- Created: 2018-09-05
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- Created: 2016-05-12
- Created: 2014-07-28
This selection guide compares the latest types of abrasives and includes a detailed discussion of grinding and finishing aluminum.By Patrick H. CarrollProduct/Market Manager - MRO, Norton | Saint-Gobain () Selecting the right abrasive and using that abrasive correctly for welding and metal fabrication applications can make a big the difference in maximizing output and minimizing cost. Still, Read full article >>
- Created: 2016-04-11
Today’s structural adhesives are an alternative to traditional joining technologies, such as mechanical fasteners.
by SHARI LOUSHIN, senior technical service specialist – structural adhesives, 3M Co.
As times change and structural assemblies become more sophisticated, manufacturing techniques must be refined to accommodate new needs in structural applications. Mechanical fasteners and welding are traditional joining methods that many manufacturers are comfortable with; however, these methods are not always the most practical solution for modern assembly. Mechanical fasteners can drive up costs, limit options for materials, and cause fatigue, metal distortion or tearing.
Solutions for structural applications are strong adhesives or tapes that can outperform mechanical fasteners and provide a clean, durable design. Industrial adhesives are an alternative that solves the problems presented by traditional joining methods.
A strength comparison between industrial adhesives and tapes and traditional mechanical fasteners when bonding aluminum to aluminum under extreme conditions.
PROBLEMS WITH MECHANICAL FASTENERS
Traditional joining methods can present difficulties in the assembly process. Metal distortion and tearing under heavy loads, or fatigue, reduce reliability and longevity of assembly parts. To address this issue, manufacturers may try and decrease the gap size between rivets or bolts, inflating the number of needed fasteners and associated purchasing and labor costs.
Welding can damage metals, by causing heat distortion or burn through, especially with lighter weight substrates. Also, energy and labor are often needed for post processing work to return the welded parts to a condition suitable for painting.
Sustainability in production is a hot button issue that is only going to escalate. Many manufacturers face pressure to decrease fuel use, energy consumption, and contaminant emissions. The best way to do that is to reduce the weight of materials being produced, and use the lighter weight materials in lieu of traditional heavier metals.
Reducing weight for sustainability’s sake becomes a difficult task because traditional joining methods usually aren’t as effective as adhesives when joining lighter-weight materials. Lighter composite or plastic materials cannot always be easily welded, and thin sheet metal parts are prone to distortion and tearing at the concentrated points where throughpart fasteners like rivets and bolts are placed.
While mechanical fasteners can often provide structural strength, the level of holding power that they provide can be overkill for many common applications. In this way, mechanical fasteners are used out of convenience rather than necessity, when there are simpler and more effective joining options available.
3M industrial adhesives and tapes are strong, but how do they stand up to a raging river? A kayak made of hard-to-bond, low-surface-energy plastic is tested.
Selecting industrial adhesives for structural applications opens the door to opportunities that would not be possible when relying on mechanical fasteners or welding options.
Where mechanical fasteners may rip though and damage a thin substrate, an adhesive bond provides even load distribution across the surface area of the joint. This reduces the potential for fatigue damage, lengthens the life of the bond and product, and drastically reduces replacement or repair costs. Thinner and lighter materials can be cheaper to manufacture than heavy metals, as well.
Adhesives allow manufacturers to diversify the materials used in their assemblies in ways other than using different metals. Using adhesives opens up options for more composite and plastic use, which can help manufacturers decrease material costs and improve the aesthetics of the end product.
Epoxy adhesives are tested by lifting this 14,550 pound shipping container for 18+ hours.
Where mechanical fasteners can limit design options for structural applications, adhesives expand them. Adhesives can be applied in that are inaccessible to mechanical fastening during final assembly, allowing new designs that further reduce weight, costs, and labor. From an aesthetics standpoint, adhesives can reduce and eliminate unsightly screw and rivet points and weld lines, leaving a smooth, uninterrupted surface without grinding after welding.
Bonding dissimilar materials, such as different kinds of metals or metal to composites or plastics, opens up a multitude of design options, and allows manufacturers to diversify their products and make them much more functional. Mechanical fasteners would not be a good option for joining metal to many composites. For example, drilling holes for fasteners would likely crack plastic material. When joining plastics to metal, thermal cycling can lead to fractures within the plastic.
