The basics: understanding different plastics

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  Key takeaways
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  What actually changes between one plastic and the next
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  How plastics behave when you machine them
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  Reading the drawing to choose the right plastic
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  Where plastics stop being the right answer
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  How we hold ±0.005 mm in plastic
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  Common engineering plastics at a glance
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  Which plastic fits which part

Every engineering plastic starts as a chain of repeating units. How those chains pack together decides almost everything a machinist cares about: stiffness, how the material fails, how much it moves after cutting, and whether a coolant will attack it.

In an amorphous plastic, the chains are tangled with no order. ABS, PC and PMMA sit in this group. They soften over a wide temperature band rather than melting at one point, which makes them forgiving to machine. They also turn transparent or translucent when the grade allows it.

In a semi-crystalline plastic, part of the structure folds into ordered regions. POM, PA, PEEK and PP belong here. Those ordered regions act like a built-in reinforcement, so the material resists wear and fatigue much better. The trade-off is that the same structure makes the material shrink and move unevenly as it cools.

There is also a third group you will meet on drawings: filled and reinforced grades. Glass-filled PA, carbon-filled PEEK and PTFE-filled POM all change the picture again. Glass raises stiffness and cuts thermal expansion, but it also grinds the cutting edge. That is a tooling decision, not just a material decision.

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  AmorphousEven shrinkage, good dimensional stability after machining, easy to polish.
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  Semi-crystallineHigher wear resistance and fatigue life, but more post-machining movement.
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  Filled gradesBetter stiffness and creep resistance, shorter tool life, higher cost.

Metal cutting removes heat with the chip and the coolant. Plastic cutting does not. Most polymers conduct heat roughly a thousand times worse than aluminium, so the heat stays at the edge. If the tool is dull or the feed is too light, the material rubs instead of cutting and the edge smears.

The practical answer is a sharp tool with a high rake angle, a positive cutting geometry and a healthy feed per tooth. On POM and PA we typically run 1,500 to 3,000 rpm with a two-flute cutter and keep the chip visibly thick. A thin chip on plastic means you are burnishing, not cutting.

Coolant choice matters more than most drawings suggest. Compressed air or a mist is usually enough for ABS and PC. Some grades stress-crack in contact with certain oils, so we confirm the coolant against the specific grade before the first cut. PEEK and PA often run dry with air blast only.

Clamping is the other half of the job. Plastics are elastic. A vise tightened the way you would tighten it on steel will bow the part, cut oversize, then spring back when you release it. We use soft jaws machined to the part profile and keep clamping pressure low and even.

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  Tool geometrySharp, high rake, polished flutes. A worn edge raises local temperature fast.
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  Feeds and speedsHigher rpm, moderate chip load. Never let the tool dwell in the cut.
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  CoolingAir or mist first. Confirm coolant compatibility with the grade.
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  WorkholdingSoft jaws, light even pressure, support thin walls from both sides.

Start with the environment, not the material name. A part that sits in a hydraulic bay sees oil mist. A part that sits in an autoclave sees steam at 134 °C. A part that sits in a medical tray sees repeated disinfectant wipe-down. Those three conditions point at three different grades before you look at any tolerance.

Next, look at the load. If the part is a sliding surface, POM or a filled PA will usually outlast an amorphous grade by a wide margin because of its lower friction and better fatigue behavior. If the part is a cover or a bracket that only carries its own weight, ABS or PC will do the job at a lower cost.

Then check the tolerance callout against the material's thermal expansion. A 100 mm POM part can grow around 0.1 mm over a 30 °C swing. If the drawing calls for ±0.05 mm across that range, the material is the wrong choice and no amount of machining skill will fix it.

Finally, ask whether the part has to be transparent, food contact approved or sterilizable. Those three requirements narrow the field faster than any mechanical property table. Tell us the end use and we will confirm the grade before quoting.

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  Chemical exposureCheck the specific fluid and temperature, not just the family name.
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  Sliding or staticSliding surfaces favor POM and filled PA. Static covers favor ABS and PC.
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  Tolerance vs expansionTight tolerance over a wide temperature band rules out high-expansion grades.

Plastics are the wrong choice when the part has to carry a real structural load, when it runs hot for long periods, or when the tolerance is tighter than the material's own movement. A 200 mm ABS beam will creep under a constant load, slowly and permanently. No machining process prevents that.

Creep is the failure mode most often missed at the design stage. A part that passes a short bench test can still deflect over months in the field. If the load is constant and the temperature is above room temperature, either derate the stress heavily or switch to a filled grade.

Environmental stress cracking is the second one. A moulded or machined PC part under load can crack weeks after contact with a cleaning solvent or a thread-locking compound. The crack looks like a manufacturing defect but the cause is the chemical plus the residual stress.

When those limits bite, aluminium 6061-T6 or stainless 304 is usually the answer. Both machine to ±0.005 mm, both hold their shape, and both handle load and heat that no polymer will. The weight penalty is real, but a failed plastic part costs more.

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  Structural loadSwitch to aluminium or steel for beams, brackets and load paths.
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  Long hot serviceAbove the material's heat deflection temperature, creep accelerates.
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  Solvent contact under stressResidual stress plus chemical equals cracking. Anneal or change material.

Plastic parts move after machining, so the process has to account for that from the first operation. We rough the part, let it rest, then finish it. The rest period lets the internal stress release and the part reach equilibrium with the shop air before the final cut.

For nylon and PEEK, we control humidity. Parts are measured in the same conditioned space they were machined in, using the same reference temperature. A part measured straight off the machine and again two days later can read differently, and the drawing tolerance has to survive both readings.

We inspect 100% of plastic parts before shipment, with reports available on request. That covers raw material check, in-process monitoring and final inspection. For a 100 mm POM part we watch the feature that sets the fit, not just the overall envelope.

Our shop runs 127 high-precision CNC machines, including 16 simultaneous 5-axis centers, with a maximum processing size of 4,000 mm. Plastics machine on the same platforms as aluminium, just with different tools and parameters. Quotation and a free DFM analysis come back within 12 hours.

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  Rough, rest, finishTwo-stage cutting lets stress release before the final pass.
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  Condition firstMeasure in the same humidity and temperature as machining.
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  Control the critical featureInspect the fit dimension, not just the outer size.