One of the problems often encountered in electrostatic powder coating applications is the Faraday cage. It is the volume that acts as an electric curtain, deflecting charged paint particles in complex areas of the part, such as corners, and often occurs, for example, in folded sheets, corners of metal boxes, etc. This effect occurs when we find corners walled at 90º. The presence of these corners creates an electromagnetic field that prevents the powder coating from settling properly.
Among electrical charge problems, lack of charge, back ionization and the Faraday Cage effect stand out. The latter is common, but can be avoided. The problem occurs because the electric field lines do not penetrate sufficiently into the more hollow areas and do not guide the electrostatically charged paint particles towards the parts. If there is a problem reaching some corners, holes or gaps, it is because we realize that the powder is not penetrating the parts sufficiently. In some cases it is advisable to use a spray gun to overcome the Faraday cage as it reduces the tension.
The Faraday Cage occurs because the electric field lines that guide the electrostatically charged paint particles towards the object do not penetrate sufficiently into the most empty spaces. When there are problems reaching certain holes, corners or gaps, it is because the powder does not penetrate the part sufficiently and sometimes it is advisable to use corona electrostatic guns by reducing the voltage to overcome the Faraday cage.
Although coating processes are based on the laws of physics, certain forces can inhibit product transfer and uniform coating coverage. For example, negatively charged particles are attracted to the nearest grounded surface, which has the advantage of both drawing the coating to the edges of a part and reducing overspray. However, the same tendency can make it difficult to properly coat the deep, recessed areas commonly seen in aluminum extrusion profiles. Particles are attracted to the protruding edges of the profile, experiencing what is known as the Faraday cage effect. The causes of Faraday cage can be listed as follows:
Reducing the spray pattern can direct the ionization field towards the recessed area. If a part is sprayed manually, the technician can focus on that work item. The gun target distance should be kept between 20 and 30 cm from the part being coated. As the distance decreases, the operator needs to reduce the liquid or air pressure (or in some cases both) to avoid applying too much coating. In addition, lowering a gun's voltage allows more of the coating to pass through the outermost edges and undercut regions of the extrusion. For optimum performance, the gun voltage should be between 50 and 70 kV.
Another consideration is the size of the coating particles. The amount of charge carried by a particle is directly related to its surface area. Powder coatings must have the right ratio of large to small particles for optimum transfer efficiency. If the powdered paint is recycled and reused, a calculated proportion of virgin powder paint must be added to the hopper to maintain the specified average particle size balance. Monitoring this balance is extremely important.
Faraday cage can be prevented by paying attention to some details in electrostatic powder coating applications. You should make sure that the equipment used such as nozzles, deflectors and guns are suitable for the process. You should check all equipment. What you need to do to prevent Faraday cage:
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