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.

11/23/2015 - Q: I'm coating a large box with a white powder. The box is dipped at the manufacturer's with electro-coat (e-coat) primer. The manufacturer assured me that this process was acceptable to use under powder. The problem I have is the finished product is turning smoky brown in spots. Is this a reaction of the two products maybe? Or could it be something else, poor cleaning, powder too thin, improper curing? T.B., Cody, Wy.

A: E-coat as a primer for powder coating is fine and is done regularly. You have to be aware of potential grounding issues because the e-coat can inhibit a ground and cause a very light coat of powder, similar to spraying with the electrostatics turned very low or off. This is easy to check with a film gauge. You should also know the e-coat thickness. Most are in the 1.0 mil to 1.2 mils range. When I've seen stains such as the ones you've described, it has been when the white powder was direct-to-metal. The stain was invisible until curing, which brought it out and caused it to bleed through the finish. I suppose this could be happening in your case. The stains I saw were caused by oils from welding. The welding process burned the metal mill oils into the substrate, and the curing brought it out to the surface. The fix? Two things: The welded area was cleaned better before welding, and the powder supplier was changed. Actually the material change did the most good. Something on the surface of the e-coat could also be at fault depending on what happens to the parts between e-coat and powder application. You could try to recreate the problem on different substrate material, making notes on each handling step in the process. I've seen a stain occur occasionally in the corners of a metal part when there was poor air circulation in an oven or when the oven was a bit too hot and the parts were really cooking to the point that the powder coating was over-curing. In your case, a lot depends on the color of the powder coating. —G.T.

11/16/2015 - Q: Can you powder coat clear zinc-plated parts or will the high heat damage the plating? L.A., Corry, Pa.

A: The only plated surface that I’ve had some difficulty with is brass plating. Brass plating sometimes turns red, depending upon cure temperature and surface contaminants under the powder coating. You shouldn’t have any problems with zinc plating. Just make sure that the surface is completely clean and rinsed with virgin de-ionized water. And cure the powder at the lowest temperature possible. —N.L.

11/9/2015 - Q: We manufacture aerospace products. One of our product lines is NiCad (nickel cadmium) batteries for aircraft. Our battery cans are made of cold-rolled or stainless steel. They're preheated and dipped in a fluidized bed. We hold the film thickness between 10 mils and 15 mils. Our brown vat has an occasional problem with powder clumping. This isn't a problem with the other colors we apply. Is this a moisture or humidity problem even though our compressed air is triple-filtered? And is there anything we can do to prevent this from happening? J.M., Waco, Tex.

A: It could be that the brown powder is a slightly finer grind, which will lump as though it's damp. Your description certainly sounds like a moisture problem. Make sure the air lines to the hopper aren't in a position to pick up condensation even though you're triple-filtering the air. You could try mechanical agitation. Some companies do this as an auxiliary method of fluidization for several reasons, one being it helps break up agglomeration. You can use an air-driven motor and shaft with a prop or wheel on the end. Stick the shaft in through the inspection opening, with the wheel about 4 inches from the fluid plate. See if that helps. —G.T.

11/2/2015 - Q: We apply our powder in one of two collector booths and require spray techs to clean their booths daily before leaving. The booths are pulse-cleaned weekly. In October, we removed all the cartridge filters and blew them out from the inside. We began to experience our contamination problem in mid-December. We noticed black specs appearing on some RAL 1013 oyster white during cooling. The specs became increasingly worse the longer they sat and were much more prevalent on horizontal surfaces. This led us to believe that the black powder we had sprayed the day before was still in the atmosphere. We’ve since cleaned the shop from top to bottom. We’ve been commended by and acquired clients due to the cleanliness of our facility. We’re at wits end. Any input would be greatly appreciated. D.A., Billings, Mo.

A: I fail to understand why you would have a powder booth with a filter blow-down system and only use the blow down on occasion. Explain that to me, please. They are there for a reason and that’s to clean the filters of the oversprayed powder. When you don't use this feature, the overspray, which is mostly the smaller powder particles, end up buried deep in the crevices of the filters. The longer they are there, the more difficult it is to expel them. Maybe you have a scarcity of compressed air. You need at least a surge tank in the system to prevent the filter blow-down system from robbing the application system of compressed air. And maybe you just don't have enough air to do the job as it should be done. If not, get a larger compressor. OK, so what else might it be? Oven dirt? Something in the air current that passes near or through the powder coating system? If you have cleaned as thoroughly as you mentioned, then you shouldn't have any shop dirt, but you never know. I'll stick by my first reason. —G.T.

