Excessive Coating Problems

Q. We are coating automotive components. We are in the process of trying to complete a new order for coating of shock absorber springs for motorcycles. The problem we are facing is that the there is repulsion of powder from the inner side of the spring in some areas and also pinholing. Because we try to reach the inner side to coat, it creates another problem of excessive coating on the outer portion, which is not acceptable to the client. What should I do to resolve the coating problem for the inner side of the spring? S. W.

A. There is a knob on your gun control that turns the charging voltage up and down. If I were you, I would use it to improve your coating results. Sorry, if this seem too simplistic, but I am trying to be funny here. I will give you a more expanded answer below.

Powder coating smaller objects can be a problem when you don’t have proper powder charging working in your favor. To best explain this, look at Figure 1.

For this circuit to work properly, you need to have the following:

1. The gun charging mechanism must be working properly. Make sure the generator and charging electrode are both up to snuff. You can check this by triggering the gun 6–8 inches away from a known ground and reading the current output at the gun. Most guns have a current reading circuit that can be activated by the operator. The current reading should be between 25 and 50 microamperes.
2. You must have a good ground on the part. The maximum resistance to this ground cannot exceed 1 megohm (1,000,000 ohms).
3. You must have an electrostatic spray-grade powder coating formula. These materials are formulated with additives to provide maximum charging characteristics.
4. You must have dry compressed air (below 32° F dewpoint). Moisture in your compressed air will not only cause fluidization and feed problems it will destroy the charging characteristics of the powder coating being sprayed.
5. Select the proper gun pattern device. Conical deflector tips provide soft forward velocity that can be very effective at coating flat panels and simple shapes. Fan spray nozzles provide faster forward velocity to penetrate into recessed areas.
6. Use slow gun motions to allow the powder time to accept an electrostatic charge and be attracted toward the part. The longer the powder particles are within the electrostatic field, the higher the electrostatic charge they will absorb. The higher the electrostatic charge the powder particle has the higher the first-pass transfer efficiency and the better the electrostatic wrap.
7. Higher powder outputs are very difficult to charge efficiently. Always adjust the powder output at the gun tip to provide a minimum amount of powder to meet production needs. Don’t forget about the powder speed, as discussed previously.
   Now that you have the basics of electrostatic charging, let’s talk about manual spraying techniques. Adjust the powder output and powder speed to ensure that you have the right amount of powder coming from the gun tip without surging or puffing. Too much powder at too high a velocity will adversely impact electrostatic attraction to the part surface. Use slow gun motions to coat the outside of the spring coil evenly in a single pass. If your part ground is sufficient and the powder velocity is low and you allow the powder to be attracted towards the part and you should have no problem having enough electrostatic wrap to coat the inside of the spring coil while you are spraying the outside of the part.