Understanding Ultrasonic Plastic Welding

Ultrasonic plastic welding relies on the materials softening and not necessarily melting with increased temperature. This makes ultrasonic plastic welding suitable for thermoplastic polymers, but not thermosets. So what does this all mean?

Most thermoplastic polymers have an amorphous structure with varying degrees of crystallinity (the small regions of the material where the chain molecules are aligned much like a crystal). Crystallinity is at a disadvantage with ultrasonic plastic welding for several reasons. First, amorphous materials melt gradually over a range of temperatures whereas crystalline materials have a more precise melting point. This makes it difficult to achieve a good quality weld without compromising the materials. Secondly, crystalline materials tend to absorb vibrations long before they’re at the welding point. This translates to more power needed to weld the materials. So Polypropylene, polyester, polyethylene and nylon–all of which are semi-crystalline—are poor candidates for ultrasonic plastic welding. Materials that are more conducive to ultrasonic plastic welding have little or no crystalline structure. They include ABS, PVC, acrylic and polycarbonate.

The best results are obtained from welding components made from the same material. But in a few instances, dissimilar materials can be welded using ultrasonics. However, for this to work, the materials must be chemically compatible. They also must have similar melting points, or results will be unpredictable. One combination that seems to work is ABS with acrylic. If ultrasonic plastic welding doesn’t result in the kind of weld desired, then other techniques related to ultrasonic plastic welding should be considered, such as reforming or staking. These processes soften and reshape only one of the components while trapping the other in place. Strong structural bonds can be accomplished with these processes, but not necessarily a tight, enduring hermetic seal. In this case, you might consider using a supplementary seal, such as a gasket or o-ring.

Learn about the inner workings of thermal assembly systems and other plastic welders at Plastic Assembly Systems, the world leader in plastic assembly solutions, by logging onto HeatStaking.com.