Complementary approach for the bubble-free seam sealing of bonded hem flanges

Inner and outer panel of automobile doors, trunks and hoods are joined via hem flange bonding. During the hemming process, air-filled cavities can form within the bonded hem flange, which in turn can lead to blisters forming in the PVC seam seal. Since the seam seal remains visible, the blisters adversely affect the optical quality of the end product. Besides this, there is a risk that these defects further develop by mechanical effects during the vehicle lifetime, which would allow direct access of electrolytes into the flange leading to the disruption of the corrosion protection for the bonded hem flange. To avoid the latter, any defect in the seam seal during their manufacture has to be repaired, a step involving time-consuming and cost-intensive finishing work. In order to avoid the costs associated with repairs, more intensive quality control, and potential damage of the manufacturer's reputation in case severe corrosion issues arise, appropriate measures have to be taken. The research project summarized herein investigates the mechanisms of defect formation; based on the observations it suggests measures to minimize such defects, and ultimately allowing to produce blister-free seam seals. This report describes the results of the FOSTA research project P909 carried out jointly by Fraunhofer IFAM and Fraunhofer IWU. The objective of this project was to develop the technological basis for the industrial manufacture of blister-free bonded hem flanges. The project involved two complementary approaches: minimization of cavities within the bonded hem flange through process optimization (minimization of the damage potential), and substitution of the defect-prone PVC plastisol by a blister-free seam sealant by means of material optimization and enhancement of the damage tolerance.