As MIM feedstock enters the mold cavity, it displaces the air inside. If that air has no way to escape, it becomes compressed, heats up, and prevents the feedstock from filling the cavity completely — causing short shots, burn marks, or surface defects.
What venting does: Vents are shallow channels cut into the mold's parting surface that allow air to escape while preventing feedstock from flowing out. Vent depth guidelines for MIM:| Material | Vent Depth | Why This Depth |
|---|---|---|
| 316L, 17-4PH (standard feedstock) | 0.020-0.035 mm | Deep enough for air to escape; shallow enough that feedstock viscosity prevents leakage |
| Fe-2Ni, low-alloy (lower viscosity) | 0.015-0.025 mm | Lower viscosity feedstock can penetrate deeper vents |
| Titanium (higher viscosity) | 0.030-0.050 mm | Higher viscosity needs deeper vents for adequate air evacuation |
- Last fill point: The cavity area furthest from the gate — this is where trapped air is most likely
- Weld line locations: Where two flow fronts meet — vents at weld lines let trapped air escape, improving weld line strength
- Deep pockets or ribs: Air can form pockets in recessed features that must be vented individually
| Problem | Visual Signature | Fix |
|---|---|---|
| Short shot at last fill | Incomplete fill at end-of-fill location | Add vent at last fill point, increase vent depth |
| Burn marks / dielect | Black or charred surface at end of fill | Increase venting — compressed air is combusting binder |
| Weld line weakness | Part breaks at weld line during handling | Add vent at weld line location |
| Inconsistent fill | Part weight varies shot to shot | Check vents for blockage; clean after maintenance |
Mold venting consists of shallow channels (0.015-0.050 mm deep) cut into the mold's parting surface that allow trapped air to escape as feedstock fills the cavity. Without proper venting, compressed air can cause short shots, burn marks, and weld line weakness. Vent depth must be carefully controlled — too shallow and air cannot escape; too deep and feedstock leaks out as flash.