Weekly Questions and Answers

By George Trigg, GRT Engineering
and Nick Liberto, Powder Coating Consultants

Welcome to Powder Coating magazine's Weekly Questions & Answers column. Questions for this column are submitted by powder coaters just like you who are seeking ways to improve efficiencies and solve every day problems on their powder coating lines.

12/15/2014 - Q: Can UV-curable powder coating be used on functional wear surfaces? Can I get information on companies that currently UV powder coat? C.P., Auburn Hills, Mich.

A: The functionality of a powder coating has little to do with the method of cure. However, it must be fully cured to provide all the expected coating characteristics. Having said that, powder coatings must be selected on their ability to meet your functional and appearance requirements. Choose a powder coating that has the properties you require, whether it’s a UV-curable or standard cure powder coating, and you won’t go wrong. Go to Powder Coating magazine’s Web site at www.pcoating.com and click on Online Buyers Guide for a list of powder coatings manufacturers of UV-curable powder coatings. —N.L.

12/8/2014 - Q: How do you powder coat hinges without taking them apart? C.G., Fogelsville, Pa.

A: It’ll be very difficult to powder coat hinges and have them functional after curing. And if you finally break loose the moving parts, there is a high chance that you’ll chip powder from a part of the hinge. You might have a chance if the powder has high edge pull, but that could be self-defeating because every edge will have low film. If you could hang the hinges from a rack that "jiggles" all the way through the oven, you may get some percentage of the hinges to be free, but don't bet any money on it. In the long run, the easiest and cheapest way is to coat the hinges as parts and assemble them after coating. —G.T.

12/1/2014 - Q: We’re a powder custom coater with several powder coating conveyors. We’re planning to install a batch-with-liquid (non-water base) application near from one of these. What are the quality issues with this project? We want to know if it's possible to apply powder and liquid coatings in the same area. C.F., Notre-Dame-du-Bon-Conseil, Que.

A: You must consider several issues before installing a liquid spray booth in an area close to a powder application booth. First, let’s talk about safety issues. Liquid systems have a completely different set of rules than powder systems. For instance, the electrical devices in the area may need to be vapor tight explosion proof; those around powder systems need to be dust tight explosion proof. Be sure your new liquid installation conforms with all safety requirements before operating it.

The second issue is visual quality problems caused by airborne particulate. If you have high appearance standards for your liquid parts, and dust and dirt contaminants aren’t allowed, then placing a dust-producing operation (powder booth) next to your liquid booth may not be the smartest idea.

These are the two primary concerns I have for your plan. Having said that, don’t forget the human factor of solvent spills affecting your powder application system or powder spills affecting your liquid operation. Otherwise, powder and liquid operations have coexisted together in many operations for many years. —N.L.

11/24/2014 - Q: For the past 6 years, I’ve been outsourcing the powder coating of the product that we manufacture. Our product is a 6061 aluminum and is used outside. It’s installed from the beaches of Florida to Hawaii. My question is what type of prep should be done to the old powder coating if we decide to repaint on site? Could it be power-washed hard or should the old finish be removed? If the old finish is failing, what is the new paint holding onto? How can the old powder-coat finish be taken off at someone's home? I’ve looked into a high performance water-based epoxy primer and finish coat recommended for aluminum and galvanized metal. Do you have any recommendations? What are your thoughts on the subject? B.E., Collegeville, Pa.

A: Right up front, you must understand that you have to properly prepare the aluminum before powder coating. Sandblasting won’t do it, especially in a salt-laden atmosphere. If the powder coating is flaking off the aluminum, it’s caused by little or no metal prep in that area. The aluminum must be properly cleaned and conversion-coated immediately before a good quality triglycidyl isocyanurate-based (TGIC-based) polyester powder is applied and cured to the recommended cure cycle. That is metal at temperature, not just oven time. If this action requires some awkward handling of the part, so be it. That’s better than the time and cost of rework.

Now, how are you going to do the field repair? You must sand away the blistered area and get back into the paint that has good adhesion. Then, you’ll need a field metal prep kit, which you can get either from an automotive refinish store or from any large pretreatment company. After following the manufacturer’s recommendations, apply a two-component liquid finish. This is also available at your automotive refinish store, or you may find it at a marina that does boat repairs. DO NOT use a zinc-rich primer. The products that you mentioned are good, but if I’m going to have to go to the field to do repairs, I want to do them only once. The two-component materials will keep you from making a future return trip. —G.T.

