Study on Heating Mechanism in Ultrasonic Welding of Thermoplastics

Heating of workpieces plays an influential role in ultrasonic welding. In the present paper, an attempt was made to further understand the heating mechanism during ultrasonic welding. Three equations in this regard were derived. Firstly, based on the viscoelastic properties of the workpieces, the equation governing heat generation was derived. Secondly, a thermal equivalent circuit model was developed in order to describe the heat transfer process from the joint interface into the surrounding air. Lastly, based on the first law of thermodynamics, the governing equation of temperature distribution in a weld sample was deduced. Finite element method was also engaged to solve these equations to reveal the transient behaviour of the heat transfer during a welding process. The observations and numerical results match well with the measured temperature distribution. It is concluded that the temperature of the joint interface is the most characteristic variable of a welding process.