Carbon fiber pultrusion runs at 100-300 mm/min with 60-70% fiber volume fraction — roughly double the glass content achievable in hand lay-up. The trade-off: a 12K carbon tow costs $20-30/kg versus $2-4/kg for E-glass roving, and the die wear rate on carbon is 3-5X higher because carbon fibers are abrasive. You don’t choose carbon pultrusion because it’s easier — you choose it when the part needs 1,200-1,800 MPa tensile strength at under 1.6 g/cm³ density, or when CTE must match steel within 1-2 ppm/°C for bonded assemblies.
Carbon vs. Glass Pultrusion: Key Parameter Differences
| Parameter | Carbon Fiber Pultrusion | Glass Fiber Pultrusion |
|---|---|---|
| Fiber volume fraction | 60-70% | 45-55% |
| Tensile strength (longitudinal) | 1,200-1,800 MPa | 400-800 MPa |
| Tensile modulus | 120-160 GPa | 35-45 GPa |
| CTE (longitudinal) | -0.5 to +0.5 ppm/°C | 8-12 ppm/°C |
| Density | 1.5-1.6 g/cm³ | 1.9-2.1 g/cm³ |
| Electrical conductivity | Conductive (10³-10⁴ S/m) | Insulating (>10¹² Ω·cm) |
| Die wear rate | 3-5X baseline | 1X baseline |
| Resin viscosity (impregnation) | 200-500 cP | 300-800 cP |
| Typical pull speed | 100-300 mm/min | 300-800 mm/min |
Resin Selection: The Critical First Choice
Vinyl ester resin dominates carbon pultrusion at 80-90% of production volume. Its low viscosity (200-400 cP at 25°C) enables complete wet-out of 12K-50K carbon tows at 60-70% fiber volume without requiring elevated temperature impregnation. Epoxy provides 15-25% higher interlaminar shear strength (65-75 MPa vs. 50-60 MPa for vinyl ester) and better fatigue resistance but requires heated resin baths (40-60°C) and longer die residence times. The cost difference is significant: epoxy resin at $8-15/kg vs. vinyl ester at $4-7/kg — multiplied by 30-40% resin content, that’s $2.50-4.50/kg additional material cost for the finished profile.
Die Temperature Profile for Carbon Pultrusion
Carbon fiber conducts heat 10-15X faster than glass (10-15 W/m·K axial vs. 1.0-1.3 W/m·K). This changes the die temperature profile fundamentally. For a 900 mm die with 8 mm wall thickness running at 250 mm/min, the recommended profile is: zone 1 (entry, first 150 mm) at 100-110°C — cooler than glass to prevent skinning at the die wall; zone 2 (middle 300 mm) at 130-140°C for gelation; zone 3 (exit 450 mm) at 160-170°C for final cure. A common mistake: using a glass pultrusion temperature profile on carbon. The result is surface blistering from the exotherm front racing ahead of the die temperature gradient.
Pull Force Monitoring as a Quality Metric
Carbon pultrusion pull forces run 30-50% higher than equivalent glass profiles — typically 15-25 kN for a 50 mm × 6 mm flat strip vs. 8-12 kN for glass. But the absolute value matters less than the variation. Pull force that oscillates ±3 kN within a 30-second window indicates inconsistent wet-out or fiber tension. A steady increase of 1-2 kN over 2 hours signals die buildup, requiring cleaning. Modern pultrusion lines log pull force at 1 Hz; the standard deviation over a production shift should stay below 10% of the mean. Trending upward? Check the die interior surface and fiber sizing compatibility.
Frequently Asked Questions
Can I pultrude carbon and glass fiber together in the same profile?
Yes — hybrid pultrusion places carbon tows in the flanges (where tensile modulus matters most) and glass in the web (where shear strength dominates). The CTE mismatch between carbon (-0.5 ppm/°C) and glass (10 ppm/°C) creates internal stresses during cooling that can cause micro-cracking if the transition between fiber types is too abrupt. Placing a thin glass veil between carbon and glass layers reduces this stress concentration by 40-60%.
What is the minimum bend radius achievable with pultruded carbon profiles?
Straight pultrusion produces constant cross-sections only. For curved profiles, radius pultrusion (also called curved pultrusion) uses a curved die and differential pulling to achieve radii as tight as 300-500 mm for thin sections (3-5 mm). This process runs at reduced speeds (50-150 mm/min) and requires specialized die design, but it produces finished curved parts without post-forming.
Why do pultruded carbon profiles sometimes have surface voids?
Surface voids (1-5 mm diameter, shallow) are almost always caused by volatiles trapped at the die wall — either moisture from inadequately dried fiber (<0.1% moisture content required) or styrene boil from vinyl ester at die temperatures above 175°C. Reduce zone 1 temperature by 5-10°C and verify fiber storage conditions (<30% RH).
What post-processing can be done on pultruded carbon profiles?
Pultruded carbon can be cut, drilled, and bonded. Drilling requires carbide or diamond-coated bits at 3,000-5,000 RPM with light feed pressure to prevent delamination at the exit side. Adhesive bonding with methacrylate or epoxy adhesives achieves 15-25 MPa lap shear strength with proper surface preparation (light sanding + solvent wipe). Mechanical fastening with bolts is possible but reduces load capacity by 30-50% at the hole due to stress concentration.
For a comparison with wet lay-up methods, see our prepreg vs wet lay-up guide. For material selection data, see our carbon fiber vs aluminum comparison. Industry references: CompositesWorld and ACMA pultrusion standards.
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Standards & References
The following industry standards are referenced in this article:
