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Section 5: Castings
Current practice often involves robotic handling of the shells and automation of the various steps. This provides advantages of cost, consistency, and uniformity of the coatings. However, where applicable, manual methods are still used.
The process for monolithic (solid) molds is very similar, the difference being that the mold is first coated and then inserted in a flask, which is then filled with investment material.
The shell process is better adapted to larger parts and has a shorter cycle time. it also has the advantage of less tendency of surface decarburization of the cast part. On the other hand, mold cracking is more likely, especially with plastic patterns. In addition, detail definition at the part surface may be poorer because of more rapid cooling of the melted metal.
TYPICAL CHARACTERISTICS AND APPLICATIONS
Historically, investment castings most likely were specified when the following part characteristics were involved: intricate shape, close tolerances, small size, and highstrength alloys. Costs of the investment material and other items inhibited its use for larger castings. However, advances in the process involving greater use of the shell method and with robotic assist have reduced the cost differential with other casting processes, especially for larger sizes. Investment castings currently range in size from 1 g (0.035 oz) to upwards of 90 kg (200 lb).
Investment castings are most likely to be used when the shape of the part involves contoured surfaces, undercuts, and other intricacies that make machining difficult or unfeasible. They are used for mechanical components in such products as sewingmachines, business machines, firearms, and surgical and dental devices. Turbine blades, valve bodies, ratchets, cams, pawls, gears, hose fittings, cranks, levers, vanes, connectors, support rings, impellers, manifolds, hand tools, golf club heads, and radar waveguides are typical parts. Figure 5.3.2 shows a collection of typical investmentcast parts.
ECONOMIC PRODUCTION QUANTITIES
Investment casting is best suited to moderately low and medium production levels. In many cases in which machining would be extensive, however, it is applied successfully to high-production applications as well.
The process also can be used for prototype quantities by having the wax patterns made by stereolithography or another rapid prototyping process (with the necessary shrinkage allowances). Prototype parts can then be cast from these patterns using the normal investment casting process.
Tooling costs for wax patterns can be low, especially if the mold is made from a master pattern by using the spray or casting methods. The lowest-cost molds are made by spraying low-temperature alloys over the pattern. Such molds are used for wax patterns only and have a life of 800 to 1200 pieces. More nearly permanent soft-metal molds made by casting will have a life of up to 12,000 pieces.
For still higher production levels, mold cavities are machined from aluminum or steel. Aluminum machined molds (for wax patterns) are quite inexpensive. With steel mold material and with multiple cavities, mold costs are the same as those for other injection-molded plastic parts.
Because of a high labor and indirect-materials content, the unit cost of investment castings tends to be higher than that of powder-metal, die-cast, or other parts made from alternative high-production methods. There are exceptions, when a part is partic-ularly complex or of a configuration or material that is not suitable for other process-es, but generally, economical quantities for investment castings do not extend to mass-production situations.
GENERAL DESIGN CONSIDERATIONS
Perhaps the greatest advantage of investment castings is the high degree of design freedom that they permit. Complex shapes that would be too costly to machine can be produced quickly and economically as investment castings. In many cases, what would otherwise be two or more separate parts can be designed as one integral casting, eliminating assembly operations. The process also allows designers to select from a wide range of alloys. Ideally, the time for deciding if a part is to be made by the investment-casting process is when it is on the drawing board. When designing for investment casting, designers should keep the toolmaker and investment caster in mind. They should remember that the pattern is injection-molded and therefore should observe good practices that apply to injection-molded parts. These include the use of a well-located, straight parting line, adequate draft, and avoidance of undercuts. Designers should use generous fillets and radii whenever possible. This not only makes a better-looking part but also produces a stronger one as well.
SUITABLE MATERIALS
A wide variety of metals, both ferrous and nonferrous, can be used to make investment castings. Any metal that is meltable in-standard induction or gas furnaces can be used. Vacuum melting and pouring can be incorporated in the process if the metal used and its application require this step.
Materials that are not easily machinable are good candidates for some investment-casting applications because the process can provide dimensional precision and intricate shapes that would otherwise require machining. Table 5.3.1 lists a number of alloys with high-castability ratings.
DETAILED DESIGN RECOMMENDATIONS
Minimum Wall Thickness
The minimum thickness of casting walls is determined primarily by the fluidity of the metal to be cast. Another factor is the length of the section involved. If the section is long, a heavier wall may be required. Table 5.3.2 provides recommended minimum wall thicknesses for various investment-castable metals.
Flatness and Straightness
Deviations from flatness and straightness can be minimized if ribs and gussets are incorporated in the part. Because of pattern shrinkage, investment shrinkage, and metal shrinkage during solidification, there is always a tendency for an investmentcast part to "dish" (develop concave surfaces where flat surfaces are specified). This condition takes place in areas of thick cross section. As illustrated in Fig. 5.3.3, dishing can be minimized by designing parts with uniformly thin walls.
Radii
Although sharp corners are achievable, generous radii are preferred whenever possible. Ample fillets and radii facilitate die filling for the pattern and mold filling for the cast part and also tend to produce better-quality, more accurate parts. Although it is possible to cast sharper comers, a minimum fillet radius of 0.75 mm (0.030 in) should be specified, and even 1.5 to 3.0 mm (0.06 to 0.125 in) is preferable. (See Fig. 5.3.4.)...
