MIM parts finish well because they start smooth: as-sintered surfaces are matte and uniform, commonly in the 0.8–2 μm Ra range, and from there the ladder runs through tumbling and vibratory finishing, mechanical polishing, electropolishing, and plating — each step buying lower Ra or a functional surface at a known cost in handling and, for material-removing steps, dimensional stock. The decision is driven less by appearance than by function: sealing, cleanability, fatigue, and corrosion behavior each have a finish that satisfies them most economically.
This page compares the practical options for MIM components — what each process does to roughness, geometry, and tolerance, which alloys behave best in which bath, and how to specify so the supplier can actually hit the requirement.
As-Sintered: The Free Baseline
The sintered surface is the MIM starting point and it is already a finished-looking surface: matte, uniform, free of tool marks, lay, and machining scratches, commonly cited in the 0.8–2 μm Ra band depending on powder and process. For housings, brackets, and internal components, as-sintered is very often sufficient — and it is the least costly state because it leaves the furnace and goes to inspection.
Two things as-sintered is not: it is not a mirror (the surface reflects powder particle size), and it is not passivated in the corrosion sense — stainless parts intended for wet duty usually get a passivation or electropolish step to build the protective oxide properly. Tolerance-wise, as-sintered is the ±0.3–0.5% state of the tolerances guide: nothing has been removed, so the sintered dimension is the part dimension.
Tumbling and Vibratory Finishing
Mass finishing — tumbling with media, or vibratory bowls — is the volume workhorse. It deburrs, softens edges, knocks down peaks, and typically moves Ra into the 0.4–0.8 μm range on accessible faces. It is cheap because it is batch: thousands of parts per run, no fixturing, no per-part labor.
- Best for: deburring after any trimming, edge radiusing, uniform cosmetic improvement on external faces.
- Limits: internal features, deep bores, and pockets see little action; media can lodge in complex geometry.
- Dimensional effect: minimal — edge geometry changes, but nominal surfaces lose only microns.
Mechanical Polishing to Mirror
Hand or automated polishing takes accessible surfaces below 0.1 μm Ra — true cosmetic mirror — and it is priced accordingly: per-part labor, geometry-dependent cycle, and operator-dependent consistency. For watch-class or display surfaces on small parts it is routine; for functional internals it is usually money spent in the wrong place.
The MIM-specific note: polishing removes stock, so parts destined for mirror finishes carry a polishing allowance, and any tight tolerance that lands on a polished face is held after polishing, not before. The 24-hour quotation prices finishing against the actual geometry — complex internal access is where polishing cost surprises come from, and a DFM pass (see the DFM guidelines) can move or open access features before the mold is cut.
Electropolishing: The 316L Sweet Spot
Electropolishing removes a thin, controlled surface layer electrochemically: peaks dissolve faster than valleys, Ra typically improves substantially, and — the functional bonus — the surface comes out clean, passive, and particle-free. On stainless grades it is the standard route where cleanability, sterility-compatible surfaces, or corrosion initiation resistance matter: 316L behaves particularly well; 17-4 PH and the 400-series are processed but with more parameter care (see the 316L and 17-4PH pages for the alloy context).
Dimensionally, electropolishing removes on the order of microns per face — enough that critical dimensions on polished faces carry an allowance, and enough to open small orifices slightly, which can be used deliberately as a fine-tuning step. On MIM density (95–99%), electropolished surfaces also expose less of the residual porosity than aggressive mechanical work, which is one reason it is the default 'clean' finish for sintered stainless.
Plating and the Selection Table
MIM parts plate like their wrought equivalents — nickel, electroless nickel, zinc, and decorative stacks all apply, with the usual caveat that surface preparation controls adhesion and that specifications are quoted per project. The selection shortlist:
| Option | Typical Ra result | Geometry reach | Tolerance impact |
|---|---|---|---|
| As-sintered | ≈0.8–2 μm | All surfaces | None (baseline ±0.3–0.5%) |
| Tumbling / vibratory | ≈0.4–0.8 μm | External faces, edges | Minimal; edge radiusing |
| Mechanical polishing | <0.1 μm mirror | Accessible faces only | Stock removal — plan allowance |
| Electropolishing | Substantial Ra improvement | Wetted surfaces, including bores | Micron-level removal; orifice opening |
| Plating / coating | Adds layer, smooths slightly | Line-of-sight varies by process | Adds thickness — dimension it |
Two rules keep the decision honest. First, specify the function (seal, clean, cosmetics, corrosion start), not a bare Ra number — overspecifying Ra on invisible faces is the classic cost leak. Second, ask which faces the process can reach: mass finishing and electropolishing are geometry-forgiving, mechanical polishing is not. Emitech's finishing scope alongside molding and sintering (17 injection machines, 22 furnaces) is on the capabilities page.
Frequently Asked Questions
Q: What Ra does MIM deliver as-sintered?
Commonly in the 0.8–2 μm range, matte and uniform, with no tool marks or machining lay — powder and process dependent, so quote-critical values should be confirmed on the actual alloy. Secondary finishing takes it down from there.
Q: Does polishing or electropolishing change my tolerances?
Both remove material — mechanical polishing visibly (allowance required on mirror faces), electropolishing at the micron level. Dimensional features on finished faces are held after finishing, and small orifices can open slightly under electropolish, sometimes used as deliberate fine-tuning.
Q: Can MIM parts reach a cosmetic mirror finish?
Yes — mechanical polishing of accessible faces reaches below 0.1 μm Ra and is routine for display surfaces on small parts. Cost is per-part labor and geometry access; internal mirror surfaces are usually not economically polishable and are better redesigned or re-specified.
Q: Which finish for cleanability and corrosion start?
Electropolished stainless — it smooths, passivates, and cleans in one step, and 316L is the best-behaving MIM grade for it. For chloride-heavy duty pair the finish with the right alloy first; no finish rescues the wrong material.
Q: Is as-sintered good enough for visible parts?
Often yes: the surface is uniform matte with no lay, which reads as a deliberate industrial finish. It is the default for internal components and brackets; move up the ladder only when the requirement (sealing, cleanability, cosmetics) names it.
Finish is a function decision with a price tag attached, and the least costly correct answer is usually low on the ladder. Send the drawing with the functional requirement — sealing face, cleanable wetted path, cosmetic surface — through the contact page, and the 24-hour quotation will come back with the finish route and its cost consequence per face.
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