Is Investment Casting Right for Me? Part 2

For the second and final post in our “Is Investment Casting Right for Me?” series we will be covering investment casting design considerations in greater detail. Our first post in the series covered the general design and economic considerations of the process and helps to identify if investment casting is the correct method of manufacture for a specific component.

So if you’ve read part one and decided that investment casting may be a good fit for your component, great! This post will then act as a guide design for manufacture of a desired component and help provide inputs into the design for manufacture process.

Detailed Design Considerations

The following characteristics are design considerations which should be taken into account when designing a part for investment casting: minimum wall thickness, flatness and straightness, radii, blind holes, through holes, draft angles, and undercuts.

Minimum wall thickness is dictated by alloy fluidity and length of the thin walled section. Table 3, shown below, provides a rule of thumb for minimum wall thicknesses of different alloys. Note that the list is rule of thumb only and some special process considerations can be used to improve these when desirable.

Minimum Wall Thickness
Alloy Type Inches Millimeters
Low Carbon Steel 0.070 1.8
High Carbon Steel 0.060 1.5
Low Alloy Steel 0.060 1.5
Stainless Steel – 300 Series 0.040 1.0
Stainless Steel – 400 Series 0.060 1.5
Cobalt Alloys 0.030 0.75
Average 0.053 1.3

Table 3: Minimum wall thickness recommendations

Flatness and straightness can be controlled to a great degree. To optimize straightness and flatness, certain design considerations should be taken into account. Thick to thin sections should be avoided and the use of ribs can help contribute to a straighter and flatter part. Geometries adjacent to large flat planes should be angled to promote solidification and to help prevent cavitation from occurring and creating “sump” on the large flat surface. Better yet, uniform section thicknesses provide the best control for flat surfaces. Again these methodologies only serve as a guide for the least expensive method of manufacture; any competent foundry can overcome these limitations with careful process consideration.

Blind holes, while adding to the casting complexity, can be cast. As a general rule of the thumb the depth of a blind hole should not exceed its width. The use of ceramic cores can allow for blind holes of any geometry to be cast; however, the use of ceramic cores will add considerable cost to the component. Through holes are easier to cast than blind holes but depending on internal geometry or diameter may also require the use of ceramic cores. In some cases, especially with threaded holes, a secondary machining operation may prove most economical.

Draft angle does not need to be considered with investment casting. Some geometries without draft may require the use of automated ejection in the wax tooling so this may add some initial cost but is easily achievable.

Undercuts can easily be achieved by investment casting. In fact the ability of investment casting to be able to produce undercuts often makes the process the ideal choice for components that require them. Though undercuts pose no problem, they do add complexity to the wax tooling and in extreme cases require the use of ceramic cores – both of which can add cost to the components through initial cost and piece price, respectively. Due to this undercuts should be avoided whenever possible.

Dimensional Accuracy

Investment casting is the most accurate of all casting processes. The only process which far exceeds investment castings accuracy is machining which is most often impractical for complex geometries, certain alloys, or medium to high level production quantities. A near-net shape casting with minimal machining (in areas where extremely high accuracy is required) is often the most cost effective solution if the tolerance of investment casting cannot suffice for the application of the component. The dimensional accuracy of castings is influenced by the magnitude of the dimensions for the features which are in question. The following table, Table 4, acts as a guide for the tolerances achievable by investment casting.

Dimensional Tolerances
Inches
Nominal Dimension Zone 1 Zone 2 Zone 3 Zone 4
0.00 0.39 0.010 0.014 0.020 —–
0.39 0.63 0.011 0.015 0.021 —–
0.63 0.98 0.012 0.017 0.023 —–
0.98 1.57 0.013 0.018 0.025 —–
1.57 2.48 0.014 0.020 0.028 —–
2.48 3.94 0.016 0.022 0.031 —–
3.94 6.30 0.017 0.024 0.035 0.047
6.30 9.84 0.020 0.028 0.039 0.055
9.84 15.75 0.022 0.031 0.043 0.063
Millimeters
Nominal Dimension Zone 1 Zone 2 Zone 3 Zone 4
0 10 0.26 0.36 0.52 —–
10 16 0.28 0.38 0.54 —–
16 25 0.30 0.42 0.58 —–
25 40 0.32 0.46 0.64 —–
40 63 0.36 0.50 0.70 —–
63 100 0.40 0.56 0.78 —–
100 160 0.44 0.62 0.88 1.20
160 250 0.50 0.70 1.00 1.40
250 400 0.52 0.78 1.10 1.60

Table 4 : Typical investment casting tolerances

The shrinkage factor of investment casting has the biggest influence on the dimensional accuracy of the process. Both the wax and metal contract significantly when changing phase from a solid to a liquid state. Due to their amorphous to crystalline nature, the phase changes that occur during solidification can cause considerable changes in volume. Alloy types and component geometry change the contraction rates from casting to casting and even feature to feature within a casting. These shrinkage rates are predictable to an extent but the variation that exists contributes to investment castings limitations for dimensional accuracy. For most components, the dimensional accuracy of investment casting meets or far exceeds the needs for the application. In the cases where casting tolerances alone are not sufficient, secondary machining of the near-net shape casting in the areas of concern is often the most economical solution.

There are very few challenges that investment casting cannot overcome. For complex geometries with tight tolerances that require high performance alloys, investment casting is more often than not the best choice for manufacture due to both the economic and performance benefits. The “Is Investment Casting Right for Me?” series serves as an initial guide for process selection and design for manufacture but is only the beginning of the limitless potential of investment casting. If you think investment casting may be right for you, please contact us to discuss in more detail. Texmo thrives on our partnerships with our customers and helping them take their designs from ideas to reality by partnering from conceptualization to manufacture. As always, we hope you have enjoyed this series and taken from it knowledge which will guide you in your future steps.