Engineering Plastics Machining Comparison: What Cuts Clean and What Fights You

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  Engineering plastics machining comparison by grade
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  Why plastics do not machine like aluminum
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  Semi-crystalline and amorphous grades behave differently at the cutter
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  Glass and carbon fill change the tooling plan, not just the strength
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  Tolerance, surface finish and thermal growth
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  Residual stress, annealing and dimensional drift
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  Cost drivers and lead time realities
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  Inspection and when plastic is the wrong choice

Thermal conductivity of unfilled engineering plastics sits around 0.2 to 0.5 W/m·K. Aluminum is above 200. Heat from the cut has nowhere to go, so it stays at the edge and softens the chip instead of letting it flow away. The result is smearing, stringy swarf and a built-up edge that rubs rather than shears.

Elastic modulus is the second difference. Unfilled POM and PA run 2 to 4 GPa. Glass-filled grades reach 10 to 20 GPa but lose toughness. Clamping force that would be normal on steel will deflect a plastic wall, and the part springs back after the vise opens. Measure after unclamping, not during.

Spindle speed is not the limit. Plastics cut 5 to 20 times faster than aluminum in surface speed terms, but feed per tooth and chip clearance decide the finish. A light chip load and a sharp, high-rake cutter beat a fast spindle every time.

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  Use sharp, polished carbideHigh rake, two flutes for chip room. Dull edges rub and melt.
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  Air blast over flood coolantCoolant helps on PEEK and PEI but can craze PC and PMMA.
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  Clamp lightEnough to hold, not enough to deflect. Support thin walls from behind.

Semi-crystalline grades (POM, PA, PEEK, PTFE, PE, PP) hold dimensions well after machining. During the cut they move because of stress relief and, in some grades, crystallization effects near the surface. A rough pass, a pause, then a finish pass removes most of that movement before the final dimension is set.

Amorphous grades (PC, PMMA, PEI, PSU) cut to a cleaner edge and usually a better finish. They are also notch-sensitive. A sharp internal corner or a drilling breakout becomes a crack starter. Radius internal corners where the design allows, and drill with a pilot and a light breakout feed.

The practical rule: semi-crystalline for wear parts and dimensions that must hold in humidity. Amorphous for optical clarity, stiffness and clean edges. When the part needs both, PEEK or PEI with a radiused corner and an anneal cycle is the usual compromise.

Adding 20 to 30 percent glass fiber raises modulus and cuts thermal expansion. It also drops tool life by a factor of three to ten versus the unfilled resin. The cutting edge dulls into rubbing, heat climbs, and the finish degrades before the dimension does. Plan for more tools and a shorter roughing cycle.

Carbon-fiber-filled grades are stiffer and conductive. The dust is conductive too, so it threatens electronics and needs extraction at the machine. Diamond-coated or PCD tooling pays back on any run above a few dozen parts. On short runs, uncoated carbide with a frequent tool change is cheaper.

Filled grades also machine drier. The fiber does not melt, so the chip breaks shorter and the edge stays cleaner. That helps chip evacuation and hurts nothing except the tool. Budget tooling as a real cost line on glass-filled PEEK, not as a rounding error.

POM is the benchmark. With good fixturing it holds ±0.05 mm on a 100 mm dimension, and Ra 0.8–1.6 μm is routine. Ra 0.4 μm is achievable with a sharp polished carbide cutter and a light finish pass. It is also stable in humidity compared with PA, which is why it is the default for fixtures and wear plates.

PEEK reaches ±0.025 mm on small features with sharp carbide and conservative depths of cut. PTFE does not. It creeps under clamp pressure and has a coefficient of thermal expansion around 100 to 130 μm/m·K, so tolerances tighter than ±0.1 mm on 50 mm are unreliable no matter how careful the setup.

Thermal growth matters on any part longer than about 200 mm. A 10 °C shop swing moves a 300 mm POM part by roughly 0.3 mm. Let the blank sit in the shop before the finish pass, and record the temperature when the final dimension is cut. That record is worth more than a tighter tolerance callout.

Extruded rod and plate carry residual stress from the mill. Machine one side and the part bows. The fix is a stress-relief anneal before machining, or a rough pass followed by an anneal and a finish pass. PEEK and PEI both respond well to a controlled ramp and slow cool.

PA is the exception that catches people. It absorbs moisture, swelling 1 to 3 percent depending on grade and conditioning. A part cut to tolerance dry will move after it equilibrates. Specify the moisture condition on the drawing, or machine oversize and finish after conditioning.

For thin walls and long parts, expect drift even after annealing. Design in a finishing allowance and plan a second op. It is cheaper than scrapping a batch because the part was measured hot and dry.

Material dominates the cost on PEEK and PTFE. PEEK is often 20 to 50 times POM per kilogram, so a design change that saves 30 percent of the blank is worth more than a faster cycle. PTFE is cheaper but slower to hold, so labor moves up the list.

Tooling is the second line on filled grades. Glass-filled PEEK can wear out several cutters on one job. On unfilled POM, tool cost is noise. The comparison only makes sense when tooling and scrap are counted with the material price.

Lead time follows the same split. Unfilled POM, PA and PC are usually in stock and ship in days. PEEK and PEI plate can need a mill order, and that sets the schedule before the machine does. Ask for the material lead time separately from the machining lead time.

Inspect plastics at the same temperature they will run at, or note the difference. A caliper reading on a warm part is not a dimension. For critical features, use a CMM with a light touch and record the shop temperature with the result.

Plastic is the wrong choice when the part needs stiffness above roughly 20 GPa, continuous service above 250 °C, or true electrical conduction. It is also wrong when the tolerance is tighter than the material's thermal movement allows. In those cases an aluminum or titanium part with an insulating coating does the job.

The honest answer for a wear pad at 80 °C in oil is POM or PA. For a chemical seal at 200 °C it is PTFE or PEEK. For a stiff bracket that must not craze, PEI or PC with radiused corners. Match the grade to the dominant load, not to the datasheet headline.