Every MIM material specification includes two density numbers: theoretical density and sintered density. Understanding the difference is essential for interpreting QC reports.
Theoretical density — the density of the material if it were 100% solid, with zero porosity. This is a calculated value based on the weighted average density of the alloying elements. For 316L stainless steel, the theoretical density is 7.96 g/cm³. Sintered density — the actual density of the as-sintered MIM part, measured by the Archimedes method (water immersion). Because MIM parts contain residual porosity (typically 2-5% by volume), the sintered density is always lower than the theoretical density. What "95% dense" actually means:| Material | Theoretical Density | At 96% Dense | At 98% Dense | Typical MIM |
|---|---|---|---|---|
| 316L | 7.96 g/cm³ | 7.64 g/cm³ | 7.80 g/cm³ | 7.56-7.80 |
| 17-4PH | 7.80 g/cm³ | 7.49 g/cm³ | 7.64 g/cm³ | 7.41-7.64 |
| Fe-2Ni | 7.84 g/cm³ | 7.53 g/cm³ | 7.68 g/cm³ | 7.45-7.68 |
| Ti6Al4V | 4.43 g/cm³ | 4.25 g/cm³ | 4.34 g/cm³ | 4.16-4.34 |
% theoretical density = (measured sintered density / theoretical density) × 100
Example: A MIM 316L part weighs 7.64 g/cm³. 7.64 / 7.96 × 100 = 96%. This means 4% of the part volume is porosity.
Quick Q: What is the difference between MIM theoretical density and sintered density?Theoretical density is the density of a 100% solid material with zero porosity — it is a calculated number based on the alloy composition. Sintered density is what a MIM part actually achieves after sintering, measured by water immersion. The ratio (sintered/theoretical × 100%) gives the % density, which is the standard measure of MIM sintering quality. For MIM 316L, typical values are 95-98% of theoretical.