Use of aluminum in auto and truck body construction is increasing rapidly. The Aluminum Association estimates the average aluminum content level will reach 500 pounds per vehicle by 2025 (up from approximately 390 pounds in 2015).
Aluminum has many advantages, including making vehicles lighter weight and therefore more fuel efficient, but it has unique properties that must be accommodated during design and construction. Aluminum is sensitive to temperature variation and will distort when exposed to extreme heat. For example, have you ever baked cookies or used a flat aluminum baking sheet? When the sheet is placed in a hot oven, thermal expansion soon causes it to warp and twist. When used as an auto body hood—one of the most visually prominent parts of a car—even a small amount of distortion in the aluminum sheet detracts from the vehicle’s aesthetics.
Hoods and other vehicle closures can suffer distortion during the electrocoating process, since coating operations involve curing parts in ovens that can reach temperatures as high as 302°F (150°C). One way to prevent distortion is by ensuring an early, strong bond along hemmed edges (i.e., seams in which the outer panel is folded over the inner panel and joined with adhesive). If the bond is not secure, there can be relative movement between the inner and outer panels, causing the entire closure to become dimensionally unstable.
Choosing the right adhesive chemistry offers a solution. One-part epoxies don’t allow the hemmed area to gain strength until the part has been heated and entirely cooled. Two-component (2K) adhesives such as LORD Versilok®, however, are epoxy-modified acrylics that achieve 85 percent of their final strength at room temperature and only rely on heat from electrocoating ovens for 15 percent of their strength.
This provides significant handling strength, holding outer and inner panels securely in place. Assembled parts can be shipped to the coater’s site without suffering slippage or loss of contact points along the hemline. The closure’s dimensional stability is therefore greatly improved. While panels will still expand and deform when exposed to high heat during electrocoating, the strong hem bond will allow them to return to their original position upon cooling.
Another advantage to achieving an early, strong bond is that it reduces “springback,” which is the tendency for the metal to unfold along the bent edge. According to Metal Forming Magazine, “The elastic modulus for aluminum is one-third that of steel. Therefore, aluminum springback will be three times that of steel.” With the increasing use of aluminum in auto body construction, it will become more important to use fast-curing adhesives such as Versilok to prevent hemmed seams from reopening.
One Auto-Maker’s Story
A premium auto manufacturer was able to eliminate a step from their curing process—thereby reducing capital investment—by using a 2K adhesive.
The auto manufacturer has a history of using LORD’s “meter, mix, dispense” (MMD) equipment to dispense adhesives from bulk containers. The proven quality of the MMD systems prompted the OEM to collaborate with LORD to improve corrosion resistance on a recent car design. Versilok 273 was selected to provide a high level of corrosion protection for the auto body.
An added advantage was the elimination of one of the OEM’s typical curing steps. Two-component adhesives require no preheating (although a low-temperature preheat can be used to accelerate the cure); therefore, a strong hem bond was achieved at room temperature and no induction cure was required.
Use of Versilok 273 resulted in a financial benefit of reducing capital equipment, floor space and energy combined with improved quality…a win-win win-win situation! To achieve these gains in your next vehicle design, talk to one of our experts.