The characteristics of particle charging and deposition during powder coating processes with coarse powder

With coarse polyurethane powder, the characteristics of particle charging and deposition during corona charging processes of powder coating were experimentally investigated by measuring the charge-to-mass ratio (Q/M) and mass-to-surface ratio (M/S) of deposited particles. It was disclosed that the deposited particles present a cone-shaped distribution on the substrate, which is mainly a direct result of the inhomogeneous concentrations of in-flight charged particles, but the edge effect leads to a rise in particle accumulation in the fringe region. It was further disclosed that the particle deposition efficiency not only has a strong dependence on the particle primary charging efficiency but suffers a strong influence from back corona as well. The highest deposition efficiency is a compromise between the primary charging efficiency and the intensity of back corona. In addition, the number of overall deposited particles increases but the deposition rate decreases with spraying duration. On the other hand, it was demonstrated that the secondary charging mechanism dominates the characteristics of the charge-to-mass ratio of local deposited particles, but may suffer some influence from their particle sizes. For this coarse powder, the charge-to-mass ratio distribution presents a concave characteristic. Furthermore, the average charge-to-mass ratio of the overall deposited particles increases with charging voltage, due to the improvement in their charging efficiency, but decreases with spraying duration, due to the intensifying effect of back corona with the growing deposited layer. Significantly, this study provides a deeper insight into the charging and deposition mechanisms during powder coating processes and will be of great help for the improvement in related technologies of corona charging as well.