The use of ultrasonic energy to weld rigid thermoplastics was developed in the early 1960s. The principle of operation is that pressure is applied to the parts to be joned and ultrasonic vibrations are transmitted through the materials. The frictional heating at the interface causes melting of the plastics and the pressure, which is normally under 1 MPa, produces a weld. A major advanatage of ultrasonic welding is that strong joints can be produced in a few seconds. Most thermoplastics, including those with glass reinforcement, can be ultrasonically welded, but for some materials, such as acetal, nylon, polycarbonate, acrylic, and rigid PVC, good joint design is critical. In general, stiffer plastics are most suitable because they have lower damping and allow greater energy transmission to the joint. Since plastics can have a wide variation in structures and melting points it is preferable to produce welds between parts made from the same material. However, it is possible to weld dissimilar materials. particularly if they are compatible, for example, polystyrene and ABS or acrylic and ABS.

An ultrasonic assembly consists of a generator which converts electrical mains frequency into ultrasonic frequencies, and a transducer, which converts this high frequency electrical signal into mechanical vibration. These vibrations are transmitted to the bond area via a horn which is usually made from titanium or aluminium alloy. The former is preferred due to its low acoustic loss, high fatigue strength and high strength to weight ratio. To provide maximum vibration amplitude at the joint area, the horn is operated in resonance and so it must be accurately machined and tuned for the vibration frequency used. Most generators provide an ultrasonic frequency of 20 KHz and output powers can vary from about 300 W to 1 KW. It is not difficult to see why welds are produced in a matter of seconds when it is realised that at a frequency of 20 KHz, a typical displacement amplitude of 0.06mm at the north is equivalent to a relative displacement of 4.8 m in one second. For example, it is possible to put a metal insert into the plastic and this can be used for inserting metal hinges into spectacle frames. Another application for ultrasonic vibration is in the staking or riveting of plastics to components made from different materials, particularly, metals. Usually a hole in the metal sheet receives a stud from the plastic which is then melted and shaped using an ultrasonic horn.