Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 158 of 331

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 electro- static 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 configura- tion. 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 di- electric strength coating with close film-thickness control to allow the wire to be wound properly. SPRAY APPLICATION Applying powder coating with elec- trostatic spray equipment is broken down into two types. In each case elec- trostatics must be used to attract pow- der to the part. There is no mechani- cal attraction or adhesion to hold powder to the part as seen in liquid spray systems. The two types of elec- trostatic spray equipment are coro- na-charged spray guns and tribo- charged spray guns. Corona Guns This device uses an electrostatic gen- erator 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 sur- face of the powder is a function of electrostatic field strength and the 157 Oilfree Scroll INNOVATION REAL

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