11/17/2014 - Q: We coat pipe with powder coating. We have to put 8 mils to 12 mils of powder on the pipe. The pipe sizes range from 1.05 inches to 2.875 inches outside diameter. We have good results on the smaller size pipes, but on the larger sizes we’re having trouble with the powder coating blistering after the pipes come out of the curing oven. Do you have any suggestions, or do you need more information? Please let me know. M.K., Shiner, Tex.

A: I don’t have much to go on here to provide an accurate diagnosis. However, since you’re stating that the problem is exhibited after the cure oven, it may be related to your cure cycle time. Because the larger pipes have more mass, they’ll require a longer cure cycle time. The cure cycle time is equal to the metal bring-up time plus the cure dwell time from the powder supplier (cure cycle time = metal bring-up time + cure + dwell time). Perform an oven profile test with the large pipe and your current oven cure cycle time. Verify that the part is at metal temperature for the prescribed time specified by the powder supplier. If not, adjust your time or temperature accordingly. —N.L.

11/10/2014 - Q: We manufacture pump tanks that are installed inside or outside, depending on the installation. We currently powder coat our tanks with a film thickness in the range of 2.0 mils to 2.5 mils. We’ve been questioned on why we go so thick with our coatings, as we’ve been told that 1.0 mil to 1.8 mils should be sufficient. Just wondering if our thicker coating offers any rust-protection advantage, or are we just wasting powder? Our powder coating is applied on cold rolled steel shells that have been prepped with zinc phosphate. W.S., Stratford, Ont.

A: All things considered, you’re giving your customer more than he’s paying for and more than is needed. There will be some added benefit to the heavier film, given adequate cure. It’ll be a bit more difficult for anything to penetrate the surface, but that benefit could be subjective. If you can lower the film thickness a tad and maintain the uniformity of the coating, you would be a better business person. That statement means that you’re giving away some powder on each part that’s coming out of your profits. Depending on how many parts are involved, you could be talking about a nice piece of change. Your powder formula will indicate the best coverage for the material you’re using. I would be a little concerned about a 1.0 mil or lower limit. That thickness courts trouble in case something goes wrong, which is inevitable. I would shoot for a 1.4-mils minimum to a 1.8-mils maximum. This will do nicely, especially over a zinc phosphate, and the orange peel should be minimal. —G.T.

11/3/2014 - Q: We have issues with nylon powder coating drawing up from the corners of parts. We preheat the parts and apply the powder so that it adheres to the part surfaces immediately. Occasionally, however, the coating will shrink and draw back when the parts are curing in the oven. T.C., Glasgow, Ky.

A: Do you have a primer on these parts before application of the nylon? You didn’t mention it, so I’m not clear on that part of the process, but that could be the problem. And too much heat may cause some pull-away from the edges and corners. So, maybe you’re trying to use the preheat in place of the primer. Any irregularity in parts, such as thickness, and any change in conditions could cause a sporadic cure problem. Maybe the preheat temperatures are inconsistent. Maybe the oven temperature or air circulation isn’t the same every time. How old is the nylon powder? I am unaware of any real issues with shelf life providing the powder is used in a reasonable time and stored in conditions dictated by the supplier. —G.T.

10/27/2014 - Q: We’re considering options for staging multiple stand-alone carts that will hold multiple pieces that are being powder-coated. After the wash process is complete, we want to load these stand-alone rolling carts with work. Because these carts aren’t attached to a conveyor and grounding is necessary for the powder to stay attracted to the work, is there a stand-alone device that we can attach to each cart to create grounding? Another option would be to attach a rolling cable from the conveyor to the cart; however, we need more flexibility with staging of work. Your input will be greatly appreciated. E.N., Midland, Tex.

