The aim of this thesis was to improve the mechanical properties of weld lines by two different approaches. On the one hand a variation of processing parameters was performed. On the other hand geometric modifications of the weld lines should enlarge the weld line area created and improve fiber orientation resulting in stronger weld lines. A double dumbbell specimen mold including a movable flow obstacle for the weld line modification was used. With this mold a specimen with a common weld line and a specimen with a modified weld line can be produced in one shot. Using a design of experiments the effects and interactions of processing parameters on the mechanical properties of specimens with "commmon" standard and modified weld lines were studied and compared to the results from literature and comparable. The investigated processing parameters seem to play an unimportant role for the improvement of the weld lines. On the other hand this makes the injection molding process robust and insensitive to minor fluctuations of processing parameters. Only the melt temperature has a significant influence on the weld line strength. Due to the geometric modification of the weld lines a drastic improvement of flexural properties and a significant increase of the tensile properties was achieved. Furthermore the structure of the modified weld lines was examined. Using incident light and darkfield microscopy polishes were scanned and a three dimensional model of a modified weld line was created. These investigations show that the weld line area of the modified weld line specimens was delocated from the original position, where the weld line area was expected to be. Furthermore, observations proved that the fiber orientation enhances weld line strength in the modified weld lines. The influence of five different obstacle head geometries was tested. The head geometries showed to have less influence on the weldline strength but some shapes have advantages in the injection molding cycle.