Two acronyms appear on every MIM dimensional capability report: CpK and PpK. They are not the same — and knowing the difference is essential for interpreting supplier quality data.
The definitions:| Index | Full Name | What It Measures | When Calculated |
|---|---|---|---|
| CpK | Process Capability Index | Short-term capability of a process that is in statistical control | During PPAP / initial qualification (30+ consecutive parts from a stable run) |
| PpK | Process Performance Index | Long-term performance including all sources of variation (setups, shifts, material lots) | Over ongoing production (minimum 100 parts from multiple batches) |
- CpK assumes the process is centered and stable — it tells you what the process can do under ideal conditions
- PpK includes real-world variation — tool wear, material shifts, temperature changes — it tells you what the process actually does in production
| Index Value | Assessment | MIM Application |
|---|---|---|
| ≥ 1.67 | Excellent | Safety-critical automotive, medical implant, aerospace |
| 1.33-1.67 | Good — acceptable | Standard automotive, general medical, most commercial MIM |
| 1.00-1.33 | Marginal — needs improvement | Non-critical features only |
| < 1.00 | Unacceptable | Process must be improved before production |
CpK measures short-term process capability under controlled conditions (what the process can do). PpK measures long-term performance including real-world variation (what the process actually does). For MIM production, a CpK ≥ 1.33 is typically required for critical dimensions during PPAP. PpK may be 0.2-0.5 lower than CpK due to batch-to-batch variation. If both CpK and PpK are ≥ 1.33, the process is both capable and stable.