Metal Finishing Guide Book

2013

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When the unit is charged to approximately 120 kV DC, a current flow develops between it and the grounded object. This current flow pulls the particles of coating off the spinning disk or bell and carries them to the product. With the introduction of paints high in solids these atomizers had to use higher speeds. Thus, electric motors were replaced with high-speed air-driven turbines. These high-speed rotational atomizers (10,000–60,000 rpm) use centrifugal force to atomize the coating and carefully directed compressed air to give the atomized paint particles forward velocity and direction. A voltage differential (120 kV DC) then takes over and allows the charged particles to migrate to the grounded part. Disk versus Bell The spray pattern needed for the kind of parts being finished, the shape of the product, and racking, or the existing facility, determine whether a disk or bell is used. For long, thin parts and flat stock, the reciprocating disk directs the horizontal spray in the thin, narrow plane that is needed to provide a fine even coat in conjunction with the omega-style conveyor and booth. For shorter, wider parts, the donut-shaped cloud produced by a bell directs a fine even coat and is mounted horizontally or used with a vertical short-stroke oscillator in a standard overhead conveyor system and booth. Advantages The principal advantage of electrostatic spraying is the savings in material and labor. This process can provide transfer efficiency ratings from 65 to 95% if all conditions are favorable. Air velocity in the spray booth where electrostatic spraying is performed may be reduced from 100 to 60 ft/min. This means a 40% reduction in air makeup costs as well as reduced emissions. Disadvantages The old saying "Where the current goes, so goes the paint" applies here. The Faraday cage effect will make painting in some corners and recesses difficult, thus manual nonelectrostatic touch up will be necessary. Contrary to some beliefs, electrostatics will not improve adhesion or provide superior gloss, nor will it always provide a complete wrap effect. Also, some users may find excessive buildup on corners and edges unacceptable. The parts you want to Fig. 4. Typical air spray guns. 177

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