> Virtually 0% shrink polyester high volume casting resins are
> available at only a slight premium - but don't know about clarity.

I use water-clear resin all the time for casting coasters. Reducing
the peroxide catalyst level does indeed reduce shrinkage. No
good being ina hurry and putting 2 or 3 or 5% peroxide in. Likely
to crack with the reaction heat

> I bought some stuff recently to cast a large flat reference surface -
> perfect for my purpose but not exactly water clear.  Would suspect
> that stuff suitable for your purpose exists though.  Or the old method
> used for those encapsulated spiders, etc. could be used - pour
> into mould in small enough amounts so that shrinkage and heating
> are minimal and build up in layers.  End result is very good, and cheap

Some parts on the original prototype are cast in resin and polished.
Looks OK, but p/e does tend to be brittle, much less than polycarb,
probably about the same as glass but it doesn't shatter. I'd like to
cast the pieces but they must have a flat surface both sides, which means
probably a glass mold, or at least glass sheet top and bottom. Possible
I could use a metal mold and cast resin instead of glass. The surface
exposed to the air as the resin cures is affected by atmospheric oxygen.
To get a hard smooth surface it needs to be covered and airtight. Too
much mucking about. And it smells and is sticky

A couple of pieces have severe re-entrant angles and I think I'll have
to cast those

> Also epoxies are generally very low shrinkage.  Formulations are
> used to repair windshields, etc, so suspect that optical properties are
> also good.  The killer here is cost though.  The windshield repair stuff
is
> *very* expensive IIRC.

Very true

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