Ultrasonic Welding Background

The reader may notice I took a long deep breath before launching into the next topic, ultrasonic welding. It is perhaps the most complex topic to address in the blog, so it deserves a pause to consider. Ultrasonic welding allegedly originally came about as an accidental discovery in a lab where an ultrasonic cell disruptor was being used in the presence of polystyrene sample containers. Apparently one of the containers was excited accidentally by the disruptor tip and bonded to another container. Thus was born one of the most common and least understood of the thermoplastics assembly methods. The basic concepts of ultrasonic welding were reasonably well fleshed out by the early 1960s. To look at some of the early equipment now reminds us of how simple the early applications were compared to what is commonly being done now, and the crudeness of the early equipment causes one to wonder that the process ever came into widespread use. The early sonotrodes, or horns, were usually conical, cylindrical, or rectangular, staying well within the 1/3 wavelength limit for transverse dimensions, the equipment was relatively low powered by today’s standards, and timing and clamp force were up to the devices of a skilled operator. The early presses (assembly stands) were little more than manual arbor presses with a vibrating probe installed, often one manufactured for cell disruption, with timing of ultrasound controlled by a foot pedal manipulated by the operator. Whatever parts could be manufactured with any consistency whatsoever would have had to be made of the most favorable of materials, polystyrene or ABS, and of very simple design. Such parts would also be severely size-limited in order to be welded with such equipment. Round or rectangular parts of perhaps not more than 30mm transverse dimension could have been welded, but only once the operator had developed an instinctual feel for the process.