Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Fig. 3. Powder delivery system. Code requirements insist that certain protection circuits be part of the system. Among these are current limitation to control arcing and grounding of all equip- ment and products that are coated to dissipate stored charges. System inter- locks 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 ac- complished by lengthening the powder route through the spray equipment in ei- ther 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 vari- ables, including (1) the amount of time the powder particle is subjected to the frictional charging apparatus; (2) surface area of the powder particle; (3) dry- ness 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. 159

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