Hem flange bonding: a challenging joining process in automotive body construction

Hem flange bonding is widely used in the automotive body shop, especially in the manufacturing of hang-on parts such as doors, hoods and tailgates. By combining the hemming and adhesive bonding processes, components can be joined by material fit and form fit. In this way, new properties are integrated into the assembly, such as higher load-bearing capacity and improved resistance to corrosion. However, both processes also influence each other in the joining process and in the further produc-tion process chain, which makes the hem flange bonding process really challenging.The paper gives an overview of the state of the art. For this purpose, based on the structure of the bonded hem flange, the requirements and criteria for quality evalua-tion are described. The common hemming technologies and adhesive application methods as well as the industrial process chain, in which the hemming operation takes place, are presented. Essential correlations between component / process pa-rameters and the quality in the hem flange bonded joint are discussed.Fraunhofer IWU has been researching various aspects of hemming and hem flange bonding for many years and supports the industry in the analysis of assem-blies and processes. Our aim is to systematically increase the quality and appearance of hem flange bonded assemblies and the robustness of hem flange bonding process-es. Two current examples from research practice will provide a brief insight:Hemming adhesive dimensioning: Adhesive application plays a decisive, quality-defining role in the hemming process. At every point of the circumferential hem seam of an assembly, the adhesive quantity and adhesive position have to match the exist-ing geometry precisely. The paper shows how the adhesive dimensioning can be car-ried out. Current test results on the effect of glass beads in the adhesive layer to en-sure a defined adhesive layer thickness are also discussed.Hem testing: A testing device was developed at Fraunhofer IWU to carry out qua-si-static strength tests on hem flange bonded joints. For the first time, the influence of various hem and process parameters on the joint strength can be determined. The functional principle of the test and first results are presented.