Laser transmission welding of absorber-free semi-crystalline polypropylene by using a quasi-simultaneous irradiation strategy

Unlike other joining techniques, laser transmission welding offers unique advantages such as selective and contactless energy deposition. This enables the fabrication of flexible seam geometries at low mechanical and thermal stresses. However, the use of absorbing additives for the lower joining partner such as carbon black is crucial as most polymers are transparent in the spectral range of typical beam sources (800–1100 nm). A novel approach is the application of beam sources emitting radiation within the polymeric intrinsic absorption bands between 1500 and 2000 nm. This enables absorber-free laser welding of transparent polymers for medical or microfluidic applications such as Lab-on-a-Chip devices. The main drawback on the other hand is the large heat affected zone (HAZ) due to the volume absorption which is extending over the entire cross section. A possible way to overcome this disadvantage is a quasi-simultaneous irradiation strategy. It could be proved in the past that the HAZ of polycarbonate (PC) can be reduced in the vertical direction by up to 30% compared with contour welding. Since the effects of light scattering on the absorber-free quasi-simultaneous irradiation strategy are still unknown, the beam propagation was simulated in polypropylene (PP). Based on the results, a thermal simulation of the welding process was carried out using the finite element method (FEM). The simulation model was then evaluated by comparing the results with experimental trials.