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Casting challenge


#1

Hi! I have a challenging situation that I hope you will be able to
help resolve. I designed a few components which we casting some items
in silver. These pieces have a hole pierced through the length of
each piece. We are getting a lot of rejects because of investment
breakage. In the rubber mold, we insert a properly-sized pin which
maintains the hole shape while the item is injected. The pin is
removed and the hole is still there as it should be. We build trees
and invest them so the holes are filled with investment and the tree
structure in wax is still there. After burnout though - we are left
with some really interesting little investment wands - all over the
flask. This makes it quite interesting during pouring of the metal
where the little investment pieces break off and end up everywhere
inside the flask wreaking havoc on the pieces inside. Investment
bowling you could call it!!! Rejection rate is understandably high!
What I (think) I need is something to insert inside each piece so
that there will be no investment entering the holes and causing this
mess. Maybe a type of compound/material that will not be affected
during burnout and can be removed somehow after casting? Am I being
unrealistic? Do any of you have a suggestion? Thanks in advance!!!
Rachel Silber www.silbers.com


#2

Graphite used for mechainical pencils can be used. Try burning the
"leads" before using them for real, as some are no longer pure
graphite and may melt long before you cast.

Bruce


#3

Rachel,

I have a couple of suggestions. First make sure that the sprues are
positioned so that the metal flow misses the cores. Second is to use
mechanical pencil lead as a core material. Available at least in
.5mm, .7mm and 2.0mm diameters, though sizes other than .5mm can be
difficult to find or expensive.

It maybe also possible to use aluminum wire, limit burnout to
1250 0 F, and etch out with lye. The heavily oxidized aluminum wire
might not fail through melting before the casting metal solidifies.
Only a test cast will tell for sure, but an interesting enough
question that I’ll try next= burnout.

Jeff
Demand Designs
Analog/Digital Modeling & Goldsmithing
http://www.aztec-net.com/~jdemand


#4

I do a similar thing in making silver beads but if I have to go over
about 12 mm in length (with a 1.6mm hole I make very small holes
through the pattern in pairs opposite each other to support the
core.

The other thing you can try is using pencil lead if you can find some
the right diameter and not made with a modern plastic binder, you
need the old fashioned clay type.

Also make sure that you pour in line with the core and maybe use a
double sprue so that the metal does not push it sideways. I have
done 25mm long holes 1.6mm wide so far! Tim Blades.


#5

Rachel, Make your molds as normal with a pin, however when you
inject, do not use the pin, but go to office depot and buy yourself
some graphite in the form of pencil lead for propelling pencils. They
come in a variety of diameters. Inject with a piece of graphite in
the mold aligned by your pin and put it on the tree and cast. Once
you acquire your piece, drilling out the graphite is like going
through butter with a hot knife. No investment breakdown or closed
holes to deal with. Every hole will be consistent time and time
again. Of course you cannot re-use the graphite, but the expense out
weighs the cost of failures and rejects on a continuous basis.

Best Regards.
Neil George
954-572-5829


#6

Rachael,

Many people in the investment casting industry ( not so much the
jewelry side of it) use soluble ceramic cores. These cores can be
inserted in the wax or even better inserted in the mold and have the
wax shot around them. They are much stronger than investment so they
won’t break off but they can be water blasted out after casting or
dissolved out with caustic.

I don’t know any company names off the top of my head right now but
I’m sure an internet search will yield results. I’ve done plenty of
hollow casting and never really had that problem. How long are the
tubes and what is your burnout cycle. Also, are you casting by vacuum
or centrifiguly.

Tino Volpe
Metallurgist, Technical Manager
Tiffany & Co.
300 Maple Ridge Drive
Cumberland, RI 02864-8707
401-288-0124


#7

To maintain a hole in a piece I have used graphite (pencil lead)
which is inserted into the wax (where your pin was in the mold) so
that it extends beyond the wax and will be supported by the
investment. The graphite I used was .7mm pencil lead. The graphite
can survive the burnout and is a little stronger than investment. The
graphite can be removed fairly easily from the silver by using a
drill bit of the appropriate size in a flex shaft.

Ron VanOstrand
VanOstrand Metal Studio
www.ronvano.com


#8

To Rachel Silber, Try putting a piece of steel or iron wire in the
hole of your wax model. Leave some sticking out at each end. The
wire will still be there during the burnout and casting. After you
have quenched the mold and cut the items from the sprues grab the
end of the wire in the hole with some pliers and pull it out.

John Wade, Wade Designs


#9

Further, if the wire tends to stick in the casting, you could coat
the wire with a thin layer of investment before casting the wax
around it so that it will withdraw easily once cast in metal.

John Dach


#10

Hi Rachel, I am not sure if this will help but it’s worth a try.
When I made hinges for bracelets years ago I would insert a graphite
rod into the holes and have these extend on either end. The
investment would hole the graphite pieces and the metal would flow
around these “rods” during casting. You would then have to drill out
the graphite to re-establish the lumen. The graphite we would use
was mechanical pencil “lead” which at that time could be obtained in
various diameters. These were solid rods. The newer “leads” are
unfortuately not solid but instead are plasticized mixtures of
graphite which would melt during the burnout. The old forms of
"lead" may still be available, but you would have to hunt it down.

Perhaps a search on the web might uncover a source.

I thought I had, but never tried since I no longer do castings, is to
use titanium wire of the proper diameter to hold the space and after
casting try to remove it. It may be worth a try, but I’m certain that
others who do castings on a regular basis will give you
a procedure that works. Good luck, Joe Dule


#11

We thought of a different approach rather than a wire insert. Is
tubing available in your size and material? Tubing could be put in
the wax and cast in place. Sort of borrowing from the stone in place
and bi-metallic casting methods.

Daniel Ballard
PmWest
800-999-7528