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Large casting question


#1

I was approached by a friend of my dad’s, who wanted me to reproduce
a large old brass belt buckle of his in sterling silver. The brass
buckle weighs 6.6 ounces, or 185 grams. I don’t have a mold frame big
enough to make a mold of the thing. And I did a weight conversion and
found that the same buckle in silver would weigh about 8.5 ounces,
unless I figured out a way to hollow it out and still maintain enough
strength in the buckle. As much as I’d love to produce (and get paid
for) a buckle that heavy, I politely declined the job, and suggested
that he get his old belt buckle silver-plated if he really wants it
in silver.

But this got me to thinking–what is the maximum weight that can
be cast in sterling on a vacuum casting machine? I don’t even know if
our machine could handle casting an 8.5 ounce object. How would
something this heavy be done in sterling if the vacuum machine
couldn’t do it?

–Kathy Johnson
Feathered Gems Jewelry
http://www.featheredgems.com


#2

Kathy, I cannot directly answer your question re what is the greatest
weight that can be cast on a vacuum caster but, several years ago I
faced a problem with casting a large buckle. It was not as large as
yours but taxed the size of my centrifugal machine. I simply cut the
wax in half, cast the two and soldered them together after casting.
Worked like a charm.

Cheers from Don at The Charles Belle Studio in SOFL where simple
elegance IS fine jewelry! @coralnut


#3

Hello Kathy, I have done several large casting projects in sterling
and some in gold.

Large objects require a large spruing system and a large button,
usually about three times the weight of the piece. They also require a
very cool casting temp. They can be done with a vac or a centrifuge
machine.

With very large pieces, I will usually reinforce the investment
using fiberglass. this is so the investment doesn’t explode when a
pound or two of silver hits it.

My machinist has created large, thick and odd shape mold frames to
meet about any molding situation. I prefer natural rubber for most
molds unless I need to minimize shrinkage. If an object is too large
for my mold frames, I make an RTV mold in a custom made box using a
two part system so I don’t have to cut a mold that big.

Getting a good wax can often be a greater challenge than casting on
an oversize piece. On a piece that is very large I use injection
times up to 5 mins, not just to fill the mold, but to keep pressure
on the wax as it is drawing and cooling. I have cobbed together some
pretty weird clamping systems to hold large molds together and in
place while injecting.

Finishing a large casting is also a challenge. I would usually opt
for a piece with lots of texture as opposed to a high polish.

The cost of the silver is less than the labor for such pieces.
Removing weight in the injected wax is not a bad idea if it improves
the casting propertiecs of the model. The customer may want the extra
weight. Maybe you could hide one of those “Abforce” things in the
back of the buckle to strengthen his stomach while he’s wearing
it!..Hope some of this helps.

John, J.A.Henkel Co., Inc., Moldmaking, Casting, Finishing and
producing solutions for jewelry artists.


#4

Hello Kathy There may be some commercial casting houses that could
handle a casting this size, or depending on the detail, perhaps a
good sandcasting could be done? I’ve spoken with a fellow who has
been in the foundry business about 40 years, and he told me they use
a very fine sand for detailed casting jobs.

Regards,
Dave Nitschke


#5

Kathy, I pour up to 25 ounces of sterling from a 100 oz electric
handy melt into a 5 inch dia by 6 inch tall flask on my vacuum
table. I don’t believe the advertised 100 ounce handy melt holds
100 ounces. The bigger question on big castings is the size of the
wax model. A 9 oz buckle would be no problem to cast if it fits into
a flask.

It would help if we knew the size of the buckle. There are mold
framed that are 4.75 inches by 3.5 inches. I had a special mold
frame made that is 6 inches by 4.5 inches. It may not be worth
buying a large frame for one project.

I would bet there are casting people that could make a mold and cast
the buckle for you.

Good Luck.
Lee


#6
 But this got me to thinking--what *is* the maximum weight that
can be cast in sterling on a vacuum casting machine? I don't even
know if our machine could handle casting an 8.5 ounce object. How
would something this heavy be done in sterling if the vacuum
machine couldn't do it? 

Hi Kathy, We have cast heavier pieces than this on Vacuum casting
machines. You do have to use a stronger investment ratio and you have
to have the ability to melt 1000 grams of silver or more to be able
to do so… Also, using a calculated weight of brass to silver isn’t
exactly accurate as you may be dealing with an additional 7 %
shrinkage factor from mold making , waxing and casting so the
silver casting may weigh less than your calculations form direct
weight conversion. Best wishes, Daniel Grandi … we do
casting/finishing and a whole lot more for people
in the trade… Contact : sales@racecarjewelry.com


#7

Kathy: have you considered casting the buckle in several pieces and
then soldering them together?

Ralph Cross
Fremont Jewelers


#8

Kathy’ A heavy piece such as you describe could be done by sand
casting. Jerry in Kodiak


#9

Kathy, Could a mold be made and then use silver clay to make a new
buckle? Teresa


#10

Hi Kathy, Sounds to me like the buckle is a prime candidate for being
cast using Delft clay to make the mold.

Delft clay casting is the jeweler’s version of sand casting. The
model, in this case the orignal buckle, is embeded in a very fine
clay. The clay is packed very tightly in 2 parts, a bottom & a top.
The model is placed in the center. After packing the clay & removing
the model, a sprue hole and any necessary vents are made in the top
part of the mold. The molten metal is then poured into the sprue,
filling the void left by the model.

After removing the cast item from the mold, the clay is reusable for
more castings of different items.

Dave


#11
Hello Kathy There may be some commercial casting houses that could
handle a casting this size, or depending on the detail, perhaps a
good sandcasting could be done? I've spoken with a fellow who has
been in the foundry business about 40 years, and he told me they use
a very fine sand for detailed casting jobs.

Or large cans (3-4000 lbs of investment is "common: in the art
industry) or ceramic shell. Ceramic shell can take most any metals,
and a hand built shell (verses a mechanical shelling system) can be
made that takes +/- 85 lbs of bronze ( a bit more in silver). Cast
iron, stainless steel, aluminym, silver, gold bronze, etc all work
well in ceramic shell. Finger print accuracy too!

John Dach