Roughness is often specified in a blanket fashion in purchasing — "Ra 0.8" or "Ra 1.6" appear on the drawing without it being clear whether the value should apply everywhere or only to function-critical surfaces. In MIM, this distinction determines 10 to 30 percent of the unit cost.
What MIM delivers directly from sintering
The surface of a MIM component after sintering is determined by three factors: feedstock particle size, tool surface and sintering parameters. Typical values on the series component:
- 316L, 17-4 PH: Ra 0.8–1.6 µm directly from sintering.
- FN02, FN08: Ra 1.6–3.2 µm — coarser due to larger Fe powder particles.
- Ti-6Al-4V: Ra 0.7–1.4 µm with an optimised feedstock formulation.
- Inconel, CoCr: Ra 0.9–2.0 µm.
These values apply across the surface. They are isotropic, i.e. independent of direction — an important difference from a milled component, which always has a preferred direction of roughness peaks.
When each finishing method pays off
Vibratory grinding (Ra 0.4–0.8 µm)
The cheapest route to surface improvement. Cost: around 0.08–0.18 EUR per part depending on geometry. Removes deburring edges and smooths slightly — ideal for consumer goods and optical parts without sharp functional surfaces.
Tumbling / vibratory deburring (Ra 0.3–0.6 µm)
Comes close to vibratory grinding, but with more of a smoothing effect. Problem: edges become rounded. Not suitable when sharp edges are functionally required.
Electropolishing (Ra 0.1–0.3 µm)
Practically always required for medical and food-contact applications. Cost: 0.15–0.45 EUR per part. At the same time it enhances the passive layer — for 316L often sufficient as a single measure.
Grinding / honing
Only worthwhile on defined surfaces. Typical: tooth flanks, sealing seats, fitting surfaces. Very fast with modern 5-axis grinding machines. Cost is strongly geometry-dependent — from 0.12 EUR (a simple flat surface) to 1.80 EUR (a gear with 17 teeth).
Polishing (Ra < 0.1 µm)
Manual or automated. Frequently used in the watchmaking industry, otherwise rarely necessary. Very expensive: 0.50–3.00 EUR per part. Only worthwhile if mirror gloss is visually relevant.
Tolerance design: zone the roughness
The best cost lever lies not in the choice of finishing, but in zoned tolerancing. On a typical drawing there are three to five surfaces that really need Ra 0.4. The rest can remain Ra 1.6 or 3.2.
In practice: on a drawing for MIM, surfaces with tight Ra requirements are explicitly marked and provided with an additional symbol per DIN EN ISO 1302. All unmarked surfaces are deemed "as sintered".
What we recommend
With MIM components, the rule is: the later in the process you decide on surface requirements, the more expensive it becomes. We advise as early as the DfMIM phase — before the tool is built. A change to the drawing costs nothing; an additional finishing stage in series production costs per part.
Further reading
- Design for MIM: 12 design rules — how to realistically define surface requirements as early as in CAD.
- Material Matrix — which material delivers which Ra values directly from sintering.
- Calculating tooling costs realistically — why surface and tool are more closely linked than often assumed.