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.

11/28/2016 - Q: Q1: I’d like to powder coat the frame of an experimental aircraft, but I’ve heard that the powder coating process can weaken the metal. The metal involved is 6061 T-6 aluminum. I’ve also read that the temperature and cure times can be set to avoid any change in the alloy. What are the correct parameters for maintaining the structural integrity of this aluminum alloy? R.M., Bradenton, Fla.

Q2: Do you have any information, recommendation, or advice regarding the concerns that powder coating and the temperatures used will weaken or affect the strength of forged rims. T.T., Kapolei, Hawaii

A: I’ll answer both R.M. and T. here because their questions are related. Organic thermoset powder coatings (what most people know as standard decorative powder coatings) have cure temperatures between 250°F and 450°F. The cure times at this temperature vary but can be as much as 30 minutes for the lower temperatures and as fast as 10 minutes for the higher temperatures. Pretty much all ferrous materials (steel and iron--tempered or not) have no difficulty with these temperatures and times. However, tempered aluminum (6061 T-6 is tempered aluminum) and softer metals (copper, lead, tin, etc.) can have problems with temperatures above 350°F. The metal temper can be annealed (softened), or the metal may begin melting if it’s subjected to temperatures higher than 350°F. Therefore, any products that have these materials must be cured at temperatures below 350°F, even if it takes longer to complete the cure of the particular powder coating. @b1:T., you have no worries with your forged steel. However, R.M., you shouldn’t have your parts cured with powder coating above 325°F just to be sure you don’t soften your aircraft structure. You should also be aware that powder coatings are heavier than liquid coatings because they’re applied much thicker. This fact may also cause some issues with adding unnecessary weight to your aircraft as well. —N.L.

11/21/2016 - Q: When I apply a second coat of powder onto a part, the powder doesn’t cover the side of it. I have an inexpensive hobby-type gun, and I think the voltage isn’t enough for make a good job on the second coat. Is this possible or not? C.T., Victoriaville, Que.

A: The equipment you’re using isn’t intended for anything more than small hobbyist applications. If you need to get a bit more sophisticated in your coating results, then you should look into an industrial application unit that’s capable of doing everything you want. You can find this equipment used sometimes, but it will take you some searching to find one. They are available. —G.T.

11/14/2016 - Q: We’ve been custom coaters in Phoenix since the 1980s. We’ve shot powders from the RAL palette since the beginning. We’ve seen improvements in powder quality across the board from all major powder manufacturers, in all formulations. But what has not improved is the uniformity in matching RAL colors from supplier to supplier. They’re all kind of close, but all do not match. Is there any effort by powder manufacturers who sell the RAL palette to come up with standardized RAL colors (similar to Federal Standard) so they all match from supplier to supplier? R.Q., Phoenix, Ariz.

A: RAL color standards can be very useful in obtaining the same color from different suppliers. However, you must also specify the tolerance you allow the supplier to drift from these standards. For this, we normally use a Cie-LAB standard of 1 ΔE, which is undetectable to the naked eye under most lighting conditions. Some colors may also require a maximum value for each of the L, A, & B values in addition to the ΔE value to ensure color accuracy.

Work with your suppliers to establish your own color standards and tolerances to ensure you obtain consistent results. —N.L.

11/7/2016 - Q: I’m using sandblasting media, air blowing with compressed air to clean sand out, then cleaning with a solvent, like a paint thinner or alcohol. I then heat the part dry for 15-30 minutes at 370°F (to outgas and dry the part), and then I remove the part from the oven and let it cool to around 150°F or sometimes cooler (tried different temps). The aluminum is usually cast, mostly motorcycle parts. I've been told that the pinholes were caused from out-gassing so that’s why I started doing the heating before powder application. It seemed to help, but it hasn’t eliminated the problem. C.S., Windsor, Ont.

A: Castings, be they ferrous or non-ferrous, will usually cause out-gassing because the metal is porous, as you know. And if the metal has been subjected to a chemical pretreatment, it’s entirely possible that some of the liquid has entered and become trapped in the pores. When heat gets to the part, as in a cure oven, the air will expand and seek a way out, and the liquid trappings can approach boiling and erupt through the coating. Pre-heating may or may not solve this problem. It will certainly reduce the amount of bubbles in the coating. Some years back, I worked with a research type engineer, and his company had a lot of problems with out-gassing. He determined that time was as important as temperature in the preheat stage. Given enough time, you can virtually eliminate the problem. However, in a production mode, you don't have the time to remove all of the gasses/liquids. So, a compromise is necessary. Sometimes, coating thickness will inhibit the out-gassing, but that adds to cost and can affect appearance. A primer will serve the same purpose and could be the best means of solving the problem. —G.T.

