CNC machined plastic parts: China Service
This guide is for design engineers and sourcing managers comparing suppliers for cnc machined plastic parts. It covers material choice, tolerance limits, finish, lead time and certification, so you can judge a China service before you release a purchase order.
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Key takeaways
When CNC Machined Plastic Parts Beat Other Processes
Use this table to pick a process before you ask for quotes.
| Situation | Better process | Why |
|---|---|---|
| 1 to 500 parts | CNC machining | No tooling cost, design can still change |
| Thin walls under 1.0 mm | CNC machining or vacuum casting | Mold flow is hard to control at this wall |
| Over 10,000 parts, stable design | Injection molding | Tool cost spreads across the run |
| Optical clarity, PMMA or PC | CNC then polish | Machined surface needs polishing to clear |
| Carbon-filled PEEK, tight bores | CNC machining | Fiber orientation stays uniform |
| Undercuts and internal channels | 5-axis CNC or mill-turn | Reaches features in one setup |
| Prototype before tooling | CNC machining | Same drawing, no mold commitment |
Which Plastic Grades Machine Well
Most engineering plastics cut cleanly with sharp carbide tooling and enough coolant or air blast. POM, ABS, PC, PMMA, PA and HDPE are the everyday grades. They hold dimensions well, produce short chips and tolerate moderate feed rates. If your part is a bracket, a housing, a fixture or a manifold, one of these is usually the right starting point.
Filled and high-temperature grades behave differently. PEEK, PEI and carbon fibre reinforced stock are abrasive. They need slower surface speeds, more frequent tool changes and sometimes polycrystalline diamond tooling. The cut is still accurate, but cycle time can be two to four times longer than the same geometry in POM. Budget for that before you compare quotes.
Hygroscopic resins need a dry cycle. PA and POM absorb moisture from the air, and a wet blank will shrink after machining. We dry blanks and rough-cut, then finish after the part has settled. On a 100 mm PA part, skipping this step can move a bore by 0.05 mm or more.
Wall thickness sets the limit. Machined plastic walls below 0.5 mm vibrate and chatter, and thin ribs deflect under clamping force. When the drawing calls for a 0.3 mm wall, vacuum casting or 3D printing may serve you better for the prototype stage.
- 1Easy to machinePOM, ABS, PC, PMMA, PA, PP, HDPE
- 2Slow but accuratePEEK, PEI, carbon fibre grades
- 3Avoid thin wallsBelow 0.5 mm, expect chatter and deflection
Tolerance, Finish and What Plastic Actually Holds
We hold ±0.005 mm on critical plastic features. That number is a machine capability, not a promise for every dimension on the print. Plastics move with temperature, and a part measured at 20 °C can shift after it sits in a warm warehouse. The practical rule is to tolerance only the features that matter and leave the rest general.
Surface finish depends on the resin. A sharp cutter leaves Ra 1.6–3.2 μm as-machined on most grades. Fine finishing reaches Ra 0.8–1.6 μm, and polishing on acrylic or PC can reach Ra 0.2–0.8 μm. Filled resins do not polish well. The glass or carbon particles tear out and leave a dull, slightly rough surface.
Threads and bores need their own attention. Cutting M2 threads in POM is routine. Cutting them in carbon-filled PEEK wears taps quickly, so we often mill the thread instead. For bearing bores, specify the fit and let the shop pick the boring strategy. A reamed hole holds size better than a milled one in soft plastics.
Annealing matters on thick sections. A 30 mm thick PEEK block carries internal stress from extrusion. Rough machine, anneal, then finish. Without that sequence the part can bow 0.2 mm over a 200 mm length after a few days.
How to Read a Quote for CNC Machined Plastic Parts
A useful quote lists material grade, stock form, tolerance, finish and inspection. If the reply only gives a price and a lead time, you cannot compare it with another supplier. Ask for the material certificate and the inspection plan as separate line items.
Stock form changes the price more than most buyers expect. A 20 mm plate is cheap. A 150 mm rod in PEEK can cost more than the machining. If your part can be redesigned around a thinner section or a standard plate size, do that before you request quotes. The savings are usually larger than any negotiation on hourly rate.
Quantity breaks are not linear. Setup and programming dominate a one-off. At 50 pieces the per-part price drops sharply, because the same program runs again with a new blank. Past a few hundred pieces, the curve flattens and the material cost takes over.
