Building a sheet-metal structure? It’s likely you might first think of welding when picking an assembly technique. But for some applications, durable adhesives deliver the desired aesthetic.
Tooling Research Inc. (TRI), Walpole, Mass., primarily designs and builds custom electrical, mechanical, and pneumatic equipment and controls as well as machining metal and plastic components. But the company also has a growing business building signs and sculptures from conceptual sketches for architectural firms and sign companies.
When TRI was tapped to provide outdoor signs for a college campus, the plans called for thin sheets of aluminum to be mounted on a structural framework. The finished sign panels had to be lightweight and removable, as well as weather resistant.
Typically, such sheets are welded to a structural backing. But welds distort the sheet metal, leaving divots on the surface. Sign makers must repair the dents with body filler. Filling and smoothing can take several applications and add time and expense to the project.
Mechanical fasteners also deform sheet metal. Sheets must be drilled or punched before joining, and installers still have filling and smoothing tasks to perform afterwards. Fasteners are also prone to leak or loosen from weather and vandalism.
For the college-campus signs, TRI used Metal Welder adhesive from Devcon, Danvers, Mass. The two-part, 10:1 methacrylate mixes as workers dispense it from manual or pneumatic mixing nozzles. There’s no need for a primer, and bonds attain functional cure in an hour at room temperature. Once the adhesive is fully cured, which takes 24 hr, it can hold 2,450 psi in tensile shear using a 0.020-in. bond-line on gritblasted substrates.
Devcon, Danvers, MA
Tooling Research Inc.,
The adhesive spreads loads over the structural framework instead of concentrating them at fastening points. This adds strength. The adhesive also acts to separate substrates electrically, diminishing the likelihood of corrosive electrolysis. The Metal Welder adhesives use 0.030-in.-diameter glass beads to control bond thickness.
“The adhesive works particularly well on panels and thin sections where clean lines and surfaces are the ultimate goal,” says TRI President Milton Florest.
Keeping that clean look can involve tight tolerances. For this level of precision, TRI uses Devcon’s Plastic Welder II. The adhesive permits bonds as thin as 0.010 in.
Plastic Welder II is a 1:1 two-part mix that cures in 2 hr at room temperature. Although Devcon recommends a primer when it is used to bond metal substrates, Plastic Welder II can join nylon alloys and polyesters, in addition to standard epoxy composites, metals, and wood. It resists UV aging and weathering.
An adhesive’s working time is also important when building complex signs. “We sometimes have three or four people working at the same time — one gluing, one holding, and one clamping — to complete an assembly before the adhesive sets up,” Florest notes.
Metal Welder has a working time of five to six minutes, while Metal Welder II sets up in 14 to 16 min. Plastic Welder II has an 18-min working time.