When to Use Pressure Pot and Vacuum Chamber

I am often asked if a vacuum chamber or a pressure pot is required in the fine art of mold making and casting. As with so many answers in life, a “yes” or “no” answer is not possible without first learning more information about the project. Except for water clear resin such as AquaClear resin, where tiny air bubbles will obscure the clarity of a piece, and such equipment is a must, my answer most often is, “It all depends.” That is unsatisfactory, I know. So the purpose of this article is to provide the specific answer you are looking for.

For casual mold making and casting, you can pour your materials in a high, narrow stream into one corner of your mold box to reduce the inevitable air bubbles. This allows air to escape as it travels down the narrow stream as you are pouring. Vibrating the mold, or mold box helps, as well, either mechanically, by knocking on it with your knuckles, a vibrating table, or even placing a vibration source against the mold box, such as a hand sander. These are all great studio tricks that will definitely reduce air bubbles. But they do not eliminate them entirely. So if that is your goal, please read on.

So if you are planning to create molds and castings on a regular basis then you should bite the bullet and acquire the right type of equipment to achieve professional results. Just as one can do carpentry using manual operating tools such as a hand saw, better and faster results are often obtained through the electric table saw or chop saw. The right tools, for the right purpose, go a long way in achieving consistent satisfactory results in any industry or hobby for that matter. The right tools in the mold and casting industry begin with the vacuum chamber and pressure pot, also known as the pressure chamber.

"What is the difference between the two and do I need both?” are the essential questions I most often receive. As the names imply one chamber provides air pressure while the other removes air pressure. But only one actually removes air from your mold making and casting material – the vacuum chamber, while the other simply hides it--the pressure chamber. The pressure chamber works by providing up to 50-psi of atmospheric pressure. If you remember your high school science, normal sea level pressure is about 14.7- psi. Thus, the higher pressure works to compress any air bubbles in your material and squeezes them down to almost microscopic size – thus making them seem to disappear. The air is still there though, but you just can’t see the bubbles now. But, once you release the air pressure back to 14.7-psi, the air bubbles will return – that is unless the air is contained as it would be if the material you were pressurizing solidified to a solid, such as a hard resin, gypsum plaster or epoxy.

If your material was a mold rubber though, such as silicone or polyurethane, the flexible rubber will not contain the compressed air bubbles and they would expand within the rubber back to normal size, even though your rubber has cured. Thus, the pressure pot is best used when your mold making or casting material cures to a solid and the vacuum chamber is used to remove air from flexible rubbers.

The vacuum chamber can also de-air solid resins and epoxies, too. But since it takes a bit more time to create a vacuum, and certain resins are fast-cured, the pressure chamber is the tool of choice in those instances as it can be quickly pressurized, faster than a vacuum chamber can be evacuated.

The last point to make is that to operate these two tools you will require a vacuum pump for the vacuum chamber and a compressor for the pressure pot. Both items are not too expensive. A compressor used to operate a nail gun and available at big box stores is all you will need. A vacuum pump used by auto mechanics to check automobile a/c units is readily available online. In conclusion, if you are serious about mold making and casting as a hobby or as an income producing profession, then a vacuum chamber and pressure chamber are in your future.

In summary, the vacuum chamber is used primarily to eliminate unsightly air bubbles in hard setting materials, whereas, the vacuum chamber can be used for both degassing rubber and hard setting materials, but is best adopted for rubbers such as silicones and polyurethanes.

Example of a compressor required to operate the pressure pot.

Example of a vacuum pump used to evacuate the vacuum chamber.