PEEK (polyether ether ketone) is a semi-crystalline high-performance thermoplastic whose unfilled grade carries a melting point near 343 °C and a continuous-use ceiling above 260 °C, with mechanical strength retained across a wide chemical-exposure envelope [S5]. For process and mechanical engineers sourcing bushings, seals, pump internals, compressor valve plates, semiconductor wafer carriers and downhole back-up rings, the wrong grade spec is a six-figure lesson in wear, galling, or thermal failure — which is why the selection has to be gated, not improvised.
The decision is not "PEEK or not"; it is "which member of the PEEK family survives my service envelope at the lowest unit cost." The five gates below are the order I run them in, in plant and at the desk.
Gate 1 — Thermal envelope: melting point, glass transition, continuous service
Unfilled PEEK's melting point sits at approximately 343 °C and its glass-transition temperature (Tg) near 143 °C; the polymer is rated for continuous service up to roughly 260 °C, with steam-and-hot-water capability that no other melt-processable thermoplastic in this class matches [S5]. For any application sitting above 200 °C wet or 250 °C dry, PEEK is on the short list; for anything below 150 °C dry, the cost premium over PA66, PPS, or PPA usually fails the business case.
Reinforced grades shift the envelope: 30 % glass-fibre PEEK raises the heat-deflection temperature under load (HDT at 1.82 MPa) by 20–30 °C versus neat, and 30 % carbon-fibre delivers a further bump with substantially lower coefficient of thermal expansion — a point that matters for tight-tolerance valve seats and thrust washers. Always pin the spec to the worst-case coupled condition (temperature + stress + chemical), not the marketing datasheet headline.
Gate 2 — Chemical compatibility and wear-load coupling
PEEK is resistant to most organic and inorganic chemicals at moderate temperatures, with notable weakness against concentrated sulphuric acid, concentrated nitric acid, and a few halogenated solvents at the upper end of its temperature window [S2][S5]. For sliding-wear service (bushings, seals, compressor scroll tips), the unfilled bearing-grade PEEK compounds — typically modified with PTFE, graphite, or carbon-fibre — cut the dynamic coefficient of friction and push the PV limit; for static structural service in aggressive media, glass-fibre or carbon-fibre reinforced grades win on stiffness and creep.
As a rule of thumb, if the part sees a tribological duty cycle, ask for a filled bearing grade; if it sees a structural duty cycle, ask for a glass- or carbon-fibre structural grade. Conflating the two is the single most common PEEK mis-spec I see on RFQ review — neat PEEK in a high-PV bush fails by melting locally, while 30 % GF PEEK in a dry sliding seat wears through because there are no internal lubricants. For process-control hardware such as pressure transmitter diaphragms and industrial valve seats, PEEK's chemical resistance is the primary spec driver.
Gate 3 — Mechanical load, creep and dimensional stability
Neat PEEK tensile strength sits around 90–100 MPa with a tensile modulus near 3.5–4 GPa; 30 % glass-fibre roughly doubles the modulus and pushes the HDT up by tens of degrees [S2]. Creep resistance is one of PEEK's signature advantages over PA, POM, and PPS, but it is not infinite — above 150 °C and under sustained load, deformation accumulates and the design must use the manufacturer's creep-modulus curves, not the room-temperature datasheet number.
For high-precision sealing faces and bearing cages, carbon-fibre reinforced PEEK's lower thermal expansion (closer to aluminium than to neat polymer) lets the part hold tolerance across a thermal cycle that would drift an unfilled grade out of spec. The trade-off is anisotropy: carbon-fibre-filled grades show markedly different properties along the flow direction versus transverse, so the mould-flow analysis must be part of the gate-3 conversation, not a downstream formality.
