At first glance, ultrasonic welding and radio frequency (RF) welding appear to have a lot in common. They’re quick, clean and energy efficient. They produce strong, durable bonds that are as strong or stronger than the parent materials. And they don’t require solvents, adhesives, or other additives.
But when it comes to material choices, weld sizes, and tooling specifications, these techniques are clearly different – and play specialized roles in the world of plastics welding. Here’s what you need to know to make sure you’re investing in the right technology.
How they work
Both ultrasonic and RF welding work by combining indirect heat and pressure to cause molecules in the materials being joined to intermingle and form homogeneous bonds. The difference is how that indirect heat is produced.
Ultrasonic welding uses high-frequency sound waves. In ultrasonic welding, standard 60 Hz alternating electrical current is converted inside a welding machine to high-frequency electrical energy and then to high frequency (ultrasonic) vibrational energy. A booster further increases the amplitude of the sound waves and transmits them to an acoustic tool called a sonotrode, or horn. Vertical vibrations are transmitted through the horn to layers of material positioned under pressure at the joint interface – at rates from 20,000 to 40,000 cycles per second.
The resulting frictional heat allows the plastic layers to melt and flow together. As the material cools, a strong molecular bond is formed.
RF welding uses high-frequency electromagnetic energy. In RF welding, the materials to be welded are placed between two metal bars called electrodes, or sealing dies. When the electrodes are pressed together, high-frequency electromagnetic energy waves (typically 27.1 MHz) are passed between them, creating a rapidly alternating electric field. Molecules in the materials begin to oscillate, realigning themselves to conform to the polarity of the energy field and releasing thermal energy in the process.
As the layers of material heat from within, pressure from the electrodes simultaneously allows the mingled molecules to fuse in place and creates a permanent seam.
Where they work
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.