Spin welding and ultrasonic welding are both used to join thermoplastic parts, but they are different in terms of the joining process and the types of parts they are best suited for.

In ultrasonic welding, high-frequency sound waves are used to generate heat and melt the parts together. The parts are held in place by a clamp, and a vibrating horn is used to apply pressure to the parts and create friction. The heat generated by the friction causes the plastic to melt, and when the horn is removed, the plastic cools and solidifies, creating a strong bond.

In spin welding, one part is held stationary while the other is rotated at a high speed. The parts are pressed together and heated using a spinning tool until they melt and fuse together. The heat generated by the friction of the spinning tool melts the plastic, and when the tool is stopped, the plastic cools and solidifies, creating a strong bond.

The main difference between the two methods is the way the plastic is heated. Ultrasonic welding uses high-frequency sound waves, while spin welding uses frictional heat generated by the spinning tool. In general, ultrasonic welding is better suited for small, complex parts with intricate geometries, while spin welding is better suited for larger, simpler parts with flat surfaces. Additionally, spin welding can create a stronger bond than ultrasonic welding in some cases, particularly when joining thick-walled parts. However, spin welding can be more difficult to control and may require more specialized equipment.