Products Finishing

NOV 2013

Products Finishing magazine is the No. 1 industrial finishing publication in the world. We keep our readers informed about the latest news and trends in plating, painting, powder coating, anodizing, electrocoating, parts cleaning, and pretreatment.

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CLINIC POWDER COATING Refurbishing Gas Cylinders Q. We refurbish LPG cylinders and apply powder coating over the shot-blasted cylinders, but we have trouble getting coverage into the deep areas where the collar and footring are welded onto the pressure vessel. We hand-spray the cylinders in those critical areas prior to conveying the part through the auto-spray, which uses reciprocators and electrostatic guns. We have tried all sorts of nozzles, but still the problem persists. Since the cylinders are being used outdoors, rust lines are developing at the gap areas. The electrostatic charging is fine, but the powder is not getting deep enough in those gaps. I am looking for a nozzle that can resolve this problem. —C.T. A. The areas you describe are hard to cover because you have a tough angle to penetrate and you also have electrostatic resistance. A fan spray tip would be the best, with a gun or guns focused on the problem area in a fixed position. Rotation of the part is helpful so that the gap is exposed to the spray pattern from every angle. You also need the right flow rate to push the powder into the recess, but avoid excess velocity and keep the micro-amps low (<25). Be aware that if you use automatic guns only, you will have heavier film build on the rest of the part, but you should be able to get the gap coated with a fan spray nozzle and the right settings. Coating Options for Steel Q. We manufacture steel and aluminum mounting structures that are used to support solar panels in a variety of environmental conditions. Historically, we have used powder coating for the steel items, but we are now redesigning this line of products and assessing using structural steel members instead of hot- or cold-rolled sheet metal punched/ formed as needed. The powder coating is not giving us as much protection from rust as we would like, and we have concerns about the prep that will be required from structural shapes. Hence, we are beginning to compare the benefits of powder with a wet-coat system.  We anticipate the need to coat several thousand parts per week while we are ramping up, but we do not have the ability to coat in-house, nor are we looking for that ability. We would prefer to find a company in the San Francisco Bay area that will be able to assist us in coating the components in volume. Where might I find information on wet-coat processes we might consider and compare to powder coat? —C.C.  A. If you are looking at liquid options to get better rust protection, you are chasing the wrong option. Liquid coating options will not enhance the corrosion protection just because it is liquid instead of powder. If you currently have premature corrosion problems it is because the process is flawed, not the powder coating. For structural steel in outdoor locations you need to do the right overall process whether it is liquid or powder. I would strongly recommend that you have the steel blasted, primed and top-coated. The 42 NOVEMBER 2013 — pfonline.com pfonline.com/experts blast operation will remove the impurities associated with rough-gauge structural steel (mill scale, carbon, rust, etc.) and provide a good anchor pattern for the coating. The primer should be a zinc-containing material. This will provide good resistance to moisture penetration, good corrosion resistance and good edge coverage. The topcoat should be a high-quality polyester material that will hold up in sunlight and provide further resistance to corrosion. The advice RODGER TALBERT for liquid coating Consultant would be the same powdercoating@pfonline.com but it will probably cost more without providing the same corrosion resistance. You may also have trouble finding a qualified liquid coater in the Bay area due to limits on the solvent content of paint, but try looking for one on the Powder Coating Institute website, powdercoating.org. Proper Coating Thickness Q. In phosphating, what coating thickness should be maintained for carbon steel components? Will the coating thickness differ from part to part after phosphating? —T.J. A. By thickness of the coating, I assume you mean the powder applied after phosphating. The coating thickness is related to the particular powder used. Each powder will have a range of thickness on the Technical Data Sheet (TDS) supplied with the coating. Typical film thickness is around 2 to 3 mils. You may want a thinner coating for cost savings, or you may want a heavier coating for better edge coverage and performance. The coating thickness is not affected much by the phosphating. Coating Process Options Q. Following are customer requirements for plating and powder coating on fluid-handling tubes, some of which are bent before powder coating and some after: 1. Rack alkaline zinc-nickel (8-16 microns) and trivalent clear passivate. 2. Customer bends some parts before powder coating. 3. Those parts bent before powder coating will have some forming oils/lubricant on the Zn-Ni plating. Quality concerns include removal of oil from the plated tube prior to powder coating and powder coating adhesion to the plating. What pretreatment process would you

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