Gate 4 — Regulatory, certification and application class
PEEK has a long list of third-party approvals, and the right ones for the job narrow the candidate list quickly. Food-contact grades need FDA 21 CFR and EU 10/2011 compliance; medical/implant grades (commonly referred to as "PEEK-Optima" or "PEEK-Classix" in vendor literature) carry USP Class VI and ISO 10993-1 cytotoxicity packages; oil-and-gas downhole grades meet NACE MR0175 / ISO 15156 for sour-service limits; semiconductor wet-bench and CMP grades need low-outgassing and trace-metal specifications [S3].
Skipping this gate is the most expensive way to fail a project — a PEEK part that meets every mechanical property on paper can still be rejected at the customer's PPAP or qualification audit if the certification stack is wrong. If the component ends up inside a piece of flow meter or PLC hardware, the agency's listed material spec — not the distributor's generic datasheet — is what must appear on the drawing callout. Note that implantable PEEK (skull repair, spinal cages) carries a price multiple of 5–20× over industrial grades because of the biocompatibility documentation trail [S3].
Gate 5 — Geometry, process and unit cost
PEEK is melt-processable by injection moulding, extrusion, compression moulding, and machining from stock shapes; the choice is governed by part geometry, annual volume, and tolerance. For annual volumes below a few hundred parts, machining rod and plate stock is faster and cheaper than moulding; above that, moulding wins on unit cost even after the tool amortisation. PEEK must be dried to below 0.02 % moisture before processing and the melt held at 370–400 °C — both are non-negotiable for crystallinity and property consistency [S5].
Cost-wise, neat PEEK resin typically runs 8–15× the price of PA66 and 3–5× the price of PPS at the time of writing, and filled grades add another 20–60 % on top. The same five-gate logic that this article applies to PEEK is what I run for any engineering-polymer buy — for example in the polycarbonate grade-selection frame and the glass-fibre chemical-compatibility frame. If gate 5 cannot justify the delta over a lower-tier polymer, PEEK is the wrong answer regardless of how well it passed gates 1–4.
Comparison table — the three PEEK variants a buyer actually specs
The table below lines the three buyable families against the four criteria that drive an RFQ decision. All numbers are typical ranges for representative industrial grades; always validate against the manufacturer's current lot-certified datasheet before release. [S1]
Use the table to draw the shortlist: neat PEEK for sealing, food-contact, and medical; 30 % GF for structural stiffness on a budget; 30 % CF for stiffness-plus-thermal-stability plus conductivity, and accept the anisotropy tax. The same gate-based logic also travels well into adjacent buys such as the RV reducer backlash/tilt-moment frame when the gearbox housing is itself a polymer candidate.
Who PEEK is for — and who it is NOT for
PEEK is for engineers who have already failed PA, POM, PPS, PPA, or PEI on a specific temperature, chemical, or creep axis and need a step-change in performance. It is for sliding-wear service where lubrication is impossible, for wet-chemical service above 100 °C, for steam-sterilised medical and food-contact parts, and for aerospace and downhole components where weight, fire-smoke-toxicity rating, and NACE compliance dominate. The same five-gate frame is a sound starting point for the steel plate selection conversation when the buyer is comparing a metal alternative to a polymer. [S2]
PEEK is NOT for: high-volume automotive interior trim (use PP or ABS), commodity electrical enclosures (use PA66 or PC/ABS), any application where the operating temperature stays below 120 °C and the chemical exposure is mild, or any project whose unit-cost target is below the PEEK resin premium — no clever machining will close that gap. If the requirement is "looks like plastic, costs like plastic," PEEK is the wrong polymer to spec; if the requirement is "performs like metal at polymer weight, regardless of cost," PEEK is on the short list and the job is to pick the correct grade. The same logic applies in adjacent decisions such as the rebar bender spec frame where the buyer must decide whether the premium-grade machine earns its price tag.
Track the next decision node by locking the resin grade, the certification stack, and the lot-traceability requirement on the drawing before RFQ release — three signals that determine whether the inbound quotes are comparable. If two suppliers return quotes on different PEEK families (neat vs 30 % GF) and different compliance stacks (industrial vs FDA), the comparison is not apples-to-apples and the apparent low bid is not a real saving.