The preformer is where the flat layers of reinforcements (typically CFM) and roving are formed into a shape that is as close to the die cavity as possible. The preformer is an array of tooling that gently shapes the materials prior to entering the die. The process is comparable to forming rolled aluminum into a rain gutter. The preformer is the step of the process that can make or break the robustness of the pultrusion process. Ideally, the preformer should continuously and consistently form the wetted reinforcements for weeks of processing without human intervention.
As an alternative to a resin bath and preformer, some pultruders use resin injection. Typically, the reinforcements are formed dry and enter a closed cavity where resin is injected. Like the preformer, once formed and adequately saturated with resin, the wetted reinforcement enters the heated die.
The heated die is the final forming step. The preformer will form the wetted reinforcement slightly thicker than the die cavity, and the die will squeeze the last bit of resin, compacting the reinforcement package to its final thickness. Pultrusion dies are usually surface hardened via nitriding or chrome plating to resist the abrasion of the reinforcements and fillers. The heated die activates the thermoset reaction, and the composite is cured as it progresses through the die. The internal release agent in the resin formula helps prevent the resin from adhering to the die surface.
The curing of the resin is usually the limiting parameter for the pultrusion line speed. Very thick parts might only run three inches per minute, where thinner parts might go 100 inches per minute or more. The degree of cure of the part must be verified through testing. Poorly cured parts offer reduced mechanical properties, corrosion resistance and elevated temperature properties.
On exiting the die, the cured profile is very hot – typically 300 F to 400 F – from the die heat and the resin exothermic reaction. Parts must be cooled before entering the polyurethane grippers of the puller. If a part is too hot when it enters the grippers, the part or the polyurethane grippers can be damaged. The cooling down process can be accomplished by extending the distance between the die and the puller (natural convection cooling) or by forcing air or water on the part between the die and the puller.
The cured profile is gripped by the puller. Pultruders typically use two continuous pulling systems; one that is a caterpillar counter-rotating type and the other a hand-to-hand reciprocating type. The puller is what makes the processes continuous. It is the same puller that overcomes the resistance of the drag of the dry reinforcements going through guides; the viscous forces of the resin being metered in the resin pan, preformer and die; the adhesive forces in the die and the force needed to push the traveling cut-off saw.