Ask what is included. Deburring, cleaning and packaging should be in the price. Laser marking is a separate operation. If you need a certificate of conformity with the shipment, say so at the quoting stage, not after the parts are packed.
What to Verify Before You Place the Order
Machine list and travel matter for large parts. A shop with only small vertical mills cannot make a 1,200 mm housing in one setup. GreatLight runs 127 high-precision CNC machines, including 16 simultaneous 5-axis centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers. Maximum processing size is 4,000 mm, with travels of 4,000 × 400 × 150 mm and 750 × 1,150 × 550 mm.
Certification should match your industry. ISO 9001:2015 covers general quality management. IATF 16949:2016 applies to automotive work. ISO 13485:2016 applies to medical devices. ISO 27001:2022 covers information security, which matters when you send CAD files. A supplier who lists all four should be able to show the scope statement for each.
Confidentiality is a real requirement, not a formality. Uploads are secure and confidential, and an NDA is available on request. If your part is an unreleased product, sign the NDA before you send the STEP file, not after.
Inspection depth is the last filter. We inspect 100% of parts before shipment, with raw material check, in-process monitoring and final inspection. Reports come on request. Ask for a first article report on the first order, then sample reports on repeat orders.
Step by Step: From Drawing to Shipped Plastic Parts
This is the sequence we follow on a typical plastic machining order.
- 1Send the 3D model and 2D printSTEP or IGES plus a PDF drawing with tolerances, material grade and finish callouts. Missing tolerances slow the quote.
- 2Review the DFM feedbackWe return quotation and free DFM analysis within 12 hours. Check wall thickness, tool reach and any feature we flagged.
- 3Confirm material and stock formApprove the grade and whether the part comes from plate, rod or near-net stock. Filled grades need extra lead time.
- 4Lock the inspection planAgree which dimensions are critical, what report you need and whether a first article is required.
- 5Machining startsProduction can start within 24 hours of approval. Rough, stress relief where needed, then finish passes.
- 6Final inspection and packing100% inspection before shipment. Deburr, clean, bag and label per your packing note.
- 7ShippingParts ship in 3–5 days. Express or air freight depending on size and destination.
Frequently Asked Questions
Can you machine a plastic part with no minimum order quantity?
Yes. There is no minimum order quantity. We run from one prototype to 10,000+ part runs on the same program, so a single unit is a normal order for us.
One-off parts carry the full setup cost, so the per-part price is higher. If you expect a production run later, tell us at the quoting stage and we will keep the fixture and program on file.
Which plastics do you keep in stock?
We machine ABS, PC, PMMA, POM, PA, PEEK, PP, HDPE and carbon fibre grades. Standard plate and rod sizes in POM, ABS, PC and PA are usually available from local stock.
PEEK, PEI and filled grades are often ordered per job. Add a few days to the lead time if the exact grade and thickness is not in stock.
How tight a tolerance can you hold on plastic?
±0.005 mm is the tightest we hold, and it applies to specific features rather than the whole part. Plastics expand and contract with temperature, so a dimension measured in the metrology room can change on your site.
For most plastic parts, ±0.05 mm on mating features is enough and much cheaper to achieve. Reserve the tight tolerance for the two or three dimensions that control fit.
Do you provide material certificates and inspection reports?
Yes, on request. We can supply the material certificate from the stock supplier and a dimensional report for the features you list as critical.
Inspection covers raw material check, in-process monitoring and final inspection. Every part is checked before shipment, not sampled.
Can you sign an NDA before I send files?
Yes. An NDA is available on request, and we can sign yours or provide ours. Uploads are secure and confidential.
For unreleased products, sign first, then send the STEP file. Do not send production drawings through a public file link.
What is the usual lead time?
Quotation and free DFM analysis come back within 12 hours. Production can start within 24 hours of approval, and parts ship in 3–5 days for standard plastic work.
Large parts, filled resins and heavy finishing add time. We will state the realistic date in the quote instead of promising a date we cannot hold.
Get a Quote on Your Plastic Parts
Send your STEP file and drawing. You get a quote and DFM feedback within 12 hours, and one engineer stays with the job from first cut to shipment.
12-hour quoteNo MOQ100% inspectionNDA on request
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