How To Design Custom Acrylic Boxes

I get calls every week from people who want acrylic boxes. They found a picture online, they have a product to display, they need it yesterday. And about half of them have already tried getting boxes made somewhere else and the boxes cracked, or warped, or the dimensions were wrong. So they call around looking for someone to fix the problem.

The problem usually isn't fixable. Once acrylic crazes, it's done. You can sand it down and polish it out if the crazing is shallow, but that takes longer than making a new box and most of the time the crazing goes deeper than people think. I've had customers bring in display cases where they tried to sand out the cracks and ended up with wavy surfaces that look worse than the original damage.

 The Cast vs Extruded Thing

I'm going to spend more time on this than anything else because this is where 90% of the problems come from.

Cast acrylic and extruded acrylic look identical. You cannot tell them apart by looking. The only way to know is the material certificate from the supplier or sometimes there's a sticker on the protective film. Cast costs more. Extruded costs less. That's why purchasing departments always want extruded.

Here's what happens with extruded: the extrusion process leaves stress in the material. Molecular stress, locked into the sheet. The sheet looks fine, cuts fine, even bends fine if you're careful. But when you solvent weld it-when you use acrylic cement to bond two pieces together-the solvent attacks the stressed areas first. Methylene chloride in the cement dissolves the surface to create a molecular bond, and when it hits those internal stress points, the stress releases as cracks.

Sometimes the cracks show up in a few hours. Sometimes a few days. I've seen boxes that looked perfect for two weeks and then the customer called saying there were lines spreading from every corner. That's the stress releasing.

Cast acrylic doesn't have this problem. The manufacturing is different-liquid monomer poured between glass plates, polymerized slowly over hours. No extrusion stress. You can solvent weld it all day without crazing.

The price difference runs maybe 15-20% depending on thickness and where you're buying. On a $500 job that's $75-100 difference in material cost. On a $5000 job it's $750-1000. When the $5000 job fails and you have to remake it, you've lost more than you saved.

 Dimensions

I could write 5000 words on dimension screwups.

Inside dimensions are the cavity. Outside dimensions are the footprint. A box with 200mm inside dimension and 5mm walls has 210mm outside dimension. This is obvious to anyone in fabrication but clients get it wrong constantly.

The conversation usually goes like this: client says they need a box 200x150x100. I ask inside or outside. They don't know. I ask what's going inside the box. They tell me. I measure the thing or ask for dimensions. We figure out whether 200x150x100 is the box size or the product size. Half the time they wanted the product dimensions and expected us to add clearance and wall thickness on our own.

Now I require a drawing with dimensions labeled inside or outside. If they can't provide that, I make them fill out a form that asks specifically. What is the item going in the box, what are the item dimensions, how much clearance do you want around the item, what wall thickness are you expecting. Forces them to think it through.

Still get it wrong sometimes. Guy ordered 50 boxes last year, provided dimensions, approved the drawing, received the boxes, items didn't fit. Turned out his "item dimensions" were from the spec sheet and his actual items were coming off a production line with +2mm variance that nobody told me about. Had to remake all 50 boxes with 5mm extra clearance.

 The Crazing Problem

Already covered the cast vs extruded piece. But crazing happens even with cast acrylic if you introduce stress other ways.

Drilling without clearance holes. Acrylic expands and contracts. A tight hole around a screw creates stress concentration. Temperature cycles and the stress builds. Eventually you get cracks radiating from the hole. Solution: drill clearance holes and use washers.

Bending too tight. Minimum bend radius is roughly 2x material thickness for cold bending, tighter if you're heat forming. Bend it sharper than that and you've got stress locked in.

For cleaning acrylic: mild soap and water, or specific acrylic cleaners like Novus or Brillianize. That's it. Put cleaning instructions on a label inside the box or on the packing slip. Still doesn't guarantee people will follow them but at least you tried.

Once it's crazed it's scrap. All the "acrylic repair" products you see sold at boat shows and auto stores are garbage. They might hide the appearance temporarily but the structural damage is done. Replace the panel.

 Edge Finishing

Laser cutting gives you a polished edge on cast acrylic right off the machine. For clear acrylic display work where the edges show, laser is the way to go.

Saw cuts and router cuts leave tool marks. You have to sand through grits-180, 320, 600, maybe 800-then buff with compound. Takes time. Flame polishing speeds this up if you have the skill. Hydrogen-oxygen torch, pass it along the edge, the heat flows the surface smooth. Screw it up and you bubble the material or warp the part or start a fire. Not a beginner technique.

For production work there are diamond edge polishing machines. High capital cost but consistent output.