PEEK

Unfilled PEEK is the default: the toughest of the three, the only one normally considered for wear against a soft mating surface, and the grade family that implantable and food-contact versions come from. Glass-filled (typically 30%) roughly doubles stiffness and improves creep resistance under sustained load, at the cost of toughness — and glass is abrasive, so tooling wears faster and the price of the machined part rises. Carbon-filled (typically 30%) gives similar stiffness plus much better thermal conductivity and wear behaviour, a lower thermal expansion coefficient, and it is electrically dissipative rather than insulating — which matters both ways: good for static-sensitive handling, wrong if you needed an insulator. Name the exact grade on the drawing, not just "PEEK"; the three behave differently enough that substituting one is a design change.

Extruded and compression-moulded PEEK stock carries internal stress from the way it was cooled, and it is semi-crystalline — so removing material lets that stress redistribute, and the part relaxes out of tolerance hours or days later. Thin walls, deep pockets and asymmetric geometry make it worse. The standard answer is a stress-relief anneal: on stock before machining for demanding parts, and again between roughing and finishing when tolerances are tight, with the finishing cut taken after the part has settled. Ask for it explicitly on tight-tolerance work — it costs time in the route and a supplier will not add it unless the drawing or the RFQ says the tolerance depends on it.

PEEK earns its price when several demands stack. Its glass transition is around 143 °C and it melts near 343 °C, which is what supports continuous service around 250 °C and repeated steam autoclaving — PEI softens well below that and is not a steam-sterilisation material in the same way. PEEK also resists nearly every industrial chemical except concentrated sulphuric acid and some halogenated media, which is where PEI gives up first. If the requirement is only chemical inertness and low friction with no structural load, PTFE is cheaper and easier but creeps badly and cannot hold a tolerance under load. If the part lives at room temperature in air under moderate load, POM machines faster and costs a fraction. The honest test: write down the maximum continuous temperature, the chemicals, the sterilisation cycles and the load. If fewer than two of the four are demanding, something cheaper will do.

Three things that do not apply to metal. Thermal expansion: unfilled PEEK expands roughly an order of magnitude more than steel per degree, so a tight fit has to state the temperature it applies at, and a part measured warm off the machine will not measure the same cold. Filled grades expand less, which is one reason they get specified for dimensionally critical work. Residual stress: see annealing above — the part can be in tolerance at inspection and out of it a week later. And cutting heat: PEEK is a poor conductor, so heat stays in the cut; dull tools or heavy finishing passes soften the surface and leave dimensions that drift as the part cools. Practically, this means dimensioning only the features that matter, stating a reference temperature for critical fits, and generous internal radii — PEEK is notch-sensitive and sharp internal corners are where stress cracks start.

Industrial PEEK withstands repeated steam autoclaving, gamma irradiation and ethylene oxide, which is a large part of why it displaced metal in reusable surgical instrumentation and lab hardware. Implantable grades are a separate matter: they are distinct product lines sold under their own certification, lot traceability and supply agreements by the resin producers, and compliance is a property of the resin and its documented chain of custody, not of the machining. If your part is implantable or long-term body-contact, name the exact implantable grade on the drawing, expect the supplier to buy it through the controlled channel and pass the producer’s documentation through unaltered, and do not accept an "equivalent". For non-implant medical and lab parts, standard grades with the producer’s USP Class VI or food-contact declaration are the normal route.