MIM shrinkage is calculated by measuring a critical dimension on the green part (before sintering) and again on the sintered part, then: Shrinkage % = (Green dimension - Sintered dimension) / Green dimension × 100. The shrink factor applied to the mold cavity is: Shrink factor = Green dimension / Sintered dimension = 1 / (1 - Shrinkage %). For a part showing 16% shrinkage: shrink factor = 1 / (1 - 0.16) = 1.19. The mold cavity is made 19% larger than the final part size.
Why experimental measurement is essential:
Theoretical shrinkage calculations are a starting point, but the actual shrinkage for a specific material + powder lot + sintering profile must be measured experimentally during first article qualification. The measured value may differ from the theoretical by 1-3%, which is enough to push tight tolerances out of spec.
Witness coupon method:- Include standardized witness coupons (bars or disks) with each sintering batch
- Measure coupon dimensions before and after sintering
- Calculate the actual shrinkage factor for that batch
- Apply the factor to the part dimensions to validate that the mold is correctly sized
Include witness coupons with each sintering batch. Measure a reference dimension (typically 50-100 mm) on the green coupon and on the sintered coupon. Calculate shrinkage % = (green - sintered) / green × 100. The measured shrinkage for the coupon applies to the full batch — if the coupon shrinks 15.8%, the parts from that batch also experienced 15.8% shrinkage.