Make a Museum Quality Acrylic Display Box

A method for making boxes out of acrylic with clear, polished edges

Acrylic sheet is a wonderful material to make display boxes. It keeps off the dust, and keeps fingers from invading delicate or precious objects. This article is about how to make “museum quality” boxes that have smooth, clear joints. I won't claim that my joints are as good as in museums, but they look pretty good for using garage workshop tools. The techniques, taught to me by a "pro", give a good result.

Acrylic is glued with a solvent, which dissolves the acrylic from each side of the joint, so that the union of two pieces is the same acrylic that the pieces are made of. This process is called “solvent welding”. The joint is close to the same strength as the pieces to be joined. TAP Plastics has a great video on joining acrylic. While their method is easy, it takes some additional tricks to get the clear, almost invisible joints that one might desire for a truly high-quality joint.

Above is a box for an LED cube, using 1/8 inch thick acrylic, and a cover for a 3D printer using 1/4 inch acrylic. The LED cube looks ok, but years later there are some bubbles at the joints that were not there when it was first made. The printer cover has much better looking joints, because of the thicker plastic. I find that 1/4 inch thick acrylic works much better. Perhaps with a larger router bit the 1/8" acrylic would do better (more on this later).

1. Acrylic sheet. I got mine from TAP Plastics, but there are many sources for this.
2. Solvent cement. Buy this where you get your acrylic sheet, so that it is compatible with your acrylic.
3. Needle-tipped squirt bottle
4. Router bit: Flush-cut with a ball bearing, and carbide tipped.
5. Sandpaper: wet-or-dry, in grits 240, 320, 400, 600, 800, 1000, and 1500. The finer grits are available at automotive parts stores.

The way to cut acrylic is with a table-saw. I use a 48 tooth carbide-tipped blade 10 inches in diameter. The smoothness of the table-saw cut is inadequate for an esthetically bonded joint. The edge of the acrylic to be glued must be smooth, very straight, square to the faces, and sharp on the edges. Since the solvent is as thin as water, it fills no space and relies on a very intimate contact between the pieces being glued. This is achieved by trimming the edge with a router and router table. You need a flush trimming bit with carbide blades and with a ball-bearing guide for trimming the edges that stick out after gluing. The larger the diameter of the bit that you use, the smoother the cut. The bigger the bit, and the faster it is spinning, then the better the edge is for joining. A larger shaft will cause less vibration and also contribute to a smoother cut. The “pros” use a large, one inch diameter carbide bit, at very high speeds. My ¼ inch shaft on my router does not allow a very large bit, so extra sanding is needed later to get the joint to its best looking potential. My “router table” is made from attaching my router to the underside of my right table saw extension. Then the fence can be on the left of the router bit, which goes well with the direction of feed and the direction of rotation of the router bit.

Cut all of your pieces ¼ inch too big in both dimensions. Use the router to trim 1/8 inch from each edge that will have bonding to the edge, and leave the other edges too long. After the pieces are bonded, the edge that sticks out is trimmed with the router. This is one of the important secrets to getting an invisible joint. Peel and then trim the protective layer that comes on the acrylic to ½ inch away from the edges to be glued, and also from the edge of the face to be glued. Wherever there is an area that won't touch the solvent, leaving the protective film intact will help keep the faces of your project free of defects. If there are areas without protective film that will be slid on the router table, some tape will keep it protected from scratches.

Before you start bonding, trim all of the edges to be bonded with your router, but not the edges that are to be trimmed off after bonding. First, consider bonding the sides to the top. The sides should be trimmed at the front, back, and probably the bottom depending on your plans for these edges. The top should be trimmed at the front and the back. If the front and back are to be bonded after the sides are bonded to the top, then it is important that the front and back edges of the top and the two sides be at exactly the same size. If you have a piece of acrylic that is wide enough to hold the top and both sides, and long enough to accommodate the length of the box (+1/4 inch), then you can trim the front and back edges on this piece. The combined width of the three pieces will stay lined up with the router bit and all three pieces will have the same length. Then, the piece can be cut into the top and two sides. After cutting it into the three pieces, now would be the time to trim the top and bottom edges of the sides.

Begin your bonding with two adjacent sides. Lay the front side that will have its face glued on two layers of an old towel which is resting on a flat surface. Call this the “face piece”. Then position the adjacent side, the “edge piece”, with its edge down on the face piece and 1/8” from the edge of the face piece. Use a square to be sure that the pieces are at 90° to each other. The edges that are at the ends of the joint that are to later be glued to another piece should be lined up with each other precisely. Check the alignment of the pieces by running a sharp pointed instrument lightly across the edges that are to be lined up. You can feel whether they are even when the point feels across the edges. When it is positioned exactly where you want it, hold it there with weights. My picture shows a square verifying the perpendicular alignment, and lead weights on a piece of wood holding down the piece to be glued. Pieces of scrap wood stack up to get the weighted piece of wood close to level. Notice that the face lying against the towel has all of the plastic protective film on it, and the film is trimmed back from areas to be bonded.

