The amount of cracking and crazing is unbelievable...
2/2/2009 - Q: What is crazing and what causes it? We clean and then send printed (screened ink) aluminum sheets, 0.018 inch to 0.030 inch thick, to a powder coater. This is a polyester powder coat applied at 1.5 mils to 3.0 mils thick. When the sheets are returned, we machine them and run them on a belt through an oven at 280°F to remove "blushing.” Unfortunately, the parts are crazing. To stop the crazing, we have to run the sheets before forming through the oven first. Sometimes, we have to run them through a hotter oven at 400°F. Any idea what causes the crazing? My understanding is that crazing is small hairline fractures in the powder coat. If we ship parts that show crazing to our customers, and they are applied to vehicles, are they susceptible to corrosion? M.H., Munich, Germany
A: Maybe. If you’re lucky, it will not go to bare metal, but I would not bet on it unless I looked closely at the cracks under a glass. It can depend on what is causing the crazing. It can be caused by too much heat. It can be caused by surface tension between two coatings that aren’t compatible, and on occasion between the substrate and the coating. The 280°F oven temperature is almost a 100 percent overbake of the powder. I wouldn’t think that would cause the material to craze, but you never know. You should check for compatibility of the two coatings with one another. One way you can do that is to use cold rolled steel test panels. Spray on the base coat, cure it out, apply the screening or the second coat, cure that, and then take a look. Use a glass to be sure there aren’t miniscule cracks. I could also make a case for the thin sheet aluminum becoming "active" during the bake cycles and causing the coating to move when it doesn’t want to. The aluminum won’t corrode, but it will develop a patina coating where it’s exposed to air. More important, if the cracks are to the substrate, you now have an entry for moisture to get under the coating and cause it to blister or lift. The aluminum will be safe, but the part will look bad and cause complaints.