Machining engineering plastics looks easy and is not. The materials are soft, so cutting forces are low, but they conduct heat poorly, they move with temperature and humidity, they carry moulding or extrusion stress that releases as material comes off, and some of them creep under the clamping pressure that holds them still. The result is a discipline of its own.
Heat is the enemy
Plastic does not carry heat away from the cut, so the heat stays in the chip and at the tool edge and can melt the surface, weld chips to the tool or leave a smeared finish. Sharp tools, high rake angles, high speeds with light cuts and generous chip clearance are all standard practice, and coolant is used to carry heat away rather than to lubricate.
Stress relief and why parts move overnight
Extruded and cast stock carries internal stress. Removing material unbalances it and the part relaxes, sometimes over hours. On tight tolerance parts the standard answer is to rough machine, anneal the part to release the stress, then finish machine. Skipping that step is the usual reason a plastic part measures correctly at the machine and wrong on arrival.
Material by material
Acetal is the easiest and holds tolerance well, which is why it is the default for machined plastic parts. Nylon absorbs moisture and grows, so dimensions depend on conditioning. Polycarbonate is tough and stress cracks if machined hot or exposed to the wrong solvent. PTFE is soft, creeps under load, and needs very sharp tools and careful workholding because it deforms under the clamp. PEEK machines well and costs a great deal.
Getting the drawing right
State the material by grade rather than by family, because the difference between grades is bigger than buyers expect. State the tolerance realistically and say at what temperature it applies. And say what the part touches in service, because chemical compatibility and stress cracking will eliminate materials that machine beautifully.
Questions people ask about custom plastic machining
Which plastic is easiest to machine?
Acetal, by a wide margin. It cuts cleanly, holds tolerance and does not absorb much moisture, which is why so many machined plastic parts are made from it.
Why did my PTFE part change size?
PTFE is soft and creeps, so it deforms under clamping and under load, and it has a large thermal expansion. Both workholding and measurement conditions have to allow for it.
Do plastic parts need annealing?
For tight tolerances, usually yes. Rough machining, annealing to release stock stress, and then finish machining is standard practice on precision plastic parts.
Can plastics be machined to the same tolerances as metal?
Not generally, and the honest limit depends on the material and the part size. Expect looser tolerances and specify the temperature at which they apply.