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.



9/26/2016 - 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.


8/29/2016 - Q: We currently run three lines with automatic spray guns and a reclaim system. My question concerns particle size. When does the particle size become a safety concern? What’s the best method of rotating recycled powder so that it doesn’t become a safety hazard or affect the chargeability/transfer efficiency? I thank you for your assistance. J.A., Kowloon, Hong Kong

A: This is a bit like powder coating 101. The overspray is made up of mostly smaller particles because they don't get a charge like the larger particles do. So that’s the dominant particle size of the reclaim. I hope your powder system efficiency is at least 60 percent. Much less than that and you’ll have material handling problems. You shouldn’t have any issues with smaller particles if you’re using a 60 percent virgin to 40 percent reclaim ratio. Too many small particles will affect the cured powder appearance some, not a lot, but some. More important, too much reclaim, that is, small particles, will cause fluidization and application problems. It can get so bad that you won't be able to fluidize the hopper or move the powder to the guns. The small particles in themselves won’t be a safety problem. Although I'm not sure what your idea of a safety problem is, they will have some small affect on dust-to-air ratios. When this happens, you can risk an explosion or fire, but this will occur when a spark is present. So, if you have a dirty hanger, and the ground is poor, there will be an arc between the gun electrode and the hanger/part. This can cause a fire. An explosion is extremely rare, and a fire is unlikely if you have the proper spark detection systems installed. Now hear this: The particles don't have to be small for this to occur. It can and usually is regular powder particles that are involved. And if the hangers and conveyor are grounded properly, you won't have a problem. —G.T.


8/22/2016 - Q: I have just had my engine and exhaust powder coated. The engine is okay so far, but the down pipes have bubbled up. Why is this? Many thanks. D.D., Cirencester, England

A: Because the powder coater did not use a heat-resistant powder coating on the exhaust. Eventually, all the powder coating on your exhaust will flake off the parts. Next time, request a dual resin with silicone heat-resistant powder coating formulated for exhaust systems (and gas grilles for that matter). Color choice will be limited, but at least it won’t flake off when the coating is used on parts where the heat is above 275°F. —N.L.


8/15/2016 - Q: I’m interested in getting feedback on coating plastic. A lot of people are looking at coating plastic based on heating the plastic first. I think there can be irregularities not only in the film build, but also in the gloss and surface of the powder, smooth or textured. Just wanted to let you know, you give great information. C.J., Markle, Ind.

A: Coating of plastics is a challenge, and I find it interesting that some people seem to think they have an answer and others don't. Sometimes, just a minuscule thing makes the difference. When I was working on an experiment for a plastic car body, we tried a liquid spray-on material that was purported to enhance electrostatics. If it did, we didn't notice it, and preheating of the parts became a necessity. We worked many long hours trying to arrive at a proper preheat temperature. Part of the problem was, immediately after preheat, we had to get the body into the powder booth. Well, the powder booth needed to have the reclaim blower on, so it was introducing ambient air under force, which began to cool the part. Allowing for this made for a lot of trial and error on the correct preheat temperature. We sprayed powder with the electrostatics on because the fan pattern was enhanced by the electrostatic field. This effort helped in uniform distribution of the powder. The powder film was within 0.03 mil from side to side on the car body. There are companies that are applying powder coating to plastic bottles. The bottles are on a spindle and rotate, so application is relatively simple. The bottles are preheated in some cases. In other cases, they’re sprayed with the aforementioned electrostatic enhancer. Each of the companies thinks its way is best. Well, who knows. Gloss and powder surface will be affected by the thickness of the plastic because of reinforcing or just plain variances in the plastic. This will show up directly in the powder, usually where preheating is the attraction for the powder. —G.T.


8/8/2016 - Q. I have some automotive headers and would like them to be powder-coated in flat white that will sustain header heat. I know of ceramic coatings, but that is another process. Any suggestions? B.P., St. Paul, Minn.

A. High-temperature organic materials are available. Even though these materials are designed for high temps, they’re good for only so long before they will begin to burn. That’s one of the reasons the serious show car exhibitors trailer their vehicles. The pipes never get to sustained high temps. —G.T.


8/1/2016 - Q: We cut, drill, and buff extruded aluminum parts in a variety of shapes in-house and then powder coat them. We’ve had a higher number of rejects than usual lately because of aluminum chips on the parts. We paint a lot of metallic and mica powder. We installed bag filters and 250-micron filters in screens, but we still see aluminum chips on the parts (under and over the paint). We noticed aluminum flakes and other airborne dirt floating in the cure oven. The aluminum chips show up only on aluminum parts and not on steel parts. What do you recommend we do to determine the cause? Is it coming from the wash, the spray-gun tip, or oven airflow or vibration? K.M., Holland Landing, Ont.

