Structural adhesives and industrial fasteners are becoming more rugged and easier to integrate into the manufacturing process. Both are becoming more resistant to temperature extremes, providing better strength, stability, rigidity, and structural integrity, as well as improved resistance to chemicals and corrosion. Some adhesive technologies are reducing or eliminating the need for mechanical fasteners, and some fastening technologies are replacing assembly methods that used traditional fasteners.
The argument for replacing basic fasteners with adhesives has several components. One is that applying adhesives takes less time than inserting fasteners. Another is that the adhesives offer superior bond coverage of an entire surface compared to bonds made only where fasteners such as bolts or rivets are located. Fasteners also punch holes in substrate materials, compromising their integrity. This creates avenues for moisture or chemicals to enter and corrode metal surfaces between fastener joints.
Engineering-grade, pressure-sensitive silicone adhesives such as FLEXcon's Densil for brake damping must resist dirt and moisture, temperature extremes, and corrosion by substances such as brake fluid.
Source: FLEXcon
The definition of a structural adhesive varies with different contexts, according to Bill Sullivan, vice president of the performance products group at FLEXcon. "What are the force and weight placed on the items to be bonded?" he asks. "Are you suspending a big chunk of concrete over a bridge or bonding two small, lightweight panels that stand upright on their own?" If the application resides in the cockpit of a plane, either chemical or pressure-sensitive adhesives would be appropriate. But thermoset or pressure types are not the best fit where a lot of weight and force are involved, since they tend to be viscoelastic.
"You can also use two-part liquid systems, such as epoxies, to hang heavy weights," Sullivan says. "Some heat-activated, chemically activated, time-release chemistries bond over time and work like pressure-sensitive adhesives."
Heather Doughty, technical application engineer for Fabrico, says one widely used definition of a structural adhesive from 3M is one that can withstand 1,000psi in overlap shear. "With structural adhesives, typically the substrate fails before the adhesive."
There are also acrylic foam tapes permeated with adhesive throughout the foam, such as 3M's VHB and Norton's Normount. "Technically, these are not structural, according to 3M's definition," Doughty says, "but they can replace mechanical fasteners in applications with a lot of vibration, such as box trucks or moving vans."
Here, viscoelasticity is a benefit. "These foam tapes have great flexibility in recovery, as well as excellent resistance to vibration," Doughty says. Many structural adhesives are so strong that their material cannot also be flexible. Moving vans or box trucks are often made of aluminum panels and steel struts, but welding aluminum to steel is not usually an option. These trucks must withstand temperature extremes, and a structural adhesive with no give ends up compromising the aluminum. In contrast, foam tapes can stretch as needed. They act almost like a gasket by padding surfaces as well as adhering to them.
Jim, Tom, and Dave: Thanks, guys, for the detailed feedback and your collective experience. The advantages of one over the other obviously depend on the specific circumstances. The big advantage I kept hearing about adhesives was simplification and therefore reduction of manufacturing times, thus savings. I will keep all this in mind when I work on the micro-fastener feature. Your feedback will help me ask better questions!
Jim makes a good point. Adhesives can resist as much load as mechanical fasteners -- provided that you have enough surface area to spread them over. But since, in round numbers, the shear strength of steel is 100 times greater than the shear strength of typical structural adhesives, this means you need to have 100 times more surface area. It might not be hard to find this much surface area on a large component, but on a small component, it could be a challenge.
Where adhesives really shine is in fatigue -- and since, in my experience, most fastener failures are fatigue failures, this can be a big advantage. Also, as Tom Drechsler points out, if adhesives are properly used, they can vastly simplify a manufacturing process.
Given all this, I would imagine we will see more and more adhesives on our products. Are there particular industries that are adopting this? Are there certain industries in which the adoption of adhesives is a particularly strong solution?
Rob, the cost differential seems to revolve mostly around application process differences that save manufacturing costs and time, which also translate into cost. But your point is well taken--if the adhesives help the product last longer since it's not poking holes in sheetmetal--which will make the metal fail sooner as well as creating a point of ingress for damaging liquids--then that's another cost savings.
Ann: We were able to reduce the cost of a small shaft assembly by eliminating machining operations for traditional fasteners by switching to an anaerobic shaft-locker. The resulting bond is stronger than the small 17-7Ph shaft it is used on. Surface prep is critical, dispensing must be done carefully but when done in a controlled environment with fixturing the assembly times can be managed. Life & overstress testing has validated the process and we continue to monitor it as this is new ground for us.
The resulting parts are stronger (no flats for stress raisers), have less vendor issues (deburring on fine ID bores from cross threading or stepped shaft diameters). and take less space (even small fasteners have length requirements that conflict with highly packed components).
I've experienced the opposite problem. In fact experiencing it now. When attempting to fasten small components with adhesives, there is less margin of error. Less surface area, more opportunity for contaminant, or surface irregularity to confound adhesive. I guess there are some caveats - such as surface preparation, use of cleanroom, etc. Sometimes you simply want there to be a "mechanical" connection.
Chuck, that's an intriguing point about adhesives replacing fasteners in ever-shrinking smaller electronics. That's what I would have thought, but it turns out there's a "micro" class of fasteners that are ridiculously small--I don't recall the exact dimensions--and that will be the subject of an upcoming feature this year.
Dave and Alex, thanks for the feedback. A whirlwind tour it was. Each of these subjects deserves its own feature, and in fact, each will get its own spot at least once again this year. I concentrated on structural solutions since those seem like the toughest area for adhesives to beat fasteners, but they came closer than I'd initially expected. The fact that adhesives spread out the load over a larger area seems to be one of their biggest advantages. Pretty awesome numbers, eh? There's a lot more going on with adhesives, and they've come a lot further, than I would have guessed.
Interesting, Ann. It could be that the material costs are small compared to overall costs. If the adhesives are particularly effective, there could be additional savings in quality through the life of the product. That could matter in terms of overall costs.
I would think that adhesives offer an advantage in smaller electronics partially because mechanical fasteners can only get so small before they become a manufacturing problem. I would imagine that some of today's razor thin phones would be a good example of this.
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