casting resin using a vacuum chamber

How many times must it be said??
You vacuum RTV rubber prior to poring into the mold. If you can get enough vacuum (26-29") the rubber boils off the embedded bubbles of air. That results in bubble free molds.
You pressurize resin after it is poured . If you attempt to vacuum it all you get is a foamy mess.
Norm

Be careful about pressurizing resin Buy a professionally built chamber. If you make one yourself you are creating a bomb that could kill someone. My company has been] casting for over 12 years. We use only certified chambers. We know from experience after we bought a used chamber that exploded sending the door from the chamber across the room to stick into the cinderblock wall. fortunately no one was hurt, as we were testing the pressure release valve. The valve was set for 80 pound but the chamber blew at approximately 50lb.Used certified Autoclaves can be of use but have them tested by a certified tank manufacturing company. My two cents
Bill Daisley wrote:

You vacuum rubber and pressurize resin.
Bill Daisley wrote:

Hi Bill,
It's much easier to create pressure for mold making and casting than to create the significant sustained level of vacuum required per other info posted here.
I use 35-40 p.s.i. pressure in a pressure paint can (from Sears) for both creating rubber molds and casting resin pieces with excellent results. This level of pressure is easily attained with a low cost household air compressor or air tank.
I hope this helps, Matt Guilfoyle

I want to assure folks, however, that vacuum and pressure are not firm requirements for home resin casting. Proper mold design will aid the removal of most bubbles. I have very little trouble with air bubbles, and the occasional one I get is easily filled with a small daub of putty.
Molds should be basically vertical, with the top of the sprue a couple of inches above the highest point in the actual part cavity. And EVERY local high spot must have a vent riser. It takes a bit more RTV to make such molds, but that is far cheaper than vacuum and pressurization equipment. Yeah, I know resin isn't that heavy, but even a small amount of hydrostatic pressure helps.
When you design your mold, think like an air bubble. Look at the design, and think, for every area of the mold cavity, "if I were an air bubble, what would be the easiest way for me to escape drowning?" Then, provide an escape vent accordingly.
Also, the sprue should be J-shaped. That is, the resin should enter the cavity from the BOTTOM. You do not want the incoming resin to mix with the outgoing air. Let the incoming resin in the bottom of the cavity force the air out the top.

yes, you only need 35-40 psi. anything over is .....over kill. :>) don is also correct as usual.

Hi! Could you, please, explain in details how the procedure with pressure should be done? I make rubber molds and cast resin pieces without either vacuum or pressure and often have problems with traped air. This would be a great help. Thanks!
Marko

If you do not use pressure, you need to make sure your mold is carefully designed. You must have some height of the mold above the highest point in the actual "part" cavity- at least an inch or inch and a half. Takes a little more resin and RTV, but worth it.
The sprue should be J-shaped. Lead it into the bottom of the "part" cavity, not the top. Every local high spot in the cavity must have a vent tube going to top of mold. Wheel = one tube, more complex shapes may need several.
Think like an air bubble. If you were trapped in the cavity and a flood was threatening to drown you, where would you like to have escape tunnel? And you sure wouldn't want to escape by swimming into the tunnel from where the water was entering.

And Don Stauffer in Minneapolis opened up and revealed to the world news: snipped-for-privacy@individual.net:
Don,
I'm not currently working with resin or casting my own parts (but after that short article in FSM I might give it a try). And I've been reading this thread and trying to visualize what you all are saying about the sprue being J-shaped. Do you mean something like this:
| Spure-->| _________ | | | | | Cavity | | | | | |_________| | | |__________|

Yep, that is it, except for the vent tubes. Let me see if I can add them.
| | | Spure-->| _|____|__ | | | | | Cavity | | | | | |_________| | | |__________|

.. but just like the RTV, the foam will collapse, creating bubble-free parts. Whether the curing time of the resin allows this is the question. Fast types will probably not allow it.
I used slow epoxy resins and always used vacuum to cast perfect parts. At least some commericial casters use vacuum instead of pressure. In my mind it is the better method, if your resin allows it.
Rob
My models: Me 163B site:

Lets get this set straight. Resin should never be exposed to a vacuum. By resin I mean any of the stuff that is mixed 50/50, has the consistency of thin syrup and becomes either white or tan when cured. The reason for this is the liquid becomes very volatile in a vacuum and certain chemicals become gaseous in this low-pressure environment-- in other words, accelerated outgassing. This is what causes the resin to foam up, not the tiny amount of air trapped in the mold. When normal air pressure is restored, the resin appears to be back to normal. Depending on the volume of the part, a lot or a little damage has been done to the material due to the outgassing and the part may or may not cure properly. It could become extremely brittle. It could weep polyol at some point down the line. It could also be perfectly fine, but the point is, the alternative (pressure or talcum powder) is much better.
Epoxy is different from resin. Way different, chemically; and due to its viscocity, using a vacuum to get rid of bubbles in epoxy is the only way to go. The difference between the two problems is that resin traps small amounts of air against the mold when it is poured; epoxy captures a rather large volume of air within the body of the material as it is mixed-- and some against the mold. It also has a much slower cure rate, which allows time to properly deair the part before the cure starts. As hobbiests, these are the facts of life. There are commercial resin compounds that allow for vacuum deairing, but we don't use those. They have been formulated for use in much fancier equipment than we have the budget or the need for.
One last comment-- someone mentioned making RTV molds using a pressure system. The only reason this is a bad idea is it is possible any trapped air will simply be compressed during the cure and when the mold is exposed to normal air pressure the air in the bubble will want to expand. If this is anywhere near the surface used to create the part, it is possible that the surface will distort, resulting in an inward bulge in the part. The same thing occurs in molds that are not deaired at all, but in reverse. The bubbles trapped near the surface of the rubber of the mold heat up and swell during repeated use and can also cause small dimples in the part.
I hate when that happens!
Scott snipped-for-privacy@AOL.com