What Is Plexiglass?
Plexiglass is acrylic sheet. Same thing. The name comes from an old brand but everyone in the shop just calls it acrylic now. Chemical name is polymethyl methacrylate. PMMA if you're reading a data sheet. Clear plastic that looks like glass but doesn't shatter the same way.
Cast vs extruded
Two ways to make the sheet. Cast acrylic gets poured into molds and cured slow. Extruded acrylic gets pushed through a die like toothpaste. Cast costs more. Also machines better. I learned this the hard way trying to laser cut extruded material for a display job. The edges came out gummy and the whole shop smelled like burnt plastic for a week.
Cast holds up better when you're doing detailed CNC work. The tool marks buff out cleaner. Extruded tends to stress crack if you push the feeds too hard. Most sign shops run extruded because it's cheaper and they're just doing simple cuts anyway.
Thickness matters
We stock everything from one millimeter up to three inches. Thin stuff is for picture frames and point of sale displays. Thick blocks go to aquarium builders and machine guards. I had a guy last month order four inch cast clear for a submarine viewport prototype. Took eight weeks to get. He didn't like hearing that either.
Standard tolerance on cast sheet is plus or minus ten percent on thickness. So your half inch sheet might actually be 0.45 or 0.55. Doesn't matter for most jobs. Matters a lot if you're doing precision optical work or trying to fit into a machined frame with tight clearances.
Working with it
Acrylic machines nice if you keep your tools sharp. Dull cutters melt the material instead of cutting it. Then you got a mess. Carbide tooling lasts longer than high speed steel. Single flute bits clear chips better than two flute on routers.
Drilling is where people screw up. Standard twist drills grab and crack the sheet. You need a sixty degree point angle or a special acrylic bit with zero rake. I've seen guys crack a hundred dollar piece of two inch cast because they grabbed a regular drill bit off the shelf.
Laser cuts clean on thinner stock. Quarter inch and under. Go thicker and you get taper on the cut edge and flame polished surfaces that look good but stress crack later. Some shops swear by CO2 lasers. Others won't touch acrylic with a laser at all.
Forming and bending
Heat it up and it bends. Strip heaters work for straight line bends. Ovens for complex shapes. The window is narrow. Too cold and it cracks. Too hot and it bubbles or sags where you don't want it. Around 340 degrees Fahrenheit is the sweet spot for most grades.
Vacuum forming works great for signs and displays. The sheet stretches thin in deep draws though. A six inch deep box form might start at quarter inch thick and end up with walls under an eighth at the corners. You gotta plan for that.
Gluing and bonding
Solvent welding is how most shops join acrylic. Weld-On 3 or 4. Capillary action pulls the solvent into the joint. The pieces fuse together chemically. Strong joint if you do it right. Mess if you don't. Solvent runs everywhere and leaves marks on the face of the sheet if you're sloppy.
Thickened cement like Weld-On 16 fills gaps better. Slower cure. Good for joints that don't fit perfectly. I use it on mitered corners where the saw cuts weren't dead on.
Scratches and polishing
Acrylic scratches easy. Looks terrible on clear parts. Fine scratches buff out with plastic polish and a soft wheel. Deep gouges need wet sanding first. Start around 400 grit and work up to 2000 before you hit it with compound.
Flame polishing edges is fast. Pass a hydrogen torch over a saw cut edge and it goes from frosty to glass clear in seconds. Takes practice. Too slow and you burn it. Too fast and you miss spots.
The stuff nobody tells you
Acrylic crazes if you use the wrong cleaner. Windex will destroy it. Alcohol too. Plain water and a microfiber cloth. Or specific plastic cleaners. I watched a cleaning crew ruin three thousand dollars worth of installed sneeze guards at a restaurant because they used regular glass cleaner.
UV exposure yellows cheap acrylic over time. Outdoor applications need cell cast with UV stabilizers or you're doing warranty replacements in two years. The customer doesn't understand why the quote is higher for outdoor rated material. Until their sign turns brown.
