Vacuum Bagging Theory       

An ideal composite layup minimizes the amount of resin because resin by itself adds weight without adding strength. When doing a composite layup by hand, it is hard to get the correct minimum amount of resin because you need enough to soak the cloth and the cloth doesn't lay completely flat against the surface. The solution to this is vacuum bagging.

Vacuum bagging uses atmospheric pressure to press the cloth tightly against the surface being covered so that the excess resin is squeezed out and soaked up in a disposable outer wrap. This technique requires a vacuum bag, vacuum pump capable of pulling a significant vacuum (at least 25 inches of mercury) and various accessories and supplies. Thus vacuum bagging has been mostly restricted to large commercial use and a few enterprising hobbyists. 
Vacuum bagging requires that the part being laminated be covered with cloth (as in hand layups). The part is wrapped with a thin film which is porous and will not stick to the resin, called "release," and a thick layer of absorbent material, called "breather." This whole assembly is then inserted into the vacuum bag and the air inside is removed by a vacuum pump. Because the air inside the vacuum bag is removed, the air pressure from the atmosphere outside the bag pushes tightly from all sides, pressing the bag against the breather. The excess resin is squeezed out of the cloth, passes through the release and is soaked up by the breather. The breather also allows the air to flow away from the tube and out of the bag.

Vacuum bagging is a technique employed to create mechanical pressure on a laminate during its cure cycle. Pressurizing a composite lamination serves several functions. First, it removes trapped air between layers. Second, it compacts the fiber layers for efficient force transmission among fiber bundles and prevents shifting of fiber orientation during cure. Third, it reduces humidity. Finally, and most important, the vacuum bagging technique optimizes the fiber-to-resin ratio in the composite part. These advantages have for years enabled aerospace and racing industries to maximize the physical properties of advanced composite materials such as carbon fiber, Kevlar, and S-glass cloths.