Local Plastic CNC Processing: 7 Checks Before You Order

• 1
  Key takeaways
• 2
  Plastic Resin Selection at a Glance
• 3
  What Local Plastic CNC Processing Can and Cannot Hold
• 4
  Materials That Machine Well and Materials That Fight Back
• 5
  Thin Walls, Deep Pockets and Other Geometry Traps
• 6
  Finish Options and What Each Resin Will Accept
• 7
  How to Judge a Local Shop Before You Send Files
• 8
  Lead Time, Order Size and Quote Pitfalls
• 9
  Seven Steps to Qualify a Shop

Plastic does not behave like aluminium on a machine table. It springs back under the cutter, it heats up and grows, and it moves again after you unclamp it. A shop that quotes the same tolerance for both materials is telling you something about how it works.

For most resins, plan on ±0.05 mm on a well-supported feature and ±0.1 mm on a long unsupported wall. POM and PC can hold tighter when the part is short and stiff. PMMA and PA usually cannot, and chasing ±0.01 mm on them wastes machine time.

Where a drawing calls for ±0.005 mm, that is metal territory. We hold ±0.005 mm on aluminium, stainless and titanium, and we say so plainly rather than promise the same number on a soft plastic.

The practical rule: tolerance follows stiffness. Short, thick, ribbed features hold tight. Long, thin, free-standing walls do not. If the function needs a tight band, change the geometry before you change the supplier.

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  Supported features±0.05 mm is routine on short, clamped sections.
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  Free-standing walls±0.1 mm or wider, especially above 60 mm tall.
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  Thermal driftLet the part cool before final inspection; measure at 20 °C.
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  DatumsMachine datums from the same setup that cuts the critical face.

ABS, POM, PC and PMMA cover most prototype and low-volume work. They cut fast, leave a decent as-machined finish, and take secondary operations without surprises. If your part is a housing, a bracket or a fixture, one of these four is usually the cheapest answer.

POM is the best of the common resins for anything that slides or meshes. It holds a sharp edge, machines to a smooth surface, and resists wear. It also carries internal stress from extrusion, so rough machine it, let it rest, then take a light finish pass.

PA and PEEK are the two that cause friction. Nylon absorbs moisture from the air and grows after machining, which ruins a tight fit measured a week later. PEEK is abrasive and expensive, so tool wear and stock cost both climb.

Carbon fibre filled grades add another problem: the fibre is abrasive and the dust needs extraction. They give high stiffness, and they eat cutters. Budget for both.

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  Cheap and quickABS, POM, PMMA for prototypes and jigs.
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  Tough and clearPC for guards and lenses when coolant is used.
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  High temperaturePEEK and PEI, at higher cost and slower feeds.
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  Moisture sensitivePA and POM benefit from a stress-relief pause.

Most plastic parts that fail inspection fail on geometry, not on the machine. A wall under 1.5 mm will deflect under clamping pressure alone, and no amount of toolpath tuning fixes a wall that thin. Thicken it or add a rib.

Deep pockets are the second trap. A pocket deeper than four times its width needs a long, slender cutter that bends and chatters. The finish goes cloudy and the corner radius drifts. Open the pocket, split the part, or accept a rougher floor.

Sharp internal corners are the third. A cutter has a radius, so an internal corner smaller than the tool leaves a fillet the drawing did not ask for. Specify a corner radius at least equal to the pocket depth divided by six, or expect a note back from the shop.

Holes have their own rules. A drilled hole in plastic tends to come out slightly oversize, and a reamed hole holds better. For a press fit, ask the shop to cut a test hole in the same resin batch.

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  Minimum wallKeep above 1.5 mm; 2 mm is safer for structural parts.
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  Pocket depthStay under 4 × tool diameter to avoid chatter.
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  Corner radiiMatch the radius to the deepest cut, not to the drawing minimum.
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  Press fitsTest on scrap from the same resin lot before the run.

As-machined plastic usually sits around Ra 1.6–3.2 μm, which is fine for hidden parts and most fixtures. Visible covers and lenses need more, and that is where finishing becomes a separate job with its own lead time.

Bead blasting gives a uniform matte surface and hides tool marks. Tumbling softens edges. Brushing leaves a directional grain that shows on PMMA and PC. Polishing can reach Ra 0.2–0.8 μm on acrylic and PC, though it needs a careful hand to avoid sink marks.

Coating is the option people forget. Electroless nickel and silver plating are normally metal processes, but we run them on machined metal parts, not on plastic. For plastic, the realistic choices are paint, laser marking and vapor polish.

Laser marking works on most dark and light plastics with a minimum character height of 1.5 mm. Below that, the mark fills in and stops being readable. Specify the mark height on the drawing so the shop can check it before cutting.

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  As-machinedRa 1.6–3.2 μm, good for hidden faces.
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  Bead blast or tumbleUniform matte, hides tool marks.
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  PolishRa 0.2–0.8 μm on PMMA and PC with care.
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  Laser markingMinimum character height 1.5 mm.

A local plastic CNC processing supplier should be able to answer four questions without checking with anyone: which machines will run the part, what tolerance they hold on that resin, how they inspect it, and when it ships. If the answer to any of them is vague, you are probably talking to a broker.

Ask about machine count and spindle type. A shop with a mix of three-axis, four-axis and five-axis centers can choose the right setup instead of forcing your part onto one machine. Five-axis work pays off when a part has features on several faces, because it cuts them in one setup and removes a stack of fixturing error.

Ask how inspection is done. A shop that measures only with calipers will miss a true position callout. Look for a shop that inspects 100% before shipment and can send reports on request, including raw material check and in-process monitoring.

Ask about certifications only if the part goes into a regulated product. ISO 9001:2015 covers general quality systems, IATF 16949:2016 covers automotive, ISO 13485:2016 covers medical devices, and ISO 27001:2022 covers information security for your files. A shop that holds all four can take work in those sectors without a new audit each time.

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  Machine list127 CNC machines, including 16 simultaneous 5-axis centers.
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  Size rangeUp to 4,000 mm maximum processing size.
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  Inspection100% before shipment, reports on request.
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  CertificationsISO 9001, IATF 16949, ISO 13485, ISO 27001.

Plastic prototypes move fast. At our plants, quotation and DFM analysis come back within 12 hours, production can start within 24 hours, and parts ship in 3–5 days. Those numbers assume the drawing is complete and the resin is in stock.

There is no minimum order quantity here, so a single prototype and a 10,000-part run both go through the same quoting process. Watch for shops that hide a setup charge until after you approve the price, or that quote finishing as a percentage instead of a real line item.

The most common quote mistake is a tolerance gap. The customer sends a drawing with a tight general tolerance block, the shop quotes to it, and the first article fails on a plastic wall that was never going to hold that band. Fixing it costs a week.

Ask two more things. First, whether the shop will flag design problems before cutting, and second, what happens to your files. A written NDA and a stated file-handling policy cost nothing to request and tell you how the shop treats customer data.

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  Quote turnaround12 hours, with free DFM analysis.
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  Order rangeOne prototype to 10,000+ parts, no minimum.
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  Late-delivery historyBelow 2% historically.
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  File handlingUploads kept secure and confidential; NDA on request.