[phucrobusl]

Epoxy does not dissolve styrofoam. Polyester resin does.

This is true. However it might make sense to use Polyester instead of Epoxy to make granite. It's hella' cheaper! Can live without styrofoam. Replace it with fibreboard structure or PUR foam (which can handle polyester, unlike styrofoam)... I am thinking about making milling machine using Polyester granite. Wonder why everybody uses expensive epoxy. Polyester might be little harder, which makes it kinda brittle. But i guess this can hardly affect thick pieces that are casted for milling machines. Such machines are commonly made using relatively fragile cast iron. They even use plain old concrete to cast such machines!

[skrubol]

This is true. However it might make sense to use Polyester instead of Epoxy to make granite.

From what I've read all polyester resins shrink on curing, whereas some epoxy resins do not. If you're going for anything precise without post cure machining, shrinkage es no bueno.

[CaptainVee]

From what I've read all polyester resins shrink on curing, whereas some epoxy resins do not. If you're going for anything precise without post cure machining, shrinkage es no bueno.

Indeed, polyester shrinks.
So does epoxy, but by very tiny amounts.
A friend of mine built a EG gantry with steel on one side to later connect the rails to.

Even with setting up the cast on a surface plate, the steel was 0.1 to 0.2 mm convex over 600 mm after curing.
That suggests a very small shrinkage indeed, but it required reworking the steel.