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.

9/15/2014 - Q: I’ve been trying to powder coat a set of 350 Chevy heads. When I shoot the heads with black powder and then cook them, the powder isn’t flowing out. It comes out as if it were never cooked. How can I get the powder on the parts to flow out? Are the heads soaking up the powder? Please help me with this. I’m baking at 500°F. C.Y., Maryville, Tenn.

A: The 350 C.I.D. Chevy motor is very reliable. I had one in several vehicles over the years. However, I prefer the horsepower or the higher revving 327 C.I.D.

The description of the problem you’re having tells me that you’re not heating the cylinder heads to a point where the powder coating even melts, let alone cures. If the powder on the part looks the same when you take it from the oven as when you put it in the oven (still a powder and not a coating), then it’s time to evaluate what you’re using to heat your part.

This problem has to do with the weight (mass) of the cylinder heads and the energy capacity of your heat source. I’ve lifted my share of Chevy heads over the years, and from my recollection, they weigh about 100 pounds. Considering that the cylinder head is made from steel and has a specific heat of 0.125 BTU per pound, it will take 4,375 BTUs to heat one cylinder head to 350°F (the average cure temperature for powder coating). If you put this cylinder head into an oven whose heat source has an energy capacity of 1,000 BTUs per hour, it would take 4.375 hours to get the cylinder head up to temperature and an additional 25 minutes to fully cure the powder coating. That’s almost 5 hours of baking time! However, if you use an oven with a heat source that has an energy capacity of 10,000 BTUs per hour, it would take less than an hour. This relation between the energy capacity of heat sources and cure time can easily be related to horsepower and speed: The more you have, the faster you’ll go.

I frequently tell my clients that the size of their oven burners determines how fast their products will achieve the desired powder cure temperature. I often tell them that you can cure a 10,000-pound part with a cigarette lighter. It will just take a couple of hundred years. The same goes for you. If you’re trying to cure your cylinder heads in a toaster oven, then I hope you’re very patient because it will take quite a while to melt, flow, and cure the powder. Remember the old racer’s adage: “Go big or go home.” Of course, they were talking about engines (horsepower), but you get the point. —N.L.

9/8/2014 - Q: We don’t have the capability to powder coat enclosures at our plant, but we do buy powder-coated enclosures. We try not to scratch them, but scratches happen. What is the best way to fix these scratches? They can range from a small scrape with a piece of metal to larger dings with a forklift. S.F., South Burlington, Vt.

A: You can use a color-matched liquid paint for almost all of the touch-up. This is an accepted industry standard. Now here are some cautions. Try not to sand down to bare metal. If you can do that, then you can maintain the integrity of the pretreatment. If you have to go to bare metal, then you should use some type of bottled chemical treatment on the metal before applying the coating. You can find this material in an automotive paint supply house. You can recoat the entire part if orange peel isn’t a problem or if close tolerances don’t present difficult fits with something. If the original powder is high-gloss, it will have to be scuffed or you’ll have poor adhesion. If it’s a low- or medium-gloss, just make sure the part is dust- and oil-free, and you should be OK. —G.T.

9/2/2014 - Q: I’m having fisheyes or pitting after powder coating. What is the cause of this? Am I not cleaning the parts properly? J.B., Seattle, Wash.

A: The most prevalent cause of fisheyes is contamination. Contamination can come in many forms. It can be on the surface of the part due to poor cleaning. It can be oil in the compressed air used to pump or fluidize the powder. It can be an airborne contaminant, such as silicone or some solvent, in the plant. Find the source of the contamination and the fisheyes will go away.

Surface contamination is the easiest one to fix, as all you have to do is clean the part better. Oil in the compressed-air supply can be very difficult to clean, as it will be in all the devices, that is, regulators, gauges, hoses, fluidized plates, gun parts, pump parts, and so on. Cleaning or flushing oil from compressed-air lines is very difficult. Work with your equipment supplier, who may have some ideas on how to do it and what to use. You may have to replace everything throughout the compressed-air system, which is a very difficult and costly chore.

Airborne contaminants are truly the worst to identify. So many things can be the source, from lubricants used elsewhere in the plant to hand creams used by workers on the line. Some people have spent months of time and boatloads of money trying to rid a system of an unknown airborne contaminant. Just hope that you don’t have this problem. —N.L.

