A common assumption about MIM is that parts shrink uniformly by the same percentage in all directions. In reality, shrinkage is almost always anisotropic — different in different directions.
Typical shrinkage by direction:| Direction | Relative Shrinkage | Why It Differs |
|---|---|---|
| X (mold parting plane, parallel to flow) | 100% (baseline) | Follows mold cavity dimensions most closely |
| Y (mold parting plane, perpendicular to flow) | 98-102% of X | Minor variation due to flow orientation effects |
| Z (mold-closing direction, perpendicular to gate) | 95-98% of X | Gravity effects during sintering; lower green density in Z direction |
- Gravity during sintering: As the part softens at sintering temperature, gravity causes more sag/settlement in the vertical (Z) direction. This is the single largest source of anisotropy — parts sintered on their side will have different X/Z shrinkage than parts sintered upright
- Mold filling orientation: Feedstock flows preferentially in the flow direction, creating slightly different green density distribution in X vs Y
- Gate location: Material flowing through the gate into the cavity has a "memory" of the flow direction, affecting shrinkage near the gate differently from the end of fill
- Part geometry: Asymmetric parts have different constraint levels in different axes during sintering
- For critical dimensions, specify the measurement direction on the drawing so the mold designer applies the correct shrinkage factor for that axis
- Expect 1-3% shrinkage difference between directions — this is normal MIM behavior, not a defect
- For parts with tight tolerances on multiple axes, shrinkage must be characterized experimentally during first article validation and the mold corrected accordingly
MIM shrinkage is anisotropic due to gravity during sintering (parts sag more in the vertical direction), mold filling orientation (flow direction creates slight density differences), and gate effects. The difference is typically 2-5% between the largest and smallest shrinkage axes. This anisotropy must be accounted for in mold design — the shrinkage factor for the Z direction may be 95-98% of the X direction factor.