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.

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.

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.

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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