The basic process of ultrasonic welding can be described by the following steps:
- The parts to be welded are placed in the anvil or fixture.
- The horn contacts the parts to be welded.
- Pressure is applied to keep the horn in contact with the welded materials and to hold them together.
- The horn delivers ultrasonic vibrations to heat up the materials. The vibrations move less than a millimeter either up-and-down or side-to-side.
- The materials are welded together.
- The horn gets retracted and the welded materials can be removed from the anvil.
The welding times, applied pressures and temperatures are controlled by a computer or microprocessor within the welding apparatus. And what actually happens during the welding process depends on the nature of the materials. In metals, the ultrasonic vibrations are delivered parallel to the plane of the materials. The frictional heat increases the temperature of the metal surfaces to about one third of the melting temperature, but does not melt the metals. Instead, the heat removes metal oxides and films from the surfaces. This allows the metal atoms to move between the two surfaces and form bonds that hold the metals together.
In the case of plastics, the vibrations are perpendicular to the plane of the materials and the frictional heat increases the temperature enough to melt the plastics. The plastic molecules mix together and form bonds. Upon cooling, the plastic surfaces are welded together. Welding times can vary, but the welds can form in as little as 0.25 seconds.
The factors that vary in ultrasonic welding are the frequency of the sound waves (usually 20, 30 or 40 kHz), the pressure applied to hold the materials together, and the time over which the ultrasound is applied (fractions of a second to more than one second).
The ultrasonic welding techniques described so far are good for materials (metals, plastics) that are similar. But what about materials that are not similar. Let's address this question by looking at how New Balance has used ultrasonic welding to assemble athletic shoes.