degassing RTV

qc · Post by qc » Sun Nov 04, 2007 9:39 pm

I'm wondering how to degas RTV. I'm about to pick up a 2.5 gal pressure paint pot, have a home vacuum cleaner, and a iwata sprint compressor. is this enough? or i will be needing more?

do a need a venturi to hook up to the compressor or will that not be enough? can home-rig the vacuum cleaner to the paint pot? i'm not agasint picking up a shop vac if needed (may want to use it for vacuforming in later porjects). what pressure is needed and consideing i'll be using a 2.5 gal pot i'm assuming i need something that can produce that vacuum in a short span of time (before the RTV sets up).

or what is the most common/recommended method of degassing RTV?

Umi_Ryuzuki · Post by **Umi_Ryuzuki** » Mon Nov 05, 2007 1:01 pm

Did you do a search in the construction section of the forum?
I think most of this may be covered in there and that is why no one has noticed, or answered your quieries.

Plus, the scratch building forum will automatically dump posts in 30 days... So no records of this will exist later.

A vacuum cleaner might be ok for a quart or less in volume, but it is not efficient enough to do a full 2.5 gal pressure pot. For one, opening for the hose would have to be carried into the pot.
You could try it, a 3/8" - 1/2" thick piece of acrylic over this smaller pot.
It shouldn't cost more than $60-$80 for a piece, Then apply the vacuum to it.

I know others have had success using those venturi systems. Probably a more sure bet if you started with that rather than betting on the shop vac.

On a side note, I just replaced my vacuum pump. We suspect that the vanes were coated in resin fumes, or that the resin fumes ate the seals.
No toxicity here, is there...

I was lucky, in that a friend had two vacuum pumps dropped into his driveway because he had mentioned my need.... I have those hooked up in parallel, and a huge modified water filter between the chamber and the pumps. Hoping that reduces any hazardous particles... I still vent to the outside when possible. They get a 12" diameter pot into full vacuum in less than 35 seconds... (this pot needs a 3/4" thick acrylic vacuum lid to prevent deflection...)

I am hoping to have a pipe put through the wall specifically for venting pots.

Joseph Osborn · Post by **Joseph Osborn** » Mon Nov 05, 2007 10:32 pm

Not to sound harsh, but you're not going to come anywhere close to degassing RTV with any vacuum cleaner nor with a venturi hooked to an airbrush compressor. You'll need a high-efficiency venturi and a BIG compressor, or better yet, a deep-vacuum pump capable of at least -27.5inHg or lower. Both options are not cheap. What is cheap, though, is using the paint pot to cure your RTV under pressure, thus negating the need to degas the RTV prior to pouring. I'm not sure what volume of air your airbrush compressor is capable of putting out, but I'm going to guess that it will not be enough. Fortunately, there are plenty of cheap air compressors to be found that can pressurize the paint pot efficiently. And if you're handy, you can convert a scrapped refrigerator compressor into a pretty decent vacuum pump that may actually pull hard enough to degas RTV.

qc · Post by qc » Wed Nov 07, 2007 12:05 am

i usually hear of degassing the rtv..
you're saying i should pressurize it?

in the pressure pot..
let it totally cure while being pressurized?

how much pressure do i need to do that satisfactorily?

Joseph Osborn · Post by **Joseph Osborn** » Wed Nov 07, 2007 3:05 pm

qc wrote:i usually hear of degassing the rtv..
you're saying i should pressurize it?

Yep.

in the pressure pot..
let it totally cure while being pressurized?

Yep.

how much pressure do i need to do that satisfactorily?

About 35 to 50psig. It's the poorman's way to make perfect bubble-free silicone molds. All you need is the pressure pot and a compressor to process both your molding and casting operations.

qc · Post by qc » Wed Nov 07, 2007 3:11 pm

scratching my head...
...
i understand the process of lowering the pressure of a liquid..to remove air entrapped...but how does pressurizing remove the bubbles?...when pressurized...bubbles would stay (much like a sealed coke can, until the top is popped)

and if this is the poor mans' method...why isn't there more mentioning of this (since most of us here are millionaires)

Umi_Ryuzuki · Post by **Umi_Ryuzuki** » Wed Nov 07, 2007 3:16 pm

If the bubbles are near your master or the surface of your piece, the RTV may not be enough to contain the bubble once it is returned to normal pressure. For pin size bubbles not a problem, but larger bubbles may pop, and you would get a small nodule on your casting where this happens.

qc · Post by qc » Wed Nov 07, 2007 3:25 pm

umi,
sorry not to follow exactly..
are you commenting on j'browns pressurizing suggestion? are you supporting the idea of pressuring the curing of the RTV? and it sounds like you're saying there WILL be bubbles left in the cured RTV once it is cured?

isn't this the issue we're trying to avoid? to RID ourselves of the bubbles...not just encapsulate them?

DX-SFX · Post by **DX-SFX** » Wed Nov 07, 2007 5:34 pm

I'm afraid I'm for degassing the RTV. You can pressure cast resin which will solidify but any airbubbles in RTV will just expand again when the pressure is released which could cause dimples in your casting if they're near the surface. You get a similar effect when your resin gets hot and makes the airbubbles in the RTV expand. The harder the RTV the less the effect but it's always best to remove air from the mix rather than merely shift it. Use a slow set RTV as this will give you plenty of time for any residual bubbles to rise out the mix whether you degas or not. You will need a high vacuum pump for degassing. Forget the vacuum cleaner. It'll hardly register.

