CNC Plastic Machining: What to Check Before You Order
Plastic parts are not metal parts with a different stock list. Wall thickness, heat, clamping and chip clearance all behave differently. This guide gives engineers and buyers six proven checks to run before releasing a purchase order for CNC plastic machining, from tolerance and material grade to finish, lead time and certification.
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Key takeaways
Which plastic for which job
Common engineering plastics and where they earn their cost
| Material | Typical use | Watch out for |
|---|---|---|
| POM (Delrin) | Gears, bushings, slides | Low moisture uptake; hard to bond |
| PEEK | Medical, high-temp seals | Needs annealing; expensive stock |
| PC | Covers, brackets, guards | Stress crazes near machined edges |
| PMMA (acrylic) | Optical windows, light pipes | Brittle; avoid sharp internal corners |
| PA (nylon) | Wear pads, connectors | Absorbs moisture; dimensions grow |
| PP / HDPE | Chemical tanks, liners | Flexes under clamping; light cuts only |
| ABS | Housings, prototypes | Soft; thread inserts often needed |
| Carbon fibre composite | Jigs, drone frames, arms | Abrasive dust; tool wear is fast |
The short version
Match the tolerance to the material, anneal anything stress-prone, and read the quote for what it excludes. If a supplier can answer those three points with numbers, the rest of the order is routine.
Tolerance and material go together
A ±0.005 mm callout on a PEEK manifold is realistic. The same callout on a 1.5 m HDPE liner is not, because the part will move as it cools and absorbs moisture. Before you send the drawing, decide which dimensions actually matter. Mark them, and leave the rest at a general tolerance. Shops quote faster when the critical list is short.
Every plastic has a different response to cutting heat. POM machines clean and holds size. PC smears if the tool rubs. PMMA chips and can crack at a sharp corner. The cutting data changes with the grade, not just the family name.
Ask the supplier to state the measurement condition. A part checked 30 minutes off the machine can read differently two days later. For nylon and HDPE, the drawing should say whether dimensions apply dry as machined or after conditioning.
Wall thickness and feature size
Thin walls are the most common cause of a plastic part failing dimensional check. Below about 1 mm, cutting force pushes the wall away from the tool. The result is chatter, a tapered wall, or a broken part on the last pass. Support from the fixture helps, but it costs time.
Deep pockets and small internal radii create the same problem. A 0.5 mm corner radius in a 40 mm deep pocket needs a small tool with a long reach. That tool deflects. If the design allows a 2 mm radius instead, cycle time and risk both drop.
Threads in plastic behave differently too. A metal-style fine thread in ABS will strip. Coarse threads, or a brass insert, last longer. Tell the shop if the thread will be assembled more than a few times.
Stress relief and post-machining drift
Extruded plastic bar carries internal stress from the mill. When you cut one side away, the part bends. Shops handle this by roughing, then stress-relieving, then finishing. On PEEK and some PC grades, that middle step is an oven cycle. It adds a day but saves the part.
If a supplier quotes a complex PEEK part for next-day shipment with no mention of annealing, ask how they control drift. The honest answer may be that they hold tolerance only in the as-machined state, which is fine for some jobs and wrong for others.
A simple test: order two pieces, measure both, leave one on the bench for 48 hours and measure again. If the second part moved, the process needs a stress-relief step.
Finish, color and edge quality
Plastic finishes are not the same menu as metal. Anodizing does not apply. What you can specify includes bead blasting, tumbling, brushing, polishing, laser marking and painting. Each has a plastic-specific limit.
Bead blasting gives a matte, uniform look and hides tool marks. It also rounds sharp edges slightly. If a sealing face must stay crisp, mask it or skip the blast. Polishing can reach a clear finish on acrylic and PC, but any internal scratch shows through.
Laser marking on dark plastics often needs a light-colored additive or a contrasting fill. Minimum character height is 1.5 mm for a clean mark. Below that, characters fill in and become hard to read.
Lead time, MOQ and quote clarity
Lead time tells you more about a shop than its equipment list. Quotation and a DFM review within 12 hours, production starting within 24 hours, and parts shipping in 3–5 days describe a shop with open capacity and a real scheduling system. If the quote takes a week, the shop is either overloaded or not set up for small runs.
MOQ is the other filter. A supplier with no minimum order quantity can take one prototype and then a 10,000-part run on the same process. That matters when the design is still moving and you do not want to commit to tooling.
Read the quote line by line. Does it include material certification, first-article inspection, and the finish you asked for? A low number that excludes inspection is not a low number.
Certification and quality evidence
Certificates only count if the scope covers your part. ISO 9001:2015 covers general quality management. IATF 16949:2016 applies to automotive production. ISO 13485:2016 applies to medical devices. ISO 27001:2022 covers information security, which matters if you are sending proprietary CAD files.
Ask what inspection happens and when. A supplier running 100% inspection before shipment, with raw material checks, in-process monitoring and a final report, is easier to audit than one that samples. Reports should be available on request.
For regulated parts, request the material certificate and the inspection record with the shipment. It costs nothing at the quoting stage and avoids a scramble later.
Six steps from drawing to shipped parts
A practical sequence for a first plastic job
- 1Mark critical dimensionsHighlight the 5–10 dimensions that control function. Leave the rest at general tolerance so the shop can plan the process.
- 2State the material grade and conditionWrite POM-C or POM-H, not just 'Delrin'. Note whether dimensions apply dry as machined or after conditioning.
- 3Send the file for DFMAsk for a DFM review with the quote. Look for notes on wall thickness under 1 mm, deep pockets and small radii.
- 4Confirm the finish on a sample areaIf bead blasting or polishing matters, mark the areas to treat and the areas to mask before the run starts.
- 5Agree the inspection planDecide which dimensions get a report. Request material certificates with the shipment for regulated parts.
- 6Measure on arrivalCheck the part at the temperature and humidity of your assembly floor. Re-measure after 48 hours on the first article.
Questions buyers ask
Can you hold ±0.005 mm on all plastics?
No. That tolerance is realistic on POM, PEEK, PC and acrylic when the geometry supports it. Soft or moisture-sensitive materials such as PP, HDPE and nylon need a wider band or a defined measurement condition.
Tell us which dimensions matter and we will confirm what the material can hold before the run.
Do you have a minimum order quantity?
No minimum. We run from a single prototype up to 10,000+ part runs on the same process.
That means a design change after the first article does not force a new tooling commitment.
How fast can I get parts?
Quotation and free DFM analysis come back within 12 hours. Production can start within 24 hours, and parts ship in 3–5 days.
The schedule depends on material stock and finish. A PEEK part with annealing adds a day for the oven cycle.
Which certifications do you hold?
ISO 9001:2015, IATF 16949:2016, ISO 13485:2016 and ISO 27001:2022.
Ask which scope applies to your part. Medical and automotive work is audited against different clauses.
How do you protect my design files?
Uploads are secure and confidential, and we sign an NDA on request.
Our ISO 27001:2022 certification covers the information-handling side of the business.
What inspection do I get?
Every order gets raw material checks, in-process monitoring and 100% inspection before shipment.
Inspection reports are available on request. For regulated parts, request them at the quoting stage.
Send a drawing and get a DFM review
Upload your CAD file. We return a quotation and a free DFM analysis within 12 hours, with the tolerance and material limits stated up front.
12-hour quoteNo MOQ100% inspection
More from the shop floor
We publish setup notes, tooling trials and inspection data from the factory floor.
