The relationship between mold cavity dimensions and final MIM part dimensions is governed by the shrink factor — the ratio that accounts for the 14-20% linear shrinkage that occurs during sintering.
The calculation:Mold cavity dimension = Final part dimension / (1 - shrinkage fraction)
Example: If the final part dimension is 20.00 mm and the linear shrinkage is 16%:
Mold cavity = 20.00 / (1 - 0.16) = 20.00 / 0.84 = 23.81 mm
The shrinkage fraction depends on material, powder PSD, powder loading, and sintering parameters.
Typical shrink factors by material:| Material | Linear Shrinkage | Shrink Factor (multiplier) | Example: 20 mm part → Mold dimension |
|---|---|---|---|
| 316L stainless | 15-18% | 1.176-1.220 | 23.5-24.4 mm |
| 17-4PH stainless | 14-17% | 1.163-1.205 | 23.3-24.1 mm |
| Fe-2Ni low alloy | 16-19% | 1.190-1.235 | 23.8-24.7 mm |
| Ti6Al4V | 14-18% | 1.163-1.220 | 23.3-24.4 mm |
| 420 stainless | 14-17% | 1.163-1.205 | 23.3-24.1 mm |
| Pure copper | 17-22% | 1.205-1.282 | 24.1-25.6 mm |
- Shift in powder PSD (D50 ± 2 µm → shrinkage shift of ± 0.5-1.0%)
- Feedstock MFI drift (± 10% → shrinkage shift of ± 0.2-0.4%)
- Sintering temperature change (± 5°C → shrinkage shift of ± 0.15%)
- Powder loading change (± 1 vol% → shrinkage shift of ± 0.3%)
- Mold is cut to the calculated oversized dimensions based on the assumed shrink factor
- First article parts are molded, debound, and sintered
- Actual dimensions are measured and compared to target
- If the actual shrinkage differs from the assumption, the mold is corrected (steel added or removed) or the process parameters are adjusted
Mold cavity dimensions are calculated as: target dimension / (1 - shrinkage fraction). For a 20 mm part at 16% shrinkage: 20/0.84 = 23.81 mm mold cavity. The exact shrinkage fraction must be determined empirically for each material + powder + sintering combination — this is why MIM mold validation typically requires 1-2 iterations before production.