Coverage Inconsistencies

We have an automatic powder coating system that uses 16 guns, eight per side, mounted on vertical gun movers. Our parts are a maximum of 84 inches high. We sometimes have trouble getting coverage at the top of the spray zone. Should we add another spray gun on each side?

Let's assume the system is in working order, with a verified ground and clean hooks, and that the work piece falls into the original design parameters of the system. The first thing I notice is the word "sometimes." This means that sometimes you do not have trouble getting coverage. Therefore, the process is out of control.

Picture this: You have a machine that drills 100,000 holes a day, and sometimes the holes become ragged and have huge burrs. Here, the drill bit is showing wear. Once the drill bit is changed, the quality comes back. The manufacturer should monitor the process and learn when to change the drill bit before rejects are produced. In a powder coating system, what wear items could cause your problem, and why only the top guns?

These wear items are in the guns and the pumps. Consult your equipment manufacturer or distributor if you are unclear as to what these items are. Like the drill bit, these items need to be replaced before they are completely worn. Keeping a maintenance log, and detailing when and what item was replaced, can help define what your maintenance schedule should be. So what does this have to do with the top guns?

The top guns and associated pumps may require maintenance before the lower guns. Therefore, they will be the first to show signs of wear, resulting in low powder output. Why the top guns? First, if we adjust the powder output to be the same for all of the guns, we have adjusted the top gun controllers differently from the lower guns. This is because the top guns have longer hoses and also need to overcome gravity for a greater distance. The increased kinetic energy required to compensate for these factors may slightly increase wear and, therefore, the frequency of maintenance on the top guns.

Also affecting the top guns is automatic gun triggering controls. Even the most rudimentary automatic powder coating system will have this feature. Automatic gun triggering turns off the lower spray guns that are not required when coating shorter parts. The top guns and associated pumps will be used more than the rest of the guns, again increasing wear, resulting in more frequent maintenance.

These factors are why modern gun controllers have built-in maintenance alerts for each spray gun. These alerts can be adjusted to meet the specific needs of each installation. Adjustability is required because the rate of wear is not the same from one installation to the next due to differences in powder flow rate and powder formulation. If your system is not equipped with this type of feature, get a calendar and track your maintenance activities.

Another possibility: There may be some "Picasso" working on the second shift. You probably know who I'm talking about. This misguided person may not understand that the top guns' controller setting should be different, and is turning the top guns down to the same setting as the lower guns. In this person's defense, the setting difference is somewhat counter-intuitive. This is why state-of-the-art control systems have recipe features that recall saved gun settings and a password protecting the recipe. (Don't tell Picasso the password.)

The culprit also could be the system design vs. how you are using it. The equipment supplier ordinarily runs tests and calculates the number of guns based on the surface area of the work piece, mil coverage and line speed. A change in one of these variables could cause complications. I don't suspect that because you have not mentioned any changes, and eight guns per side over 84 inches is a fairly robust system. If something has changed, contact your equipment supplier or manufacturer.