In the production of injection moulded parts, weld lines are often unavoidable. They form in the confluence area of two melt fronts caused either by several gates or by different melt front velocities due to varying wall thicknesses. In the confluence or weld line area, interfacial disturbances and molecular orientation often lead to a significant reduction of the material strength. At present, there are no methods available for the product development of plastic parts that offer a reliable prediction of the weld line strength. The resulting uncertainty of the mechanical properties of the part in the weld line area often leads to quality defects as well as premature and unexpected component failure and loss of function. Therefore, an integrative simulation method for the prediction of weld line strength in unreinforced amorphous thermoplastics has been developed. Using data from injection moulding simulations, the location and orientation of the weld line is determined and a reduction factor for the material strength of each finite element in the weld line area is calculated. The model for the calculation of the reduction factor is based on an interdiffusion model by Kim and Suh [KS86] which has been augmented to describe the influence of several process parameters such as melt temperature and holding pressure. For the calibration and validation of method and model, PC, PMMA and SAN tensile bars with defined weld lines were produced with varying process parameters using a double-gated mould. The tensile strength was determined and compared to the simulation results. The simulated weld line strengths show good accordance with the experimental results for the investigated materials. References: [KS86] Kim, S.-G.; Suh, N. P.: Performance prediction of weldline structure in amorphous polymers. Polymer Engineering and Science 26 (1986) 17, S. 1200-1207 - DOI: 10.1002/pen.760261707