The most common cause of failed MIM first article inspections is not a manufacturing deviation. It is a drawing written for a machining process and handed to a sintering process unchanged. It then contains requirements that cannot technically be met, without anyone having objected — and omits information that determines the outcome.
This checklist walks the drawing field by field.
The figures the review is based on
| Item | Value | Source |
|---|---|---|
| Standard tolerance | ±0.3 % of nominal | EPMA, guide value for the as-sintered condition |
| Tightest tolerance | ±0.1 % of nominal, on a single fine-tuned dimension | EPMA |
| Linear sintering shrinkage | 14 – 20 % | Industry guide value; a single measurement on a standard feedstock gave 14.3 ± 0.3 % |
| Batch scatter of shrinkage | a few tenths of a percentage point | Experience value; causes are powder distribution and binder content |
| Surface in the as-sintered condition | Ra ≈ 0.8 µm | EPMA; MIMA states "32 rms or better" |
| Density | 95 – 98 % of theoretical density | plant range; MPIF Standard 35 sets a minimum density per material |
Twelve review points on the drawing
Datums and orientation
| No. | Review point | Requirement |
|---|---|---|
| 1 | Complete datum system present | primary, secondary and tertiary datum named, not just one face |
| 2 | Primary datum on the sintering support face | the face resting in the furnace is the only one with reproducible position |
| 3 | Datums away from the gate | the gate carries the vestige and the largest local density deviation |
| 4 | Datum targets spatially close together | datum targets on widely separated regions add up direction-dependent shrinkage |
Dimensional tolerances
| No. | Review point | Requirement |
|---|---|---|
| 5 | Function-critical dimensions marked | three to five at most, explicitly identified as such |
| 6 | Tight tolerances placed in one direction | ±0.1 % is achievable along the flow direction, considerably harder across it |
| 7 | Remaining dimensions at standard | ±0.3 % or wider; every unnecessarily tight dimension ties up inspection effort |
| 8 | Dimensions to be machined named | including stock allowance, so the tool can carry it |
Form, position and surface
| No. | Review point | Requirement |
|---|---|---|
| 9 | Flatness set realistically | it is hardest to hold across the longest span — check there first |
| 10 | Roundness and coaxiality | demand only where function requires it; otherwise paid for expensively through sizing |
| 11 | Surface requirement split by face | Ra 0.8 µm applies to flat faces, not to the gate vestige and support points |
| 12 | Gate position and support face released | both belong on the drawing or in the release agreement |
The three entries most often missing
The condition in which the dimension applies
A MIM part passes through up to four conditions: sintered, heat treated, sized, coated. Heat treatment changes dimensions and flatness; a coating changes outer dimensions at the micrometre level. If the drawing does not state the condition of measurement, the supplier measures as-sintered and the customer measures the coated part — and both are right.
The measurement method
At a tolerance of ±0.03 mm the method decides the result. Tactile, optical and computed tomography readings deviate systematically from each other at sintered edges, because edge rounding is captured differently. For first article inspection the method has to be named, otherwise the discussion is about instruments rather than parts.
The Cpk expectation per dimension
A blanket Cpk of 1.67 across all dimensions is usually not economically achievable in MIM. What is common and defensible is a split requirement: Cpk 1.67 and above on the named functional dimensions, Cpk 1.33 and above as the general standard. That split belongs before sampling, not in the discussion afterwards.
What actually makes shrinkage scatter
Compensation in the tool is uncritical as long as shrinkage is constant. It is not entirely. Four influences sit behind it:
- Feedstock batch. Powder distribution and binder content vary slightly from batch to batch. This is the largest single contribution.
- Fill level and packing pressure. They determine local green density and therefore how much a given area shrinks.
- Furnace position. Within one batch the temperature field is not exactly identical.
- Part orientation. Shrinkage differs parallel and perpendicular to the setter plate.
In practice: assuming a shrinkage scatter of a few tenths of a percentage point is correct — and demanding ±0.02 mm on a 30 mm dimension unintentionally demands batch-specific readjustment of the sintering parameters.
Our approach
We give tolerance feedback on every drawing before we design a tool. It names the dimensions achievable as-sintered, the dimensions that should be sized, and the dimensions that will need machining. That split is therefore fixed before the tool is cut — not during first article inspection.
Further reading
- Tolerancing for Cpk — how tolerance classes affect the unit price.
- PPAP for MIM parts — how shrinkage and batch scatter are documented in the evidence package.
- Geometry and wall thickness checklist — the geometry review that comes before this one.