PEEK remains the default high-performance thermoplastic for aircraft secondary structure, with carbon-fibre-reinforced variants such as VICTREX PEEK 90HMF40 delivering up to 40% weight and cost reductions versus aluminium 7075-T6 in primary structural brackets on the Airbus A350, per the Victrex case study dated 2018-03-23 [S2].
Selection in 2026 is a three-axis decision: unfilled, glass-fibre, or carbon-fibre grade; part form (machined stock, injection-moulded, extruded, additive); and qualification envelope (continuous use, flame, smoke, toxicity, outgassing). The A350 door-bracket case shows that even point-loaded primary structure can be re-specified in PEEK once buy-to-fly and fatigue data are demonstrated, with the injection-moulded CF-PEEK part reporting up to 85% better buy-to-fly ratio than machined aluminium [S2].
Material family and the PAEK context
PEEK belongs to the polyaryletherketone (PAEK) family, with a glass transition around 143°C, melting point at 343°C, and inherent UL 94 V-0 flammability rating without additives, per the Infinita Lab grade guide of 2026-07-29 [S4]. The semi-crystalline structure carries 25-40% crystalline regions that provide strength, stiffness, chemical resistance, and heat tolerance, balanced by 60-75% amorphous regions that absorb impact and prevent brittle fracture, as detailed in the Aon3d material review of 2026-04-13 [S3].
Within PAEK, polyetherketoneketone (PEKK) sits as a higher-ketone-ratio sibling whose slower crystallization rate and wider processing window make it the preferred PAEK for fused-filament-fabrication (FFF) aerospace parts where warping and Z-axis delamination are disqualifying defects [S5]. For machined and injection-moulded secondary structure, neat, GF, and CF PEEK continue to dominate specification, with typical PEEK density of 1.26-1.32 g/cm³, tensile strength 85-100 MPa, and Young's modulus 2.8-3.9 GPa [S3].
Grade selection by load and function
Unfilled PEEK is the baseline for seal rings, bearings, bushings, cable protection tubing, and similar wear and dielectric components where elongation (25-40% at break) and chemical resistance matter more than peak stiffness [S3][S4]. Glass-fibre-reinforced PEEK, typically 30% glass, lifts stiffness and dimensional stability for structural aerospace brackets where creep at elevated cabin or avionics-bay temperatures is the design driver [S4].
Carbon-fibre-reinforced PEEK at 30-40% fibre loading is the workhorse for primary and secondary structural brackets, high-performance bearings, and high-temperature wear surfaces. The 90HMF40 grade cited in the A350 case reports up to 100x longer fatigue life and up to 20% higher specific strength and stiffness than aluminium 7075-T6 under matched test conditions, supporting its selection for load-bearing door fittings [S2]. For components that must bleed static charge, electrically conductive PEEK filled with carbon fibre or carbon nanotubes provides controlled surface resistivity for ESD-sensitive avionics bays and semiconductor-process tooling [S4].
Property envelope and design limits
Continuous service temperature reaches 260°C in unfilled PEEK, with heat-deflection and creep data that hold stiffness well above the cabin pressurization and engine-aperture envelopes typical of commercial aircraft [S4]. Tensile strength of 85-100 MPa, flexural strength of 100-170 MPa, and fatigue strength of 30-100 MPa at 10⁶ cycles bracket the static and dynamic design space for non-reinforced grades [S3].
The coefficient of friction of 0.35-0.45 for unfilled PEEK rises or falls predictably with reinforcement, and the material is chemically resistant to most organic solvents, acids, and hydrocarbons, which matters for fuel- and hydraulic-system components that see Skydrol, phosphate-ester fluids, and jet fuel exposure [S3][S4]. Density at 1.26-1.32 g/cm³ is roughly 1/5 that of aluminium 7075-T6 (≈2.81 g/cm³), which is the starting point for any mass-savings business case before reinforcement and geometry gains are added [S3].
Processing route: moulding, machining, extrusion, additive
Injection moulding of CF-PEEK delivers the cycle time, surface finish, and buy-to-fly ratio that the A350 door bracket case exploited: up to 85% better buy-to-fly than machined aluminium means most of the expensive carbon-fibre-reinforced melt ends up in the part, not in chips [S2]. Machined stock remains the right answer for low-volume brackets, prototypes, and components with thick cross-sections where mould-flow simulation would push the design past the limits of the high-flow 90HMF40 melt [S2][S4].
