A defining characteristic of MIM versus conventional powder metallurgy is particle size. MIM powder is typically under 20 µm (D50) — about 5-10x finer than PM powder. Why?
The three reasons:- Sintering kinetics: MIM parts are sintered without mechanical pressure. The driving force for densification comes from particle surface energy. Smaller particles have higher surface area per unit volume, meaning they sinter faster and to higher density at lower temperatures. A powder with D50 = 20 µm sinters to 96-98% density under standard conditions. A powder with D50 = 50 µm would reach only 88-92% under the same conditions — the same as conventional PM.
- Feedstock flow through thin cavities: MIM produces parts with wall thicknesses as low as 0.3 mm. The powder must be small enough that the feedstock can flow into these thin sections without bridging or filtering at the gate. If powder particles were larger than approximately 25 µm, they would bridge at the gate and prevent complete fill of thin-wall sections.
- Surface finish: As-sintered surface roughness is proportional to the powder particle size. A finer powder produces a smoother as-sintered surface (Ra 1.6-2.5 µm for D50 = 15 µm vs Ra 3.5-6.0 µm for D50 = 40 µm).
| Parameter | MIM Powder | PM Powder |
|---|---|---|
| D50 | 10-20 µm | 50-150 µm |
| Surface area | 0.5-1.5 m²/g | 0.05-0.2 m²/g |
| Sintering density | 95-99% | 85-92% |
| Feedstock flow | Thin-wall capable | Cannot flow through a mold gate |
| As-sintered Ra | 1.6-3.2 µm | 3.2-6.3 µm |
MIM uses powder under 20 µm because: (1) fine particles sinter to higher density without external pressure, (2) the feedstock must flow through thin mold cavities (0.3+ mm), and (3) fine powder produces smoother as-sintered surfaces. The trade-off is cost — fine MIM powder costs 3-5x more per kg than coarse PM powder.