Issue link: https://www.e-digitaleditions.com/i/1051901
Bubbling black powder coating during cure Q We powder coat an extruded aluminum bar with a black epoxy powder. In almost all the parts, the powder seems to bubble up in spots during baking. We've tried cleaning it with solvents and running it through our three-stage washer, but we still get the same thing. We've tried a different brand of black polyurethane powder and experienced the same problems. We powder coat the same parts with the same cleaning method in other col- ors and don't experience this problem. We see it only with the black powder. We have adjusted the bake time and temperature from 375°F to 400°F with no success. If we sand the parts before powder coating them, we greatly re- duce or completely eliminate the prob- lem. Any ideas why we don't have the same problem in the other colors? D.M., Mentor, Ohio A Sanding the parts reduced the problem? It may be the heat is affecting my thinking, but I can't think of a reason why that would help. This might imply that the clean- ing method isn't adequate enough for those parts. The black would be slightly more heat absorbent and that might affect the outcome as well. Try reducing the temperature just a bit, on the black only, to see what happens. This may sound far-fetched, but give it a try. —G.T. Applying powder and liquid coatings in the same area Q We're a custom coater with several powder coating con- veyors. We're planning to install a batch-with-liquid (non-water base) application near one of these con- veyors. What are the quality issues with this project? We want to know if it's possible to apply powder and liquid coatings in the same area. C.F., Notre- Dame-du-Bon-Conseil, Que. A You must consider several issues before installing a liq- uid spray booth in an area close to a powder application booth. First, let's talk about safety issues. Liq- uid systems have a completely different set of rules than powder systems. For instance, the electrical devices in the area may need to be vapor tight explo- sion proof; those around powder sys- tems need to be dust tight explosion proof. Be sure your new liquid installa- tion conforms with all safety require- ments before operating it. The second issue is visual quality prob- lems caused by airborne particulate. If you have high appearance standards for your liquid parts, and dust and dirt contaminants aren't allowed, then placing a dust-producing operation (powder booth) next to your liquid booth may not be the smartest idea. These are the two primary concerns I have for your plan. Having said that, don't forget the human factor of sol- vent spills affecting your powder appli- cation system or powder spills affecting your liquid operation. Otherwise, powder and liquid operations have co- existed together in many operations for many years. —N.L. Spraying a two-coat powder system Q I have a customer who requires a two-coat color (mirror silver and then cop- per translucent). I usually do a one- coat process. I've received different opinions from powder coaters and powder salespeople on the proper way to achieve an even coat. After applying the basecoat, I apply the second coat, but it "puffs off" the part. When it's finished baking, I can see small spots on the part. A fellow coater says to partly bake the basecoat, take the part out of the oven, immediately apply the second coat, and then put it back in the oven for a full bake. A powder salesperson says to wait a day between coats so that the part loses its charge. Is there a way to remove the charge from the part quicker, and do you have any suggestions on how to get a more even topcoat? B.H., Simcoe, Ont. A I'm sure it's no consolation that you are working with two colors that aren't very easy to apply as single coats, let alone as a two-coat system. The curing process will remove any charge that's left in the coating, so I doubt that's the answer. Is the silver a metallic material? Powders can be made with nonmetallic flakes that act and look like a metal material. I don't know that this has anything to do with your problem, but I know that spraying a metal flake material that uses real metal flakes will do all sorts of weird things, all of which are bad. You 24 POWDER COATING, November 2018 George Trigg GRT Engineering Nick Liberto Powder Coating Consultants Questions & Answers