(05-09-2010, 10:48 AM)427 Cobra Wrote: OK, Joe. This is what we're saying:
You can't pressurise the silicone as it's curing. This causes distortions in the rubber once the mould master is removed. The air isn't sucked out in the pressurising process, instead it's pushing the air into the rubber deeper to where it touches anything solid inside the liquid. Does this make more sense? This means that you will have a lot of air holes along the entire surface of your mould = RUINED!!!
Unfortunately, many people claim they've done this without a problem, which is a lie!!! There's no way to pressurise silicone, as this would push a lot of the material out of the mould box, and onto the sides. This too, will compound your problems. There's no way to control the liquid once under pressure. These compounds are formulated to be used one way only, and any deviation from it will be disasterous!! Does this answer your question more thoroughly? Don't give up - if there's a way to get around something, we'll try to help you find it. That's what we're all here for.
~ Cobra Chris
Oh ok, I see, said the blind man, LOL. I guess you did know what I was saying,
And I wasn't going to give up on casting just on this question, 
I wonder why Smooth On recommends this on their site. it says right in there Faq, that you can do it. That's weird. Oh well, i wasn't questioning you guys knowledge, just throwing around ideas. I always try to find the easiest way to do things, cause I'm lazy, LOL. I also understand that there's a right way to do stuff and anything else will fail. I will have a Vac pump, so I can de gass. I was just looking for a shortcut. I guess it doesn't take any more time to de gass, versus pressurizing anyway. Thanks again guys, for the responses. I bow to your wisdom. I'm a total beginner at resin casting, haven't cast anything in silicone yet. Done plaster, fiberglass, and the assorted other mediums, no silicone though. Don't think this is the last crazy idea you'll hear from me. I'm a mad scientist, Cheers,Joe
PS. Here is the section on the Smooth On site I referred to.
Quote:Suggested by professional model makers; if you are going to use a rubber mold on a regular basis for pressure casting, make the rubber mold under pressure. This means mixing and pouring mold rubber over your original model, placing the structure in your pressure chamber and applying 60 psi until the mold cures.
Reason; If you pressure cast resin in a rubber mold that has not also been pressurized, any bubbles present in the rubber mold may implode under the pressure and the collapsed bubbles may be reflected in the finished casting as bubble positives or "dimples".
Limitation; when making a mold of a hollow model under pressure it may collapse under pressure
Warning; use commercial pressure chambers only. More than one resin caster has learned the hard way that "home-made" pressure chambers can explode violently and cause serious injury.
Important; ensure that the air running from your compressor (used to pressurize the pressure chamber) is dry. Moisture in the air line will react with urethane resins and rubbers causing bubbles or foaming.
Air line driers are available from industrial supply stores.