This is HarborFreight Tool's 2 1/2
gallon paint pot's lid. Here I replace the hard rubber gasket that comes
with the paintpot. This material is actullay ok, I used it for the vacuum chamber
set up. It's a good piece of rubber, so don't toss it! The thing
is you have to clamp it down hard to keep 40 psi all night (resin really only
needs pressure during its cure time though). But as resin drips
here-and-there, and it will, the hard rubber will not seat well
enough to make a good seal. Best to replace it and avoid wishing
you had after a batch of casting comes out bubbly. Now when mixing
rubber to make a new seal it is best to degas the rubber. My first lid
used a poured oomoo 30 rubber gasket. It has since worn out. Looking
at it I could see where it had torn in many places, thousands of micro-bubbles
where present in the mixture and they allowed the rubber to tear just like
perforated paper tears. It might be possible to get a 'good' enough pour
with lower viscousity rubbers like Moldmax 30, however we want the cured rubber
to be, well, rubbery (alumilte rubber is much 'harder' post cure so its not a good
choice). So one could make a gasket without a degassing setup.
Now we will need to seal the inner ring/lip of the
lid. Looking closely
one can see that this piece is just a guide to wedge the hard rubber gasket
in place. It is welded in patches around its circumference so liquid
rubber will run out in some places. I used Mold Builder rubber from
the local hobby store. This rubber can only be applied in thin coats
and is useless to the novice mold builder (maybe useless to
everyone). It is ammonia based and stinks to high heaven. I could've
used a thin bead of Kleen Clay all the way around. Anything will do to seal
the gap but be sure it won't react with the rubber. I also sprayed a
coating of mold release (Mann 800, Ease Release 800 from Smoth-on) into the
lip, this way I could pull the gasket when/if it goes bad, or maybe pull the
gasket and use it somewhere else (new vacuum chamber lid).
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