Metal Finishing Guide Book


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spray electrostatically must always be conductive, either by virtue of the material from which it is made or through the application of a conductive pretreatment. Recent equipment developments now make it relatively simple to apply water-reducible coatings with electrostatic equipment. Both handheld and automatic systems are available. Since the material is highly conFig. 5. Gun should be moved parallel to surface ductive, it is necessary to isolate the being sprayed. material supply. The fluid supply will be highly charged, and it is, therefore, necessary to erect barricades to prevent physical contact with this portion of the system. Sometimes the added expense to apply this coating may not justify the material savings. Also, added maintenance — cleaning the hangers, supports, or conveyors — to assure conductivity to ground is necessary. All electrically conductive items, such as containers and spray equipment, within 10 ft of the spray area should be grounded to prevent static buildup. Operators grounding out any static buildup may experience a severe shock. COMPONENTS OF A SPRAY SYSTEM A spray finishing system consists of the following components: a spray gun, a compressed air supply, a fluid supply, and other necessary items such as hoses, fittings, regulators, and proper respiratory equipment Spray Gun Spray guns (see Fig. 4) are available in a wide variety of performance capabilities and costs. The three factors in order of importance are function, service, and price. As with anything, you pay for quality. Cheap spray guns, which may look like the expensive guns they imitate, have no place in a production finishing shop. Consult your spray gun supplier for correct air and fluid nozzle recommendations. After all, this is the part of the spray gun that develops the atomization desired. Compressed Air Supply The compressor you use does not make air, it only compresses it. The condition of your air supply usually tells a lot about the compressor and the air-supply lines. A compressor is rated for its volume, measured in cubic feet per minute (cfm), and its pressure, measured in pounds per square inch (psi). To assure maximum performance of any spray finishing system, the compressed air supply should always exceed the required psi and cfm requirements of its components. This is especially true when using HVLP spray guns. The rule of thumb is to expect about 4 cfm for Fig. 6. Fanning will result in an uneven coating. 178

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