Pultrusion is a process in which dry, continuous fibers are pulled through a bath of resin and then through a die. The die serves two purposes: it forces the bundle of wet fibers to conform to the shape you want to create and, since the die is heated, it will cure the resin to set the bundle of fibers into its final shape. After the composite comes out of the die, it is allowed to post-cure while being pulled to the saw where it will be cut to stock lengths.
How pultrusion works:
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.
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.
Advantages of Pultrusion:
High stiffness to weight ratio
Also known as specific stiffness, it allows materials of different mass to be compared quickly in rigidity-sensitive applications where weight is still a factor. Carbon fiber does extremely well in this area, being about 3 times stiffer than steel and aluminium for a given weight.
With all the fibers running unidirectionally, pultruded tubes take optimal advantage of this characteristic.
High strength to weight ratio
Also known as specific strength, this is similar to the stiffness to weight ratio. This ratio allows you to compare materials of different mass for applications where resistance against breaking has priority.
The ability to easily make tubes with specific wall-thicknesses
The inside diameter of a pultruded tube is determined by a mandrel, which is easy to exchange for a different sized one, making it easy to produce tubes with varying wall thickness.
Easy to machine
Because of the low density of composites, they lend themselves to be machined well.