Weighing models for casting

Jewelry Discussion
21

First of all I want to thank everyone who gave me such helpful and
valuableadvice, both on-line and off-line. I truly appreciate it
very much.

I did my first vacuum casting and wish I could say that it was a
success. Unfortunately I had the dreaded blow out—a term that
meant nothingto me until I experienced it. Fortunately I had only
burned out one flask in order to see how it would turn out. Good
thing, as I only lost onewax model.

I wasn’t too concerned even though the blowout totally melted the
plastic tubing under the casting table, and the silver was fused to
it–an ugly mess. I had some extra tubing, and figured I would cut
off theruined piece, replace it with a fresh piece. That was easily
done. Next I noticed that the in-line filters were clogged with
investment, so decided that they needed to be replaced. Fortunately
they stoppedthe investment from entering the pump, but just to be on
the safe side I will drain the oil, and replace it with fresh.

Now for the bad news. I called Rio, and learned to my dismay that
they no longer are able to get the size In-line filters I needed,
and that all the newer vacuums now have smaller diameter tubing and
In-line filters. I spoke at length with their very helpful
technicians and it developed that the only course of action was for
me to ship them the vacuum table and that they would replace the
larger diameter hoses and in-line filters with the newer smaller
ones. As the brass barbed connectors on the pump, and the bell jar,
have the larger, permanently installed brass connectors, they will
arrange some adapters so that they will accommodate their larger
size.

Now for some good news. I did a search of the archives to find out
the causes of blow-outs, as I had done everything according to the
book. Properly measured, mixed, and invested the investment, careful
measurements of metal and an 8 hour burnout with a nice cleanlook to
the investment in the flask.

However, the one thing I was unaware of, and which was the cause of
the blow-out was that there needed to be at least 1" or more of
investment above the model. I was using a small flask, and after I
scooped out some of the investment to get a good seal, it was much
too thin resulting in a blow-out.

It was a lesson I shall never forget—as well as an expensive one.

The vacuum table will be shipped to Rio on Monday, and I shall
eagerly await its return.

When next I try vacuum casting it will be with much taller flasks,
so that I am sure there is enough investment between the top and the
model, something I was totally unaware of, and never thought to
inquire about.

Alma

22

Oh Alma! I am so sorry this happened on your first try. I should
have told you about having a good amount of space between the top of
the model and the top of the flask.

Keep trying. It’ll work.

Jo Haemer
timothywgreen.com

23

Hi Alma

I am going to give you some old fashioned info that others may think
is terrible. I have been vacuum casting with the same pump for at
least 25 years. It does not have any kind of filter and I have
changed oil maybe 15 times. I have a trap under the table which is a
pipe that goes straight down about 8" from the table with a pipe plug
on the end. The vacuum line is connected to the pipe about 1/2 way
up. That way in case of a blowout (luckily only 1 time) the metal
goes to the bottom of the pipe and not into the line. Hope this makes
sense. If you want to send the unit back go ahead, not saying not to
but might be overkill.

When I had my blowout, because the gold went to the bottom of the
pipe it did not shut me down. I cast 4 or 5 more flask that day
before I even took it apart to get the metal out.

Good luck
Bill Wismar

24

You have to make sure there is enough space above your model in the
flask to have strength from the investment. Happens to everybody once
in a while. Make sure you don’t move the flasks while they are curing
also.

Good luck.

25

In my experience the blow out usually happens when the vacuum is
first pulled on the flask. If that happens the sound of the vacuum
pump will give youa clue the blowout has occurred. The pump sound
will go from a heavy pulling noise to a free running noise. The pump
will no longer be pulling a vacuum.

You can also check the seal by trying to move the flask while the
vacuum is being pulled. If you can move it you don’t have a seal.

Lee Epperson

26

-stop guessing!

Does your wax float in water? If it does, the density is less than
1.0 g/cc. From my research and testing, most of it is 0.92 or
within.01 of this.

