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Vacuum casting seal problem

Thank you everyone who responded to my deep-draw question! Your
collective wisdom is ‘deeply’ appreciated.

I had a very exasperating problem with my last batch of castings,
and I am at a loss as to what’s going on.

I have a table-top vacuum machine, the kind that uses a solid flask.
Lately, I have not been able to get a seal on my flasks. Thanks to
the Ganoksin tips, my spruing has improved to the point where
bubbling is minimal even without a seal, but it is still a problem in
some cases. Since I’m under a deadline to get out a bunch of pieces,
I can’t afford to have a significant failure rate.

Changing pads hasn’t helped. The machine is pulling a vacuum (I’ve
tested it every time), but I just can’t get the flasks to seal. I’m
thinking of gluing the pad down in case air is seeping underneath.
Pushing down with the tongs has no effect. The investments don’t
appear to have any cracks, and there aren’t any blow-outs through the
bottom.

I would be grateful for any tips or suggestions.

The flasks are not flat on the bottom sometimes they warp from the
heat and they will not seal I have a large disc sander and i
regularly gind the bottom of the flasks to make sure they are flat
you can use a file as well also make sure that the investment isnt
all the way to the top of the flask I know you have probably checked
that but I figured I would just say it in case, leave about a 1/4
inch from the top and make sure that the rim is clean if that doesnt
work get new flasks.

Kevin

Fr. George:

It could be the flasks them selves, if the ends are uneven and/or
corroded this could cause a leak, Also I’m not sure if they still
handle it but Rio Grande used to have a product called wax web that
was used to line the vacuum flasks with prior to investing, it gave
good flow channels and helped evacuate the gasses. this worked much
better than using straws and gave excellent evacuation, and it wasn’t
terribly expensive, I imagine Stuller carries a similar product as
well. If the ends of the flasks aren’t smooth, try turning them
around with the ends on a piece of fairly coarse sandpaper about 220
grit which is sitting on a good flat surface and keep the sandpaper
lubricated, (use the wet or dry type sand paper or even a sheet of
emery cloth. Water or light oil will suffice for a lubricant. This may
help until you can get new flasks, don’t glue the silicone pads down,
possibly just a light spritz with water on the bottom will help with
a seal as well.

Kenneth Ferrell

I saw the sun today; I’d forgotten what it looked like!!!

Hello, I used to have the same problem. Before each cast, I wipe down
each piece on the machine ie;the rubber ring, the steel plate and the
rubber top with a damp cloth. Keep them clean. Also, try and hollow
out the top of the flask before burnout. It creates a better seal.

Good luck, Scott

Changing pads hasn't helped.  The machine is pulling a vacuum
(I've tested it every time), but I just can't get the flasks to
seal. 

check that the bottom edges of your flasks have not become too rough
and nicked to seal with the gasket, and be sure also, that you’re not
filling the flasks quite level with the bottom edge. You need that
edge to be at least slightly higher than the level of the investment,
so the flask is resting on that edge, not the investment. What’s
sometimes gotten me is that though the investment can be the right
level, it sometimes crawls up the wax web a little, and after
burnout, the holes around where the web used to be are slight peaks
of investment that can rise slightly above the rim of the flask.
Just scrape those off before casting, if you didn’t notice it before
burnout.

Also, take a fine scouring pad (like scotch brite) to the metal
surface that the gasket pad rests on, to be sure there’s no
corrosion or some thin deposits or build ups that might keep the pad
from properly sealing. And if the pad doesn’t well seal to the
surface, you can also lightly oil (vac pump oil) that surface to the
pad seals better.

And, in checking your vacuum pump, how are you checking it? same
table/connections as the casting one? Some machines that cast on
one surface and invest on another with a valve to choose between
might have a fine pump and do just fine investing, yet have some leak
in the connection to the other table, and then have trouble pulling a
good vacuum on one side only. I know that sounds simplistic, but
it’s sometimes easy to overlook the most obvious things while
pursuing the most esoteric possibilities…

HTH
Peter

Make sure that the investment is scraped down to a slightly concave
surface. This will insure that the flask edges form the seal and the
vacuum is in fact created All around the investment surface.

If you are using a table top vacuum caster and not one that allows
you to drop the flask down into the machine you may want to try this
as a possible solution.

First do not fill the flask even to the top ( actually bottom) with
investment. Leave about 1/4 inch without investment. This will allow
the vacuum to seal better.

