Fixing Penetration Issues in Recessed Areas of Parts with Electrostatic Applicators

Q. We recently installed an electrostatic bell applicator and are having problems getting coatings into recessed areas of the parts. Is there anything we can do to help penetrate those areas?

A. Your problem is not specific to bell applicators. All electrostatic applicators (bell, disk, gun and so on) can experience something called the Faraday cage effect. When a coating is electrostatically charged and atomized, the charged atomized particles are attracted to the nearest grounded surface. When the target area is in a recessed portion of a part, it typically results in a higher film build along the edges of the recess, with little to no coating fully penetrating it.

Some non-specific applicator solutions include reducing the target distance, lowering the voltage, increasing the resistivity of the material and adjusting the atomization level. Looking at each of these in some detail could potentially solve the problem, no matter what type of electrostatic applicator you are using.

Reducing the target distance. Moving the applicator closer to the target area is the easiest solution. A general rule of thumb when using electrostatics is to maintain a safe distance of approximately 1" per 10 kV (kilovolts) from the grounded target surface. For example, when operating at 80 kV, a minimum distance of 8" would be recommended. If your current target distance to the recessed area is greater than the minimum recommended distance, you may be able to move in closer and get better penetration. This reduces the likelihood that charged particles will be pulled to the edges of the recess.

Lowering the voltage. In addition to producing lower particle voltage, this easy solution also potentially reduces the amount of the coating being pulled to the edges (the momentum of the particle more easily overcomes the associated net charge and penetrates the recessed area).

Increasing the resistivity of the material. When using any electrostatic application equipment, the resistivity of the coating material should be tested to ensure that it is within an acceptable range for the specific manufacturer’s equipment. Generally, for manual applications, the recommended range for resistance is 0.1 – ∞ MΩ (megaohms). For automatic application equipment, resistance in the range of 0.05 – ∞ MΩ is acceptable. Obviously, these are large ranges. Typically, the resistivity of the material can be changed by the type and amount of solvent used to reduce the material (the lower the resistance of the material, the more easily it is able to accept a charge). Highly charged coating materials are more apt to be pulled to the nearest grounded surface, leaving recessed areas relatively untouched. When the coating material is made more resistive, it does not readily accept as much of a charge and relies more heavily on particle momentum to reach the surface. This increases the potential to reach recessed areas.

Adjusting the atomization level. Smaller-sized particles are more easily pulled to the nearest grounded surface. Atomization affects particle size. Lowering the atomization air pressure to a gun results in a larger particle size. In your case, reducing the bell speed will result in a larger particle size, which makes it more difficult for the net particle charge to pull the coating material to the nearest grounded surface. This also increases the potential to reach recessed areas.

In your particular case, there are also a couple of additional, bell-applicator-specific options that could help to better penetrate the part recessed areas. These include changing the bell cup size and/or adjusting the shaping air. 

Typically, bell applicators support a wide variety of bell cup types and sizes, usually ranging from 15 to 70 mm. A 57-mm bell cup will produce a much larger spray pattern than a 30-mm bell cup, which could better reach recessed areas.

Shaping air is a bell applicator-specific parameter that will change both pattern size and particle velocity. This parameter introduces air around the edge of the bell cup surface. Increasing the shaping-air pressure will tighten and focus the spray pattern into a smaller area. A smaller, more focused pattern could also better reach recessed areas. In contrast to changing the bell cup size, shaping-air adjustments can be made while the bell is spraying, allowing greater flexibility during the coating process. If the part you are coating has both flat and recessed areas, the shaping air can be programmed to automatically adjust while spraying to better match the topography of the part.

Shaping air also affects particle velocity. A higher shaping-air pressure results in a smaller, tighter pattern and a higher particle velocity. A higher particle velocity will better “push” the coating material into the recessed areas while overcoming the electrostatic charge.

Because of the Faraday cage effect, it can be difficult to penetrate recessed areas when spraying with an electrostatic applicator. Sometimes just changing one of the above parameters will produce the results you are looking for, although the solution often requires making multiple adjustments to multiple parameters (for example, you may need to lower the voltage and increase the shaping air pressure). Every combination of application and coating is different. Your best approach is to adjust one parameter, check the effect and then adjust other parameters as necessary.