Metal Finishing Guide Book


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PROBLEMS COATING OVER CERTAIN SUBSTRATES Q: We have a problem when we apply metallic silver powder coating over black E-coated automobile truck wheels. During tire assembly an iron rod is used to [seat] the tire. After fixing the tire when the rod is withdrawn from the parts, we could see a heavy scratch mark at the edge of the part. Is it due to compatibility over e-coat.? How can we avoid this? Lastly, is there any need to improve the powder system? Note: My scratch resistance is 3,000 gms and impact resistance is 250 kg-cm. A: This sounds like a tough one. First of all, the surface properties of the powder topcoat do not appear to be a function of the compatibility with the e-coat primer. Incompatibility with the e-coat might cause intercoat adhesion issues, but not surface slip problems. The scratch resistance and surface slip of the powder coating can be improved. Your powder supplier can increase the crosslink density and, thus, the hardness of the powder by using a more functional (containing more chemically reactive groups) resin. They can also increase the surface slip of the coating by incorporating a polytetrafluoroethylene/wax blend into the formula. Both measures will help. However, I do not think that this will completely eliminate scratching caused by an iron bar. You may not have the ability to influence the manner in which the tires are installed, but using a more forgiving tool may be the best answer. If the tire installer can use a bar that has a softer surface (nylon or PTFE rich) the scratching could be eliminated. TROUBLESHOOTING PROBLEMS PERTAINING TO OUT-GASSING ON ALUMINUM Q: We have been painting these parts for another company. Lately we have had trouble with out-gassing. I think the quality of the aluminum has decreased. We are using Rohm & Haas Midnight Black Wrinkle. Our pretreatment chemicals are from DuBois. We have a fivestage wash. I really think it is the aluminum, since the steel parts painted at the same time are perfect. What are your thoughts? A: Indeed, it sounds like your aluminum may be declining in quality. Before you conclude this, you should also take a close look at your own process. Has the pretreatment system changed? Is it in control (i.e., pH, solids, temperatures, etc.)? Are you running your production line at the same speed as before? Are you running the same amount of parts through the finishing system? Is your oven steady and in control? Out-gassing is most common with cast alloys (aluminum and magnesium) and galvanized substrates. A high level of porosity can spell trouble. If you suspect the aluminum is getting worse, I suggest you take a few parts and preheat them, allow them to cool to just above ambient temperature, then powder coat them. The preheating should expel any entrained volatiles, and the finished part should not exhibit any blisters from out-gassing. If this is the case, then you should get in touch with your part supplier to investigate a change in the quality of their parts. Alternately, you can also investigate the use of an ���out-gassing-forgiving��� powder coating, which many powder suppliers offer. These minimize the effect of inconsistent porosity in substrates. 619

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