Most of the work I do with the laser is 2D: a flat piece of plastic or wood is engraved, maybe cut into a shape. But I’ve assembled plenty of 3D objects using laser-cut pieces, especially by paying close attention to kerf to make sure that box joints snap together tightly. But even if your settings are kerfect you’re not really going to get the kind of strength you’d want to carry any weight beyond the piece itself.
Up until recently, I solved this problem using cyanoacrylate, every day run-of-the-mill super glue, which works in tandem with my usual box jointing to provide a solid enough piece for most purposes. But in all but perfect conditions, super glue tends to cause a haze on the surface of the acrylic, especially anywhere you’ve touched it. If you’ve ever wondered why something you glued together has your fingerprint stuck on it even though you don’t have a finger full of super glue, there’s your answer! In fact, cyanoacrylate is used in forensics to reveal latent prints.
That’s not all that useful for what I’m trying to accomplish, though. I’ve had some minor success cleaning this fog off using Novus 1 plastic polish, but it’s an obnoxious process that requires a fair amount of elbow grease and you’re liable to damage the acrylic surface unless you’re being careful. It’s just not worth the trouble! So I did some research and decided to give acrylic cement a shot.
I’ve dabbled a little bit with some two-part epoxies and other adhesives, but I’ve always been apprehensive about welding as opposed to gluing materials. But I took the plunge after reviewing some video tutorials by TAP Plastics on how to use their acrylic cement. Just in time, too, as I had a new client project on the way that would need that stronger bond.
Enter the acrylic backpack. One of my clients wanted a backpack—of sorts—made out of two types of acrylic: five faces in clear acrylic and a back face made with a mirrored material. As usual, I’d be using box joints for the project, but this time I’d also have to attach hinges to the top of the box so that the lid was functional. Conveniently enough, TAP also sells clear acrylic hinges, specifically designed to be cemented to an acrylic surface like my lid.
Like with most projects, we went through a few iterations before settling on what would eventually be made physical. Shown above are 3D mockups of two potential backpack sizes, as designed in Adobe Illustrator and imported into Autodesk Tinkercad. In these images the small red cubes were placeholders for where the acrylic hinges would eventually be placed. We went with the smaller design over concerns that the larger design would be too heavy on its own.
Once the pieces were cut, all I had to do was put them together! I made sure to offset the cutting path—accounting for kerf as usual—just enough that the box joints were tight. This way I wouldn’t have to hold the pieces in place or use clamps and could focus on applying the cement. The container it arrived in was tricky to open, but thankfully TAP made a video about that, too. In fact, most of the steps I took were taken straight from their series of tutorial videos, so I recommend checking them out if you want a more detailed breakdown of how to use acrylic cement, in video form.
During assembly, some of the acrylic cement dripped onto some surfaces of the acrylic. It evaporates quickly, but will leave a mark that cannot be removed, so if it happens to you don’t wipe it off—it’ll only make the mark bigger. Thankfully, the marks are small and hard to see, a far cry from the fingerprinted fog we were getting with super glue.
When attaching the hinges, I used some painter’s tape to hold the lid in place. I held each hinge in place for about a minute after flowing the cement underneath it; it’s enough time to bond the materials together such that you don’t need to hold them anymore, but TAP recommends 24–48 hours for maximum strength. I’d originally planned on affixing the hinges to the inside of the backpack, but it wouldn’t allow me to open the lid far enough to lay flat against the back surface, so we settled on leaving the hinges on the outside, despite the protrusion. The hinges welded perfectly to the clear acrylic of the lid, but they did mar the silvered back surface of the mirror material. It’s subtle, but noticeable, from the front side.
With that, the backpack was finished! The larger 1″ box joints helped make a cleaner aesthetic compared to the blue acrylic pencil holder above, but the clearest difference is the lack of haze. This process is obviously a little more expensive than the superglue, both in material and in labor, but it sure seems to be worth it! Well, I think so, anyway; check out the video below and let me know if you agree!