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.



5/26/2015 - Q: What methods are used to control the finish color of metallic powder? I'm aware of the 3:1 ratio for consistency of virgin powders to reclaim. I've also read much about metallic powder consistency of color from one batch to next not being stable. What in-process methods are employed to maintain a consistent product with metallic powder? From a quality control point of view, what are typical standards for color match and gloss for metallic powder finishes? C.M., Worcester, Mass.

A: The pigmentation of a given metallic color is tightly controlled, as it would be in any other finish color. The problem with metallics is the "metal" flake that gives this finish the metallic look. It used to be that the metal flake in metallic paint was an aluminum or some type of nonmetallic material serving as metal flake. Electrostatics, which are the most efficient means of spray painting, caused the metal flake to stand up on end so to speak, especially in powder coatings. This was hardly an issue in wet coatings, but it was a huge problem with powder coatings. As a result, attempts have been made to substitute a nonmetallic flake for the same purpose but one that wouldn’t be an electrostatic antenna. Of course, this rearrangement of the metal flake gives an entirely different appearance to the finish, mostly unwanted. Mica was one of the substitutes, for example.

There are still issues with metal flake, but the powder suppliers have done a good job of reducing the problems. However, reclaim resurrects appearance problems because the reclaimed flake now contains a bit of the electrostatic charge, as does all reclaim, and it won’t react on the part being coated, quite the same as virgin material. This causes a color shift or the perception of a color shift. Not using reclaim isn’t an option if you’re spraying large quantities of powder. You might reduce the color shift by reducing the voltage slightly when you’re spraying a mix of reclaim and virgin. Beyond that, you’ll have to talk with your powder supplier to see what can be done to help.

By the way, most powder coating systems have a reclaim-to-virgin mix ratio of 60 percent virgin to 40 percent reclaim. If you can or are getting 70 percent to 30 percent, you’re doing great. I would suggest that you verify that the ratio is 3 to 1. I would be impressed if it is. And this difference may be what is causing your color shift problems.

Keep in mind that the powder coatings supplier is taking out anything below 5.0 microns as part of the classifying process. Powder particle size is based on a bell curve, with the tip ends of the bell on both sides removed. If your powder is standard grind, the average micron size is probably about 32.0 microns. Most powder suppliers remove particles 5.0 microns or less from the lower end and particles 55.0 microns or more from the higher end. The velocity of the booth air will erode some of the overspray particles as they come in contact with objects along the path to the reclaim hopper. The very large particles will readily take a charge, so very few large particles make it back into the reclaim hopper. If you have a sieving device, you can begin a program of reducing the screen micron size until the screen begins to plug. In a manufacturing atmosphere, you don't want screen that is too fine because of this packing condition, which will overload the sieving device. If it’s electrically powered, it’ll cause a load on the motor, which causes heat, which will eventually cause the powder to solidify. Then you really have a problem. Hope this helps. —G.T.


5/18/2015 - Q: I’m replacing galvanized sheet metal on a cooling tower. The metal will be subjected to water with biocides, algaecides, and metal corrosion inhibitors in it. In addition, at any given time it may be submersed, drying or wet from splashing. Would powder coating be a way to rust and corrosion proof the metal to lengthen its service life? B.O., Richmond, Va.

A: Depending on the quality of the galvanizing, powder coating may not provide a significant improvement to protect the steel from corrosion. Normally, hot dipped galvanized steel is powder-coated to improve the aesthetics of the part more than to significantly improve the corrosion resistance of the design. Galvanizing applies molten zinc to the steel surface. This zinc is designed to be sacrificed to save the steel from corrosion. Over the life of the product, the amount of zinc lessens as it degrades (like a dissolving bar of soap). Applying powder coating to the surface can inhibit this method of protection. Eventually, the powder coating will delaminate from the steel as the galvanized layer dissolves. When this happens, there is often enough galvanizing left on the steel surface to provide continued protection for some time to come. It’s true that the powder coating will protect the galvanizing from immediate corrosion effects. However, the product’s aesthetics will already be compromised as the coating delaminates and exposes the underlying galvanized surface, often in spotty areas. I hope this information helps. —N.L.


