| Weldability of Polymers The principle of ultrasonic assembly involves the use of high-frequency mechanical vibrations transmitted through thermoplastic parts to generate a frictional heat build-up at an interface. This bulletin provides guidelines on the welding characteristics of thermoplastics as well as an understanding of how polymer structure and other factors affect the weldability of various polymers. The term "weldability" is used generically and includes the ability to stake, swage, insert, or spot weld the resin. Polymers: Thermoset Versus Thermoplastic A polymer is a repeating structural unit formed during a process called polymerization. There are two basic polymer families: thermoset and thermoplastic. A thermoset is a material that, once formed, undergoes an irreversible chemical change and cannot be reformed with the reintroduction of heat and pressure; therefore, thermosets cannot be ultrasonically assembled in the traditional sense. A thermoplastic material, after being formed can, with the reintroduction of heat and pressure, be remelted and reformed, undergoing only a change of state. This characteristic makes thermoplastics suitable for ultrasonic assembly. All the information contained in this information sheet and others covering ultrasonic processes is based on the use of thermoplastic polymers (resins). Factors That Affect Weldability When discussing the weldability of thermoplastics, it must be recognized that there are a number of factors that affect the ultrasonic energy requirements and, therefore, weldability of the various resins. The major factors include polymer structure, melt temperature, melt index (flow), modulus of elasticity (stiffness), and chemical makeup. Polymer Structure Amorphous polymers have a structure characterized by a random molecular arrangement (Figure 1a). |