Including CNC Plastic Machining: 6 Checks Before You Order

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  Key takeaways
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  Process choice by part count and geometry
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  Why including CNC plastic machining changes the drawing review
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  Six polymers that decide the quote
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  Tolerance and surface finish you can actually hold
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  Lead time, MOQ and what the quote should contain
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  Inspection and certifications that matter for plastics
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  Finishing and secondary operations on polymers
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  How we run an including CNC plastic machining job

Metals forgive a lot. Plastics do not. A 6061 bracket with a 0.8 mm wall machines fine. The same wall in POM will deflect under clamping pressure and spring back after the vise opens. That is why including CNC plastic machining in your supplier search is not a matter of adding a material name to a quote form. It is a different set of rules.

The first rule is thermal. Plastics expand roughly five to ten times more than steel per degree. A PEEK part machined at 25 °C and measured at 25 °C can sit 0.05 mm out of print after a shift in a warm inspection room. Good shops measure at a controlled temperature and tell you what that temperature was.

The second rule is stress. Every cut leaves residual stress in the surface. On a thin plastic rib, that stress shows up as bow or twist a day later, not on the machine. Suppliers who rough, rest and finish avoid this. Suppliers who take one heavy pass do not.

So the useful question is not whether a shop owns a CNC mill. It is whether the shop knows what a polymer does after the tool leaves the cut. Ask for a first article report and a note on how the part was held. The answer tells you more than a machine list.

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  Check the coefficient of thermal expansionAsk for the measurement temperature in the inspection report.
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  Ask about rough and rest cyclesStress relief between passes holds flatness on thin sections.

Polymer choice drives cost more than machining time. PEEK machines slowly, needs sharp carbide and often a stress-relief anneal. ABS cuts fast and cheap. The price gap between the two on the same geometry can be three to one, and neither is wrong.

POM, also called acetal, is the workhorse for gears, bushings and sliding parts. It holds ±0.005 mm well, has low friction and machines clean. It does not like strong acids or hot water. PC is the choice when you need impact strength and optical clarity, but it stress-cracks around sharp internal corners, so add a fillet.

PA, or nylon, absorbs moisture. A nylon part can grow 0.2 to 0.3 percent after sitting in humid air, which matters on a press fit. PMMA gives you optical clarity and scratch resistance, and it chips if your feed is too light. PEI and PPSU cover the high-temperature end and are common in medical and aerospace interiors.

Carbon-fibre-filled grades give stiffness close to aluminium at half the weight. They also eat tooling. Expect shorter tool life, a higher piece price and a shop that knows to use diamond-coated cutters.

A ±0.005 mm tolerance on a plastic part is achievable, but not everywhere on the part. It holds on a rigid boss or a thick flange. It does not hold across a 300 mm unsupported span, where thermal drift and clamping alone can consume the whole band.

Surface finish follows the same logic. Ra 0.8–1.6 μm is a normal machined finish on POM or PC. Ra 0.2–0.8 μm needs a finishing pass, sharper tooling and often a polished cutter, and it costs time. As-machined at Ra 1.6–3.2 μm is fine for brackets and housings that will not be seen.

Be careful with cosmetic requirements on transparent plastics. A tool mark on PMMA shows up under light in a way it never does on grey ABS. If the part is a lens cover or a light pipe, say so on the drawing. Otherwise the shop optimizes for size and you reject for looks.

Put the critical dimensions on the drawing and mark the rest as reference. A drawing where every dimension is tight forces the shop to quote the worst case for all of them.

A real quote for including CNC plastic machining should tell you four things beyond price: the stock form and grade, the tolerance band, the finish callout and the ship date. If any of those is missing, you are comparing numbers that will not survive contact with production.

MOQ is worth asking about directly. Many shops will not touch a single prototype because setup dominates. A shop set up for short runs will machine one part and still give you a firm date. That is the practical test.

Lead time depends on stock. Common grades like POM, PC and ABS are usually on the shelf. PEEK, PEI and PPSU in a specific thickness may need to be ordered, which adds days before the first chip. Tell the shop the grade and thickness early.

On confidentiality, the upload path matters. If the drawing is a new product, get an NDA in place before you send it. Ask where files are stored and who can open them.

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  Stock form and gradePlate, rod or near-net shape, and the exact grade number.
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  Tolerance band per featureNot one global number for the whole part.
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  Finish calloutRa value or a visual standard, whichever the part is judged on.
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  Ship dateA date, not a range of weeks.

A certificate on a wall proves a system, not a part. What you want on a plastic job is dimensional data from the actual part. Ask for a first article inspection report with the measured values, not just pass or fail. On a medical or automotive program, that report is usually a contract requirement anyway.

For medical device work, ISO 13485:2016 covers the quality system, and material traceability from resin lot to finished part is part of it. For automotive and EV, IATF 16949:2016 is the baseline. ISO 9001:2015 covers general industrial work. If your drawings leave the building, ISO 27001:2022 covers information security.

In-process checks matter on plastics because the material moves. A shop that measures only at the end will find a bowed part too late. A shop that checks after roughing can correct the finishing pass before the part is scrap.

Ask how many parts are inspected and when. On a 50-piece run of PEEK parts, full inspection is affordable. On 10,000 pieces it is sampled. Both can be right, but you should know which one you bought.

Most plastic parts ship as machined. Some need more. Vapor polishing smooths PMMA and PC without touching dimensions much, and it is the usual route to optical clarity. Bead blasting gives a matte, uniform look and hides tool marks on opaque grades.

Laser marking works on most engineering plastics with a minimum character height around 1.5 mm. Below that, the mark smears on POM and PA. If you need a small logo or a data matrix, raise it or plan for a printed label instead.

Painting plastics is possible but adhesion is the issue. Polyolefins like PP and HDPE need a flame or plasma treatment first. PC and ABS take paint far more readily. If colour is critical, confirm the process before the parts are cut, because a failed paint batch wastes the machined parts too.

Deburring deserves a mention. A sharp burr on a PC housing is a handling injury and a fit problem. Specify whether edges are broken, and by how much.