How to Choose a Plastic CNC Processing Service

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  Five things to settle before you send an RFQ
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  When plastic CNC beats molding, and when it does not
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  Which plastic to machine, and why it changes everything
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  What 3-axis, 4-axis and 5-axis really buy you in plastic
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  Tolerance and surface finish you can actually hold on plastic
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  How to screen a plastic CNC processing service before you commit
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  How to read a quotation without getting surprised later
• 8
  Seven steps to qualify a plastic machining supplier

Plastic CNC processing starts with the polymer, not the machine. A grade that machines cleanly at one feed rate can smear or chip at the same settings if it has glass or carbon filler. When you write the RFQ, name the grade and the filler content, not just the family. ABS and POM are the forgiving ones. PEEK and carbon fibre filled grades are not.

Thermal behavior decides the cutting strategy. POM and HDPE have low melting points and cut with a gummy chip if the tool dwells. PEEK and PMMA crack rather than deform, so the tool needs a positive rake and a light finishing pass. On a 5-axis machine the same logic holds along the curved surfaces, where a heavy stepover leaves marks you cannot polish out of a filled grade.

Hygiene and chemical exposure matter more than hardness in some industries. Medical device housings often run in PEEK or PC because they survive repeated sterilization. Aerospace ducts lean toward lightweight, dimensionally stable grades. Automotive under-hood parts need heat resistance, which usually means a different grade than the interior trim right beside it.

A useful rule: pick the grade the end product needs, then let the shop tell you the machining consequence. Do not let a shop swap your grade for one that is easier to cut. If they suggest an alternative, ask what property changes, and get it in writing.

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  ABS, PC, PMMA, POM, PACommon engineering grades, good availability and predictable cutting.
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  PEEK, PP, HDPEChemical and heat resistance; PEEK needs slower feeds and sharp tooling.
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  Carbon fibre filledStiff and light, but abrasive on cutters and harder to finish.

A 3-axis mill cuts pockets, slots, flats and profiles from one direction. For a bracket, a spacer or a simple cover, that is all the capability the part needs, and the setup is short. Most plastic parts in the 100 mm to 400 mm range never need more.

The limit appears on parts with features on several faces. A pump housing with bores on two sides, or a manifold with angled ports, needs either multiple setups or a rotary axis. Every extra setup adds a fixture, an alignment step and a chance for error. On plastics the alignment risk is worse than on metal because soft materials can shift slightly in the vise.

Simultaneous 5-axis changes the plan. Sixteen simultaneous 5-axis machining centers let us cut contoured surfaces and side features in one continuous pass, with the tool kept at the right angle to the surface. That matters for ducting, shrouds, ergonomic grips and any part where the surface finish changes direction. The trade-off is programming time, so it is worth it on complex geometry and not on a flat plate.

Fixture design deserves a line in the quotation. Thin plastic walls deflect under clamping force. A shop that machines plastics well will use soft jaws, vacuum plates or support material rather than cranking a vise down. Ask how the part is held, especially if the wall is under 1.5 mm.

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  3-axisFlats, pockets, profiles; 27 three-axis machines on the floor.
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  4-axisAdds indexed faces; 12 four-axis mills and 16 mill-turn centers.
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  5-axisContoured and angled features in one setup; 16 simultaneous centers.

Tolerance on plastic is not one number for the whole part. A printed ±0.005 mm applies to a specific feature, usually a small bore or a short datum. Stretch it across a 1,000 mm length and thermal expansion, residual stress and cutter deflection all push the real result wider. Good drawings separate the critical features from the general ones.

Finish is the same story. Ra 0.8–1.6 μm is the normal as-machined range for a fine finishing pass. Ra 0.2–0.8 μm takes slower feeds and often a second operation, and it is best reserved for sealing faces, sliding surfaces and optical areas. Ra 1.6–3.2 μm is fine for brackets and internal parts. Ask which surfaces need the fine callout, because polishing an entire part doubles the time.

Plastic adds two failure modes that metal does not have. Filled grades chip at the exit edge of a cut, and transparent grades show every tool mark. Both are managed with cutter geometry, feed rate and a finishing pass, but they cannot be removed from the drawing after the fact. Mark the cosmetic surfaces on the model.

Inspection should match the tolerance claim. We inspect 100% of parts before shipment and can supply reports on request, covering raw material check, in-process monitoring and final inspection. If your drawing has a critical dimension, say so, and the report should list it.

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  ±0.005 mm ( ±0.0002 in )Small features and datums; state which ones.
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  Ra 0.8–1.6 μmStandard fine machined finish on most plastics.
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  Ra 0.2–0.8 μmSealing and sliding surfaces, extra operation.

Start with certifications that match your industry. ISO 9001:2015 covers general quality management. IATF 16949:2016 is the automotive expectation. ISO 13485:2016 applies to medical devices. ISO 27001:2022 covers information security, which matters if your drawings are sensitive. A supplier that holds the ones your industry needs has already been audited against them.

Ask about capacity in the range your part sits in. The maximum processing size here is 4,000 mm, with a 4,000 × 400 × 150 mm travel on the large machines, 750 × 1,150 × 550 mm and 600 × 600 × 600 mm on the medium machines, and 500 × 500 × 450 mm plus 500 × 310 × 200 mm on the compact ones. A Ø400 mm rotary table handles round work. A shop with the right envelope will say so quickly.

Quantities are a screening question too. Some suppliers quietly price small runs high to push you toward volume. We run from one prototype to 10,000+ part runs with no minimum order quantity, which keeps an early design iteration affordable. Confirm the number in writing before you plan a build.

Confidentiality is often the last thing asked and the first thing regretted. Uploads should be secure and confidential, an NDA should be available on request, and the shop should not show your parts in a public portfolio. If your program is under NDA, say so on day one.

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  CertificationsISO 9001, IATF 16949, ISO 13485, ISO 27001.
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  Capacity127 high-precision CNC machines, 3 wholly-owned plants, 7,600 m².
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  QuantitiesOne prototype to 10,000+ parts, no minimum order quantity.

A useful quotation separates material, machining, finishing, inspection and freight. A single lump sum hides where the cost sits, and you cannot compare two suppliers who bundled differently. Ask for the material grade and the stock form, since a 20 mm plate and a 60 mm block of the same polymer can differ a lot in price.

Lead time should come with its own breakdown. Here the standard flow is a quotation and free DFM analysis within 12 hours, production can start within 24 hours, and parts ship in 3–5 days. Treat those as the shop's normal rhythm, not a guarantee on your specific part. Complex 5-axis work or an unusual grade can take longer, and a supplier should say so up front.

Watch for missing line items. Finishing is often left out: anodizing, plating, powder coating, bead blasting, polishing and laser marking all add operations. Laser marking has a minimum character height of 1.5 mm, so a tiny logo may need a different method. If finishing is not on the quote, it is not in the price.

Finally, ask what happens if a dimension is out. A shop with a documented inspection process will tell you how it reworks or remakes. One that only promises to try harder is telling you something about its process control.

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  BreakdownMaterial, machining, finishing, inspection, freight as separate lines.
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  Timeline12-hour quote and DFM, 24-hour start, 3–5 day shipping.
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  FinishingName the finish and the surfaces; it is a separate operation.