A: Contaminants in powder-coated products can come from numerous sources. Looking at your coated and cured products will help identify what the sources of these contaminants are. For instance, contaminants on the surface of the coating (not in or under the coating) come from sources after the coating operation (applying the powder). Conversely, contaminants in or under the coating can come from sources either before powder application or during powder application.

Contaminants on top of the coating are typically deposited in the cure oven and are airborne in nature. Contaminants in or under the coating can be mixed in with the reclaimed powder and applied before or during powder application. Visual inspection of parts before they enter the powder coating booth will tell you if there are contaminants on the parts before spray. Finally, spraying clean test panels with reclaimed powder will tell you if your reclaim is contaminated.

Once you’ve identified where the contaminants are in your coating process, you can begin to eliminate them as coating defects. The best way to eliminate these contaminants is to trap and collect them at their source, which in your case in at the fabrication stage. Air hoods and dust containment systems will contain and collect contaminants as they are made by machining operations. If you can’t eliminate the contaminants at their source, then isolate your powder coating process from your manufacturing process. This is done by installing an environmentally controlled room around the powder coating process. This room should be made of easy-to-clean materials and have positive air pressure to the surrounding plant airspace to keep airborne contaminants from entering the room.

Finally, it’s unlikely that these contaminants are being deposited by your wash system. However, your wash system may not be removing them entirely from the part surface. Increasing dwell time and impingement pressure should fix this issue. —N.L.


7/25/2016 - Q: Where can I get a powder production process manual? What is the most recommended warehouse storage temperature? J.E., Apodaca, Nuevo Leon, Mexico

A: Several universities offer symposiums on the formulation of powder coatings and the operation of powder processing equipment. However, I don’t know of any handbooks on the subject. There is a white paper available from Bruno Fawer, associate consultant at Powder Coating Consultants, that may shed some light on your questions. You can reach him by emailing pcc@powdercoat.com.

As for the best storage temperature for a warehouse containing powder coatings, I recommend 70°F to 80°F for most normal formulations. Higher reactive systems (low-temperature-cure powders) should be stored at 60°F to 70°F. —N.L.


7/18/2016 - Q: What’s the best way to get rid of used powder when we spray to waste? Is there anyone that uses this powder? Right now we put it in the dry-off oven until it gets hard, then throw it away. J., Peoria Heights, Ill.

A: That's the best way unless you can find someone who’ll buy it from you. Finding these people isn't easy because it usually requires some type of sifting or filtering system to clean up the material. If you have a lot, maybe you should consider a reclaim system for high-use powders. —G.T.


7/11/2016 - Q: What’s the most effective way to remove old powder coating without having to invest in chemical process equipment? I've tried media blasting with coal slag, chemicals called "stripper" in an aerosol can, and aircraft paint remover in a liquid form. Most of the products I powder coat are new steel or aluminum, which I prep by media blasting. This is the first time I’ve had to deal with already powder-coated parts. It’s a pain! Are there different removers for steel and aluminum? D.K., Bagley, Minn.

A: There aren’t different strippers for aluminum and steel that I know of at least. A myriad of companies claim to have metal strippers. Some work, some don't. The two most effective ways to clean cured powder from a part is by burn-off (not good for aluminum, depending upon the parts) and chemical. Stripper in a can absolutely won't work as you have apparently found out. The hot-sand method is a variant on the burn-off method. Parts are placed in a container with sand in it and a burner tube. The sand fluidizes, and the heat aids and abets the sand in removing the coating. You’re finding out why, when properly applied, powder is such a good coating. Depending upon the number of parts you have to strip, search for a stripping company in the area and have the company chemically remove the coating. Once the powder is removed from the aluminum, make sure the parts are cleaned of any residuals. Then, pretreat and paint immediately. Aluminum will begin to create its own patina very quickly, and this will negatively affect the adhesion of the paint you’ll apply. —G.T.


7/5/2016 - Q: I have a part that has two issues. First, the part is stainless steel. Second, it requires 10 to 15 mils of paint. What issues can I expect when powder coating stainless steel, and how difficult will it be to achieve the thickness spec? B.S., Toronto, Ont.

A: Stainless steel comes in several finishes. The most common is a high-gloss surface. Less common is a muted finish. I don't suppose you’re lucky enough to be coating the lower-gloss material, which is friendlier to paint than the high-gloss finish. For a lasting finish, you should scuff-sand the area to be coated; otherwise, the finish will scratch easily, and the paint will begin to peel. Depending on the designed film thickness of the powder you’re using, you’ll probably have to build the film in several passes. Trying to get the coating that thick in one pass will most likely create "starring," or kilovolt rejection. You can apply about 4 mils, maybe 5, in one pass. Place the part in the oven and set the powder, then coat again. Continue until you get the desired film thickness. It certainly would help if the powder is designed for heavy film build. If that’s the case, then you may be able to achieve the thickness in no more than two passes. If you’re going to use an off-the-shelf material, it may require multiple passes. This won’t be a cheap application. I hope that you’ve made your customer aware of the cost. —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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