Coating Aluminum Castings

Ask an Expert From: Products Finishing, , from Powder Coating Consultants, Div. of Ninan, Inc.

Posted on: 1/1/2003

Question: We are seeing a problem with coating aluminum castings.

Question:

We are seeing a problem with coating aluminum castings. We can paint these castings for over two hours and have great results, then we start seeing what looks like rough paint, or better yet what looks like the paint had not covered the part. The part is a speaker frame and has a rounded surface.  We do not change any settings and the oven room temperature remains stable.  We are using a Nordson Versa sprayer and controller.  The paint is silver metallic.  This is what we did to fix the problem: Changed the gun, put a complete system change (Versa sprayer, controller and gun). Did a complete change of the powder, using different batch number but with the same type of paint.  We are at a loss, and hope that you can shed some light on this to help us solve the problem.  We are a small company and do several types of powder coatings. I have been a painter for this company for nine years and have never seen this problem.  We use an IR oven. We do not have any problems with any other product that we coat.  Thank you in advance for your help. J. M.

Answer:

It sounds like you have tried to isolate and fix the problem by changing both the gun and the powder. Both of these changes were good ideas, but they did not solve the problem. I suspect that there is something else that changed over time in your system. Two items that come to mind that would affect this light spray or “rough paint” problem are powder reclaim quality and part grounding.

When you start out the day you are probably using mostly virgin material and as the day goes on the powder in the hopper is mostly reclaim. As the particle size distribution changes in the powder, the film thickness of the coating will also change. It will get thinner as the particle size gets finer. Without a good particle size distribution, powder coatings become more difficult to fluidize, electrostatically charge, pump, spray and build film thickness. Therefore, it is very important to reintroduce some virgin powder coating material into the feed hopper to mix with the reclaim powder coating material. The best ratio is 70% virgin and 30% reclaim. But if you have a first pass transfer efficiency below 70%, you will never achieve this goal without generating more reclaim than you can use. However, you will find that most systems have few problems when they use a 50/50 mixture of reclaim to virgin powder.

The next issue that can affect film build on a part over time is part grounding. Part hangers start out clean at the beginning of the day and the contact point gets coated during each pass through the system. A couple of hours later and the part ground is significantly reduced. As part grounding gets worse, the application efficiency becomes lower and the same gun settings will not build the same film thickness on the part. If the resistance to ground exceeds one megohm (1,000,000 ohms), then you are in violation of NFPA 33 Guidelines and have a process that is unsafe. Having an extra set of hangers and changing them out every two hours with clean ones will make this problem go away. Cleaning the contact points after each pass will definitely make this problem go away, but can be an impractical solution.

 



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