Electrostatic powder coating of insulating surfaces using an alternating polarity internal corona gun

An internal-corona (IC) gun operated at 1-Hz alternating polarity (AP) was used in an attempt to powder coat insulating plastic plates without using conductive primers, metal backings or ion "backings." The IC gun was effective in charging the powder particles and emitted very few free ions, but it required frequent cleaning. The Q/M ratios obtained using DC voltage were very good, with negative polarity better than positive. The use of charge separation electrodes verified that the 1-Hz AP gun emitted "waves" of oppositely charged particles. The charged powder average velocity between the gun and the target (3.8 m/s) was calculated from the measured transit time (0.13 s), and was dominated by the airflow. The current waveform produced by the charged particle motion yielded results consistent with the Q/M data and the measured transit time. The measured saturation surface charge densities (/spl sigma//sub s/) were used together with the total Q/M ratios to calculate the saturation surface mass densities, which were in good agreement with the measured values (0.3-0.5 g/m/sup 2/).