What Is Acrylic Sheet?

Acrylic sheet is PMMA in flat form. Polymethyl methacrylate. The same stuff optometrists call "hard contact lens material" back in the 80s.

Most people call it plexiglass. Plexiglas is actually a brand name from Rohm and Haas, like Kleenex for tissue. The generic term is acrylic sheet or cast acrylic. I still catch myself saying plexi on job sites.

The Two Types You Actually Buy

Cell cast and extruded. That's it.

Cell cast comes from pouring liquid monomer between glass plates and baking it. Takes hours. The molecules chain up long and tight. This is what you want for laser cutting, for engraving, for anything that needs to look good up close.

Extruded sheet runs through rollers like pasta dough. Faster, cheaper. The molecular chains come out shorter. Easier to thermoform because it softens more evenly. Harder to get a clean polished edge.

Here's the thing nobody tells you at the supply house: extruded sheet has internal stress. Heat it wrong and it warps or cracks. I've seen guys ruin $400 worth of material trying to bend extruded on a strip heater designed for cast.

Numbers That Matter

92% light transmission. Glass is around 90%. So acrylic actually beats glass for clarity, which surprises people.

Half the weight of glass at same thickness. A 4x8 sheet of quarter inch runs maybe 30 pounds. Same size in glass, you're looking at 80. Makes a difference when you're installing overhead.

Softening point around 160 Fahrenheit. Forming range 275 to 350. These numbers shift depending on manufacturer. Plaskolite runs slightly different from Acrylite runs different from the imported stuff out of Taiwan. Always test scrap first.

Impact resistance gets quoted as "17 times glass" which is technically true under laboratory conditions. Real world, acrylic cracks. It doesn't shatter into daggers like glass does. Big difference when you're spec'ing a machine guard or a public barrier.

Cutting

Three ways: saw, router, laser.

Saw cutting needs a triple chip blade. Negative rake or zero rake. Positive rake grabs and chips the edge. Feed rate matters more than speed. Too fast creates heat, heat creates gumming, gumming creates a mess on your blade and a melted edge on your part.

Router works fine with a single or double flute straight bit. Spiral bits work but they can grab. Climb cutting helps on the finish pass if your setup is rigid enough.

Laser is the cleanest. CO2 at 10.6 micron wavelength. The beam actually flame polishes as it cuts. You pull parts off the machine ready to use. No secondary finishing. This is why every sign shop in America owns a laser now.

One warning about laser: the fumes. Acrylic smoke contains MMA monomer. Not something you want to breathe. Run your exhaust. Change your filters.

Joining

Solvent cement is standard. Weld-On 3 and Weld-On 4 are the industry workhorses. Number 3 is thinner, wicks into tight joints by capillary action. Number 4 is syrupy, fills gaps better.

The joint surfaces need to be flat. Machine marks show through a cemented joint. If you're doing display work, the customer will see every flaw.

Capillary cementing works like this: dry fit your parts, hold them together, touch the applicator bottle to the seam. The solvent wicks in and fuses the surfaces. Done right, the bond is almost invisible and nearly as strong as solid material.

Mechanical fasteners work when you can't solvent weld. Drill oversized. Acrylic moves with temperature. A tight hole cracks when the seasons change.

Forming

Strip heater for line bends. Oven for complex shapes.

The mistake I see most often: heating too fast. The surface gets soft while the core stays rigid. You bend it, looks fine, then it cracks overnight as the internal stress releases.

Heat slow. Heat even. Let the whole thickness come up to temperature. For quarter inch material I give it 8 to 10 minutes on a strip heater. Thicker takes longer.

Vacuum forming pulls heated sheet over a mold using atmosphere pressure. That's about 14.7 psi doing the work. Not much force, so your draw depth has limits. Deep draws need thicker sheet and hotter forming temps.

What Goes Wrong

Crazing. Fine surface cracks from solvent exposure or stress. Looks like a spiderweb under the surface. Once it starts, it spreads. No fix except replacement.

Scratching. Acrylic scratches easier than glass. Minor scratches buff out with Novus 2 or similar plastic polish. Deep scratches need wet sanding through grits, then polish. Sometimes it's faster to just cut a new part.

Stress cracking around fasteners. Almost always from holes drilled too tight. Drill 1/16 over for screws. Use rubber washers to cushion.

Yellowing from UV. Standard acrylic yellows outdoors over years. UV-stabilized grades exist for exterior applications. Costs more. Lasts longer.

Buying Right

Domestic sheet costs more than import. Plaskolite, Trinseo, Acrylite from Roehm-these run premium prices but the consistency is there. Sheet-to-sheet color match, thickness tolerance, no weird inclusions.

Import sheet varies. Some suppliers are fine. Some ship material with waves, bubbles, or inconsistent thickness. If you're doing production work, test a sample before you commit to a container.

Masking comes paper or poly film. Paper pulls off easier when fresh. Gets stubborn after UV exposure. Poly film costs more, removes cleaner, worth it for outdoor storage.

Standard sizes are 4x8, 4x10, and 5x8 feet depending on supplier. Thickness from 0.060 inch up to 4 inches for aquarium grade. Most fabrication work uses eighth through half inch.

Colors: clear, white, black, and maybe a dozen stock colors. Custom colors require minimums, usually 20 sheets or more. Lead time runs 4 to 8 weeks.

Where It Goes

Sign shops use more acrylic than anyone. Channel letters, push-thru faces, display cases, dimensional logos.

Point of purchase. Cosmetic displays, electronics risers, product holders. Retail wants clear sight lines to merchandise.

Aquariums. Cell cast only. The big public tanks use panels inches thick, chemically welded into seamless viewing windows. This is specialized work-most fabricators don't touch it.

Medical and food service barriers. Post-2020 this market exploded. Every restaurant, clinic, and checkout counter needed sneeze guards.

Machine guarding. Impact-modified grades for when OSHA walks through. Has to take a hit without shattering.