When I just started this business, I underpriced a casting job
because I assumed the density of the wax was1.0, I. e., 1 g per cc,
like several of the commenters on this thread. The margin was very
small in this job, and I madevery little money because the specific
gravity of the wax that I was injecting with was actually 0.91–it
was 10% less. so I began to look for manufacturers wax products’
densities. In most cases, it was on their website or in their
product brochures.

It is easy to get exactly the amount of gold or silver that you want
in a casting. But you must use the correct value for the specific
gravity (density)of the wax before you multiply it by the density of
the silver or gold you’re using. My main carving wax, made by
Ferris, in either blue, green or purple has a density of 0.92. My
sprue wax is almost identical at just a hair less, 0.915g/cc.

For my small fast flasks, centrifugally cast, I like a button of 5g.
I use the following formula for exact results, with no guessing, or
ending up with buttons that are too large or too small:

Wax / density of wax X density of metal + Button weight = metal
needed

If my carved wax item and the sprue weighed 2.0 g, and I was casting
14 karat yellow gold (Stuller CA03 alloy), this calculation would
look as follows: 2/0.92 * 13.07 + 5 = 28.41 grams.

So you don’t need to guess, or add some rough percentages, or use a
rule of thumb. Just tap out four numbers on a calculator and you’ll
have precise results every time.

Joris

27

As I stated in my earlier post, I had a blow-out with my Rio Vacuum,
and as Rio no longer carries the size hose and in-line filters, I
was advised to ship the vacuum to them and they would replace all the
hoses with the newersmaller diameter ones. One of my concerns is
that the smaller diameter hoses will affect the pressure needed for
vacuum casting.

Also, the replacing of the hoses and the shipping back and forth of
the vacuum will be costly.

I believe I could make the necessary repairs if I could get the
original size polyurethane hoses and filters.

Also, in the event that I can get the correct size hoses to fit the
existing barbed brass connectors but am unable to get the correct
size filters, could the vacuum be used without them.

I can easily replace the hoses myself provided I can get the correct
diameter, but the in-line filters pose a problem.

Should I just forget the notion of trying to fix it myself and just
ship it back to Rio?

I will appreciate your suggestions.

Alma

28

Hi Alma,

Which vacuum caster do you have? Depending on which one, you may be
able to not only fix it yourself, but make it better.

Way back in the mists of time, table top vacuum casters came with
what are called ‘traps’, to trap the molten metal in case of a
blowout, rather than letting it fry the hosing and filters.
Unfortunately, the bean counters didn’t understand why there was $10
worth of plumbing pipe under the tabletop, and ‘rationalized’ it out
of existence. For some machines, there’s still room under the
casting table to put a new trap. I’ve never seen the guts of one of
Rio’s VIC machines, but there may be room under there too.

You can re-create a trap for about $20 worth of plumbing parts and
high temp gasket. (need a good auto-supply place for that) The only
real metalworking would be drilling and tapping 3 holes into the
metal plate you cast on, and making the hole in the plate a little
bigger.

Other than that, it’s just using a wrench to put the pipe together.

Before I get into the whys and wherefores, you want the biggest tube
you can find for a vac caster, so switching to a smaller tube is a
bad idea unless you don’t have a choice. The “inline filter” that
most of the tabletop units use is actually an automotive fuel
filter. Take the fried unit to a good auto supply place, and they
can probably set you up with something similar. The only thing you
really care about is tubing diameter. (Any filter good enough for
car engines is more than enough to filter investment crud.)

Think about this before you send it all off to Rio. Mark and Sessin
are good guys, so this isn’t in any sense a slam on them, but there
are easier ways out of this hole.

