Metal Finishing Guide Book


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Page 223 of 903

, and high-solids solvent borne coatings, enamels,lacquers, and two-component coatings can be applied electrostatically. The various types of electrostatic systems can apply coatings regardless of their conductivity. Waterborne and metallic coatings can be highly conductive. Solvent-borne coatings tend to be nonconductive. Any metallic coatings can contain conductive metal particles. These metallic coatings must be kept in circulation to prevent a short circuit in the feed line. As high voltage is introduced into the system, the metal particles can line up to form a conductive path.System modifications may be required because of coating conductivity to prevent the charge from shorting to ground. (See Fig. 3.) Operating Electrostatics Safely Electrostatic finishing is safe if the equipment is maintained properly and safety procedures are followed. All items in the work area must be grounded, including the spray booth, conveyor, parts hangers, application equipment (unless using conductive/waterborne coatings), Fig. 1. Electrostatic application circuit for solvent-based coatings and the spray operator. As electrical charges come in contact with ungrounded components, the charges can be absorbed and stored. This is known as a capacitive charge buildup. Eventually, enough charge is built up so that when the ungrounded item comes within sparking distance of a ground, it cand is charge as a spark. Such a spark may have enough energy to ignite the flammable vapors and mists that are present in the spray area. An ungrounded worker will not know that the capacitive charge has been absorbed until it is too late. Workers should never wear rubber- or cork-soled shoes, which can turn then into ungrounded capacitors. Special shoe-grounding devices are available. If workers are using hand-held guns, they should grasp them with bare hands or with gloves with cut-outs for fingertips and palms that allow adequate skin contact. Proper grounding of all equipment that is not used for the high-voltage process is essential. Grounding straps should be attached to equipment and connected to a known ground. A quick inspection of all equipment, including conveyors and part hangers, can reveal improper grounding. Good housekeeping can pay dividends. Removing paint buildup from parts hangers can help ensure that workpieces are grounded. Ungrounded objects, such as tools and containers, should be removed from the finishing area. PAINT PARTICLE CHARGING Electrostatic charging of paint particle got its start back in the early 1950s. Engineers were looking for methods to reduce the cost of finishing products. Harold Ransburg, the inventor of the electrostatic process, reasoned that since unlike electrical charges are attracted to each other, the same idea would apply to charged paint particles and a part to be painted. Everyone���s heard the saying that ���opposites attract, and likes repel.��� This is true with both a magnetic field and with the electrostatic process of charging paint particles. 218

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