General
Structural bonding in the body shell means that the adhesive assumes functions of stiffness, crash strength and endurance strength, whereby the characteristics of the body shell are positively influenced in a significant way. In a conventional steel body shell, use is made of a high-strength, heat-hardening single-component epoxy resin adhesive that fully hardens in the cathodic immersion paint drying oven. In the case of a repair, this adhesive is replaced by a cold-hardening two-component epoxy resin adhesive, since the hardening temperatures necessary for the single-component epoxy resin adhesive are not conceivable on an assembled vehicle. For both series production of bodies-in-white and repair, structural bonded joints are regularly used in combined jointing processes.
Resistance spot-welding combined with adhesive bonding and mechanical jointing processes In the case of spot weld bonding for repair and series production, the entire extent of adhesive surfaces and spot welds is replaced. In this case, before actually spot weld bonding, the unhardened adhesive must be displaced from the joint area by an appropriate squeeze time. In the case of two-component adhesives, the hardening reaction starts immediately after mixing and application; this increases the viscosity and so there is only a certain time window in which spot weld bonding can be performed if the process is to be reliable. For this reason, all time-intensive preliminary work (drilling, cutting to size, fixing etc.) on the parts to be joined must be performed before application of adhesive and jointing. This also applies to mechanical jointing processes where the adhesive has to be displaced from the joint area.
Scheme 863
In the case of 3-layer spot-welded panel joints with a hardened bonded layer in which only the outer panel needs to be replaced, drill the outer panel in the area of the spotwelds sufficiently well without damaging the spotweld and bonded joint in the second component layer.
Scheme 864
In the case of 3-layer spot-welded panel joints with a hardened bonded layer on the outer panel, drill the outer panel in the area of the spotwelds sufficiently well without damaging the spotweld in the second component layer. After removing the loosened body part, old adhesive must be removed and any uneven spots sanded down. The flange area must then be pre-treated for bonding.
Scheme 865
Note. In areas in which bonded flanges are spot-welded in an unhardened state, suction devices must be used due to combustion products. Areas that can only be accessed from one side must be joined with MAG plug-welds or MAG welded seams. In these cases, no adhesive can be used in the vicinity of the welded seam because this would burn and would have a negative effect on the joint. The areas without adhesive must be protected using appropriate anti-corrosion and sealing measures.
Once the spare parts have been attached, repair spotwelds must be placed on the old spotwelds because there is no current flow possible in areas with a hardened bonded layer. Welding procedures other than resistance spot-welding are not permitted since the thermal effect would destroy the bonded joint over a wide area.
