Ultrasonic welding is particularly suited for bonding medical devices because it uses the device material itself to form a bond and does not introduce glues or adhesives into the device. A fast, clean, efficient and repeatable process, ultrasonic welding is ideal for joining just about any set of plastic parts for many applications. To obtain these advantages, however, you need to select a readily weldable material, design and develop the joint, design and develop the fixtures, design and optimize the welding process and then control the process.
Ultrasonic welding joins thermoplastic parts by using high-frequency (typically 20 to 40 kilohertz) vibration to melt material at the junction of the parts. First patented in the 1960s, ultrasonic welding has since been developed and refined for use in many applications. Related processes include ultrasonic staking, swaging, and spot-welding.
Many thermoplastics, both amorphous (such as polystyrene) and semi-crystalline (such as nylon), can be ultrasonically welded. Ideally, both parts in a weld are made from the same material. However, many combinations of unlike plastics can be ultrasonically welded if their melting temperatures are fairly close.
In the ultrasonic welding machine the two parts are fixtured so they are aligned and touching. A “horn”, that transfers ultrasonic energy, is moved to apply pressure to the top part. The horn then vibrates at high frequency and the friction of one part vibrating against the other produces heat. This softens and melts the plastic at the interface between the parts, joining them.