Once you have the pieces lined up just as you want them, solvent used for welding the joint is added with a needle tipped squeeze bottle. You don’t want to get any of the solvent on the surface of the acrylic, only on the joint. Point the needle up, and squeeze some air out of the bottle. Then when you turn the bottle over with the needle facing down, you can let air back in the bottle and keep any liquid from escaping out of the needle tip until it is in position to apply solvent to the joint. As you add the solvent, start at one end and dispense the solvent along the joint, while you push gently on the underlying piece. The cushion of the towel will allow your finger to open up the gap between the two pieces by pushing on the protruding edge of the lower piece. That way you can get an even film of solvent between the pieces, without bubbles. The protruding edge of the face piece will also keep the solvent from going around the edge to mess up the face. My photo shows the application of the solvent, and the drawing shows and exaggerates how your finger bends the face piece down a little to open up space to get the glue into it. You want just enough solvent to fill the joint, and maybe a tiny bit more to keep the joint wet. If too much is pooled at the edge of the bond, it will make your joint a little more noticeable. This is one of the critical steps in getting a beautiful joint.

In a very short time the weld will hold so well that you will not be able to get the pieces apart. Despite this, leave the joint undisturbed for an hour. Then you can remove the weights, and leave the pieces as you had them on the towel. Then bond the second side that butts up against the front face that is lying on the towel. Align it, angle it, weight it and bond it like the one you just did. Once you have the sides bonded, flip it all over onto the back face and bond the sides to the back. Do one side at a time: align, angle, weight and bond each joint separately. That way can be sure of the precise aligning of each joint. If one piece is just a hair off from its adjacent piece where the top joins, then your joint will have a very noticeable void.

After all the sides are bonded together, wait 24 hours before setting up to glue the top, for the joints to get strong enough. The joint will seem strong, but the solvent deeper in the joint will not have dissipated for some time, and the trimming action can lead to air bubbles appearing later because the inside of the joint was not fully cured. Now the 1/8 inch excess is trimmed off with the router. You don’t need the fence: the ball bearing guide allows you to run the piece along it while trimming off the excess very smoothly and flush with the surface. The router depth is set so that the ball bearing ridesabout 1/16 inch above the piece to be trimmed. If the bearing rides too far away from the joint, your pressure holding the pieces against the bearing of the router bit will cause the bit to trim a little too much. If the bearing does not turn freely enough on the router bit, the bearing can spin and rub against the face that it is riding on, and leave a mark. This would be fixable, but make more work for you later.

Once the sides are trimmed, the top can be bonded in place. Align it, weight it, start at one corner and go all the way around the top with the solvent welding process. After waiting 24 hours, trim the edges of the top that are sticking out with the router.

The next step is to get the edges polished. Polishing is the process of making big scratches into little scratches. Sandpaper is first used to get the edges very flush with the faces, and then a flame is used to get the final polish. Start by putting a strip of Scotch Magic tape ¼ to ½ inch from the edges to be smoothed. This will keep you from sanding too far onto the face of your box. Begin with 240 grade wet-or-dry sandpaper wrapped around a block, and wet sand the edge that has been trimmed with the router so that it is flush with the face, and all the waviness from trimming with the router is gone. Using slightly soapy water with a little bit of dish soap in it helps keep the sandpaper from loading and helps wet the surface better. A spray bottle is convenient for keeping it all wet. You then wet sand with progressively finer grades of sandpaper, but now you skip using the block, and just use your fingers to support the sandpaper. This will make you less likely to sand a groove next to the tape. I use grades 240, 320, 400, 600, 800, 1000 and 1500. For the last grade of sandpaper, number 1500, take off the Scotch tape so that you can sand it right up to the area that was slightly protected by the tape.

Once the edges are all very smooth, it is time to get the final polish with a torch. Remove all tape, and wash the project with mild soap and water, and using a soft cloth to dry it. Use a brush flame, and a MAPP gas torch. Propane does not work as nicely, and increases your chance of getting little bubbles that blister up on the surface. The brush part of the flame is held about ½ inch from the surface of the acrylic, and moved with a steady pace along the edge. The speed is important, and the way to get the hang of it is to practice. Too slow makes the acrylic bubble and blister. Too fast, and nothing seems to happen. The better you do with the sanding, the less you have to do with the torch, and the less chance of blistering because you don't need as much torching. A few practice pieces will help you make the clear joints that are very nice to look at. Some brands of acrylic do not respond well to the flame, and bubble very easily.

There is a great temptation to go too quickly with the sanding, and use the torch to get it smooth. In practicing, give it a try! Doesn’t work too well, does it? The torch is great for the final finish, but too much heat in trying to make up for not enough sanding will give poorer results.

An alternative method to flame polishing is to use finer grades of sandpaper, wet sanding to about 2400, and then polishing with a rag wheel and polishing compound such as Bendix. Get the rag wheel going, and hold the Bendix against it for a second. Then apply the rag wheel to the acrylic, charging it with Bendix every now and then. A very light touch is needed, moving from one spot to another fairly quickly to avoid burning the surface. While this method also works well, it is a lot more work than using the torch. Also, once you use this method, you should not use the torch on areas that have been polished in this manner, or it is likely that crazing will occur and ruin the whole thing.

I highly recommend that you cut some extra pieces, and practice each procedure with those pieces first: trimming the edges to butt against the adjacent pieces, aligning and bonding the pieces, trimming the edge that is left sticking out, and finishing and polishing the edges. A little bit of practice will help you get a feel for how it works, and pay back big dividends in the quality of your box. It will help get a feel as to how important the sanding is, and also for the speed, heat, and distance for the flame polishing. There are steps where waiting is required. A high quality joint requires some patience.

I have enjoyed making things all of my life. My website is www.chaneyproductions.com and has many of my creations with pictures and videos.