings are to be made. The pattern is divided into two segments along a single parting plane, which will correspond to the parting plane of the mold. Match-plate patterns, such as the one in Figure 14-5, further simplify the process and can be coupled with modem molding machines to produce large quantities of duplicate castings. The cope and drag segments of a split pattern are permanently fastened to opposite sides of a wood or metal match plate.

Types Of Patterns

Types Of Patterns When the product has protruding sections arranged such that a one-piece or split pattern could not be removed from the molding sand, a loose-piece pattern can sometimes be developed. Loose pieces are held to the primary segment of the pattern by beveled grooves or pins. After molding, the primary segment of the pattern is removed by direct withdrawal. The hole that is created then permits the remaining segments to be moved in the direction necessary for their extraction. In some loose-piece patterns, a single sliding pin is used to hold all the segments in place.

Types Of Patterns

Sand Conditioning The sand used to make molds must be carefully prepared if it is to provide satisfactory and uniform results. Ordinary silica (SiOz), zircon, or olivine (forsterite and fayalite) sands are compounded with additives to meet four requirements: 1. Refractoriness: the ability to withstand high temperatures 2. Cohesiveness: (also referred to as bond): the ability to retain a given shape when packed into a mold 3. Permeability: the ability to permit gases to escape through it 4. Collapsibility: the ability to permit the metal to shrink after it solidifies and ultimately to free the casting by disintegration of the surrounding mold.

Sand Conditioning 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 obtained by adding cereals or other organic material, such as cellulose, that bum out when they come in contact with the hot metal. The combustion 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.

Sand Testing Standard tests and procedures have been developed to evaluate grain size, moisture content, clay content, and compactability, as well as mold hardness, permeability, and strength. 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 230°F (l100C) for sufficient time to drive off all the water.

Sand Testing Clay content can be determined by washing the clay from a 50-g sample of molding 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-in.-diameter steel tube so that after a 14-lb weight is dropped three times from a height of 2 inch the final height of the specimen is within 1/32 in of 2 in.

Sand Testing The hardness of the compacted sand can provide a quick indication of mold strength and give additional insight into the strength-permeability characteristics. 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%.

Sand Properties and related Defects The characteristics of the sand granules can be very influential in determining the properties of the 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 wide distribution of sizes enhances surface finish.

Sand Properties and related Defects When hot metal is poured into a silica sand mold, the sand becomes hot, undergoes one or more phase transformations, and has 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 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 having large, flat surfaces are more prone to sand expansion defects since a considerable amount of expansion must occur in a single, fixed direction.

Sand Properties and related Defects Sand expansion defects can be minimized in a number of ways. Certain sand geometries permit the grains to slide over one another, thereby relieving the expansion stresses. Excess clay can be added to absorb the sand expansion, or volatile additives, such as cellulose, can be added to the mix. As the sand becomes hot, the cellulose bums, creating voids that can accommodate the sand expansion.

Sand Properties and related Defects Castings can also contain voids that form where the molten metal is held back by trapped or evolved gas. These are usually attributed to low sand permeability and/or large amounts of gas evolution caused by high 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, a procedure that adds significantly to the mold-making cost.

Sand Properties and related Defects The molten metal can also penetrate between the sand grains, causing the mold material to become embedded in the surface of the casting. Penetration can be the result of high pouring temperatures (excess fluidity), high metal pressure, or the use of high-permeability sands with coarse, uniform particles. Fine-grained materials, such as silica flour, can be used to fill the voids, but this reduces permeability and increases the likelihood of gas and expansion defects.

Desirable Mold Properties Strength ‑ to maintain shape and resist erosion Permeability ‑ to allow hot air and gases to pass through voids in sand Thermal stability ‑ to resist cracking on contact with molten metal Collapsibility ‑ ability to give way and allow casting to shrink without cracking the casting Reusability ‑ can sand from broken mold be reused to make other molds?

Green Sand, Dry Sand and Skin Dried Molds In green-sand molding, the mold 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 mold material and the moisture that is present in the binder.

