Guides / Grades

Carbon Filled or Glass Filled PEEK: How to Decide

September 20, 2026

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

Once a drawing says filled PEEK, the real decision is carbon or glass. They cost differently, machine differently and fail differently, and picking the wrong one usually shows up late, either as a warped fixture or as an insulator that conducts.

The numbers side by side

Property	PEEK GF30	PEEK CF30
Density	1.49 to 1.51 g/cm³	1.40 to 1.44 g/cm³
Tensile strength	150 to 170 MPa	130 to 140 MPa
Tensile modulus	about 10 GPa	about 13 GPa
Thermal expansion	about 22 µm/m·K	about 15 µm/m·K
Thermal conductivity	about 0.4 W/m·K	about 0.9 W/m·K
Electrical behaviour	Insulating	Static dissipative
Dry sliding	Poor, abrasive to the mate	Good, and better in bearing blends
Relative stock cost	Lower of the two	Higher

Typical published values. The pattern matters more than any single figure: glass buys strength cheaply, carbon buys stability, heat transfer and a conductive surface.

Four questions that decide it

1. Does the part have to insulate?

If yes, GF30. Carbon fibre makes PEEK conductive enough to bleed charge, which rules it out for insulating washers, connector bodies and standoffs. If the part instead has to dissipate static, as in wafer handling and display equipment, that same property is why CF30 exists.

2. Does the part have to hold a dimension?

CF30, clearly. At roughly 15 µm/m·K it moves about a third as much as unfilled PEEK and noticeably less than GF30. On a 200 mm fixture running between 20 °C and 150 °C that is the difference between a part that locates and one that does not. Higher thermal conductivity also pulls heat out of the part instead of letting it soak in.

3. Does the part slide against something?

CF30, and better still a bearing blend with PTFE and graphite. Glass fibre is hard and angular, so a GF30 part chews the surface it runs against. This is the mistake that ruins a shaft, not the bushing.

4. Is the part simply loaded, cold and cheap?

GF30. It is the lower cost of the two, it is stronger in tension and it resists creep well at temperature. For a bracket, a housing or a clamp block that does not slide, insulate or hold a tight fit, glass is the sensible answer.

Where each one is normally found

GF30: pump and valve bodies, structural brackets, housings, clamp blocks, insulating hardware, parts loaded at temperature in process equipment.

CF30: wafer handling combs and end effectors, CMP retaining rings, test and inspection fixtures, metrology parts, dry running bushings, seal parts in downhole tools.

What changes in the shop

Both grades wear tooling faster than unfilled PEEK, and both need attention that an unfilled part does not:

• Fresh, sharp tooling. A worn edge in filled stock rubs, and rubbing in PEEK means heat and a poor surface.
• Separate tooling from unfilled work. Many shops keep filled and unfilled tools apart so fibre dust never reaches medical parts.
• Dust handling for carbon. It is conductive and fine, so shops running it near electronics extract it properly.
• Expect fibre at the machined surface. Neither grade polishes to a mirror. If you need a smooth sealing face, plan a finishing operation or use unfilled.
• Anisotropy. Short fibre stock is stronger along the extrusion direction. On long parts, say which direction carries the load so the shop can choose the blank orientation.

Cutting data and tooling geometry for both are in machining PEEK: tooling, speeds and feeds.

The mistakes worth naming

Specifying GF30 for a bushing. It will wear the shaft. Use a bearing grade.

Specifying CF30 for an insulator. It will pass enough current to matter in a high impedance circuit.

Specifying a filled grade for a part that gets autoclaved hundreds of times. Unfilled PEEK is the tougher material and the fibre ends at a machined surface give steam a path in.

Specifying filled stock for a small chunky part with open tolerances. You are paying for properties the part never uses.

This choice is one step of our guide to PEEK grade selection, which also compares moulded-bar and stock-sheet figures.

Not sure which side your part falls on? The grade summary in which PEEK grade should be on your drawing covers all five grades, or send the drawing with the temperature, load and media and the shops we match will 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