I understand acrylic products are *commonly* made by three different
methods. Cast, extruded and continuous with continuous being used for making
sheets using the acrylic polymer made for extrusion. Extruded and cast
acrylic have different properties, I have found. What chemicals are added to
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.
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
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
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.
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
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.