Post Processing CNC Plastic Machining Services
This guide is for engineers and buyers sourcing post processing cnc plastic work from a contract shop. It covers which finish suits which resin, the tolerances you can actually hold after finishing, and the seven points to check before you release a purchase order.
In this article
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Key takeaways
Post-processing method vs. plastic and use case
Use this as a first filter. A method that works well on one resin can craze or warp another.
| Method | Best-fit plastics | Typical finish | Watch out for |
|---|---|---|---|
| Bead blasting | ABS, PC, POM, PMMA | Matte, Ra 1.6–3.2 μm | Embedded media; edge rounding on thin walls |
| Tumbling / vibratory | POM, PA, HDPE | Edge break, uniform satin | Slow on deep pockets; can dull sharp corners |
| Vapor polishing | PC, PMMA, ABS | Gloss, near-optical clarity | Solvent attack; needs vented setup |
| Laser marking | Most thermoplastics | Dark or frosted mark | Min. character height 1.5 mm; low contrast on white POM |
| Stress relief anneal | PEEK, PEI, PA, PC | No visual change | Dimensional shift; needs a re-check after cooling |
| Painting / coating | ABS, PC, PA, PEEK | Color match, gloss control | Adhesion on PP and HDPE without primer |
What a complete post-processing quote should contain
If any row is missing from the quote, ask before you approve the order.
| Item | What good looks like | Red flag |
|---|---|---|
| Finish named per surface | Cosmetic and functional faces listed separately | 'Deburr and finish as required' |
| Trim allowance stated | Coating thickness or material removal given in μm | No allowance mentioned |
| Sequence | Finish and machining order defined | Finishing assumed to be last |
| Inspection | Dimensional report on request | Visual check only |
| Lead time | Quotation and DFM analysis within 12 hours | Vague 'TBD' on the schedule |
The verdict on choosing a plastic finishing supplier
If your part has a tight fit, a visible surface, or a clean-room requirement, choose a shop that sequences finishing before the final cut and inspects after it. If the part is a simple bracket with no functional surface, a basic deburr and tumble is enough and you should not pay for more.
Why post processing cnc plastic parts changes the design intent
A CNC machine leaves a specific set of artifacts on plastic: tool marks, burrs on the exit side, a whitened edge where the cutter rubbed, and internal stress from the cutting forces. None of these show up in the CAD model. Post processing cnc plastic work is the step that removes or controls them.
The stakes are higher than on metal. Plastics have lower thermal conductivity, so heat stays at the cut and softens the surface. They also creep. A burr you leave on an ABS housing will not stay a burr; it will fold over after a few thermal cycles and may shed particles into a clean assembly.
Cosmetic surfaces matter in different ways too. A visible PC lens cover or a PMMA display bezel is judged by the eye, while a POM gear is judged by wear and noise. The same shop needs two very different finishing routes for those two parts.
The practical consequence: finish should be specified at the RFQ stage, with the resin, the functional surface, and the cosmetic surface all named. Retrofitting a finish after the parts are cut usually means a second setup, a new fixture, and a dimensional re-check.
- 1Functional surfaceSealing faces, bearing bores, gear flanks. Finish is chosen for friction and wear, not looks.
- 2Cosmetic surfaceVisible faces on housings and covers. Finish is chosen for gloss, color, and texture uniformity.
- 3Burr-critical edgesEdges that touch skin, optics, or a sliding fit. These get a controlled edge break, not a generic deburr.
Common finishing routes for machined plastics
Media blasting is the workhorse. Glass bead or plastic media at 2–4 bar removes tool marks and gives a uniform matte without loading the surface with heat. On PC and PMMA, keep the pressure low and the nozzle moving; a stationary blast will frost the surface unevenly and you cannot polish that out on a textured part.
Vapor polishing uses a solvent vapor to reflow a thin surface layer. It produces the clearest result on PC and PMMA and can bring a machined window close to optical clarity. The trade-off is dimensional: you lose a few microns of skin, so do not vapor polish a part with a tight press fit unless the fit was calculated with that loss in mind.
Tumbling and vibratory finishing break edges across a whole batch without hand work. They suit POM, PA, and HDPE parts where a rounded edge is acceptable or desirable. Deep pockets and blind holes see much less action, so a part with internal features usually needs a hybrid route: tumble for the bulk, hand finish for the pockets.
Laser marking is the usual route for part numbers, traceability codes, and orientation marks. The minimum character height we work to is 1.5 mm. Below that, contrast drops on most filled and unfilled resins, and the mark becomes hard to read under shop lighting. If the mark must survive handling, test it before the full run.
- 1Anneal before final cutOn PEEK and PA, relieve stress first, then take a light finishing pass so the final geometry is stable.
- 2Mask the functional facesProtect sealing and bearing surfaces from blasting media and paint overspray.
How finishing shifts tolerances on CNC plastic parts
Every mechanical finish moves the surface. Bead blasting removes material, typically a few microns on a soft resin. Painting and coating add material, often 15–25 μm per coat. If a bore is held at ±0.005 mm and then painted, the paint decides the final fit, not the cutter.
The safe pattern is to finish first, then cut the tight features. A shop with 16 simultaneous 5-axis machining centers can blast the blank, re-fixture, and machine the sealing face and dowel holes in the same setup. That keeps the tight dimensions on the finished surface and avoids a second tolerance stack.