8/25/2014 - Q: We have been powder coating for several years. Our oven sits on top of a concrete floor. Over the years, powder has built up on the floor during the curing process. As a result, the concrete surface is getting very rough, and our cart wheels are sticking to the powder that falls to the floor. I rented a concrete sander, but it didn’t phase the cured powder on the floor. Do you have any suggestions on how to get the cured powder up? Is there something to prevent the powder from sticking to the floor after being baked? Please help. A.G., Roanoke, Va.

A: What a mess. Most methods of removing unwanted powder, such as cleaning hangers, can be done by burn off or chemical stripping. Your mess can’t be cleaned by chemical methods unless you provide strong temporary exhausting and safety clothing for the people who would have to perform this duty. So, I really don’t recommend this method. Burning with a torch is just as hazardous because some powders will burn when exposed to a flame. The burning will cease when the source of ignition is removed, but, hey, you never know. My only suggestion is to rent an air chisel with as broad a blade as you can get and start hammering away. Any method you choose will be time-consuming and risky. I wonder why you have so much powder on the oven floor? True, several years will make a mess if you don’t keep after it, but you shouldn’t be getting that much powder falling from the parts unless the oven air is too turbulent. Maybe you should spend some time adjusting the ducts so that the airflow isn’t directly on the rack and parts. You don’t want to have to do this on a regular basis, and unless you can reduce or stop the accumulation of powder, you’ll have to repeat this process again. I’ve been around many, many batch ovens that have very little powder on the floor after years of use. Better get to the root of the problem. —G.T.

8/18/2014 - Q: I've had an internal request to specify some powder coatings to coat over electroless nickel plating. I would like to know if this has been done and what defects arise when trying this. Other guidance and tips would also be beneficial to our progress on this issue. E.H., Cedar Rapids, Iowa

A: The plating will present a hard, smooth surface to the powder coating, which will cause some adhesion issues. I won’t ask why you would go to the expense of plating this part and then powder coating it. The adhesion issues could make you wish you had never coated any of the parts. Nothing is uglier than peeling paint, and nothing else gets you an angrier customer, even if the part performs mechanically better than expected. I’m unaware of any other problems powder coating over plating. You shouldn’t get any out-gassing because of the nature of the plating action. My favorite old trick is to run a sample and check it closely for defects and especially adhesion. —G.T.

8/11/2014 - Q: One of my cast aluminum parts looks and feels rough. I cleaned it first by sandblasting it and then wiping it with acetone. I know about out-gassing, so I heated the part in the oven for 45 minutes at 450°F. I took it out, wiped it again with acetone, and let it cool. I then applied silver-colored powder, stuck it in the oven, and waited for flow out. After flow out had occurred, I lowered my temp to 400°F and cooked it for 20 minutes. I didn't notice the part gassed out until I had about 5 minutes to go on the bake. My customer is going for a "candy look,” so this part might be cooked 3 to 4 more times. Is this unusual--one silver coat as a base and three coats with a translucent? Sometimes, it seems as though the powder won’t stick, which is the reason for the extra coats. My niche is automotive parts, under the hood as well as exterior accessories. H.H., Rosharon, Tex.

A: I would change several things here, at least from what you’ve told me. The second acetone wipe shouldn’t be necessary, and it’s possible that you’re putting some solvent back into the pores of the aluminum casting. Next, I wouldn’t use a two-stage bake temp when powder is on the part. Further, I wouldn’t completely cure each of the coats. If the silver is a basecoat, heat it until the powder begins to flow out or gel. Then apply the next coat. It would be nice to have fewer coats because if you fully cure each coat, you’re frying the basecoat, which will cause some adhesion problems. You also may have some inner-coat adhesion issues. I assume you’re media blasting to smooth out the surface. You want to be sure that the blast media is clean at all times. Aluminum is self-protecting with a patina that forms on the surface of the part. This patina will be very detrimental to coating adhesion. Blasting will remove it, but if the part sits around the shop for many hours, this patina will begin to form again. Keep the time from blast cleaning to coating short. Test some panels or parts with the methods I’ve suggested and see how you do. —G.T.

