Thermal Effects on Ultrasonic Joining of Thin Plastic Films Using Torsional Vibrations

Heat generation caused by high-amplitude ultrasonic vibrations at the interface has long been believed to be the most dominant factor for the mechanism of ultrasonic plastic joining. However, the authors have not found any conspicuous thermal effects in experiments of ultrasonic joining of thin very low density polyethylene (VLDPE) films using torsional vibrations. In the process of joining, the temperature at the interface of two VLDPE films of 0.1 mm thickness has increased up to 73 °C only and no trace of melting of the material has been observed at this interface under a polarizing microscope. Investigation using a differential scanning calorimeter (DSC) has revealed that the "melting point" of VLDPE is about 110 °C, and an ultrasonically joined specimen, unlike a heat-sealed specimen, shows no significant difference in thermal characteristics compared with an intact VLDPE film. The VLDPE films cannot be been joined, even after being pressed together for a period of 30 min or longer at a temperature approximately 80 °C.