Questions and Answers
George Trigg, GRT Engineering
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Identifying defects properly to solve problems
Q: I recently had a customer contact me regarding coating a high-visibility part for a retail item. The customer had tried three other powder coaters in the area and complained of particulate matter embedded in the finished coating from all three companies. Because the customer had used coaters with conveyor systerms, the customer thought I might be able to help with quality control as I operate a job shop and do everything manually. When I recieved the sample parts my customer had coated by the other companies, my first thought when inspecting them was that there where problems with out-gassing. the part was made from 18-guage cold rolled steel (CRS). I took a shot at coating it, and I ended up with the same problem but not as bad. The parts where pretreated in a heated alkaline aqueous washer, rinsed, iron phosphatized, and rinsed again. I also tried different cure temperatures, ramping up the heat in increments and even out-gassing the parts with a preheat cycle just in case.
I then told my customer that I suspected the steel had some contaminants that I wasn't getting out in pretreatment. So, the customer switched to aluminum. I still had the same problem. I was able to successfully coat some 20-guage CRS I had without the defects showing. the defects look like out-gassing as they are volcano-shaped with a pinhole in the middle. they are very small, but these guys want them gone totally. I eventually was able to get the steel parts perfect, but I had to heat the aqueous washer to 180*F and wash them for 20 minutes. then I left them in the iron phosphatizer for 3 minutes. This affected the outcome of the color of the cured coating to a darker shade. Have you ever heard of this before? The aluminum was 5052 cut in about a 16-guage sheet. L.S., Hastings, Minn.
A: I've never heard of CRS out-gassing. To my knowledge, it can't because it's nonporous. My suspicion is that there is a mill oil on the metal that's put there by the steel warehouse to prevent oxidation, and as it ages, it becomes more difficult to remove. That's why you had success by cranking up the cleaner stage to 180*F. Because the rust preventive (or whatever else the warehouse might have applied) didn't come off at lower cleaner temperatures, the defects on the parts where more like fisheyes than out-gassing. Your description fits this condition perfectly. As for the longer exposure in the phosphate stage, there is such a thing as too much coating build from the phosphate, and that's what you got by leaving the parts in the phosphate tank for more than the recommended time. the heavy iron coating (or zinc, whatever) will cause the change of color in the powder. And you can actually do more harm than good by applying too much phosphate. Besides, it's a waste of chemical, which means a waste of money. I believe you can go back to the CRS, unless your customer really wants the aluminum, in which case you will need to slightly modify your pretreatment system.
Preventing conveyor oils from falling on parts
Q: What's the best way to keep your conveyor track wheels lubricated without having excess oil falling on the parts? Thanks. P.D., Northeast, Pa
A: Ok, you asked the best way, which is to use C hooks on the conveyor and put in sanitary pans. I didn't say anything about cost to do this or the maintenance required when you use sanitary pans, but this will do it. The second best way is an automatic lubricant system, set up by the supplier of the lubricating oil, which will be based on your temperatures and the conditions involving the conveyor. When done right, this method will not get lubricant on the parts. If you get lubricant on the parts, a maintenance person is tinkering with something he doesn't know anything about. In this case, a little bit of grease is good, but a lot is not. The location of the lube system will have an impact on the situation as well. Don't put the oiler just ahead of the oven. If you're manually greasing the trolleys, then look for several things; Is the lub the correct application for your line, such as the right temperature, nozzle conditions in the washer, and so forth? Once again, is the manual application using too much grease? this is a boring job, and the maintenance people get tired of standing on a ladder for hours squirting lubricant into a bunch of little wheels. What happens when a Zert fitting is missing? Do they apply grease anyway? Ooops! Bottom line, there's no reason to have grease falling on parts.
Arranging guns and product orientation
Q: I powder coat aluminum extrusions for the building industry, making doors and windows. Which is the best way to arrange the guns in my booth: vertically or horizontally? I use six guns from each side. Also, which plant has better productivity for aluminum extrusions: the vertical plants or the horizontal plants? With a vertical installation, is it possible to pass the American Architectural Manufacturers Association (AAMA) 62 test? L.K., Athens, Greece
A: First, I have never compared the best way to actually powder coat extrusions, vertical versus horizontal. What really dictates this situation is plant conditions. Do you have room to run parts either way? Having said that, however, I have seen more vertical systems than horizontal. It's called a "Christmas tree," and the parts are hung vertically, some as long as 20 feet. Guns are mounted on oscillators and staggered so that the entire part gets coated, but not all at the same time. You must carefully study what happens to the overspray as it falls away from the top gun(s). You don't want the overspray causing an appearance problem on portions of the extrusion. Sounds to me as if you are already powder coating in the horizontal position with the extrusions stacked on a rack. And fixed guns can be made to work, versus oscillating guns. Usually, you trade the cost and location of a fixed gun (or guns) for the cost of an oscillator or reciprocator. The design of the part you're coating also affects this decision. Don't put opposing guns directly opposite of one another. They should be staggered through the length of the booth. I prefer 18 inches between opposing guns. I know this can't always be done, but at least don't put them directly across from each other.
Applying a bright powder coating color on MDF
Q: I need to know how I can get a bright powder coating color on medium-density fiberboard (MDF). E.D., San Martin, Villa Rosa, Argentina
A: Your powder supplier would be the first place to try. I assume you've tried that or you wouldn't be writing. It may well be that the pigmentation isn't tolerant for use with MDF coatings. And it may be possible that a bright color won't show well unless you can put a primer coat on the MDF. This means curing the board twice. Maybe a Supplier of MDF coatings has a solution and will respond to this. If so, we will forward any information we receive. PC
For further reading on the problems discussed in this column, see Powder Coating magazine's Web site at [www.pcoating.com]. Click on Article Index and search by subject category. Have a question? Click on Problem solving to submit one.
George R. Trigg is president of GRT Engineering, 6314 Hughes Road, Prospect, OH 43342; 740/494-2496. He has been involved in the powder coating industry for more than 37 years. He holds a BSBA degree from Muskingum College, New Concord, Ohio. His e-mail address is [firstname.lastname@example.org]. (If you e-mail Trigg directly, please include Powder Coating in the subject line so that your e-mail is not seen as junk mail.)