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Hot Glue 101: Intro to Thermoplastic Cement
Hot glue is one of the most versatile glues you can buy. I use it for almost every project in some way or another. Whether you’re picking up a glue gun for the first time, or you’re an experience glue-slinger, this post should provide some inspiration and information that will stick with you.
Eric Hart did some research on his site, Prop Agenda, into the history of the first “Portable Thermoplastic Cement Dispenser.” Hot glue was original invented around the mid 60’s as a way of packaging that avoided harsh chemical adhesives and water-based glues, which were susceptible to humidity.
Today you can easily find glue guns at any store selling craft supplies. Most glue guns on the shelves are craft grade and labeled either low temp (248 °F) or high temp (374–410 °F). There are also two-temp guns that can switch between high and low.
The guns are essentially hand-held heating elements used to melt the glue as it’s fed in. Most guns use a trigger mechanism but some are just thumb-fed.
There are industrial grade glue guns, which are typically better constructed and have higher wattage for faster warm up. High-volume industrial guns can use pellets fed through a hopper or large “slugs” to reduce cost and re-load time. We’re just going to be talking about standard high/low temp craft grade guns which accept sticks with diameters of 7/16 (“mini”) inch or 5/8 inch.
The sticks are made of polymers (rubbers, vinyls, resins, etc.) that are stable at a wide range of temperatures, but that are viscous and tacky enough to adhere to most surfaces when molten. Sticks come in a huge variety and are formulated to maintain a hold in different temperature ranges, onto different materials, or are made with novel characteristics, like metallic, glow in the dark, glitter, etc.
The hot glue I used as a kid used to get yellow when it got too hot, saw a lot of UV, or just got old. It may be a problem with low-temp sticks, but with the standard multi-temp sticks, I haven’t noticed the phenomenon in a long time. I found it funny that you can buy a non-yellowing formula stick that start out more yellow than any of the others.
Now that you know a little bit about this wonderful stuff, here are my favorite ways to use it, along with a few hot glue do’s and don’ts.
Even though “hot” is in the name of the product, it’s easy to forget just how hot it is, once you start to get comfortable using it. While it won’t result in missing limbs, hot glue can easily cause second degree burns. Follow these two safety tips to help avoid problems.
TIP 1: Don’t touch the gun tip or fresh glue
Remember that the glue comes out of low temp guns at a temperature higher than boiling water, and high temp gun at the same temp you would bake a cake in the oven. If you get a drop on bare skin, it can transfer a lot of heat in a short amount of time. Wiping it away can be tricky because it sticks to skin and hair so well. Any left behind will finish transferring heat into the skin, then remain as an insulating barrier to prevent the air from absorbing that heat back. Not to mention, if you try to wipe it away with another body part, you may just be spreading the pain.
It can be tempting to touch the glue to test if it is still tacky. At worst you risk a blister, at best you’ll deform or weaken the glue. It’s always better to just wait another minute, or try some of the cooling techniques below.
TIP 2: Pay attention to where and when a gun is plugged in.
Try to designate a place to set the gun down and put it there after every time you pick it up. Make sure it’s on something that you don’t care about, like a scrap of cardboard, or a surface that’s easy to get the glue off of, like glass. Keeping the gun in its designated place will make sure you don’t accidentally step on it or brush up against it.
If you’re a real neat freak I suppose you could make or buy a dock like this.
I imagine it would just get messy and be more trouble than it’s worth. I’m more of a pragmatist, so if anything I’ll just grab a paper plate.
Whatever you set the gun on will get messy because most guns have a slow drip that leads to what I call “polymites”: tiny little stalagmites of glue.
Aside from just dripping, a well seasoned gun will also develop a coating of glue on the outside. I don’t know how exactly this happens, but after about a year, it may start to accumulate scraps from various projects and leave residue where it’s laid down.
In this state it can ruin cloth, carpet or other materials that it’s set on. If you have a heat gun or hair dryer you can heat the glue on the outside and wipe it off with a paper towel.
Don’t leave a gun plugged in for more than 10 minutes if you’re not using it. Not only is it a waste of electricity and I guess a fire hazard, but the gun get’s extra hot and the glue inside starts to become very fluid, seeping into the internal parts of the gun.