If you are casting large flasks and you are not getting complete
fills on your castings you can try adding some sprue tunnels that
are open at the bottom of the flask. To do this add a few pieces of
wax rod into the flask from the bottom and leave it over hang the
bottom. Make sure you attach it to the edge of the flask so it
doesn’t move when you fill it with investment. Also make sure that
you do not have it touching any of your wax models and try to keep
it as far away from them as possible ( at least 1/2 inch ). Run the
wax rod approximately 1/3 to 1/2 way into the flask. Use two to
three rods.

This along with not filling the flask level will allow you to get a
better vacuum and pull.

Good Luck
Greg DeMark
email: greg@demarkjewelry.com
Website: www.demarkjewelry.com
Custom Jewelry - Handmade Jewelry - Antique Jewelry

Just before burnout, try rubbing the investment-filled flask (not
the button end), across sandpaper on a flat surface. A machined
surface would be the best choice, but use something as flat as you
can. Check the flask to make sure it has been roughed up around the
entire rim. That may help you get a better seal.

James in SoFl

quaro

Try using 4pieces of the local news paper folded in half and in half
again til it is 1 or 2 inches larger then your flask. Cut a hole in
the center half the diameter as your flask let it soak in water till
saturated 2 to 3 min pless out extra water .Now you have a gasket
that will not leak. Yes you will get a bit of water in your pump but
this is removable as oil floats on water . also even if you have to
change out your oil more often it is better then loosing a flask to
no vacuum

I do alot of casting and my fellow jewellers who do in shop casting
use this method .it works well

any questions please email them to me @samuel_olin

Dear George,

I would bet if you were using a new flask you would have no
problems. As flasks wear you need to resurface the bottom on the
flask that contacts the pad. This can be done with a larger disc
sander or anything that will flatten the end. Here’s another tip that
will guarantee success. Before you place the flasks in the oven, try
them cold. If they vacuum cold they will vacuum hot. I’ve done this
for thirty four years with continued success.

Best Regards,
Todd Hawkinson

    Changing pads hasn't helped.  The machine is pulling a vacuum
(I've tested it every time), but I just can't get the flasks to
seal.  I'm thinking of gluing the pad down in case air is seeping
underneath. Pushing down with the tongs has no effect.  The
investments don't appear to have any cracks, and there aren't any
blow-outs through the bottom. 

Hi George,

The easiest thing to make your flasks seal perfectly on a flat top
vaccuum casting table is to buy a table top belt sander from your
local hardware shop. Usually, I recommend a 6 inch wide belt sander
so you can smooth the large 5 inch diameter flasks down to the
smallest flask easily.

I have bought Sears brand belt sanders and ruined them quickly as
the motors are not strong enough ( cheap motors). I would recommend a
floor model from Grainger if you can afford it.

The sears unit should work fine if you don’t do huge volume like we
do… We run typically 60 to 80 flasks (4x7") daily so we resurface
our flasks every 6 - 8 months. When we have to clean up that many
flasks, the bigger belt sander is neccessary.

Heat will warp any flask… standard ones as well as perforated
flasks. The standard flasks are easy to maintain and fix in a volume
situation and very inexpensive to buy.

The perforated flasks with or without flanges are much more
expensive and can be problematic when the flanges become warped .

Hope this is helpful.

Daniel Grandi
Racecar Jewelry Co. Inc.
Tel: 401-461-7803

I had a similar problem with peforated flasks. Turns out the weld on
them was kind of sloppy and woould let air in. They are too
expensive to just throw out, so I filed the welds down. Seems to
stick better now, although I dread mixing investment so I havent
played with them yet. I’ve been using solid flasks with 100% seal.
Hard to switch gears when a process takes this long and possibly
won’t work…

-Stanley

Had a similar problem… for years!!!>… It was one of 2
problems…

1…Wasn’t mixing the casting material according to specification…

2… The pump and the motor are joined together with a flexible
link… the link was slipping …‘On Occasion’ so I 'sometimes’
couldn’t cast…always could pull a vacuum on the air extraction side
of the caster…

Not sure which was the dominate problem as I fixed both at the same
time… Drove me Nuts… so am very sympathetic…

Jimc

Dear Friends,

Thank you all for your input. I discovered that all of my flasks,
including the new ones, were (in some cases seriously) out-of-flat.
So, as I’m getting ready for another batch of casting, I am grinding
them flush.

I am also planning to try the newspaper gasket method. All the
lines are rock solid. I will also do a run-through before they go
into the oven.

