Rub your hands together rapidly. Notice anything? They warmed up, right? If you take a hammer and pound a metal surface rapidly and repeatedly, you will find that the place where the hammer strikes the metal warms up, too. In both these examples, the heat is due to friction. Now imagine rubbing your hands or pounding that hammer thousands of times per second. The frictional heat generated can raise the temperature significantly in a very short time. Basically, high-frequency sound (ultrasound) causes rapid vibrations within the materials to be welded. The vibrations cause the materials to rub against each other and the friction raises the temperature at the surfaces in contact. This rapid frictional heat is what sets the conditions for the materials to bind together.
Ultrasonic welding equipment has four main parts. A power supply converts low-frequency electricity (50-60 Hz) to high-frequency electricity (20 - 40 kHz; 1 kHz = 1000 Hz). Next, a transducer or converter changes the high-frequency electricity into high-frequency sound (ultrasound). A booster makes the ultrasound vibrations bigger. Finally, a horn or sonotrode focuses the ultrasound vibrations and delivers them to the materials to be welded. Besides these pieces, there is an anvil upon which the welded materials are stacked and held. There is also some method to apply force (usually air pressure supplied by a pneumatic piston) to hold the materials together during welding.
So what materials and industries take advantage of this clever process? Ultrasonic welding of plastics is used widely in making electronics, medical devices and car parts. For example, ultrasonic welding is used to make electrical connections on computer circuit boards, and assemble electronic components such as transformers, electric motors and capacitors. Medical devices, such as catheters, valves, filters and face masks are also assembled using ultrasonic welding. The packaging industry uses this technique to make films, assemble tubes and blister packs. Even Ford Motor Company has explored using ultrasonic welding to make aluminum chassis in cars.