5/11/2015 - Q: The powder spray section of my powder coating line contains a cyclone recovery and spray-to-waste system for short runs. Before discharge to atmosphere, both of these filter through a water scrubber system that is supposed to remove all airborne powder before venting. The problem is the water in the scrubber is frothing badly, and the froth is taken through the system to atmosphere. This froth contains the powder color and thus is settling on vehicles in the parking lot. What is causing the frothing, and how can I permanently stop it? I am currently spraying the froth with mineral turps to minimize it. A.G., Smithfield, New South Wales, Australia

A: The chemistry of the powder, much like the chemicals in the pretreatment system, is reacting with the water and turbulence, and causing the frothing. The only way I know how to prevent this is to buy an anti-foaming agent from a chemical house, such as your pretreatment company, and add it to the system. You can get equipment that will make this addition automatically so that you don't have to be concerned about it. Water suppression systems are very unusual in the US, so we don't see much of what you’re experiencing. We would have a problem getting rid of the contaminated water, but we can spray to waste, sweep up the overspray, and then legally dispose of it at a waste landfill. —G.T.


5/4/2015 - Q: I’m trying to get powder to stick to corrugated e-flute. It works fine on the flat, but of course, it drops off vertical surfaces. Any tips on how to get it to hold on verticals before it gets in the oven? L.H., Camarillo, Calif.

A: I had to look up e-flute online first to answer your question. What I found was that e-flute is a type of corrugated cardboard used in box manufacturing. Because the substrate is paper, the electrostatics used to attract and hold the particulate on the surface are useless and the paper is non-conductive and ungrounded. Therefore, you can try wetting the paper to provide some surface tension to hold the particles or spray a conductive material on the paper before applying the powder. The conductive material that is normally used for this process is called “Rans-Prep” and is applied by using typical liquid application equipment. Good Luck. —N.L.


4/27/2015 - Q: What’s the average shelf life of a powder coating? S.S., Gainesville, GA.

A: The industry standard is usually 1 year. This standard is qualified by the conditions under which the powder is stored and the formulation. Low-cure-temperature materials won’t like a year of storage but may be OK. Uncontrolled atmospheric conditions can cause the powder to age quickly, depending on the ambient temperature of the storage area. I’ve seen powder that couldn’t be used when it was 6 months old. On the other hand, I’ve seen and sprayed powder that was almost a year and a half old that looked OK. —G.T.


4/20/2015 - Q: We’re a project management firm that oversaw the installation of a new powder line, and we’re trying to help our customer. What are some of the causes of intermittent pinholes on a finished product? We’ve looked at time and temperature, rate of temperature rise, powder thickness, and de-ionized rinse. The problem doesn’t occur on the older powder line but does occur on the new line. Any suggestions? The parts with pinholes are cast iron. Aluminum and light sheet metal parts on the same hook look beautiful, so we think the pinhole problem may be due to casting porosity or inadequate drying. It could also be due to the heat-sink nature of the casting, which may not be cool enough before the application of the powder. It’s the intermittent nature of the problem that is puzzling. I appreciate any suggestion no matter how simple or obvious. There are no egos here. T.C., Thornbury, Ontario, CD.

A: At first blush, it would seem that you have an out-gassing problem. Any type of casting, because it’s porous, will out-gas. There are ways to minimize the problem, but all of it may never go away. The powder supplier can modify the powder to gel and cure in a way that prevents the air pockets from getting to the surface. Heavy film on cast parts can help reduce some out-gassing. And preheating the parts to drive out the gas or liquid before coating will help. A lot depends on the porosity of the casting. Don’t preheat the parts and then put them through the washer. That defeats the preheat conditioning. The parts should be 100°F or lower before applying the powder; otherwise, you will build unnecessary film thickness, which is a waste of powder and money. In addition, warm or hot parts won’t build an even film, but will build an erratic film. Infrared curing on the casting could help because it cures only the powder and immediate part surface. However, if you just installed a new line, your customer will likely scream if you suggest some additional equipment. Coating castings has always been an issue with powder coatings. Liquid coatings can also suffer from this condition, but liquid flows out differently than powder and will mask the bubble. —G.T.


4/13/2015 - Q: I have a customer who is asking about the performance of powder coating at -40°F. We’re coating coil springs (high carbon steel) that are cleaned before powder coating through an aggressive shot peen necessary for spring properties. We currently are using silver TGIC-based (triglycidyl isocyanurate) polyester with a full-gloss, clear TGIC-based polyester topcoat. Have powder coatings ever been tested at such extreme temperatures? Any educated guess as to what would happen at this temperature? The customer is being rather evasive about the specific end use. I appreciate any light you can shed on this issue. P.M., Pittsburgh, Pa.

