Metal Finishing Guide Book


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heating the surface to be coated. Additionally, electrostatic charging of conductive materials (i.e., blended metallic powders) can be difficult since they can short-circuit the applicator���s charging circuit. However, most equipment manufacturers provide electrodes outside the powder path to overcome this problem. Both positive and negative polarity electrostatic guns are available from most manufacturers to provide efficient charging of widely divergent coating materials. It is worth noting that 98% of all applicators used in powder coating operations are negative polarity devices. Code requirements insist that certain protection circuits be part of the system. Among these are current limitation to control arcing and grounding of all equipment and products that are coated to dissipate stored charges. System interlocks are required for automatic equipment. Guidelines for this equipment are listed in National Fire Protection Association Code (NFPA) 33. Tribo Guns Tribo-charged spray equipment uses the principle of frictional electrostatic charging. This type of charging is best explained by the following analogy: When you shuffle your shoes on a carpet in the winter, you create an electrostatic charge that is stored in your body. This charge is usually dissipated when you come into contact with a ground, such as a light switch. This phenomenon will only occur in a dry (not humid) environment. This is why we are not bothered by static electricity in the humid summertime, but only in the dry air of winter. Tribo-charge spray equipment will direct the powder stream through a path that it will tumble and rub against a dielectric surface within the applicator, yielding a frictional electrostatic charge on the powder particle. This path is accomplished by lengthening the powder route through the spray equipment in either a straight, radial, or oscillating path. The amount of electrostatic charge that builds up on the surface of the powder particle is a function of several variables, including (1) the amount of time the powder particle is subjected to the frictional charging apparatus; (2) surface area of the powder particle; (3) dryness of air the powder is transported with or comes into contact with; and (4) the type of resin material from which the powder is made. Fig. 2. Principle of corona charging. 244

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