If your gun’s trigger won’t function when cool, but works okay when warm, this is probably what happened.
It may continue to serve that way, once it warms up enough to allow the components to move again, but I can’t think of any way to clean it that is worth the time it would take, so I usually retire them at that point.
Hot glue is great for holding certain things together, but has it’s limits. As discussed above, there are specialty sticks to hold at higher temperatures, but if the object being bonded to or repaired will be exposed to temperatures over what humans can handle, the glue may not fare well either. If there’s any risk of re-melting, consider a different glue. This video from Prop Shop shows how easy it is to ruin a project with exposure to heat.
As Bill explains, his projects were made mostly of EVA foam or PLA plastic and the damage would have been much worse if hot glue had been used. Just something to keep in mind.
So when is hot glue the best option? Once exposure to temperature is ruled out, consider other factors, like materials being bonded, stress load, and wear. See the tables below for a simplified look at when hot glue is most useful.
In general, the bond is all about surface area on a very fine scale. If the surface is smooth, the surface area is low and the glue has less to hold on to.
Materials that are more porous or fibrous give the glue more to hold onto.
Sanding, scoring, or priming the surface with chemicals can sometimes create grooves or open pores enough to make a much stronger bond. Also, make sure the surface has no grease on it (even just from handling it) that will be a barrier to the bond.
Because the glue is designed to resist moisture it can work well for waterproofing. It can’t handle high pressure except possibly serving as a kind of gasket, but for making a water-proof seal between two edges or protecting a component from water, it can often fit the bill. This is seen in a lot of small-scale molding tutorials.
The plastic and rubber polymers also make it useful for quickly insulating electronics. If you take apart electronics you’ll notice that factories use it for this all the time. It’s great for electronics on costumes or where there will be a lot of movement because it not only insulates the contacts, but also keeps them from bending or wiggling at the connection.
Cooling The Glue
Waiting for the glue to cool down and hold a bond or hold its shape can be really frustrating, especially when components have to be held in position. There are a few tricks that can be used to speed up the cooling process of hot glue. The heat keeping the glue molten and tacky is going to dissipate into whatever it comes into contact with, so how long that takes will depend on the thermal properties of those surfaces.
Cardboard and foam core have a lot of air trapped in them, insulating against the heat absorption, so it can take a lot longer than you might expect for it to set completely.
Aluminum and glass, on the other hand, absorb the heat surprisingly quickly, and you may have a hard time getting the other surface onto the glue before it’s at least partially set.
As an example, you can see how long a full trigger squeeze of glue took to set on each surface (I let the gun reach peak heat between tests for accuracy of results).
I repeated the experiment with the hot glue sandwiched between two pieces of each material.
If the glue is sandwiched between surfaces so that you can’t get to it while it cools, the best way to speed cooling is to start with cooler glue. You can use a lower temp gun (or just unplug it and wait a bit). Otherwise, just leave it exposed or blow on it for a few seconds before applying it to the other surface.
If the seam is going to stay exposed, you have a lot more options. Blowing on or fanning the glue helps, but I’d recommend saving your breath and using a can of compressed air. This is especially effective if you use short bursts while the can is inverted (be careful, you can get blisters from extreme cold almost as easily as from heat).
If no part of the thing you’re gluing is sensitive to water, you can spray or run cold water over the glue for a pretty immediate set.
Note that the glue insulates itself and cures from the outside in, so large blobs can stay hot for a longer time.
Make sure not to put strain on the glue until it’s set all the way through. You can use several layers of glue to help it cure quicker. It comes out hot enough to melt onto the glue below it, so the layers fuse well.
Painting Hot Glue
Hot glue can be tricky to keep paint on because it is so rubbery. If the piece small and not made to be handled or flexed, the type of paint is not as important, as long as you can get it to stick. If it is likely to be touched or brushed against a lot, you may want to use certain paints or sealers. If it will experience a lot of wear and flex, you may want to consider other options, like buying colored glue sticks or using died silicone.
Below are some examples of different types of paint applied directly to glue sticks. To simulate wear, each stick was scraped with a blade to check for susceptibility to flaking. Each stick was also rubbed with a wet cotton ball, and one soaked with 91% rubbing alcohol. Finally, you can see the flexibility of the paint when the stick is bent.
