A process capability study is the formal method for proving that a MIM production process can consistently produce parts within specification.
Short-term vs long-term capability:| Aspect | Short-Term Capability Study | Long-Term Capability Study |
|---|---|---|
| What it measures | Process capability under controlled conditions — one setup, one material lot, one operator | Process performance over extended production — multiple batches, setups, material lots |
| Sample size | Minimum 30 consecutive parts from a stable run | Minimum 100 parts from 5+ production batches |
| Reported index | CpK | PpK |
| When performed | During PPAP / initial tooling qualification | Ongoing — monitored monthly or quarterly |
| What it proves | The process CAN hold tolerance | The process DOES hold tolerance in real production |
- Run the process at normal production parameters (no special tweaking for the study)
- Collect 30-50 consecutive parts (for short-term) or pull from 5+ batches (for long-term)
- Measure the critical dimensions using a calibrated CMM or vision system
- Calculate CpK and PpK using standard formulas
- Compare to the acceptance criteria (CpK ≥ 1.33 for automotive)
| CpK Value | Estimated Defect Rate | Verdict |
|---|---|---|
| 1.67 | 0.54 PPM | Excellent — process is well-centered and stable |
| 1.33 | 63 PPM | Acceptable — meets most automotive requirements |
| 1.00 | 2,700 PPM | Marginal — needs improvement for production |
| 0.67 | 45,500 PPM | Unacceptable — process not capable of holding tolerance |
A MIM capability study collects 30-50 consecutive parts from a production run, measures critical dimensions, and calculates CpK (short-term) to prove the process can hold tolerance. A long-term study collects 100+ parts from multiple batches and reports PpK. For automotive PPAP, CpK ≥ 1.33 is required for all critical and significant characteristics. The study is performed during initial tooling qualification and repeated periodically to monitor process stability.