My gratitude to you all, especially Hanuman, for making such a
brilliant site!

Did the same thing… to grind the bottoms flat… place on Emory
paper, on a flat service and turn in circles… THEN to check place
on your caster, on the metal ring, and shine a pen light flash light
through the top…in the dark… when you cant see any light…

Jim

Did the same thing.... to grind the bottoms flat... place on Emory
paper, on a flat service and turn in circles.... THEN to check place
on your caster, on the metal ring, and shine a pen light flash light
through the top...in the dark.... when you cant see any light...

Really easy to simply keep a sheet of sandpaper handy and give each
flask a light scrub every time you put them in the oven.

I have been told by lifelong jewelers and casters that the perforated
flasks are a gimmick. I tend to believe them. I have tried both
perforated as well as solid and I can’t tell a difference. The
perforated have frequently not had good seals while my solid flasks
have performed flawlessly.

Fr. Alexis Duncan

I have been told by lifelong jewelers and casters that the
perforated flasks are a gimmick. I tend to believe them. I have
tried both perforated as well as solid and I can't tell a
difference. The perforated have frequently not had good seals while
my solid flasks have performed flawlessly. 

Not a gimmick, but more, a timesaver and productivity improvement.
Especially with larger flasks, you need good air flow through all
parts of the tree to get uniform, consistant, repeatable results.
Without perf flasks, you can get the same results using wax web, or
similar products, but it’s simply an extra step, and another product
you have to buy. While many individual jewelers used to casting
little 2.5 x 2.5 inch or smaller flasks don’t need, nor use,
perforated flasks, you’ll find that most of the larger commercial vac
casting machines are designed for them. This is a more costly way to
build a casting machine than a flat table. They wouldn’t do it if it
didn’t work better for them. You certainly CAN get equally good
results with solid flasks, if you prepare it correctly. But a
perforated flask needs less space between the models and the flask
walls (since patterns don’t have to also clear the wax web or other
such air vent product), and the holes in the flask also grip the
investment column securely, which coupled with the straps across the
bottom, reduce the potentials for an investment blowout. All in all
its simply an incremental improvement in the process. The castings
are not somehow better than equally filled castings from solid
flasks. It’s just slightly easier to get them consistantly filled,
and you can get more in a flask. Plus, with manual pouring types of
setups, locating the flask down in a casting well may offer some
improvements in safety, as there’s less hot metal exposed to
accidentally brush against, and you can’t knock a flask over by
mistake, etc. Minor considerations, but they all add up.

Peter

I have been told by lifelong jewelers and casters that the perforated
flasks are a gimmick. I tend to believe them. I have tried both
perforated as well as solid and I can't tell a difference. The
perforated have frequently not had good seals while my solid flasks
have performed flawlessly.

The perforations hold the investment in the larger flasks. That is
the main difference. Especially with larger flasks the difference in
thermal expansion between the investment and the flask will make the
investment loose in the flask and on a couple of occasions I have
seen it slide right out of the flask with a solid flask.

Jim

Jim Binnion
James Binnion Metal Arts
Phone (360) 756-6550
Toll Free (877) 408 7287
Fax (360) 756-2160


@James_Binnion
Member of the Better Business Bureau

Dear Peter & All,

The perforated flasks may cast more but I can only load four large
perforated flasks compared to six or seven large non perforated one
in my larger oven at one time. Small flasks also will fit next to the
large plain ones with more ability to stack.

The perforated flasks are very expensive compared to the solid
flasks and if the lip becomes uneven is very difficult to correct. I
have thrown out several while I grind the old sold ones as good as
new. The perforated flasks the last time I looked were at least twice
as expensive as the solid also. Plus the gaskets are expensive and
wear out more rapidly than the flat ones.

When taking a tour of Rio Grand a few years ago I noticed all of
their very large flasks were not only solid, but of varying size to
fit in the oven. By varying size, I saw some were almost rectangular
in shape. A wax web was not apparent. But was not able to look that
close.

I think the main difference in performance in my opinion is more in
the strength of the vacuum pump and the size of the air line in the
casting unit. I made my own casting machine with a 3/4 inch brass air
line and a five horse power Vaughn pump and motor. My vacuum castings
are excellent. The hose used in most commercial casting unit so small
I wonder how anyone could cast with success and consistency. I think
the vacuum casting machine designers have never even set foot in a
casting room.

Best Regards,
Todd Hawkinson