Optical properties of plastics and their role for the modelling of the laser transmission welding process

Laser transmission welding of plastics is a joining technique which permits the welding of plastic parts with low process inherent thermal and mechanical stresses. In order to compute the temperature fields during the welding process a simulation model of the process is established. Especially the optical and thermal properties of the welded materials are necessary as input parameters in order to model the process properly. While the thermal properties of polymers are well-investigated, there is little knowledge about the optical properties of plastics. The optical properties of different amorphous and semi-crystalline thermoplastics are therefore investigated in dependence on material temperature and additive concentration. Based on these data it is possible to analytically describe the absorption of laser energy in plastics. The computation of temperature fields during welding is carried out for polypropylene and the results are then compared with experimentally accomplished welding tests. The calculated results show good accordance with the experimentally determined melt pool geometries of welded specimens and prove the compatibility of the computation model with the experimental analysis.