🧪 PEEK
CNC Machining PEEK: Unfilled, Glass-Filled and Carbon-Filled Grades
PEEK is the high-performance thermoplastic that earns the 'engineering plastic that replaces metal' label honestly, holding strength and chemical resistance up to roughly 250 C and qualifying for implant and aerospace use. Machining it well is mostly an exercise in heat and stress management, because PEEK's low thermal conductivity and built-in residual stress will warp parts and ruin tolerances if the cut is treated like a soft plastic. The grade you choose also changes the rules.
ISO 13485AS9100ISO 9001
Heat and stress: why annealing and cooling matter
PEEK conducts heat poorly, like most polymers, so frictional heat from the cut concentrates locally and can soften, gum or even melt the surface if speeds are too high or chips are not cleared. The remedy is sharp tooling, healthy feed to keep the chip carrying heat away, air blast or light coolant, and avoiding rubbing or dwelling. Polished, positive-rake tools made for plastics, often the same geometry used for acetal, give the cleanest cut.
Residual stress is the bigger trap. Extruded and molded PEEK stock carries internal stress, and removing material unbalances it, causing parts, especially thin or asymmetric ones, to warp after machining. The professional practice is stress-relief annealing: heating the stock or the rough-machined part through a controlled cycle to relax internal stress before final machining, so the finished part stays dimensionally stable. For tight-tolerance PEEK parts, machinists often rough, anneal, then finish.
For buyers, the implications are concrete: tight-tolerance PEEK parts require annealing in the process plan, which adds time, and skipping it is a common cause of warped, out-of-tolerance plastic parts. A quote for precision PEEK should include this step.
Unfilled, glass-filled and carbon-filled PEEK
Unfilled (virgin) PEEK is the baseline: good strength, excellent chemical and hydrolysis resistance, and the form used for medical and food-contact parts and electrical insulators. Implant-grade PEEK (PEEK-OPTIMA and similar) is unfilled and used for spinal cages and other implants because it is biocompatible, radiolucent (X-ray transparent), and has a modulus close to bone. It machines cleanly and is the choice when purity and biocompatibility dominate.
Glass-filled PEEK (typically 30 percent glass fiber) adds stiffness, dimensional stability and improved load and creep resistance at temperature, used for structural and high-load parts. The glass fibers are abrasive, so they accelerate tool wear noticeably, demanding more frequent tooling and sometimes diamond-coated cutters; the finish can be slightly rougher as fibers are cut.
Carbon-filled PEEK (typically 30 percent carbon fiber) goes further on stiffness and strength while adding thermal and electrical conductivity and excellent wear resistance, favored for bearings, bushings, seals and aerospace structural parts. Carbon fiber is also abrasive on tooling but less aggressively than glass, and the conductivity can matter for ESD-sensitive applications. The buyer choice tracks the need: virgin for purity and biocompatibility, glass-filled for stiffness and stability, carbon-filled for wear, stiffness and conductivity, with both filled grades costing more in tool wear.
Tolerances, applications and the cost picture
Machined PEEK holds good tolerances for a plastic, +/-0.005 in (0.13 mm) is realistic and tighter is achievable on annealed, stress-relieved stock, but it will never match metal's dimensional stability because of higher thermal expansion (several times that of steel) and moisture and temperature sensitivity. Parts must be measured at a controlled temperature, and designers should avoid relying on metal-tight tolerances across large dimensions. Surface finish is good with sharp tooling; filled grades show fiber texture.
Applications concentrate where PEEK's properties justify its high cost: medical implants and surgical instruments (unfilled, implant grade), semiconductor wafer-handling and process parts needing chemical purity and high-temperature resistance, aerospace structural and interior components valued for strength-to-weight and flame/smoke/toxicity performance, oil-and-gas seals and back-up rings surviving heat and aggressive fluids, and bearings and bushings (carbon-filled) running dry at temperature.
On cost, PEEK stock is very expensive, far above common metals and most plastics, so material is a real share of part cost, and annealing plus careful machining add labor. But it machines faster and with less tool consumption than tough metals, and it delivers a metal-replacing part where corrosion, weight, chemical attack or biocompatibility would defeat steel. The honest buyer test: PEEK is justified when its specific high-performance properties are needed; for ordinary plastic parts, acetal or nylon costs a fraction as much.
Frequently Asked Questions
Because PEEK stock carries internal residual stress from how it is extruded or molded, and removing material during machining unbalances that stress, causing the part to warp, especially thin, large or asymmetric geometry. For tight-tolerance parts this warping puts dimensions out of spec after machining even if they were perfect on the machine. Annealing is a controlled heating cycle that relaxes the internal stress so the material becomes dimensionally stable, and the professional practice for precision PEEK is to rough-machine, anneal to relieve the newly unbalanced stress, then finish-machine to final dimensions, sometimes with a final stress-relief as well. The annealing schedule follows the manufacturer's recommended temperatures and slow ramp/soak/cool cycle to avoid introducing new stress. Skipping annealing is one of the most common causes of warped, out-of-tolerance machined plastic parts, and it cannot be corrected after the fact. For buyers, the practical implication is that a quote for precision PEEK should explicitly include annealing in the process plan, that it adds lead time (the cycles are slow), and that very loose-tolerance or simple PEEK parts may not need it, while anything with tight tolerances, thin walls or critical flatness almost certainly does.
