Short-fiber nylons are a common route to stiff fixtures, brackets and tooling on a filament printer. Prusa says fibers improve dimensional stability and high-temperature resistance, and that carbon-fiber nylons warp much less than unfilled ones. The trade-offs are abrasion, a higher risk of clogs, and, according to Prusa, usually more brittle parts with weaker layer-to-layer adhesion. Carbon and glass fibers shift those trade-offs in different directions.
This page compares Polymaker's Fiberon PA6-CF20 and Fiberon PA6-GF25. Both use a PA6 (nylon 6) base, share the same recommended print settings, and were tested the same way: annealed at 100 °C for 16 hours, then measured dry and after 48 hours in 60 °C water. The differences reflect the two products as sold, not carbon and glass at a matched fiber content.
Carbon-fiber-filled nylon and Glass-fiber-filled nylon side by side
| Property | Carbon-fiber-filled nylon | Glass-fiber-filled nylon |
|---|---|---|
| Print settings | 280–300 °C nozzle, 40–50 °C bed, room-temperature chamber, fan off (Polymaker) | 280–300 °C nozzle, 40–50 °C bed, room-temperature chamber, fan off (Polymaker) |
| Nozzle wear | Brass nozzle life about 9 h; hardened steel or ruby nozzle highly recommended (Polymaker) | Brass nozzle life about 9 h; hardened steel or ruby nozzle highly recommended (Polymaker) |
| Drying, annealing and storage | Dry 100 °C for 10 h; anneal 100 °C for 16 h; store and use below 20% RH | Dry 100 °C for 10 h; anneal 100 °C for 16 h; store and use below 20% RH |
| Heat deflection, ISO 75 (1.8 / 0.45 MPa) | 173 °C / 215 °C | 157 °C / 191 °C |
| Vicat softening / glass transition / melting | 219.2 °C / 74.2 °C / 218.5 °C | 211.7 °C / 70.4 °C / 214.5 °C |
| Tensile modulus, dry (X-Y / Z) | 8636.5 / 3759.5 MPa | 5356.9 / 3375.9 MPa |
| Tensile strength, dry (X-Y / Z) | 109.3 / 54.0 MPa | 80.1 / 60.7 MPa |
| Bending strength, dry (X-Y / Z) | 161.0 / 71.3 MPa | 133.8 / 99.9 MPa |
| Elongation at break, dry (X-Y / Z) | 2.1% / 1.9% | 2.4% / 4.0% |
| Notched Charpy, X-Y (dry / wet) | 11.0 / 35.6 kJ/m² | 10.0 / 28.0 kJ/m² |
| Unnotched Charpy, Z (dry) | 7.5 kJ/m² | 16.2 kJ/m² |
| Tensile modulus, wet (X-Y) | 2508.1 MPa at 5.30% moisture | 1793.6 MPa at 4.57% moisture |
| Equilibrium water absorption | 3.3% | 3.33% |
| Density, ISO 1183 | 1.17 g/cm³ | 1.20 g/cm³ |
| Flammability and surface resistivity | UL 94 HB at 1.5 mm; surface resistivity above 10¹² Ω (ANSI ESD S11.11) | UL 94 HB at 1.5 mm; surface resistivity above 10¹² Ω (ANSI ESD S11.11) |
Property values come from Polymaker's Fiberon PA6-CF20 and Fiberon PA6-GF25 technical data sheets (V1.1; ISO methods; specimens printed at 300 °C with 100% infill and annealed at 100 °C for 16 h; wet values after 48 h in 60 °C water). General notes on fiber-filled filaments come from Prusa's composite and polyamide articles.
Stiffness in the print plane
Carbon fiber's clearest advantage is stiffness along the layers. Dry Fiberon PA6-CF20 has an in-plane tensile modulus of 8,637 MPa against 5,357 MPa for PA6-GF25, and a bending modulus of 7,038 MPa against 4,314 MPa. It is also stronger in-plane, at 109.3 MPa tensile and 161.0 MPa bending strength, against 80.1 MPa and 133.8 MPa.
That makes the carbon grade a good fit for long, flat parts loaded along their length, such as fixture arms, where deflection matters more than ultimate strength. The figures are for specimens annealed at 100 °C for 16 hours, which Polymaker recommends after printing; unannealed parts may differ.
