Engineers coming from CNC machining are accustomed to designing parts with zero-draft vertical walls. When they move to MIM, the requirement for draft angles (0.5-2°) can seem like an unnecessary constraint. Here is why MIM needs draft and CNC does not.
The fundamental difference:| Factor | MIM | CNC Machining |
|---|---|---|
| How the part is formed | Molten feedstock injected into a closed steel mold | Cutting tool removes material from a solid block |
| How the part is removed | Mold opens; the part is ejected by pins pushing it off the core | Tool retracts; no ejection needed (part is already free) |
| Shrinkage direction | Metal shrinks ONTO the mold core during cooling | No shrinkage — the part dimension is the same as machined |
| Surface contact | Full surface contact between part and mold cavity | Only the cutting tool touches the part |
When MIM feedstock cools from 150-200°C to mold temperature (40-80°C), it shrinks. This shrinkage causes the part to grip the mold core tightly — like a shrink-fit assembly. To release the part, ejector pins push it off the core. Without draft, the friction between the part surface and the core is high enough that:
- The part may not eject at all (machine jams)
- The part surface may be scored or damaged during ejection
- Ejector pins may push through the part (leaving dents or holes)
| Feature | Minimum Draft | Why More May Be Needed |
|---|---|---|
| Outer wall (cavity side) | 0.5° | Part shrinks away from cavity — less draft needed |
| Inner wall (core side) | 0.75-1.0° | Part shrinks onto core — more draft to break grip |
| Deep rib or boss | 1.0-2.0° | Longer contact surface = more friction |
| Textured surface | 1.0-3.0° | Micro-undercuts from texture resist ejection |
MIM parts are formed by injecting feedstock into a closed steel mold. As the material cools, it shrinks onto the mold core, creating a tight grip. Draft angles (0.5-2°) allow the part to release from the mold during ejection without damage. CNC machining removes material from a solid block — there is no mold to stick to, so draft is optional. For MIM, insufficient draft is one of the most common DFM errors, leading to ejection damage or stuck parts.