One of the most common defects in the injection molding of plastics parts are weld lines, which are formed in the confluence of two melt fronts. Due to the interfacial disturbances that occur in the confluence, a significant reduction of mechanical properties is often linked to weld lines. At present, there are no methods available for the product development of plastics parts that offer a reliable mechanical design including the weld line strength. The resulting uncertainty about the actual mechanical properties of the intermediate layers of the weld lines often leads to quality defects as well as premature and unexpected component failure and loss of function. For these reasons, a concept is being developed for the numerical prediction of the weld line strength of unreinforced thermoplastic parts. In a first step, a semi-empirical model is derived which describes a reduction factor of the weld line strength as a function of relevant process variables based on fundamental material investigations. In the model, the physical description of interdiffusion processes is also included to create a generalized and material-independent approach for the prediction of the mechanical properties of the weld line. To create the model, the strength of both stagnating and flowing weld lines as a function of mold and melt temperature, injection speed as well as holding pressure time and amount are determined. Investigating flowing weld lines, the geometry of flow obstacles is varied to analyse the influence of the angle between the converging flow fronts. The model is used to compute local reductions of material properties due to weld lines by importing and converting data from injection moulding simulations to structural simulations.