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Robustness encompasses every aspect of the way in which a 2K (two-part) structural automotive adhesive is made and used, from manufacturing and storage to dispensability, adaptability to different substrates and stamping lubricants, cure characteristics, strength, and durability. Illustration of the many facets of adhesive robustness are made using primarily 3M #5047, a 2K epoxy hem flange adhesive developed for bonding oily untreated metals. Areas of robustness covered include storage stability, surface oil accommodation, open time, and performance under various curing conditions. The recent introduction of adhesives having unique mix ratio control and NDT capabilities heralds new levels of adhesive robustness.

Two-component epoxy adhesives that cure at room temperature but are responsive to induction heating offer tremendous processing flexibility in the manufacture of hemmed closure panels. Using a two-component, hemmed closure panels can be induction heated to quickly cure the adhesive. Since two-components are generally very reactive, lower induction temperatures can be used and panel handling strength will still develop within minutes. Also, if panel deformation due to induction heat is a concern, or if spot induction coils are being used, two-component epoxy adhesives build strength with time at ambient temperatures, allowing parts to be handled within 3-4 hours after hemming. Experiments described here will illustrate the differences between single-side coils and L-coil induction heating systems. Other induction experiments will show the sometimes large differential between metal skin temperature and the temperature actually seen by the adhesive in the bondline.