We all know glue is for sticking stuff to other stuff. But how does it work?

Glue works because it provides adhesion- that is, the glue sticks to your stuff and to itself. The more it sticks to your stuff, and the more it resists pulling away from your stuff, the better the bond is.

There’s actually a lot of unsettled science on how adhesion actually works- why are certain compounds sticky? At this point we only have some guesses around complex molecular-level interactions. But what we can explain is what’s called “mechanical adhesion”, and that’s the basis for most of today’s glues. We can create mechanical adhesion if we take a product that’s liquid or gel-like, let it ooze into the surface imperfections of both sides of a joint, then solidify it. Like tiny claws, these hardened spikes dig into both sides to keep the two pieces from separating. And bam! They’re glued.

Drying vs. curing glues

To get a glue to be liquidy at first but then hard later, there’s a couple primary approaches.

First, you can take something that’s hard or rubbery and dissolve it in something that will make it liquidy for now, but evaporate over time. For example, Elmer’s glue, Tacky glue, and wood glue are all PVA- polyvinyl acetate, mixed with water and/or the water-soluble polyvinyl alcohol. Over the course of the glue drying the liquids in the glue evaporate, leaving the hard PVA behind. Upsides to this method- the glue is easy to thin to a desired consistency, and easy to clean up if spilled. As a downside, the glue often shrinks more than cured glues since all the liquid has to get lost.

Alternately, you can take something that’s liquidy and use a chemical reaction to turn it hard. Bathroom caulk, silicone adhesives, and superglue all react with moisture in the air to solidify. 2-part adhesives like epoxy mix two liquid, gel, or putty compounds together that react to form a solid. Sometimes these reactions can take time- some caulks can take up to 7 days to really set. Or they can be fast- if exposed to moisture superglue will cure almost instantly, and some epoxies will set in 60 seconds and cure fully in under half an hour.

Or, you can use a substance that melts at a conveniently high temperature but is solid at the temperatures you’re going to use it at. This is the philosophy behind hot glue as well as solder. Get it hot, melt it into those crannies to make glue-spikes, and let it solidify in place.

However you get your liquid thing to become a solid thing, it’s important that you’ve prepped your surface appropriately to ensure your bond is as strong as it can be. Still, stuff doesn’t always go as planned…

Reasons Bonds Fail

There’s a number of ways your glue joint could fail, and understanding these will help you prep appropriately.

  • Your glue-spikes don’t form because your piece repels the glue. This will happen with oily, greasy, or waxy parts. Something as small as a finger smudge can cause issues with some bonds. I clean most of my parts with a little bit of rubbing alcohol on a rag to remove any oils before applying glue!
  • Your glue-spikes don’t form because they can’t penetrate your piece. This happens with very smooth surfaces like glass, plastic, or glossy paint. The glue will dry and just peel off. I lightly sand or scratch most surfaces if possible before gluing. This gives more for the glue to “hold on to”- more area for the glue-spikes to form. If you absolutely need to glue something very smooth without scratching it, like clear glass, check out my upcoming guide “The Science Of Glue II: Surface Energy”. Or just buy some 3M 300LSE.
  • Your piece absorbs too much of the glue and now the two pieces are barely even touching. This is common with fabric and with some foams. If your glue soaks entirely into your piece, there’s very little or nothing left on the surface to hold the two pieces together. Your glue needs to form glue-spikes, but it also needs to contact both pieces at the same time!
  • Your glue-spikes are too small and fragile, and break off. This can happen when either not enough glue spikes formed (see above), or your glue-spikes were not deep enough. Perhaps you only made a couple scratches, or sanded with a very fine grit.
  • Your glue sticks to stuff on the surface instead of to the piece, and the stuff on the surface breaks free. This is most common with dirt and sanding dust, but could also happen if your piece is painted. I usually leave my pieces unpainted before bonding if I can. If I need to paint first, I will mask off the areas to be glued with masking tape before getting started there.
  • Your glue itself isn’t strong and breaks or shatters. Some glues are flexible when dried or cured. Some are very rigid. A rigid glue is usually stronger, but a rigid glue in a flexing joint is a recipe for bad times. This can also happen if something penetrates and weakens your glue- water dissolving a water-based glue, or heat softening a heat-type glue.
  • Your piece actually fails. Sometimes it isn’t the glue’s fault, but what it’s stuck to. Foam and fabric can tear. Plastic can crack. Make sure what you’re gluing to is strong enough, too!

