Vacuum chamber
A vacuum chamber is a rigid enclosure from which air and other gases are removed by a vacuum pump. The resulting low pressure, commonly referred to as a vacuum. A vacuum environment allows researchers to conduct physical experiments or to test mechanical devices which must operate in outer space (for example) or for processes such as vacuum drying or vacuum coating. Chambers are typically made of metals which may or may not shield applied external magnetic fields depending on wall thickness, frequency, resistivity, and permeability of the material used. Only some materials are suitable for vacuum use.
Chambers often have multiple ports, covered with vacuum flanges, to allow instruments or windows to be installed in the walls of the chamber. In low to medium-vacuum applications, these are sealed with elastomer o-rings. In higher vacuum applications, the flanges have hardened steel knives welded onto them, which cut into a copper gasket when the flange is bolted on.
A type of vacuum chamber frequently used in the field of spacecraft engineering is a Thermal Vacuum Chamber, which provides a thermal environment representing what a spacecraft would experience in space.
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[edit] Vacuum Chamber Materials
Vacuum chambers can be constructed of many materials. "Metals are arguably the most prevalent vacuum chamber materials."[1] The strength, pressure, and permeability are considerations for selecting chamber material. Common materials are:
- Stainless Steel
- Aluminum
- Mild Steel
- Brass
- High Density Ceramic
- Glass
- Acrylic
[edit] Vacuum Degassing
"Vacuum degassing is the process of using vacuum to remove gases from compounds which become entrapped in the mixture when mixing the components."[2] To assure a bubble-free mold when mixing resin and silicone rubbers and slower setting harder resins, a vacuum chamber is required. A small vacuum chamber is needed for de-airing (eliminating air bubbles) for materials prior to their setting. The process is fairly straight forward. The casting or molding material is mixed according to the manufacturers directions.
[edit] The Process
Since the material will expand 4-5 times under a vacuum, the mixing container must be large enough to hold a volume of four-five times the amount of the original material that is being vacuumed to allow for the expansion. If not it will spill over the top of the container requiring clean-up that can be avoided. The material container is then placed into the vacuum chamber; a vacuum pump is connected and turned on. Once the vacuum reaches 29-inches (at sea level) of mercury, the material will begin to rise (resembling foam). When the material falls it will plateau and not rise any more. The vacuuming is continued for another 2-3-minute to make certain all of the air has been removed from the material. Once this interval is reached the vacuum pump is shut off and the vacuum chamber release valve is opened to equalize air pressure. The vacuum chamber is opened and the material is removed and ready to pour into the mold.
To keep the material air free it must be slowly poured in a high and narrow stream starting from the corner of the mold box, or mold, letting the material to flow freely into the box or mold cavity. This method will usually not introduce any new bubbles into the vacuumed material. To insure that the material is totally devoid of air bubbles you can place the entire mold/mold box into the chamber for an additional few minutes. This will assist the material to flow into difficult areas of the mold/mold box.
[edit] High Altitude Vacuum
Though a vacuum of 29-inches of mercury is desired for de-airing most mold making and casting materials, it cannot be achieved at higher elevations. It is a sea level target. For example, in Denver (The mile-high city) only about 24-inches of mercury can be achieved even with the most efficient vacuum pump. In those instances, vacuuming will be required for a longer period to achieve proper de-gassing.
[edit] Vacuum Drying
Water and other liquids may accumulate on a product during the production process. "Vacuum is often employed as a process for removing bulk and absorbed water (or other solvents) from a product. Combined with heat, vacuum can be an effective method for drying."[3] [4]
[edit] See also
[edit] References
- ^ Danielson, PHil. "Choosing the Right Vacuum Materials". The Vacuum Lab. http://www.vacuumlab.com/Articles/VacLab36.pdf. Retrieved 2/10/12.
- ^ "Vacuum Degassing Epoxy & Silicone". LACO Technologies, Inc.. http://www.lacotech.com/ProductFiles/SMT-04-1013%20revA1%20(App%2006-14%20Vacuum%20Degassing%20Epoxy%20and%20Silicone).pdf. Retrieved 2/10/12.
- ^ "Vacuum Drying". LACO Technologies, Inc.. http://www.lacotech.com/ProductFiles/SMT-04-1011%20revA1%20(App%2005-07%20Vacuum%20Drying).pdf. Retrieved 2/10/2012.
- ^ Danielson, Phil. "Desorbing Water in Vacuum Systems: Bakeout or UV?". The Vacuum Lab. http://www.vacuumlab.com/Articles/VacLab22%20.pdf. Retrieved 2/10/12.
