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6/1/2019 | 7 MINUTE READ

Powder Coating Hot Dipped Galvanized Steel

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If you are not sure if the 3-stage washer is the best way to prepare the metal for powder, then Rodger Talbert has some tips to get the material ready.

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Q: We run a job shop powder coating facility with a 3-stage iron phosphate washer for pretreatment. We have been doing mostly mild steel products and a few aluminum parts, but now we are getting some hot-dipped galvanized steel parts and I am not sure if the 3-stage washer is the best way to prepare the metal for powder. Can you please comment on how well our washer will work for galvanized steel? Is there something else we should be considering for these parts? It is an important order and we do not want to make any mistakes with this customer.

A: The 3-stage spray washer is a minimum type of pretreatment system for cold-rolled steel (CRS) product that will be used indoors. The cleaner coater that is used in the first stage can remove rust inhibitor and add an iron phosphate coating so you have a decent surface for adhesion on CRS. If you use that washer for galvanized steel, you will remove shop dust and light oil, but you may have serious trouble with long-term adhesion. Hot-dipped galvanized (HDG) steel has several challenges that could be a problem—the zinc coating is not uniform and it may include clusters of zinc or other elements that are not tightly adhered and not helpful for adhesion; the surface is very porous and may trap oil or other materials in the pores of the metal; or the zinc will not react to the iron phosphate and will not have any adhesion promoting properties. In short, the washer is not suitable for HDG. If you want to ensure good performance on the HDG, you should do a brush blast over the metal and then blow it off before powder coating. It may be that the parts will be used in a noncorrosive environment, but that is not likely because it makes no since to use a zinc-coated metal for indoor applications. If the parts will be used outdoors (highly likely), you will want to apply a primer coat to help with corrosion resistance, adhesion and to help overcome the rough and porous surface of the zinc-coated metal. Apply an epoxy primer coat and partially cure it; then apply your topcoat and run a full cure cycle. That is the right way to treat this metal. If you want to better understand what “brush blast” means, check out the National Association of Corrosion Engineers (NACE) number 4 or the Society for Protective Coatings (SSPC) SP 7. It is a light blast to remove inclusions and create a more uniform surface. If you cannot accomplish this blast process, I suggest you stay away from the zinc-coated steel substrate.

Film Build Control

Q: We are a manufacturer and we run our own powder coating facility. We have a manual batch-type coating process with parts hung from carts and moved from one station to the next by pushing/pulling the wheeled cart around. Our pretreatment process is a three step spray wand. We have two powder booths and we use manual spray guns. We have been having a lot of trouble lately with light coating and heavy coating. Our minimum film build requirement is 2.5 mils and we have some films measured under 1.5 mils. We also have some product that has exceeded 9 mils. We run two shifts to try to keep up with our volume but we still fall behind. The light coating has forced us to do a lot of rework and also created a lot of customer returns. We seem to be falling farther behind instead of making progress and have run out of ideas to improve our film build consistency and reduce our rework. Any suggestions are welcome.

A: The first thing you should consider is if the system is the right size and design for your work. If you are running two shifts, cannot keep up and have a lot of rework the answer is most likely that you do not have a system that is big enough or arranged correctly and you need to either add to it or rearrange it so that you are set up for the right volume. Look at the flow of work in the facility; do you have bottlenecks where carts cannot be easily moved to the next station? Do you see a lot of sharp turning and interference that slows down movement? Are there a lot of racked parts in progress that get in the way of one another? If the answer to these questions is yes, you have a poor system. You need more space or you need a better layout to open up the area and move freely. Work on a better use of space or the addition of more space or equipment to get things to open up and move better. Pay attention to the equipment. Do you use a separate drying oven? You should be using a separate oven for drying. Does to your equipment have flow through from end to end or does everything enter and exit from the same end? These may be areas to improve flow.

Another thing that comes to mind is application technique and the amount of second coating going on. If your two shifts are behind and know it they may be rushing the process and not inspecting the coating before moving the parts to the cure oven. Encourage the coaters to make sure they get it right before cure. Light coat often leads to heavy coat because the second layer adds a lot of film. These can also give you more orange peel, drips and dry spray. Application technique should focus on consistent patterns with modest flow rate and consistent gun-to-target distance. Training and practice can develop good technique and then it has to be reinforced and encouraged. “Hurry” is never a good idea. Patient and accurate application is better in the long run.

Get the system flowing well and encourage sound application practices as a priority. And take a good look at that second shift to see if you have a drop off in skill level. If so, work with them to get them trained and make sure there is no difference between the two shifts.

Need to Reduce Rejects

Q: We are struggling to get our first-pass yield up and reduce the number of parts that we have to rework or reject. We have a monorail system with a five-stage washer, dry-off, automatic spray guns and manual touch-up, and a convection cure oven. We seem to have the ability to get out good parts, but we are not as consistent as we need to be. In reviewing our problems, we have begun to think that it may be related to our racking arrangement.  We use some simple hooks and few utility-style racks. Some of our racks have a lot of buildup on the contact area, and rack maintenance is inconsistent. We have some bent and damaged racks; and we use some hooks that do not hold as many parts as we could fit on the line with better racking. Is there any way to qualify the impact of racking on the powder-coating process? Are there any other factors to consider for racking?

A: Racking is one of the most important and least respected parts of successful powder coating. You must have good ground at all times to avoid rejects. You should position the parts in ways that make them easy to see and cover. Racks should be well maintained. The spray zone should be used effectively for maximum throughput with minimum rejects.

You can evaluate the cost of racking to determine the difference between an optimum racking scheme and a questionable racking scheme. You need to evaluate what your throughput could be if you had proper racks and compare it to how you run it now. You can build samples to test theories and compare the defect rates to any other racking arrangement. If you can quantify the current throughput with your existing racking and quality, you can compare them to any other rack option. It is simple math to evaluate the number of parts per foot and the number of good parts versus bad. Many companies waste a lot of dollars by using suboptimum racks because they do not want to invest in tooling; do not want too many different rack designs around; or just do not even evaluate it at all. Evaluate the options and there is a good chance you can improve the throughput and reduce costs by using the right racking scheme. If you like rejects, use bad racking. If you do not like rejects, concentrate on good racking schemes and keep the racks in good condition.

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