A short shot (incomplete mold filling) is one of the most common MIM molding defects. It is almost always fixable — and usually caused by one of five factors.
Common causes ranked by frequency:| Cause | Frequency | Mechanism | Fix |
|---|---|---|---|
| Insufficient shot volume | 30% | Screw recovery incomplete; not enough feedstock in barrel | Increase shot size; check screw back pressure |
| Low injection pressure | 25% | Material solidifies before fill is complete | Increase injection pressure by 10-15% |
| Restricted gate or runner | 20% | Gate too small or partially blocked | Clean gate; increase gate cross-section |
| Cold mold temperature | 15% | Material freezes on contact with cavity wall | Raise mold temperature by 5-10°C |
| Feedstock viscosity too high | 10% | MFI below nominal range | Increase barrel temperature 5-8°C; verify MFI |
- Weigh the short shot: Compare to a known-good part weight. If significantly lighter, the issue is material volume. If close to target weight, the issue is flow/fill
- Examine the freeze pattern: Where did the flow stop? If flow stopped at a thin wall or long flow path, increase injection speed or temperature. If stopped at a gate, check gate dimensions
- Check the mold venting: Trapped air prevents complete fill. Add venting at the last fill point (0.02-0.05 mm depth, 5-10 mm wide)
- Increase injection speed: Higher fill rate pushes material further before freezing. Increase by 10-20% and evaluate
- Verify material temperature: The actual barrel temperature may differ from the setpoint. Measure with a pyrometer
The most common cause is insufficient shot volume (30% of cases) — the injection screw is not recovering enough feedstock per cycle. The second most common is low injection pressure (25%). Raise injection pressure to 80-90% of machine maximum and verify screw recovery before each shot.
For persistent short shots after adjusting pressure and shot volume, examine the gate for partial blockage — carbonized binder or powder buildup can restrict flow.