Ultrasonic welding involves the use of high frequency sound energy to soften or melt the thermoplastic at the joint. Parts to be joined are held together under pressure and are then subjected to ultrasonic vibrations usually at a frequency of 20, 30 or 40kHz . The ability to weld a component successfully is governed by the design of the equipment, the mechanical properties of the material to be welded and the design of the components.
        Since ultrasonic welding is very fast (weld times are typically less than 1 second) and easily automated, it is a widely used technique. In order to guarantee the successful welding of any parts, careful design of components and fixtures is required and for this reason the technique is best suited for mass production.
        An ultrasonic welding machine consists of four main components: a power supply, a converter, an amplitude modifying device (commonly called a Booster) and an acoustic tool known as the horn (or sonotrode). The power supply changes mains electricity at a frequency of 50-60Hz, into a high frequency electrical supply operating at 20, 30 or 40kHz. This electrical energy is supplied to the converter. Within the converter, discs of piezoelectric material are sandwiched between two metal sections. The converter changes the electrical energy into mechanical vibratory energy at ultrasonic frequencies.
        The vibratory energy is then transmitted through the booster, which increases the amplitude of the sound wave. The sound waves are then transmitted to the horn. The horn is an acoustic tool that transfers the vibratory energy directly to the parts being assembled, and it also applies a welding pressure. The vibrations are transmitted through the workpiece to the joint area. Here the vibratory energy is converted to heat through friction - this then softens or melts the thermoplastic, and joins the parts together.
        Benefits of the process include: energy efficiency, high productivity with low costs and ease of automated assembly line production. The main limitation of the process is that the maximum component length that can be welded by a single horn is approximately 250 mm. This is due to limitations in the power output capability of a single transducer, the inability of the horns to transmit very high power, and amplitude control difficulties due to the fact that joints of this length are comparable to the wavelength of the ultrasound.