Pultrusion is a rather mature process that has been used in commercial applications since the 1950s. In the pultrusion process, a continuous fibrous reinforcement is impregnated with a matrix that is continuously consolidated into a solid composite. While there are several different variations of the pultrusion process, the basic process for thermoset composites is shown in Fig. 7.49. The reinforcement, usually glass rovings, is pulled from packages on a creel stand and gradually brought together and pulled into an open resin bath, where the reinforcement is impregnated with liquid resin. After emerging from the resin bath, the reinforcement is first directed through a preform die that aligns the rovings to the part shape and then guides it into a heated constant cross-section die, where the part cures as it progresses through the die. Curing takes place from the outside of the part toward the interior. Although the die initially heats the resin, the exotherm resulting from the curing resin can also provide a significant amount of the heat required for cure. The temperature peak caused by the exotherm should occur within the confines of the die and allow the composite to shrink away from the die at the exit. The composite part emerges from the die as a fully cured part that cools as it is being pulled by the puller mechanism. Finally, the part is cut to the required length by a cut-off saw.
While pultrusion has the advantage of being an extremely cost-effective process for making long constant cross-section composite parts, it is definitely a high volume process, as the set-up time for a production run can be rather costly. In addition, there are limitations in that the part must be of constant
Fig. 7.49. Pultrusion Process30
cross-section, and the flexibility in defining reinforcement orientation is somewhat limited. While glass fiber/polyester materials dominate the market, a considerable amount of work has been done to develop the process for the aerospace industry with higher performance carbon/epoxy materials. Floor beams for commercial aircraft are a potential application. Pultrusion is capable of making a wide variety of structural shapes as shown in Fig. 7.50, including hollow sections when an internal mandrel is used.
The major advantages of the pultrusion process are low production costs due to the continuous nature of the process, low raw material costs and minimal scrap, uncomplicated machinery, and a high degree of automation. Disadvantages include: the process is limited to constant cross-section shapes; set-up times and initial process start-up is labor intensive; parts can have higher void contents than allowed for some structural applications; the majority of the reinforcement is oriented in the longitudinal direction; the resin used must have a low viscosity and a long pot life; and in the case of polyesters, styrene emissions can create worker health concerns.
Due to the nature of the pultrusion process, continuous reinforcement must be used, in the form of either rovings or rolls of fabric; however, discontinuous mats and veils can be incorporated. To facilitate set-up, creel stands are often placed on wheels so that a majority of the set-up can be done off-line, reducing the down time for the pultruder. A consideration for the preimpregnation guide mechanism is that the reinforcement is usually fragile, and in the case of glass and carbon abrasive. Dry rovings are often guided by ceramic eyelets to reduce wear on both the fibers and guidance mechanism. Fabrics, mats, and veils can be guided with plastic, or steel, sheets with machined slots or holes. Chopped
strand mat is often used at an areal weight of 1.5oz/yd2 in rolls up to 300 ft, with a minimum width of 4 in. Several sets of guidance mechanisms may be required to gradually shape the reinforcements prior to impregnation. In a process called pull-winding, moving winding units are used to overwrap the primarily unidirectional reinforcement, thereby providing additional torsional stiffness.
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