In basic ultrasonic welding, ultrasonic vibrations are transmitted to the component and travel to the joint area where the parts meet. At the joint area, energy is dissipated in the form of frictional heat which meets the plastic, melts and welds the joining surfaces. Therefore, the shape of the two joining surfaces, called joint design, is extremely important for obtaining optimum results. Joint design varies with each application and depends on factors such as type of plastic to be bonded, part geometry and requirements of the weld (strength, hermetic seal, etc.).

Step Joint
Ultrasonic step joint is ideal for hiding/trapping external flash.

A variation on the use of an energy director is found in the shear joint. This type of joint is preferred for parts which require a hermetic seal or for plastics that change rapidly from a solid to molten state over a very narrow temperature range, such as crystalline resins (on such plastics, an energy director joint may not be the best design solution because the molten plastic from the triangular director solidifies before it can fuse with the second part). With the shear joint, welding is accomplished by first melting the small, initial contact area and then continuing to melt with the controlled interference along the vertical walls as the parts come together. Exposure to air is reduced in this smearing action, thus preventing premature solidification of the molten plastic.

Shear Joint 2

Shear Joint 5