Water Based Paints
Water-based paints will not adhere very well. The water used in the emulsion does not allow the paint to fuse with the polymers in the glue like an accelerant would, so it tends to bead on the surface of the glue. Sometimes multiple coats can help.
The idea of painting hot glue with watercolor is pretty laughable, but I’ve made an attempt just to be thorough. I could hardly get it to stay on the stick to dry.
Once dry it was hardly noticeable, and rubbed away easily.
As you might have guessed, there wasn’t enough paint on the stick to flake off, and it was highly susceptible to water and alcohol.
Because the vinyl that binds the acrylic pigment is so similar to rubber it is very flexible. Multiple coats may be needed to get a good even coat. It adheres alright, as long as there is not much wear.
The acrylic wasn’t completely immune to water and came right off under the alcohol swab, but was definitely the most flexible of the paints I tested.
Spray paint can contain different formulas, such as oil-based enamels, vinyl dyes or water-based acrylics. The emulsion can help get an even coat and act as a kind of primer to encourage the paint stick, but ultimately the polymers used to bind and seal the pigment are what matter. A cheap spraypaint will be more likely to rub off of a surface, not having a hard enamel finish, but at the same time, be more flexible and dry much faster. Because it lacks the hardening agents and oils that make enamels more water resistant, it can be considered water-based, even though it it contains accelerates, propellants and binding agents.
I tried two water based sprays, a black generic and a crimson Krylon.
Being water based, the paint itself was at least somewhat flexible. The problem here is that it lacked the body to hold itself together and the bonding ability on the polymer that oil might have. Both paints began flaking after being flexed, but the Krylon was by far the stronger paint of the two.
The generic seems to have more vinyl because it didn’t flake as much when scraped. Both were mostly impervious to the water swab but dissolved readily in alcohol.
Enamel (in paint) refers to the finish being hard and resistant. The enamel finish will make paint much more durable to wear, but much less flexible. Most enamel paints use an oil-based emulsion, which means much longer dry times.
I used this Rustoleum to paint my brake calipers, with no major flaking a year later, so that should give you an idea of how tough the bond and finish is.
It passed the flex test much better than I expected. I’m not sure if the 24 hours I gave it was enough for it to fully harden, but it may just have more oil than enamel compound.
It resisted flaking, wasn’t even phased by water, and left more on the stick after the alcohol than any of the others.
Model paints are designed for models made of plastics, not so much for more flexible polymers like rubber or vinyl. They have enough of a bite to bind with the hot glue, but not enough flexibility to stay on through much flexing.
Just like we saw with the spray paints, having oil in the mix helped it bind much better, going on more redily and sticking to the hot glue better, but ultimately flaking off when bent.
Just out of curiosity, I thought I would try a few things to prime hot glue and see if it’s effective for getting any of the paints we’ve looked at to stick a little better
I read somewhere that letting this stuff dry onto hot glue can form a nice primer base.
It goes on and forms a sticky sort of spackle, so obviously that affects the texture of the paint. More coats of paint could even it out. In general though, this definitely gave the water based spray paint and the acrylic something to hang onto. They did better at the flex and scrape tests.
This is a chemical similar to acetone that breaks down PVC to soften it and allow a better bond with PVC cement. I wanted to see if it works on any of the polymers in hot glue
It dyed the stick pretty easily, but I didn’t notice that it was any softer or tackier afterwords. It had no effect on the acrylic, still beading when applied, flexing and scraping about the same. If anything, the spray paint might have stuck a little better, passing the flex test better than before.
Overall, I was much more impressed with the cool translucent purple color it gave the glue.
This is one of my favorite materials ever put in a can. It is incredibly flexible and sticks well to most stuff. I know what kind of paints stick to Plastidip, and if you find it in the right color there is no need to paint it, so really, I just want to know how well it holds on to hot glue.
I would bet that most paints above would stick better to the Plastidip than the raw glue, but that wouldn’t change their base flexibility to keep up with the Plastidip.
So what else can we do with hot glue? Knowing the properties we talked about above will allow you to really get creative with how you use it. Here are a few things I’ve tried or seen others do that I thought are worth mentioning.
We’ve looked at how to paint hot glue, but there are a lot of other ways to mess around with hot glue and color. I mentioned that you can buy novelty sticks like these glitter ones.
