For automotive programs requiring PPAP (Production Part Approval Process), MIM typically requires significantly more documentation than CNC machining. Here is why.
PPAP submission scope — MIM vs CNC:| PPAP Element | CNC Machining | MIM |
|---|---|---|
| Dimensional report | All dimensions | All dimensions (higher volume due to multi-cavity) |
| Material certification | CoA from bar stock supplier (one source) | CoA from powder supplier + feedstock batch verification + sintered property testing |
| Process flow | Simple: load bar → cut → inspect → ship | Complex: 8-12 steps including powder QC, compounding, molding, debinding, sintering |
| PFMEA | Low RPN — tool wear, fixturing | Higher RPN — multiple process steps with critical parameters |
| Control plan | 3-5 inspection points | 8-12 inspection points across the process chain |
| Capability study | One setup, one material lot | Must include multiple cavities (multi-cavity mold) |
| Gauge R&R | Simple | Must cover CMM, vision, density, hardness |
| Reason | Explanation |
|---|---|
| Multi-step process | MIM has 4 major process stages (mold → debind → sinter → finish) vs CNC's 1 (cut). Each stage has its own control requirements |
| Shrinkage validation | The 14-20% sintering shrinkage must be validated per cavity per material — adds 3-5x the dimensional inspection compared to CNC |
| Multi-cavity molds | A 4-16 cavity MIM mold requires per-cavity capability data. CNC typically produces one part at a time |
| Material transformation | The material's mechanical properties are developed during MIM processing. In CNC, the properties come from the bar stock |
| Delayed results | Cannot verify final dimensions until 12+ hours after molding — PPAP must prove the process will stay in control during that delay |
MIM PPAP requires more documentation because the process has multiple controlled stages (feedstock → molding → debinding → sintering), shrinkage must be validated per cavity, material properties are developed during processing (not inherent in the raw material), and the 12+ hour delay between molding and measurement requires SPC-based process control rather than inspection-based quality.