Vacuuming: A vacuum pump is like a vacuum cleaner. A vacuum cleaner is designed to pick up things such as dirt off of carpet using suction or vacuum. A vacuum pump is identical to a vacuum cleaner, except it will have MUCH more suction. A vacuum system is measured in the amount of vacuum it will pull. In technical terms, it is measured in inches of mercury. A common household vacuum will pull 1-2 inches of mercury. A good vacuum pump will pull 29-30 inches of mercury. A vacuum pump is used to suck the air from the mixed silicone to assist in pouring air/void free molds. The concept is that when we mix the silicone we mix in tons and tons of air into the mix. Ideally, we want to remove the air from the mixed rubber so it doesn?t end up in our cured rubber mold and we're really trying to avoid any air bubbles from sticking onto our original parts surface that would show up in our cured silicone rubber mold. When you put the mixed rubber that is full of tiny air bubbles under vacuum, it makes those air bubbles expand into big air bubbles which now have the buoyancy to float to the top and pop which removes them from your liquid rubber. If they remain small, they do not have the buoyancy required to float through the thick consistency of the rubber and get to the top. The thicker the rubber the harder it is for small bubbles to float and hence the need for the vacuuming rises. Vacuuming Silicone: Our QuickSet Silicone, High Strength 2, and High Strength 3 has a thin enough viscosity the majority of air bubbles will rise to the surface without a vacuuming system. Plat 55 is too thick to allow air bubbles to rise. Therefore, you must pull a vacuum to assure no air is entrapped. Once your vacuum reaches 29-30 inches of mercury, the rubber will foam up. You will need a container around 4 times the size of the amount of silicone you are mixing. Once the rubber has risen, it will break (meaning to fall quickly). After the silicone falls, you will want to keep it under vacuum for another 2-3 minutes. Then remove the material from the vacuum and pour the rubber over your master. You may want to pull a second vacuum after the mold has been poured. This will remove any air that you may have entrapped while pouring. Remember, once the rubber has risen and fallen, it will not rise again.