One of the most important factors in optimizing the ultrasonic plastic welding process is the right joint design. 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 (ED), a sharply pointed triangular rib molded onto the surface of one of the parts. Because the ED is designed to provide a small pointed contact area, its size and shape—or rather the sharpness, roundness, or flatness of the tip—are critical factors for producing a good weld. The geometrical consistency of the ED, both within a single part as well as on a part-to-part basis, can significantly affect process repeatability, joint quality and weld strength.