The basic idea of a trap is to set up a “T” joint, sideways, under
the casting table. Set it such that one of the arms of the “T” faces
up, and attaches to the bottom of the table. The vacuum line
attaches to the upright of the “T”, and another stub of pipe
attaches to the downhill arm of the “T”, with a cap on the end. If
you have a blowout, the metal dumps into the hole, and goes shooting
past the vac line (attached sideways, right under the table) and
ends up in the bottom of the pipe attached to the downhill arm of
the T. Thus collecting all of your valuable metal, right where it’s
easy to get to, all in one spot, and NOT burning out the guts of
your machine!. Depending on how much metal you dumped, it may not be
the easiest thing to get that bottom cap off the pipe and get it
out, but I guarantee it’seasier than picking silver BB’s out of the
guts of your machine for half a day. The nice thing is that once
you’ve retrieved the metal, you just screw the cap back on, and
you’re back in business.

The short form of ‘how to’ is to get a 3/4 pipe “T” joint, a 3/4
close nipple, a 3/4 plate fitting, as well as a 3/4x2 nipple, and a
3/4 end cap, along with a 3/4male to (whatever size) hose barb
connector. The plate connector is just a round flat plate, with a
hole in the middle for the pipe. Mount that under the hole in the
casting plate. You’ll have to drill 3 holes in the plate, and then
tap them for whatever size machine screw you decide to use to mount
the plate fitting. The important thing is that the screws end up
perfectly flat on the other side of the table, so it makes sense to
get them a little long, then file them off on the far (top) side of
the casting table, once you’re sure they’re OK. If you want to get
seriously deluxe about it, put a ring or two of silicon gasket
forming compound on the back of the plate fitting before you screw
it home for the last time. (Use the high-temp tube stuff from an
auto supply house.) You can also open out the hole in the casting
table to fit the hole in the plate fitting.

Then use the close nipple to attach one arm of the “T” to the plate
fitting.

Put the hose barb connector (sized to your hose) into the leg of the
"T". (Sticking out sideways.) wrench it tight.

Put the 3/4x2 nipple on the bottom arm of the “T”, and then close it
off with the end cap.

That’s all there is to it. Connect your vac line to the hose barb,
and away you go. Never need to worry about trashing your machine
again.

(Ask me why I know this…)

Regards,
Brian

29

Thank you Jo. It is certainly not your fault. You have always been
so helpful and encouraging. I should have done some more research
before I attempted the vacuum casting.

At least I now know how blow outs occur, and in the future will use
taller flasks to be sure that I have ample investment all around. The
glob of silver that I was able to retrieve is so ugly, that by
nostretch of the imagination can it be called an “art piece.” I
shall enshrine it in a jar above my caster as a reminder to make
sure I have plenty of investment in my flask. Thanks again for all
your good advice, and encouragement. Alma

30

You don’t need a huge amount of head space but 1/4 to 1/2 inch is
good

31

Is it new? You should be able to clean this out by yourself. My
vacuum machine has access panels to get to the good stuff. You can’t
send in your machine for every blowout. It’s just dust in there.
Rinse out the hoses and dry them and try again tomorrow.

32

Before you ship it off to Rio for an expensive repair, I suggest you
check out Home Depot, Lowes, and plumbing supply places to see if you
can cobble together a set of workable valves, tubing and fittings.

It’s worked for me.

Twice. J

This time (the third), I’m planning to add a metal jar into the
piping, directly below the intake pipe. The tubing will enter and
leave the jar by the top. The hope is that the metal will fall into
the jar instead of get sucked further into the tubing. That way, I’ll
only have a small amount of tubing to replace the next time.

(I have friends come over and learn to cast, so I have lots more
beginner mistakes at my place than usual.)

Before you ship it off to Rio for an expensive repair, I suggest you
check out Home Depot, Lowes, and plumbing supply places to see if
you can cobble together a set of workable valves, tubing and
fittings. It’s worked for me. Twice. J This time (the third), I’m
planning to add a metal jar into the piping, directly below the
intake pipe. The tubing will enter and leave the jar by the top. The
hope is that the metal will fall into the jar instead of get sucked
further into the tubing. That way, I’ll only have a small amount of
tubing to replace the next time. (I have friends come over and learn
to cast, so I have lots more beginner mistakes at my place than
usual.)