The use of CAD/CAM technology has greatly improved the manufacturing of contoured fixtures to support the part during ultrasonic assembly. Exact mirror replicas of the part can be created to provide excellent support to the part. Due to complex contours seen on many parts, it is recommended that the horn and fixture be purchased as a set, so proper alignment is achieved between the two. The fixture used for ultrasonic assembly is often overlooked, but usually very critical to the success of the application.

The use of a fixture that is not properly designed will usually result in very poor welds or no weld at all. Typically you want the fixture to support the part half to be welded that has the deepest draw. The horn will contact the other part to be assembled, so that the distance from the horn contact surface to the weld joint is minimized. The primary purpose of the fixture is to hold the part secure, in position and in alignment to the ultrasonic horn that will descend upon the assembly. It is important to support the bottom of the part to prevent it from deflecting under the pressure applied by the ultrasonic welder. If the part does not fit correctly, alignment can be impacted and energy that is needed for the weld can be dissipated into locations not necessary for the weld. A sloppy fit can actually create a condition in which the parts vibrate in unison instead out of phase with each other. It is preferred that the part in the fixture be held sufficiently to allow the mating piece to vibrate against the fixtured part and create a melt flow. If the parts vibrate in unison during ultrasonic vibration, the frictional activity is decreased resulting in less melt flow.