Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 143 of 331

coating applicators to improve the TE of their operations. Until recently, most coating materials were solvent based and did not conduct elec- tricity readily. This made it easy to use electrostatics with these materials by simply placing a high-voltage electrode in the coating material at the atomizer nozzle. The coating material picks up a static charge of electricity as it is atomized. Waterborne coating materials conduct electricity much more readily than solvent-based materials, however, and the electrical charge drains off down the paint hoses. None of the charge gets into the particles of atomized coating ma- terial, so there is no electrostatic attraction between the particles and the part be- ing coated. Without electrostatics, TE drops to unacceptably low levels. The challenge in converting to waterbornes is getting comparable TE with these conductive materials as with the nonconductive solvent-based materials they replace. This means finding a way to get the electrostatic voltage into the at- omized particles rather than letting it drain away through hoses and equipment made conductive by the waterborne coating material. Any equipment that con- tacts an electrical ground, such as a pipe or a damp concrete floor, provides an electrical pathway that drains off the electrostatics. This means that the elec- trostatics won't work in a waterborne system unless all the wetted equipment is isolated from potential grounds. SYSTEM ISOLATION Waterborne systems are commonly isolated in one of three ways. (1) Complete isolation of all equipment that contacts wet coating material. (2) Isolation of the charging electrode from the wet coating material. (3) Isolation of only the at- omizer and its feed hose by using a voltage blocking device. Each method has ad- vantages and disadvantages. Complete Isolation The advantage of completely isolating the entire application system is that the coating material can be directly charged with electrostatic voltage. If isolation is successful, the resulting TE will be the highest possible for the specific applica- tion. The exact TE that can be achieved in a specific application depends on the part geometry, line speed, application equipment, and other factors, the same as it would with a solvent-based coating material. To isolate a complete waterborne system, every pump, tank, pipe, atomizer, or other piece of equipment that sees wet coating material must be set on a plastic table or hung from a plastic rod or stuffed in a plastic pipe sleeve. Suitable com- mon and inexpensive plastic materials for this purpose include polyvinyl chlo- ride (PVC), polyethylene, and polypropylene. Teflon, some nylons, and Delrin are also good isolation materials for high voltage, but are relatively expensive. Dry air is one of the best isolation materials. A 12-in. air gap will isolate equip- ment charged with electrostatic voltage, except in cases of extreme humidity. Air has some advantages over plastic as an isolator for an electrostatic system. A paint spill down a plastic table leg can make it conductive. Humidity in a thick coating of dust on a plastic pipe can make it conductive. The disadvantage of air as an isola- tor is that it is an easily penetrated barrier between a charged part and a grounded part — too easily penetrated by personnel or by loose hoses or other equipment. Unfortunately, it's almost impossible to design an isolated system that won't accidentally ground out, and a system that grounds out is less efficient. In one case, for example, an 18-in. long, hollow PVC table leg provided a direct path to 142

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