Metal Finishing Guide Book


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Page 191 of 843

Fig. 3. Isolated electrostatic application circuit for waterborne and metallic coatings. tioned differently to the workpiece.Bells are positioned with their axis horizontal to the part, and disks are positioned vertically. A rotating disk or bell distributes a thin, even coating to the edge of the atomizer. There the coating is atomized either by the electrostatic force or centrifugal force. A low speed rotary atomizer utilizes almost all electrostatic forces, a high speed rotary atomizer relies on the centrifugal force of the atomizer to atomize the coating material. A DC high voltage charge is then supplied to the rotating atomizer, creating an electrostatic field between it and the grounded target object. The negatively charged paint particles are attracted to and deposited on the positive grounded workpiece. The forces between the charged particles and the grounded target are sufficient to turn normal overspray around and deposit it on the back surface of the target; therefore, a very high percentage of the paint particles are deposited on the part. Paint resistivity, often referred to as conductivity, is critical when spraying materials electrostatically. Waterborne materials are very conductive; therefore, measures such as voltage blocking devices,external charging probes, or completely isolating the fluid supply and fluid lines must be taken or the paint particles will not be able to maintain the electrostatic charge. Due to the low resistance of waterborne materials, all of the electrostatic voltage will drain off to ground and short out the system. If one of the three previous methods mentioned are not used, the paint particles cannot be charged electrostatically. 186

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