A: The simplest and cheapest thing you can do is attach a spring-loaded alligator clip on the other end of a ground cord to the powder system. When you wheel the cart to the application area, just clip the ground wire to the cart frame. The ground wire must make a complete circuit with the spray gun to be effective. I don’t know of a device, per se, that can do that any better. Having said that, if the floor of the spray area is metal, and if the carts have metal wheels, you might get a ground that way. That’s a lot of ifs. The worst part is there are a lot of contact points here that can cause loss or reduction of ground, so this method isn’t the most reliable method to use. You must surely know that spraying the parts while they’re on the cart is a sure way to get a lot of powder on the cart. Eventually, the cart should be cleaned. How will you do that? Or maybe the cart is small? —G.T.

10/20/2014 - Q: I have two sets of wrought iron gates that are newly fabricated and galvanized — two 3-foot-by-8-foot panels and two 3-foot-by-4-foot panels. The subcontractor brought it to a powder coater to get it painted, but then informed us that the paint has all these bubbles and holes in it after coming out of the oven. The general contractor brought it to another painter who said the same thing would happen. What are they doing incorrectly or what do they not know in order to powder coat these galvanized wrought iron gates properly? I understand that there can be issues, but that they can be mitigated with the correct surface preparation methods. Any advice or recommendations? How about using an epoxy sealer before powder coating or trying special degassing powders? Our general contractor has all but given up on getting these done. Help would be much appreciated. J.D., Lakeland, Fla.

A: Powder coating over hot dipped galvanizing is a sketchy proposition and results can vary widely. Hot dipped galvanize is an unstable substrate for powder coating as outgassing normally occurs during the cure process. Some people have success when powder coating within 24 hours after galvanizing after performing a quick brush blast to remove surface contaminants and preheating the parts in the cure oven above the cure temperature for twice the cure time before coating to drive out internal gasses. However, this methodology does not always produce predictable results and you may still have bubbles and pinholes in the coating. As the galvanizing ages, these problems can get worse, since the galvanizing can absorb moisture over time and the brush blast may not remove all the surface soils. However, galvanizing that has aged up to 2 years can become more stable and easier to coat without pinholes, but still requires aggressive cleaning to remove the zinc oxide. All these stages and cleaning methods are outlined in ASTM D 6386, a specification for preparing hot dipped galvanized parts for painting.

Please realize that powder coating over hot dipped galvanized steel does not provide any appreciable corrosion resistance, it just makes the product look more appealing. Galvanizing protects ferrous substrates by sacrificing itself to protect the substrate. Since the hot dipped galvanizing is often applied at 12 mils or thicker, this can result in 30 years of protection, depending upon what conditions the product sees. This sacrificial reaction with the elements will occur even under a powder coating. However, when the sacrificed zinc oxide material is formed and “sloughs off” the part, it will carry the coating with it and expose a new layer of galvanizing. This situation will require constant maintenance and recoating (or at least touch-up) over the life of the product, whereas galvanizing alone will provide similar corrosion protection without any maintenance over the product life, as only the patina of the zinc will change. This has been the subject of many a lawsuit, as customers expect maintenance-free product life from galvanized products but want them to look more aesthetically pleasing by applying a coating over the galvanized surface, resulting in “premature” coating failure and costly repainting. —N.L.

10/13/2014 - Q: What is crazing and what causes it? We clean and then send printed (screened ink) aluminum sheets, 0.018 inch to 0.030 inch thick, to a powder coater. This is a polyester powder coat applied at 1.5 mils to 3.0 mils thick. When the sheets are returned, we machine them and run them on a belt through an oven at 280°F to remove "blushing.” Unfortunately, the parts are crazing. To stop the crazing, we have to run the sheets before forming through the oven first. Sometimes, we have to run them through a hotter oven at 400°F. Any idea what causes the crazing? My understanding is that crazing is small hairline fractures in the powder coat. If we ship parts that show crazing to our customers, and they are applied to vehicles, are they susceptible to corrosion? M.H., Munich, Germany

A: Maybe. If you’re lucky, it will not go to bare metal, but I would not bet on it unless I looked closely at the cracks under a glass. It can depend on what is causing the crazing. It can be caused by too much heat. It can be caused by surface tension between two coatings that aren’t compatible, and on occasion between the substrate and the coating. The 280°F oven temperature is almost a 100 percent overbake of the powder. I wouldn’t think that would cause the material to craze, but you never know.

