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Bending and piercing holes in powder-coated tubes Do you have any sugges- tions for successfully bend- ing and piercing holes in 1- inch round powder-coated tubes? Are there specific types of powder coatings, techniques, or process parameters that should be used or avoided? We have ex- perienced flaking powder coating and poor adhesion to steel tubes after bend- ing and piercing holes. We have re- sorted to powder coating following these operations but find it creates longer lead times, more handling of parts, and more expense. Any info on this would be appreciated. T.N., Sey- mour, Conn. You should be able to bend and drill tubing with none of the failures you're experienc- ing. However, there are some qualifica- tions. The pipe must be properly cleaned and chemically treated. De- greasing or grit blasting won't cut it! Then, you need to select a powder coat- ing with the proper resin type. Polyester resins, when formulated into powder coatings to meet your specifications, can perform well. Epoxies will also withstand the bending and drilling you need to do. If the product is to be used in sunlight, and you can't tolerate chalking, the epoxies wouldn't be a resin of choice, however. I know that bending a pipe causes a lot of distortion and stress on the metal. Without proper metal prepping, you'll have poor adhesion, as you have discov- ered, and the powder will flake, crack, or peel. So, I recommend that you start b y l o o k i n g a t y o u r p r e t r e a t m e n t method. If you are already using a chemical metal prep system, be sure it's up to standards. Let me know your progress. —G.T. Solving an adhesion problem on powder-coated extrusions We've been powder coat- ing aluminum extrusions for over 25 years, and we just recently started having problems with adhesion. Why would a 12-foot part sporadically flake off in certain areas but still pass photo acoustical tech- nology (PAT) and MEK rub tests in re- maining areas? We're applying a metallic polyurethane powder coating that we cure in our in- frared (IR) boost, followed by gas-fired convection. We use gas-fired convec- tion only for our smaller orders, which we do data profiles on periodically to calibrate as needed. We're cleaning these parts in a five-stage immersion pretreatment system, which is titrated daily. Your feedback on this matter would be greatly appreciated before we confer with our powder and chemical supplier. S.W., Summerville, S.C. You're right to assume that the delamination problems you're experiencing are most often caused by improper pretreatment (cleaning) and insufficient curing. Of course localized coating failures are the most difficult to correct, especially when they're sporadic from part to part and even on different areas of the same part. This typically means that you don't have system-wide failures with your process. Interestingly enough, you don't say if the areas that failed on your part pass the PAT or MEK testing. My guess is they may not pass this testing to deter- mine the cleanliness of the surface under the powder coating (PAT) or the complete cure of the coating (MEK). If they don't pass this testing, then I would look first to your IR boost oven (before the gas convection oven). Often, IR ovens, especially electric ones, have reflectors as part of the emitter array. These can become dirty or coated, reducing the effectiveness of the IR energy being imparted onto the product surface. This reduced efficiency can affect some extrusions more than others, depending upon the complexity of the shape. Some of your emitters may be faulty as well. Of course, the area affected by these dirty or faulty emitters is focused within the view of that emitter, leaving the rest of the product unaffected. IR energy and IR problems won't be de- picted on your oven profiles unless the part is coated with powder. (In which case, you will be buying new thermo- couples.) Furthermore, you state that you don't always use this IR booster. All of these make it a hit or miss situation that is causing your sporadic problems and your puzzlement. Considering that you're immersion cleaning and pretreating the surface be- fore powder coating eliminates the pos- sibility that the delamination problems are chemistry or pretreatment process specific. However, what happens to the parts before and after you pretreat them can have a remarkable affect on the ad- hesion of the powder coatings. For in- 40 POWDER COATING, October 2016 Questions & Answers George Trigg GRT Engineering Nick Liberto Powder Coating Consultants Q Q A A