Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 159 of 331

amount of time the powder particle is in the field. Also of importance is the surface area of the powder particle, as finer powder particles hold less electrostatic charge. The following equations (see Fig. 2) best explain how the powder is charged: Field Strength: E=V/d Charge on Particle: Q= 1/2 CEt2 Notice that some factors are more important that others. For instance, elec- trostatic field strength is directly proportional to applicator electrode voltage. Also, the distance between the part and the applicator (sometimes called the target dis- tance) will directly affect electrostatic field strength. The charge on the powder particle (which causes the attraction) is most affected by the amount of time the particle is in the field (by its square). The time and field strength will determine how much powder is attracted to the part (i.e., first pass transfer efficiency). The time the powder particle is within the electrostatic field is most easily controlled by adjusting the velocity of the powder pumped through the gun, or applicator, and reducing the speed of the applicator motion. It is a known fact that systems that use reduced powder velocity and slow gun mo- tion will provide the best coating efficiency with the least effort. The powder coating process is most often used to apply a charged dielectric material (powder coating) and onto a conductive (grounded) part. However, electrostatic powder coating on nonconductive materials (i.e. plastics, rubber, glass, etc.) can be performed using a conductive primer or aiding powder attraction by heating the surface to be coated. Additionally, electrostatic charging of con- ductive materials (i.e., blended metallic powders) can be difficult since they can short-circuit the applicator's charging circuit. However, most equipment man- ufacturers 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. Fig. 2. Principle of corona charging. 158

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