I have a question about lost wax casting. I’ve probably cast less
than ten pieces so far, all small waxes in 2 1/2 x 2 1/2 flasks
casting sterling silver. Casting single pieces at a time also. I am
wondering if anyone out there does this on a regular basis and is
doing it sucessfully without porosity problems. Sterling seems to be
more prone to this than gold.
Any tips or advice on how to get porous free casts would be really
helpful.
John H.
Escape Designs
John,
Good question. Have you researched the Orchid archives on this? We
need descriptions of what the porosity looks like in order to
describe the solution to the problem.
Chuck from Asheville
Over the last 30 or so years we have done over 15,000 single
castings, mostly in 18 and 14 karat gold but a few hundred in
sterling silver. If we get porosity it is because we have done
something dumb like improper sprueing, too high a flask temperature,
or too much old metal in the melt. Of course, you should always cast
as cold as you dare and still fill the hole. We use a Kerr
centrifugal casting machine that has been modified with extra weight
on the end of the flask arm to cut down on the acceleration a bit.
This helps to keep the investment from breaking when casting delicate
designs.
Our flasks are 1 1/2 inches in diameter and from 1 1/2 to 3 inches
long. These will accomodate almost all rings. We make our own mold
bases out of vulcanized rubber.
Smaller flasks can be a real advantage, since they have less mass we
can use a shorter burnout cycle. We routinely go from mixing plaster
to casting in 4 hours. We have found our casting methods to be be
very forgiving and we rarely get a bad casting.
John
John
We use a Kerr centrifugal casting machine that has been modified with extra weight on the end of the flask arm to cut down on the acceleration a bit. This helps to keep the investment from breaking when casting delicate designs.
Could you give us more detail on the modification of the
counterweight. What kind of problem were you having and why was this
the solution.
Charles Friedman DDS
Caster of small functional sculptures
About 7 years ago we started using wax models from our home made
computer controled mill. This allowed us to make the waxes with a lot
of fine detail. As we learned how to take advantage of the system’s
capabilities we kept cutting deeper into the waxs to give more relief
to the design. This changes the look and personality of the finished
jewelry. Our customers like it and so do we. The down side is that
the investment would be more likely to break off in the fine deep
areas of the rings causing unusable castings. This would happen
mostly near the sprue where I suspect the force of the onrushing
metal entering the mold is greatest. When we started slowing the rpm
of the casting machine down to lessen the force we reached a point
where we were getting incomplete castings. Remember, we are using
small flasks with out much mass so the acceleration at the start of
the casting machine’s rotation is greater than if we were using a
larger flask (and more counterweight). Adding weight to the flask arm
(and counterweight to the other side) pretty much solved the problem,
although we still have to be mindful of our sprueing on some pieces.
The modification is easy enough, I just drilled and tapped a 1/4-20
thread into the center of the back plate of the flask arm. I bolted
different weights on until I was happy with the results. One of the
extra counterweights that comes with the machine is about right.
John
How about vacuum casting? That’s all I’ve been using for 20 years and
it seems to work fine, even with detailed computer carved waxes from
Blue Star Wax carving. It seems like there’s die hard centrifigal
casters and die hard vacuum casters and never the twain shall meet.
Thank You,
Michael Babinski
info@foxfirejewelers.com
We make our own mold bases out of vulcanized rubber.
John, Please tell us how to make the bases. I would like very
much,to have a base with a deep dome, instead of the shallow dome
bases most suppliers carry.
thank you, Andrew
John, Please tell us how to make the bases. I would like very much,to have a base with a deep dome, instead of the shallow dome bases most suppliers carry.
If you have access to a metal lathe you can machine the negative of
the shape you want the sprue base to be. I used a block of aluminum.
I then pack it with mold rubber (castaldo white) and vulcanize it. I
have rubber bases we have been using for over 20 years.