Ultrasonic welding of thermoplastic composites with flat energy directors: Influence of the thickness of the energy director on the welding process

This paper presents a detailed experimental examination of the influence of the thickness of flat energy directors (ED) on the ultrasonic welding (USW) process for carbon fibre/polyetherimide composites. Three thicknesses of flat ED were compared: 0.06 mm, 0.25 mm and 0.50 mm. Power and displacement data for 0.06 mm-thick EDs did not clearly show the stages of the process and the location of the optimum for best weld quality. Consequently, an investigation of samples welded at different stages in the welding process had to be performed. For 0.06 mm-thick EDs, the optimum was determined to occur at the beginning of the downward displacement of the sonotrode in the vibration phase. The output parameters at the optimum conditions for all thicknesses were compared. Average lap shear strength was found to be lowest for 0.06 mm-thick EDs. Based on the analysis of the fracture surfaces, resin flakes and voids were observed when using the thinnest energy directors, indicating thermal degradation. These observations suggest that thin energy directors are not as efficient as thicker EDs (i.e. 0.25 mm) to achieve preferential heat generation at the weld line, leading to less consistent weld quality