Green Sand, Dry Sand and Skin Dried Molds

Green Sand, Dry Sand and Skin Dried Molds The problems of the green-sand process can be reduced by heating the mold to a temperature of 300°F or higher, and baking until most of the moisture is driven off. This strengthens the mold and reduces the amount of gases generated when the hot metal enters the cavity. These dry-sand molds are not very popular, because of the long times required for drying, the added cost of that operation, and the availability of practical alternatives. An attractive compromise is to produce 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 one-half inch.

Green Sand, Dry Sand and Skin Dried Molds Molds used for the casting of steel are almost always skin-dried, because the pouring temperatures 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-stable zircon sand can be used as a facing. Additional binders, such as molasses, linseed oil, or corn flour, may be added to the facing sand to provide additional strength to the skin-dried segment.

Sodium Silicate- CO2 Molding Molds (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 discussed previously. It remains soft and moldable until it is exposed to a flow of C02 gas, when it hardens in a matter of seconds by the reaction.

Sodium Silicate- CO2 Molding The C02 gas is nontoxic and odorless, 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 mold even stronger (a phenomenon similar to the firing of a ceramic material). Additives that will burn out during the pour are often used to enhance the collapsibility of sodium silicate molds. In addition, care must be taken to prevent the carbon dioxide in the air from hardening the sand before the mold-making process is complete.

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 molding operation, and the curing reactions begin immediately. Because the mix is workable for only a short period of time, the molds (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.

No-Bake, Air-Set, or Chemically Bonded Sands 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 molds, no-bake offers high dimensional accuracy, good hot strength, and high resistance to mold-related casting defects. Patterns can incorporate thinner sections and deeper draws. In contrast to the sodium silicate material, however, the no-bake molds decompose readily after the metal has solidified, providing excellent shakeout characteristics.

Shell Molding Many molds are now being made by the shell-molding process, which offers better surface finish than can be obtained with ordinary sand molding, better dimensional accuracy, and a higher production rate with reduced labor requirements. The six steps involved in Shell Molding are as follows; 1. A mixture of sand and thermosetting plastic binder is dumped onto a metal pattern that has been heated to 300 to 450°F . It is allowed to stand for a few minutes while the heat from the pattern partially cures a layer of the sand-plastic mixture. This forms a strong, solid-bonded region about 3.5 mm thick adjacent to the pattern.

Shell Molding 2. The pattern and sand mixture are then inverted. All of the excess sand drops free, leaving only the layer of partially cured material that adhered to the pattern. 3. The pattern and partially cured "shell" are then placed in an oven for a few minutes to complete the curing process. 4. The hardened shell is then stripped from the pattern. 5. Two or more shells are then clamped or glued together to produce a mold. 6. The bonded shells are often placed in a pouring jacket and surrounded with metal shot or sand to provide extra support during the pour.

Shell Molding

Shell Molding Because the sand is compounded for almost no shrinkage and a metal pattern is used, the shell has excellent dimensional accuracy. Shell-mold sand is finer than ordinary foundry sand and, in combination with the plastic resin, produces a very smooth shell and casting surface. Cleaning, machining, and other finishing costs can be reduced significantly. In addition, the shell mold process offers a product consistency that is superior to that of green-sand casting.

Other Sand-Based Molding Methods Over the years, various processes have been proposed to overcome some of the limitations of the more traditional methods. While few have become commercially significant, several are included here to illustrate the nature of these efforts. In one method, known as the V-process or vacuum molding, a vacuum is used in place of a sand binder. Figure 14-19 depicts the production sequence, which begins by draping a thin sheet of plastic over a special pattern, which is then drawn tightly to the pattern surface by a pattern vacuum.

Other Sand-Based Molding Methods 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 mold. 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 mold halves are assembled. The mold is poured while maintaining a vacuum in both the cope and drag segments of the flask

Other Sand-Based Molding Methods 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. Shakeouts characteristics are exceptional; the mold 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.

14.3 Cores and 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 mold. Figure 14-20 shows an example of a product that could not be made by any process other than casting with cores. 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.

14.3 Cores and Core Making

14.3 Cores and Core Making Consider the simple belt pulley shown schematically in Figure 14-21. 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 through a substantial amount of costly machining. A more economical approach would be to make the pulley with a cast-in hole of the approximate final size.

