PEEK 30% carbon fibre: when it beats unfilled PEEK

PEEK 30% carbon fibre is the grade you reach for when an unfilled part is stiff enough on paper but not in service, or when thermal expansion is moving a dimension out of tolerance. It is a genuine step change rather than a small tweak: roughly three times the stiffness of unfilled PEEK, and around half the thermal expansion.

The trade is impact strength and cost. Carbon fibre makes PEEK stiffer and more dimensionally stable, but more brittle, and the price per kilo climbs. This guide sets out where the trade is worth it, and where unfilled or glass-filled PEEK is still the better buy.

What 30% carbon fibre changes in PEEK

PEEK 30% carbon fibre raises the tensile modulus from roughly 3.7 GPa to about 11 GPa, and lifts the flexural modulus in a similar proportion. It also cuts the coefficient of thermal expansion sharply and improves creep resistance, because the stiff fibres carry load that the polymer matrix would otherwise carry alone. The exact figures depend on the grade and the fibre length used, but the direction of the change is consistent.

The fibres do not bond perfectly to the matrix, which is where the downsides start. Notched impact strength falls by roughly half, and the material becomes more sensitive to sharp internal corners. Where that matters, radius the corners and review the impact case before committing to the grade. A fillet that is generous by metal standards can be the difference between a part that survives a knock and one that cracks.

Stiffness and creep under sustained load

Creep is where the carbon grade earns its place. Under a sustained load at elevated temperature, unfilled PEEK continues to deflect over time, and the deflection can be large enough to change a fit or open a seal. Carbon fibre restrains that movement, so a part that would creep out of tolerance in unfilled PEEK can hold its shape here. The effect grows with temperature, so the hotter the service, the more the carbon grade pulls ahead.

For a structural bracket, that difference often decides the design. If you can tolerate a small, predictable deflection, unfilled PEEK is cheaper and tougher. If the part must hold a dimension under load for years, the higher PEEK modulus of a carbon grade usually pays for itself. Where the load is intermittent and the temperature is moderate, the same logic does not apply and unfilled PEEK is often the better answer.

Thermal expansion and dimensional stability

Unfilled PEEK expands by roughly 47 microns per metre per degree, while the 30% carbon grade sits nearer 15. Over a 300 mm part and a 60 degree swing, that is the difference between about 0.85 mm and 0.27 mm of growth. For anything that has to stay aligned across a temperature range, the gap is decisive. Dimensional stability is often the real reason a designer moves to the carbon grade, ahead of stiffness on its own.

The same stiffness that restrains expansion also improves machining stability, since a stiffer material deflects less under cutting load. Parts are supplied in standard stock shapes from our stock shape range and can be cut or machined to size. Cutting a carbon-filled grade requires the same dust control as any other filled polymer, since the fibre is abrasive and the dust is not something you want airborne.

Electrical and thermal conductivity side effects

Carbon fibre is electrically and thermally conductive, so the composite is too, at least partly. Surface resistivity typically falls into the dissipative band rather than the insulating one, which is a benefit in ESD-sensitive handling and a problem where electrical isolation is required. If the part must be earthed or must hold a defined resistivity, that has to be specified explicitly rather than assumed.

Thermal conductivity rises as well, which can help a part shed heat but also spreads heat into adjacent components. Neither effect is usually the reason to choose the grade, but both should be checked, especially in electrical and electronics work where the industries we serve often need one or the other. A heat path that looks helpful in one assembly can be a nuisance in the next, so the thermal side is worth modelling for anything that runs hot.

Wear and PV limits in bearings and seals

Carbon fibre improves wear resistance against hard counterfaces and raises the achievable pressure-velocity limit. In bearing and seal service a carbon-filled grade can run at a higher PV than unfilled PEEK, provided the counterface is hard and well finished. Against a soft or rough counterface the abrasive fibre can wear the steel instead. Counterface hardness and finish matter as much as the polymer choice in sliding service.

The rule of thumb is to match the grade to the counterface. A hard shaft, fine finish and high load favour carbon filling; a soft or poorly finished counterface may suit unfilled PEEK or a lubricated blend better. Our PEEK grade selection notes cover the variants in more detail, and you can send us the duty for a recommendation.

Questions buyers ask about PEEK 30% carbon fibre

Is a carbon-filled PEEK suitable for load-bearing parts?

Yes, and it is often the point of the grade. The higher modulus and lower creep let it carry sustained load at temperature where unfilled PEEK would deflect. Check the impact case first, because the carbon grade is more brittle.

Does carbon fibre affect the electrical properties?

It makes the material partly conductive. Surface resistivity usually lands in the dissipative range rather than the insulating one, which suits ESD handling but rules the grade out where electrical isolation is required.

Can the carbon-filled grade be machined like unfilled PEEK?

It machines cleanly but is more abrasive on tooling, so expect shorter tool life and use carbide or diamond tooling with good dust extraction. Support thin sections well, because the material is stiffer but less tolerant of vibration.

When should I stay with unfilled or glass-filled PEEK?

When impact toughness matters more than stiffness, when the part must insulate electrically, or when the load is light. Unfilled PEEK is tougher; glass-filled grades are a middle ground with better stability than unfilled at lower cost than carbon.