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Ultrasonic welding (USW) is a welding technique that uses ultrasonic vibrations of high frequency to weld the two pieces together. It is most commonly used to weld thermoplastic materials and dissimilar materials. A high frequency (15 kHz to 40 kHz) ultrasonic vibration is used to join two plastic pieces together. The high-frequency vibration generates heat energy at the interface of the two pieces and melts the material. The melted materials fused with each other to form a strong weld on cooling and solidification. The typical frequency used is 15, 20, 30, 35 or 40 kHz.

In order to fuse thermoplastic components with rotationally symmetric joints, the spin welding process uses heat generated by rotational friction at the joint line. The spin welding machine applies pressure axially while rotating one part against its stationary counterpart, and the resulting friction generates heat that melts the two pieces together.