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What Are the Size Limits of Investment Cast Components?
Investment castings can be produced in all alloys from a fraction of an ounce, for dental braces, to more than 1,000 lbs. (453.6 kg) for complex aircraft engine parts. Smaller components can be cast at hundreds per tree, while heavier castings often are produced with an individual tree. The weight limit of an investment casting depends on the mold handling equipment at the casting plant. The majority of U.S. facilities cast parts up to 20 lbs. (9.07 kg). However, many domestic facilities are increasing their capability to pour larger parts, and components in the 20-120-lb. (9.07-54.43-kg) range are becoming common.
 Through investment casting, this aluminum re-agent rack for the medical industry was converted from a machined part to a one-piece casting with 0.004-in. tolerances and a 50% cost reduction.
What As-Cast Dimensional Tolerances Can Be Achieved?
Typically, a linear tolerance of ±0.005 in/in (0.127 mm/in.) is standard for investment castings (Table 1). For example, if a 1-ft. component were cast, it likely would have a tolerance of ±0.06 in. (1.52 mm). This would vary depending on the size and complexity of the part. Post-casting procedures, such as straightening or coining, often allow for tighter tolerances to be maintained on several specific dimensions.
Table 1. Normal and Premium Linear Tolerances for Investment Castings
| Dimensions (in.) |
Normal Tolerances (+/- in.) |
Premium Tolerances (+/- in.) |
| Up to 0.25 |
0.005 |
0.003 |
| Up to 0.5 |
0.005 |
0.004 |
| Up to 1 |
0.005 |
0.005 |
| Up to 2 |
0.01 |
0.01 |
| Up to 3 |
0.015 |
0.013 |
| Up to 4 |
0.019 |
0.015 |
| Up to 5 |
0.022 |
0.017 |
| Up to 6 |
0.025 |
0.02 |
| Up to 7 |
0.028 |
0.022 |
| Up to 8 |
0.031 |
0.024 |
| Up to 9 |
0.034 |
0.026 |
| Up to 10 |
0.037 |
0.028 |
By working with an investment casting firm’s engineering staff, design engineers can produce an investment casting drawing for a part that substantially reduces or completely eliminates the previous machining requirements to produce an acceptable component.
What Kind of Surface Finishes Are Produced?
Because the ceramic shell is assembled around smooth patterns produced by injecting wax into a polished aluminum die, the final casting finish is excellent. A 125 microfinish is standard and even finer finishes (63 or 32) are not uncommon on aircraft engine castings. The size of shot particles used also factors into the final surface finish.
Individual metalcasting facilities have their own standards for surface blemishes, and facility staffs and design engineers/customers will discuss these capabilities before the tooling order is released. Certain standards depend on a component’s end-use and final cosmetic features.
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