Also, heard nothing about the clear resin question. Should it be "glass clear" or is milky white normal?
From
Casting Resins into Molds :
One of the Holy Grails of rubber mold resin casting has always been the search for a method of casting an optically clear part. While many resins come in clear formulas, it seems impossible to cast them and have them come out clear enough to read through. As said before, resins always shrink and the last bit to cure is the surface-- this conspires to
prevent the reproduction of a glassy surface no matter how perfect the mold. (The one exception to this is a fully drafted vacuum-formed polyethylene mold, with a simple hemisphere shape-- as the resin shrinks, the casting slides further down into the bowl, maintaining contact with the surface of the mold. Alas, more interesting shapes lack this
quality.) While you can fill resins to minimize shrinkage, filling them will ruin the clear quality you're after. For decades the only way to get an optical surface was to cast it close, and then laboriously hand polish it to a rouge finish. Today, several companies, such as Synair Corporation or BJB Enterprises, (see suppliers list at bottom) offer "water clear" urethane resins. They are tricky to work with. Some have inhibition problems with certain silicones, for example. All are tremendously toxic sensitizers. They achieve a far more faithful unfilled casting fidelity by taking a long time to cure, so that the surface is not soft when the core cures. Synair's product takes 8-12 hours to set, and then requires a 2-4 hour "post-cure" at 275 degrees. These products are not perfect. There is nominal shrinkage, and the part comes out a little less than completely clear. However, they are close enough that adding one or two coats of clear gloss coat will achieve near-perfect clarity: a first in casting materials technology. Of course, being the holy grail, they cost an arm and a leg, but at least they're available.
