Metal Finishing Guide Book


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to be directly charged with electrostatics. In practice, that potential is rarely achieved unless the application system is very small because it is difficult to keep the electrostatic charge isolated. Fully isolated systems can store too much electrical energy and become dangerous. To prevent operator injury, such systems need to be caged and equipped with interlocks to prevent access while the system is operating. Unfortunately, this means that even minor maintenance to the equipment is impossible while any part of the system is operating at high voltage because all the equipment is electrically connected by the fluid hoses. This is also why only one handgun can be permitted in a completely isolated system. Indirect Charging Indirect charging avoids many of the problems of completely isolated systems, but at a price. Indirect charging systems charge the coating material between the nozzle of the atomizer and the part, rather than at the atomizer. This is done by placing the high-voltage electrode in the air stream near the nozzle but not in direct contact.The coating material particles pick up a charge after they leave the atomizer. Because the high voltage never directly contacts the application equipment, there is no opportunity for the charge to drain away down paint hoses. On the other hand, any charge inadvertently imparted to the application system drains away harmlessly to ground because the system is not isolated from ground. In fact these systems can, and should, be intentionally grounded to prevent storage of electrical energy. There is little capacitive storage of electrical energy in an indirect charged electrostatic system so any electrical hazard is greatly reduced. This means that safety caging and interlocks can be less intrusive, or eliminated completely. With handguns no longer connected electrically by their hoses, there is no need to limit the number of handguns in a particular application system. Several coating application equipment manufacturers offer atomizers specially designed for indirect charging. These devices position the electrostatic electrode away from the coating material stream so that there is no direct electrical contact between the application equipment and components charged with high voltage. Some conventional atomizers can also be retrofit with indirect charging apparatus, making the conversion to waterbornes easy and relatively inexpensive. The downside of indirect charging is lower than optimum TE. Indirect charging does improve TE over comparable nonelectrostatic systems. Unfortunately, tests prove that the TE with indirect charging is less than the TE that can be achieved by direct charging in any given application — that means using the same applicators, coating material, part shape, etc. The difference can be considerable, up to 40% improvement in TE in extreme cases. Even the best indirect charged systems rarely achieve TEs within 10 percentage points of what is possible for the same application but using direct charging for the electrostatics. To maximize TE, the coating material should be directly charged with electrostatic voltage, but to minimize shock hazards and operational problems, the size of the charged parts of the system should be minimized. This can be achieved by using a voltage block at each atomizer. Each atomizer then becomes a miniisolated system with no electrical connection to any other atomizer in the system. Voltage Blocks Voltage blocks are devices that allow coating material to pass through to the atomizer but prevent voltage from leaking back the other way. They allow coat201

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