Welding of foam injection molded parts - Analysis of the process - material - structure - property relations

More and more applications of everyday life consist of foamed thermoplastic components. The increasing requirements in terms of weight, material and time savings such as component integration and functionalization can be met using the FIM (Foam Injection Molding) process. The resulting characteristic three-layer structure of the components (foamed core, surrounded by a solid skin layer) offers advantages such as weight reduction, increased specific bending stiffness, less tendency to warp and optimized acoustic and thermal properties. The automotive, aerospace, transportation and construction industries in particular are therefore very interested in these applications or are already using this technology. For a subsequent joining process, however, difficulties arise due to a thin, solid skin layer. The joining process and the parameters as well as the mechanical properties strongly depend on the thickness of the solid skin layer. The aim of this study is 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, different foam technologies and foam injection parameters were used to manufacture microcellular polyamide and polypropylene specimen with and without fiber reinforcement. For the subsequent welding process, vibration and infrared welding were carried out to weld the foamed specimen. This study also examined the influence of a parameter variation, different welding arrangements and the dependence of the storage time on mechanical and optical properties. The aim was to investigate structure-process-property relationships and to create guidelines for the transfer to industrial applications. The results show an inhomogeneity of the characteristic three-layer structure of all materials depending on the flow path position and the injection molding process and parameters used. The most influential parameters for the mechanical properties of the welded components are the thickness of the solid layer, the welding process, the arrangement and the parameters as well as the storage time between the FIM and the welding process.