Your questions are good ones, but the answers are a bit tricky.
If the model you want to mold is essentially flat, or multiple
layers of flat sheet, by all means try to fabricate and pierce your
model out of sheet stock ( I prefer sterling, it forms and cuts
easily, and solders cleanly). That will make the most precise model,
in my opinion.
The question about how thick to make your model to get a consistent
wax injection is harder to answer. If your piece is small, its
thickness can be thinner than a larger piece. You see, the problem is
not whether a rubber mold can be made of the model, but whether your
wax injection can fill the mold cavity completely. Imagine your
liquid wax making its way through the rubber mold while it is still
hot and liquid. That wax has to make it from one end of the mold
cavity to the other quite quickly, or it will cool and solidify
before it finishes filling the mold. The injection wax you use will
contribute to its flow and solidification properties. Lots of
powdered vents will be important, I think.
If you were to ask the Orchid community specific questions about
what you are proposing to mold, then you would get some very good
answers, I am certain. How large or small are your models, and what
are the thicknesses of the sheet(s) used in the model ( in. or mm)?
Also, those who are experts at casting and mold making have some
incredible “voodoo” they use to make wax models cast incredibly thin.
Now that I think of it, I’d love to know some of the secrets casters
use to shoot consistent super-thin waxes and have them cast
completely. Anyone out there want to share this