14.3 Cores and Core Making

14.3 Cores and Core Making Figure 14-21b depicts an approach where each half of the pattern includes a tapered hole, Which receives the same green sand that is used for the remainder of mold. While these protruding sections are an integral part of the mold, they are also known as green-sand cores. Unfortunately, green-sand cores have a relatively low level of strength. If the protrusions are narrow or long, it might be difficult to withdraw the pattern without breaking them, or they may not have enough strength to even support their own weight.

14.3 Cores and Core Making Dry-sand cores can be used to overcome some of the cited difficulties. These cores are made independent from the remainder of the mold and are then inserted into core prints to hold them in position. The remaining sketches in Figure 14-21 show dry-sand cores in the vertical and horizontal positions.

14.3 Cores and Core Making To function properly, cores must have the following characteristics; 1. Sufficient hardness and strength (after baking or hardening) to withstand handling and the forces of the molten metal. Compressive strength should be between 100 and 500 psi. 2. Sufficient strength before hardening to permit handling in that condition. 3. Adequate permeability to permit the escape of gases. Since cores are largely surrounded by molten metal, they should possess exceptionally good permeability.

14.3 Cores and Core Making 4. Collapsibility, After pouring, the cores must be weak enough to permit shrinkage of the solidified casting as it cools, thereby preventing cracking. In addition, they must be easily removable from the interior of the finished product via shakeout. 5. Adequate refractoriness, Since the cores are largely surrounded by hot metal, they can become quite a bit hotter than the adjacent mold material. 6. A smooth surface. 7. Minimum generation of gases when heated during the pour.

14.3 Cores and Core Making Various techniques have been developed to enhance the natural properties of cores and core materials. Internal wires or rods can be used to 'impart additional strength. Collapsibility can be enhanced by making the cores hollow or by placing a material such as straw in the center. Enhanced collapsibility is particularly important in steel castings, where a large amount of shrinkage is observed. All but the smallest of cores must be vented to permit the escape of trapped and evolved gases. Vent holes can be made by pushing small wires into the core.

14.4 Other Expandable- Mold Processes with Multiple-Use Patterns Plaster Mold Casting: In plaster molding the mold material is plaster of paris (also known as calcium sulfate or gypsum), with various additions to improve strength, permeability, and castability. The mold material is first mixed with water, and the resultant slurry is immediately poured over a pattern and allowed to set. Hydration of the plaster produces a hard solid that can then be stripped from the pattern, which is usually made from metal.

Plaster Mold Casting Table 14.4 summarizes the features of Plaster Mold Casting.

Ceramic Mold Casting Ceramic mold casting is similar to plaster mold casting, except that the mold can now withstand the higher-melting-point metals. Cope-and-drag molds are formed around withdrawable patterns (which can be metal or wood), using a ceramic slury as the mold material. As with the plaster process, ceramic molding can produce thin sections, fine detail, and smooth surfaces and eliminate a considerable amount of finish machining.

Ceramic Mold Casting Table 14.4 summarizes the features of Ceramic Mold Casting.

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 cement, starch, and water and compacted around a pattern. The pattern is then removed and the mold is fired at I800°F (1OOO°C) to consolidate the graphite. After pouring, the mold is broken to remove the casting.

Rubber-Mold Casting Several types of artificial elastomers are available that can be compounded in liquid form and then poured over a pattern to form a semi-rigid mold. The molds are sufficiently flexible to permit stripping. Unfortunately, rubber molds are suitable only for small castings of low-melting-point materials. Wax patterns for investment casting can be made as well as finished castings of plastics and metals which can be poured at temperatures below 5OO°F(260°C).

14.5 EXPENDABLE-MOLD PROCESSES USING SINGLE-USE PATTERNS Investment Casting: While investment casting is actually a very old process and has been performed by dentists and jewelers for a number of years. Developments and demands in the aerospace industry, such as rocket components and jet engine turbine blades, required high-precision complex shapes from high-melting-point metals that are not readily machineable. Investment casting offers almost unlimited freedom in both the complexity of shapes and types of materials.

Investment Casting Table 14.6 summarizes features of investment casting.