Guides / Grades

PEEK Material Properties, and What They Mean on a Part

September 20, 2026

Sources: manufacturer datasheets and published standards. See editorial standards.

Most PEEK datasheets give you 40 numbers and no help deciding which matter. This is the short list, with what each one actually changes on a part.

All values are typical published figures for unfilled PEEK at 23 °C unless stated. Filled grades differ and are noted where the difference decides a design.

Mechanical

Property	Unfilled PEEK	What it changes
Tensile strength	90 to 100 MPa	Load a part can carry before it yields
Tensile modulus	3.5 to 4 GPa	How much it deflects. About 1/50 of steel
Elongation at break	20 to 45%	Tolerance for impact and snap fits
Flexural modulus	3.8 to 4.3 GPa	Stiffness of beams, brackets and thin plates
Izod impact, notched	5 to 8 kJ/m²	Survival of a knock. Filled grades are lower
Rockwell hardness	M99 to M105	Resistance to indentation, not to abrasion
Density	1.30 g/cm³	Part weight. GF30 rises to 1.50

The number people misread is modulus. PEEK is strong for a plastic but it is not stiff. A bracket designed with aluminium sections and made in PEEK will deflect visibly. Either the section grows, or you move to GF30 at around 10 GPa or CF30 at around 13 GPa.

Thermal

Property	Unfilled PEEK
Glass transition, Tg	143 °C
Melting point	343 °C
Continuous service temperature	about 250 °C
Heat deflection temperature, 1.8 MPa	about 152 °C
Thermal expansion below Tg	about 47 µm/m·K
Thermal conductivity	about 0.25 W/m·K

Tg at 143 °C is the number to design around. Below it PEEK behaves like the datasheet. Above it the material softens, expansion roughly doubles and creep under constant load becomes the limit rather than strength. A part loaded at 200 °C needs a filled grade or a lower stress, not a bigger safety factor on room temperature strength.

Thermal conductivity of 0.25 W/m·K is about 1/500 of aluminium. That is why PEEK is used as a thermal break, and also why it traps machining heat. Carbon filled grades reach roughly 0.9 W/m·K, which is one of the reasons they machine to tighter tolerances.

Chemical and environmental

• Resists nearly all organic and inorganic chemicals at room temperature
• Survives continuous hot water and steam, including repeated autoclave cycles
• Attacked by concentrated sulfuric acid, and by some strong oxidising acids
• Halogenated solvents can cause stress cracking in a highly stressed part
• Water absorption about 0.1% in 24 hours, around 0.5% at saturation
• Handles high gamma and electron beam doses without losing much strength
• Ages slowly under sustained UV unless the grade is black or shielded

The steam resistance is the property that wins medical work, because PEI and polysulfone lose it. Details and the short list of things that do attack PEEK are in PEEK chemical resistance.

Electrical

Property	Typical
Volume resistivity	above 10¹⁵ Ω·cm, unfilled
Dielectric strength	about 19 to 23 kV/mm
Dissipation factor at 1 MHz	around 0.003
Flammability	UL94 V-0 at typical wall thickness
Limiting oxygen index	about 35%

Unfilled and glass filled PEEK insulate. Carbon filled PEEK does not: it is conductive enough to bleed off static, which is wanted in wafer handling and unacceptable in an insulator. This single property decides the grade on a lot of drawings.

Wear and friction

Unfilled PEEK has a dry coefficient of friction around 0.3 to 0.4 against steel, which is not low. Bearing grades that blend carbon fibre, PTFE and graphite reach roughly 0.1 to 0.2 and carry far more PV. If the part slides, the grade matters more than the geometry, as covered in bearing grade PEEK.

Reading the numbers honestly

Three cautions that save projects:

Datasheet values come from moulded test bars. A machined part cut from thick extruded stock is a slightly different animal, mostly in stress and in fibre orientation.

Filled grade data is directional. Short fibre stock is anisotropic. Strength and expansion along the extrusion direction differ from across it, which matters on long parts.

Temperature changes everything. Ask for the property at your service temperature, not at 23 °C. Suppliers publish strength versus temperature curves, and they are worth reading before you set a wall thickness.

Which rows to trust, and for which grade, is covered in our guide to PEEK grade selection.

Working out which grade these numbers point to? Start with which PEEK grade should be on your drawing, or send the part and let the shops that quote suggest one.

Frequently asked questions

Keep reading

• X-Ray Opaque PEEK: What Barium Sulfate Changes in a Medical Part
• When PEEK Runs Out of Heat and Polyimide Takes Over