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Incomplete castings

Hello everyone! I have recently tried doing vacuum casting and am
having a little bit of trouble. So far ever step seems to be doing
well (for example, when I put my investment under the vacuum table,
it boils rapidly).

I’m having some issues with the actual casting part, however. When I
put the hot flask over the vacuum table and turn the table on, my
vacuum gauge is showing no suction. I’m using a thick silicone pad as
well. When I use my bell jar to boil my investment, it shows the
right amount of vacuum. Am I doing something wrong? I have tried to
cast despite this vacuum rating of 0 twice and both castings were
incomplete. The first time I used a wax replica of a shell, which had
multiple craters throughout the casting (and the casting cut off
halfway through.) The second time I tried casting half of a tree
branch and the metal (I’m using fine silver) only went halfway up the
tree branch mold (this one had a huge button if it helps; it was VERY
difficult to try to get the metal to even go through the mold.) If it
helps I’m using 2 stage, 3 CFM, 1/3 HP pump. Any idea of what
I’m doing wrong? Thank you! :slight_smile:


Are you SURE that there is actually a vacuum being pulled on the can?
Does your table have one side for use with investment and the other
side for casting and you can move the ball valve handle to either the
investment side or the casting side? If this is your set up, you
might have a problem on one side and not the other. If you have
double sided unit, check the casting side by turning the ball valve
handle to that side and place a piece of steel or very heavy plastic
over the vacuum hold and see if you get a vacuum registering on the
vac. gauge. If not, you have a hole in a line on that side, or it
might not be hooked up or the fittings are not tight or??? If you
get a vacuum to register, then you just are not getting a seal
somewhere… a nick on the edge of the can, an old, hard silicon pad
or a rough pad from past hot cans, something… You just need to
work thru a process of elimination to find the leak if there is one.

If you still cannot find the problem, repost with any other
additional you can think of that might be causing this

If you want to call me my number is 360-681-4340. I am on the
Pacific Coast for a time zone. It really shouldn’t bee too difficult
to find the problem.

John Dach

Hi Jane,

Pretty clearly, you’re not getting a good seal around your flask.

First question: is you caster of the type where you set the flask on
top of a flat pad, or is it the type where you use perforated flasks
with a side collar, that sit down into a well?

If it’s the first type, (flat pad) my trick was always to get 5-6
sheets of newspaper, and tear them into a pad bigger than the flask.
Tear a hole in the center for the vacuum to draw thru, and then soak
them in your blowout bucket. Get your hand wet, and make sure the
bottom side of the silicone pad is wet too. (helps seal) Once the
paper’s soaked, put it on top of the silicone, and cast as normal.
The paper will form a more flexible seal than the silicone will.
It’ll dry out, and the area in contact with the flask will char, but
I’ve never had one burn yet. (Saves wear on the $$$ silicone pad

Also, it’s worth checking that you have a flat, smooth surface on
the top of your flask. Rub on concrete floor or sandpaper. You should
see bright metal all the way around the top of your flask. If you
don’t, that’s half the problem right there.

Pit vacuums rarely have these sorts of problems. If that’s what you
have, follow up, and we can try to suss that out.


Sounds like your vacuum pump is capable of pulling a vacuum.

The first thing you might want to check is to see if the vacuum pump
is pulling a vacuum to the table. Put your finger over the vacuum
hole on the casting table. You should get a solid vacuum reading.

There could be a clog in the line between the vacuum pump and the
casting table or the valve that directs the vacuum to the casting
table might not be working.

My guess is the investment is not flush with the flask edge.

A vacuum on the flask can not be pulled if there is any investment
below the flask edge. The investment below the edge of the flask
allows the vacuum to leak out.

Before I put my flasks in the burnout oven I use the edge of a
trowel to scrape the investment flush with the flask edge.

The trowel I use is curved so the investment is slightly concave. I
think the concaved surface in the investment provides better
distributes the vacuum to the flask.

Even with that I will every now and then have a flask on which I
cannot pull a vacuum. I take the flask and rub the bottom edge on the
rough stepping stones I have as a floor in my casting area. That
always fixed the problem.

My paper blog shows my casting procedure. A ways into the paper you
will find drawings of what I mentioned above.

If you have difficulty finding the sketches let me know and I will
send sketched off line.

I did have a warped vacuum table once. I was using the table
regularly until one day I could not get a vacuum on the flask. I
tried all sorts of fixes but none worked. I finally checked out the
flatness of the table and found it warped. Luckily I had a backup
table and casting continued without interruption.

To this day I don’t have any idea what warped the table.


Hi Jane

It would seem that there isn’t a problem with your flask of the
debubblising works the way you say. It would seem that you have a
problem with the seal between your flask and the vacuum chamber when
casting hence the low reading on your vacuum gauge. When you do the
investing you need to make sure that the flask surface that goes
against the vacuum chamber gasket is clean of investment whether this
is the bottom of the flask or the underside of the flange. When
casting I always check the vacuum gauge before pouring the metal. If
there is insufficient vacuum then it is best to turn off the pump and
remove the flask and check it. You can clean flange or flask bottom
with a sharp scraper and return the flask to the kiln for 15 minutes
or so to bring it back to casting temperature. The problem might also
be some muck on the gasket. BTW I use two gaskets, the usual silicone
rubber one on the top of the casting chamber and a special high
temperature gasket on top of that under the hot flask. It is
important to ensure the top of the casting chamber and both gaskets
are clean before casting.

Another possible problem might be that you might be using too large
a flask for the capacity of your vacuum pump. A greater volume is
drawn through a large flask than a smaller one. In practice I have
found with my casting machine that though it can take quite a large
flask, the capacity of the pump is only sufficient to give a good
vacuum for up to a 90mm flask

Incomplete fills are also caused by low metal or flask temperatures
but this does not seem to be your problem.

All the best

Jane, the level of the investment should be slightly (2mm?) below the
edge of the flask and that edge must be clean and smooth.

Have fun.
Tom Arnold


It sounds like you may have some switch that needs to be engaged to
activate the suction. Some machines activate one side or the other
depending on function. Is you flask on top of a hole on the vacuum
table? That is what sucks the air out of the flask and pulls the
metal in. For the units that are just one table there may be two
holes in the plate. If you invest, you have one hole open, if you
cast you need to stop off that hole to activate the vacuum hole.

You flasks have to have a flat surface that creates a seal on the
casting pad.

Here is the simplest test to do. Before you load the oven, try the
flask cold. If it works cold it will work hot.

Best regards,

Todd Hawkinson
Southeast Technical College

Hi Jane,

You need to find out if you have vacuum available at the vacuum port
in the cast mode. If you don’t have vacuum available to pull the
gases from the investment in the flask, the metal will be blocked
and you won’t get the complete metal flow. I have had similar
problems and I found my gaskets were leaking vacuum. Once I got
complete seals around my gaskets and casting pad the results were

Good luck
Ken Moore

One thing that I need to know is what kind of flask and what vacuum
table are you using. I will use an example of a flat table and plain
small flask. The most common thing is making sure the investment is
not over lip of flask. This will keep you from getting a vacuum. Try
after it sets take a flat scraper or old butter knife and make sure
you get good metal to rubber contact. I slightly wet the underside of
rubber to keep it stuck on table and get better contact, just very
lightly moist not where you can shock the flask. The investment will
seem to boil but it is air bubbling just bring it to bubbling state
and reduce vacuum. Dol not let it bubble to much, it is debubblized
after it is bubbling good. Hope this helps.