You should check for compatibility of the two coatings with one another. One way you can do that is to use cold rolled steel test panels. Spray on the base coat, cure it out, apply the screening or the second coat, cure that, and then take a look. Use a glass to be sure there aren’t miniscule cracks. I could also make a case for the thin sheet aluminum becoming "active" during the bake cycles and causing the coating to move when it doesn’t want to. The aluminum won’t corrode, but it will develop a patina coating where it’s exposed to air. More important, if the cracks are to the substrate, you now have an entry for moisture to get under the coating and cause it to blister or lift. The aluminum will be safe, but the part will look bad and cause complaints. —G.T.

10/6/2014 - Q: Our powder coating operation is currently facing a problem with impact test failure. Does zinc phosphate pretreatment play any role in this type of failure? If so, how do we get our coating weight in the correct range to pass the impact test? A.M., Navi, Mumbai

A: Impact fusion is a mechanical property that is formulated in the powder coating. Once the formula is made you can only degrade this maximum level of impact by process problems. Therefore, the first thing you need to do is to verify that your powder coating formula is capable of achieving the target impact resistance (direct and indirect) you want to attain.

After you have verified that the formula is capable, then you can look at process issues. The most common process issues that affect impact and other mechanical properties are under/over cure, high coating thickness, and poor pretreatment/cleaning of the substrate. All of these process related issues must be performed within tolerance for the coating to perform under mechanical stress.

You specifically asked about your pretreatment (zinc phosphate) as being the root cause of your impact failures. The only way zinc phosphate can affect coating impact performance is if it has been applied too thick (too high a coating weight). If you stay under 140 mg/square foot you should have no problems with impact resistance caused by the zinc phosphate. For reference, iron phosphate can be applied up to 70 mg/square foot before it can be problematic. —N.L.

9/29/2014 - Q: I need to ask a question that I’m sure everyone who applies powder coating is struggling with. Do you know how to eliminate the extreme flashing around threaded holes when plugged with a silicone plug? The buildup that we’re seeing is with heavy-mil powder coating. If anyone knows of a special plug system, or another way of masking these holes, I’d greatly appreciate it. J.M., Churchville, N.Y.

A: Other than experimenting with masking from a dozen suppliers, the only suggestion I’d have is to use a very small vacuum line to remove the powder that is at the bridge between the current thread mask and the body of the part. This can be labor intensive unless the situation is consistent and you can automate. There is some commonality in what you’re doing, yet each company has its own part design, and therefore, a lot of time is spent in experimentation. —G.T.

9/22/2014 - Q: We’re using forced air (fans and portable evaporative coolers) to cool our product after it leaves the oven so that it can be packaged, a process done by hand. We’re having a problem getting the product cool enough to handle when the ambient temperature rises in the summer months. (The plant isn’t environmentally controlled.) I was wondering if using chilled air would work better. Will it affect the product by interfering with the curing cycle? What are your thoughts on this? Thanks. I’m learning a lot by reading your column. R.B., Harvey, Ill.

A: The powder coating should have reached its cure-time-and-temp cycle before it leaves the oven. It won’t post-cure as a solvent-based material will. It’s common to use chilled air for cooldown after the oven. This won’t be the cheapest thing to do because you’ll need some type of chiller. Make sure there aren’t any chilled air currents getting to the oven that will affect temps inside. This shouldn’t be an issue but just a note about things to be aware of! Should I assume that you have used fans to aid in the cooling? Forced-air moving across the parts will do wonders in lowering the temp of the parts. —G.T.

Further reading on the problems discussed in this column can be found in our Article Index and Bookstore.

George R. Trigg is president of GRT Engineering, 6314 Hughes Road, Prospect, OH 43342; 740/494-2496. He has been involved in the powder coating industry for more than 38 years. He holds a BSBA degree from Muskingum College, New Concord, Ohio. His email address is molly95@earthlink.net.

Nick Liberto is president of Powder Coating Consultants (www.powdercc.com), a division of Ninan Inc., 1529 Laurel Avenue, Bridgeport, CT 06604; 203/366-7244. He has more than 3 decades of experience in the powder coating industry. A registered professional engineer in Connecticut, he holds a bachelor’s of science degree in mechanical engineering with a minor in physics. His email address is pcc@powdercoat.com.

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