When comparing MIM parts to PM (powder metallurgy) or investment casting parts side by side, the first thing most people notice is the surface finish. MIM parts are noticeably smoother. Why?
Surface finish comparison:| Process | Typical As-Produced Ra (µm) | After Finishing Ra (µm) |
|---|---|---|
| MIM (spherical powder) | 1.6-2.5 | 0.2-0.8 (polished) |
| MIM (standard powder) | 1.6-3.2 | 0.2-1.6 |
| PM (press-and-sinter) | 3.2-6.3 | 1.6-3.2 (coining) |
| Investment casting | 3.2-10 | 1.6-6.3 (polished) |
| Die casting (zinc) | 1.0-2.0 | 0.4-1.0 (polished) |
| Die casting (aluminum) | 2.0-4.0 | 1.0-2.0 |
| Reason | MIM | PM | Investment Casting |
|---|---|---|---|
| Starting material | Fine powder (D50 <20 µm) duplicates mold surface | Coarse powder (50-150 µm) leaves interparticle voids | Wax pattern surface quality limits final finish |
| Mold/tool surface | Hardened steel cavity (mirror-polished or textured) | Die cavity (can be polished, but coarse powder limits transfer) | Wax injection die (aluminum, lower precision) |
| Surface replication | Excellent — feedstock replicates mold surface at sub-micron level | Poor — powder bridging at surface creates roughness | Good — wax replicates die surface; pattern assembly adds roughness |
| Porosity at surface | < 3% — micro-pores only | 8-15% — visible pores at surface | Variable — shrinkage porosity can appear |
MIM's surface finish advantage comes primarily from its much finer powder (D50 <20 µm vs 50-150 µm for PM). The surface of a MIM part replicates the polished steel mold surface at a resolution limited by the powder particle size. With 20 µm powder, the "step" between particles at the surface is approximately 2-3 µm Ra. With 100 µm PM powder, that step is 5-10 µm Ra.
Quick Q: Why is MIM surface finish better than PM and casting?MIM uses fine powder (D50 <20 µm) that replicates the polished steel mold surface with high fidelity, producing as-sintered surfaces of Ra 1.6-3.2 µm. PM uses coarser powder (50-150 µm) that creates visible interparticle voids and surfaces of Ra 3.2-6.3 µm. Investment casting surface quality is limited by the wax pattern surface and ceramic shell roughness, typically Ra 3.2-10 µm.