Developed in the 1950s by the person considered by many to be “the father of composites,” W. Brandt Goldsworthy, pultrusion is the process of “pulling” raw composites through a heated die to create a continuous composite profile.

The term pultrusion combines the words, “pull” and “extrusion”. Extrusion is the pushing of material, such as a billet of aluminum, through a shaped die. Pultrusion is the pulling of material, such as carbon fiber and resin, through a shaped die.

HOW PULTRUSION WORKS

The typical pultrusion process starts with racks or creels holding spools of bundled continuous fiber (roving). Most often the reinforcement is fiberglass, but it can be carbon, aramid, or a mixture. This raw fiber is pulled off the racks and guided through a resin bath or resin impregnation system.

The raw resin is almost always a thermosetting resin, and is sometimes combined with fillers, catalysts, and pigments. The fiber reinforcement becomes fully impregnated (wetted-out) with the resin such that all the fiber filaments are thoroughly saturated with the resin mixture.

As the resin rich fiber exits the resin impregnation system, the un-cured composite material is guided through a series of tooling. This custom tooling helps arrange and organize the fiber into the correct shape, while excess resin is squeezed out, also known as “debulking.” This tooling is known as a “pre-former.” Often continuous strand mat and surface veils are added in this step to increase structure and surface finish.

Once the resin impregnated fiber is organized and removed of excess resin, the composite will pass through a heated steel die. Precisely machined and often chromed, the die is heated to a constant temperature, and may have several zones of temperature throughout its length, which will cure the thermosetting resin. The profile that exits the die is now a cured pultruded Fiber Reinforced Plastic (FRP) composite.

This FRP profile is pinched and pulled by a “gripper” system. Either caterpillar tracks or hydraulic clamps are used to pull the composite through the pultrusion die on a continuous basis.

At the end of this pultrusion machine is a cut-off saw. The pultruded profiles are cut to the specific length and stacked for delivery.

A variation is Pull-braiding where, in addition to the continuous fibers being pulled into the die, a braiding machine will apply a layer of fibers that acts to improve structural properties in other fiber orientations and/or adds an aesthetically pleasing look to the profile.