Prediction of the Bond Strength of Thermoplastics Welded by Laser Transmission Welding

Abstract

Laser transmission welding is one of various welding techniques used to join thermoplastics. Low heat introduction into the welded parts and a high welding speed are the reasons why laser transmission welding established itself as a joining process in the plastics processing industry. To minimise defective parts and maximise productivity, it is essential to determine a set of ideal welding parameters that allow maximum bond strength at the lowest possible cycle times. To facilitate this process, simulation models provide detailed analysis without the need for destructive and costly part testing. To predict the weld strength of two thermoplastic parts joined by laser transmission welding, the Institute for Plastics Processing has developed a model that combines the simulated thermal properties of the material during and after welding with the molecular behaviour of plastic melts. Based on the results of the thermal modelling of the welding process a mathematical model describing the movement of polymer chains is used to calculate the resulting bond strength depending on material properties as well as heating and cooling rates. The temperature data of nodes situated at the interface of both welding partners are extracted from the simulation for every time increment of the simulation. The model, which is based on the reptation theory of polymer melts, is then used with these data and the bond strength is calculated. The results are validated by tensile tests on welded parts with the same input parameters used in simulation. In first results, the calculated bond strength shows a good agreement with the values measured in tensile tests. Occasional deviations can be explained by the fact that the material decomposition occurring in experimental welds is not considered in the thermal simulation and the reptation theory.