DX-SFX · Post by **DX-SFX** » Wed Nov 07, 2007 5:39 pm

BTW don't ignore your airbrush. If you paint a layer of rubber on your master pattern first, any small air bubbles trapped in the detail can be popped by a quick blast of air from your airbrush (obviously without any paint in it).

qc · Post by qc » Wed Nov 07, 2007 5:45 pm

i keep seeing the magic number of "28" of mercury"...i'm not familiar with that so much (i'm a structural engineer) but i do have a better feel for psi...
and 28" of mercury is only 14 psi
that doesn't seem like much at all
would think a good shop vac could pull that..
and if a venturi only gets 1/3 of the positive pressure blown by it (40 psi) off a regular air brush compressor, it should work as well

this seems too simple..or am i overlooking something?

Joseph Osborn · Post by **Joseph Osborn** » Wed Nov 07, 2007 6:11 pm

qc wrote:scratching my head...
...
i understand the process of lowering the pressure of a liquid..to remove air entrapped...but how does pressurizing remove the bubbles?...when pressurized...bubbles would stay (much like a sealed coke can, until the top is popped)

and if this is the poor mans' method...why isn't there more mentioning of this (since most of us here are millionaires)

The pressure crushes the bubbles in the RTV to microscopic size and then holds them there while the rubber cures. If you're using normal RTV that we use for hobby casting (Shore A 20 or firmer) the bubble stays crushed. I guess if you were using VERY soft rubber-- less than Shore A 10-- then bubbles could re-expand and cause mold problems. Exact same thing happens with resin when it's cured under pressure. The bubbles are crushed, but if you remove the pressure before the casting has cured, bubbles can bulge out of the soft casting. I guess pressure curing isn't mentioned here because it's more fun to watch RTV boil. There are some caveats to pressure-curing rubber, like your master has to be either solid or a very strong hollow piece. But if you don't want to invest big bucks in a vacuum pump, then pressure-curing both your RTV and your resin is the way to go. You're not out anything but a little bit of RTV if you try it and it doesn't work for you. Then you can go spend the $200 - $300 on a vacuum pump that is capable of properly degassing the RTV. One thing I can guarantee is if you don't either vacuum or pressurize the rubber, you will not be able to pressure-cure your castings.

Joseph Osborn · Post by **Joseph Osborn** » Wed Nov 07, 2007 6:19 pm

qc wrote:i keep seeing the magic number of "28" of mercury"...i'm not familiar with that so much (i'm a structural engineer) but i do have a better feel for psi...
and 28" of mercury is only 14 psi
that doesn't seem like much at all
would think a good shop vac could pull that..
and if a venturi only gets 1/3 of the positive pressure blown by it (40 psi) off a regular air brush compressor, it should work as well

this seems too simple..or am i overlooking something?

Okay, think of it this way: if you put a shop vac on your arm, it might make a red spot that will last for a few minutes. If you put a deep-draw vacuum pump on your arm, it'll pull out your bodily fluids right through the skin. And venturis require air volume (cfm) as well as pressure.

USS Atlantis · Post by **USS Atlantis** » Wed Nov 07, 2007 7:10 pm

Joseph Osborn wrote:
qc wrote:i keep seeing the magic number of "28" of mercury"...i'm not familiar with that so much (i'm a structural engineer) but i do have a better feel for psi...
and 28" of mercury is only 14 psi
that doesn't seem like much at all
would think a good shop vac could pull that..
and if a venturi only gets 1/3 of the positive pressure blown by it (40 psi) off a regular air brush compressor, it should work as well

this seems too simple..or am i overlooking something?
Okay, think of it this way: if you put a shop vac on your arm, it might make a red spot that will last for a few minutes. If you put a deep-draw vacuum pump on your arm, it'll pull out your bodily fluids right through the skin. And venturis require air volume (cfm) as well as pressure.

Add to the fact that we're talking a 28" Vacuum here... which mean's the pressure inside the vacuum chamber is near 0 PSI - or what you have outside the Shuttle or ISS outside the airlocks. Can't get any lower than zero PSI - negative pressure doesn't exist.

Atlantis

DX-SFX · Post by **DX-SFX** » Wed Nov 07, 2007 7:32 pm

Exactly. Atmospheric pressure is only 13-14 psi so total vacuum can only have that much differential. That's why there are limits to vac forming. One tends to think of it as sucking the plastic down but although that's true in one sense, what's actually happening is you're removing the air from underneath and atmospheric pressure above is pushing the plastic down. In practice it's almost impossible to attain a 100% vacuum without very expensive equipment.

Umi_Ryuzuki · Post by **Umi_Ryuzuki** » Wed Nov 07, 2007 10:07 pm

Essentially, I prefer vacuum de-airing the RTV

As far as Vacuum and 14psi, it is per square inch...
have a read through this thread....
viewtopic.php?t=54659

Also realize that the calculations there are only for one small side of the fish bowl... You could probably multiply the number by at least 3...