When welding plastics using Ultrasonic, the interface of the two parts is specially designed to concentrate the melting process. The vacuum form (lid) usually has a spiked energy director which contacts the second plastic part (back side).

 

The Ultrasonic energy melts the point contact between the parts, creating a joint. This process is a good automated alternative to glue and adhesive materials, connected the most of thermoplastics like PVC, PET, OPS, PP.
 
It has environmental protection and is typically using with small parts like cell phones, consumer electronics, disposable medical tools, toys.