Troubleshooting Bubbles in Powder Coating

Q. I am using a spraying system with a pressure tank to coat the inside of an upper receiver with a 1-inch diameter, but I am having a few problems. First, when I increase the pressure, the paint is too thick, and when I decrease, the pressure is too low. Second, I am getting bubbles after curing. How can I get rid of these bubbles? Will the pressure tank work for this application? The paint contains high solids concentration and is being agitated. Controlled variables: atomizing pressure, tank pressure, time delay and spray time. There are 10 orifices on the spray tip.

A. The issue you are referring to is multifaceted. While the pressure pot is your fluid delivery, it is not necessarily the root cause of your issue. It sounds like the issue has more to do with viscosity than with fluid delivery. When paint is too thick or viscous it does not flow and atomize correctly. Even excessive amounts of pressure will not break up thick material correctly. The best remedy is to mix the material with the manufacturer’s recommended spraying viscosity. You will also need to validate the proper spray gun setup. This usually includes the fluid nozzle, needle and air cap configuration.

In general, the thicker the material to be sprayed, the bigger the spray gun set needs to be. If you are spraying a low viscosity material in a pressure pot, for example, a 1.4 to 1.8 fluid nozzle can be used. But if you are spraying more solid material, the size should increase to 2.4 to 2.8 or more. Be sure to work closely with your coating supplier who will give you the proper recommendation.          

Your second issue regarding the bubbles after curing will typically disappear when the viscosity and spray gun issues are thoroughly addressed. The velocity you are driving the material to the surface with is trapping air when the paint is too thick. This air has to go somewhere, and when the material is too dense, bubbles get trapped in the coating as they try to escape before the coating can level itself.

Also, consider the speed of agitation. Often, over agitating paint can introduce excessive amounts of air. When coupled with improper viscosity and leveling capabilities this air will, you guessed it, become trapped in your coating. When the material is at the proper spray recommendation, you can address proper atomizing and fluid pressure. I highly recommend that you work closely with your coating supplier to get the best information.

Originally published in the July 2015 issue.

Q. We recently have had some trouble with an area of the part where small bubbles will appear in the film. The parts look all right after they are coated and cured but the bubbles start popping up a day or so later. The bubble can be popped and the aluminum surface is bare under the film. We cannot figure out what the cause is and we need advice on what to look for. 

A. You could have a surface contaminant but from your description I think that what you have is humidity blisters. The aluminum is not treated properly and an oxide layer forms on the surface prior to coating. The oxide layer creates a low pressure zone under the film. After the parts have sat around for a while the low pressure zone begins to draw moisture through the film though osmotic pressure. The small areas of moisture swell over time and lift the coating into the bubble you see.

You should examine every detail of your pretreatment system. Do you have the right chemistry for treating aluminum? The process starts with a good acidic or alkaline cleaner. Since aluminum rarely has heavy oils or grease you may want to consider an acid cleaner to help etch the aluminum. Rinsing is very critical. All residual chemistry and soils must be removed before the part enters the next stage.

You cannot allow the part to dry-down in the drain zones. If you are in the drain zone too long and the part dries it will begin to form an oxide layer again. Wetting nozzles are sometimes helpful to make sure the part stays wet. Never stop the line with parts in the washer. This will almost certainly cause problems of the sort you describe.

The conversion coating should be designed for aluminum only. Many companies use iron phosphate with a fluoride additive for the aluminum. This may be adequate for some indoor use products but it is hard to control the parts per million of fluoride and not reliable for sensitive products, especially products that will be used outdoors. A chrome or non-chrome conversion coating for aluminum is best. If you have to run multi-metals (ferrous and non-ferrous) through the washer you should see if you are a good candidate for a transitional metal treatment such as zirconium oxide. These products can work well for steel and aluminum in the same system.

The final rinse is critical. You need deionized or reverse osmosis quality water to ensure that no residuals are left on the surface to dry in place and leave minerals on the surface.

If you do have a surface contaminant that the washer cannot remove you will need to do a thorough soil audit and identify the type of material causing the blisters and find the source.