I wanted to get something cast that had tubes through which a
chain would pass. Since I wanted a mold made so I could have lots
of them I thought sticking something in place before molding and
before casting would solve my problem. The caster said they used
to use graphite to do that (so I knew the idea I had "invented"
was nutso) but that the graphite had lost quality and was sticking
inmetal.Well, I got an idea to ask about ceramic rods. I wrote to
a few manufacturers and I am enclosing an excert from a reply that
I got. I'd like ya'll to chew on this, toss it around and let
everybody else know what you think.
I have a few options for you. Precious metals will not adhere to
the majority of ceramic materials. The problems you will encounter
are thermal shock and the low thermal expansion coefficient of the
ceramics relative to the metals. That said, I can offer three
materials that I believe will survive the application. Aluminum
oxide, mullite and boron nitride. Aluminum oxide and mullite must be
cut with an abrasive diamond wheel. The boron nitride can be cut
with a knife. Of the three, boron nitride will be the most thermal
shock resistant, followed closely by the mullite and the alumina a
distant third. Fortunately for you, the geometry is small so I think
all three would survive the application. If you are concerned about
reuse and mechanical strength, the alumina is strongest with mullite
a close second. Boron nitride is significantly weaker. Thermal
expansion may be a problem even with the small diameters. Once the
metal reaches solidus, it will shrink faster than the ceramic thus
becoming locked into the metal ring. Geometries may be small enough
that this is not a problem or you can pull out the ceramic shortly
after solidus is reached.
As an alternative, have you considered the use of coatings on a
steel pin? If you have an acetylene torch about, try making a rich
flame and depositing carbon onto a metal pin and use it. Carbon and
graphite are remarkably good release agents. Acheson colloids sells
graphite laden liquids that can be painted on also. You might try
your local weld shop and see if they sell a braze stop-off. The
stop-offs are ceramic powders in a vehicle designed to stop the
adherence or flow of braze alloys. They are readily available in a
number of forms from multiple sources. You can also obtain a boron
nitride spray from us with outstanding release characteristics.
I like the fact that several options seem reasonable.
Even with ideas to solve the chain channel problem, the two
commercial casters I talked to did not want to get involved with the
project (piece too large) in fine silver.
Enjoy - Justine