10/31/2016 - Q: I have a customer who wants us to put THREE coats of our TGIC powder paint onto his sheet metal parts. That seems like a disaster waiting to happen. We cure the paint at around 350°F (the max our oven will go to). Do three coats sound reasonable? What is the maximum paint thickness recommended with these powder paints? J.B., Tulsa, Okla.

A: This question is better directed to your powder supplier, as they know the particulars about your coating. However, suffice it to say three coats may be difficult to provide, as each coat after the first will be difficult to apply to a cold part, considering the reduced electrostatic attraction that will occur with the reduced part ground. The next problem is cure; as each coat is cured the preceding coat is over-cured. Because most powders have 100 percent over-bake resistance, you’ll have to cure the first coat only half way before applying the second coat and curing it half way. Even doing this and fully curing the final coat, the first coat will be 150 percent cured, 50 percent higher than recommended. At this point the coating will start becoming brittle.

Therefore, discuss with your customer what is the final film build they want. If it’s 6 mils or less, then consider applying it in two coats. You won’t have over-bake problems, and it’ll perform better than three coats at 2 mils each. —N.L.

10/24/2016 - Q: Just wondering if you can help with a problem we’re having with our powder not attracting to our parts. As far as we can tell, we’re getting a good ground on our parts that are hanging in our booth, but when we spray the powder on the parts, you can see powder just kind of falling off. And if we try to spray in corners or anywhere there is a bend, the powder won’t stick. This is causing us to overspray the parts to get full coverage, and the powder is very thick after the parts are baked. Our operator for our powder coating said it looks like the powder wants to attract directly to the booth instead of the parts. We’ve tried adjusting our settings on the powder coating machine, but nothing seems to help. Your help would be appreciated. C.M., Lewiston, N.Y.

A: From here, this is a classic case of little or no ground. It bothers me that you say "as far as you can tell, we have good ground." You should know precisely whether you have good ground or not. You need a megger meter, or megohm meter, to easily take a reading at the various points along the part stack. If you have 1.0 megohm or less, you have a good ground. Anything over that will gradually be less attractive to the powder. Another way to check ground is to make up a ground wire with a set of alligator clips on each end, and connect to the part and a KNOWN ground. You should see a marked difference. If that isn't the answer, then the problem immediately gets a bit weird. It could be a powder problem. If the powder is old, improperly stored, or damp (you should see other problems if this is the case), or had a problem right from the start, then there are other ways to test for the cause. However, it sure sounds like a ground problem to me. —G.T.

10/17/2016 - Q: Are there cool powders? We powder coat aluminum extrusions for making doors and windows, and we want a powder that will reflect the infrared radiation, and the window will remain cool. Let me know how they work and where I can buy them. L.K., Piraeus, Attica, Greece

A: All of us in the finishing industry think powder coatings are way cool! The physics behind your question are pretty simple: Dark colors absorb light; light colors reflect light. Light absorption will result in heat energy; therefore, architects for years have selected lighter colors for their buildings in climates where light reflection aids in reducing building energy requirements. Selecting metallic colors can further improve light reflectance, which is why most energy-efficient buildings have highly reflective colors on their exterior surface.

In your application, the best solution is not to apply any color at all but use a clear powder to allow the underlying aluminum color to reflect the sunlight. —N.L.

10/10/2016 - Q: We’re coating automotive components and are in the process of trying to complete a new order for coating shock absorber springs for motorcycles. The powder is repelling from the inner side of the spring in some areas and pin-holing. Because we try to reach the inner side to coat it, we get excessive coating on the outer portion, which isn’t acceptable to our client. What should I do to resolve the coating problem for the inner side of the spring? S.D., Aurangabad, Mahrashtra, India

A: The pinholes could be caused by improper cleaning of the inner portion of the spring. Some oils may still be present. Or, if the space is really tight, you may be getting kilovolt rejection because the gun/electrode is too close to the metal. Springs should be easy to spray. I haven’t tried bike springs, but I have sprayed auto coil springs. You can use one gun to spray the entire spring without moving the spring or the gun. Anyway, spray the interior first, then lightly spray the outside. You can take a good look at the springs when you coat only the interior to get a good idea how much powder will be needed to finish the exterior. I would think that it would take only a light pass to finish coating. If I'm not getting the picture, then take a few photos and e-mail them to me so that I can take another stab at it. —G.T.

10/3/2016 - Q: I was wondering if you can paint over a powder-coated surface. If so, which type of paint is better--enamel or acrylic? Thanks in advance. S.J., North Ryde, New South Wales, Australia

A: It’s an industry-accepted standard to touch up powder coating with a good quality touch-up paint. It’s been that way for years. Try very hard not to sand defects to bare metal. Just scuff it. If you sand to bare metal, you should apply a pretreatment chemical to the bare metal before painting. These treatment chemicals may be purchased at most automotive refinishing stores. Enamel will work just fine; however, if your product is used in a corrosive atmosphere, the touch-up should be a two-component material. This of course makes the touch-up a bit more complicated and cumbersome, but it’s the best practice. —G.T.

