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.
The authors previously determined that thermal effects are not a dominant factor in the ultrasonic joining of very low density polyethylene (VLDPE) films using torsional vibration. Now, to confirm that the plastic materials are not ''melted'' by mechanically generated heat in the joining, they have conducted joining experiments for thin poly(ethylene terephthalate) (PET) films. The temperature at the interface of two PET films of 0.1 mm thickness only increased to approximately 100 C, and no trace of liquidation of the material was observed at the interface under a polarizing microscope. Investigation using a differential scanning calorimeter (DSC) revealed that the ''melting point'' of PET is about 260 C, and an ultrasonically joined specimen showed no significant difference in thermal characteristics compared with an intact PET film. It was also determined that the PET films cannot be joined even after being pressed together for a period of 30 min or longer at approximately 150 C. From the results obtained using the microscope and the DSC, the authors conclude that melting of the materials plays essentially no role in ultrasonic plastic joining.
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