Study of vibration and viscoelastic heating of thermoplastic parts subjected to ultrasonic excitation

Ultrasonic welding is the most popular technique for joining thermoplastics, however, the vibration and heating mechanisms during ultrasonic welding are not fully understood. The main objective of this work is to study the relations between vibration and viscoelastic heating of thermoplastic parts during ultrasonic excitation. This also involves investigating the effect of the dynamic moduli of the thermoplastics on ultrasonic vibration and heating. This work also demonstrates the feasibility of using computer-aided-design methodology to detect design problems in parts prior to molding and welding. The dynamic moduli of polystyrene were measured by the flexural resonance and forced non-resonance methods. Finite element analysis was used to predict the mode shape, resonant frequency and forced vibration response of a thin-walled polystyrene box. Using the dynamic storage modulus as an input into the FE modeling resulted in good agreement between predictions and experimental measurements. To improve understanding of vibration and viscoelastic heating, the simpler structure of a flexural beam was studied. Both the elastic solutions of the Bernoulli-Euler and Timoshenko beams were derived. The correspondence principle was applied to obtain the viscoelastic beam solutions. The Bernoulli-Euler beam solution was inadequate because it neglects the effects of rotary inertia and shear deformation. The viscoelastic Timoshenko beam solution, which includes these effects, was in good agreement with the measurements. Then the equations for predicting the energy dissipation due to bending and shear were derived. The predicted energy dissipation due to bending was almost two orders of magnitude higher than that due to shear so that the heating was dominated by bending. The temperature of the beam was measured along the length by using a non-contact infrared temperature transducer. It was found that the hot spots were at the displacement antinodes which correspond to the peaks in strain due to bending. The FEM results were in good agreement with the Timoshenko beam solution and measurement. Ultimately this work will enable the use of computer simulations to design parts for ultrasonic welding instead of the trial and error process of today. This will reduce the development time and costs associated with part design for ultrasonic welding.