I need to vacuum degass some silicone rubber before shooting it into a mold, I've seen some canisters (looks like I could make my own pretty easy out of a paint pot or other such device) and the pumps...big money.

Anyone know of a way to do it cheap? I have a big air compressor, seems like there would be a way to turn that thing into a sucker? Guess there is always the shop vac :).

You need a good 28 inches of vacuum to degass that stuff, the shop vac has no hope of getting anywhere close to that. There are venturi devices to turn your air compressor into a vacuum pump but you can buy a brand new robinair type 6cfm vacuum pump for about $230 so you may want to compare the price of each. Buy a pressure tank aka paint pot aka pressure pot, dont go screwing around trying to fashion one on the cheap and risk serious injury to shop and/or yourself. Small paint pots can be had at Sears for next to nothing. I bought a 5 gallon on ebay for $199.

Note that liquid silicone (they are really stretching calling it a liquid) will expand 400% during degassing so be sure to use the appropriate sized container.

Here's a pic of my pressure/vacuum tank next to my vacuum forming table, I use it to degass liquid silicone among other things, you can also pressure form with it.

http://www.coilbuilders.com/pics/vac1.jpg

Robinair it is! Thanks for the info...sweet setup by the way.

I already have a 5 gal paint pot I use to cure under pressure, it doesn't get out some really fine bubbles though, I'm still getting a small number at the top of the molded part, since the stuff is clear it really shows.

Forgot to ask...after I apply the vacuum, how much and how long for silicone?

nice photo, how does this system work exactly? what does it mean to degass something? sorry for my rookie questions, just curious to understand.

Bubbles in the mold look bad and make a weak part, to get the bubbles out you can take a can of mixed material ready to pour into a mold, put it under vacuum and the bubbles will expand (PV=nT or PV=nTR if you want to include the gas constant PV=k for Boyles law remember the physics class?) and come out of the solution. You can also cast the part under pressure and contract the bubbles so you don't see them.

In my situation 60mmhg pressure does not seem to be enough to get the bubbles out so I'll need to degass the mix prior to shooting it into the mold.

I do silicone molds, and use an old milking machine pump that i got for $20 at a junk store . For a chamber I use a presure cooker pot with a 1/2" lexan lid that I made . The clear lid alows me to watch the rubber boil and expand . When the boiling mass colapses it is ready to pour . I also have a presure connector on the presure cooker lid that alows me to let the mold cure under pressure . Presure is usefull for casting clear acrylic with no bubles .

Ah.. ok, thanks for explaining

Degassing time may vary, check your product specs it should tell you. The silicone mold material I use from GT Products in TX specifies 2 minutes at 29 inches.

Forgot to ask...after I apply the vacuum, how much and how long for silicone?

Pressure casting works for a rigid resins but I think whats happening with the silicone is that you compress the air bubbles but the larger ones simply expand again when the pressure is released because the silicone is so flexible, especially at the top where the bubble surface would be thin. Just a thought.

Bubbles in the mold look bad and make a weak part, to get the bubbles out you can take a can of mixed material ready to pour into a mold, put it under vacuum and the bubbles will expand (PV=nT or PV=nTR if you want to include the gas constant PV=k for Boyles law remember the physics class?) and come out of the solution. You can also cast the part under pressure and contract the bubbles so you don't see them.

In my situation 60mmhg pressure does not seem to be enough to get the bubbles out so I'll need to degass the mix prior to shooting it into the mold.

BAD BAD BAD idea, under vacuum this is a good way to get all your front teeth knocked when the lexan lid fails, under pressure its a bomb waiting to explode.

Pressure cookers are only rated to about 15psi, there have been many cases were under pressure the lid, even the pot litterly blow apart sending scrapnel in all directions, most often the lid fails. Don't be fooled by the big heavy alum casting, its not very strong.

When researching this I found many horror stories of using a pressure cooker for both vacuum degassing and pressure casting. I just read one on yahoo a couple weeks ago of guys that had been using 1/2 inch lexan for vacuum degassing lids and viewports. Even the small view port failed eventually. One guys lexan lid lasted for a year then BOOM.

