Faraday Cage In Powder Coating
Let’s start looking at what happens in the space between the spraying gun and part during the electrostatic powder coating application procedure. In Figure 1, the high potential voltage applied to the tip of the gun’s charging electrode creates an electric field (shown by red lines) between the gun and grounded part. This bring about the development of corona discharge. A great amount of free ions generated by the corona discharge fills the space between the gun and the part. Some of the ions are captured by powder particles, resulting in the particles being charged. However, multiple ions remain free and traveling along the electric field lines to the grounded metal part, mixing with powder particles propelled by the air stream.
As stated earlier, a cloud of charged powder particles and free ions created in the space between the spraying gun and part has some cumulative potential called space charge. Much like a thunder cloud creating an electric field between itself and the earth(which ultimately leads to lightning development),a cloud of charged powder particles and free ions creates an electric field between itself and a grounded part. Therefore, in a conventional corona-charging system, the electric field in close vicinity to the part’s surface is comprised of fields created by the gun’s charging electrode and the space charge. The combination of these two fields facilitates powder deposition on the grounded substrate, resulting in high transfer efficiencies.Positive effects of the strong electric fields created by conventional corona-charging systems are most pronounced when coating parts with large, flat surfaces at high conveyor speeds. Unfortunately,stronger electric fields of corona-charging systems can have negative effects in some applications. For example, when coating parts with deep recesses and channels, one encounters Faraday cage effect (see Figure 2).When a part has a recess or a channel on its surface, the electric field will follow the path of the lowest resistivity to ground (i.e. the edges of such a recess). Therefore, with most of the electric field (from both the gun and space charge) concentrating on the edges of a channel, powder deposition will be greatly enhanced in these areas and the powder coating layer will build up very rapidly.
Unfortunately, two negative effects will accompany this process. First, fewer particles have a chance to go inside the recess since powder particles are strongly “pushed” by the electric field towards the edges of Faraday cage. Second, free ions generated by the corona discharge will follow field lines toward the edges, quickly saturate the existing coating with extra charge, and lead to very rapid development of back ionization.It has been established earlier that for powder particles to overcome aerodynamic and gravity forces and be deposited on the substrate, there has to be a sufficiently strong electric field to assist in the process. In Figure 2, it is clear that neither the field created by the gun’s electrode, nor the field of space charge between the gun and the part penetrate inside the Faraday cage. Therefore, the only source of assistance in coating the insides of recessed areas is the field created by the space charge of powder particles delivered by the air stream inside the recess (see Figure 3).If a channel or recess is narrow, back ionization rapidly developing on its edges will generate positive ions which will reduce the charge of powder particles trying to pass between the Faraday cage edges to deposit themselves inside the channel.Once this occurs, even if we continue spraying powder at the channel, the cumulative space charge of powder particles delivered inside the channel by the air stream will not be sufficient to create a strong enough electric force to overcome the air turbulence and deposit the powder.
Therefore, the configuration of the electric field and its concentration on the edges of Faraday cage areas is not the only problem when coating recessed areas. If it were it would only be necessary to spray a recess for a sufficient length of time. We would expect that once the edges are coated with a thick layer of powder, other particles would be unable to deposit there, with the only logical place for powder to go being the inside of the recess. Unfortunately this does not happen due, in part, to back ionization. There are many examples of Faraday cage areas which cannot be coated regardless of how long powder is sprayed.In some cases, this happens because of the geometry of the recess and problems with air turbulence, but often times it is due to back ionization.