Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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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 per- centage 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 elec- trostatic 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 mini- isolated 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 at- omizer but prevent voltage from leaking back the other way. They allow coating material to flow from the grounded pumps or kitchen to charged atomizers, yet block voltage from leaking back from the atomizers to the pumps or kitchen. This means that the hardware in the pump house and distribution system can be vir- tually the same as for a conventional solvent-based system, or for an indirect charged waterborne system. Since the primary advantage of voltage block technology is that it limits the amount of hardware at high voltage, it is important to install these devices as close to the atomizer as possible. The connecting hoses between the voltage block and the atomizer are at high voltage, so keeping them short minimizes both the ca- pacitance and the opportunity for accidental grounding. A voltage block for one atomizer is compact, requiring about as much space as a small electrical control box, so it can be mounted inside the spray or ventilation booth close to the atomizer. The mini-isolated systems created by voltage blocks do not have the prob- lems found in large isolated systems because less hardware is charged with elec- trical energy. Capacitance is greatly reduced, making the system inherently safer. Safer systems mean easier access to the inside of the spray booths. Often a sim- ple guard rail and warning sign can replace elaborate caging and interlocks. Volt- age leakage problems are minimized, since only the atomizer and a short hose are charged, making it easy to keep the TEs up to a high level. By isolating atomiz- ers from each other, mini-isolated systems have some unexpected advantages. First, the NFPA limitation concerning handguns no longer applies. Each handgun is independently isolated from every other handgun so the voltage to idled guns can be turned "off." In fact, spraying waterbornes with a handgun and voltage block can be easier than spraying the old solvent-based material with the handgun. Solvent-based materi- al is charged at the gun barrel so a high-voltage cable to the gun is required. Since waterborne coating material conducts electricity, however, it can be directly charged at the voltage block and the cable to the gun can be eliminated. With the cable gone, the gun feels lighter and the hose bundle flexes more easily. Even automatic atomizers, such as rotaries or disks, require less maintenance if the coating material is charged at the voltage block rather than at the atomizer, as it was when spraying solvent-based material. This is because the high-voltage cables last longer when they don't get flexed over and over by the motion of the gun mover or robot. A second unexpected advantage of making each atomizer into a mini-isolat- 145

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