Strength between layers
Across the layers, the order reverses. PA6-GF25 has higher Z-direction tensile strength (60.7 against 54.0 MPa), bending strength (99.9 against 71.3 MPa) and elongation (4.0% against 1.9%). Its unnotched Charpy value in the Z direction is more than twice the carbon grade's, at 16.2 against 7.5 kJ/m².
Prusa says fibers usually worsen layer-to-layer adhesion compared with the unfilled polymer. Polymaker describes both of these grades as keeping good layer adhesion, which is a manufacturer claim. For a part loaded across its layers, such as a bracket printed upright, the glass-fiber grade's Z values are the more relevant numbers.
Heat and moisture
Both grades' heat deflection temperatures sit far above their glass transition temperatures of about 70 to 74 °C. Polymaker lists heat deflection at 1.8 MPa of 173 °C for PA6-CF20 and 157 °C for PA6-GF25, and 215 °C and 191 °C at 0.45 MPa.
Both are still nylons, and water softens them. After 48 hours in 60 °C water, the in-plane tensile modulus fell to 2,508 MPa for the carbon grade (5.30% moisture) and 1,794 MPa for the glass grade (4.57% moisture), roughly a third of their dry values. Polymaker asks for both to be stored and used below 20% relative humidity and dried at 100 °C for 10 hours, and warns that a support structure printed in the same material can bond permanently if left to absorb moisture.
Nozzles and printing
Polymaker says both grades wear a brass nozzle out in roughly 9 hours of printing and highly recommends a hardened steel or ruby nozzle. Prusa says fiber-filled filaments need a hardened nozzle, carry a higher clog risk, and print best with at least a 0.4 mm nozzle and 0.2 mm layers; it also suggests cleaning the nozzle first, for example with a cold pull.
The print recipe is otherwise identical: a 280 to 300 °C nozzle, a 40 to 50 °C bed, a room-temperature chamber and the fan off, with Polymaker's PolyDissolve S1 as the recommended support material. Neither grade is sold as static-dissipative; both data sheets list surface resistivity above 10¹² Ω.
Which should you choose?
Choose Carbon-fiber-filled nylon if…
- The part must deflect as little as possible under load along its layers, such as a long fixture arm.
- Heat under load matters: 173 °C heat deflection at 1.8 MPa against 157 °C.
- The highest in-plane tensile and bending strength matter more than strength across layers.
- You already run a hardened or ruby nozzle and want the stiffest of the two.
Choose Glass-fiber-filled nylon if…
- The part is loaded across its layers, where PA6-GF25's Z-direction strength is higher.
- Some stretch across layers helps: 4.0% elongation in Z against 1.9%.
- Impact across layers matters: 16.2 against 7.5 kJ/m² unnotched Charpy in Z.
- You want a balance of stiffness and layer strength rather than maximum in-plane stiffness.
Common questions
Is carbon fiber nylon stronger than glass fiber nylon?
Along the layers, yes, in Polymaker's data: dry Fiberon PA6-CF20 has 109.3 MPa in-plane tensile strength against 80.1 MPa for PA6-GF25. Across the layers, PA6-GF25 is stronger, at 60.7 against 54.0 MPa.
Do both need a hardened nozzle?
Yes. Polymaker says either grade wears a brass nozzle out in about 9 hours and highly recommends hardened steel or ruby. Prusa also says fiber-filled filaments need a hardened nozzle.
Is carbon fiber nylon electrically conductive?
Not this grade, by Polymaker's data. Fiberon PA6-CF20 lists surface resistivity above 10¹² Ω under ANSI ESD S11.11, the same as PA6-GF25. If you need static control, use a grade sold specifically for ESD use and check its data sheet.
Sources
- Fiberon PA6-CF20 technical data sheet (V1.1) Polymaker · Checked 2026-11-02
- Fiberon PA6-GF25 technical data sheet (V1.1) Polymaker · Checked 2026-11-02
- Composite materials (filled with carbon, kevlar or glass) Prusa Research · Checked 2026-10-15
- Polyamide (Nylon) Prusa Research · Checked 2026-10-15
This comparison summarizes the cited documentation; it is not a lab test. Properties vary by brand, grade, print settings and orientation, so check the data sheet for the exact product you buy.