The unique advantages of pultruded fiber reinforced polymer composites have enabled them to penetrate markets where other materials could not meet the design or end-use requirements efficiently. For high-volume applications, it is tough to beat the economics of the pultrusion process.
Pultrusion is a manufacturing process for producing continuous lengths of FRP structural shapes with constant cross-sections. Raw materials used in pultrusion include a liquid resin formulation, flexible textile reinforcing fibers and a polymer surfacing veil. In general, the pultrusion process involves pulling these raw materials – rather than pushing, as is the case in extrusion – through a heated steel forming die using a continuous pulling device. The reinforcement materials are in continuous forms such as rolls of continuous filament mats (CFM) or doffs (hollow spools) of fiberglass roving.
As the reinforcements are saturated with the resin mixture in a resin bath (“wet-out”), they are shaped by a preformer and pulled through a heated die. The cure of the resin is initiated by the heat from the die, setting off a catalytic reaction that results in a rigid, cured profile corresponding to the shape of the die cavity.
The pultrusion process can vary, ranging from a caterpillar puller (counter-rotating belt drives) to a reciprocating puller. However, the basic pultrusion process is described below and follows the flow shown in the chart below:
Starting from the tail end of the pultrusion line, roving racks and CFM creels hold the fiber reinforcements, which are delivered in wide rolls that are pre-slit to prescribed widths. The CFM is continuously pulled off the roll, similar to pulling paper towels. These creels can also hold other rolled goods such as stitched mats or tows of carbon fiber. The creels essentially stage the reinforcements for subsequent feeding into guide plates.
The guide plates precisely position the unrolled mats and rovings before entering the resin bath, which wets-out the reinforcements with a liquid resin formulation. The resin formulation generally includes a base resin, fillers, catalysts, pigments, wetting agents and an internal mold release.
The interior of the resin bath is carefully designed to optimize the wet-out of the reinforcements. Wet-out can be further optimized by separating the reinforcements as they are submerged in the bath. The resin bath is replenished with the resin formulation as the wetted reinforcement draws from the bath. On exiting the resin bath, the unrolled resin-saturated reinforcements are oriented in flat sheets before they enter the preformer.