Investigation on the melting of the weld interface in continuous ultrasonic welding of thermoplastic composites

Continuous ultrasonic welding is a new promising high-speed joining technique for thermoplastic composites. At the moment no straightforward method exists to find an optimum welding speed to easily obtain high strength joints. However, for static ultrasonic welding a systematic approach is available. Therefore, the aim of this study is to understand whether the optimum welding time in static ultrasonic welding can be related to the welding speed in continuous ultrasonic welding. The duration of the welding stages in static and continuous welding were compared in order to find similarities and discrepancies. The duration of the welding stages in continuous ultrasonic welding was estimated by analysing the weld interface and measuring the distances of the stages in the fractured interface around the location where the weld was stopped. No significant difference in weld duration was found for the first welding stage in which the mesh energy director is flattened. However, the second stage in which the energy director and adherends melt simultaneously, lasted much longer for the continuous welding process. This was possibly caused by the more constraining boundary conditions in the continuous welding process. In conclusion, the optimum welding speed in continuous welding was lower than the expected optimum welding speed based on the optimum welding time in static ultrasonic welding, because the second welding stage lasted longer in the continuous ultrasonic welding process.