---
type: process
name: "Pultrusion"
category: "Forming"
subcategory: "Composites"
materials: ["Composite"]
tolerances: "Governed by ASTM D3917 for glass-reinforced pultruded shapes (EN 13706 in Europe). Wall thickness is held closest; overall width, straightness, and twist carry progressively looser bands. Do not assume machining-class tolerances on any pultruded dimension."
volumes: "Thousands to millions of linear feet; a custom die requires a long run to amortize"
lead_time: "10–20 weeks for a new die; standard catalog profiles ship from stock. Production runs are measured in feet per minute rather than parts per hour."
url: https://manufacturingprocesses.org/processes/forming/pultrusion
---

# Pultrusion

Pultrusion pulls continuous fiber through a resin bath and a heated die, curing it into a constant-section composite profile of unlimited length.

- **Category**: [Forming](https://manufacturingprocesses.org/processes/forming.md)
- **Family**: Composites
- **Materials**: Composite
- **Typical tolerances**: Governed by ASTM D3917 for glass-reinforced pultruded shapes (EN 13706 in Europe). Wall thickness is held closest; overall width, straightness, and twist carry progressively looser bands. Do not assume machining-class tolerances on any pultruded dimension.
- **Surface finish**: Die-formed surfaces are smooth and glossy where a surfacing veil is used, and show fiber texture without one. Cut ends expose raw fiber and are normally sealed for chemical or outdoor service.
- **Typical volumes**: Thousands to millions of linear feet; a custom die requires a long run to amortize
- **Lead time**: 10–20 weeks for a new die; standard catalog profiles ship from stock. Production runs are measured in feet per minute rather than parts per hour.

## Overview

Pultrusion pulls continuous reinforcement through a resin bath and then through a heated steel die that shapes and cures it, producing a constant-section composite profile in continuous length. Line speeds run roughly 12–60 in/min (0.3–1.5 m/min) with die temperatures of 250–390 °F (120–200 °C), and the product is cut to length on the fly, so length is effectively unlimited.

It is the highest fiber volume process in commercial composites — 40–70% depending on the reinforcement package — and the most anisotropic. Longitudinal roving carries the axial load; continuous strand mat or stitched fabric is added specifically to give the profile any transverse strength at all. Standard E-glass/polyester structural shapes are commonly published at around 30 ksi (207 MPa) longitudinal tensile strength and 2.5–3.0 Msi (17–21 GPa) flexural modulus, with transverse properties a small fraction of that.

Applications are structural and continuous: ladder rails, walkway grating, cable tray, concrete rebar, window lineals, utility crossarms, and I-beams and channels for corrosive environments where steel would not survive.

## How it works

1. **Creel and guide.** Glass or carbon roving is drawn from a creel of hundreds of packages, along with continuous strand mat, stitched fabric, or veil, and threaded through guide plates that place each reinforcement at its correct position in the cross-section.
2. **Impregnate.** The reinforcement passes through an open resin bath or, increasingly, a closed injection chamber immediately ahead of the die. Resins are unsaturated polyester for general structural work, vinyl ester for chemical resistance, epoxy for higher mechanical performance, and phenolic where fire, smoke, and toxicity performance governs.
3. **Preform.** A series of shaping plates progressively squeezes the wet package toward the final cross-section and drives out excess resin and air before the die.
4. **Cure in the die.** The chrome-plated tool steel die is typically 24–60 in (0.6–1.5 m) long and heated in zones to 250–390 °F (120–200 °C). Resin gels partway along and cures completely before the exit, so the profile leaves the die as a rigid solid. Exotherm inside the die is the process's controlling variable — too fast and the profile cracks internally, too slow and it exits uncured.
5. **Pull.** Reciprocating or caterpillar pullers grip the cured profile and provide the driving force for the whole line. Pull force, monitored continuously, is the primary process health indicator.
6. **Cut.** A flying cut-off saw with a diamond or carbide blade cuts to length without stopping the line.

