When casting epoxy in a mold, placing the mold into a vacuum chamber and putting it under a very high vacuum forces any air bubbles to float up to the surface of the liquid epoxy to pop on the surface. The higher the vacuum, the more effective it will be at removing bubbles. This is usually done when the liquid epoxy materials have been dispensed into mixing cups, or as soon as the mixed epoxy is poured into a mold. Once all the bubbles are expressed, normal pressure can be restored.
When a pressure pot is being used, it should be used at at least 50psi to put the liquid epoxy under pressure. This pressure compresses the bubbles until they are so small that they are not visible. Pressure pots are used when mixed epoxy has been placed into a mold. Consideration should be taken with molds under pressure though, as any air bubbles in the mold itself can be compressed, distorting a mold. The epoxy needs to stay under pressure until it has cured to a hard, firm, stable material.