9/26/2016 - Q: What percentage (maximum) of recovery powder can be mixed to virgin powder with the same characteristics or same appearance after baking? What are the advantages and disadvantages of recoating parts such as rejects and how many coatings could be applied? V.M., Quezon City, Rizal, Philippines

A: The industry standard for application efficiency is 60 percent. This means that about 40 percent of the powder will go to reclaim. Part configuration and hanger design will impact this efficiency either up or down. Anyway, 40 percent reclaim will normally not cause a problem. If you get more than 50 percent reclaim, you’ll have fluidization problems and impact problems in the hoses and guns, and you’ll have a modified appearance on the parts. Powder chemistry and age can also have affect the reclaim, which will in-turn affect the appearance of the powder on the parts. The number of recoats allowed will depend to a certain degree on the use of the parts. If there are any machined areas, or fit areas with other parts, you shouldn’t apply more than a second coat, depending on the film thickness of the first coat. Orange peel will become more prominent with additional film; therefore, if appearance standards are strict, you may not wish to do a recoat. If the reject is small, you can do a liquid touch-up with a color-matched material, but you’ll have to be the judge of that. —G.T.

9/19/2016 - Q: We are a small effect powder coater and thermal barrier (ceramics, etc.) applier in Germany. We also sell powder coating machines as a niche product solution for small shops. We are always trying to get a perfect solution for sealing our coatings. We are now at a pretty good stage where we use an industrial one-component nano clear seal (wet paint) that is also cured at 180°C (356°F) for half an hour. Do USA coaters have experience with coatings like that? Can you tell me which powder producers supply powder coatings that produce a real chrome look that can be sealed without fading into yellow or gray? M.W., Luebeck, Germany

A: First, I’m not sure what you mean by nano. If you mean an organic clear coat, then, yes, we have that here in both powder and liquid form. Powder clear is used on automobile wheels, plumbing fixtures, and many other items. Liquid clear is used on automobiles, bikes, and many other items as well. I'm reasonably certain that nano isn't a ceramic material because the temperatures aren't high enough. To find a powder supplier for chrome lookalike powders, you should try the list of powder coatings manufacturers in the magazine’s online Buyer’s Resource. —G.T.

9/12/2016 - Q: Are powder coatings flammable? Thanks. R.A., Brandon, Fla.

A: NFPA classifies powder coatings as combustible materials. Combustible materials can ignite and support flame in the right conditions, while flammable materials can ignite more easily. This does not mean that similar precautions are not necessary when using combustible versus flammable materials. In the case of powder coatings, the material must be atomized in air to ignite, a condition that occurs when spraying, fluidizing, or collecting powder coatings. Refer to NFPA 33 guidelines when using and storing powder coatings and you won’t have a problem. —N.L.

9/6/2016 - Q: What kind of alternatives exist for using tape for masking? Plugs at load didn’t work, and tape ahead of pretreatment didn’t hold up in the washer. I’ve thought of permanent masks on the hooks but worry about part location when hanging and mask distortion when burning off the hooks. This also would make my hooks VERY product specific and complex. Yes, it’s doable, but, yech. And yes, I did ask our design engineers if the part had to be masked. However, I got no support there, even when I told them how much it was costing us. For now, I’d just like to know what alternatives exist to eliminate masking after pretreatment. T.C., Milwaukee, Wis.

A: It’s a pity that some design engineers don’t know or care about the details of the products they design. There are probably more parts in existence than not that are difficult, if not impossible, to coat because the design considerations didn’t include how to finish the product. So much for the sermon, and you didn’t ask for one.

Maybe the good news is that you can fixture the parts in the same position every time. That means you may also fixture a method of removing unwanted coating in a specific area. So, what I suggest is a minivacuum system that uses a very small hose to suck away the powder that is unwanted in a particular area. Don’t mess around with a shop-vac system. This will require a more sophisticated system than that. Check this magazine’s Special Supplier Links at www.pcoating.com/suppliers_vacuum.asp. You’ll find several companies that make devices for this purpose, and they will be happy (or at least pleased) to work with you in developing what you need.

I don’t know of another means for doing what you want to do. Yes, hanger-attached masks will likely have a short service life in the burn-off oven. And that dedicates the hangers to specific parts. Tape is not feasible. Synthetic masks will probably cause a powder ridge and be difficult to remove. Blow-off lines will remove too much powder. Although that method is a possibility, I wouldn’t count on it. —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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