8/4/2014 - Q: My question is regarding powder coating polybutylene terephthalates (PBTs). Our customer recently switched to PBT-molded parts from nylon, and we’re having issues with powder coating transfer and appearance. When we first ran parts through our booth, powder particles that first hit the part formed an envelope of charged particle (this combined with high resistivity of the material) started repelling the powder, resulting in poor appearance. The first trial we ran involved removing electrostatic voltage and then sending parts through the spray booth. The result again wasn’t so good. There was uneven coating distribution on the part and heavy mottling around holes or grounding areas. Please let me know what you think and if there is something we can try to improve the process. My next step is to run a trial with slow line speed and powder coating with reduced voltage, which is now set at 95 to 100 kilovolts. T.T., Mississauga, Ont.

A: What is the polarity of the powder spray gun you’re using? Most often, nylons are applied via a positively charging gun. If that’s the case, then you should try a standard powder gun, which is a negative charge, to see if that makes a difference. Of course, you can flip this scenario if the opposite is the case. I doubt that a slower line speed will have any effect. Lower voltage might, but that’s a big might. And I doubt that a tribo-charge gun would work either. Your powder supplier should be able to give you the application characteristics of the material, which may or may not be of any value. —G.T.

7/28/2014 - Q: I build large aluminum globes of the world and I have a 5-foot-diameter globe that we need to first powder coat with clear and then have the outsides of the continents painted to resemble the 'Blue Marble' NASA image from space. My questions would be: Beyond scuffing, what other prep do I need to do? Is there a powder type/primer chemistry that works well together? Do I need a primer? My artist will be mixing paints on a palette for the top coat, so she can't use high VOC or two-part paints. M.B., Chicago, Ill.

A: I have to say that this is one of the more unusual questions I received lately. Having said that, I think I can help you. If these globes are used indoors, a primer is unnecessary for good product life. A standard polyester powder coating formula will provide great service life during the use of the product.

Scuffing the surface to ensure good intercoat adhesion is important before you do the artistry. For extra measure, a clear topcoat using normal liquid technologies will encapsulate and protect the artwork.—N.L.

7/21/2014 - Q: We make relatively thin gauge open-top aluminum tanks that hold hot salt or chlorinated water. We specified a fusion bond epoxy lining because of our experience with this product on steel piping. Our power coating contractor is having problems getting satisfactory results. Is there a product or process that will give the corrosion resistance of the epoxy that can be applied to surfaces that cool rapidly? The surface needs to be cosmetically appealing and resistant to submersion in hot water. M.S., Surrey, B.C.

A: By definition, Fusion Bonded Epoxy (FBE) describes a method by which a functional epoxy powder coating is applied to an object that is preheated to more than 600°F. The applied coating will melt and flow on contact with the preheated part and cure in approximately 30 to 40 seconds. The applied coating is normally applied thicker than 12 mils (often 20+ mils) using this process. The epoxy is formulated with a fast curing catalyst and is ground to a larger than normal particle size to allow for a quick build of the intended coating thickness.

The coating can be applied using guns or fluidized bed methods, depending upon the application. This coating process is normally used to coat concrete reinforcement bar, pipe, bus bar, and other functional products.

Although this is the definition of the FBE coating process, this does not mean that the methodology cannot be modified to accommodate process issues, like quickly cooling substrates. The thermal characteristics of aluminum are very different than steel. Aluminum conducts heat about twice as fast as steel and, therefore, cools twice as fast. We have many clients who manufacture products with this substrate or other products that cool faster than the FBE applied material can fully cure. Considering that fully curing the epoxy is imperative to achieve full functionality (including corrosion resistance), you must ensure that the epoxy is fully cured on the product. This is why most people will employ a post-application cure step in the FBE process to reheat the coated part up to 500°F or hotter to ensure that the coating is fully cured. How much time the product must spend inside this post-cure oven is dependent upon how much cure is attained at the time of coating application. Performing an oven profile on a test part through preheat, application, and post-heat steps will ensure you develop an appropriate recipe to follow. —N.L.