## Design guidelines

### Constant cross-section, no exceptions
Pultrusion produces one profile per die and cannot vary the section along the length. Holes, notches, and cutouts are secondary operations, and every one of them cuts continuous fibers.

### Design for the fiber direction
Longitudinal properties are excellent and transverse properties are poor — often less than a quarter of the axial value. Never load a pultruded profile in a way that puts principal tension across the fibers, and never rely on the transverse strength of a flange to carry a bolt bearing load without added mat or fabric in that direction.

### Wall thickness
0.060–0.500 in (1.5–13 mm) is the normal range. Hold wall thickness as uniform as the section allows: a thick junction cures more slowly and exotherms more strongly than the thin walls around it, which is the usual source of internal cracking.

### Corner radii
Use inside radii of at least 0.060 in (1.5 mm) and preferably 0.125 in (3 mm). A sharp internal corner in the die is both a fiber-bridging site and a wear point in the tool.

### Connections
Bolted connections need generous edge distance — at least 3× hole diameter — and are limited by bearing strength, which is far below the axial tensile strength. Drill with carbide or diamond tooling and support the back face to avoid breakout delamination. Bonded and mechanically clamped connections often outperform bolts.

### UV and weathering
Polyester and vinyl ester surfaces chalk and fiber-bloom outdoors. Specify a surfacing veil, and a UV-stabilized resin or a painted or coextruded coating for exposed service.

### Tolerances
Dimensional tolerances for pultruded shapes are covered by ASTM D3917 (and EN 13706 in Europe); wall thickness is typically held closest, with overall width, straightness, and twist looser. Consult the standard rather than assuming machining-class tolerances.

| Feature | Recommended | Limit | Why |
| --- | --- | --- | --- |
| Cross-section | Constant along length | No variation possible | Fixed die geometry |
| Wall thickness | 0.125–0.250 in (3–6 mm) | 0.060–0.500 in (1.5–13 mm) | Cure exotherm and pull force |
| Section uniformity | Within 2:1 | 3:1 | Thick junctions exotherm and crack |
| Inside radius | 0.125 in (3 mm) | 0.060 in (1.5 mm) | Fiber bridging and die wear |
| Bolt edge distance | 4× hole diameter | 3× | Low bearing and shear-out strength |
| Transverse load | Add mat or fabric | Roving alone | Roving carries no transverse load |

## Cost drivers

The die is the fixed cost and it is significant: a chrome-plated tool steel die machined to the exact cured profile, one per cross-section, plus the preform plates and guides that feed it. That cost only makes sense across long production runs, which is why pultrusion is a catalog business — most users buy standard shapes rather than commissioning a die.

Running cost is dominated by material, because the process converts feedstock into product with very little waste and very little labor. Glass roving and polyester resin are inexpensive; carbon and epoxy raise material cost several-fold and typically also require slower line speeds.

Line speed is the throughput lever, and it is set by cure kinetics rather than by machinery. A thicker section cures more slowly and runs slower, so cost per linear foot rises faster than cross-sectional area alone would suggest.

Volume breakpoints: a custom die needs thousands of linear feet of production to amortize. Below that, use a standard catalog profile, or fabricate from [composite laminating](/processes/forming/composite-laminating) if the quantity is genuinely low.

1. Use a standard catalog profile wherever possible — the die is the whole capital argument.
2. Keep wall thickness uniform to run the line as fast as cure allows.
3. Specify glass unless stiffness or weight genuinely requires carbon.
4. Design connections as clamped or bonded rather than bolted; bearing strength, not tensile strength, governs bolted joints.
5. Add a surfacing veil for any outdoor application — it is inexpensive insurance against fiber bloom.

## FAQ

### Can pultrusion make a variable cross-section?

No. The die is a fixed geometry and the profile is pulled through it continuously, so the cross-section is constant along the entire length. Any variation — holes, notches, tapers — is a secondary machining operation that cuts continuous fibers and locally weakens the profile.

