Reasons and solutions for powder falling phenomenon after powder coating

Reasons and solutions for powder falling phenomenon after powder coating

The general response from coating production line manufacturers is that the main factor affecting the acceptance and retention of charges by powder particles is the dielectric constant of the powder. The lower the dielectric constant of the powder, the easier it is for the particles to become charged, but the easier it is for them to lose charge. This is reflected in the weak adsorption force of the powder on the workpiece, which causes it to fall off under slight vibration. For electrostatic spraying powder coatings, high dielectric constant should be used as much as possible, which will greatly increase the adsorption force of the powder.

From electrostatics, it is known that the distribution of surface charges on a charged isolated conductor is related to the curvature radius of the surface. The charge density is higher in areas with larger curvature (i.e. sharp surfaces), and the electric field strength in the surrounding space is also higher. When the electric field strength reaches enough to ionize the surrounding gas, the upper end of the conductor will discharge. If it is a negative high-voltage discharge, electrons leaving the conductor will be accelerated by a strong electric field, causing them to collide with air molecules and ionize them to produce positive ions and electrons. The newly generated electrons are accelerated and collide again, causing air molecules to form an "electron avalanche" process. Electrons have a small mass, and when they leave the ionization region, they are quickly attracted by much heavier gas molecules, which become free negative ions. This type of negative ion rushes towards the positive electrode under the action of electric field force, producing a layer of halo discharge at the ionosphere, which is called halo discharge. When the powder passes through the corona periphery, it will be charged by the collision of negative ions rushing towards the positive electrode.

Most industrial powder coatings are structurally complex polymer insulators, and negative ions can only adsorb onto the surface of the powder particles when there are suitable positions on the powder surface to receive charges. For negative ions, this site can be a positively charged impurity in the powder composition or a potential energy pit in the composition, or it can be purely mechanical. However, regardless of the adsorption mechanism, the deposition of ions on each powder particle 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, they are blown out close to the workpiece (positive electrode) by the conveying force of compressed air, and guided by the electric field force, the coating is firmly adsorbed on the workpiece. Generally, it only takes a few seconds to achieve a coating thickness of 50-100 μ m. At the same time as the powder layer reaches a certain thickness, a thick negative charge shielding layer is stored on the surface, causing the subsequent negative charge particles to be repelled back and the coating to no longer thicken. The coating process is now completed.