Now where’s that vial of powdered toad’s liver?
Yeah, I’m fresh out, myself. This thread has had about as great of
advise on the subject as you’ll ever find - I’d like to start from
square one, though, for those who can use it.
There have been a couple of threads here about, “How to get rubber
molds to fill properly.” The answer to that is simple - put the mold
to the nozzle and push. If your model is well made, and the mold is
well made, you should get 99-100% waxes, at first glance. You might
reject a few for details or finish. So, the point I am making is that
it is the wrong question. The real question is, “How to make a
successful model?”. I get beginners coming in who want their pieces
molded, and I reject 50% of them as being unmoldable, and the other
50% need to be reworked, and it is always (ALWAYS) the same reason -
they are too thin. I certainly can’t delve into model making
seriously, here, but there is one essential thing to understand: When
you make a piece of jewelry, you make it, and it’s done. When you
make a model, you are dealing with plumbing. The wax and metal are
fluids, and they need pipes to flow through, and that’s your job - to
make pipes. Most often jewelry has a fat side and a skinny side, or
is uniformly shaped, and you can just sprue the fat side and be OK.
But you just can’t do things like an hourglass shape without thinking
about the consequences. You’ll get what I call the coke bottle
effect: Get a full coke bottle, open it and quickly turn it upside
down and you’ll see. This would apply to a big shank and a big top
connect by skinny little wires and the like. In that case you would
mold the shank, perhaps with the wires, mold the top separately (can
be in the same mold on a separate sprue) and assemble after casting.
You have an 8% shrinkage in the direction of pressure with Castaldo
gold, and then don’t forget that you need to finish it after. If you
are molding a 3 1/2" circle, as in the recent thread, you should
probably be starting with 16 gauge metal at least for a good fill.
That means that it becomes 18 gauge or so out of the mold, from
shrinkage, and then 20 gauge after filing, give or take. Well, I’m
rambling a little, but you just need to think about plumbing - If you
have a 14 gauge wire, it will fill for 2" (Not really, just for
proportion), if it’s 16 ga. - 1 1/2", and a 22 ga wire will only fill
for 1/2 inch or something - never forget shrinkage - that 22ga wire
is now 24 ga. You need to go up a gauge in sheet metal for every inch
bigger (again not really, just proportion). You can mold a 4" circle
out of 22 ga metal just fine, but you will never get it to fill
unless you sprue it every 1/2" or something around - plumbing. If it
were 14 ga. it might work, but 12 ga. is what I’d want for that.
Finally, a wise man once said that you can cast anything if you sprue
it properly. Very rarely there is such a thing as a bad mold - poorly
cut or executed, and we all call molds that don’t shoot well “Bad
Molds”. They almost always are not, though, they are usually bad
models. Put your minds eye in the sprue, and swim up the model. Do
you flow like the lines of a Ferrari? Or do you have to squeeze up
and down and go around corners and have the maker expect you to jump
gaps or (heaven forbid) go backwards? Or flatten yourself impossibly
thin? If you can’t see it going “Whoosh” and filling in 1 second,
then it might be a problem - and don’t be afraid to make 2 part models
to be assembled later - that can often solve many problems in a
flash.
http://www.donivanandmaggiora.com