They trade purity for mechanical and functional performance, and they machine differently. Unfilled or virgin PEEK is the pure polymer: good strength, outstanding chemical and hydrolysis resistance, biocompatible in implant grades, radiolucent, and the choice for medical implants, food-contact parts and electrical insulators where purity matters; it machines cleanly with the least tool wear. Glass-filled PEEK, typically 30 percent glass fiber, adds stiffness, dimensional stability and better load and creep resistance at high temperature for structural parts, but the glass fibers are abrasive and noticeably accelerate tool wear, sometimes requiring diamond-coated tooling, and the finish can show fiber texture. Carbon-filled PEEK, typically 30 percent carbon fiber, gives even higher stiffness and strength plus thermal and electrical conductivity and excellent wear resistance, making it ideal for bearings, bushings, seals and aerospace structural parts and for ESD-sensitive uses; carbon fiber is abrasive on tooling too, though generally less aggressive than glass. So choose virgin PEEK for purity and biocompatibility, glass-filled for stiffness and stability, and carbon-filled for wear resistance, maximum stiffness and conductivity. Remember the filled grades cost more in tooling wear and are not biocompatible the way implant-grade virgin PEEK is.
PEEK holds good tolerances for a plastic, with +/-0.005 in (0.13 mm) realistic and tighter achievable on properly annealed, stress-relieved stock, but it will not match metal's dimensional precision and stability. The reasons are inherent to the polymer: PEEK's coefficient of thermal expansion is several times that of steel, so its dimensions shift meaningfully with temperature, meaning parts must be machined and measured at controlled temperatures and designers should avoid specifying metal-tight tolerances across large dimensions. It also has some sensitivity to moisture and to the residual-stress warping discussed above. The practical path to the tightest achievable PEEK tolerances is to use stress-relieved or annealed stock, rough-machine then anneal then finish, keep cutting temperatures down with sharp tooling and good chip clearing, and inspect at a stable temperature. Filled grades, especially glass-filled, are dimensionally more stable than unfilled because the fibers reduce thermal expansion, so for structural tight-tolerance parts a filled grade can actually help. For buyers, the guidance is to specify only the tolerances the function needs, expect that very tight tolerances require annealing and add cost, and understand that PEEK's strength is high-temperature and chemical performance, not metal-level dimensional precision.
PEEK is worth it only when you genuinely need its high-performance properties, because its raw stock costs far more than common engineering plastics and many metals. PEEK is justified when a part must survive continuous service near 250 C, resist aggressive chemicals, steam and hydrolysis, meet medical biocompatibility and radiolucency for implants, satisfy aerospace flame-smoke-toxicity and strength-to-weight requirements, maintain purity for semiconductor processing, or run as a dry bearing at elevated temperature in carbon-filled form. In those roles nothing cheaper performs, and PEEK legitimately replaces metal where corrosion, weight or chemical attack would defeat it. The honest counter-case is that PEEK is frequently over-specified. If a part lives at moderate temperature with ordinary chemical exposure and just needs to be a tough, machinable plastic, acetal (Delrin), nylon or polycarbonate will do the job at a small fraction of PEEK's material cost. Buyers sometimes default to PEEK as a premium plastic when the application does not require its temperature, chemical or biocompatibility performance, which wastes significant money. The decision rule: identify the specific extreme property your part needs, and if no PEEK-level property is actually required, choose a cheaper engineering plastic and reserve PEEK for the demanding applications that truly justify it.
Machining PEEK is generally faster and easier on tooling than machining tough metals like stainless or titanium, but it requires its own discipline around heat and stress. PEEK is soft compared to metal, so cutting forces are low and material removes quickly with sharp, polished, positive-rake tooling, and unfilled PEEK in particular cuts cleanly with little tool wear. The complications are not cutting force but thermal and dimensional behavior: PEEK conducts heat poorly, so frictional heat concentrates and can soften or gum the surface if speeds are too high or chips are not cleared, requiring good chip evacuation, air or light coolant, and avoidance of rubbing and dwelling. Residual stress means precision parts need annealing steps that add time. Filled grades, especially glass-filled PEEK, are abrasive and wear tooling faster, sometimes needing diamond-coated cutters, which slows things and raises tooling cost relative to unfilled PEEK. So the cutting itself is quicker than for hard metals, but total lead time can still be substantial once annealing cycles and careful finishing are included for tight-tolerance parts. For buyers, the takeaway is that PEEK machining cost is driven more by expensive material and by annealing and careful process control than by raw cutting time, which is the opposite of the tough-metal cost picture.
Last updated: July 2026
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