Metal Finishing Guide Book


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gravity cup mounted to the gun or by a pressure feed device such as a pressure tank or pump. When the gun is triggered, the material is discharged through the fluid nozzle of the gun in the form of a liquid stream. Upon exiting the gun, this stream is immediately surrounded by a hollow column of compressed air, usually under high pressure, emitted from the center of the air nozzle of the gun. The action of this column of air on the fluid stream converts it into small droplets and imparts forward velocity to them. Additional jets of high-pressure compressed air from the face and horns of the air nozzle are directed into the droplets, forming even smaller droplets and an elliptical or fan spray pattern. The ability to control these forces at work at the head of an air spray gun is the key to a successful spray application. The proof that you have this control is in the quality of the spray pattern produced. HIGH-VOLUME LOW-PRESSURE ATOMIZATION Worldwide concern over increased air pollution has necessitated numerous changes, including in how we finish our products. HVLP air atomization and electrostatics are now the only accepted methods of production spraying in certain parts of the country. Although all HVLP spray guns operate with the same objective in mind, how they accomplish this goal may differ. First, air used in the atomization process reaches the HVLP spray gun's nozzle in one of four ways: (1) standard highpressure compressed air, which has its pressure restricted within the gun body; (2) standard high-pressure compressed air, which is assisted with a venturi feed and then filtered ambient air prior to its pressure restriction within the gun's body; (3) standard externally fed HVLP turbine air; and (4) compressor-assisted externally fed turbine air. Items 1 and 3 have seen the most growth and ultimate acceptance in recent years. All HVLP spray guns should operate at air pressures between 0.1 and 10 psi (at the air nozzle) and consume air volumes of 15���30 cfm to be considered true HVLP spray guns. Although some HVLP guns with internal restrictors can exceed 10 psi air pressure, it is up to the operator to follow local regulations when necessary. Advantages The benefits of HVLP atomization are improved transfer efficiency, often approaching 65���75%, compliance with local finishing regulations, a softer spray that penetrates easily into recesses or cavities, reduced material (costs) consumption as well as reduced spray booth maintenance and reduced hazardous waste. Turbine-operated HVLP systems enjoy great portability and ease of operation where compressed air is not available. HVLP spray guns with internal restrictors use existing air supplies, are easy to operate, and are low in cost. Disadvantages The most notable limitation to the HVLP process is that the finish quality from some HVLP spray guns is not as fine as conventional air spray. This may not pose a problem to some finishers, but to others it may mean additional polishing, a change in the material formulation, or switching to electrostatics. Turbine-generated HVLP systems may be expensive to purchase and to operate. HVLP guns using internal restrictors must have an adequate supply of clean and dry air to operate efficiently. Turbine guns use a larger air hose, which may be difficult to work with. Some high-volume production lines may find HVLP to 208

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