Particle Charging and Absolute Measurement of Charge to Mass Ratio

Commercial electrostatic powder coating techniques have been used since the early 1960' s in various operations using numerous powders and a variety of coating systems. The success of the process, apart from that inherent in the workpiece geometry, is determined by the efficiency of powder charging and its subsequent transportation to close proximity of the workpiece where electrostatic forces ensure good adhesion of the powder to the grounded workpiece. Since particle transportation is normally adequately provided by an air stream, the success of coating generally depends upon the efficiency of the gun corona charging system. Traditionally, the charge-to-mass (q/m) ratio of a powder has been used as a measure of the system's performance. However, meaningful interpretation of this method depends upon the effective removal of free ions present in the powder cloud. In the past, this was accomplished by placing a grounded grid between the corona gun and the charge/mass measuring apparatus, usually a Faraday cup. The inherent weakness of this method, however, is that back ionization will occur on the grid after a relatively short time, particularly with highly resistive powders. Accurate and reliable qim measurements are therefore often difficult to establish. To overcome this limitation, a technique has been developed which removes the free ions from the charged powder cloud and, at the same time, eliminates all back ionization on the ion screen.