Although the rivets used to attach the aluminum skins to the frames of Featherlite's custom trailers protrude a mere 0.20 inch, some discerning customers were asking for a smoother surface.
"If you're looking at the trailer straight on, you don't notice it so much," says Bob Gast, engineering manager. "But if you're looking down the side, the rivets-which are located between 2 and 8 inches center-to-center (depending on the construction criteria)-break up the contour somewhat. Companies who want to put high-end graphics on our trailers were telling us they wanted the sides of the trailer to be as smooth as glass."
A high-bond-strength adhesive tape from 3M looked like a good alternative to rivets in this panel bonding application. Featherlite engineers were familiar with the double-sided, acrylic foam adhesive, which they had used on some interior lining applications in the past. One major requirement, however, was that the tape perform well under a wide range of operating temperatures-from frigid Minnesota winters to the summer-time heat of Death Valley. The tape also had to provide a sufficiently strong bond to withstand the tensile and shear stresses produced by air flow and trailer movement.
An invisible fastener. As a builder of custom and specialty products, Featherlite builds trailers in a variety of sizes and formats. The typical frame is a rigid structure consisting of hat-shaped side posts located on approximately 18-inch centers attached to top and bottom rails. Mounted to this frame are aluminum side panels or skins measuring approximately 4-ft-wide and anywhere from 0.04 to 0.09 inch thick.
After prepping and cleaning the areas to be bonded, an operator applies the tape with uniform pressure along the length of each side post. Then he or she removes the liner from the tape and puts the aluminum panels in place. To make sure that uniform pressure is applied over the length of the section, Kevin Weinacht, senior design engineer, explains that Featherlite built its own fixtures.
"There is an immediate adhesion, and then a full cure that takes 36 hours," says Weinacht. "One of the things we've learned is that the extent to which tape performs depends highly on the quality of the assembly process. If you don't do your homework in the factory, you are not going to get good performance in the field."
Temperature also plays a role in that process, since to form a strong bond the pressure-sensitive adhesive cold flows across the surface of the substrate.
Another temperature-related benefit stems from the fact that the tape is more forgiving than a mechanical fastener in allowing the posts and panels to move independently. Small, differential movements are caused by the different thermal expansion rates of the two metals (aluminum vs. steel), and also because the outer panels heat up faster in the sun than the inner posts.
A mechanical fastener is used to attach the panels to the top and bottom rails of the frame, lending strength to the design. But the tape's dynamic tensile strength still has to withstand the forces of the wind on a trailer as it is zipping along the highway. Although the exact magnitude of the forces cannot be easily quantified, the strength of the tape used by Featherlite in the assembly process appears to be more than enough as no failures in the field have been reported.
In fact, so good is the tape's peel strength that removing the tape has been more of a challenge than getting it to stick. "Actually, 3M has an abrasive wheel that works well, as does an air chisel. It's really more of an issue of bond accessibility with our interlocking panels," says Weinacht.
One last unique property of the tape is that it is viscoelastic in nature, and therefore is a great energy absorber. "We're catering to a different type of customer and their expectations are a little higher across the board. They are sensitive to even minor vibrations or rattles," says Gast. He explains that the tape has helped to reduce both noise levels inside the trailer and vibrations, leading to a quieter, more comfortable ride.
In a typical panel-bonding application, the frame is a rigid structure consisting of upright side posts attached to top and bottom rails. Load-bearing and non-load bearing aluminum side panels are mounted to the posts, using an acrylic, pressure-sensitive, double-sided adhesive tape. Mechanical fasteners add strength to the assembly, although the tape has sufficient strength to withstand tensile stresses on the joint. "Bonding panels onto frames is a common application for 3M's VHB Tape across a variety of industries," says Ted Steiner, technical service manager, 3M's Bonding Systems Division. "Any application that involves putting skins over the surface of a frame or box is a good application for these tapes, because they have fantastic strength."