"CHAPTER 1 Introduction SHAU-TARNG LEE 1.1 THERMOPLASTIC FOAM Thermoplastic polymer is well known for its useful properties and thermal-reversible morphology. Modern processing systems have been developed to fully capture its structural transformation benefits for mass production. When gas is successfully incorporated, thermoplastic foam became a popular member in the thermoplastic family. Basically, foam can be defined as a gaseous void surrounded by a much denser continuum matrix, which is usually in a liquid or solid phase. It exists widely in nature; such as: in cellulositic wood, marine organisms, and other phenomena, and it can be made using synthetic processes (i.e., foamed plastics). The presence of gas voids can be outside, frothing, or inside, irreversible volume expansion. In most cases, gas phase possesses dramatically different properties and structures (or states) than the surrounding solid phase, as opposed to composite blends, to make a lighter heterogeneous cellular structure [1, 2]. A material property and density chart is shown in Figure 1.1 [3, 4]. It, more or less, seems to follow a linear band in the log-log performance with density chart. Foamed material evidently extends the lower limit of the property spectrum of the solid by at least a magnitude. When very tiny voids are evenly dispersed in the solid matrix without seriously disrupting its continuity, the parent property hardly varies when weight reduction is slight. As cell size, quantity, and its distribution vary, a much different composite property spectrum can thus be obtained. In other words, foamed material's performance/weight ratio can markedly vary from the solid material depending upon cell density, integrity, and morphology"--provided by publisher