A: In one of my former lives, I worked in the appliance industry. We installed one of the first major automated powder systems for refrigerator liners, which is the interior shell of a refrigerator. This liner is designed to include the freezer portion of the unit. Temperatures in a freezer compartment can reach nearly zero, depending on the setting by the homeowner. We had absolutely no problems. We deliberately flexed the metal to see if we lost adhesion, and everything was OK. Now, I hasten to add that these units are sheet metal and not spring steel; they don’t see daily flexing such as a spring might. I tried cryogenic hook cleaning for a system that I installed, also in the appliance industry. This is a process that was being promoted by a large supplier of the gases used in this process. Couldn’t touch the powder on the hanger. Parameters were about 3 mils of powder; time ranged from a few minutes up to just short of an hour. At about an hour, we saw some cracking and crazing of the outer coating, but it wouldn’t peel from the hanger, even with a hammer blow. This process has been used on liquid paint, at fairly low film, and works. I don’t know how well it works over time. think it’s interesting that you need to put a high-gloss clear over a metallized first coat on a spring. Oh well, none of my business. I sure wouldn’t do anything unless I tested it first. One concern would be the inner-coat reaction between the clear and the silver. The other is, of course, can the coating flex along with the spring steel. Having been intimate with the refrigerator tests, I’m inclined to say it will work OK, at least for a time. Constant flexing, day in and day out, may be something else. I queried a technical director for one of the larger powder coatings suppliers who said that unless the clear is formulated for 0-T, which means the capability to withstand a sheet metal bend back on itself, it’ll craze when the spring does a tight compression. I wouldn’t commit to anything until the customer revealed some additional information. —G.T.


4/6/2015 - Q: I coated a fancy decorative metal piece with a polyester clear that bonded with aluminum. I decorated it in the backyard. It got wet during the last rain, which changed the appearance. It looks like the aluminum oxidized. It looks darker. Is there any way I can restore its original color by cleaning with alcohol or another cleaner? I really appreciate you answer. P.R., Midland, Tex.

A: At what temperature did you cure the clear coat? Aluminum forms a self-protective coating that acts as a preservative. If you didn’t clean that off just before painting, then you will not have any adhesion. If that’s so, then moisture likely got between the clear coat and the aluminum. If the coating was cured at its recommended temperature, alcohol won’t remove it, even with poor adhesion. What may happen if the adhesion is poor? If you try to remove the paint with a mild solvent, some of it will come off and some of it won’t. You need a much stronger solvent, like methylene chloride (MC) or methyl ethyl ketone (MEK), but both of these are nasty to work with manually. Take the proper precautions. If you live near one of the larger cities, try to find a company that does stripping and have them do it for you. It’ll cost a little bit, but it’ll be so much easier than hand stripping. —G.T.


3/30/2015 - Q: I work for a recreational company, and we powder coat many different items within an order. This may be 10 to 100 different tube lengths, but these tubes are scattered throughout the line depending on the color. My question is, what's the best way to identify parts before they get coated and then be able to identify them at the packing end of the line? M.U., Lewisburg, Pa.

A: Well, let's try this. You can use a metal tag maybe the size of a quarter or whatever, depending on how you identify the various parts. Affix this tag to a small magnet, and place it on the rack or wire that holds the parts. When the parts are unloaded (cool I presume), place the tag on the part. At the packing end, remove the metal tag. Caution here: The cure heat will shorten the service life of the magnet, and that metal will need to be remagnetized in the near future. Maintenance can set this up for you. I haven't a clue as to how you handle the parts through your shop, but another way is to use the tag system on the rack but shielded from powder spray. After cure, use a tie wrap on the finished part, which will have the metal tag hanging from it. Again, shipping people can remove this tag and send it back for reuse. Not knowing what the parts look like limits my assistance. Are there assembly holes available? Are these open-ended tubes? If so, you have a lot of possibilities to identify parts. —G.T.


3/23/2015 - Q: We are powder coating aluminum dock ladders in candy colors from TIGER Drylac. We are having problems with the powder peeling off in sheets as well as poor uniformity in the color throughout each ladder. We wash each part with a degreaser and then rinse it with softened water. Next, we use an acid cleaner to etch the part and then rinse it again with softened water. How do we stop the powder problems in the future? R.P., Jamestown, Ohio