### How strong are pultruded profiles?

Very strong along the fibers and weak across them. Standard E-glass/polyester structural shapes are commonly published around 30 ksi (207 MPa) longitudinal tensile with 2.5–3.0 Msi (17–21 GPa) flexural modulus, while transverse properties are a small fraction of that. Design loads must follow the axial direction.

### How fast does a pultrusion line run?

Roughly 12–60 in/min (0.3–1.5 m/min) for typical glass/polyester profiles. Speed is set by cure kinetics inside the heated die, not by machine capability, so thicker sections and slower-curing resins such as epoxy run more slowly.

### Why does pultrusion need continuous strand mat as well as roving?

Roving runs only along the pull direction and carries no transverse load at all. Continuous strand mat or stitched multi-axial fabric is added specifically to give the profile transverse and shear strength, which is what allows a bolted connection or a flange to work.

### What tolerances apply to pultruded shapes?

ASTM D3917 covers dimensional tolerances for glass-reinforced pultruded shapes in the US, and EN 13706 in Europe. Wall thickness is held closest, with width, straightness, and twist looser. Pultrusion is a structural process, not a precision one — machine any feature that needs a fit.

### When is pultrusion cheaper than aluminum extrusion?

Where corrosion, electrical insulation, or thermal break performance matters. Pultruded profiles do not corrode, do not conduct, and have a fraction of aluminum's thermal conductivity, which is why they dominate in cooling towers, chemical plants, and window thermal breaks despite a comparable or higher material cost.

## Alternative processes

- [Metal Extrusion](https://manufacturingprocesses.org/processes/forming/metal-extrusion.md): Metal extrusion pushes heated billet through a shaped die, producing continuous aluminum or copper profile of constant cross-section.
- [Plastic Extrusion](https://manufacturingprocesses.org/processes/forming/plastic-extrusion.md): Plastic extrusion pushes molten polymer through a shaped die and cools it, producing continuous profile, sheet, pipe or film of constant cross-section.
- [Composite Laminating](https://manufacturingprocesses.org/processes/forming/composite-laminating.md): Composite laminating lays reinforcing fiber into a mold and consolidates it with resin, curing into a stiff, light monocoque part.
- [Roll Forming](https://manufacturingprocesses.org/processes/forming/roll-forming.md): Roll forming passes coil stock through a sequence of contoured roller stands that bend it progressively into a constant cross-section profile.
- [Filament Winding](https://manufacturingprocesses.org/processes/forming/filament-winding.md): Filament winding wraps resin-impregnated continuous fiber onto a rotating mandrel in a controlled pattern, producing tubes and pressure vessels.

## Related processes

- [Filament Winding](https://manufacturingprocesses.org/processes/forming/filament-winding.md): Filament winding wraps resin-impregnated continuous fiber onto a rotating mandrel in a controlled pattern, producing tubes and pressure vessels.
- [Composite Laminating](https://manufacturingprocesses.org/processes/forming/composite-laminating.md): Composite laminating lays reinforcing fiber into a mold and consolidates it with resin, curing into a stiff, light monocoque part.
- [Plastic Extrusion](https://manufacturingprocesses.org/processes/forming/plastic-extrusion.md): Plastic extrusion pushes molten polymer through a shaped die and cools it, producing continuous profile, sheet, pipe or film of constant cross-section.
- [Metal Extrusion](https://manufacturingprocesses.org/processes/forming/metal-extrusion.md): Metal extrusion pushes heated billet through a shaped die, producing continuous aluminum or copper profile of constant cross-section.
- [DMC and SMC Molding](https://manufacturingprocesses.org/processes/forming/dmc-and-smc-molding.md): DMC and SMC molding compression-molds a pre-mixed dough or sheet of chopped fiber and thermoset resin into a stiff, dimensionally stable part.

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*Source: [Manufacturing Processes](https://manufacturingprocesses.org/processes/forming/pultrusion)*

*Last updated: August 11, 2026*