Monitor these key parameters per the table below. Use X-bar and R charts for variable data (shot weight, dimensions) and p-charts for attribute data (defect rate). Set control limits at ±3 sigma. Sample every 30-60 minutes for molding parameters. Review charts weekly for trends. Investigate any point outside control limits or runs of 7 on one side of the centerline.
Recommended MIM SPC plan:
| Process Step | Parameter | Chart Type | Sampling Freq | Sample Size | Control Limits | Reaction Plan |
|---|---|---|---|---|---|---|
| Molding | Shot weight | X-bar & R | Every 30 min | 5 consecutive shots | ±1% of nominal | Out of limits: check screw recovery, material feed |
| Molding | Injection peak pressure | I-MR | Every shot (auto) | 1 | ±5% of nominal | Adjust pressure setting |
| Molding | Mold temperature | I-MR | Continuous | 1 per zone | ±2°C from setpoint | Check controller, cooling channels |
| Debinding | Weight loss | I-MR | Every batch | 5 coupons | ±2% of target weight loss | Adjust cycle time or temperature |
| Sintering | Soak zone temp | I-MR | Continuous | 1 per zone | ±5°C | Check thermocouple, recalibrate |
| Sintering | Belt speed | I-MR | Daily | 1 | ±2% of setpoint | Adjust drive, check belt tension |
| Sintering | Dew point | I-MR | Continuous | 1 | Below -40°C | Check gas purity, dryer function |
| Dimensional | Critical feature | X-bar & R | Per control plan | 5-10 parts | CpK ≥ 1.33 limits | Investigate cause, segregate affected batch |
| Rule | Definition | Probable Cause in MIM |
|---|---|---|
| One point beyond ±3σ | Process is out of control — stop and investigate | Feedstock change, furnace malfunction, gauge error |
| 2 of 3 points beyond ±2σ | Process is shifting — increase sampling frequency | Tool wear, temperature drift, material lot change |
| 7 consecutive points same side | Process has shifted off-center — adjust | Shrinkage factor change, new powder lot, calibration drift |
| 7 consecutive points trending | Process is drifting — investigate before it reaches limit | Gradual mold wear, binder degradation, furnace aging |