Fixing Adhesion Failures and Bubbles in Powder Coating on Zinc
Expert Rodger Talbert helps a coater who is having issues with the application of powder on zinc castings.
Q: We are having some issues with application of powder on zinc castings. We use the same five-stage washer that we use for steel parts, which cleans and then applies an iron phosphate conversion coating. Our chemical supplier tells us this should also work for these zinc parts, but we keep getting adhesion failures and bubbles in the powder coating. We are wondering if iron phosphate is effective for zinc, or are there other options for zinc treatment?
A: It sounds like you are seeing outgassing due to porosity in the casting. A cast part has trapped pockets of air in the body of the casting that may cause blemishes in the coating during the cure cycle. Air that is trapped near the surface may expand and rupture the film when the part is heated. There are several ways to help mitigate the issue: 1) Preheat the part to drive off some of the trapped air that causes the problem; heat it to a temperature around 50°F greater than the ultimate cure temperature, cool it down and then apply the coating. 2) Cure at the lowest temperature possible within guidelines supplied by the Technical Data Sheet (TDS) for the powder to limit the problem. 3) Use a powder that is formulated for a flow cycle that will help release air without leaving a blemish and limit the impact of outgassing.
As for adhesion, if the powder does not stick, it is because you have not cleaned and prepared the surface correctly. You need to get rid of all organic soils (grease, oil, dirt), and you may have to polish or blast the surface to get rid of inorganic soils (die release or similar compounds).
Look at the location and area in which the failure occurs. Is it random areas of the part’s surface, or is it more likely to occur in the same areas all the time? Does it only occur on certain parts or many different parts? If it occurs everywhere and on all parts, then the parts are not getting clean. You need a more aggressive cleaner with more heat.
If the problem occurs in isolated areas on the part, it is probably a die-release product. Iron phosphate leaves a film on zinc, but it is not a true conversion coating for zinc. I suggest that you try some sort of abrasion (tumble the parts in a vibratory device, or use ultrasonic cleaning or some similar method) to see if the problem is, in fact, die-release agents.
Talk to the chemical supplier again about a thorough audit of the part and options for preparation. The supplier may have something that can improve the coating performance on the zinc, such as a transitional metal treatment like zirconium that could still work in your washer.
Q: We are having trouble getting consistent quality and yields. Our first-pass yield is inconsistent, and our rejects are too high. I think the problem may be related to how we rack our parts. Some of our racks have a lot of buildup on the contact area, and rack maintenance is not always a high priority. I have noticed that sometimes racks are bent or have missing contacts, so we run them not fully loaded or lose ground contact on some parts. We also sometimes have parts that do not look like they are spaced as close as they could be or are too close together. Is there any way to qualify the impact of racking on the powder coating process?
A: Racking costs can be evaluated to determine the difference between an optimum racking scheme and a racking pattern that does not work as well. Racks can help you be profitable if you understand good principles and take good care of the racks. Parts should be racked close enough to take advantage of the space available but far enough apart to allow plenty of time for the guns to get good coverage. Parts should be racked consistently, and all hooks should be full. Parts should be held to provide good access to all surfaces, and they should be stable in the spray zone. Never rack on dirty hooks unless you want to have a lot of rejected parts for light coat and orange peel, and you like to waste time, powder and money.
You should evaluate what your throughput could be if you had proper racks and compare it to how you run it now. You can build samples to test different racking set-ups and compare the results from one option to another. It is simple math to evaluate the number of parts per foot and the number of good parts versus bad. Many companies waste a lot of dollars by using sub-optimum racks because they do not want to invest in good tooling. Also, they may not want too many different rack designs around. Evaluate the options and there is a good chance you can improve the throughput and reduce costs by using the right racking scheme. Simple hooks are fine if they hold the right number of parts and meet the other goals for good racking. But take your time to get racking right and you should find more profit.
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