Pultrusion Process: Steps, Diagram and Limits

What pultrusion is

The word combines pull and extrusion. In extrusion, material is pushed through a die. In pultrusion, continuous reinforcement is pulled through the die, and the reinforcement itself carries the pulling load. The fiber is usually glass, sometimes carbon, basalt or aramid. The matrix is usually a thermoset resin such as unsaturated polyester, vinyl ester, epoxy, phenolic or polyurethane. Because the reinforcement runs the full length of the part, a pultruded profile is strongest along its axis. That single fact explains most of the process limits described below.

The process step by step

Read the list below as the diagram. A pultrusion line is laid out in a straight row, and the material passes through every station in this order.

1. Creel and reinforcement feed. Rovings come off bobbins on a creel; mats, fabrics and veils come off rolls. Tension devices keep each strand at a set pull so the fiber arrives at the die in the right place.
2. Guides. Plates with drilled holes and slots sort hundreds of roving ends and mat strips into the pattern the cross-section needs. The guide set is specific to the profile.
3. Impregnation. The reinforcement is wetted with resin, filler and additives. Two routes exist: an open bath the fiber is dragged through before the die, or an injection box where resin is pumped into the fiber inside or just ahead of the die. Injection keeps styrene and isocyanate vapor contained and is the usual route for polyurethane.
4. Preformer. A series of shaped guides squeezes out excess resin and folds the wet pack toward the final geometry before it enters the steel die.
5. Heated die. The die is a steel block with a polished cavity of the profile shape. Heaters along its length bring the resin to its cure temperature. The resin gels, cures and shrinks slightly away from the wall, and the solid profile exits the far end.
6. Puller. Two kinds exist: reciprocating hydraulic clamps that grip and release in turn, or a caterpillar belt that grips continuously. The puller sets the line speed and must overcome the friction of the curing profile against the die wall.
7. Cut-off saw. A traveling saw clamps to the moving profile, cuts it to length and returns. Cut parts are stacked, inspected and packed.

What happens inside the die

EPTA describes the sequence this way. The wet reinforcement enters a cooled section so the resin does not start to react in the inlet. Heaters then raise the material to the temperature at which the peroxide initiator decomposes, and curing spreads from the hot die surfaces toward the center of the section. Curing is exothermic, so the center of a thick wall can run hotter than the die wall. As the resin gels it grips the cavity wall, and in hollow profiles it grips the mandrel too. Internal release agent in the resin mix and a polished, chrome-plated cavity keep that friction low enough for the puller. The die is typically 0.75 to 1.5 m long and is split into sections so it can be opened, cleaned and reground.

Published operating ranges

These figures are the ranges the European pultrusion association publishes for the industry. They are not a recipe for a specific profile, and they are not ratings for any machine sold through pultrx. A real parameter set depends on wall thickness, resin system, reinforcement structure and die length, and it is established in trials.

For how temperature, speed and pull force are recorded and interpreted during a trial, read the pultrusion process parameters guide

Advantages of the process

• Continuous output. Once the line is stable it runs for a shift with little operator input, and length is limited only by handling.
• High axial strength and stiffness. Continuous rovings run the full length of the part, so tensile and bending properties along the axis are high for the weight.
• Low tooling cost compared with molds for discrete parts. One die makes one cross-section in any length.
• Built-in corrosion resistance and electrical insulation from the resin and glass, without coatings.
• Dimensional repeatability. The steel die fixes the outside geometry, so cut parts from the same die and parameter set match.

Limits of the process

• Constant cross-section only. Tapers, steps and bosses have to be machined afterward or made another way.
• Hollow sections need a mandrel held from the inlet side, which adds friction and limits very thin walls on small hollows.
• Transverse properties are set by mats and fabrics, not by rovings, and are lower than axial properties. Fastener bearing and shear need checking.
• Speed falls as wall thickness rises, because heat has to reach the center of the section and the exotherm has to be controlled.
• Each profile needs its own die, guide set and preformer. Short runs carry the full tooling cost.
• Start-up produces scrap until temperature and speed settle, and die changes take hours, so the economics favor long runs of a few profiles.

Variants you will meet in quotations

Open-bath impregnation remains common for polyester and vinyl ester. Resin injection is required for polyurethane and increasingly used for other resins to control emissions. Pull-winding adds a station that wraps hoop fibers around the pack before the die, used for tubes that need transverse strength. Vertical pultrusion exists for symmetric hollow sections. Thermoplastic pultrusion uses a different die and cooling arrangement and runs faster. When a quotation names one of these, confirm that the die, the impregnation unit and the puller are rated for it, because each variant changes the pull force and the cure path.

To decide between a pultruded and an extruded section for a given part, see the pultrusion vs extrusion comparison

Where the process stands in a purchase

Buyers of finished profiles do not need to specify the process; they specify the section, the material system, the tolerances and the evidence. Buyers of a production line specify the profiles they intend to make first, because the die, guides, preformer and puller rating all follow from the section. Henan Zhongsheng, the manufacturer behind pultrx, runs pultrusion lines for its own profile production, and pultrx quotes profiles, dies, tooling and equipment from that single source.