Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 147 of 331

ed system is that every atomizer can operate at a different voltage, or at zero voltage, as desired. For example, in-plant experience might show that the rotary atomizers in a paint line run best at 60,000 V, but the handguns perform better at 45,000 V. With voltage blocks, the handguns and the rotary atomizers can run at different voltages, yet all can be supplied from a common paint distrib- ution system. Finally, in an application system for waterbornes and using voltage blocks, any atomizer in the system can be shut down and repaired or changed out, even though the other atomizers are operating at high voltage. The ease of access to production application equipment is comparable between a voltage-blocked wa- terborne system and the solvent-based coating material system it replaces. CONCLUSION An often unstated goal when converting a coating application system to water- bornes is to disrupt "the way it's done now" as little as possible, particularly if the existing system has good equipment and is performing well. That goal is not out of reach because the existing process, and much of the existing equipment, can often be used for waterbornes. Usually only the atomizers will need to be mod- ified for waterbornes, or replaced with atomizers specifically designed to handle waterborne materials. The remaining equipment and distribution system can be reused unless made of materials that will corrode in waterbornes or be dam- aged by exposure to them. Well-engineered conversions from solvent-based coating materials to water- bornes result in the highest possible operating efficiency at low cost and with max- imum operator safety. The operating cost, in terms of TE, should be about the same as that of a good solvent-based paint system. To get high TE, electrostat- ics must operate at peak efficiency. This means directly charging the material with electrostatic voltage, but limiting the hardware that gets charged. Voltage-blocking devices confine high electrostatic voltage to only the atom- izer and hoses to the atomizer. This means that the rest of the coating material application system can be the same or similar to the system before the conver- sion is made. The electrostatics will still operate at high efficiency because the coat- ing material can be directly charged so the TE will be comparable before and af- ter conversion. Because system capacitance, or the capacity to store electrical energy, can be controlled to "safe"' levels, safety issues with the converted systems are not prohibitive. In other words, a voltage-blocked waterborne system is as close as possible to the solvent-based material system it replaces with a coating material that conducts electricity. To summarize, here is how to convert an application system from solvent-based coating materials to waterbornes: (1) Reuse the existing process and hardware if it is up to date and performing well for the existing solvent-based system. Change com- ponents where materials are not compatible with waterbornes. (2) Turn each at- omizer into a mini-isolated system by installing a voltage block in the coating ma- terial hose, as close to the atomizer as possible. Directly charge the material for max- imum TE. (3) Lower the voltage to maximize TE, extend equipment life, and reduce shock hazard. (4) Take advantage of the fact that waterbornes conduct electricity. Remove the electrostatic cables from the atomizers and charge at the voltage blocks. Cables will last longer and the guns will move easier. 146

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