Thermal exposure is the other half of the problem. A stress-relief anneal on PEEK or PEI will change dimensions slightly, because the part relaxes as the internal stress leaves. If the anneal happens after final machining, you may lose the fit. If it happens before, the final cut holds.
For most plastic parts we hold ±0.005 mm on machined features and Ra 0.8–1.6 μm on functional surfaces. When a drawing calls for Ra 0.2–0.8 μm, ask whether the surface is functional or cosmetic. Polishing to that level on a large plastic face takes time and can round an edge you wanted sharp.
- 1Finish, then cutSequence coating or blasting before the final machining pass on tight features.
- 2Anneal, then cutRelieve stress before the last pass so relaxation does not move the part afterward.
- 3Check after finishingRe-measure tight features after any process that adds or removes surface material.
What to check in a post processing cnc plastic supplier
Ask what the shop does in-house. A supplier that machines plastic well but sends every part out for blasting and marking adds a week and loses control of the edge break. In-house finishing means one fixture, one inspection record, and one party accountable for the final dimensions.
Check the equipment list against your part. Large parts up to 4,000 mm need a machine with the travel to match: 4,000 × 400 × 150 mm for long extrusions, 750 × 1,150 × 550 mm for boxy housings. Small, detailed work runs better on a 500 × 500 × 450 mm or 500 × 310 × 200 mm machine with a Ø400 mm rotary table.
Ask about the inspection flow. Raw material check, in-process monitoring, and final inspection before shipment are the baseline. For medical and automotive work, the paper trail matters as much as the part. ISO 9001:2015 and IATF 16949:2016 cover the process side; ISO 13485:2016 applies when the part touches a medical device; ISO 27001:2022 covers how your files are handled.
Finally, ask how the shop handles finish on a mixed batch. If you order 20 prototypes and 2,000 production parts, the finishing route should not change between them. A finish that looks right on one part from a hand setup may not repeat across a tumbled batch.
- 1In-house vs. outsourcedOutsourced finishing adds transit time and splits dimensional responsibility.
- 2Machine travel vs. part sizeMatch the machine envelope to the part before you assume one setup is possible.
- 3Certifications that fitMatch the certificate to the industry, not to the marketing page.
Step by step: specifying post processing on an RFQ
Follow these seven steps in order. Each one removes a common cause of rework.
- 1Name the resin and its gradeWrite POM-C, PC, PA66-GF30, or PEEK, not just 'plastic'. Filled grades finish differently from unfilled ones. Glass-filled PA will show a different surface after blasting than unfilled PA.
- 2Separate cosmetic from functional surfacesMark them on the drawing. A sealing face at Ra 0.8–1.6 μm and a visible cover at Ra 1.6–3.2 μm are two different operations and two different costs.
- 3Set the edge requirementState a chamfer or radius range, for example 0.2–0.5 mm on handling edges. 'Deburr all edges' is not measurable and gets interpreted differently on every shift.
- 4State the trim allowanceIf you are coating, allow 15–25 μm per coat. If you are blasting before a tight cut, say so, and let the shop sequence the operations.
- 5Call out the annealFor PEEK, PEI, PA, and PC parts with tight fits, request stress relief before final machining. Expect a small dimensional shift and plan a re-check.
- 6Define the markingGive character height, location, and contrast requirement. Minimum 1.5 mm character height is a practical floor on most thermoplastics.
- 7Ask for the inspection recordRequest dimensional reports on the tight features after finishing. 100% inspection before shipment is the baseline; the report is what you keep.
Frequently asked questions
Can you hold ±0.005 mm after bead blasting or painting?
Yes, if the finishing step is sequenced before the final machining pass on the tight features. Blasting removes only a few microns, but painting adds 15–25 μm per coat, which is larger than the tolerance band on a precision fit.
The practical rule is finish first, then cut. That keeps the tight dimensions on the finished surface and avoids a second tolerance stack.
Which plastics warp most during post processing?
Thin-wall PA and PC parts are the usual offenders, because they carry machining stress and have low stiffness. A stress-relief anneal before the final cut reduces the risk.
PEEK and PEI also move during anneal, but the shift is predictable if the part is supported properly and cooled slowly.
Is vapor polishing safe for a press-fit part?
It removes a thin surface layer, so a press fit calculated on the as-machined dimension will come out looser. Either size the fit for the polished dimension or mask the fit area.
Vapor polishing is best reserved for visible windows and covers where clarity is the point.
What is the minimum order quantity for post processing cnc plastic parts?
There is no minimum order quantity. We run from one prototype to 10,000+ part runs, and the finishing route should stay the same across both.
If a shop changes its finishing method between the prototype and the production batch, expect a visible difference.
How do you handle confidentiality on customer drawings?
Uploads are secure and confidential, and an NDA is available on request. File handling is covered under ISO 27001:2022.
Send the drawing and we return a quotation with a free DFM analysis, usually within 12 hours.
What lead time should I plan for finishing?
Quotation and free DFM analysis come back within 12 hours. Production can start within 24 hours, and parts ship in 3–5 days.
Add time for any outsourced finishing step. Keeping blasting, marking, and inspection in-house is the main reason we can hold that schedule.
Send your drawing and finish requirements
We review the resin, the finish, and the tolerance stack, then come back with a quotation and a free DFM analysis. No minimum order quantity, from one prototype to 10,000+ parts.
12-hour quote100% inspectionNDA on request
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