How to Design MIM Parts for Post-Sintering Assembly: Snap Fits and Interference Fits

Quick Q: Can MIM parts have snap-fit features?

Yes — MIM 17-4PH (aged) and 316L can function as spring elements for limited-cycle snap-fit applications. Key design parameters: maximum strain 0.5-1.0% for 316L (will yield above this), 0.3-0.6% for 17-4PH aged (brittle above this). Use a cantilever beam snap fit with a taper (2-3°) on the mating surface and a 45° return angle for the latch face.


Snap fit design guide for MIM materials:
Material Max Strain Max Stress Max Deflection (10 mm beam) Cycles to Fatigue
MIM 316L as-sintered 0.8% 170 MPa 0.08 mm 1000-5000
MIM 17-4PH H900 0.4% 480 MPa 0.04 mm 500-2000
MIM 17-4PH H1025 0.6% 520 MPa 0.06 mm 1000-3000
Interference fit tolerances:
Fit Type Hole Diameter (mm) Shaft Tolerance Recommended Interference MIM Tolerance Required
Light press fit 5.00 +0.02/+0.04 0.01-0.03 mm ±0.05 mm
Medium press fit 5.00 +0.04/+0.06 0.03-0.05 mm ±0.05 mm
Heavy press fit 5.00 +0.06/+0.10 0.05-0.08 mm ±0.05 mm with coining
Assembly design considerations specific to MIM:
  • As-sintered surfaces may have Ra 2.0-3.2 µm — this increased friction means interference fits need 20-30% less interference than machined parts to achieve the same retention force
  • Gate vestige on snap fit beams may interfere with assembly — locate gates on non-functional surfaces
  • Draft angles on snap fit features reduce the effective cross-section at the root — compensate by increasing beam width at the root by 10-20%

Contents

Contact: Cindy