Effect of contact area with fixture on dynamic behaviour of joint interface in ultrasonic welding of thermoplastics

Abstract

Ultrasonic welding is one of the most common methods for joining thermoplastics. The two plastic parts to be joined are placed on the fixture before high frequency mechanical vibration is applied to the parts through an ultrasonic horn. This generates heat at the joint area and locally melts thermoplastics. The parts are welded together. In this paper, we investigate the effect of the contact area between the lower part and the fixture on the dynamic behaviour of the joint interface, which affects heat generation. The displacements and elastic strains of the interface are predicted using finite element dynamic contact analysis and compared for different contact areas. The results show that the dynamic behaviour of the interface depends on the dynamic characteristics of the two parts. When both natural frequencies are close to the horn vibration frequency, the displacements and strains are small. Conversely, when the lower part has no natural frequency near the horn vibration frequency, the displacements and strains are large. The contact area significantly affects the dynamic behaviour of the interface. This is because its dynamic behaviour depends on the natural frequencies of the parts to be joined while the natural frequency of the lower part is easily improved by the contact area.