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Help.. large gold casting


#1

I’m trying to cast a 14oz ,18k rod braclet. The rod is 12mm round
.Each time I’ve tried to cast I keep getting shrink porosity.I’m
using a vacuum casting machine.And the flask temp. is 1150,sprue
size is 6x12mm x1inch

OO <–sprue
I’m sprueing at the center–>
OOOOOOOOO

O O

OO OO

please,if you have any ideas or thought write me ASAP thanks.


#2

Hi , I had same problem some time ago with a 14 k large heavy
bracelet answer seemed to be casting in two halves.


#3

Try more sprues, and a lower flask temp - about 1000F


#4
  I'm trying to cast a 14oz ,18k rod braclet. The rod is 12mm
round .Each time I've tried to cast I keep getting shrink
porosity.I'm using a vacuum casting machine.And the flask temp.
is 1150,sprue size is 6x12mm x1inch

Your flask temp seems to be too high to me. I would think 900 for
something so heavy. I would call my alloy supplier, they would
recommend the proper temp for their alloy.

Mark P
Wisconsin


#5

D! Try casting with your flask temp down around 700 or 800 F. Use
a round sprue at least 13 mm in diameter. What appears to be
happening is your sprue freezes before the rest of the bracelet and
there is no liquid metal left to feed into the interstices of the
dendrites. Hm. Where to start? Molten metal crystallizes when it
solidifies. The crystals are shaped like dendrites…sort of like
treetrunks with branches or like branches of a snow flake. If
there’s always a supply of molten metal available to a
solidification front (where the metal is solidifying, hopefully at
the edge of your melt), then the interstices draw molten metal into
them to solidify into a dense rather than porous solid. I imagine
that the reason we have snow fall rather than ice chunk fall is
because the supply of water where the snow is forming is limited.
Only small bits of solid water can form. The porosity in your
bracelet could be the spaces between the branches which never
filled in with fresh molten metal. It is likely that the center of
your bracelet, even when cast at the lower temp, will still have
porosity in it. But, for most customers, out of sight is out of
mind. If you would like to cast the perfect casting, make your wax
form in the shape of a wedge, thin at one end, thick at the other.
Attach your gate (sprue) to the thick end. The bracelet would be
shaped like a double cone ( a round wedge, if you will). The center
where your sprue attaches, would be slightly thicker (2mm) than the
ends. The gate must be slightly larger in diameter than the largest
cross-section diameter of your ‘wedge’. In this way, molten metal
will flow into the form, begin to solidify at the edges of the
form, and progressively solidify to the thickest part of the
form…the sprue attached to the wide part of the ‘wedge’. All
shrinkage porosity should appear in the sprue and not in the
’wedge’. Interestingly, casting is a system. So, if your metal was
clean, no dissolved gasses or extaneous solids, and there wasn’t a
reaction with the walls of your investment, you’ll probably get a
dense casting. I’m sure there will be other additions to this
thread…it’s a fun subject. Chuck Hunner


#6

Hi DWarehous, I cast large gold pieces with some frequency. You’ll
need to use more but indirect sprues.are you using any vents?
Bring your flask down to 900F. You can e-mail me off-line for more
info or call me. 207-729-3599 John at the J.A.Henkel Co.,Inc (see
our ad in Metalsmith)


#7

might be your alloy. unless you are using one designed for 18kt
casting it will often cause the problem you are describing. try
United Precious Metals they have a very good 18kt casting alloy.
they are listed in the 800 directory. Frank Houston, tx.


#8

try a flask temp. @ 650 f


#9

Robert;

The responses about sprues and flask temp are all right on. Use
big sprues, like 3/8" and a low flask temp. Linus suggested 650 -
that’s what I’d try. I just wanted to add that you should have as
big a button as possible at least 75dwt in addition to the sprue,
the more the better. With such a large heavy piece you want to be
certain that it is being fed with metal as it cools. Even with
large sprues, your button could solidify first since it is exposed
to air and you would still get shrinkage. If you are torch melting
you may want to keep the flame on the button for a minute after you
pour just to make sure the button stays liquid until the piece has
solidified.

Brett


#10

Hi! make a if you can give me alittle more as to the
shape of your peice that your casting. We cast 18k all the time and
shrinkage porosity is just part of the game. But I may help you get
it ti a minimum. Try sprueing past the peice. If you need more info
e-mail me.

Mr.Matt
Contemporary
Industrial Arts