Improving weld uniformity in continuous ultrasonic welding of thermoplastic composites

Continuous ultrasonic welding (CUW) is an innovative high-speed joining method for thermoplastic composites. Currently, thin flat energy directors (EDs) are used to focus the heat generation at the weld line. The resulting fracture surfaces exhibit large areas of intact ED, resulting in a non-uniform weld, and significantly lowering the strength. The goal of this study is to improve the weld uniformity of continuous ultrasonically welded joints. In the first part of this paper we found that a 0.20 mm-thick woven mesh ED significantly improved the weld uniformity and strength in comparison to a 0.08 mm thick flat ED. The second part the paper focuses on understanding why the mesh gives this improved weld uniformity by analyzing the feedback data from the welder and by performing a microscopy analysis of the weld line at different moments during the static welding process. It was found that at the beginning of the welding process the mesh filaments expand within the open areas of the mesh while flattening; the mesh is being pre-formed in between the adherends. This pre-forming most likely created a good uniform intimate contact between the ED and adherends, which most likely resulted in a uniform heat generation and therefore created a uniform weld line. Because energy directing meshes make it possible to create uniform weld lines, they are expected to play an important role in the future for the continuous ultrasonic welding of thermoplastic composites.