When auditing a MIM manufacturer or setting up acceptance criteria for a new program, two numbers come up constantly: Cpk (process capability index) and PPM (parts per million defective). Here is what is realistic and what you should demand.
Typical Cpk requirements for MIM production:| Application | Critical Features | Non-Critical Features | Measurement Standard |
|---|---|---|---|
| General commercial | ≥ 1.33 | ≥ 1.00 | Per print tolerance |
| Automotive (Tier 1) | ≥ 1.33-1.67 | ≥ 1.33 | AIAG PPAP Level 3 |
| Automotive (safety-critical) | ≥ 1.67 | ≥ 1.33 | AIAG + customer-specific |
| Medical device (non-implant) | ≥ 1.33 | ≥ 1.00 | Per device master record |
| Medical implant | ≥ 1.67 | ≥ 1.33 | Per FDA/QSR requirements |
| Aerospace (non-structural) | ≥ 1.33 | ≥ 1.33 | AS9102 first article |
| Aerospace (flight-critical) | ≥ 1.67 | ≥ 1.33 | AS9100 + customer-approved |
| Cpk | Estimated PPM Outside Spec | Process Quality Level |
|---|---|---|
| 1.00 | 2,700 PPM | Marginal — process needs improvement |
| 1.33 | 63 PPM | Acceptable for most applications |
| 1.67 | 0.54 PPM | Preferred for automotive and medical |
| 2.00 | 0.002 PPM | Six Sigma quality level |
- Dimensional non-conformance: < 200 PPM is good; < 50 PPM is excellent (achievable with stable process and mature tooling)
- Visual / surface defects: < 500 PPM typical; < 100 PPM with controlled finishing and inspection
- Material property failure: Effectively 0 PPM for qualified processes — mechanical testing is batch-level verification
For critical dimensions on automotive MIM parts, Cpk ≥ 1.33 is the minimum requirement. For safety-critical features, Cpk ≥ 1.67. Non-critical features should meet Cpk ≥ 1.33.
ATMIK maintains CpK tracking on all critical dimensions with monthly monitoring and corrective action on any downward trend before it reaches the control limit.