Glass fibers processing

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The hand lay-up or spray-up process, used universally for the production of laminar composites incorporating glass fiber reinforcement, is most efficient for the manufacture of large parts, such as boats, bathtubs, tanks, architectural shapes, and recreational accessories. Resins intended for spray-up processes are usually modified with thixotropic additives, such as fumed siHca (1%), to reduce the risk of drainage when appHed over large vertical mold surfaces. Molds are also made from ERP for short-mn products usually surfaced with a tooling gel coat to provide consistent surface quaHty and appearance. 

When surfactant-coated glass fibers are included to improve product flexibility and reduce thermal expansion, these semibatch processes are termed reinforced reaction injection mol ding (RRIM). 

In the spray-up process a reinforcement, usuaHy glass fiber, is substituted for the mat and a special spray gun simultaneously chops the glass fiber and appHes it with catalyzed resin to the mold surface. Hand rolling techniques then consoHdate the fiber and resin to conform to the mold surface contours. The shorter chopped fibers aHow for more intricate design detaHs than do mats. Both processes rely heavily on the operators skiHs for product quahty. These two processes require the least capital investment and have the largest product size capabHity of aH the processes. A single-surface mold produces a part with one controHed (usuaHy the visible) surface. 

Reinforced Thermoplastic Sheet. This process uses precombined sheets of thermoplastic resin and glass fiber reinforcement, cut into blanks to fit the weight and size requirements of the part to be molded. The blanks, preheated to a specified temperature, are loaded into the metal mold and the material flows under mol ding pressure to fiU the mold. The mold is kept closed under pressure until the temperature of the part has been reduced, the resin solidified, and demolding is possible. Cycle time, as with thermosetting resins, depends on the thickness of the part and the heat distortion temperature of the resin. Mol ding pressures are similar to SMC, 10—21 MPa (1500—3000 psi), depending on the size and complexity of the part. 

Polypropylene can be fabricated by almost any process used for plastics (see Plastics processing). The extmsion of pipe and injection mol ding of fittings present no unusual problem. However, there is no way to bond the fittings to the pipe except by remelting the polymer, which is impractical on most constmction sites. The resin can be reinforced by glass fibers, mineral fillers, or other types of fillers and can be pigmented readily. 

Polyurethane. Polyurethanes (pu) are predominantly thermosets. The preparation processes for polyurethane foams have several steps (see Urethane polymers) and many variations that lead to products of widely differing properties. Polyurethane foams can have quite low thermal conductivity values, among the lowest of all types of thermal insulation, and have replaced polystyrene and glass fiber as insulation in refrigeration. The sprayed-on foam can be appHed to walls, roofs, tanks, and pipes, and between walls or surfacing materials directly. The slabs can be used as insulation in the usual ways. 

Some of the common types of plastics that ate used ate thermoplastics, such as poly(phenylene sulfide) (PPS) (see Polymers containing sulfur), nylons, Hquid crystal polymer (LCP), the polyesters (qv) such as polyesters that ate 30% glass-fiber reinforced, and poly(ethylene terephthalate) (PET), and polyetherimide (PEI) and thermosets such as diaHyl phthalate and phenoHc resins (qv). Because of the wide variety of manufacturing processes and usage requirements, these materials ate available in several variations which have a range of physical properties. 

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