Question for some one who uses perforated vacum flasks

I want to make a vacum chamber to use in conjunction with some "off the
shelf " store bought perforated vacum flasks. I am wondering what
keeps the vacum from pulling air through the top of the mold around the
pouring cup. Is this area some how sealed. It seems to me that vacum
would only be created in the mold cavity if the entire mold was sealled
from atmosphric pressure, except for the area above the metal after it
has been poured in and seals off that area as well so that atmospheric
pressure would then push the metal down. This bings to mind a basic
puzzel I can't figure out how is an vacum created in the mold cavity
before the metal is poured; as air will be drawn in throug the pouring
cup? Does all the vacum really build up after the metal is poured in.
If this is the case what about the the expose investment at the top of
the flask after you pour the metal would'nt you lose vacum from air
being sucked in from that area aroun the cup. What size and shape
pouring cup do you use?

Thanks Bruce

On Mon, 29 May 2006 17:43:40 -0700, in rec.crafts.jewelry "Bruce"
wrote:

I want to make a vacum chamber to use in conjunction with some "off the
shelf " store bought perforated vacum flasks. I am wondering what
keeps the vacum from pulling air through the top of the mold around the
pouring cup. Is this area some how sealed.

It's not sealed. The air takes the path of least resistance, which is the
shortest distance through the investment. This is because although the
investment is somewhat permiable, it doesn't really allow high rates of air flow
through it. So long as the capacity of the vacuum pump is sufficient, the
amount of air drawn through the investment by all routes will still be somewhat
less than that removed by the pump, so negative pressure is maintained. If
you'll look at your flasks, you'll note the holes are only below that collar
around the flask, and even then, slightly below that collar, not all the way up
to it. Your wax models project below the sprue cup too. The idea is that the
holes in the flask come up to about where the models (waxes) extend to, so the
shortest distance for air flow will be through the mold cavities. Total
efficiency is not required.

It seems to me that vacum
would only be created in the mold cavity if the entire mold was sealled
from atmosphric pressure, except for the area above the metal after it
has been poured in and seals off that area as well so that atmospheric
pressure would then push the metal down.

Here you've got a common, but important misconception. The vacuum's function is
only to assist in removing air from the mold cavity, so it does not impede metal
flow. The vacuum does NOT materially increase the pressure on the metal by
enough to matter. Vacuum assist casting is essentially a gravity pour casting
method, with the vacuum removing the back pressure from air trapped in the mold.
The reason for this becomes clear if you remember your basic hydraulics. The
atmospheric pressure on the metal acts, seemingly, on the entire back surface of
the sprue button, but only after the pour is complete and everything is settled
down. During the pour, effectively, the only surface area measurement that is
acted on by the atmospheric pressure is the cross sectional area of the sprue
stem itself, and at that, the smallest section of the sprue. So this will often
amount to atmospheric pressure being exerted, at 14 pounds PER SQUARE INCH, over
an area that often is only about an 1/8th of an inch round. That's a tiny
surface area, and the pressure there will be almost unimportant. Instead, the
force exerted is the weight of the metal itself, and the vertical height of the
column of metal from the bottom of the mold cavity to the top of the sprue. The
metals we use are pretty dense, and so this will amount to a significant amount
of weight stacked up on top of the metal in the mold cavity itself. Enough, if
done right, to ensure that the fluid metal does indeed do what fluids do,and
that is to fill voids into which they are poured, especially when there's
nothing, like air, already there. The investment is gas permiable already, and
the vacuum draw of the casting setup greatly enhances the speed with which air
moves through the investment, thus facilitating the casting process.

This bings to mind a basic
puzzel I can't figure out how is an vacum created in the mold cavity
before the metal is poured; as air will be drawn in throug the pouring
cup?

The vacuum pump is sized so it's air moving capacity exceeds the rate at which
air pressure can push air through the resistance of the investment. That
resistance is much higher through thicker sections of investment, so as the
metal is poured, the vacuum guage will go up as the amount of air that can enter
the investment through the sprue system drops off (as it's blocked by themolten
metal). Some will continue to be drawn all the way through from the top,but
it's much less simply because this requires air to move through a much longer
distance of investment.


Does all the vacum really build up after the metal is poured in.

Usually you never get to quite a full vacuum, even after the metal has fully
filled the sprue system. But you'll get within a pretty small increment, on
the vacuum guage, of what you'd have if you simply closed off the top of the
vacuum system. The air leakage through lots of solid investment isn't much.

If this is the case what about the the expose investment at the top of
the flask after you pour the metal would'nt you lose vacum from air
being sucked in from that area aroun the cup.

As described above, yes you loose a little vacuum. This has no effect on the
casting. And besides, once the metal has fully filled the sprue system, the job
is done, and air leakage no longer matters.


What size and shape
pouring cup do you use?


Whatever meets your needs best. Most efficient will be puring cups that lead to
a single central sprue extending down from the cup, from which the modelsthen
branch off. That's the classic sprue tree. But when using smaller flasks with
fewer models, it's gnerally just as effective to branch off from the cup,and
pretty much any of the designs you might use, will work. I personally prefer
either a cone shape leading to a single tree base, or a hemisphere shapedcup.
the latter allows a faster pour with more metal. Occasionally I'll have
occasion to use a donut shape, if I don't wish to use a tree, and wish to
minimize the excess metal in the button. This seems slightly harder sometimes
to get a complete fill, but the difference isn't all that much.

Peter

Thanks Peter, Very informative. Is there a reading that you should
wait for ont vacum gauge before you pour? I have a 6cfm. pump that
pulls a high vacum that I use to degass silicone rtv. rubber do you
think it's large enough.

Bruce

On Tue, 30 May 2006 22:35:24 -0700, in rec.crafts.jewelry "Bruce"
wrote:

Thanks Peter, Very informative. Is there a reading that you should
wait for ont vacum gauge before you pour? I have a 6cfm. pump that
pulls a high vacum that I use to degass silicone rtv. rubber do you
think it's large enough.

Bruce


I'd think that should be enough, easily. The vac guage reading you need isn't
critical. Somewhere within an inch or two of it's maximum draw should befine.
It's common for the reading to max out at some slightly lower level before you
start to pour, but to increase a bit when you start to pour the metal. The
main thing to remember is that since all the vacuum is doing is removing the
potential back pressure from air trapped in the mold, the precise reading has
little to do with whether the casting works. So long as you're in the ballpark,
you should be fine.

One hint, if you're building your own, devise the vacuum port from the chamber
in such a way that if you do have a flask blowout, you won't get investment dust
into the pump. Put the port high up the side of the chamber wall, and include
an easily accessed and cleaned trap, with particulate filter of some sort,
between the chamber and the pump. A small detail that can save costly damage to
the pump if something blows out.

Peter