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.



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.


2/23/2015 - Q: How do I calculate the heat load for a powder curing convection oven? Is there a relation between heat load and the circulating fan? If yes, what is the relation? A.H., Bangalore, Karnataka, India

A: This information is contained in various articles I’ve written over the years in this publication. (See the Article Index at www.pcoating.com; click on Authors and scroll to L. You’ll find the articles listed under my name.)

As I’m not in favor of helping unqualified persons from designing equipment that can cause injury or death when it fails, I’ll provide my answer in engineering terms and not provide the actual formulas. My liability insurance company is very comfortable with this personal and professional choice. I hope that you are as well.

The heat load for a convection cure oven is determined by the mass of the products to be cured multiplied by the specific heat of the substrate. This value is then multiplied by the delta temperature (room temperature to cure temperature). The mass of the hangers and conveyor chain must be added to this calculation in conveyorized systems. In addition, the radiant heat loss through the oven walls must be included in the heat load. Finally, the heat lost through the exhaust fan and stack must be included, as well.

Any well-designed convection oven must deliver the BTU energy from the burner efficiently to the parts through the use of a properly sized circulation fan. The circulation fan must provide at least three turns per minute to be efficient, but more is better.

My recommendation is that you use this information to judge an oven design from a qualified equipment supplier and not design your own oven. The consequences of doing the oven design incorrectly range from using too much energy or not properly curing your parts to having to rebuild your plant after the explosion and paying for the funerals of your shop personnel.—N.L.


2/16/2015 - Q: I’m using an alkaline cleaner on two different lines. On one of the lines, I’m able to run it at 110°F; however, on the other line, I’m unable to run below 140°F as it foams madly. What could be checked to remedy this foaming so that I can run the stage at 110°F like the other line? K.M., Holland Landing, Ont.

A: If we assume that both lines are identical mechanically, the only logical reason is that you’re trying to remove a different soil on the foaming line. Conduct a soil and substrate audit for both lines. What soils are applied to the metal before processing? You may find that a fatty acid soil or an animal fat lube is being applied to the metal. This creates saponification when a heated especially caustic containing cleaner is used. You create "soap" and thus foaming. If this is the case, change soils. Also ask the chemical supplier this: What is the cloud point of the surfactant system in use with my cleaner? My guess is that he’ll say 110°F to 115°F. —G.T.


2/9/2015 - Q. Are there any specific quality issues or process requirements a powder coater needs to be aware of when powder coating over Zintec mild steel? I’m a buyer in the middle of a sheet metal producer in England, who uses Zintec to manufacture parts for us, and a powder coater in the US, who will eventually coat the parts. I realize the question is so general that the answers may be limitless. Any help in pointing me to resources would be appreciated. J.W., Lincolnwood, Ill.

A: Zintec is a brand of zinc-coated steel similar to Galvanneal, galvanized, or aluminized steel. Depending on the coating performance specifications, a zinc phosphate pretreatment would give the best results although it’s not uncommon to use iron phosphate. You should be aware that you can get outgassing from these types of substrates during the cure cycle for the powder. Often, preheating the part will reduce or eliminate the problem. Heavy powder film build can also inhibit the gasses from getting to the surface. You might want to make sure the supplier in England doesn’t coat the parts in a heavy rust preventive for resistance to oxidation when shipping by boat. A heavy coating of oils and so on will make it difficult for your US coater, and you will end up in a finger-pointing contest. The shipper would be most helpful by using a watertight container and maybe some desiccant drying materials. —G.T.


2/2/2015 - Q: We’re considering an expansion of our powder coating room, and questions have arisen regarding the need for explosion proofing. Are there any general guidelines available that we can consult for information regarding what, if any, regulations there may be for explosion proofing that we have to abide by in this project? M.B., Oak Creek, Wis.

