05-09-2010, 09:52 AM
(05-09-2010, 08:40 AM)427 Cobra Wrote: Ron, and Jase are both correct. When vaccum degassing is done, this makes any tiny airbubbles rise to the surface. This keeps them from distorting the silicone mould, so that it can produce an exact reproduction. The problem with pressurising is that it only pushes the air into any available space, as Ron said. This only ruins the mould surface, and therefore making it impossible to make a good cast. This only leaves the surface uneven with bumps all over it. Not only that, but it might not even cure completetly with pressure casting, and this too will leave soft spots in the rubber.
Your best bet is that if you don't want to degas is what Ron suggested - use the OO-MOO brushable silicone, or a platinum based silicone to keep the airbubbles out, and the uniformity, or integrity of your mould intact. This way, you won't need a separate degassing chamber. This is only for small projects. If you have something big, you'll have to use a different method of making a reinforcing jacket for your mould, and use epoxy instead of urethane resin.
You can still use the regular silicone without degassing, but you'll never be able to pressure cast with this if it's not degassed. Meaning, that you can only use epoxy, or polyester type resins, or fibreglas with this mould. I hope this helps in some small way.
~ Cobra Chris
Everybody is missing my point
Probably because of the way I'm phrasing it. What I'm suggesting is pressure casting the mold as your making it. You pour the rubber around your part and pressurize the rubber as it cures. This forces the bubbles that are there to microscopic size and into suspension. Then when you pressure cast the parts in resin, the bubbles in the rubber are to small to effect the casting. I hope that is clearer, This is why typing out a question on the internet sucks. I'm a terrible writer, and can't get the idea across, LOL. This is the same thing they discuss in the Smooth on article I linked to. I'm hoping there's a bunch of head slapping goin on, with people saying, " Oh I get what you're saying", I definitely know the that vacuum degassing removes most of the air from the rubber, This was just a question about the merits of pressurizing the rubber itself. i hope I explained it better. If we were on the phone, it would have been clearer. It's this damned typing thing. If we don't get it this time, well I give up, LOL. Thanks for all the replies, I'll just try it and see what happens, cheers,Joe