The Pultrusion Process

Where the name comes from

The word combines the English pull and extrusion . The difference sits right there: in extrusion the material is pushedthrough the die, while in pultrusion the curing profile is pulledout of it. The units applying the pulling force sit at the end of the line, so the profile is drawn rather than pushed.

That one detail sets the whole character of the method. Because fiber is fed continuously from creels the line never stops; the profile runs on for meters, even hundreds of meters, and is cut to whatever length is required. The die sets the shape of the section; the stability of the process sets its quality.

Step by step

• 1 · Fiber feed (creel): Continuous glass rovings, plus mat, woven or surfacing veil layers where needed, are fed from a rack of spools called the creel. How the layers are stacked determines the transverse strength and the surface quality of the profile, so the "recipe" for the line starts here.
• 2 · Resin bath (wet-out): The fiber bundles pass through a bath where resin, filler, pigment, UV additive and the cure system are mixed. The aim is to wet out every single strand; a dry spot means a weak point later. Some lines use a closed injection unit instead of an open bath — resin is fed straight into the die entry, which cuts emissions.
• 3 · Heated die: The wetted bundle first passes through preform elements that bring it close to its final section, then enters the heated steel die. There are usually several separate temperature zones along the die: the resin first thins, then gels, and finally completes its cure. By the time the profile leaves the die it is solid.
• 4 · Pulling unit (puller): Caterpillar or twin-clamp pullers draw the profile out of the die at a steady, vibration-free speed. Pull speed and die temperature are tied together: raise the speed and the cure will not finish; drop it and surface and color suffer. That balance is the heart of the line.
• 5 · Cut-off: Because the line never stops, cutting is done on the move; the saw travels with the profile and cuts it to length. Length tolerance, packing and labeling are completed at this station.

The parameters that govern the process

Pultrusion rests on a set-up line giving the same product over and over. The main variables that change the outcome are these:

• Fiber content and layup: As fiber content by weight rises, longitudinal strength rises with it; but mat layers are still needed for transverse behavior and surface quality. The balance is set according to where the section will be used.
• Die temperature profile: Not a single temperature but a curve planned across entry, middle and exit. It changes with the resin type.
• Pull speed: Directly tied to how long the cure takes; it drops on thick sections and rises on thin ones.
• Resin and cure system: Polyester resin and the cure profile are planned around the chemical, temperature and fire requirements of the environment.
• Surfacing veil: The fine mat in the outer layer keeps fiber from coming to the surface and raises UV and chemical resistance markedly.
• Die maintenance: The condition of the inner die surface directly affects the smoothness of the profile and its dimensional stability.

Why pultrusion?

Where the method stops

What makes pultrusion strongest is also its limit: the section has to stay constant along the length . Variable sections, tapered transitions and parts with local thickening cannot be made this way; those belong to other composite methods such as hand lay-up, RTM or filament winding.

The second limit is the die. For standard sections the die is ready and production starts quickly; when a project-specific section is wanted, the die is made first. That is why total meterage and delivery schedule are the two decisive headings for custom sections. If you have a sketch, we can tell you at a glance whether a die is needed.

↳ On a standard section we produce right away; on a custom section we cut the die first, then move to series production.

Which sections can be made?

Almost any geometry that satisfies the constant-section rule can be pultruded. The most frequently requested are:

C-channel, angle, tee and project-specific sections run on the same line. If you want to read more closely about the material itself, see What is a GRP profile? and What is an FRP profile? ; for the scope of the two terms see the difference between GRP and FRP profiles page is worth a look.