Greenwood, MS-Engineers often feel the heat to improve their existing designs, and that was nowhere more true than at Viking Specialty Products, a maker of high-performance ventilation systems for cooking equipment. After putting a lot of effort into making sure that the surface finish on the ventilation hoods for the company's line of stainless steel cooking ranges was flawless, engineers were frustrated at having to undo the damaging effects of a high-temperature, spotwelding process.
"Because aesthetics are extremely important to our customers, an excellent factory finish is critical," says Design Engineer Kirk Clements, "So we've spent a lot of time figuring out how to produce the right surface characteristics, including the color, grain size, and surface profile, on our stainless steel parts."
Unfortunately, throwing heat at stainless steel--no matter how short the duration--has all kinds of deleterious effects on those characteristics. In the fraction of a second electrical current flowed to melt the metal to form a spot weld between the front and side panels of a canopy hood, the stainless steel discolored, grain size changed, and indentions formed on the surface. Even thermal stress patterns could be seen. "All these things were obvious to the naked eye, which meant that we actually had to sand down the surface of the parts and regrain them to match the factory finish," says Clements. "It was time-consuming and expensive."
When Viking decided to redesign its line of kitchen ranges, engineers Bob Harrell and Kirk Clements had an opportunity to rethink the process for assembling the hood. "We were curious about structural tapes, so we asked 3M to give us some samples for us to experiment with," recalls Clements.
For this panel-bonding application, 3M engineers recommended a pressure-sensitive, thermosetting adhesive tape consisting of an epoxy-impregnated acrylic. Cure time to temperature varies from 95 minutes at 250F to 2 minutes at 400F. Along with high bond strength, 3M Product Engineer Doug Lamm explains, a key characteristic of the tape is viscoelasticity.
"Substances that are viscoelastic have the ability to absorb energy and are therefore quite tough," says Lamm. "The best description I can give is that the tape has a kind of leathery quality to it that is forgiving under repeated stress."
Putting the pieces together
| This canopy for a line of Viking kitchen ranges consists of a stainless steel hood front and two side panels. In the initial design (middle), the front flange overlaps the side flange. The seam is spot-welded together. In the new design (right), the bottom flanges come together to create a flush, mitered corner. The seam is joined with a 0.5-inch-wide strip of structural bonding tape. The tape allows engineers to use thinner-gauge metal sheets (22 gauge as opposed to 18 gauge), but in order to ensure that the miter has a tight joint, engineers added a mechanical fastener at the bottom corner.
High-strength, pressure sensitive foam tapes that are considered excellent for fatigue typically measure 5,000 cycles in stress tests. According to 3M, this tape has survived hundreds of thousands of cycles without failure. Lamm says that the technology was originally developed to bond rearview mirrors in place on windshields. The liquid adhesives used previously often failed prematurely, causing many mirrors to wind up in the driver's lap. So effective is the tape for this application that Ford uses it predominantly.
A key consideration in any assembly application is the mode of loading to which the bonded joint is subjected. A 0.5-inch-wide, 0.020-inch-thick strip of this structural tape is rated to support 500 lb/inch2 in static loading, which was more than enough strength to support the 22-gauge stainless steel panels in Viking's application. The mechanical fastener Viking engineers added to the mitered corner is merely to ensure a tight joint and eliminate any gapping. According to 3M, the structural bonding tape is an effective replacement in many applications involving traditional liquid adhesives and mechanical fasteners such as rivets, screws and bolts, and spot welds.
Although it is said to be ideal for panel applications such as this one, Lamm notes that the tape is not recommended for bonding structural members, where the mode of loading is concentrated in a local area, or applications involving high impact stresses at temperatures less than -40F or long-term shear stress at above 180F.
At Viking, operators cut the tape to the desired length and apply it in angled sections along the abutting edges of the hood front and sides, using fixtures designed in-house. A special fixture designed by Viking engineers helps operators hold the parts flush and in place. Heat is applied and the tape cures in 8 to 12 minutes, leaving a clean edge.
"From a production standpoint, the tape has cut our cycle times. We eliminated the spot welding and metal finishing operations, and even with the curing time added in we come out ahead," says Clements.
Tape technical data
Tape type: 3M Structural bonding tape 9245
Adhesive type: Acrylic/epoxy
Tape thickness: 0.010, 0.020, 0.040 inch
Tape width: 0.25 to 22 inches
Suggested surface: Glass, ceramics, most metals
Peel adhesion to stainless steel 10.0 lb/inch
180 deg pull on FPL etched aluminum 28 lb/inch
Temperature performance Supports 2.2 lb in static shear for 10,000 minutes at 300F