In fabrication the main way we create permanent joins is by welding. Welding has it’s drawbacks though, some of which we’ve discussed here. Thin materials, dissimilar materials, aluminum, even stainless can present challenges. And then there’s all the prep and clean-up, which takes time and adds cost. So how would you feel about gluing things together?
In fairness, gluing isn’t really the best word to use. Adhesive bonding is better. Gluing brings to mind assembling cardboard cartons and elementary school craft projects. Adhesive bonding is how cars, aircraft and skyscrapers are put together, (or parts of them anyway.) So let’s talk about adhesive bonding in fabrication, highlight a few applications, and discuss how it compares with welding.
Get a Grip on Structural Adhesives
If you’re putting your mother-in-law’s best vase back together a cyanoacrylate is probably what you reach for. It’s fast and forms an incredibly strong bond, which is why in some quarters they call it Super Glue. What it isn’t though is durable. Heat, humidity, vibration and just time will lead it to fail. (Hopefully you’re far away when that happens!)
For repair jobs in your own house you probably go with a two-part epoxy. There’s a little bit of prep to do, (the mixing,) then you spread a thin film and push the two surfaces together. Give the epoxy some time to cure and voila! That tool/toy/household item is back together and will probably last for years.
So epoxies are good, but would you trust your life to them? Well if you fly in a plane, ride in a car or go up a tall building you may already have done so. It probably wasn’t a hardware store two-part that was holding them together though. More likely, it was an industrial-grade structural adhesive.
Structural adhesives are those which work under loads. Some industrial versions will take up to 4,000 psi before failing. (CompositesWorld defines structural adhesives as those that can take up to 1,000 psi in a lap-shear test.)
Epoxies, cyanoacrylates and even polyurethanes can all form strong bonds, but today the strongest bonds are formed with a class of adhesives called methyl methacrylates or MMAs. (Some people just call them acrylic adhesives.)
Structural Adhesive Applications
Several paragraphs ago we mentioned that cars, aircraft and skyscrapers all incorporate adhesive bonding. Now let’s take a deeper dive.
Composite materials used in various high-end, low volume cars have long been bonded together. (If you know a way to weld carbon fiber there are a few companies waiting to hear from you!) Likewise, sealing and gasketing materials have been bonded onto metal, often with a pressure sensitive adhesive (PSA.) Where they haven’t been used until recently is for metal-to-metal bonding.
That’s changing, as described in an Assembly Magazine article. This describes how VW have adopted adhesive bonding in body-in-white assembly of several Audi models. Read the article yourself for the details, but the bottom line is, VW believe bonding offers advantages over welding in some specific applications.
Also, in the context of metal fabrication, a few years ago adhesive producer Henkel built a trailer where the panels were joined with adhesives rather than conventional fasteners. Read about it and watch a video at “Work Truck and Trailer.”
Weight minimization is paramount in aerospace, which is why they use a variety of different composite materials. These could be joined with fasteners but in many cases adhesive bonding is preferred. Adhesives avoid the need for fasteners, which means no drilled holes, (themselves stress-raisers,) and no nuts and bolts, which add weight.
In addition to bonding honeycomb structures, other aerospace applications include bonding fasteners to metal hard points.
While steel frames will be bolted and welded for the foreseeable future, adhesives are making inroads into construction. Wood is very amenable to gluing, with the added benefit that it doesn’t split, but you will also find adhesives in modern glass and metal buildings. One of the biggest applications is in bonding exterior cladding to the frame. Here architects like adhesives because they eliminate stress-risers and can also improve the overall appearance.
Using Structural Adhesives
Before we dive into the welding comparison there are a few caveats we need to mention with regard to adhesives.
- Design for compression. Adhesives don’t fare well in peel or tension applications. Instead, try to design joints where the adhesive is in compression. (There are some good diagrams at “Designing for adhesive bonding.”)
- Check the environmental conditions. Another limitation is in high temperature and high humidity applications. Here, high temperature means over 150°F, and some adhesives won’t go that high.
- Check the Safety Data Sheet (SDS.) Formerly known as the MSDS, this describes the hazards associated with your chosen adhesive. Some can be nasty, so wear gloves, face-mask etc as recommended.
- Consider the bondline thickness required. Optimum is 0.004” to 0.01”. Go thicker and strength drops, unless you’re using an adhesive with filler that’s engineered for larger gaps.
Adhesive Bonding Versus Welding
Welding is a heat process, and that’s both it’s strength and it’s weakness. When metal pieces being joined are large and have similar melting points it works well: the pieces fuse in the melt pool and two become one. (Wasn’t that a song?) Having plenty of metal minimizes distortion and with little risk of cracking the resulting joint is strong and durable.
Strikes against welding include:
- Takes skill to do it well
- Needs good surface prep
- Needs “fit-up” to position and fixture the pieces being welded
- Needs a lot of clean up
- Not easy to weld thin materials. (High temperatures tend to burn through and cause distortion.)
- Not easy to weld dissimilar metals. (A consequence of different melting points.)
- Not easy to weld high thermal conductivity metals.
- Strips away galvanized coatings/surfaces
Adhesive bonding overcomes some of these issues. It’s very good with dissimilar materials and works well with thin sheet, (subject to good joint design!) It doesn’t cause distortion or cracking, it can be applied in hard-to-reach locations and it doesn’t damage galvanized surfaces.
Points that appeal to designers in particular are:
- Eliminates unsightly fasteners
- Can create a “liquid shim.” This takes up gaps between mating pieces and so allows wider tolerances. (Not all adhesives can be used this way: look for paste adhesives if this function is needed.)
- Can provide a sealing function. (Think of it as a liquid gasket.)
- Can reduce the transmission of vibration.
However, there are also some downsides to using adhesives, most notably:
- Cure time. Depending on the specific adhesive, this can range from minutes to hours and adds a delay into the manufacturing sequence.
- Needs precise application. Of course, welding also needs good hand-eye coordination, so is this an argument against adhesive bonding?
- Surface preparation needed. But the same is true for welding.
Think About Adhesives For Your next Fabrication Project
If you’ve never considered adhesive bonding in your next fabrication project perhaps this blog post has opened your eyes to the possibilities. Modern structural adhesives join products with strength comparable to that of welding and at the same time open up more design possibilities.
One thing to remember is that while cyanoacrylates and epoxies have a role, in general we’re talking about methyl methacrylate adhesives. These are extremely powerful industrial-grade glues and they need careful handling. If you’d like to learn more about what they could do for you, give us a call or click the button below. We’d love to talk.