Investment Casting
Investment castings have some of the advantages of the sand casting and some of the advantages of the closed-mold processes. The process begins by making a metal die (mold), often of aluminum, that is the shape of the finished part—plus normal shrinkage factors for the material. The mold is injected with wax or low-melting-point plastic. Several of these "wax" parts are attached to a central shaft, or sprue, forming a cluster of parts attached to the sprue. Each wax pattern has one or more gates, which are used to attach it to the sprue. The entire cluster of parts is then coated with a ceramic slurry or placed in a metal flask and the flask filled with a mold slurry. After the mold material has set and cured, the mold is heated, allowing the wax pattern to run out, leaving a hollow core. This is the origin of the term "lost wax" method. Molten aluminum (or other metal) is then poured into the cluster through the sprue and all cavities are filled. Upon solidification, the mold material is broken away and the parts broken off the sprue. Figure 2.25 shows both the investment flask and investment shell processes.
Design for Investment Casting
Wax or plastic temperature, pressure, die temperature, mold or shell composition, backup sand, firing temperature, rate of cooling, position of the part on the tree, and heat treatment temperature all bear directly on tolerances required in the investment casting process. The amount of tolerance required to cover each process step is dependent, basically, on the size and shape of thes casting and will vary from foundry to foundry. This is because one foundry may specialize in thin-walled, highly sophisticated castings, another in mass-production requirements, and yet another in high-integrity aerospace or aircraft applications. One factor, however, is constant. The cost of any casting increases in proportion to the preciseness of specifications, whether on chemistry, nondestructive testing, or tolerance bands.
Tolerances
As a general rule, normal linear tolerance on investment castings can be up to 1 ± 0.010 in. For each additional inch thereafter, allow ±0.003 in. Figure 2.26a shows a chart with expected normal and premium tolerances. Normal tolerances are tolerances that can be expected for production repeatability of all casting dimensions. Premium tolerances are those which require added operations at extra cost, and which provide for closer tolerances on selected dimensions.
Flatness, or the effect of dishing, tolerances cannot be quoted, as they vary with configuration and alloy used. Figure 2.26b is a rough guide in areas under 6 in.-. The amount of dishing allowed is in addition to the basic tolerance. Thus, on a block 1 ± 0.005 in. thick, the tolerances as shown in Figure 2.26c would apply.
Roundness, or "out of round," is defined as the radial difference between a true circle and a given circumference. Tolerances are shown in Figure 2.27a. Figure 2.27b shows the relationship between roundness and concentricity. Two cylindrical surfaces sharing a common point or axis as their center are concentric. Any dimensional difference in the location of one center with respect to the other is the extent of eccentricity. When the length of a bar or tube does not exceed its component diameters by a factor of more than 2, the component diameters will be concentric within 0.005 in. per inch of separation. The roundness of a cast hole as shown in Figure 2.27c is affected by the mass of surrounding metal. If an uneven mass is adjacent, the hole will be pulled out of round. If the surrounding metal is symmetrical, holes up to Vi in. in diameter can be held to ±0.003 in. when checked with a plug gauge. Larger holes may be affected by interior shrinkage or pulling, and the foundry should be consulted. The longer the hole or the more mass of the section around it, the more pronounced will be the effect. Although the above notes are mostly taken from the Investment Casting Handbook, which is published by the Investment Casting Institute, it is strongly recommended that the design team consult with the supplier for a particular part if there might be a question regarding a particular feature not described above. The general castability rating of investment casting alloys is shown in Figure 2.28.
Drawing Notes
Linear tolerances, unless otherwise specified, ± 0.010 in. for first inch plus ± 0.003
in. per inch for every inch thereafter. Comer radii 0.030 in. max., fillet radii 0.060 in. max. unless otherwise specified. Wall thickness 0.080, +0.015 -€.000 in. Surface finish 125 rms.
Add 0.040 to 0.060 in. stock to surfaces identified by machining mark. Material: aluminum alloy, MIL-C-l 1866-T6 Comp. 2 (356-T6).

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