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.

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.

6/27/2016 - Q: What temperature should galvanized steel reach when degassing it before powder coating over it? S.M., Louisville, Ky.

A: Preheating galvanized steel may or may not completely degas the substrate. You should experiment with time and temperature settings to eliminate most of the gas. A good starting point is to select the cure temperature and cure time for the powder coating you’re using first and go from there.

Beyond the gas problem, make sure the surface has been “brush blasted” or chemically etched to remove the zinc oxide on the galvanized surface before applying the powder coating. Otherwise, you’ll have an adhesion problem with the powder coating. —N.L.

6/20/2016 - Q: We’re looking for a material that could be machined and used as part of our powder racks for masking areas of product where no powder is allowed. Masking a part is too costly. We would like this protection to be part of the racks. We have the ability to machine and build special racks. We have a standard powder process with a five-stage washer. We cure parts up to 475°F in our cure oven. B.J., Jasper, Ind.

A: I know of no material that will resist powder coating and serve as a mask at the same time. The problem is that some things you could use, such as petroleum jelly (shudder), will cause horrendous problems in a powder coating shop. Will petroleum jelly keep the powder off the area? Sure, but then it has to be cleaned off, to say nothing of the contamination problems it would cause. Plastics, such as nylon and a dozen other synthetics will protect the area and survive the heat, for a while, but they have to be cleaned or tossed out and replaced. Exotic metals? They would still draw the electrostatics, and they’re too expensive. So, here are several suggestions. You probably won't like some of them. You can powder coat first and then machine. It's done all the time (on engine blocks for example). You can vacuum away the powder from the critical area. This works nicely when the person doing it cares, or if you’re lucky, you can automate. Maybe the critical area can be shielded by something on the hanger. Bad news here is if it needs to be a tight fit, it probably can't be done. And the hanger will need frequent cleaning. Don't even think of making a moving piece as part of the hanger because that will get powder coated and cease to function. I suspect you’ll just have to bite the bullet and try some version of the above mentioned items. —G.T.

6/13/2016 - Q: After powder coating, especially with black color, the powder coating becomes white after a period of time. The parts are mild steel. M.A., Mafraq, Jordan

A: The problem you describe is called chalking

. Powder coating formulas that aren’t formulated for outdoor exposure (ultraviolet-light [UV] resistance) will readily turn from black to white in a relatively short time (a couple of weeks). Epoxies and epoxy-polyester hybrids aren’t formulated for UV exposure and will readily chalk in sunlight.

Select a coating formula that has the UV resistance you need, that is, a polyester or an acrylic formula. These formulas are manufactured to provide excellent UV resistance, meaning that both the resin and the pigments are UV-stabilized. —N.L.

6/6/2016 - Q: I was wondering if you have any idea where we could have independent testing done on some aluminum parts. We have reports of the paint peeling off aluminum rails and need to find out why this is occurring. The powder we're using is a custom color through our regular powder distributor, which is the only color we’re having trouble with. I’m hesitant to have the distributor test the powder because I'm not sure how impartial it would be. I thought of our chemical company, but if it’s a problem on that end, I may not receive accurate information either. Any information you can provide will be appreciated. B.F., Hamilton Township, N.J.

A: The most common reason for a coating to peel off of a substrate is poor metal pretreatment. Actually, that’s about 95 percent of the reason. If you have failure on aluminum, that percentage rises to about 99.9 percent. You have to clean aluminum before coating it. You don't say what process you’re using, if any, but a description of your current system would be enlightening. There are several sources for testing parts or panels. One is the paint guy. Another is the pretreatment guy. And another is an independent laboratory. The latter is very expensive, which makes one or both of the previous two attractive. I could find a lab for you, but be prepared to pay in the four-figure range for the lab services. If you’ll send a description of your current metal prep method, maybe I can at least eliminate some things that could be at the root of the problem. —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.

View More Question and Answers |  Submit a question |  Problem Solving Index
Powder Coating Home Page |  Print |  Close Window