Metal Finishing Guide Book


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Charged powder particles and excess ions are both deposited on the grounded workpiece. The charged powder particles are held onto the workpiece electrostatically until it is transported into an oven for curing. Heat causes the powder particles to flow together and fuse into a continuous film. The Pauthenier equation describes the charge, over time, accumulated by a powder particle exposed to a corona discharge: Q(t) =Ar2Et/(t+␶) where, A = a constant, which depends on the particle composition, r = particle radius, E = electric field strength = electrode voltage/gun-to-workpiece distance, t = time, and ␶ = charging time constant. The charging performance of a corona gun can be affected by the gunto-workpiece distance. Today, corona guns are available that use specially designed high-voltage generators or gun-control modules to reduce or eliminate this dependency. The high-voltage generator may be located remotely from the gun in the gun-control module or, alternatively, part of it may be located within the gun body itself. When the high-voltage generator is located in the gun-control module, a high-voltage cable is used to transmit the power to the gun. When a portion of the high-voltage generator is located within the gun body, a lowvoltage cable is used. For spraying most types of finishing powders, a negative-polarity voltage is produced in the high-voltage generator. This results in the powder particles accumulating a negative charge. Positive-polarity generators are also typically available as an option and are used primarily for charging nylon powders. Two basic types of spray heads are available for shaping the powder particles into a cloud as they exit a corona gun. They are called conical deflectors and flatspray nozzles. Conical deflectors shape the powder cloud into a circular, hollow, dome-shaped pattern. These spray heads can produce a large, low-velocity spray, 360° SD in circumference. They are best for simple-shaped workpieces and can produce a very high transfer efficiency. Flat-spray nozzles typically have a single slot through which the powder particles exit. The resulting powder cloud is fanshaped from the side, but has a narrow width. These nozzles may have a higher velocity than a conical spray head and are, therefore, best for spraying parts with deep recesses and corners. Many equipment manufacturers design their electrostatic corona powder spray guns to comply with the different codes governing the manufacture and use of these products worldwide. Some of the worldwide agencies that test and issue approvals on these spray guns are Factory Mutual (United States), Canadian Standards Association, European Committee for Electrotechnical Standardization (CENELEC), and the Research Institute of Industrial Safety (RIIS, Japan). In the United States, a local fire marshal would typically look for "Approved" equipment or compliance with National Fire Protection Association (N.F.P.A.) Standard 33, Spray Application Using Flammable and Combustible Materials, before permitting an installation to start production. TRIBO GUNS Tribo or triboelectric charging guns charge powder particles as a result of the intimate contact and subsequent separation of the powder particles from the gun walls. The word "tribo" comes from the Greek word tribein meaning to rub. The tribo charging process is illustrated in Figure 2. 220

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