Engineers designing parts for MIM sometimes ask: can I have zero draft (vertical walls with no taper) on my part? The answer is technically yes, but practically risky.
The straight answer: Zero draft is possible on shallow features (<5 mm height) with highly polished cavity surfaces and multiple ejector pins — but it carries real risks. Risk assessment by feature height:| Feature Height | Risk Level | Likely Outcome with Zero Draft |
|---|---|---|
| < 3 mm | Low | May eject OK with polished cavity and strong ejector pins |
| 3-8 mm | Moderate-High | 20-40% risk of surface scoring or ejection distortion |
| 8-15 mm | High | 50-80% risk of part sticking or ejection pin push-through |
| > 15 mm | Very High | Not recommended — almost certain to cause mold damage or part failure |
| Surface Condition | Minimum Draft | Preferred Draft |
|---|---|---|
| Standard cavity (polished H13) | 0.5° | 1.0° |
| Core surface (part shrinks onto core) | 0.75° | 1.5° |
| Textured surface (EDM, etching) | 1.0° + texture depth × 1.0° per 0.025 mm | 1.5-3.0° |
| Deep ribs or fins (height > 10 mm) | 1.0° | 2.0° |
| Small holes (core pins < 2 mm diameter) | 0.5° | 1.0° |
Zero draft is possible for features under 3 mm height on polished cavities, but is not recommended for production. Adding just 0.5° draft eliminates most ejection problems with no measurable effect on part function. The old rule from the molding world holds: no draft = no part.
If you truly cannot add draft (a rare constraint — usually only for precision interference fits), expect to pay for: higher-cost polished cavity steel (S136 or NAK80), more ejector pins, and potential scrap from ejection damage during process development.