MIM part density is the single most reliable indicator of sintering quality. The standard method is the Archimedes (water immersion) method per ASTM B311 (MPIF Standard 42).
Step-by-step procedure:- Dry weight (Wdry): Weigh the clean, dry MIM part on a calibrated balance (0.0001 g resolution for small parts)
- Saturated weight (Wsat): Immerse the part in water and place in a vacuum chamber (25 inHg minimum) for 30 minutes to pull water into open pores. Weigh the part suspended in water
- Suspended weight (Wsus): Without removing from water, weigh the part on the submerged weighing pan
- Calculate density:
where ρwater = 0.997 g/cm³ at 23°C
Interpreting results:| Material | Theoretical Density | MIM Target | Acceptable Range |
|---|---|---|---|
| 316L stainless | 7.96 g/cm³ | >7.64 g/cm³ (96%) | >7.56 g/cm³ (95%) |
| 17-4PH stainless | 7.80 g/cm³ | >7.49 g/cm³ (96%) | >7.41 g/cm³ (95%) |
| Fe-2Ni steel | 7.84 g/cm³ | >7.45 g/cm³ (95%) | >7.37 g/cm³ (94%) |
| Ti6Al4V | 4.43 g/cm³ | >4.25 g/cm³ (96%) | >4.16 g/cm³ (94%) |
- Surface bubbles: Air trapped on rough surfaces gives falsely low density. Pre-wet with a drop of wetting agent
- Incomplete saturation: Water must penetrate all open pores. 30-minute vacuum soak is minimum. Thick parts may need 60 minutes
- Temperature variation: Water density changes 0.02% per °C. For critical measurements, control water temperature to ±1°C
- Oil or contamination: Grease on the surface repels water. Clean parts in acetone before measurement
For MIM 316L, the typical target is >96% of theoretical density (>7.64 g/cm³). Premium gas-atomized powder with optimized sintering achieves 97-98% (>7.72 g/cm³). Density below 95% indicates insufficient sintering — the part will have reduced mechanical properties and corrosion resistance.