Investment casting creates metal parts with excellent surface finish and tight tolerances. To get the best results, you need to follow specific design rules.
Wall Thickness
Wall thickness is one of the most critical factors in investment casting design. You can achieve walls as thin as 0.75 mm (0.030 inches) with specialized techniques, though most applications require thicker walls for reliability.
Here’s what works best for different materials:
| Material Type | Minimum Wall Thickness |
|---|---|
| 300-series Stainless Steel | 1.0 mm |
| 400-series Stainless Steel | 1.5 mm |
| Carbon/Low-Alloy Steel | 1.5-1.8 mm |
| Aluminum/Brass | 1.0-1.5 mm |
| Cobalt & Copper Alloys | 0.75-1.0 mm |
| Bronze | 1.5 mm |
Keep your walls uniform whenever possible. This helps the metal cool evenly and prevents defects.
When you need to transition from thick to thin sections, taper them gradually. Sharp changes create hot spots that can lead to shrinkage and porosity.
Corner Design and Fillets
Sharp corners create stress points and can cause cracks during cooling.
Always use rounded corners (fillets) with a minimum radius of 0.75 mm. Better results come from using larger fillets between 1.5 and 3.0 mm wherever your design allows. These generous radii improve metal flow and reduce stress concentrations that cause hot tears.
Outside corners benefit from slight rounding too. Even a small radius helps prevent defects and improves the overall quality of your casting.
Draft Angles
Adding 1 to 3 degrees of draft on vertical surfaces helps remove the ceramic shell without damage. For tall or deep features, increase the draft angle up to 5 degrees.
Some small, symmetrical parts can get away with nearly zero draft thanks to wax’s natural thermal contraction. But relying on this is risky for complex shapes.
Even a minimal 0.5-degree draft prevents shell breakage and protects your part during removal.
Designing Holes and Slots
Holes and slots follow specific rules based on their size and depth.
For through holes:
- Small holes (1.5-2.3 mm): Keep depth under 2x the diameter
- Medium holes (2.3-5 mm): Depth up to 3x diameter
- Large holes (5-10 mm): Depth up to 4x diameter
- Extra-large holes (over 10 mm): Can go up to 6x diameter with ceramic cores
Blind holes need even more care. Small blind holes can go 5 times their diameter deep, but larger ones should stay shallow at 1 to 2 times their diameter.
Always round the bottom of blind holes. A good rule is making the corner radius about half the hole diameter.
Slots follow similar guidelines. Through slots can be 2 to 6 times their width in length, depending on size. Blind slots should stay around 1 to 1.5 times their width with rounded ends.
Tolerances
Investment casting delivers impressive accuracy, but linear tolerances vary with part size.
For imperial measurements:
- Parts up to 0.5″: ±0.007″
- Parts up to 1″: ±0.010″
- Parts up to 2″: ±0.015″
- Add ±0.005″ for each additional inch
In metric:
- Up to 15 mm: ±0.20 mm
- 15-25 mm: ±0.25 mm
- 25-50 mm: ±0.40 mm
- Add ±0.13 mm per additional 25 mm
Surface Finish
Investment casting produces excellent surface finish right out of the mold. You’ll typically get about 125 RMS (3.2 μm Ra), which feels smooth to the touch.
Larger or thicker parts might be slightly rougher. If you need a mirror finish, plan for polishing or grinding after casting.
