A molded MIM green part is typically 118-122% of its final size. After sintering, it shrinks to 100%. This 14-20% linear shrinkage is not a defect — it is the direct result of densification from ~60% (powder packing density) to ~96-98% (sintered density).
The physics:The green part contains metal powder at 55-65 vol% (the rest is binder). After debinding, the brown part is essentially a porous metal compact at ~60-65% density. During sintering, the particles fuse together and the pores shrink. The driving force is surface energy reduction — the particles want to minimize their surface area by bonding together.
The volume relationship:- If powder loading is 60 vol%, the green volume = final volume / 0.60 = 1.67× final volume (in 3D)
- But because the binder is also present, the relationship is more complex
- The 14-20% linear shrinkage corresponds to approximately 35-50% volume reduction
| Aspect | With 15-20% Shrinkage | Without Shrinkage |
|---|---|---|
| Mold detail resolution | Mold features are 1.18× larger — finer detail transferred to sintered part | Mold features must be at final size |
| Surface finish | Mold surface texture is relaxed 15-20% during sintering — smoother final surface | No improvement from mold to part |
| Internal features | Holes and cavities formed by larger, stronger core pins that can withstand higher injection pressure | Smaller, weaker core pins needed |
| Dimensional tuning | Shrinkage factor can be adjusted by ±1% to fine-tune final dimensions | No tuning available |
MIM parts shrink because the green part is only 55-65% metal powder by volume (the rest is binder). During sintering, the binder is removed and the metal particles densify to 95-99% density. The 14-20% linear shrinkage corresponds to a volume reduction of approximately 35-50%. This shrinkage is predictable and compensated for in the mold design — the mold cavity is made 1.16-1.20× larger than the final part size.