Vacuum pumps


I’m using LiquaCast(pink) to make molds and need to vacuum the
bubbles out of the rubber. I can’t find any on the specs
for the kind of vacuum pump needed. I see very inexpensive vacuum
pumps on Amazon, but have no idea if they can do the job. Any
guidance would be appreciated.

Thanks, Jan

Hi Jan:

The Rio Grande Catalog shows a 3 cfm (cubic feet per minute) pump on
their vacuum system for mold making. Another small unit probably for
casting uses a 5 cfm pump.

Use that as a guideline.


I think you still need a platform and a bell jar. Otherwise I think
even the cheap pump will work

Hi Jan,

I never really kept track of what vacuum rating the chamber was at
when the rubber started to boil, but it was pretty well pumped out
before anything started to happen, so I’d say you need a reasonably
serious pump. Michael Knight reads Orchid, at least occasionally, so
I’m hoping he’ll hop in here and say what pressure you need to get
down to definitively.

The short form is that 30 inches of mercury (on the gauge) is
absolutely empty. (29.7 something, to be precise.) Zero inches is
room air. I’d guess the rubber didn’t really start to boil until
28-29 inches. I was doing mine in my investment chamber. So any pump
that’ll do for investment will pull hard enough for the rubber. But
for that, you need either a rotary pump, or a vane pump, which are
usually a couple of hundred dollars, even used. There are cheapo
little venturi pumps that use running water, and I’ve heard of
people using those for investment, but I don’t think they’d develop
enough vacuum to be really useful for rubber. (It being a lot
thicker than investment.)

On the other hand, if you spend money for a decent pump, you’ve got
the core of an investing system at least, and maybe a vac cast
system if you get a fast enough pump.

(For investing, you need low pressure. For casting, you need high
flow. (for just investing, you need 2(ish) CFM (Cubic Feet per
Minute) for vac casting, you want more like 5 or more CFM if you can
manage it, but you don’t need anything like as low of an absolute
pressure. I’ve seen vac casting done successfully with a shop vac.
Not low pressure, but very fast at what it does do.)

Actually, I just went and looked at ebay, to see what pumps you were
talking about. Found a couple of vane pump and chamber kits for degassing silicone that look like they’d be just the thing, all for
only $160. Which is MUCH cheaper than I remember real pumps being.
I wouldn’t hold much hope that the pumps will last for more than a
few years, but they should get you down the road pretty well until
then, and it’s a cheap way to get your feet wet.


Hi Jan,

When I am making RTV moulds I use the vacuum pump and bell jar that
comes with my vacuum casting machine for debubblising investment. I
first debubblise the liquid compound after mixing, and again when I
have poured it into the mould. I use a disposable plastic cup to mix
the compound and measure precisely by weight. Basically you need a
pump that can draw a good vacuum of at least 71 cm (28 of those
American inches) connected to a vacuum gauge, vacuum chamber that can
withstand atmospheric pressure and a release valve. The usual arrange
is a bell jar mounted on a heavy base with a rubber mat to provide a
seal. The vacuum pump is usually connected to the base.

When you are using the chamber.

One note of caution. For the second stage debubblising you need to
ensure that the object you are making a casting of can withstand a

Obviously this is not a problem with hard solid objects such as
metal, plastic or wax patterns but could be problematic with soft or
hollow objects, or ones containing water or other liquids.

All the best
Jennifer Gow
Tears of the Moon Artisan Jewellery


be advised that Liquacast is a relatively inexpensive urethane
elastomer and your molds will eventually (2+ years depending on
ambient humidity and temperature) soften radically and MELT! We used
it years ago and there was no disclaimer that it would turn to Goo
and make terrible messes. We’ve lost 100’s of molds to the
degradation. The shelf life of molds also seems to be a function of
how carefully the 2 parts are measured before mixing.

Smooth-On has/had a similar product (possible the same without the
pink color) called 724. Caveat emptor!

Hi Frank,

I have a series of moulds made with Liqua Glass RTV that are well
over two years old and are still in perfect condition. I have now
finished with the project I made them for and still have bags of
injected waxes just in case, but I still have the moulds.

I am very particular about accurate measurement when making up the

I mix in a clean disposable plastic cup and measure by weight
precise to 0.2 grams.

All the best
Jennifer Gow

Jen, I appologize. I got your reference to Liquaglass confused with
Castaldo’s Liquacast. Glass = clear, cast = opaque pink. It’s the
Liquacast that leaves MUCH to be desired. The European equivalent of
a MSDS for the part B for Liquacast

reveals it contains mercury, which can be released if any mold
burning modifications are to take place. And the part A has a
cyanate component.

So, sorry again about mixing the two names. I really don’t have any
familiarity with Liquaglass

Still vulcanizing is still better.


I don’t know about CFM and bars of pressure and all that; I defer to
Brian when I really need the technical stuff. Basically Jan, what you
need is to be able to make water boil at room temperature within 45
seconds of applying the vacuum if debubblizing rubber and investment
is the goal. I would recommend getting a pump, spring table and bell
jar set made specifically for the purpose of investing unless you
have lots of time and patience set aside for experimentation. No
reason not to get a used setup, I bought a used investing/casting
table, bell jar and pump set for $100 in 1986, and the darned thing
is still crankin’ out the air. Swest made stuff to last, I guess.
Look for a “Rey” or “ProCraft” label.

I can verify the warning about Liquicast molds breaking down. Keep
them cool and in the dark (like most metalsmiths I know ;-)) and
they’ll survive longer, but no where near as long as vulcanized
rubber. They are also not up to making more than a few dozen pieces
before they start getting torn up. If the objective is to make one or
two pieces for which minimal shrink is critical, like reproducing
Grampa’s signet ring so his two grandkids can each have one, or if
you want to make a mold of an exceptionally time consuming
hand-carved wax so you have a back-up, it’s perfect. If it’s for mass
production at any level, you’re really better off using vulcanized
rubber for your molds.

Dave Phelps

The short form is that 30 inches of mercury (on the gauge) is
absolutely empty. (29.7 something, to be precise.) Zero inches is
room air.

Remember that the 29.5 inches of mercury is at sea level. As you go
higher it will be less. In Denver it is something like 24 inches so
that is about all I can pull. To find out what it is for you area
Google the barometer for a local airport. They will give the absolute
and relative. Absolute is the true pressure. Relative is adjusted as
if you were at sea level.