The science and technology of electrostatic powder spraying, transport and coating

The electrostatic powder coating process is reviewed. Three basic regions of a powder coating system are considered. The charged powder source region embraces corona and triboelectric guns. These are reviewed and particle charging processes considered. The second region where charged powder transport to the workpiece occurs is one in which aerodynamic, space charge, and gravitational forces act to determine particle trajectories. With corona sources ions generally outnumber charged particles by 10 to 1 so that the space charge due to charged particles is negligible compared to that of ions. Charged particle deposition onto the workpiece, the third region, is affected by the electric field of particles already deposited. Back ionisation may occur if ions are present and lead to poor coating finish. The adhesion of charged particles to a workpiece is often greater than expected by consideration of image forces alone.