Ultrasonic welding of plastics is performed through vibrations which are presented perpendicularly to the joint. Heating is dependent on the loss modulus of the polymer, the frequency and amplitude of the ultrasonic vibrations.

Advantages:
Internal heat generation means that less material is heated so there is a smaller heat affected zone and less chance for deformation
Very quick cycle times, typically less than a second
Well-established process with quite sophisticated equipment available and plenty of industry knowledge