Fixing all of these things will give your bond the best chance it can but sometimes, bonds just aren’t meant to be. When thinking about what glue to use on a piece, it’s important to consider the force that bond is going to be subjected to, and to spread that force over the largest bond contact-patch you can. Sometimes even the strongest glues won’t hold, especially with heavy pieces or bonds with a lot of levering action- long pieces anchored only on one end. That’s why it’s important to start the process with a little bit of a plan.

Designing for glue

Whenever you know you’re going to need to attach thing A to thing B in a build, you should be thinking about how that’s going to happen. Important questions to ask are:

  1. How much of part A is going to touch part B?
  2. What are the usual forces that are going to try to pull A and B apart? (Things like gravity, swinging motions, wrapping around a part of your body that expands and contracts, etc.)
  3. What are the secondary forces that might pull A and B apart? (Getting bumped into at a con, banging into stuff, packing props or costumes for transport, bouncing around in a car or suitcase)
  4. What are A and B made of?

Many people only consider the last question when choosing a glue. They go online, search “glue plastic to foam”, and wonder why their giant heavy gem on their sword keeps popping off. It doesn’t matter if you used the best glue for the materials if your contact patch between pieces is minuscule or the forces involved are huge.

For the best durability, you want a large contact patch and you want to minimize those forces. This may mean making a larger part out of a lighter material to keep gravity at bay- EVA foam instead of Worbla, plastic instead of resin, perhaps. Or it may mean designing an under-structure out of something durable like plastic or wood, and attaching lightweight foam facing pieces to large contact areas to create the desired final shape. Most bonds benefit greatly from pins or screws through both pieces, which can then be hidden behind small foam or plastic decorations or painting.

It’s also important to make sure you leave room for glue between your pieces. If the pieces fit exactly, there’s no room for the material that’s going to bond them together. Design in a little clearance for your glue. This is especially critical with rigid parts like 3D prints.

And finally, flexing is the death of bonds. Flex is when your pieces move, putting stresses on your glue and the bond between it and your pieces. Anything you can do to reduce the flex and stress on a bond, and reduce the number of times the bond flexes, is beneficial. If you’re gluing something to the tip of a pole, use a cap shape to keep the bond from flexing side to side. If you’re gluing 3D printed parts together, use screws or pins to hold each piece in place so the glue isn’t absorbing all those forces. Reduce flexing, and your bond will last that much longer.

Bonus time: welding

Welding is almost-but-not-quite-entirely-unlike adhesion, but ‘weld’ is often sold as glue- and due to the strength of welding, the word ‘weld’ appears in many products that don’t weld, such as DAP Weldwood (a contact adhesive) and JB Weld (a metal epoxy).

So what is welding? And how do I know if I’m actually doing it? Welding is essentially melting two parts together into one. Usually we hear this in the context of metal welding, but plastic can be welded too. One of the most effective ways to weld plastic is with a plastic solvent cement. This is a “glue” that is actually plastic dissolved in a solvent. When you put the “glue” on your plastic, the solvent actually dissolves the surface of your plastic too. The two plastic surfaces from your pieces melt into each other and into the plastic in the solvent cement. Then the solvent evaporates, leaving a bunch of solid plastic chemically melted together.

This bond is very, very strong- assuming the solvent actually penetrated and softened both surfaces. I love to plastic-weld-together an under-structure and then apply foam facing to make the right shape.