A sintered MIM part is not always ready for final assembly as soon as it leaves the furnace. The required surface condition is often part of the functional design: a sealing face may need machining, a lock component may need wear resistance, a medical or appliance component may need a clean, corrosion-resistant finish, and a structural part may need hardness that only heat treatment provides.
This guide covers the post-sinter operations SINTS reviews most often when a customer sends a drawing — mechanical finishing, chemical and coating treatments, and heat treatment by material family — with the specification notes that prevent the most common RFQ mistakes.
Start with the service requirement, not the finish
The first question in any finishing review is what the part actually has to do. Ask whether it will slide, seal, carry load, contact fluid, resist corrosion, or remain visible to the end customer. The answer determines whether the requirement is cosmetic, dimensional or both — and it prevents a decorative finish from being specified where dimensional control is the real issue.
For example, a locking component that wears against another metal part needs a surface and hardness designed for friction, not a mirror polish. A medical or food-contact part needs cleanability and corrosion resistance, which usually points to a stainless grade with a controlled surface. An appliance part visible to the customer needs appearance consistency across every batch. The environment the part operates in matters as much as the geometry.
Mechanical finishing options
Mechanical finishing changes the surface of the sintered part without adding material. It is the most common category of post-sinter work and the easiest to over-specify, so it helps to know what each option actually buys.
A practical rule is to separate the part into faces that are critical-to-function and faces that are cosmetic. Only the critical faces need machining, controlled roughness or polishing. Everything else can run as-sintered or tumbled, which keeps both unit cost and lead time under control.
Chemical and coating treatments
Chemical and coating processes add a controlled layer or alter the surface chemistry to manage corrosion, wear, friction or appearance. They are specified with the same discipline as machining: the drawing should state what is treated, what is masked, and what thickness or performance is expected.
Black oxide is another common option for low-alloy and carbon steels where mild corrosion protection and a controlled appearance are needed without the thickness of plating. Whichever treatment is chosen, masking definitions matter: a coating applied where it is not wanted can change a critical dimension as surely as one left off a surface that needs it.
Heat treatment by material family
Heat treatment should always be linked to a material grade and a measurable target. The same instruction — for example, “hardened” — means different things for 17-4PH, a low-alloy steel, or an austenitic stainless, so the grade and the required condition belong together on the drawing.
Specify heat treatment the way a machine shop specifies a surface: with a target range, a test method and a test location. “Hardened” or “heat-treated” on its own forces the supplier to guess. A clear statement such as “17-4PH, H900, hardness 36–42 HRC, verified on the boss face” turns a subjective requirement into a measurable acceptance criterion.
How finishing and heat treatment affect tolerances and inspection
Every post-process adds something to verify. Plating and PVD add thickness that must be allowed for on mating dimensions, and the coating tolerance belongs on the drawing. Heat treatment can introduce slight dimensional movement, so critical features may need to be machined after treatment rather than before. Roughness, hardness and coating thickness each have their own measurement methods and acceptance criteria.
The practical result is that finishing and heat treatment decisions change the inspection plan. SINTS verifies surface roughness with a roughness tester, hardness on the specified surfaces, and critical dimensions by CMM, and reports the results on first-article samples. When the acceptance criteria are written down before quoting, the supplier prices the real requirement instead of adding a risk allowance for an unknown one.
Common specification mistakes to avoid
- Specifying “hardened” without a target: No range, no method, no location. Add a hardness band, the test method and where it is measured.
- Specifying “polished” without roughness or area: A mirror finish on a sealing face is different from a matte polish on a visible surface. State Ra and which faces.
- Assuming passivation protects everything: Passivation supports stainless corrosion resistance; it does not make low-alloy steel corrosion-proof.
- Coating without masking or thickness tolerance: Coated threads, bores and datum surfaces can fail in assembly. Define what is masked and the allowed thickness.
- Heat-treating a grade that does not respond: 316L will not harden. Select the grade for the property, then the treatment for the grade.
What to include in the RFQ
A finishing and heat-treatment RFQ is only as good as the information behind it. The checklist below is what SINTS engineering looks for when reviewing a drawing for post-sinter requirements:
- Drawing or CAD file with the material grade and condition (for example, 17-4PH H900, or 316L as-sintered).
- Critical surfaces identified, with target roughness (Ra) and which faces need polishing or machining.
- Hardness or strength target with test method and location if heat treatment is required.
- Corrosion environment (indoor, outdoor, salt, food contact, sterilization) so the right stainless or coating can be selected.
- Coating or passivation requirements, including masked areas and thickness tolerance.
- Annual volume and inspection method (FAI, dimensional report, material certificate) so the plan matches the program.
With these details, the supplier can decide which requirements are best handled as-sintered, with selective secondary machining, or with a defined surface treatment — and quote the real scope instead of a worst-case assumption.
When SINTS helps you decide
The finishing and heat-treatment decision almost always interacts with the material grade, the geometry and the inspection plan. For OEM programs, the most efficient step is to send the drawing with a short note on how the part is used. SINTS engineering reviews the part against the finishing and heat-treatment options, confirms what is measurable, and proposes the most cost-effective combination of grade, treatment and inspection before tooling is approved.
Programs where the treatment is already locked by regulation or end-use requirements still benefit from the review: the focus shifts to geometry, masking and verification. Either way, writing the requirements down early is the cheapest way to avoid a rework loop after sampling.
Have a drawing to discuss?
Send it to our engineering team for a finishing and heat-treatment review. SINTS confirms what is measurable, proposes the most cost-effective treatment, and responds with a clear quotation scope.
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