Because plastic parts of sufficient complexity could be molded, joining of plastics did not receive much attention in the past, and joiningmethods were considered secondary operations to be avoided if possible. Important exceptions were high-performance composites made of thermoset resin-based unidirectional laminates, mainly used by the aerospace industry, for which adhesive bonding and mechanical fastening technologies were developed. Now joining of plastics and plastic composites has become important for several reasons. First, such materials are increasingly being used in complex structural assemblies, in which joining considerations and cost are important. Second, the emerging structural (load-bearing) applications of polymeric materials require structural joints that must withstand static and fatigue loads. Third, weldable filled and unfilled thermoplastic resins are being used in many structural applications. A good example is the first all-plastic 8-km h−1 (5-mph) bumper, made of an unfilled thermoplastic polycarbonate (PC)/poly(butylene terephthalate) (PBT) blend (Section 12.2.7), which was fabricated by vibration welding two injection-molded parts (Figures 2.4.1 and 21.7.5). Welding is also an important consideration in high-performance thermoplastic composites.