A: Powder coating aluminum poses special challenges not normally seen when dealing with ferrous substrates. First of all, aluminum readily oxidizes when exposed to air, causing aluminum oxide on the surface (white powdery residue). Of course, ferrous substrates will also oxidize in air, but the oxidation (red rust) is more easily seen before coating and therefore can be removed before it causes a problem. This is the reason you are using an acid etchant to remove this oxide as the final step in preparing your parts. A second issue that is somewhat unique to aluminum is that the surface of extrusions are remarkably smoother than ferrous parts. This surface smoothness means reduced adhesion as the coating requires good surface “tooth” for the coating to “bite” onto. Often, aluminum parts are chemically conversion coated (using either zirconium or chromate) to provide both better surface “tooth” and improved corrosion resistance. Of course, you can rough-up the surface using Scotch pads or sandpaper to improve the “tooth”. A third issue that causes poor adhesion and low corrosion resistance is rinse water quality. You mentioned that you use “softened” water to rinse the parts. While softened water is suitable for household use, it is less desirable for metal surface preparation prior to powder coating. The water softening process uses salts to remove/neutralize the hardness contaminants. These salts can leave a residue on the surface if not properly balanced. The residue prevents proper coating adhesion. Using a DI or RO water process will provide suitable quality water and eliminate the problem associated with water softening systems. Finally, undercured powder coating will be exhibited as poor adhesion to the substrate. Ensure that you are fully curing the products using the PCI Solvent Cure Test Procedure #8. —N.L.


3/16/2015 - Q: Do you have any information on electrical bus bar insulation by fluidized epoxy coatings? I’m looking into what’s required for the process and technical specs on the base material. J.C., Holland, Mich.

A: Bus bars are coated via fluidized bed by some very large companies. It’s fairly easy, depending on the size of the bus bar. Epoxy is a good choice because it has some insulating factor, but be aware that it chalks if used in sunlight. Contact a reputable powder coatings supplier for the material you need. You should consider heating the bar before spraying to get maximum film build. You can do this with induction heating, or, if you’re on a tight budget, with an oven. The nice thing about induction heating is you preheat, coat, water quench, and then load right on the truck. No hands except to load it on the truck. Fluid-bed applications will work on straight-line bus bar, but "dog leg" or contoured bus will present a problem. —G.T.


3/9/2015 - Q: We’re now producing an outdoor part (steel or aluminum) with a five-stage zinc phosphate pretreatment finish and a polyester powder coating topcoat. I want to know for both substrates if a mechanical pretreatment such as sandblasting is as good as our chemical pretreatment. Also, what is the best way to finish these parts? Apply a thicker topcoat (if yes, how much?) or use a powder primer before. C.F., Notre-Dame-du-Bon-Conseil, Que.

A: Zinc phosphate pretreatment is just about the ultimate chemical pretreatment for metal. For aluminum however, I wouldn’t call it overkill, just a bit wasteful. It will not hurt, and if the line is set up for all metals, then fine, go with it. I can’t accurately tell what the best metal cleaning and preparation is for your system without knowing what it is you expect the coating to do in the field. Blasting works well for some products, not so well for others. If you have a moisture issue when the parts are in use by your customer, then the added benefit of chemical pretreatment will certainly help adhesion and inhibit corrosion. If that isn’t an issue, then you may wish to look at and test media blasting and powder coating. Depending on end use, you might consider a two-coat system: Media blast, zinc primer, then polyester topcoat. You know that media blasting might alter the surface of the metal, depending on what it has to remove from the metal and on the type of media material you use. Powder coatings manufacturers design powder coatings for certain film thicknesses. Applying the material heavier than recommended will lead to a negative result, and thicker won’t solve an adhesion problem. If you think you could have field failures due to atmospheric conditions, then a two-coat system would work well. And as I mentioned, if you go that route, you could media blast and not use chemical pretreatment. Of course, you probably wouldn’t want to use the same media particle size or material on the aluminum that you would use on the ferrous metal. —G.T.


3/2/2015 - Q: A new customer discussed a problem he had with his old powder coater. He explained that his old guy only processed his oven time at 30 minutes max no matter how thick the material was. How can I best explain to him how the finished product will be with under-heating? I’ve only been in business for 1 year and am still learning myself. B.S., Yucca Valley, Calif.

A: At the risk of getting extreme, the proper method of curing is to know what the metal is doing. If the powder coating is scheduled to be cured at 360°F for 10 minutes, then that’s what you need. If you don’t have any idea what the part temperature is, you’ll have cure all over the chart. Thirty minutes at some temperature is pretty safe from some aspects, but it could be grossly over-curing the part. If the parts are heavy castings, or in other words, heat sinks, they could be under-cured, which would be the worst of the two conditions. If the former coater has been doing this for a long time, he can probably get fairly close to time and temperature by experience alone. However, to arbitrarily cure everything for 30 minutes? I wouldn’t want him coating my 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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