Cause of Blisters and Pinholes

We currently have a powder operation where we applied powder to a part that is made of cold rolled steel with zinc plating over it. After the curing process we notice small blisters and pinholes in the finish. Could this be impurities in the cold rolled steel trying to get out? Could it be the zinc? Most of the problems are in areas that have been welded and ground off prior to plating.

It is possible that contaminants from the steel have worked their way through the coatings during the cure cycle causing blemishes. If the steel has been properly cleaned prior to plating, this probably isn't the culprit.

You ask if it could be the zinc. Blistering during the cure cycle can occur if the plating's film build is too high. You can run test panels or parts through your system with various film thicknesses of zinc plating and powder coat as you would normally. This will illustrate how a higher zinc film build vs. a lower zinc film build reacts during the cure cycle. Generally, a lower film build of zinc is preferred. It would be a good idea to check with your plating supplier for their recommendations.

You say most of the problems are in areas that have been welded and ground off prior to plating. This is a typical "hot spot" for pinholes but there are some things that can help. Cleanliness of the weld area is crucial. Flux and slag left behind while welding are insoluble and must be removed by mechanical means. While blasting is the best method, some prefer grinding. If grinding is your preference, choose a fine grit. Coarser grit grinding wheels could leave abrasives imbedded in the metal adding to the problem. Also, the silicone content of the weld rod can cause an influx of plating at the weld resulting in blisters.

Liquid Vs. Powder; Vertical Vs. Horizontal

I am a manager of an electrostatic, vertical, liquid painting operation using discs and painting with high solids polyesters (AAMA 2603). We used to have a horizontal powder coat line, but discontinued it due to lack of business at that time for the length of aluminum extrusions we were coating. By the way, we paint aluminum extrusions. I have two questions: We are seeing powder coated parts being offered at cheaper prices than liquid painted products. How do powder coaters paint for less than liquid paint? Is it labor, better transfer efficiency? What?

There are significant differences in the steps needed to use powder coatings compared to liquid coatings. Powder is ready to use. It doesn't require mixing solvents or the monitoring of viscosity, pH, and the level of solids. First pass transfer efficiency, whether a liquid or a powder, should be similar but with powder's ability to be instantly reclaimed - more of what you've purchased ends up on your part. Almost all of a powder coating is usable solids, where liquids are composed of solids and at least one-third solvents or water. You can ask a liquid coatings supplier and a powder coatings supplier to run an economic comparison analysis (liquid vs. powder). This can help determine which coating would work best for you.

Another factor to consider is energy savings. Because powder coatings contain negligible amounts of volatile organic compounds (VOCs), the spray booth exhaust can be recirculated. This reduces the cost to heat or cool the makeup air. A liquid system, on the other hand, requires venting of the spray exhaust, which also vents the air of the spray room. The result is additional costs to heat or cool the ambient air. As a bonus, airborne contaminants in a powder system are greatly reduced by returning the booth exhaust into the spray room. Likewise, with the lack of VOCs, the venting of oven exhaust for powder is considerably less than is required for curing liquids. This allows a powder system to reach higher cure temperatures while using less energy. Powder lines also can run at higher speeds with greater part density and less rejected parts due to sags, runs or drips. The ongoing cost of controlling the environmental impact of solvents by using an afterburner to incinerate hazardous air pollutants (HAPs) and the waste disposal for liquid overspray doesn't apply to powder coatings. Although not all, these are a few of the differences between powder coating and liquid coatings that illustrate how a powder coater is potentially capable of delivering a less expensive finished product.

Answers provided by Chris Parks, technical service representative at DuPont Powder Coatings USA Inc., Houston. Chris has been with DuPont Powder Coatings for 11 years.