Issue link: https://www.e-digitaleditions.com/i/964311
Powder coating hiding problems after spot welding Q We fabricate 20- to 22-gauge cold rolled steel bookcases and storage cabinets as part of our product line. We spot-weld the cut sheets together, then dip them in a phosphate bath at 140°F. We then rinse them in two separate rinse tanks. All the water is deionized. Next, we place the parts in an oven to be dried before coat- ing. After powder coating, they are baked at 400°F for 14 minutes. When we check the coating thickness, it's very uniform at 2.0 mils to 2.5 mils. The problem is this: When the spot welder positioned the sheet metal, the bottom swivel head electrode was dragged around, leaving a contrast trail in the coating at some points. Our pow- der coating vendor calls it "telegraph- ing." When the steel comes out of the baths, you can see these snail trails. We think it's a coating hiding problem. It's most apparent in black and darker col- ors. Any ideas as to what's causing this? B.W., Temecula, Calif. A If you can't feel it with a flat rag in the palm of your hand, it's a coverage, or hiding, problem. Let me explain my comment. Take any smooth cloth, such as a handkerchief, place it (wrinkle free) in the palm and on the fingertips of your hand, and move the cloth across the drag mark from the welder. If there's anything more than a burn mark, you should be able to feel it. If the spot welder is getting proper cool- ing, the weld tip should be cool enough not to burn the metal as it's dragged across the surface. I assume from your description that when the metal is high- lighted, there's a glossy appearance to the coating that strictly follows the drag mark. At more than 2.0 mils, the coat- ing should cover this problem, unless mill oils are getting burned into the metal, or the welder is actually creasing the substrate. I know that welding wheels can burn the mill oils into the metal and create a hiding problem for the coating. Actually, it looks like a stain. The powder coating vendor can correct the latter problem. Maybe you should spend a bit more time defining the problem that the welder is causing. —G.T. Powder coating transformer plates Q We're a job coater usually coating mild steel sheet metal parts. Recently, a vendor con- tacted us to powder coat top plates for transformers. During the discussion, the vendor told us that each powder coated plate must have 10.8 mils to 12.0 mils of film thickness, it must pass the High Voltage Test (8 KVA), the paint must not be chipped off while fix- ing the nut on the studs, and so on. We use an eight-stage zinc phosphate system to pretreat the substrate. Could you please suggest what other measures we should take to avoid rejection from quality control on these specifications. What is the right process for coating top plates of transformers? I'm reluctant to accept this job offer because we would have to strip the paint and recoat the plates at our expense. S.G., North Karachi, Sindh, Pakistan A The transformer plate coating requirements describe a func- tional epoxy coating that is up to 12.0 mils thick. This is further proved by the dielectric test require- ment (High Voltage Test at 8 KVA). All of this makes sense if you're making transformers, as dielectric strength (in- sulation) is very important to prevent the transformer from shorting out. However, if you have no experience in providing this type of functional coat- ing and only provided decorative fin- ishes in the past, you may be without the proper experience and equipment to support this effort. The process to apply this functional coating is as follows: 1. Fixture the part onto a hanger and mask any areas where you don't want the coating applied. 2. Clean and pretreat the part. (Zinc phosphate is fine for this application.) Some functional coating operations use media blasting and don't apply a pretreatment (zinc phosphate) at all, as the surface roughness from the media blasting provides excellent ad- hesion, and the thick epoxy coating eliminates any need for phosphate. 3. Preheat the part to 400°F to 500°F. 4. Immerse the part into a fluidized bed to apply the epoxy powder coating. Several dip cycles with hanger shak- ing may be required to build the de- sired film thickness (in this case 12.0 mils). You'll notice that the powder is melting and flowing on contact with the part. This is normal for the fusion bonded epoxy process. 28 POWDER COATING, April 2018 George Trigg GRT Engineering Nick Liberto Powder Coating Consultants Questions & Answers