Additive manufacturing is the segment where the PEEK-versus-PEKK choice is sharpest. PEEK's fast crystallization in a non-constrained FFF build produces warping, corner lift, and cold-joint Z-axis delamination; PEKK-A's slower crystallization keeps the polymer amorphous and malleable long enough for layer-to-layer molecular entanglement, with Z-strength approaching X/Y strength in tall builds [S5]. For 100%-infill, thick-walled, large-format aerospace prints, this translates into flat parts that stay flat and cross-sections that hold dimension without specialized enclosed-chamber hardware, which is why PEKK-A is gaining specification in 3D-printed brackets, clips, and ducting [S5].
Qualification, testing, and supply chain
Mechanical qualification of PEEK grades follows ASTM D638 (tensile), ASTM D790 (flexural), ASTM D256 (Izod impact), ASTM D648 (heat deflection), ASTM D3418 by DSC (crystallinity and melting), and ASTM D1238 (melt flow rate), with biocompatibility per ISO 10993 required for medical-grade PEEK-OPTIMA components [S4]. For aerospace parts, the relevant overlay is OEM material specifications, flame-smoke-toxicity (FST) per applicable aircraft certification rules, and any programme-specific outgassing limits, none of which substitute for the base property data above.
On the supply side, the 2026-08-15 ZYPEEK (Joinature Polymer) product page lists 1000-ton PEEK capacity, 5000L reaction vessels, 20 years in the polymer business, 35 PEEK-related patents, A-share listing, and main-drafter status on China's first national PEEK standard, which together signal a credible second-source option for tier-1 and tier-2 aerospace moulders that need to qualify a PEEK polymer supplier outside the historical single-vendor base [S1]. The same vendor's 2026-07-09 aerospace note covers typical ZYPEEK grades for seal rings, bearings, bushings, cable protection, and structural brackets, with the explicit positioning that lightweight design, high-temperature resistance, and flame retardancy are the three selection drivers for new aerospace programmes [S6].
Selection decision matrix
Use unfilled PEEK for seal rings, bushings, valve seats, and dielectric components where elongation, chemical resistance, and purity dominate; pick 30% glass-fibre PEEK for structural brackets where creep and dimensional stability above 150°C matter more than specific strength; choose 30-40% carbon-fibre PEEK (90HMF-class) for load-bearing primary structure and high-cycle fatigue parts where the 100x fatigue-life and 20% specific-stiffness uplift over aluminium 7075-T6 are the business case [S2][S3][S4].
Switch to PEKK-A when the manufacturing route is FFF additive and the part is large, thick-walled, or Z-axis loaded, because the slower crystallization avoids the warping and cold-joint failure modes that disqualify PEEK on the same printer [S5]. Specify conductive PEEK for avionics bay brackets and ESD-sensitive equipment racks where static dissipation is a documented requirement [S4]. Within this matrix, density stays at 1.26-1.32 g/cm³, continuous use at 260°C, and UL 94 V-0 flammability without additives as the baseline invariants for the whole PAEK family [S3][S4].
When PEEK is the wrong choice
PEEK is over-specified for non-structural cabin trim, low-load fairings, and interior decorative parts where a lower-cost thermoplastic or thermoset composite delivers adequate performance at a fraction of the polymer cost. PEEK is also the wrong pick for first-article additive builds on a desktop-class FFF printer: the chamber cannot hold the thermal envelope PEEK needs to suppress warping, and PEKK-A or a lower-melt PAEK will out-of-the-box print what PEEK cannot. [S5]
For static discharge on a non-critical bracket, the conductive carbon-fibre filler premium is wasted; for a fuel-line clip that never sees a 100 MPa load, unfilled PEEK is wasted. Match grade to load, not to reputation. PEEK remains expensive, so the procurement question is always: which component on the airframe actually needs the 260°C continuous-use, V-0 flammability, and 100x-fatigue envelope, and which can be specified in a cheaper engineering polymer or aluminium?
For engineers building an internal specification for the next aero programme, the verified signals to track are: (1) further qualification of CF-PEEK grades on primary structure beyond the A350 door-bracket precedent, with buy-to-fly and fatigue data published for new programmes; (2) expansion of PEKK-A into AS9100-qualified additive supply chains as FFF printer OEMs release aerospace-grade machines; and (3) FST and outgassing data on conductive PEEK grades for next-generation avionics bays, where ESD control is becoming a documented airworthiness requirement. On the spec side, pressure transmitter housing and flow meter body manufacturers are evaluating PEEK and PEKK for low-weight fuel and hydraulic skids, and industrial valve trim trials have moved from neat PEEK to CF-PEEK at 150-200°C service, the exact temperature band where PEEK's creep and fatigue data start paying back the polymer premium.
For related coverage, see Spiral Duct Sizing and Selection Guide for HVAC and Process Exhaust.