The effect of ring electrodes attachment to a corona gun on control of free ion concentration and back corona for improving powder paint appearance

In a powder coating process using corona guns, only a small fraction (6%-10%) of the total ions generated during the corona discharge contribute to powder charging. Most of the remaining free ions deposit on the powder layer and on the grounded workpiece, increasing the charge-to-mass ratio (Q/M) of the powder layer, which contributes 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 to the ring electrode and the powder did not deposit on the ring. 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 level of Q/M acquired by the powder. By adjusting the position of the ring electrodes and the gun voltage, Q/M could be controlled from -0.15 to -2.15 /spl mu/C/g, which shows a much wider range of Q/M control compared to that which can be achieved with corona gun without a ring electrode. However, the application of a ring electrode reduced the electric field for powder deposition, resulting a low first-pass transfer efficiency (FPTE). While the magnitude of the electric field near the target decreased, the distribution of the field near the corona gun covered a wide area, thus expanding the angle of the conical powder spray pattern from the nozzle. The electric field modifications introduced by the grounded ring electrodes were simulated by using Lorentz 2D software for different geometrical configurations. The reduction of field intensity between the gun and the substrate and a strong radial field introduced by the ring, explain the cause of the FPTE reduction and the expansion of the conical powder spray pattern. The electrical field distribution around the corona electrode and the target panels are presented along with the experimental data on ion currents for different applied voltages to the corona gun and for different ring electrode positions. The appearance of the cured powder films was found to be influenced by the voltage applied to the corona gun, the type of ring and its position relative to the corona tip. The best appearance was obtained by using a ring with a smooth outer surface biased at -20 kV, and with the corona voltage at -100 kV.