Degassing Silicone at High Elevation

[Wheat King]

Greetings.

I came upon this wonderful forum quite by accident. I'm a sculptor who lives in Western Canada near the Rocky Mountains and I'm planning a project that requires me to make some silicone molds. Over the years, I've made many waste molds using a variety of materials such as plaster and wax, but I've never made reusable silicone molds, so there are a few unknown elements that I need to clarify.

I want to degass my silicone (slow, tin catalyst curing) before I pour it, but from the reading I've done it seems that living at a higher elevation, 3500 feet above sea level, means that I will only be able to achieve a maximum vacuum of 26 inHg, even with the very best of equipment (which I haven't bought yet). At least one of the experts I've watched on YouTube said that, "...you are wasting your money and your time if you cannot reach a vacuum of 29 inHg."

What I need to know is if I will be able to degass my silicone at 26 inHg (or quite possibly 25.5)? My thinking is that it should be easier to get the bubbles out of the mixture at higher elevation because there's not as much pressure pushing down on them, and so 26 inHg should be as effective at 3500 feet as 29.5 inHg is at sea level.

Any advice would be very much appreciated. Thank you.

[mangozac]

Hi Wheat King, welcome to RA!

This is certainly a very interesting question but alas I'm not sure I have a definitive answer.

The "Youtube pro's"are right - if you can't reach a decent vacuum it's a waste of time. I would say however that -29 in/Hg isn't as necessary as they make out (for our applications anyway). I used to use a vacuum pump that only ever got to around -28 in/Hg and that was fine. Using a slower curing silicone also helps, as it gives more time to help coax the bubbles out.

But the big question is whether -26 in/Hg is enough. I suspect not, but the only way to find out would be to give it a go. If you want to wait a few days next time I make a mould I can double check at which point the silicone really starts to bubble during vacuum and note what level of vacuum that is?

There are other options though: you can paint the silicone into all of the detailed areas, fill up the mould box and then place the whole lot under pressure. The painting will ensure that no detailed areas get any air pockets and the pressure will crush all of the bubbles. Until recently that's the way I did pretty much all of my moulds (without the painting part, but I was using a very thin silicone).

I hope that was helpful?

[Blindhorizon]

What are you casting? Don't need to share if it's a secret, but what are the dimensions. Does it have a lot of surface detail? The painting technique works very well, but also you might not even need to degass. If you hold the cup with the silicone in it high up so a thin strand pours down into your mould into one corner so that airbubbles are pulled out and the silicone slides slowly over your item(if a fan is on or wind blows in any way the strand will spray around the room). This will cause some minor air bubbles but those will push out mostly in the corner, doing this with combing the paint on technique i don't think you'd run into any major problems.

[HeatXfer]

Wheat King - you are correct. The same level of vacuum (29"hg) registers a lower value on the gauge at higher altitudes. Absolute atmospheric pressure at 3500' is 26.3"hg. so a vacuum at that altitude will read 26.3"hg on your gauge.

[RTVMagik]

Hello folks, I am new to this forum also. When searching for molding information, this site comes up quite a bit. Anyway, to comment on this question of altitude vs. vacuum I do have a good resource. A chart that shows the maximum vacuum you can get at different altitudes.
We have some folks in Denver Colorado making molds with our silicone. Almost exactly 1 mile high at 5,280 ft. I'll try to reach out to them to see what results they are getting. They can only reach a maximum vacuum of around 24".

[mangozac]

Welcome to RA and thanks for your comments, RTVmagik!