vacuum degassing rubber

[nervis1]

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 .

[CNCPlastic]

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.

[nervis1]

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.

[nervis1]

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

[trubleshtr]

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

[nervis1]

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.

[windsor]

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 .

[trubleshtr]

Ah.. ok, thanks for explaining

[CNCPlastic]

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.

Originally Posted by nervis1 Forgot to ask...after I apply the vacuum, how much and how long for silicone?

[CNCPlastic]

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.

Originally Posted by nervis1 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.

[CNCPlastic]

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.

Originally Posted by windsor 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 .

[nervis1]

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

I'll stick to the rated stuff.