Not too long ago, the claim was that all vehicles would be adhesively bonded. However, the din surrounding this technology has died down such that you wonder whether any OEM has an adhesive bonding initiative in place. “Actually,” says Greg Korchnak, product market manager, Body Engineered Systems, North and Latin America, Dow Automotive, “the exact opposite is true.” And the picture is even brighter in Europe: “This year we will have some OEMs fully penetrated with structural bonding technology on all of their vehicles,” says Thomas Mettler, product market manager, Body Engineered Systems, Europe, Dow Automotive. “That means every platform will have some amount of structural adhesives.”
Traditional uses for adhesives include bonding the windshield and rear window in place, and in hem flanges around the vehicle. Increasingly, however, adhesives are being used to increase stiffness, seal gaps, and increase structural integrity in a crash. “You see its use in reinforcement areas, within the engine compartment–especially the shock towers and firewall–the A- and B-pillars, and in the side aperture,” says Mettler.
This doesn’t mean mechanical fastening will suddenly disappear, especially since adhesives are more prone to peeling than shearing, and need a mechanical system to hold the panels together before the adhesives cure. “As automakers get more comfortable with adhesives,” says Korchnak, “I suspect they will increase the weld spacing, and–perhaps–downgauge the steel.” The strength of adhesives’ continuous bond line, and the introduction of crash-durable glues should speed up this process. Korchnak suggests these are money-saving opportunities “that OEMs are willing to recognize at this time.” Also, automakers could improve crash performance by optimizing joint design for this fastening method. “We’re still using joints that were designed for mechanical fastening technologies,” says Glenn Eagle, product development leader, Structural Adhesives, North America, Dow Automotive, “but optimizing them for adhesive performance characteristics–which can be modeled quite accurately–is too far out of their comfort level. It will require a brave OEM to take that first step before others even begin to consider moving in this direction.”
One reason for this reticence can be summed up in a single word: Faith. Automakers can test the strength of welds by examining nugget tear. They can’t see whether or not the adhesive is on place, or–of it is–that it has cured properly. “Unfortunately,” says Eagle, “welding high-strength steel to high-strength steel makes it harder to get nugget tear, and changing or mixing substrates changes the parameters completely. This has opened their eyes to alternative joining methods like bonding, and the need for vision systems to track adhesive application.” He expects the technology will grow via low-volume, high-margin programs that reduce risk exposure and provide a real-world test of the adhesive and application process.