Pultrusion

Fibers pulled through resin and a heated die, nonstop

Pultrusion pulls continuous fibers through a resin bath and then a hot die, where the resin hardens around them. Out comes an endless rod, tube or beam with the same cross-section all along, cut to length on the fly. Fiberglass ladder rails, walkway grating and cable trays are made this way.

Photo: Thomastechnik, CC BY-SA 3.0, via Wikimedia Commons

Pultrusion line, side view: six stations from creel to saw, plus typical profilesCircled numbers match the steps below. The row of shapes shows typical profiles; each one needs its own die.

How it works

Feed the fibers. Rovings (untwisted bundles of continuous fiber) unwind from a creel, a rack of spools. Mats or woven tapes are often added so the profile is also strong across its width.
Soak them in resin. The fibers run through an open resin bath, or the resin is injected straight into the die. Polyester, vinyl ester and epoxy are common choices.
Gather the shape. Guide plates called preformers squeeze out extra resin and bring the wet fibers close to the final cross-section.
Cure in the heated die. A steel die, typically about 0.5–1.5 m long, gives the exact shape. Heat of roughly 100–200 °C makes the resin harden before the profile leaves the die.
Pull continuously. Caterpillar belts, or two clamps that take turns, grip the cured profile and pull the whole line. This pulling is what gives the process its name (pull plus extrusion).
Cut to length. A flying saw, one that travels along with the profile while it cuts, chops pieces to length without stopping the line.

Key facts

Materials: Mostly glass fiber, also carbon or basalt; polyester, vinyl ester, epoxy or polyurethane resin
Typical parts: Rods, tubes, I-beams, channels, angles, flat bars, ladder rails, grating bars, cable trays
Part size: Cross-sections from a few millimeters to roughly 1 m wide; any length, cut as needed
Precision: Cross-section typically within about ±0.2–0.5 mm, depending on size
Surface finish: Smooth and resin-rich; often covered by a surface veil (a thin fiber tissue) for weather resistance
Batch size it suits: Long runs of one profile, often kilometers
Tooling and machine cost: Dies: roughly $5,000 to over $100,000 each; complete lines: about $100,000 to over $1 million
Speed: Typically about 0.2–3 m per minute, around the clock

Strengths and limits

Strengths

Continuous and automated: low cost per meter, little waste
Very strong and stiff along its length, often 60–70 % fiber by weight
Glass-fiber profiles don't rust, are light and don't conduct electricity
Any length in one piece, cut on the fly

Limits

Only straight profiles with one constant cross-section
Much weaker across the width unless mats or fabrics are added
Every new cross-section needs its own die, and changeovers take time
Cured thermoset profiles are hard to recycle

Where it's used

ConstructionWalkway grating, handrails, window and door frame profiles, fiber rebar
ElectricalCable trays, ladder side rails, insulating rods and poles
InfrastructureFootbridges, bridge decks, cooling-tower structures
EnergyCarbon strips for wind-turbine blade spars, utility poles
Everyday itemsTent poles, tool handles, plant stakes

How to recognize it

A long piece with exactly the same cross-section everywhere, with saw-cut ends.
Fine lengthwise fiber streaks at the surface, sometimes under a thin tissue-like veil.
At a cut end, a dense mass of fiber dots: the rovings seen end-on.
Classic look: yellow, gray or green fiberglass grating, ladder rails and handrails.