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Vacuum casting gaskets

Hi, I help young women in Mexico learn to make jewelry so they have
work, a living, and to help keep their families together. I am
currently teaching what little I know of vacuum casting in our
workshop. We got ahold of a nice Kaya Cast Vacuum Table Top Machine
and have been doing with limited success casting of 3 1/2" wide x 4"
tall flasks. We only have 4 flasks. We are doing Bronze castings. We
have had some nice items from this work, but not always. Part of the
problem is that I have no instruction guide to this machine, it is
rather simple I know, but for example. We burnout the wax by starting
at 300 degrees, go up to 600 degrees then up to 1350 degrees (over 6
hours). Then we lower the temp to 1100 and we are ready to cast. My
problem is the silicon gasket rings. I have the right 3 1/2" metal
ring that goes on top of the casting chamber, I am not sure what else
goes there, I use the red silicon ring between the flask and the
metal ring, but they don’t last at all, I am only able to do 6 flasks
or so and have to replace the ring. I read here on Orchid that there
are high temperature rings (graphite or asbestos) but for the life of
me I can’t find out where they sell them to order some in. I read
where one man uses two rings, the red silicon one below and the high
temp one on top. This sounds like this would work better for me. The
red silicon rings are rather expensive here. But I suspect I need
more than this one ring. Anyone else use these types of rubber or
silicon rings? I ordered a casting book, but it is not here yet, so
for now I am on my own trying to do the best I can! Rio Grande has a
ring that is not flat, maybe this ring would work with my setup and
last longer? Any ideas to a solution!? Thanks so much!

Laura Brito

Hello Laura - I’m part owner of a vacuum chamber manufacturing firm
in Salem, Oregon. I should be able to help you out. What model Kaya
machine do you have? As for the red silicone rings, I can probably
give you some advise on making your own without having to purchase
them from the supplier or after-market. I hope I can solve your


Hi Laura,

I had problems with the silicon pads melting as well when casting
silver. I found out that I needed to use 900 deg. F as the casting
temp. It was hot enough to cast but did not melt the silicon pad as
bad. Just my opinon. Good luck,

Ken Moore

I assume you mean the rubber pads that the hot flask rests on while
the vacuum pump pulls the air out the bottom of the flask?

Here’s a trick I learned. Take a wet paper towel, tear a hole in it
so the hole in the pad leading ot the vacuum pump is not obstructed,
and place it on top of the pad. Then place the hot flask on the paper
towel. I’m still on my first rubber pad some fifty castings later.

Hi Laura,

I also use a tabletop vacuum casting machine. When casting I use 2
gaskets under the rim of the flask - firstly a red silicone rubber
one on top of the vacuum chamber and a black heatproof gasket on top
of that. Both of these are readily available from my jewellery
equipment supplier in Australia. The black heatproof gasket looks
like it’s made of some combination of glass fibre and graphite,
certainly not asbestos. With this setup I have been using the same
set of gaskets for a year or so.

However there is an additional issue that your burnout and casting
temperatures are too hot for the usual plaster silica investments.
The temperature at which the gypsum in investment starts to break
down and begins to erode the casting cavity and release sulphur is
730C or 1340F. You indicate that use a soak temperature (the maximum
burnout temperature) of 1350. This is likely to contribute to rough
and tarnished castings. In my burnout cycle I usually soak at 650C
(1200F) and 700C (1300F) as an absolute maximum when I am burning
out patterns that contain organics other than wax. Your casting
temperature is also very high at 600C (1100F). This is probably the
reason for the short life of your gaskets. I usually cast with a
flask temperature of 450C (850F) or as high as 500C (930F) where the
castings have thin sections. These are temperatures I use for
sterling silver. You possibly need a flask temperature higher than
450C for bronze but I would suggest 500C as an upper limit to
preserve your gaskets

All the best

As for the red silicone rings, I can probably give you some advise
on making your own without having to purchase them from the
supplier or after-market. I hope I can solve your problem! 

I would love to know this too!

Laura, I do some very high temp vacuum casting because of the
delicate and very thin pieces I work with. I have found that the red
gaskets fail even on the first attempt. The jewelry suppliers have
high temperature gaskets just for this purpose. I think I got mine
from Rio Grande. They work like a charm over and over again.
Unfortunately I had to ruin a big casting to find this out.

Good luck.

I would like to know this, too. I have a Durabull table top vacuum
casting unit, which was previously called Kaya Cast model #10. The
last time I cast, I had a friend that is learning the process was
assisting. Some of his metal got on the gasket that touches the flask
burning into it. I would like to replace the gasket. Please share
resources for making or purchasing new silcone rubber gasket.

Thank you,
Melissa Stenstrom

I wanted to thank everyone who sent me some of the most wonderful
ideas, and I am going to try all of them! It is so neat that so many
of us think outside of the box, and for me this is so necessary being
I am in such a remote spot and have difficulty finding so many items
or if I do find them the cost prohibits me from purchasing! So all of
this is so helpful and I appreciate every single comment about what I
could do to solve my problem! Again Thank you to everyone, you guys
are the bomb! I even was contacted by a man nearby here who does
castings! I had tried to find him (I knew of him) and failed to
contact him! So now we are connected! Thank you again!

Laura Brito

We’ve had metal melt-through (blown out bad investing) that burn
rough areas into the rubber gaskets. Our flasks soak at 1000 F prior
to casting and the vacuum is used for those large flasks that don’t
fit the centrifuge. Our solution is to use 5 or 6 inch squares of
grey cardboard soaked in water. The type of cardboard used for the
backs of sketchbooks or notepads. We poke a half inch or larger hole
in the center and place the damp cardboard onto new or damaged
silicone pads. Set the flask like normal, the flasks pull into the
cardboard a little and we’ve never had problems. The cardboard pads
are reusable and more sturdy than paper towels. This saves budget
for other replacement items like crucibles and graphite rods.

Ruthanne Robertson