Laser diode transmission welding of polypropylene: Geometrical and microstructure characterisation of weld

This study concerns the welding of polypropylene with diode laser using the overlap joint method. The influence of process parameters, both the laser power (20-40 W) and the welding speed (3-6 mm/s) on the geometry and the microstructure of the weld zone were investigated. The objective of this work is to evaluate the effects of selected welding parameters on the seam geometry, defects and material crystallinity. The out coming results help with the choice of the welding parameters that can satisfy the demands of users and consumers with respect of good quality and safety of the process. Microscopic observations performed on the cross section of the joining area reveal that the increase of the laser power and the decrease of the scanning speed lead to a larger volume of the weld zone with a more important depth penetration. The geometry of the weld zone is elliptic. Microscopic Fourier transform infrared (FTIR) spectroscopy method shows that diode laser welding induces thermal degradation of the polypropylene by random chain scissions. An increase of the crystallinity along the cross section of the welding joint was observed with a maximum reached in the centre of the weld zone. Some process parameters produced the occurrence of a void due to the thermal decomposition and the vaporisation of the polypropylene.