Corona guns with ring electrodes: reduction of free ion concentration for back corona control in powder coating

In a powder coating process with corona guns, only a small fraction (6-10%) of the total ions generated during the electrostatic discharge contribute to powder charging. Most of the remaining free ions deposit on the powder layer on the grounded workpiece and increase the charge-to-mass ratio (Q/M), contributing to the early onset of back corona. To reduce the number of free ions reaching the powder coated work piece, two different ion traps made of copper ring electrodes were tested. The first ring had a smooth surface and the second ring had 11 sharp points for concentrating electric field lines. Adding the sharp points was found to provide self-cleaning properties from powder deposition. By increasing the distance from the ring electrodes to the corona gun electrode, or by increasing the powder flow rate, it was possible to control the amount of QIM acquired by the powder. By adjusting the position of the ring electrodes, QIM could be controlled from -0.15 to -0.65 /spl mu/C/g. However, the application of a ring electrode decreased the powder first pass transfer efficiency (FPTE) while expanding the angle of the conical powder flow pattern. The electric field modifications, due to the grounded ring electrodes, were simulated by using Lorentz 2D software and are considered to be the cause of FPTE reduction and the expansion of conical powder spray pattern. The electrical field intensity distributions around the corona electrode and the target panels are presented along with the experimental data on ion currents for different applied voltages to corona gun and the ring electrode position.