Despite extraordinary advances in adhesive technology, many manufacturers continue to doubt adhesive strength. In this video from 3M Assembly Solutions, adhesives and tapes are shown to outperform metal fasteners in a test of strength and durability.
The video shows that adhesives and tapes such as 3M’s VHB Tapes can out-perform mechanical fasteners for applications with lighter weight substrates through their load-distribution capabilities. Adhesive sealants provide high elongation, flexibility and gap filling. Toughened epoxies are suited for applications that need a more rigid solid combination of impact resistance and strength. 3M’s provide impact load resistance on thin substrates.
When put to the test against mechanical fasteners and rivets, 3M adhesives and tapes emerge victorious with superior hold, even stress distribution and seamless design.
Tape allows for construction from panels to windows as it allows for high holding power and long-term reliability. Die-cut tape from 3M allows for easy assembly.
ADHESIVE SOLUTIONS TO BONDING CHALLENGES
If a manufacturer were to choose substrates in lieu of traditional metals for a particular project, the right adhesives can be solutions for hard-to-bond materials.
Low surface energy (LSE) plastics like polyethylene, TPO, HDPE, PP and some powdercoats can be hard to bond because of their chemical composition. Typically, these materials will require extensive surface preparation processes before even beginning the joining process.
Adhesives can join dissimilar materials including different metals to plastics and other composites.
Thermal methods like friction or ultrasonic welding are considered solutions for LSE plastics. However, these methods are typically expensive and require ongoing tooling, which is not ideal for products that have short runs or need frequent design changes.
Even adhesives don’t always solve the surface preparation issue, however some specialty adhesives, like 3M Scotch-Weld Structural Plastic Adhesive DP8010 Blue, can bond LSE materials and even reduce or eliminate the surface preparation steps, such as is seen in this video.
Adhesives can be used for a variety of uses, such as for panel-to-frame applications.
Another challenge is trying to bond oily metal. Oily metals often require extensive preparation and cleaning, which creates extra work. Unfortunately, these steps are necessary because the oil can interfere with robust bond formation.
That said, two-part structural acrylic adhesives are able to absorb most oils from metal surfaces, allowing the metals to bond without first undertaking extensive cleaning procedures. This eliminates those tedious steps, saving the manufacturer time and money.
Adhesives can eliminate the problems of metal distortion and tearing.
Though many manufacturers are comfortable with traditional fastening methods, modern design and sustainability factors render mechanical fasteners and welding to be less effective than alternative joining methods for many applications.
Adhesives are extremely effective to remedy the issues presented by mechanical fasteners, including metal distortion and tearing, and expensive repairs or replacements. Not only do adhesives provide a more effective alternative to traditional joining methods, they also allow manufacturers to choose from a wider variety of substrates, bond dissimilar or hard-to-bond substrates, and explore different design avenues to create unique and superior products.
Many people continue to misjudge the strength of adhesives. How could glue be suitable for a huge application? People may think that way, but an adhesive bond is not to be underestimated. In this video, a thin layer of adhesive holds thousands of pounds:
Imagine what adhesives could do for you and your design.
- Created: 2015-05-27
Peel-test coupons show one of the primary goals of structural adhesives – peel strength. These are an exaggeration of what real structural adhesives can tolerate, but the idea comes through: an elastomeric capability is necessary for strong adhesives to withstand the extreme load at the very edge of a peeled joint.
By Ed Huntress, Editor
Automotive applications are driving advances and the proliferation of adhesive assembly. They’re now a primary. No Longer a Stepchild fastening method, and the fallout will affect many segments of fabricating.
Anyone who produces sub-assemblies or intermediate products for consumer-product OEMs probably has encountered industrial adhesives, but for most, it’s been a sideshow to mainstream fabrication, or a specialty. But if you’re part of the automotive supply chain, or are involved with other transportation equipment or appliances, you know that’s not the case anymore. Adhesive assembly for metal-to-metal bonds has gone mainstream – not just for convenience or cost-saving, but increasingly for greater structural stiffness and strength.
Adhesives: Salt-spray tests are used to determine effective ways to prevent road salt and other environmental hazards. Corrosion in an adhesive joint can be hidden and insidious.
