For this master thesis an in-mould-actuator tool was further developed in order to produce specimens with weld lines for tensile tests. The weld line strength of the specimens should be increased by using this tool which includes the actuator. Additionally the tool is fitted with ceramic heating elements to reach higher temperatures in the weld line area. Two materials were used in this thesis, a pure polypropylene and a polypropylene reinforced with 30 % glass fibres. 19 test series varying actuator parameters and actuator geometry were made and the specimens were tested afterwards by tensile testing in order to get information about the Young’s modulus and the yield strength. The results of the tensile tests show that for unreinforced polypropylene a special set of parameters and geometrical design leads to higher values of the yield strength and the Young’s modulus. If fibre reinforced materials are used a combination of special modes of the actuator increase the weld line strength. For further test series the heated areas as well as the heating elements were enlarged. The design and planning work for this has been done. Also some μCT-measurements has been planned in order to get more information about the orientation of the fibres at the weld line. With those results it should be possible to find out clear correlations between strength and fibre orientation in dependence of different modes of operation of the in-mould actuator.