Due to increasing demands on component integration, functionalization, saving weight or material, the density and weight of thermoplastic parts could be influenced significantly by using the thermoplastic foam injection molding process. The characteristic three-layer structure offers numerous advantages for applications, such as weight reduction, increasing the specific bending stiffness with a simultaneous low tendency to warp and optimizing thermal and acoustic properties. For a subsequent joining process, however, difficulties arise due to the thin solid skin layer. Minimum joining distances during welding can no longer be met geometrically and the mechanical properties of the components are reduced. The present study is intended to analyze the interaction between the microcellular structure of the injection molded parts, the influence on the joining process and the resulting mechanical properties. Therefore, a cooperation of Chemnitz University of Technology and University of Bayreuth was founded to investigate the correlation between injection molding and welding by vibration and infrared for microcellular polypropylene and polyamide materials. In addition to various materials with and without fiber reinforcement, the influence of storage time and different joint types were investigated in this study. The aim was to improve the knowledge of process - material - structure - properties as well as to prepare guidelines for the transfer to industrial applications. The results have shown that the characteristic three-layer structure has a considerable inhomogeneity, depending on the used material, the foaming process and the process parameters. However, the weldability of foamed thermoplastics strongly depends on the internal structure due to the injection molding parameters, the storage time between foam injection molding and welding process, the joint type and the welding process itself.