The field has become so complex that, like welding, there is no way one can put his arms around the whole subject in an article, or even a book. We can, however, give an overview of the materials, processes and applications in the industry that’s driving the technology – building lighter cars – along with a sense of where the implications may lead us. That’s what we’ve done here.
Adhesives for metalworking got a couple of shots in the arm back in the 1970s, when aircraft manufacturers started to glue airplanes together, and hand power-tool manufacturers designed adhesive-assembled motors and other components to compete with the Japanese invasion. Recently developed high-strength epoxies and fast-cure acrylics and cyanoacrylates had made it all possible. There was a proliferation of adhesive types and methods of curing them, including chemical reactions, heat, moisture, and ultraviolet light. So now we have a smorgasbord of materials and processes available. We can trace them back to those earlier high-strength, fast-cure developments, but automotive applications, in the new Cadillac CT6 chassis, 60% aluminum, shows the variety of fasteners used: rivets, flow-screws, spot- and stitch-welding. Under almost all of them are adhesives.
So now we have a smorgasbord of materials and processes available. We can trace them back to those earlier high-strength, fast-cure developments, but automotive applications, in particular, have gone off on their own path. It’s a product of the materials being bonded and special performance requirements, particularly the needs to withstand the impact of collisions; the downstream processes of coating and curing; and combinations of structural loads, including the tendency of strong adhesives to peel apart. Corrosion resistance has been another issue.
The new Cadillac CT6 chassis, 60% aluminum, shows the variety of fasteners used: rivets, flow-screws, spot- and stitch-welding. Under almost all of them are adhesives.
Dealing with dirt and oil
Look at the particular adhesive requirements of cars. First, it’s not a clean manufacturing environment, and adhesives for metals have historically required extremely clean surfaces. Stamping lubricants and other contaminants are death to traditional high-strength epoxies. They required a freshly cleaned and even a chemically etched surface to get a good bond. No more. The major adhesive suppliers formulate their newer materials to get right through oil and even some corrosion, and to get a strong, chemical bond despite it.
Except with aluminum – sort of. Aluminum starts to oxidize within seconds of being exposed to air, and that aluminum oxide is itself weak, and is weakly bonded to the base metal. If you put ordinary adhesives on it, they have no bonding strength. The time- honored treatment was a conversion coating – specifically, phosphoric-acid- anodizing (PAA), which was a standard in aluminum aircraft manufacture for decades. It required a separate, messy pre-treatment but it was necessary.
Now we have three new solutions. First, pre-treated aluminum coils, with a conversion coating already applied. These pre-treatments are becoming sophisticated and they make aluminum bonding more practical, saving a step for the car builders. Two of the biggest players in this business – aluminum supplier Novelis and multinational adhesives company Henkel – are collaborating on new coatings and have one product on the market, Bonderite M-NT 8453, that can even be applied in-process, by dipping, spraying, or roll- coating. It’s a multi-purpose conversion coating that gives a strong bond to both adhesives and to paints.
In addition to those production materials, there also are new adhesive formulations that incorporate an etch and/or conversion coating right in the adhesive. These are especially handy for repair work, but they also portend simpler processes for low-volume production. Among the suppliers producing or developing such materials is Lord Corp., which has been certified by Ford for supplying repair materials for the new, aluminum-bodied Ford F-150 pickup.
We all know that steel in automobiles keeps getter stronger and thinner, which causes problems for welding. First, those steels are weakened where they’ve been heated to welding temperatures. Second, as thin as they are, stiffness, if not strength, becomes a limiting engineering factor. (See “Welders Respond to Thin, Coated ‘Lightweighting’ Steels” in this issue). In other words, they can be too floppy with intermittent welds, such as spot- or stitch welds, and continuous- seam welding exacerbates the heat- weakening issue.
Enter adhesives. With a modest amount of overlap, the shear strength of the joint in an adhesive-bonded steel panel can be as strong as the parent metal. Applying adhesives to continuous seams is easy, either by hand or by robot.
Aluminum, once again, has its own problems. The auto makers are using combinations of heat-treatable and non-heat-treatable aluminum grades, and the heat-treatable ones can lose strength from welding, like the high- strength steels. Adhesive bonding solves that problem.
