Precision CNC plastic processing: how tight-tolerance plastic parts are actually made

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  Key takeaways
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  Why precision CNC plastic processing behaves differently from metal
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  Which thermoplastics machine well, and which fight back
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  What ±0.005 mm really means on a plastic part
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  Fixturing, toolpaths and the mistakes that scrap parts
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  How a precision plastic job runs here
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  CNC plastic processing vs injection molding

A machined plastic part fails for reasons a metal part rarely does. Thermoplastics conduct heat poorly, so the cutting edge cannot push heat into the workpiece. It has to leave with the chip. When the chip stays in the cut, the material rubs, smears, and re-welds behind the tool. The result looks like a rough patch, not a chip.

Stiffness is the second difference. Unfilled POM and PA have a modulus roughly one twentieth that of aluminium. Push a 12 mm end mill too hard and the tool deflects, not the part. That deflection shows up as taper on a deep wall or a bowed floor in a pocket. The machine holds position; the tool and the part are moving.

Thermal expansion matters as much as cutting force. Plastics grow and shrink far more than steel per degree. A part machined warm and measured cold will not match its drawing. We let parts rest before final inspection so the number on the report reflects the part, not the shop temperature.

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  Sharp, polished flutesUncoated carbide with a high rake angle cuts cleaner than a coated general-purpose tool.
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  Air or mist, not floodFlood coolant can shock some plastics; compressed air clears chips and controls heat.
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  Climb millingReduces rubbing on the finished wall and improves surface finish on most thermoplastics.

ABS, PC, PMMA, POM, PA, PEEK, PP, HDPE and carbon fibre all run on our machines. They do not run the same way. POM and HDPE cut cleanly and hold a sharp edge. They make good starting points for anyone new to plastic machining. Both form long stringy chips, so chip evacuation needs attention more than tool wear does.

PEEK and PA absorb moisture from the air. Machine a wet nylon part and it will shrink as it dries, moving dimensions after the last cut. We dry nylon before machining and keep the finishing passes light. PEEK is dimensionally steadier but abrasive on tooling when it carries glass or carbon fibre, so tool life drops and we plan a fresh cutter for the final pass.

PMMA and PC are transparent and scratch easily. Any chip dragged across a finished face leaves a mark you cannot polish out without changing the geometry. We protect surfaces with film where geometry allows and keep the finishing pass separate from roughing. Carbon fibre is a different case: it wears carbide quickly and the dust needs extraction.

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  EasyPOM, HDPE, ABS, PMMA
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  ModeratePC, PA (dry first), PP
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  DemandingPEEK, carbon fibre, glass-filled grades

Our process capability reaches ±0.005 mm (±0.0002 in) and surface finishes from Ra 0.2–0.8 μm on the fine end, with Ra 0.8–1.6 μm as the usual machined target. Those numbers are real, but they are not a blanket promise across every feature. Tolerance is a property of the feature, not of the shop.

A 6 mm bore in POM, held in soft jaws and bored in one pass, can sit inside ±0.005 mm. A 300 mm flat plate in the same material will not. It moves with humidity and temperature, and its own mass makes it flex under the tool. On larger plastic parts we quote realistic limits and mark the critical dimensions instead of tightening everything.

Call out the features that matter. A drawing with twenty tight tolerances costs more than a drawing with three, because every extra one needs a separate setup, a temperature-controlled check, and sometimes a second operation. Send the mating part or the assembly fit. That tells us which numbers are functional and which are habit.

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  Holds tightSmall bores, slots, and short walls
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  Needs careThin ribs, deep pockets, long unsupported spans
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  Plan for itFlat panels over 200 mm, parts with wall under 1 mm

Soft jaws machined to the part profile are the default. Aluminium jaws with a rubber or urethane liner work for most jobs. Vacuum plates suit thin flat parts but lose grip as the part thins, so we leave a sacrificial web and cut it last. Double-sided tape helps on prototypes and small runs; it does not survive heavy roughing.

Roughing removes bulk with a generous radial stepover and a light axial depth. One rule of thumb: keep radial engagement low enough that the tool sees a steady chip load, not a sudden bite. For finishing, a single continuous pass along a wall beats several overlapping passes. Overlap marks show up on transparent and matte finishes.

Deburring is where plastic differs most from metal. A scraper leaves a white stress mark on dark PC or ABS. We use a sharp blade at low angle, then a light abrasive pass. For threads, a cut tap works in POM and PA; form taps can split thin walls. Every part gets a check before it ships, and inspection reports are available on request.

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  Watch the chipColour change means heat is building. Stop and adjust.
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  Measure coldLet the part reach room temperature before final inspection.
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  Separate finishingNever let a roughing pass touch a finished wall.