Ultrasonic welding is widely used to join polymers because it is fast, economic and suitable for mass production. This technique joins parts through friction and viscoelastic dissipation in the polymer.
The parts to be welded often have a small initial contact at the joint interface to concentrate the ultrasonic energy and initiate melting. For many applications, this small contact area is achieved with an energy director, a sharply pointed triangular rib molded onto the surface of one of the parts. But, because creating this feature can increase the cost of the parts, research has focused on ways to weld parts, particularly polymeric composites, without energy directors.