What is Cast Acrylic?

People call my shop asking for acrylic sheet and half the time they don't know what they actually need. Cast or extruded. Big difference. Most acrylic sheet suppliers carry both, but the sales guy on the phone might not explain why one costs twice as much.

Cast acrylic starts as liquid. MMA monomer, which smells terrible by the way. The stuff goes between two pieces of plate glass with a rubber gasket around the edge. That gasket sets your thickness. The whole assembly goes into a curing oven and sits there for hours while the liquid turns solid.

Rohm and Haas invented this back in the 1930s. Their brand was Plexiglas. Lucite was DuPont. Different companies, same basic process. You still see both names on spec sheets even though ownership changed hands multiple times over the decades.

The glass mold gives cast sheet its surface. Smooth as the glass itself. You can't get that finish any other way. Extruded sheet runs through steel rollers and sometimes you see faint lines in it if you look at the right angle. For picture framing or museum cases, that matters. For a router template, nobody cares.

Why fabricators prefer cast

I've run both materials through my CNC router. Cast machines like butter. Clean chips, smooth edges, holds tolerance. Extruded gets gummy. The bit heats up, plastic melts instead of cutting, and you end up with a welded mess stuck to your tooling. Slow down the feed rate and it helps some but you're still fighting the material.

The molecular weight is different between them. Cast has longer polymer chains. Makes the material stiffer and more heat-stable. When I'm thermoforming a part, cast gives me a wider window. I can heat it past where extruded would sag and drip off the frame.

Crazing is the other thing. Some customers want to solvent-weld acrylic with Weld-On or similar cement. Works fine on cast. Extruded crazes around the joint half the time. Those little surface cracks spread and the whole piece looks like garbage. Had a guy bring in a retail display his intern built with extruded sheet and the wrong solvent. Completely ruined.

The production side

Custom acrylic fabrication shops order from distributors who stock standard colors and thicknesses. Clear, white, black, maybe some fluorescents. Anything else and you're looking at a mill order. The nice thing about cell casting is a factory can pour whatever color you want in relatively small batches. Fifty sheets of a specific Pantone match is doable. Try that with extrusion and they'll laugh at you or quote a minimum buy that bankrupts your project.

Thickness range is wider too. I've bought cast sheet over three inches thick for a custom aquarium job. Extruded tops out around 20mm at most distributors. The extrusion die and cooling setup limits how thick they can run.

The tradeoff is tolerance. Cast sheet thickness varies more than extruded. Your 6mm cast might actually measure anywhere from 5.5 to 7mm depending on where you check it. The gasket shifts, the glass flexes slightly, whatever. Extruded holds tighter because the die is rigid steel. For precision work I've had to surface both sides of cast sheet on the mill just to get it flat and consistent.

Where the material goes

Aquarium builders use cast almost exclusively. The panels need to be thick, optically perfect, and handle solvent welding for the seams. Point of purchase displays, if the client has budget. Museum cases obviously. Aircraft windows are cast acrylic with specific certifications.

Sign shops use a lot of extruded because it's cheaper and they're cutting letters on a laser anyway. Edge quality from laser cutting doesn't differ much between cast and extruded. It's all melted either way.

Wholesale acrylic distributors stock mostly extruded in common thicknesses because that's what moves. Cast sits on the rack longer and ties up capital. Specialty orders go direct to acrylic manufacturers like Plaskolite or Trinseo or whoever bought what used to be Altuglas.

Raw material supply chains start with MMA monomer from the big chemical companies. Mitsubishi, Dow, a few others. The casting factories buy tanker loads of the stuff. It's flammable and the fumes are nasty so they treat it carefully. Most manufacturers run their own polymerization rather than buying pre-made syrup.

Problems nobody mentions

Stress in the sheet. Cast acrylic cures under heat and the outer layer sets before the middle. Internal stress stays locked in. Anneal it if you need to, but most shops skip that step. Laser cutting releases stress at the kerf and sometimes the part warps slightly. Not usually a big deal unless you're making precision parts.

Protective masking is hit or miss. Some suppliers use paper-based masking that tears and leaves adhesive residue. Others use polyethylene film that peels clean. Ask before you buy a hundred sheets and discover you have to scrape every piece.

Storage matters. Stand sheets vertical in a rack. Lay them flat and the weight causes creep over time. I've seen sheet from a distributor's bottom of the pile with a permanent bow in it.

The stuff scratches easier than glass despite what the marketing says. Novus plastic polish helps but deep scratches need sanding and buffing. Cast polishes out better than extruded because again, that molecular weight difference.

Thermal expansion is higher than glass. Frame accordingly or the panel buckles in summer heat. I've fixed a few installations where somebody set the glazing tight to the rabbet and the whole thing oil-canned when the sun hit it.

Bottom line

Cast acrylic costs more because it takes more labor to make. Glass cells assembled by hand, one pour at a time, hours of curing. The properties justify the price for the right applications. Optical work, machined parts, thermoforming, chemical exposure. For basic glazing and signs, extruded works fine and saves money.

Ask your supplier which one they're quoting. Cheaper price probably means extruded unless they specify otherwise. Get a sample if you're not sure. The surface feel and edge quality tell you immediately which one you're holding.