There is a reason for ASME certification, tanks under pressure are very dangerous. I saw pictures of a pressure tank that was at 110psi, it was used for pressure casting bricks. The door failed and it leveled the brick building it was in, e.g. blew the roof off and walls down. The tank shot out of the building like a cannon ball and blasted through the side of a school 200 yards away.

Before you use your pressure cooker again I urge you to do some research on this.

I do silicone molds, and use an old milking machine pump that i got for $20 at a junk store . For a chamber I use a presure cooker pot with a 1/2" lexan lid that I made . The clear lid alows me to watch the rubber boil and expand . When the boiling mass colapses it is ready to pour . I also have a presure connector on the presure cooker lid that alows me to let the mold cure under pressure . Presure is usefull for casting clear acrylic with no bubles .

Blew the house down eh? That would really piss my wife off.

:) I'll stick to the rated stuff.

There is a reason for ASME certification, tanks under pressure are very dangerous. I saw pictures of a pressure tank that was at 110psi, it was used for pressure casting bricks. The door failed and it leveled the brick building it was in, e.g. blew the roof off and walls down. The tank shot out of the building like a cannon ball and blasted through the side of a school 200 yards away.

110 PSI huh ? , Well I do not use any pressure pots but I would have to see the above to believe it, perhaps it was a leaking 500 pound propane tank with a 110 PSI :)

Ken,

Maybe it had a significant volume. A big tank would make quite a pop! :D

I got my pumps from a couple of old photocopy setups. The ones they had before the scanners entered the market. If you have a printshop in your area that did not throw out their old spares, they may still have one. For sure they don't have any use for it. So if you find one, it's going to be cheap.

They are also used in the woodwoking industry for laminating. But those are your "competitors", so no use in asking there for anything in good condition.

Back when I was "thinking" of fashioning my own pressure vessel for pressure casting someone sent me a link to the pics, that scared me right out of building my own. I can't find it but the building was destroyed, just a pile of bricks left. This was a very large tank, they shoved pallets of bricks into it for curing under pressure.

I did a google though and found some smaller explosive examples...

Check out what this little 5 gallon water heater did to a school when it blew (http://www.safteng.net/Accident%20Photos/Hot%20Water%20heater%20explosion.htm)

A video store water heater blew and landed 460 feet away blowing out the front of 3 stores (http://server.firehouse.com/news/2001/7/28_APheater.html)

This guy got the bright idea of converting his old water heater tank to an air compressor, it raised the roof, blew out the walls, and the roof collapsed back onto the pile of rubble. (no pics)

Lifted the roof off the house and blew out the walls with a water heater tank (http://yarchive.net/metal/compressor_tank.html)

The 5 gal heater incident was quite impressive, but also noticed that is was extreme pressure built up from boiling temperatures held under control because of the pressure but when it let loose, boom.

Not to make light of what pressure can do, it can be seriously deadly, your warnings are well justified.

Hi guys.

I have used an old fridge motor as a vacuum pump with great success but it does take time. If you coupled it to a reservoir and left all the air get out of it before you mix and degass your rubber it could be a very cheap way of doing it. This however depends on the quantity you want to degass at a time. Hope this helps

Lee

Careful with lexan windows for pressure casting various resins. Lexan is sensitive to some chemicals which make it less prone to crazing. Definitely bad for something you depend on being tough.

It sounds like you're not planning on pressure casting. There are vacuum venturi devices for compressed air lines which draw a pretty hard vacuum. You will need a decent sized compressor, not a dinky 1gal type portable comp. I've used a similar device to this for silicone rubber casting.

http://www.wicksaircraft.com/catalog/product_cat.php/subid=654/

Works nice, but the combined shriek of the vacuum generator and comp' can get annoying.

Depending on how serious of a degassing chamber you want you..

I was able to make a degassing vacuum by poking a hole in the top of a mason major and simply sucking out the air with a brake bleeder i was able to pull a max of 27 in of mercury just enough to degass my HS III silicone

it cant boil water but it seemed sufficient for removing bubbles from the silicone. I was able to set it up in literally one minute and for only $22