Double-Pulse Ultrasonic Welding of Fiber-Reinforced Composite
Ultrasonic welding is widely used to join polymers because it is fast, economic and suitable for mass production. This technique joins parts through friction and viscoelastic dissipation in the polymer.
When welding polymers without an energy director, the energy is not concentrated, and coulombic friction at the faying interface is less than that of a joint with an energy director. As a result, the parts need more time to melt, and overall welding time is longer. However, thermoplastics produce viscoelastic dissipation under ultrasonic welding, which leads to an increase in part temperature with the extension of weld time. When the temperature rises above a certain point, thermal decomposition of the polymer will occur and a porous region will form, which severely deteriorates weld quality.
We have developed an alternative solution, double-pulse ultrasonic welding. In our technique, the first pulse is used to preheat the parts, while the second completes the weld. The first pulse generates heat at the joint interface. The materials at the interface melt first, and the temperature is higher there than in the middle of the workpiece. After cooling for a while, another ultrasonic pulse is applied at the same location to extend the size of the weld. The temperature in the middle of the workpiece is maintained below the decomposition temperature of the plastic by adjusting the weld time and cooling time between the two pulses. As a result, weld quality is improved by increasing energy dissipation at the joint interface while inhibiting thermal decomposition.