Products Finishing

APR 2016

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PFonline.com/experts 40 APRIL 2016 — PFonline.com THE VOICE OF FINISHING 80 YEARS P F o n l i n e . c o m / e x p e r t s POWDER COATING C L I N I C RODGER TALBERT powdercoating@PFonline.com Proper Coating for Steel Tubing Q. I currently have a client who manufactures trampolines for backyard use. The structure is made of steel pipe and coated with black powder, which is applied for corrosion protection and appearance. The steel is a Chinese grade DC51D+Z, and the composition is an equivalent ASTM grade (the +Z denotes a zinc-plated coating). The current material is a smooth black polyester triglycidyl isocyan- urate (TGIC) powder coating. Currently, a zinc-rich epoxy primer is used with a half cure, followed by the polyester top coat with a full cure. Do you have a recommendation for a powder coating/possible vendor? A. TGIC polyester is a good idea for the topcoat material. I would use a high-grade epoxy primer without the zinc, unless you are blasting the steel before coating. Zinc in the resin is most effective when it can make intimate contact with the steel to provide the needed cathodic protection. On a smooth surface, not much of the zinc will contact the steel, so the benefit is marginal. The zinc will also make the coating more brittle in areas of stress, so a standard, high-grade epoxy primer may make it easier to avoid cracking. The supplier should be able to help you with the cracking problem. Powder Coating Assembled Parts Q. We currently use an air-dry liquid paint. We want to powder coat our parts for better appearance and dura- bility, but we are concerned about the high temperatures needed to cure a powder coating. Are there any powder coatings that can be cured at a temperature below 200°F? If not, do you have any other ideas about how we could cure a powder coating without causing damage to the sensitive parts? A. The lowest curing powder coatings are in the range of 250°F and that is for epoxy resin systems that do not have good resistance to sunlight exposure. You will need a polyester resin system to get the ultraviolet protection. The lowest temperature polyester products will probably need to be cured at 325°F or higher, so I do not think low temper- ature curing is going to help with the heat-sensitive parts. Consider coating all of the metal parts before assembly. I have done this with more than one manufacturer and had excellent success.This requires a lot of changes to the manufacturing process, but the results are so positive that it is well worth the effort. Parts are coated inside the areas that overlap in assembly, so no rust occurs between two parts that lie flat against one another. Holes have some coating in them, too, providing some additional corro- sion resistance around fasteners. The powder coating is a thermal-set material, so it will be much tougher than most air-dry products. If you store parts outside in a yard, you will be impressed by how much better they handle sun and rain compared with its air-dry counterparts. Adding More Cure Capacity Q. We have an old cure oven that we use for a liquid paint that gives us about eight minutes of metal temperatures between 365°F and 400°F on our heaviest part. We have trouble with consistent color and gloss with some of our colors, and we think it is related to the cure oven. We recently took on some work from another manufacturer, and we cannot get a full cure on the powders that are approved for its products. The powder film looks good when it comes out of the oven, but it is not scratch resistant, and a cleaner that we use before packaging takes off some of the coating. Our oven is too short, and we are not sure of the best way to add more capacity. Can the powder material supplier change the new powders so they will work in our oven? Should we consider adding an infrared section or adding more convection time to the oven? A. First, you could slow the line down, which will give you more exposure time. Second, the supplier may be able to give you a lower cure temperature/faster cure powder that will help. That may add a little cost, and the powder is less stable in a hot atmosphere, but it is a viable way to help with a short oven. If you exhaust these options and think an oven addition is the only way to get the needed capacity, then you will need to evaluate several issues. More convection will probably work well if you have enough space in the line to install it. Convection curing is reliable across a variety of parts, and the cost operation is modest. If you do not have the space, consider an infrared (IR) booster oven, which can be an excellent way to get more cure power in less space. Keep in mind that IR curing is less flexible across a variety of part shapes and masses, so you will want to test it before you commit. With IR, you can get more cure impact in a smaller space. The challenge is to make sure the part geometry has enough surface, low enough mass and hidden surfaces to take full advantage of the radiant energy from the IR emitter. Compare the size, cost and benefit carefully to determine what type of oven will work best for your application. Contact Rodger Talbert at rtalbert@colmetsb.com.

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