Ultrasonic & Radio Frequency Welding Facts

The choice between ultrasonic welding and RF welding begins with the product. Is it made of flexible or rigid plastic? Are the joints flat or dimensional? Does it require small, precise welds or long continuous seams? Here are a few key things to keep in mind.

Material compatibility. Material selection is the most important consideration in choosing ultrasonic welding or RF welding. For example, ultrasonic welding is typically limited to harder, more rigid plastics because they carry acoustic sound waves better than softer materials. RF welding requires materials that contain polar groups in their molecular structure; non-polar materials are invisible to radio frequency energy unless additional polar plastics are incorporated into the welding process. (See the chart below for more details.)

One interesting fact to note is that ultrasonic welding and RF welding both work best using identical (or similar) layers of material. Welding dissimilar plastics is possible with both methods, but requires specialized equipment and/or process adaptations in order to be successful.

Weld size. This is another important selection criteria. Ultrasonic welds are limited by the size of the horn (sonotrode) that makes them. Horn size, in turn, is determined by the wavelength of the ultrasound energy being used. As a result, ultrasonic welds are rarely larger than 3 inches, unless adaptations are made to the welder or specialized forms such as spot welding or rotating horns are used. RF welding can produce continuous welds measuring many feet long as well as very small and precise welds via custom-shaped dies.

Tooling design. A final point to consider is the complexity of the tooling design. Proper part and joint design are important in both ultrasonic and RF welding, but the geometry of the joints (or weld points) is more complex in ultrasonic welding and proper joint design is critical to ensuring successful welds.