Re: Pressure tank wanted

From: Peter W. Rowe (
Date: Fri Apr 18 2003 - 07:37:58 EEST

On Thu, 17 Apr 2003 19:12:04 -0400, you wrote:

>>This approach works fine if you are using low viscosity LONG pot life
>>materials.....I like the idea of using an old fridge though...clever..
>>Pressure casting is the only way to make high quality pieces from an RTV
>>mould in a short timeline...
>>We do a huge amount of urethane casting and have found that it is just not
>>economically feasible to vacuum cast parts if you can only do 2 parts a day
>>because you are either waiting for the machine to mix and degas or you're
>>waiting 6-8 hours for demold..

In the jewelry industry, we often use RTV rubber molds, degassed via vacuum.
The process of pouring the mold and degassing it takes only about fifteen
minutes, max. The rubbers we use often take 24 hours to cure fully, but that's
another issue, and not a problem. One can, with jewelry scale molds, pour a
dozen at a time easily. Most often I use the G.E. Silastic E type rubber, or
similar types.

degassing the stuff, which is hardly low viscosity, is a several step process.
When you initially pull a vacuum on a container of the stuff, it rises up a
tremendous amount. We use a container three or four times or more, the volume
of the compound. Let it rise up almost to the top, and release the vacuum, so
it slumps down. Repeat this process five or six times. Each time it rises up,
it will do it less and less. Once it rises up only about half as much as it
initially did, or less, you can pour the molds. We use mold frames made of
sheet metal, usually aluminum, in U shaped pieces, with plexiglass sides, so one
can see the model before pouring, and watch the rubber flowing around it while
pouring. Plain old rubber bands clamp the mold frames together. (note that
silicone rubbers in contact with natural rubber, as in the bands, doesn't cure,
so spills on the rubber bands can be messy later, but this doesn't affect what's
in the molds themselves. After the mold is poured, to just barely cover the
model, it's again vacuumed several times, each time the rise of the rubber is
less and less. After doing this five to seven times, more rubber is then poured
in to top up the mold, and it's set aside to cure, for however long the rubber
needs. The whole process of initial vacuuming takes about 7-8 minutes, a
couple minutes to initially pour the molds, another 5-7 minutes of vacuuming the
molds, then topping off the molds, and you're done till it's time to remove the
molds from the frames and cut them open.

Hope that's of use.

Peter Rowe

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