Ultrasonic welding is one of the most popular techniques for joining thermoplastics. In this paper, a process analysis was conducted to describe the ultrasonic welding mechanism for thermoplastics. An in-situ sensor-less method was used to measure the welding force and the velocity applied to the workpiece. Using this method, the mechanical impedance variation and the force versus displacement curve at any time during ultrasonic welding can be generated and quantified. A detailed description of ultrasonic welding process for thermoplastics was then proposed based on the experimental data obtained. In this description, the welding process is divided into four distinct phases: the viscoelastic-plastic phase, the energy director melting phase, the melting completing phase and the upper & lower parts coupling phase. The process analysis clearly reveals that the mechanical impedance at the joint interface is the most characteristic variable of ultrasonic welding. Both the analysis method and the description of the process in the study are demonstrated to be useful tools for better understanding of ultrasonic welding mechanism.

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