Smooth-On 27A

[Craftdiggity]

Just got my 27A and I am reading that I need to use vacuum to degas after mixing. Is that necessary? Do any of you who work with this stuff cast molds without vacuum? I'm thinking that if I stir slowly and avoid introducing too much air, I might be okay without it. Any thoughts?

 

[Craftdiggity]

Stumped y'all, huh?:tongue:

 

[NewLondon88]

Sorry.. I just saw this.

You can do it without vacuum, especially if you pour it in layers. Pour
thin ribbon (the bubbles pop on their way down) And it helps to add
pure silicone oil. Do a partial pour slowly, let it settle, pour more, let it
settle etc. THe oil helps bubbles clear, but only goes so far. The deeper
the pour, the more stages you need.

 

[workinforwood]

I just mix it mega thorough and pour then 40 lbs pressure. Never had bubble problem because its 24 hrs to cure and the set tim is so long that any bubbles float to top and disappear. On rare ocassion gets a few very small bubble crators on top but that is really bottom of mold so it affects nothing

 

[workinforwood]

I took an old spade drill bit and ground off point and use it in drill as a mixer

 

[jason_r]

While I do vacuum mine now, in the past I would set the mold and the pot of mixed rubber on the drill press. Pour some into the mold, set both on the drill press and turn it on so the vibration helped the bubbles rise. Come back in a few minutes, pour more... put back on drill press. Repeat until done.

But in the larger scheme of things, you don't really need to worry about any bubbles. If you were pouring to get a mold that would give you a finished surface, that's when you need to worry about bubbles in the rubber. But since you turn the casting after taking it out of the mold, any bumps caused by bubbles in the mold will be turned away.

 

[NewLondon88]

But in the larger scheme of things, you don't really need to worry about any bubbles. If you were pouring to get a mold that would give you a finished surface, that's when you need to worry about bubbles in the rubber. But since you turn the casting after taking it out of the mold, any bumps caused by bubbles in the mold will be turned away.

That's true, and besides the bubbles don't form against the master anyway..
they're suspended within the mix.
But if you're going to use pressure then you'd want the silicone de-gassed
so that the air bubbles don't compress and warp the mold. If using high
pressure, you wouldn't use this for a mold anyway.