Metal Finishing Guide Book


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Fig. 2. Faraday cage effect The electrostatic process is almost identical to the way a common magnet works. By creating an electrostatic field between a negatively charged paint particle and a positive grounded workpiece, the paint particles are attracted and deposit themselves onto the workpiece. The basic building block of electrical energy is the charged particle. All matter is made from electrically charged particles. These par- ticles are either neutral, negative, or positive. Back in the early days of particle charging, a process referred to as the Number One Process was developed by Harold Ransburg to charge paint particles. Paint particles were sprayed into an electrostatic field by conventional air spray guns. Two wire grids were aligned parallel to each other at a certain distance, then the parts were conveyed through these grids. At one end of the grids, atomized paint particles were sprayed into the electrostatic field. The paint particles would become negatively charged and would be attracted to the positively grounded parts. These wire grids are now the wire electrode in an electrostatic spray gun. The three most common ways of charging paint particles are the electrostatic spray gun, a rotary bell, or a rotary disk. All three of these methods work by the same common principle of the electrostatic field between the atomizer and the workpiece then introduce atomized paint particles into the field and they will be attracted to and deposit themselves on the positive grounded workpiece. With an air spray or an HVLP electrostatic spray gun, a high voltage DC charge is supplied to the applicator's nozzle electrode, creating an electrostatic field between the gun and the grounded target object. (See Fig. 4.) The coating materials are charged at the point of atomization. The charged paint particles are attracted to and deposited on the grounded target object. This electrostatic charge allows a more efficient, uniform application of the coating material to the front, edges, sides, and back of the product. The electrostatic forces allow for a high percentage of the charged paint particles to be deposited on the workpiece. The electrostatic process can also be used to charge paint particles using airless and air-assisted airless electrostatic spray guns. The only difference is the coating material is atomized by different methods. An air spray or HVLP electrostatic gun utilizes much lower air pressure to atomize the coating material, the airless and airassisted airless methods use a much higher pressure. Coating material is delivered at high pressure to the atomizer. There, the material is atomized by passing through a very small orifice under high pressure. The resulting spray mist particles then become electrostatically charged and are attracted to the workpiece in the same manner as electrostatic air spray or electrostatic HVLP. Today, rotary bells are generally about 1 to 3 in. in diameter and rotary disks are about 6 to 12 in. These atomizers operate on the same principle except they are posi185

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