Metal Finishing Guide Book


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is transported above the fluidized bed and the powder is attracted to it. The part requires no preheating prior to being placed above the bed. Powder is attracted to the part by an electrostatic charge on the powder particle. This electrostatic charge is developed in an electrostatic field either above or in the fluidized bed. Film thickness on the part now is controlled within tighter tolerances not only by the amount of time the part is in the fluidized bed but also according to how much electrostatic charge is on the powder particle. Sometimes, heat still is used in this process to overcome Faraday cage problems caused by part configuration. This process routinely applies powder from 5 to 10 mils thick. Electrostatic fluid bed application is used for coating electrical motor armatures. These require a high dielectric strength coating with close film-thickness control to allow the wire to be wound properly. SPRAY APPLICATION Applying powder coating with electrostatic spray equipment is broken down into two types. In each case electrostatics must be used to attract powder to the part. There is no mechanical attraction or adhesion to hold powder to the part as seen in liquid spray systems. The two types of electrostatic spray equipment are corona-charged spray guns and tribo-charged spray guns. Corona Guns This device uses an electrostatic generator to create an electrostatic field between the gun and a grounded part. Powder is sprayed through the field, picks up an electrostatic charge, and is attracted to the part. The amount of charge that is transferred on the surface of the powder is a function of electrostatic field strength and the 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, electrostatic field strength is directly proportional to applicator electrode voltage. Also, the distance between the part and the applicator (sometimes called the target distance) 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 motion 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 209

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