China CNC Processing Plastic Parts: A Practical Guide

Most plastic parts in our shop are cut from stock plate or rod. That already narrows the list, because a polymer that flows well in a mold is not automatically a polymer that machines cleanly. ABS, PC, PMMA, POM, PA, PEEK, PP, HDPE and carbon-fiber-filled grades all cut on a CNC, but they fail in different ways. Some gum up the cutter. Some chip at the edge. Some move after the cut because internal stress was locked into the extruded plate.

POM (acetal, often called Delrin) is the easy one. It holds tight tolerances, produces short chips, and finishes well. PC is strong and transparent but softens under friction, so toolpaths and coolant need attention. PMMA gives optical clarity if you polish after machining. ABS is cheap, forgiving, and a good default for enclosures and brackets. PA (nylon) absorbs moisture and will change size after machining if it is not dried and conditioned first.

Then come the high-performance grades. PEEK and PEI hold stiffness and chemical resistance at high temperature, which matters in medical and semiconductor fixtures. Carbon-fiber-filled PEEK or PA is abrasive; it wears carbide fast, so we plan tool changes and sometimes use diamond-coated cutters. PVDF and PTFE are soft and creep under clamp pressure, so light passes and custom soft jaws are the norm.

The practical question is not which plastic is best. It is which plastic survives your service temperature, chemical exposure, and load case while still being machinable at the geometry you drew. If you are unsure, tell us the environment and we will suggest two or three candidates before quoting.

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  Good general-purpose choicesABS, POM, PC, PA, PMMA cover most prototypes and fixtures.
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  High-temperature or chemical exposurePEEK, PEI, PVDF, PTFE when metal is not an option.
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  Wear or stiffnessCarbon-fiber-filled grades, with faster tool wear accepted.

Plastic is roughly ten times more thermally expansive than steel. A part machined at 25 °C and measured in a cold inspection room will not match the drawing. We machine to a defined temperature, measure at the same temperature, and note the condition on the report. For tight features we sometimes let the part rest overnight before the finishing pass, because stress relief changes the shape.

Clamping is the second problem. Plastics dent, and a dent is a dimensional error. We use soft jaws, vacuum chucks, or sacrificial backing plates so the part is supported across its full face. Thin walls below 1 mm are machined with light radial cuts and high spindle speed to avoid chatter. Deep pockets need a smaller tool with longer reach, which deflects; we compensate in the CAM model rather than pushing the feed.

The tolerance your drawing asks for should match the feature. A mounting hole pattern can sit at ±0.05 mm and work fine. A bearing bore or a mating slide may need ±0.005 mm. Asking for ±0.005 mm across a 400 mm plastic part is usually wasted cost, because the material will move with humidity and temperature once it leaves the shop.

Inspection is not a final step for us. We check raw material certificates, monitor critical dimensions during the run, and inspect 100% of parts before shipment. Reports are available on request. That matters more in plastics than in metals, because one bad batch of resin or one wet nylon bar can ruin an otherwise correct program.

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  Soft jaws and vacuum chucksDistribute clamp load so plastic does not dent.
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  Temperature controlMachine and measure at the same temperature.
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  Feature-based toleranceTight only where the function demands it.

A 3-axis mill can cut most plastic brackets, covers and plates. It struggles with undercuts, compound angles, and features on five faces. Each extra setup means re-clamping, and every re-clamp in plastic risks a witness mark or a shifted datum. That is where simultaneous 5-axis pays for itself.

Our shop runs 16 simultaneous 5-axis machining centers, 12 four-axis mills, 27 three-axis machines, and 16 mill-turn centers. Large travels reach 4,000 × 400 × 150 mm, which covers long extruded profiles and large panels. For contoured ducts, intake housings, and complex manifolds, 5-axis lets us cut the whole surface in one setup with a ball-nose tool, keeping the surface continuous and free of step lines.

5-axis is not automatically better. For a simple flat plate with a few holes, a 3-axis machine is faster and cheaper. The honest rule is this: if the part has three or more angled faces, a deep undercut, or a curved surface that must blend smoothly, 5-axis usually wins on total cost. If it is flat and prismatic, it does not.

Mill-turn adds another option. A plastic bushing or threaded insert can be turned and milled in one machine, so the bore and the cross-features stay concentric. That removes a second setup and the error that comes with it.

Machined plastic comes off the machine with visible tool marks. For many parts that is acceptable. When it is not, we bead blast, tumble, brush or polish, and we can laser mark with a minimum character height of 1.5 mm. Anodizing and plating are metal processes, so they do not apply to plastics; painting, printing and vapor polishing are the usual options, and we will say plainly what a given polymer can and cannot take.

Cost in plastic machining is driven by setup count, feature depth, and tolerance. A part with a deep narrow pocket takes many hours of light passes. A part with a loose tolerance and open geometry runs fast. If the budget is tight, the first question to ask is whether the tolerance can relax and whether two setups can become one.

Lead time is short because there is no tooling. DFM feedback and a quotation come back within 12 hours, production can start within 24 hours, and finished parts ship in 3 to 5 days. There is no minimum order quantity; one prototype and a 10,000-part run go through the same quoting process.

Send a STEP file and a 2D drawing with tolerances and critical features marked. If you only have a print, send the print. We can suggest material, flag features that will not machine cleanly, and quote from there. Uploads stay confidential, and an NDA is available on request.

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  Finishes for plasticsBead blasting, tumbling, brushing, polishing, laser marking.
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  Main cost driversSetup count, pocket depth, and how tight the tolerance is.
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  What to sendSTEP plus a drawing with critical dimensions marked.