Mitch Hedberg calls glitter the “herpes of craft supplies,” because it is so impossible to clean up. Keeping the sparkles bound up in glue from the start will keep them from infecting every surface in the room, like traditional glitter. Be warned though: put glitter sticks in a glue gun and that gun will now emit glitter long after the glitter stick is done. It will take about 3 clear sticks to flush all the glitter out, and even then don’t be surprised if, even way later on, you get a random
flair up sparkle. Remember when I said I retire old guns that have gotten a little too gunky? I keep them around for things like this so that my normal gun can keep spitting out nothing but clean clear glue.
I’m sure there are dyes you can buy for flexible polymers, but I’ve never really needed to find any.
You can get Sharpie to color hot glue, but it goes on streaky and wipes off pretty easily.
I have used the dye from a Sharpie mixed with acetone to stain PVC with great success, so based on the results from the PVC primer above, I imagine you could do the same to dye hot glue.
As we saw in the section about paint, some paints refuse to stick to hot glue. You can actually use it as a kind of resist, like I did in this painting where it prevented the watercolor from reaching the paper everywhere I wanted the web. I also used raised bumps as dew drops. Concentrated watercolor on the droplet blobs looks like the refracted colors captured by real life water drops.
A friend of mine teaches classes on creating really amazing custom greeting cards.
Mostly she incorporates her awesome line of original stamps, which you can check out here, but she also uses other interesting techniques to add texture and color. She showed me how gold foil can be applied to hot glue to create an impressively metallic raised border.
You can watch Adam savage use the same type of foil when gilding the hilt of his Excalibur sword for ComiCon17.
Hot glue can be dressed up or even used strictly as a way to add textures. With some practice, it can be dispensed artistically or impressed with materials it won’t bond to in order to create
One of the most natural textures it creates is a welded look. We’ve already talked about ways to get hot melt looking metallic, and the method of application isn’t entirely unlike actual welding, so this is easier than it might seem.
Another thing hot glue loves doing is making a lot of annoying little strings. You can avoid creating these by pulling the tip away from the glue you applied by about a quarter inch, waiting just a second for hair-sized string to solidify, then pulling away quickly or circling the place you just applied glue. The solid string should break where it is still molten, either at the gun tip or the blob of glue. Whichever one it separates from will have enough heat to absorb the hair, so that it disappears.
However, sometimes these little hairs can create a desirable texture. With a little practice you can anchor and drape the strings into a cobweb.
You can even get a blob to stretch enough that it forms fine strands that look like actual hairs.
In the right context it can even look a viscous liquid like saliva strands or Brain Venom!
When hot enough, the glue can also droop and drip. In most cases you want to either speed up cooling, keep the surface level, or even rotate it counter to the developing drip, in order to prevent this. If, however, you want a drip effect, it’s easy to dispense in lines with a blob at the end to encourage or mimic a drip.
The solidified glue can be melted with the tip of the gun, wood burners (unplug and wait for the right temp so it doesn’t burn) or really anything that can be heated above the glue’s melting temp, to make all kinds of textures.
Alternatively, anything smooth enough to not stick to the glue can be pushed into it while still molten to create texture.
Wax seals, for example, work really well with hot glue. You can use colored, metallic, or even wax sticks that are sized for glue guns, to make envelope seals.
I used a siliconetexture stamp on some hot glue to great effect.
With a little paint it made a passable faux leather.
Finally, hot glue can be used as a molding or casting material in small scale. This could actually be a practical application as well. You could cover a small metal piece in hot glue and cast it in resin, but in most cases there are better fixes. Mostly it’s just an easy way to make a mold or duplicate of something from soft polymers.
My friend made a Gelatinous Cube for our Dungeons and Dragons campaign by simply dripping the glue over a metal cube and painting it. It’s even big enough to fit the character pieces inside it to mimic a character being absorbed into it.
In which case, we can roll saving throws with this new hot glue D20!
The principle of heating and dispensing melted polymers is the basis of 3d printing. I haven’t personally built up anything impressive out of hot glue, but others have sculpted some amazing things. Look at these, search around and get inspired.
Thanks for reading!
I’ll leave you with one last piece of advice:
I do not glue with my hand.
He who glues with his hand has forgotten the face of his father.
I glue with my mind.