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.