A growing issue is bonding dissimilar materials: steel to aluminum, or either one to plastics and composites. There is some development work going on for welding steel to aluminum, but adhesives have a natural advantage here, too. They can be formulated to “give” elastically, which takes care of the different thermal expansion rates of aluminum and steel. The ratio is 3:1, and it can develop enough force to shear steel right off of aluminum, if they aren’t bonded with the right materials.
Some adhesive materials
The adhesives we’re talking about are primarily of two families: epoxies and polyurethanes. We think of epoxies as two-component adhesives but they can be formulated with the chemical cure agents already mixed in, but which won’t activate until they’re heated.
This is the common structural adhesive in use for cars: a one-part, heat- cured epoxy. Because it’s often applied down the supply chain and won’t be cured until assembly is complete, it’s being formulated to handle room- temperature “open times” of 4, 6, or even 8 weeks. A supplier can apply a bead of it, hold the assembly together with a few rivets or welds, and send it up the line.
When the assembled car reaches the bake oven for curing the e-coat paint primer, the epoxy cures along with the primer, typically at around 180 deg. F. Two-part epoxies that cure at room temperature have their uses in car manufacturing, but many of these post-bake applications are being fulfilled with two-part polyurethanes. These are mostly non-structural applications, such as applying trim to the car. Several two-part materials also have uses in repair work.
Application is flexible. The adhesives typically have the consistency of caulk, and they can be applied manually, with a pneumatic caulking gun. Some supply chain companies are doing it that way. But the caulking-gun approach is a natural, too, for robotic applications. In the OEM body shops, robots apply miles of adhesive beads at very high feed rates.
Welds and rivets
The photo of Cadillac’s CT6 chassis exposes a variety of fasteners – self- piercing rivets, flow screws, spot- and stitch-welds, and, of course, adhesives. You can’t see the adhesives; they’re in the joints, of course. Wherever you see an overlap in materials, there’s probably adhesive in there. Yards and yards of it throughout the chassis.
The welds can go right through the epoxy. They leave a little burned spot, but it doesn’t have a substantial effect. It was common at one time, says Dow adhesives expert Selamawit Belli, to use skip-application of adhesive to leave room for welds. No more. They just weld right through it.
The welds and mechanical fasteners are there for two reasons. First, there is some tendency within parts of the automotive manufacturing community to want a little extra insurance. Old ideas die hard.
Second, with the long open times before final assembly, you need some other fasteners to hold the whole assembly together as it moves down the line. Once the adhesive is cured, though, their role is incidental.
Strength is a complicated issue with adhesives. They respond differently to different types of loads. Without getting into it two deeply, these are the basic considerations.
First, the primary load typically is shear. Second, if a high-strength adhesive starts to peel or cleave at the edge while it’s under a load, the joint is going to the joint is going to fail, right now. There is an old rule of thumb with adhesives: the stronger they are in tensile strength or shear, the weaker they are in peel (think about peeling adhesive tape off of your desk) and cleavage (think about splitting a log). In aircraft, where the structural adhesives have well over 5,000 psi tensile strength – sometimes twice that much -- they often apply a row of rivets to the very edge of an assembly to prevent the onset of peel or cleavage.
In cars, things are different. First, you don’t need quite as much tensile or shear strength, so you can formulate them to have much greater peel and cleavage strength. The lead photo to this article conveys the idea, although that is an exaggeration for structural adhesives.
Also in cars, you have to think about impact-peel, which means tolerating the shock of a crash. And you have to be able to tolerate wet, salty environments, like road salt. Adhesive manufacturers do extensive testing and formulating to tolerate it.
Testing, testing, testing…it’s a big part of the technology. And each automobile OEM has its own ideas about what performance must be met. So Dow’s adhesives for the Cadillac in our photo, for example, are different from those they’re supplying to Ford for their F-150.
That’s the overview on structural adhesives in cars. FAB Shop is introducing some specific, fabricating- related adhesives coverage to our scope, which will move beyond cars but which, necessarily, will be closely tied to it. Cars are where most of the action is on structures. They’ve taken adhesives to their heart, and we must, too.
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