7/14/2014 - Q: My company’s metal panel surfaces end up with water marks on them after our coating process is complete. For pretreatment, our process includes pre-degreasing, degreasing, hot water rinse, iron phosphate, cool water rinse 1, cool water rinse 2, and DI water. Our cool water rinse 1 pH level is 5, cool water rinse 2 pH level is 6, and DI water pH level is 7. Please help. Z.X., Selangor, Malaysia

A: Water spots are caused by minerals in the rinse water or when the cleaner stage did not provide a water-break-free surface. DI and RO water generation systems generate mineral-free water that should eliminate spotting issues. However, pH only measures the acidity or alkalinity of the water (with neutral at 6) and does not measure mineral content. Therefore, check the TDS (total dissolved solids) or conductivity of your water to determine mineral content. This measurement, along with a complete laboratory analysis of mineral content in your water will tell you if your DI or RO system is working properly. If the spotting is caused by an ineffective cleaning stage that does not provide a water-break-free surface, then fix that with better chemistry, heat, or mechanical force. A water-break-free surface is by definition spot-free (no beading of water), and therefore will not have any spotting issues. It also means that surface is organic soil free, an important condition to ensure proper coating adhesion. —N.L.

7/7/2014 - Q: I’m looking at a rusting issue with our powder-coated products. In the fabrication process, we spot-weld several small items, such as hinges, to the surfaces of our product while in the raw state. Once this equipment is placed in service and exposed to water in the form of rain or otherwise, rust stains will begin weeping out from between the hinges and the main product. We do not wish to use primer paint before spot-welding because of extensive handling and cost. Is there a product that can be wiped or brushed onto the parts to achieve a higher degree of rust protection than we’re getting now and won’t impede spot-welding performance? We use a wash-rinse system to place an iron phosphate coating onto our carbon steel parts before powder coating. A.R., Fayetteville, N.C.

A: This is a very common problem with two metal pieces joined together via welding. At an automotive refinish store, you can get a zinc-rich material that is designed for application on bare metal to act as another barrier to rust. This material will accept paint. You can swipe this on with a rag or brush or cotton swab. Do this after the spot-weld operation. Give it time to dry and then paint. Also, give the parts a few seconds to cool after the spot-welding. Maybe you can work it in so that the guy spot-welding can also perform this duty. This seems to be a very successful application. At least I have never been told otherwise, not to my face anyway. —G.T.

6/30/2014 - Q: I’m using textured powder. I need a uniform appearance in texture on the entire part and a film thickness of 4.0 mils to 8.0 mils. When the part comes out of the powder coating line, some areas look different in texture. (I can see different texture on the part--heavy versus normal.) But when I measure these two areas with different textures, they have the same film thickness. What could be the reason or root cause of this issue? A.R., Lithonia, Ga.

A: You have a film variation of 4.0 mils, starting with a base film build of 4.0 mils, and you wonder why it looks different? You powder guys can correct me, but a textured powder at 8.0 mils is way beyond what it should be. No wonder it looks different. What is the design film build of the powder? Before I get too mouthy, I should ask that. I think you need to do a better job of controlling the film (see your supplier’s film recommendations). Know what the film thickness readings are. Getting a good, true film reading on texture isn’t easy because of the texture. You can read a "valley" at one reading, and then read a "peak" the next time and get confused. Without spending a lot of money on a fancy film gauge, you can take a reading over many locations and average them out to see what the average is. I still think a lot of the problem is the huge difference in film thickness. —G.T.

6/23/2014 - Q: The substrate is 304L stainless steel, B finish sheet. The weldment is a squirrel cage style fan unit with 24 inch wide by 72 inch curved blades. We clean the weldment in a three-stage wash consisting of iron phosphate, water rinse, and rinse seal. Coating is a light green polyester with TGIC (triglycidyl isocyanurate). We have to rotate this fan unit when applying powder to reach all of the areas. This causes us some problems. Also, the visual appearance at the time of application of the powder appears to be covering well (3.0 mils to 6.0 mils uncured). After curing, we have areas that appear light but have a cured mil of 1.5. Any application suggestions? L.H., Carrington, N.Dak.

A: Have you checked the powder supplier to see where the hiding power film thickness is? In other words, if the powder is designed to cover nicely at 2.0 mils to 2.5 mils, something less than 2.0 mils will look blue. As I visualize that part, it seems to me that there are a lot of edges that can rob powder from where you are aiming. All those edges act as antennas and will draw away some of the coating. It goes without saying, but I will say it anyway, the very best film check is on the cured film. That is what you have to go by no matter what it appears to be in the uncured state. If it’s Tuesday and you got a little bit sideways on Monday night, the uncured film appearance might not be what it is! Got that? I don’t envy you trying to coat a squirrel cage fan. It would be time-consuming and full of Faradays. —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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