A: Check with National Fire Protection Association (NFPA) codes, Chapter 33, which are the generally accepted requirements for safety items. However, you should also check local codes with your community fire marshal. The NFPA codes are adequate, but many times the locals insert their own requirements, which can be more stringent than NFPA's. A powder booth must be explosion vented. Some requirements demand that it be vented to the outside. Some just require that you vent to an unoccupied area, such as a high ceiling. This can cause you to modify your application system layout, so check this carefully. In smaller communities, you can find all kinds of variances in the codes. Some fire marshals (inspectors) don’t have any idea what constitutes a powder coating system and will demand that you meet wet paint regulations. If you get a hardheaded inspector, you’re in for a real treat, so it’s best to review what you’re doing before you do it. Maybe that will be the last and only time you hear from that person. —G.T.


1/26/2015 - Q: What powder coating and cleaning process do you recommend for corrugated brass tubes? These brass tubes are used for gas connectors and need to be ammonia-resistant. T.M., Rancho Cucamonga, Calif.

A: The powder of choice is a triglycidyl isocyanurate-based (TGIC-based) polyester, I believe. That’s the easy part. The cleaning step is a little different. A similar project that I worked on used different tubing. This stainless steel tubing was annealed for improved flexibility, and the annealing burned away any oils or impurities on the metal. As a result, they didn’t pretreat the tube. They just applied the powder. I don’t know what you’re doing with the tube you manufacture. If you’re annealing, then don’t do anything else. If you’re not annealing, then some type of pretreatment would certainly improve adhesion. Chemically pretreating will be difficult with the tube already corrugated. Powder coating with the parts hanging horizontally works best. If you hang vertically, the parts will sway and bump into each other (wind-chime) and cause defects and line wrecks. However, pretreating horizontally will cause a lot of dragout. You probably have that all figured out. If there is any kind of contamination on the parts, you’ll lose adhesion, and for this part, which needs to flex, you certainly don’t want adhesion loss. This material is a bit tricky. Be sure you look in the valleys of the corrugating, using a glass, because the paint may crack so fine that you won’t see it with the naked eye. But it will certainly be a failure. If you need additional help with the pretreatment, send me a more detailed description of how you’re now cleaning. —G.T.


1/19/2015 - Q: We currently have three powder lines. All three use a cartridge reclaim system. We have been running the same recycled powder for an unknown amount of time. To my knowledge, we have never actually dumped powder and started with new on some colors. Thus, the particle size ranges are affecting the chargeability of the powder and in turn affecting the transfer efficiency (TE). When should I take a stand, so to speak, to reduce this effect? Could this also be affecting the amount of powder that is escaping from the booths? Any input you can offer would be taken on board with thanks! J.A., Zhongshan, Guangdong Province, China

A: With some slight reservation, I say it’s time to take a stand. My reservation is that poor part design, that is, a part not designed with painting in mind, can affect the outcome, but you probably can’t do anything about that. It seems from here that poor TE is the problem, maybe not the root of the problem, but nonetheless a problem. You know that the reclaim is out of whack, but the guy who pays the bills probably doesn’t--and doesn’t care. But poor TE is costing you money and time, which is still money. You’re right in that the smaller the particle size, the poorer the chargeability of the powder and the more difficult it is to get the required film on the parts. Eventually, you’ll have no fluidization, or at best you’ll have to jumpstart the fluidization with a paddle or some such device. You might be able to help by redesigning the hangers so that they will present the parts to the application equipment in a better way to increase powder transfer. You should study this and be prepared to offer some ideas when the boss asks what you propose to do to fix the situation. Yes, poor TE and reclaim overuse (which will increase small particles) can result in powder escaping the booth. Part of that problem is likely caused by the small particles clogging the filters. Take an airflow reading at the part openings. If it’s low, as I suspect, that will also cause powder drifting. This costs money and makes for a messy shop. So, you need to install new filters, remodel the hangers, dump the powder now in the hoppers, and realign the guns for better coverage. If the boss doesn’t want to do any of these things, you should look for another job where they will appreciate your understanding of powder applications. —G.T.


1/12/2015 - Q: Can motorcycle exhaust pipes be powder-coated? Will they withstand the heat? Can they be in colors? J.H., Gadsden, Ala.

