Vacuum pumps are often classified in the flow capacity which is indicated as CFM (cubic feet per minute). This refers to the rate at which the pump is capable of moving or withdrawing air and is most often measured at zero pressure. A pump rated at 1 CFM will be fine for flat panel jobs in vacuum bags up to 4 'x 4'. For bags up to 4 'x 8' a pump of 3 CFM or greater is required.
If your vacuum press is going to pull down curved plate shapes or folded laminations, the general rule is that the pump should be three times faster. This means that a pump of 3 CFM or greater is best for vacuum bags up to 4 'x 4' for this type of use. Why? Because there is, on average, 3 times more free air inside a bag containing a folded lamination that is inside a bag containing a flat panel.
Vacuum pumps are also classified by their maximum vacuum level attainable at sea level, which is often expressed in inches of mercury or "Hg". For vacuum coating, the minimum acceptable vacuum level is 18 "Hg. The ideal vacuum level is 21" Hg. At the upper end, the maximum vacuum level for sheet metal work is 25.5 "Hg. Anything over this amount is not only excessive, it is also harder on the pump. Also, extreme vacuum levels can cause sheet metal To develop small glue pustules On the face of the plate.
Types of vacuum pumps
The next consideration in constructing a pump-based vacuum pressure system is the style of the pump. Here are some of the options:
• Diaphragm pumps are very quiet and durable. This type of pump is oil free and usually has a small footprint. They are pumps ideal for a vacuum system. The typical CFM rating is 1 to 3.
• Piston pumps are not as quiet as diaphragm pumps, but they are so durable. They are almost always without oil and also work well for a vacuum press. The typical CFM range is 2 to 5.
• Oil bath pumps are less noisy than piston pumps but not as quiet as diaphragm pumps. This type of pump has a tendency to emit a plume of oil in the air. To the best of my knowledge, there is no way to contain the oil smoke. For most users, this can be a huge problem that causes problems with the finish applied to the veneer project or any other unfinished project in the immediate vicinity of the pump. This type of pump requires occasional oil changes and can draw a large amount of amperage. Generally, these pumps range from 3 to 6 CFM.
• Refrigerant compressors can also be used for limited vacuum press work. They are quiet, but slow to pull a complete vacuum.