Causes and treatment of powder dropping after powder spraying coating

Causes and treatment of powder dropping after powder spraying coating

General coating production line manufacturers answer that the main factor affecting the acceptance and retention of charge of powder particles is the dielectric constant of the powder. The lower the dielectric constant of the powder, the easier it is to charge the particles, but the easier it is to lose the charge. This is reflected in the weak adsorption force of the powder on the workpiece, and the powder will fall out if it is slightly vibrated. For electrostatic spraying powder coatings, high dielectric constant should be used as much as possible, which will greatly improve the adsorption force of the powder.

From electrostatics, it can be seen that the distribution of surface charge of charged isolated conductor is related to the radius of surface curvature. The charge density at the place with large curvature (i.e. the place with sharp surface) is large, and the electric field intensity in the nearby space is also large. When the electric field intensity reaches enough to ionize the surrounding gas, the upper end of the conductor will discharge. If it is a negative high voltage discharge, the electrons leaving the conductor will be accelerated by a strong electric field to collide with air molecules, ionize air molecules and produce positive ions and electrons. The newborn electrons are accelerated and collided again, making the air molecules form an "electron avalanche" process. The mass of the electron is very small. When it rushes out of the ionization region, it is soon attracted by gas molecules much heavier than it, and the gas molecules become free negative ions. Under the action of electric field force, the negative ions rush to the positive electrode and produce a layer of halo at the ionosphere, that is, the so-called halo discharge. When the powder passes through the periphery of the corona, it will be charged by the collision of negative ions running to the positive electrode.

Most industrial powder coatings are polymer insulators with complex structure. Negative ions can be adsorbed to this part of the powder surface only when there is a position suitable for receiving charge on the powder surface. For negative ions, this part can be a positive charge impurity in the powder composition or a potential energy pit in the composition, or it can be purely mechanical. However, no matter which mechanism causes the adsorption, the deposition of ions on each powder is not easy. The surface resistance of powder particles is very high, and the charge will not be redistributed due to conductivity, so the surface charge distribution is uneven.

Powder coating particles are negatively charged near the electrode due to corona discharge. When the powder particles just leave the muzzle and are blown out close to the workpiece (positive electrode) by the conveying force of compressed air, the coating is firmly adsorbed on the workpiece by the guidance of electric field force. Generally, the coating thickness can reach 50 ~ 100 after only a few seconds μ m。 When the powder layer reaches a certain thickness, a thick negative charge shielding layer is stored on the surface, so that the later negative particles are repelled back and the coating is no longer thickened. Thus, the coating process is completed.