A: The answer to all three questions is yes, yes, and yes. Go to this magazine's website and click on the Buyer's Guide to search for powder coatings companies that supply high-heat-resistant powders. In fact, just recently I was checking out some auto engine exhaust headers that were powder-coated with a chrome-appearing material. This material won’t last forever, but most people who want this kind of dressing on their vehicles use them for a limited time and never experience the eventual powder coating failure. —G.T.


1/5/2015 - Q: It’s a common practice for us to use liquid paint to do touch-up. Why should tack rags not be used? Does this apply only to certain types of liquid paint? Does this also apply to recoating with powder over powder? For your information, we use polyester powder and automotive-grade acrylic enamel in aerosol cans for recoat. We also have problems recoating with powder if we have blown off the part with compressed air after it has been scuffed. It seems to collect a static charge perhaps. If this is what is happening, is it more common with certain weather conditions? One more question: Does the reason for the recoat dictate the best method to recoat or the preparation that should be used (that is, insufficient coverage or burned finish)? D.B., Leola, Pa.

A: My comments about tack rags tend to be overkill because more often than not I’ve seen shops reusing tack rags until they disintegrate. After just a few uses, a tack rag will leave a residue on the surface, and the sprayer will coat over this collection of gummy material and then have another rejected part. You can also wash or solvent wipe, but the same conditions apply. You can’t continue to use the same solvent for weeks on end, and the same rags for that long, too. If you’re using tack rags and replace them frequently, you should be OK. Your choice of rework liquid is great. The compressed air can aggravate a static charge condition, which will be worse in cold weather because the cold air is dryer and encourages static conditions. And the scuffing can also create a static condition on the surface of the coated part. You can buy a de-ionizing tip for use on a compressed air line that will help reduce some of the static problem, but don’t expect miracles. If you sand to bare metal, you should apply a metal prep material before painting. This can be done with an automotive refinish chemical, or you can run the part back through the washer. The only caution is that if you leave all of the chemicals on, you can get water spotting on the painted portion of the parts. So, you should carefully check parts run in this manner for spotting. The spots will leave a mineral ring that will show through the finish. —G.T.


12/29/2014 - Q: In the powder booth application zone, will lower humidity make the application process more susceptible to back ionization? A.Y., Southfield, Mich.

A: The humidity would have to be very low for you to notice problems. Actually, low humidity causes the same type of problems as high humidity, except for agglomeration. Generally, low humidity will make the powder more difficult to charge. We’re talking about humidity down in the 20 percent range. Back ionization is caused predominantly by too much powder or a gun too close to the part. What makes you think that isn’t the problem? What is the humidity in the powder booth? What is the dew point of the compressed air coming into the hopper and feeding the guns? A 35 percent dew point is typical, and a humidity range of 40 percent to 60 percent is normal. If you’re anywhere near those figures, you’re probably looking in the wrong place. —G.T.


12/22/2014 - Q: I have a customer who has steel poles rusting in the field. The powder coating is still on the poles, and it seems like the rust is coming through the powder. Their pretreatment is shot blast and iron phosphate. I got different mil readings from 3.5 to 6.0. Got any answers? A.L., Arlington, Tex.

A: I have more questions than answers, but maybe my questions will help find an answer. What was the condition of the poles before shot blasting? How was the shot blasting done? What media was used? How long after blasting were the parts pretreated? What, exactly, is the pretreatment process: Clean, rinse, phosphate, rinse, seal, rinse? Are all stages up to specifications? Did the parts get dried after pretreatment and before powder coating? I mean really dry. Any hiccup along this process could cause the problem.

I certainly would think that at 3.5 mils or better, there should be no rust bleeding through unless the powder is designed for significantly lower films, in which case very heavy film might be a contributor. Might, I said. Are you certain that this isn’t oven dirt? Have your best detective check all of these points in detail: chemical titration readings, shop conditions, oven charts, and so on. Can you duplicate this condition in the lab by using accelerated tests? When I hear of issues such as this, my first reaction is poor conditions involving the pretreatment. On the surface (so to speak), it would appear that pretreatment is done properly, but the devil is in the details. If you have any further information, forward it to me, and I will take another look. —G.T.


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.


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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