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Casting porosity Priority: normal


#1
 Maybe I should raise the temp another 100 deg?

I’d really have to see the pattern to properly judge temps.

Something just occurred to me. You previously mentioned you use
3 turns on your centrifugal machine. The only time I use 3 turns
is when I’m casting a large heavy flask (3x5 inches at least).
The high velocity of the gold entering the ring cavity might be
eroding sharp corners in the investment, and mixing broken
investment particles with the gold. That could create sulphur
dioxide gas inclusions in the casting, showing up as pits in
heavy sections . I usually use 1 turn, only a very gentle force
is needed, just enough to get the gold into the flask and hold
it there until it solidifies. I mostly vacuum cast now because
it is so much gentler and many of my pieces have fragile
details.

When you refer to a "ball" next to the ring, are you referring to
a resevoir on the sprue? I just ordered in some plastic sprues
that have a resevoir on them from a dental supply outfit. I am
going to try them on Thursday, when I cast again.

Yup, that’s the idea. I used to use those. Many of them are
hollow (to help with bending them). You’ve got to be aware of
that because you could possible get investment into the hollow.
After I ran out, I just started building wax up. You might also
try using blind vents with a resevoir. You can get away from
multiple sprues that way.

Do I  need special regulators for hydrogen or can I use the same ones
I currently use?

Hydrogen requires it’s own regulator. I’ve been told it’s
actually an adapted oxygen regulator with left hand threads and
some other internal changes. It’s interesting that I go through
at least 4 hydrogen bottles for every one oxygen bottle. You
should also know that you need to open the valve extremely
slowly, the gauge should never pop, but take a second or more to
register full pressure. I’ve had my hydrogen regulator in
regular use for almost 10 years without a single cracked
diaphram (or explosion). You should check your fittings very
carefully (soapy water or the like) because hydrogen is odorless
and extremely explosive.

I will contact AAA Precious Metals and order some alloys from
them. Hopefully these tips will help. I appreciate all the good
advice.Maybe I should stick to fabrication...Thats one thing I
do pretty well. :-)

Ask for Mary Helen, and tell her I sent you. :slight_smile: Jeff from
Iowa… She’s about the sweetest person I’ve ever ordered from.
Tell her I said HI!

Good luck solving the casting problem Ken, I know how
frustrating it is…

Jeffrey Everett