Every MIM powder certificate of analysis reports both apparent density and tap density. Most engineers look at the D50 and move on — but these two numbers reveal important information about how the powder will behave in processing.
Definitions:- Apparent density (AD): The mass per unit volume of freely settled powder (ASTM B212). Measured by allowing powder to flow through a Hall flowmeter into a 25 cm³ cup. Tells you how powder packs under gravity alone
- Tap density (TD): The mass per unit volume of powder after mechanical tapping (ASTM B527). Typically 1000-3000 taps until no further volume change. Tells you the maximum packing density achievable through vibration
| AD (g/cm³) | TD (g/cm³) | TD/AD Ratio | Interpretation | Impact on MIM |
|---|---|---|---|---|
| > 3.5 | > 4.5 | > 1.3 | Excellent packing — spherical, well-graded powder | Low binder demand, consistent shrinkage, good flow |
| 2.5-3.5 | 3.5-4.5 | 1.2-1.3 | Good packing — typical for standard MIM powder | Normal processing, acceptable sintering |
| < 2.5 | < 3.5 | < 1.2 | Poor packing — irregular particles, excessive fines | High binder demand, more shrinkage variation, potential molding issues |
Apparent density measures how powder packs when freely poured (before tapping). Tap density measures the maximum packing after vibration. The ratio between them (TD/AD) indicates powder quality — a higher ratio means better packing, less binder needed, and more consistent sintering shrinkage.
For MIM, target an apparent density above 2.5 g/cm³ and a TD/AD ratio above 1.3. If the ratio falls below 1.2, expect higher binder demand, reduced powder loading in feedstock, and potentially lower sintered density.