Experience Plastic CNC Machining: What to Check Before You Order

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  What decides a plastic machining order
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  Which plastic suits which requirement
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  Why experience plastic CNC machining starts with the resin
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  Tolerance and surface finish you can actually hold
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  Equipment and setup that fit plastic parts
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  Lead time, quantity and what the quote should include
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  Inspection, certification and documentation
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  Common pitfalls when sourcing plastic machining
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  When plastic machining is the wrong process
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  Step by step: from RFQ to shipped parts

A metal shop and a plastic shop look similar on the floor. They are not. Plastics cut at 3 to 10 times the surface speed of aluminium, they spring back after the tool passes, and they move when the temperature in the room changes. A shop that quotes your PEEK bracket the same way it quotes an aluminium bracket will miss the tolerance.

Start by naming the resin and the stock form. Extruded plate carries internal stress from the extrusion die. Cast plate is annealed and stays flatter after you remove material. For a 200 mm long part with a flatness callout, that difference alone decides whether the part passes.

Then check the wall thickness. Below about 1.5 mm in most filled resins, the cutting force pushes the wall away from the tool instead of cutting it. You get chatter marks and a taper you did not ask for. If your design needs a 1 mm rib, say so in the RFQ so the shop can plan a support or a finishing pass.

Moisture matters too. Nylon and POM absorb water from the air. A dry-machined nylon part can grow 0.2 to 0.5 percent within days. If your drawing has a tight bore, ask whether the shop machines wet or dry and whether it conditions the stock first.

±0.005 mm is a real number we hold on metal parts, and on rigid plastics like POM, PC and PEEK in small sizes. It is not a blanket promise across every resin. A 300 mm long PP duct will not hold ±0.005 mm because the material moves more than that on its own.

Ask the shop to separate the tolerances by feature. A bore diameter, a bolt hole pattern and an overall length do not cost the same to hold. Tightening one critical dimension is usually cheap. Tightening every dimension on the drawing by habit is what doubles a quote.

Surface finish follows the same logic. Ra 0.8–1.6 μm is a normal machined finish in most engineering plastics. Ra 0.2–0.8 μm needs a finishing pass, sharper tooling and more time. As-machined at Ra 1.6–3.2 μm is fine for brackets and internal parts nobody sees.

One caution on callouts. A Ra number on a plastic drawing is harder to verify than on metal because the material smears. If the surface is cosmetic, describe it as a visual standard or a sample instead of a single Ra value.

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  Critical features onlyMark the two or three dimensions that matter and let the rest run to general tolerance.
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  Datum choicePick a datum you can actually clamp on, not a thin flange.
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  Edge breakPlastic edges chip. Specify 0.3 to 0.5 mm chamfers or radius notes.

For plastic work, the useful questions are about spindle speed, vacuum fixturing and tool geometry. A spindle that tops out at 8,000 rpm cannot run a 3 mm cutter in PEEK at the surface speed the material wants. You end up feeding slow and burning the edge.

Multi-axis capability helps when a part has undercuts, angled holes or a curved face that would otherwise need three setups. Each setup is another chance to introduce a locating error, and in plastic those errors accumulate fast because the part is light and easy to shift.

Ask how the shop holds the part. Soft jaws, vacuum chucks and sacrificial tabs are normal for plastic. A shop that clamps a thin PC cover in a standard vise will bow it and then measure the bow as if it were the part.

We run 16 simultaneous 5-axis machining centers, 16 mill-turn centers and 27 three-axis machines, with a maximum processing size of 4,000 mm. That range covers everything from a 20 mm PEEK seal to a long PVC duct.

Plastic parts usually move faster than metal parts because the material is softer and tool wear is lower. We quote and return a free DFM analysis within 12 hours, and production can start within 24 hours. Parts ship in 3 to 5 days on standard work.

There is no minimum order quantity. A single prototype and a 10,000-part run go through the same DFM review. That matters when you are validating a design and do not want to commit to a mold or a large batch.

Read the quote line by line. Material grade, stock form, machining hours, finishing, inspection and packaging should each be visible. A single lump-sum price hides where the cost sits, and you cannot negotiate a line you cannot see.

Watch for quotes that name a resin family but not a grade. PC is not one material. A 20 percent glass-filled PC machines very differently from an unfilled optical grade, and the price gap is real.

Ask what gets measured and how it is recorded. We inspect 100 percent of parts before shipment, covering incoming raw material, in-process monitoring and final inspection, with reports available on request.

For medical and automotive work, the paperwork matters as much as the part. We hold ISO 9001:2015, IATF 16949:2016, ISO 13485:2016 and ISO 27001:2022. The last one covers information security, which is what your legal team will ask about before they send a CAD file.

If your part is cosmetic, agree on the inspection method up front. A dimensional report will not tell you whether a scratch is acceptable. Send a boundary sample or a photo standard and have it referenced in the PO.

For regulated programs, ask early whether the shop can supply material certificates and traceability back to the lot. Getting that after the run is often impossible.

The most expensive mistake is treating plastic like slow aluminium. Drawings copied from a machined metal part often carry tolerances, radii and wall thicknesses that make no sense in polymer. The shop quotes them, then struggles, then the schedule slips.

The second is ignoring finishing. Bead blasting, tumbling and laser marking all interact with the material. Laser marking on dark POM gives a low-contrast mark. If the mark must be readable, say so before the parts are cut. Minimum character height for laser marking is 1.5 mm.

The third is shipping without a fixturing plan for the finished part. A machined plastic component can warp in transit if it is packed flat under load. Ask how parts are packed and whether they are supported.

Finally, do not let the shop anneal or stress-relieve without telling you. Some plastics need it, some warp more if you do it wrong. It should be a stated step on the routing, not a surprise.

CNC is a subtractive process. It removes material from a solid block. If your part has a hollow internal channel, a lattice or an internal cavity you cannot reach with a tool, machining will not produce it in one piece.

For very high volumes, injection molding is cheaper per part once tooling is amortized. Machining wins on low and mid volume, on large parts, on tight schedules and on designs that are still changing.

If the part is thin-walled and complex, 3D printing may be the better first step. We run SLM, SLA and SLS printers alongside the CNC floor, so a prototype can be printed for form and fit and then machined in the final resin for function.

And if the requirement is purely cosmetic with no dimensional load, vacuum casting in a silicone tool can produce a small batch of urethane parts that look like molded plastic at a lower cost.