Bubbles in the 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.—H.N.
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.
Masking is employed in most any metal finishing operation where only a specifically defined area of the surface of a part must be exposed to a process. Conversely, masking may be employed on a surface where treatment is either not required or must be avoided. This article covers the many aspects of masking for metal finishing, including applications, methods and the various types of masking employed.
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