Metal Finishing Guide Book


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Fig. 5. Typical transefer efficiencies for various electrostatic and conventional spray processes. desirable characteristics being medium to high delivery rates, ability to spray heavy viscosities at low velocities, and high transfer efficiency. Electrostatic Electrical Atomization Electrostatic electrical atomization is accomplished by using a rotary bell on the end of a gun to evenly dispense paint to the edge of the bell. Once the coating material reaches the edge of the bell it is introduced to an electrical charge. The electrical charge at the sharp edge (approximately 100 kV) causes paint of a medium electrical resistance range (0.1 to 1 megohms) to disperse onto the product. The pure electrical application is a slightly slower process than an air spray or air-assisted airless technology and requires a rotational type spray paint technique, due to the bells spray pattern, but is the most transfer efficient spray gun process in the industry today. The ultrasoft forward velocity of the spray pattern achieves transfer efficiencies of nearly 100% on most products. This high transfer efficiency spawned the industry of painting and refurbishing machinery and furniture in place. Electrostatic Rotary-Bell-Type Atomization An electrostatic bell atomizer is a high-speed rotary bell that uses centrifugal force as well as electrical atomization to atomize material and efficiently transfer material from the bell edge to the target being painted. (See Fig. 6.) The bell is used on a turbine motor where the pattern is carefully directed by the use of compressed air,introduced to the pattern at the edge of the bell cup. The compressed air gives the material forward velocity to aid in penetrating recessed areas. The bells are usually mounted stationary or reciprocated to coat products on straight line conveyors. The bells may also be positioned on both sides of the conveyor. Rotary-bell-type atomization provides transfer efficiencies in the 70 to 95% range. Electrostatic Rotary-Disk-Type Atomization An electrostatic rotary-disk atomizer is a high-speed flat rotary atomizer that uses centrifugal force along with electrical atomization to atomize coating material and efficiently transfer the material from the disk edge to the target being painted. The 189

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