Issue link: https://www.e-digitaleditions.com/i/954267
Switching powder coatings and causing a coverage problem on inside edges Q We powder coat our cold rolled steel sheet metal with powder that has high texture and gloss. From time to time, we receive our parts with missing coating on the inside edges, and bare metal can easily be seen. This is not acceptable to us. Our units are used indoors and should last at least 20 years in service, so no rust should be seen during that service. We were happy with the powder we were receiving from our previous supplier; however, when we switched to a new one, we started having coverage prob- lems on the inside edges. Our film thickness requirement is 0.002 inch to 0.004 inch thick. We also see pinholes on the inside edges from time to time. Can you please comment on the cause of these problems? I.K., Calgary, Alta. A Light coverage on the inside of an edge sounds like a Faraday cage problem. And the newer powder may be "hotter" than the previ- ous material so that the Faraday cage ef- fect is much worse. It may also be that the new powder coating is the cause of the pinholes. Why don't you go back to the other supplier if that one worked? If that isn't possible, you should have the new sup- plier modify the formula to duplicate what you had previously. Otherwise, you're going to have to experiment with spray gun settings and maybe gun posi- tioning to overcome the lack of cover- age in the corners. You shouldn't have to do this if you were problem-free with a former powder vendor. —G.T. Finding reasons for moisture in a powder coating Q We're having trouble with a particular powder that we use. When we had the pow- der analyzed, the moisture content was at 0.06 percent. It tested at 1.2 percent when it came in the door. After fluidiz- ing it for a few days, it dropped signifi- cantly. This seems very low to me. What would the normal range be for a polyester-hybrid-type powder? K.H., Muskegon, Mich. A The moisture in a powder coating is affected by climatic and storage conditions. Pow- ders are normally shipped in sealed bags within boxes or drums to keep moisture from being absorbed into the material. Rapid changes in climatic conditions (from hot to cold, from dry to humid) can have a dramatic effect on the mois- ture content in the powder. This is why it's always recommended to move the powder from a storage room to the point of use 24 hours ahead of time to ensure that the powder has acclimatized to the application environment. Open- ing a sealed bag of powder before it has acclimatized will ensure rapid moisture development in the material. Moist powder has a tendency to clump, making fluidization and pumping diffi- cult. This clumping can cause clogged pumps, blocked hoses, and "seeds" on the part, as well as an increase in the in- cidence of impact fusion. The typical way to reduce moisture in powder is to fluidize it with clean and dry (less than or equal to 38°F dew point) compressed air for a period of time greater than or equal to 3 hours, depending upon the amount of moisture in the powder coat- ing. This is why the moisture content reduced after fluidizing it for a while (a good thing). Now to your questions: "How low can the moisture be in powder coating?" It can never be too low. The dryer the powder is, the easier it is to fluidize, pump, atomize, and control film thickness and surface appearance. Moisture will also affect the electrosta- tic charging efficiency. Too much moisture will lower the electrostatic charge on the particles and reduce transfer efficiency and film build, therefore, you cannot be too dry. The only issue with "over-charge" powder particles is that Faraday cage problems may increase, and thicker coatings will exhibit back ionization. Both of these problems are easily corrected by using the gun charging controls provided on the gun control panel. —N.L. Burning off hooks Q Do you have any informa- tion on standards for how long metal hooks can be burned off before the strength of the hooks has been compromised? J.L., Litchfield, Minn. A A metallurgist could tell you, and an engineer familiar with standard engineering books could look up some information that could tell you what size steel rod would hold up to the temperatures of a burn- off oven. A 1/2-inch rod will hold up for some time, but eventually it will warp to some degree. The design of the rack will influence the onset of warping, too. George Trigg GRT Engineering Nick Liberto Powder Coating Consultants Questions & Answers POWDER COATING, March 2018 37