Cast and Extruded Acrylic - What is difference?

Extruded acrylic is a thermoplastic and is melted by applying heat so it can be shaped. It can be re-melted. Cast acrylic is a thermoset which is hardened by chemical reation and is not reversable. Cast acrylic will soften with heat but will not melt. I'm sure someone in this group can give you a more scientific explanation.

While some cast acrylic may be cross-linked (thermoset), my understanding is that the major difference between cast (which may be continuous cast on a belt, or batch cast between glass plates) and extruded is molecular weight. To be able to extrude the acrylic resin, it has to have reasonable melt viscosity, which puts limitations on molecular weight. There is no such limitation on cast sheet.

The cast product is likely to have slightly better toughness and less likely to have problems with solvent stress cracking. Due to differences in the process (and not to molecular weight) the cast sheet is likely to have better optical properties. Thicker sheet (>1/2 inch) is likely to be cast rather than extruded.

Regards,

Ernie

Some cast acrylic may have small amounts of crosslinkers added that make them infusible. If you overheat any acrylic it may decompose to mostly monomer. There are acrylic sheets that are cast or extruded and some are meltable. Manufacturers will not tell you which. Usually the embedded castings you see are pure polymer but cast sheets normally are copolymers. There are also specially made extrusion grades and casting grades - in casting grade, resin is dissolved in monomer. There are also acrylic resins formulated for paint use and most contain comonomers. If you have a specific interest you need talk to the manufacturers. Frank

Thanks for the replies. I did not realize that cast acrylic is considered a themoset. It can be re-heated and bent - that describes a thermoplastc, but if you consider the crosslinking, it is a thermoset!

I also found on the Internet, that by flame testing, Cast will crackle when burning, but not drip, extruded drips and no crackle. I have several scrap pieces I tested with the results:

1) Blue tinted 1/8" thick: crackles loudest, no drip (will eventually drip if left burning awhile). Probably cast. 2) Bronze tinted 3/16": crackles, lots of bubbles. Drips after a few seconds. cast? 3) clear 3/32": burns nearly silent, lots of bubbles, drips. extruded? 4) clear 3/32": burns silent, little bubbling, most fluid when hot. appears to be extruded. bg

Many thermosets like polyurethane, epoxy, polyester, get somewhat flexible when heated, but as temperature rises, they eventually char and burn without melting.

If you heat the samples on a metal plate, the thermoplastic will melt, while the thermoset will soften but not melt. I guess this test is similar to the burn or drip test. If you have a temp. control on the plate I think the melt or not melt is easier to see without the flames.

I would not call cast acrylics thermosets. As the name thermoset implies a thermoset is set by heat, that is, cross-linked by heat curing. Compare a true cross-linked thermoset, like phenolic or melamine type resin to cast acrylic sheet: the acrylic is thermoformable the cured thermosets are not. My guess is that most cast acrylic sheets will dissolve in a solvent like toluene. Branching the resin may improve thermoformabilty, cross-linking destroys it.

Dripping is a function of melt viscosity: the extruded sheet has lower molecular weight and lower melt viscosity, hence it drips. The crackling/bubbling is a function of depolymerization, and melt strength of the resin. Most cast sheet is essentially 100% pMMA, which does not have sufficient thermal stability for melt processing such as extrusion or injection molding. The melt processable resins contain 2-10% comonomer (usually ethyl acrylate) to improve the melt stability; the comonomer acts as zipper jammer in depolymerizaton. The higher melt viscosity of the cast sheet means that it requires a greater force (vapor pressure) for the developing bubble to break the melt, and the crackling is probably the bubbles tearing the melt.

Regards,

Ernie