| The first step in the formation of a weld bond is interfacial molecular contact; subsequently, the chains interdiffuse across the interface and become entangled with one another. Weld bonds are limited by the cohesive strength of the components. Thus, in order to understand the weld strength of various polystyrenes, it was first necessary to study their physical and mechanical properties. Tensile strength and elongation increase with increasing molecular weight and strain rate, but decrease with increasing temperature until the brittle-ductile transition is approached. For high MW and low temperature, crazes are very numerous and fine in texture. The number of crazes decreases and they become short and jagged with decreasing MW or increasing temperature.;The welding factor increases with increasing welding time and welding temperature. The effect of welding pressure on the welding factor falls into three categories, based on the welding temperature. At low temperature, welding factors increase monotonically with increasing welding pressure. At higher temperature, the welding factor passes through a maximum with increasing pressure. At still higher temperatures, the maximum is shifted to lower pressure. The flow pattern influences chain orientation and the microstructure of the heat affected zone. These were characterized by optical and polarized microscopy. Also, increasing molecular weight shifts the welding window to higher temperature; a broad molecular weight distribution widens the welding window.;In order to eliminate melt flow in the welding zone, a confining jacket was designed for the welding process. With confinement, the welding factor increases as welding pressure increases, and it can reach unity. |
