Regarding the necessity of lightweight construction, performance and design, composites are well established. At this moment, the joining of these composites still is often a major challenge. While suitable technologies like welding, sticking or riveting do exist, there are several drawbacks of these technologies such as damaging the fiber structure and low bonding strength. The combination of welding and sewing is a novel approach to improve the bonding of glass fibre-reinforced polypropylene and the influence of the processing parameters is not yet well understood. This study aims at analyzing the influence of parameters such as operating temperature, overlap length, needle temperature and thread material on the tensile strength of thermoplastic composites joined in the previously described hybrid approach. Furthermore, the interaction between thread and composite material is investigated by a microscopical analysis. The joint strength has been found to depend on the overlap length of the joint and the results suggest a contribution of the seam to the joint strength, especially for lower overlap lengths. Influences of the processing temperature, the thread material and needle temperature are also present, however, due to the complex interaction, no systematic conclusions could be drawn for these yet. The optical analysis confirms the findings of thermoset composites stitched before impregnation, where additional seams cause dislocations and local agglomeration of fibers. Additional investigations are necessary to determine the effect of the stitching on the joint and to further the understanding of the interaction between composite and thread material.