CNC plastic processing quality index

• 1
  What the CNC plastic processing quality index actually measures
• 2
  Dimensional accuracy and where plastic stops cooperating
• 3
  Surface integrity: roughness, burrs and crazing
• 4
  Geometric tolerance: flatness, parallelism, position and runout
• 5
  Retained material properties and batch consistency
• 6
  Which plastic quality indicator to tighten for your part

The CNC plastic processing quality index is a set of five indicators that together describe whether a machined plastic part will work in its assembly. They are dimensional accuracy, surface integrity, geometric tolerance, retained material properties, and batch consistency. No single number stands alone. A part can hold ±0.005 mm on every feature and still fail because the cut surface crazed two days later.

Plastic behaves differently from aluminum or steel on the same machine. It springs back under the tool, it heats up fast, and it moves after the fixture is released. That is why the index is written as a range of indicators rather than one tolerance callout. The drawing tells us what the part must do. The index tells us what the process can hold while it does that.

We machine ABS, PC, PMMA, POM, PA, PEEK, PP, HDPE and carbon fibre on 127 high-precision CNC machines, including 16 simultaneous 5-axis centers. Every job runs 100% inspection before shipment, with raw material check, in-process monitoring and final inspection. Reports are available on request.

• 1
  Dimensional accuracyFeature size against the nominal, measured at 20 °C.
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  Surface integrityRoughness, burrs, and whether the cut surface stays sound.
• 3
  Geometric toleranceFlatness, parallelism, position and runout.
• 4
  Retained propertiesWhether the polymer still has its strength and color.
• 5
  Batch consistencyPart 1 and part 10,000 measure the same.

Dimensional accuracy compares a measured feature to the nominal on the drawing. On metal, the limit is usually the machine. On plastic, the limit is the material. POM and PEEK hold tight numbers because they are stiff and have low thermal expansion. PP and HDPE move with every degree of shop temperature, so a ±0.025 mm callout on a 200 mm PP part is a conversation, not a given.

We routinely hold ±0.005 mm (0.0002 in) on critical plastic features when the geometry and material allow it. That number is not a default for every feature on every part. It is the floor of what our machines and metrology can verify. Standard tolerances such as ISO 2768 medium, around ±0.1 mm, are comfortable on a fully equipped machine.

Three things drive the achievable number. Wall thickness, because thin walls deflect under clamping. Feature depth-to-width ratio, because deep pockets need long tools that push sideways. And the measurement itself, because a caliper on a soft polymer reads differently than a CMM touch probe at controlled temperature.

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  Stiff polymersPOM, PEEK, PA with glass fill hold the tightest sizes.
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  Soft polymersPP, HDPE, LDPE need looser callouts or a temperature-controlled check.
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  Long partsAbove 500 mm, thermal growth dominates the tolerance stack.
• 4
  Thin wallsBelow 1.5 mm, clamping force moves the part before the cutter does.

Surface integrity covers roughness, edge quality and whether the cut surface stays sound over time. Roughness is expressed as Ra. Machined plastic typically lands in Ra 1.6–3.2 μm as-cut, Ra 0.8–1.6 μm with a finishing pass, and Ra 0.2–0.8 μm after polishing or a fine stepover. The polymer decides which of those is realistic.

Burrs form differently in plastic than in metal. A sharp cutter shears acrylic cleanly. The same cutter on PP leaves a frayed edge that a deburring tool makes worse. We switch to a climb cut, a smaller chipload, or a support fixture behind the exit face. Sometimes the answer is a chamfer on the drawing instead of a square edge.

Crazing is the failure most engineers miss. A cut surface can look fine under shop light and show micro-cracks under magnification. It happens when the tool rubs instead of cutting, or when coolant attacks the polymer. PC and PMMA are the usual suspects. If a transparent part is going into a stressed assembly, we inspect the cut edge at magnification before it ships.

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  Acrylic and PCWatch for crazing; use sharp tooling and compatible coolant.
• 2
  POMMachines clean, but chips wrap; air blast beats flood coolant.
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  Carbon fibreEdge fraying and dust control matter more than Ra.
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  PEEKHolds a fine finish but generates heat; control the feed.

Geometric tolerance is where plastic machining separates from metal machining. A flat plate in POM may measure flat on the CMM and bow 0.3 mm the next morning. The cause is internal stress released by removing material. The fix is not a tighter tolerance. It is a different roughing strategy, a stress-relief cycle, or a change in material stock.

Flatness and parallelism on plastic parts above 300 mm are usually called out at 0.05–0.1 mm, not at the dimensional tolerance. Position tolerance on hole patterns holds better because the holes are drilled in one setup. Runout on a turned plastic shaft depends on how the part is held, not on the lathe accuracy.

For parts that must stay flat, we rough, let the part rest, then finish. The rest period depends on the polymer and the amount of material removed. PEEK and PAI benefit from an anneal between operations. PP and HDPE settle faster but move more overall. Both facts should be on the process sheet, not discovered at assembly.

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  Rough and restRemove bulk stock, let stress relax, then finish.
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  Symmetrical stock removalMachine both faces evenly to balance internal stress.
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  Fixture on a finished faceNever clamp on a surface that will stay visible.
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  Vacuum or soft jawsPoint contact deforms plastic; spread the load.

Retained material properties ask whether the polymer still behaves like the polymer you specified. Machining generates heat. Too much heat at the cut zone can change crystallinity in PEEK, leave burn marks in PC, or drive moisture out of PA and cause later dimensional drift. The part passes inspection today and fails the drop test next month.

Batch consistency is the index item that matters most to buyers running production. Part 1 and part 10,000 must measure the same. That comes from fixed process parameters, documented setups, and in-process monitoring rather than operator feel. We run 100% inspection before shipment and hold a 99.99% qualification rate across this work.

The practical test for batch consistency is a first article inspection report plus a documented process. If a supplier cannot show you the setup sheet, the fixture photo and the in-process check interval, the tolerance on the drawing is a hope. Ask for the report before you place the second order, not after.

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  Heat controlSharp tooling, moderate speed, air or mist over flood where possible.
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  MoistureDry PA and PEEK stock before machining and seal after.
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  Documented setupSame fixture, same parameters, same inspection interval.
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  First article reportAvailable on request for production runs.