10 1 Composite Material Structure and Processing (PEI),and polyphenyl sulfide(PPS),the processing temperature typically ranges from 300 to 400C. Thermosets(especially epoxy)have long been used as polymer matrices for car- bon fiber composites.During curing,usually performed in the presence ofheat and pressure,a thermoset resin hardens gradually due to the completion of polymer- ization and the associated crosslinking of the polymer molecules.Thermoplastic polymers have recently become important because of their greater ductility and processing speed compared to thermosets,and the recent availability of thermo- plastics that can withstand high temperatures.The higher processing speed of thermoplastics arises from the fact that amorphous thermoplastics soften im- mediately upon heating above the glass transition temperature(Ta),and so the softened material can be shaped easily.Subsequent cooling completes the process- ing.In contrast,the curing of a thermoset resin is a reaction that occurs gradually. Short-fiber or particulate composites are usually fabricated by mixing the fibers or particles with a liquid resin to form a slurry,and then molding to form a com- posite.The liquid resin is the unpolymerized or partially polymerized matrix material in the case of a thermoset;it is the molten polymer or the polymer dissolved in a solvent in the case of a thermoplastic.The molding methods are those conventionally used for polymers by themselves.For thermoplastics,the methods include injection molding(heating above the melting temperature of the thermoplastic and forcing the slurry into a closed die opening through the use of a screw mechanism),extrusion(forcing the slurry through a die opening via a screw mechanism),calendering (pouring the slurry into a set of rollers with a small opening between adjacent rollers to form a thin sheet),and thermoform- ing(heating above the softening temperature of the thermoplastic and forming over a die using matching dies,a vacuum or air pressure,or without a die using movable rollers).For thermosets,compression molding or matched die molding (applying a high pressure and temperature to the slurry in a die to harden the thermoset)is commonly used.The casting of the slurry into a mold is not usually suitable because the difference in density between the resin and the fibers causes the fibers to float or sink unless the viscosity of the resin is carefully adjusted.To form a composite coating,the fiber-resin or particle-resin slurry can be sprayed instead of molded. Instead of using a fiber-resin slurry,short fibers in the form of a mat or a contin- uous spun staple yarn can be impregnated with a resin and shaped using methods commonly used for continuous fiber composites.Yet another method involves us- ing continuous staple yarns in the form of an intimate blend of short carbon fibers and short thermoplastic fibers.The yarns may be woven,if desired.They do not need to be impregnated with a resin to form a composite,as the thermoplastic fibers melt during consolidation under heat and pressure. One method of forming unidirectional fiber composite parts with a constant cross-section (e.g.,round,rectangular,pipe,plate,I-shaped)is pultrusion,in which fibers are drawn from spools,passed through a polymer resin bath for impregnation,and gathered together to produce a particular shape before enter- ing a heated die.10 1 Composite Material Structure and Processing (PEI), and polyphenyl sulfide (PPS), the processing temperature typically ranges from 300 to 400°C. Thermosets (especially epoxy) have long been used as polymer matrices for car￾bon fiber composites. During curing, usually performed in the presence of heat and pressure, a thermoset resin hardens gradually due to the completion of polymer￾ization and the associated crosslinking of the polymer molecules. Thermoplastic polymers have recently become important because of their greater ductility and processing speed compared to thermosets, and the recent availability of thermo￾plastics that can withstand high temperatures. The higher processing speed of thermoplastics arises from the fact that amorphous thermoplastics soften im￾mediately upon heating above the glass transition temperature (Tg), and so the softened material can be shaped easily. Subsequent cooling completes the process￾ing. In contrast, the curing of a thermoset resin is a reaction that occurs gradually. Short-fiber or particulate composites are usually fabricated by mixing the fibers or particles with a liquid resin to form a slurry, and then molding to form a com￾posite. The liquid resin is the unpolymerized or partially polymerized matrix material in the case of a thermoset; it is the molten polymer or the polymer dissolved in a solvent in the case of a thermoplastic. The molding methods are those conventionally used for polymers by themselves. For thermoplastics, the methods include injection molding (heating above the melting temperature of the thermoplastic and forcing the slurry into a closed die opening through the use of a screw mechanism), extrusion (forcing the slurry through a die opening via a screw mechanism), calendering (pouring the slurry into a set of rollers with a small opening between adjacent rollers to form a thin sheet), and thermoform￾ing (heating above the softening temperature of the thermoplastic and forming over a die using matching dies, a vacuum or air pressure, or without a die using movable rollers). For thermosets, compression molding or matched die molding (applying a high pressure and temperature to the slurry in a die to harden the thermoset) is commonly used. The casting of the slurry into a mold is not usually suitable because the difference in density between the resin and the fibers causes the fibers to float or sink unless the viscosity of the resin is carefully adjusted. To form a composite coating, the fiber-resin or particle-resin slurry can be sprayed instead of molded. Instead of using a fiber-resin slurry, short fibers in the form of a mat or a contin￾uous spun staple yarn can be impregnated with a resin and shaped using methods commonly used for continuous fiber composites. Yet another method involves us￾ing continuous staple yarns in the form of an intimate blend of short carbon fibers and short thermoplastic fibers. The yarns may be woven, if desired. They do not need to be impregnated with a resin to form a composite, as the thermoplastic fibers melt during consolidation under heat and pressure. One method of forming unidirectional fiber composite parts with a constant cross-section (e.g., round, rectangular, pipe, plate, I-shaped) is pultrusion, in which fibers are drawn from spools, passed through a polymer resin bath for impregnation, and gathered together to produce a particular shape before enter￾ing a heated die