Statistical Process Control (SPC) is the system that keeps a MIM production line within specification — not by inspecting parts after they are made, but by monitoring process parameters in real time and intervening before the process drifts out of tolerance.
What MIM SPC typically monitors:| Process Step | Parameter Monitored | Sampling Frequency | Control Limit |
|---|---|---|---|
| Injection molding | Shot weight | Every 30-60 minutes | ±1% of nominal |
| Injection molding | Injection pressure peak | Every shot (automated) | ±5% of nominal |
| Injection molding | Mold temperature | Continuous | ±2°C |
| Debinding | Weight loss | Every batch | ±2% of target |
| Sintering (continuous) | Temperature at each zone | Continuous chart recording | ±5°C at soak zone |
| Sintering | Belt speed | Daily verification | ±2% of setpoint |
| Sintering | Dew point | Continuous | Below -40°C |
| Dimensional | Critical feature on CMM | Per control plan sample size | ±1.33 CpK |
| Chart Type | What It Plots | Upper / Lower Limits | Detects |
|---|---|---|---|
| X-bar (average) chart | Average of a subgroup of measurements | ±3 sigma from process mean | Shift in process center (e.g., shrinkage changing) |
| R (range) chart | Difference between highest and lowest in the subgroup | Based on average range | Increase in part-to-part variation (e.g., tool wear) |
| Individual (I) chart | Single measurements | ±3 sigma | Outliers, sudden changes |
A process is "out of control" if any of these occur:
- One point beyond ±3 sigma control limits
- 2 of 3 consecutive points beyond ±2 sigma
- 7 consecutive points on the same side of the centerline
- 7 consecutive points trending up or down
SPC (Statistical Process Control) is a real-time monitoring system that tracks key MIM process parameters — shot weight, injection pressure, mold temperature, debinding weight loss, sintering temperature — and alerts operators when the process drifts toward the control limits. It allows corrective action before any out-of-spec parts are produced. For automotive MIM, SPC is an IATF 16949 requirement.