Using the same vessel for vacuum and pressure


I’m thinking of taking the plunge and purchasing some equipment for molding & casting. I was looking at this degassing chamber, and I’m wondering if it could also be used for pressure casting. I know I need a different pump. I’m asking about the vessel itself. It has to be air tight for the vacuum, and the lid looks fairly secure.
Or for that matter, starting with a pressure pot and also using it for degassing.
Is this type of thing done often?
Bad idea?


I wouldn’t use the degassing chamber as a pressure pot, the lid is too weak for that. Conversely, you can’t see though the lid of the pressure pot to see if your material is done degassing. I only use a pressure pot as I do not degas any of the casting materials I use.


I had started off wanting the pressure pot, then I got to looking at the surfaces of some of my castings that had little pits - the result of air in the mold. So I started looking at degassing. I suppose the proper thing would be to use both.


To eliminate the pits also cure your mold under pressure. …they are caused by air pockets trapped just below the surface of the rtv mold then pressure casting pushes the casting into those In perfections. …not sure if it makes sense


So, you’re saying I could use just the pressure pot? Make the mold; place in pot; pressurize; allow to cure = no bubbles? At what pressure?


The general rule is if you put your RTV under pressure when you make the mold you must always put it under pressure when you pour your resin. Furthermore you shouldn’t put a RTV mold that wasn’t under pressure when made into a pressure pot when pouring resin into it. If you mix and pour your RTV carefully you can avoid trapping air into it. Pressure casting resin will compress the bubbles that form while the resin is curing and eliminate the surface pitting they can cause but it will not help with trapped air bubbles caused by an improperly vented mold. Learning to vent a mold well will insure perfect parts every time.


My pots are set to 35psi. just because that seems to work for me. I’ve read about using higher pressure. I don’t know if there is an advantage to that.


What Joe said. …I use 40 psi. …again just me :wink:


You should always Degas your silicone. If you can/ have a vacuum setup. Now you could buy the pressure pot and the vacuum pump and then buy a 1inch think piece of plex/acrylic and add a valve and gauge . But if you are going to do that just buy the 2 separate units and have something safe to use. It is not always best to DIY it.
The pressure pot from harbor Freight is $98 and the vacuum unit off eBay is about $179.
Now if you need bigger then DIY it as then you are looking at some real money.
The harbor freight one can go to 65lb.
Just my 2 cents.


I have a 2 1/2 gallon pot similar to the one Mike mentioned. I also have a old 7 gallon pot I got from work. Both have worked well for years. If you decided to use a paint pot you will need to modify it. They are not meant to hold air in. Rather they act as giant spray cans. You’ll have to remove the siphon pipe attached to the underside of the lid then add ball valves to the air inlet and outlet. Also add a pressure gauge to air inlet side.


Thanks for your advice, everyone. I think Mike made the most sense. Just spend the extra money for a proper, safe setup.

JoeB - I saw a very good tutorial on converting the Harbor Freight paint pot for pressure casting. Seems do-able. Thanks


Take my advice and avoid the Harbor Freight tank, it’s cheaply made and it’s an explosion waiting to happen. The clamps are cast in soft metal, mine became so bent out of shape that the clamping bolts wouldn’t turn anymore.

I now use the Grizzly pressure tank it’s much better made. The lid alone weighs almost as much as the whole HF.

Like Joe I’ve never used vacuum, I use pressure for both molds and casts. Pressure alone will eliminate bubbles in your silicone just fine, the only caveat is that your master part must be able to withstand pressure. If it’s a hollow part with thin walls it’ll need to be filled with something to make it solid (or vented by holes in the bottom of your mold box to allow for equal pressure inside).

Also molds made under pressure do not need to be pressurized when casting. It’s a good idea for the best cast, but not a necessity.


What do you think of this one?


The max pressure of 45 psi seems low to me. My small tank is rated at 80 psi. A lower psi might mean the tank isn’t as robust.


I have no experience with that tank so I can’t really comment on it. The one I have is very robust, I use it at 60psi and it’s been just fine.


Rob, is it difficult to remove all the unnecessary hardware? It looks more complicated than the Harbor one.
Is the bottom of the pot concave? Did you need to make a flat platform?

Also, can you use a mold that has not been degassed in the pot? Will it deform in any way once it’s under pressure?


Not difficult, just a bit more to do because of the paint mixing handle which needs to be removed. I detailed the conversion of my Grizzly pot here. I also went over my experience with the HF pot and why I don’t recommend it.

Yes the bottom is rounded so I cut out a circle of 1/4in MDF to make a bottom panel. I also used the MDF to make racks that can stack up three levels high to make the most of the tank’s space.

As for the mold all that matters is that it’s free of bubbles, curing the silicone under pressure eliminates the bubbles just as well as vacuum. Vacuum also does the job but it’s extra equipment. In my case I already had a large compressor so the pressure pot was the only other thing I needed. So far it’s worked great. Just make sure that your original pattern can be pressurized without damage or distortion. A hollow plastic piece could be crushed, or a vinyl doll head will distort under pressure. I just add a small hole or two in the mold box bottom that the part is attached to so the pressure can equalize.


One more… I’ve been using Smooth-Cast 300 lately. Is there enough time to get the unit sealed up & pressurized before the resin kicks? Should I maybe switch to 305? I’m not used to 30 minutes for a casting :stuck_out_tongue_winking_eye:



A three minute post life is pushing it. I use 305 which gives me plenty of time to get multiple molds into the pot and pressurize it before the resin kicks.


I didn’t want to invest in expensive equipment for the few castings I do. I build a vacuum chamber out of an old pressure cooker and a plexi plate. Works just fine.