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.

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.

12/1/2014 - Q: We’re a powder custom coater with several powder coating conveyors. We’re planning to install a batch-with-liquid (non-water base) application near from one of these. What are the quality issues with this project? We want to know if it's possible to apply powder and liquid coatings in the same area. C.F., Notre-Dame-du-Bon-Conseil, Que.

A: You must consider several issues before installing a liquid spray booth in an area close to a powder application booth. First, let’s talk about safety issues. Liquid systems have a completely different set of rules than powder systems. For instance, the electrical devices in the area may need to be vapor tight explosion proof; those around powder systems need to be dust tight explosion proof. Be sure your new liquid installation conforms with all safety requirements before operating it.

The second issue is visual quality problems caused by airborne particulate. If you have high appearance standards for your liquid parts, and dust and dirt contaminants aren’t allowed, then placing a dust-producing operation (powder booth) next to your liquid booth may not be the smartest idea.

These are the two primary concerns I have for your plan. Having said that, don’t forget the human factor of solvent spills affecting your powder application system or powder spills affecting your liquid operation. Otherwise, powder and liquid operations have coexisted together in many operations for many years. —N.L.

11/24/2014 - Q: For the past 6 years, I’ve been outsourcing the powder coating of the product that we manufacture. Our product is a 6061 aluminum and is used outside. It’s installed from the beaches of Florida to Hawaii. My question is what type of prep should be done to the old powder coating if we decide to repaint on site? Could it be power-washed hard or should the old finish be removed? If the old finish is failing, what is the new paint holding onto? How can the old powder-coat finish be taken off at someone's home? I’ve looked into a high performance water-based epoxy primer and finish coat recommended for aluminum and galvanized metal. Do you have any recommendations? What are your thoughts on the subject? B.E., Collegeville, Pa.

A: Right up front, you must understand that you have to properly prepare the aluminum before powder coating. Sandblasting won’t do it, especially in a salt-laden atmosphere. If the powder coating is flaking off the aluminum, it’s caused by little or no metal prep in that area. The aluminum must be properly cleaned and conversion-coated immediately before a good quality triglycidyl isocyanurate-based (TGIC-based) polyester powder is applied and cured to the recommended cure cycle. That is metal at temperature, not just oven time. If this action requires some awkward handling of the part, so be it. That’s better than the time and cost of rework.

Now, how are you going to do the field repair? You must sand away the blistered area and get back into the paint that has good adhesion. Then, you’ll need a field metal prep kit, which you can get either from an automotive refinish store or from any large pretreatment company. After following the manufacturer’s recommendations, apply a two-component liquid finish. This is also available at your automotive refinish store, or you may find it at a marina that does boat repairs. DO NOT use a zinc-rich primer. The products that you mentioned are good, but if I’m going to have to go to the field to do repairs, I want to do them only once. The two-component materials will keep you from making a future return trip. —G.T.

11/17/2014 - Q: We coat pipe with powder coating. We have to put 8 mils to 12 mils of powder on the pipe. The pipe sizes range from 1.05 inches to 2.875 inches outside diameter. We have good results on the smaller size pipes, but on the larger sizes we’re having trouble with the powder coating blistering after the pipes come out of the curing oven. Do you have any suggestions, or do you need more information? Please let me know. M.K., Shiner, Tex.

A: I don’t have much to go on here to provide an accurate diagnosis. However, since you’re stating that the problem is exhibited after the cure oven, it may be related to your cure cycle time. Because the larger pipes have more mass, they’ll require a longer cure cycle time. The cure cycle time is equal to the metal bring-up time plus the cure dwell time from the powder supplier (cure cycle time = metal bring-up time + cure + dwell time). Perform an oven profile test with the large pipe and your current oven cure cycle time. Verify that the part is at metal temperature for the prescribed time specified by the powder supplier. If not, adjust your time or temperature accordingly. —N.L.

11/10/2014 - Q: We manufacture pump tanks that are installed inside or outside, depending on the installation. We currently powder coat our tanks with a film thickness in the range of 2.0 mils to 2.5 mils. We’ve been questioned on why we go so thick with our coatings, as we’ve been told that 1.0 mil to 1.8 mils should be sufficient. Just wondering if our thicker coating offers any rust-protection advantage, or are we just wasting powder? Our powder coating is applied on cold rolled steel shells that have been prepped with zinc phosphate. W.S., Stratford, Ont.

A: All things considered, you’re giving your customer more than he’s paying for and more than is needed. There will be some added benefit to the heavier film, given adequate cure. It’ll be a bit more difficult for anything to penetrate the surface, but that benefit could be subjective. If you can lower the film thickness a tad and maintain the uniformity of the coating, you would be a better business person. That statement means that you’re giving away some powder on each part that’s coming out of your profits. Depending on how many parts are involved, you could be talking about a nice piece of change. Your powder formula will indicate the best coverage for the material you’re using. I would be a little concerned about a 1.0 mil or lower limit. That thickness courts trouble in case something goes wrong, which is inevitable. I would shoot for a 1.4-mils minimum to a 1.8-mils maximum. This will do nicely, especially over a zinc phosphate, and the orange peel should be minimal. —G.T.

11/3/2014 - Q: We have issues with nylon powder coating drawing up from the corners of parts. We preheat the parts and apply the powder so that it adheres to the part surfaces immediately. Occasionally, however, the coating will shrink and draw back when the parts are curing in the oven. T.C., Glasgow, Ky.

A: Do you have a primer on these parts before application of the nylon? You didn’t mention it, so I’m not clear on that part of the process, but that could be the problem. And too much heat may cause some pull-away from the edges and corners. So, maybe you’re trying to use the preheat in place of the primer. Any irregularity in parts, such as thickness, and any change in conditions could cause a sporadic cure problem. Maybe the preheat temperatures are inconsistent. Maybe the oven temperature or air circulation isn’t the same every time. How old is the nylon powder? I am unaware of any real issues with shelf life providing the powder is used in a reasonable time and stored in conditions dictated by the supplier. —G.T.

10/27/2014 - Q: We’re considering options for staging multiple stand-alone carts that will hold multiple pieces that are being powder-coated. After the wash process is complete, we want to load these stand-alone rolling carts with work. Because these carts aren’t attached to a conveyor and grounding is necessary for the powder to stay attracted to the work, is there a stand-alone device that we can attach to each cart to create grounding? Another option would be to attach a rolling cable from the conveyor to the cart; however, we need more flexibility with staging of work. Your input will be greatly appreciated. E.N., Midland, Tex.

A: The simplest and cheapest thing you can do is attach a spring-loaded alligator clip on the other end of a ground cord to the powder system. When you wheel the cart to the application area, just clip the ground wire to the cart frame. The ground wire must make a complete circuit with the spray gun to be effective. I don’t know of a device, per se, that can do that any better. Having said that, if the floor of the spray area is metal, and if the carts have metal wheels, you might get a ground that way. That’s a lot of ifs. The worst part is there are a lot of contact points here that can cause loss or reduction of ground, so this method isn’t the most reliable method to use. You must surely know that spraying the parts while they’re on the cart is a sure way to get a lot of powder on the cart. Eventually, the cart should be cleaned. How will you do that? Or maybe the cart is small? —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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