10/26/2015 - Q: A small part of our powder coating business deals with the coating of antique car parts used on chassis. Currently, we prep these parts by washing them in solvent and sandblasting. We then coat them with polyester powder coating at a high film thickness to let the powder flow into the bumps and craters caused by corrosion being removed by blasting. The parts come out of the oven looking spectacular, but we find them easy to chip. The chipping exposes bare metal and needs to be touched up occasionally with paint. Is the chipping caused by the lack of proper surface prep (such as a zinc or iron phosphate)? Or is it caused from excessive film thickness? We have tried varying the cure temperature and time, with no success. M.M., Fall River, Mass.

A: My heavy bet is on the heavy film thickness. Your chipping problem is a classic example of loss of adhesion due to heavy paint. There’s a lot of paint applied to parts prepped as you’re doing with no adhesion problems, although I prefer to add a chemical pretreatment for a total finish system. Because my garage is full of tools that were handed down from my grandfather, and I have tinkered with old cars, mostly out of necessity, I know what that metal surface looks like, and you’re expecting too much from the coating. Phosphate coatings won’t compensate for the excess film failure. —G.T.

10/19/2015 - Q: We are having shade variation problems during powder coating. We are using the same powder on different components, one is aluminum plate and the other is square tube, but after powder coating in the same batch there is slight shade variation. What is the root cause of this and how we can avoid it? D.P., Maharashtra, India

A: The most probable cause for color variance using the same batch powder is poor curing. Color and appearance of powder coatings are affected by both under and over cure conditions. Determining which condition you are facing can be as easy as performing a solvent resistance cure test (PCI Test Method #8). This can be a common problem when coating different materials (sheet and tube) or different substrates (steel and aluminum) where bring-up time in the cure oven can be dramatically different. As bring-up time changes, the time the part is at the desired metal temperature changes (longer or shorter) for a given conveyor speed and cure oven setpoint. These changes can be dramatic enough to cause color and smoothness variations with the same coating material on different substrates and different mass products. —N.L.

10/12/2015 - Q: We’re powder coating a rectangular aluminum extrusion that is 30-millimeters (mm), or 1.18-inch, wide by 18-mm (0.71-inch) thick by 1,228-mm (48.35-inches) long. At the center of the 18-mm thickness, there’s a U-type slot that’s 6.00-mm (0.24-inch) wide by 6.00-mm (0.24-inch) deep through the length of the extrusion. We’re unable to powder coat this slot properly. Powder coating is being done manually with several different brands of powder coating spray guns. P.M., India

A: Sounds like a Faraday cage problem, if I get the picture correctly. You should be able to get coverage with a manual spray application. Back off on the atomizing air and possibly the voltage. You want to ease the powder into the U turn. Don't try to flood it. Even with an aluminum part, the edges will act as antennas and draw the powder away from where you want it. —G.T.

10/5/2015 - Q: How can we clean our spray booth cyclones without blowing them with air? A.G., Kempton Park, South Africa

A: Your first choice should be a GOOD vacuum system. You might have to fabricate a pickup head that will match the contours of the cyclone, but don't do that until you have tried whatever comes with the unit. Don't buy a Shop-Vac or a hardware-store-type vacuum. They aren't designed for the fine particles that are inherent in a powder coating system. Don't let anybody tell you otherwise. You can find several suppliers of vacuum systems designed for powder coatings in the magazine's Online Buyer’ Guide on its Web site [www.pcoating.com]. The only other means would be to fabricate a dry-wipe system that someone would have to manipulate through the collector. I don't advise this as you’ll spend more time working and experimenting with a design that could fail, and that money could purchase a vacuum system. —G.T.

9/28/2015 - Q: I have a customer who has no set quality procedure in place for inspecting coated items. The customer is rejecting items for flaws in the appearance after using a flashlight to find them. Is there a procedure set by a powder coating authority that I can use to demonstrate to this customer what should be rejected and what should pass? We don’t have a clean room, and the customer isn’t willing to pay for that service. A.N., Monroe, N.C.

A: I feel sympathy for you and disgust at your customer for not having specifications. There are as many generic specifications as there are companies. I don't know of any standard that you could use for reference. Using a flashlight is completely out of line, especially with no set standards. I could say shame on you for taking on a project that has no specifications for the coating, but I won't say that. If you want to maintain this account, I recommend that you request a meeting with your customer’s pertinent personnel and state your case for the establishment of specifications. In some cases, even with specifications you can get divergent ideas on what is acceptable, so with no spec you’re aiming at a moving target. You have every right to say to the customer that without specs, whatever it is you’re coating is acceptable. If the customer wishes to create a spec, then you need to review it and confirm whether you want to coat the parts to that standard. Everyone has to be on the same page about that. You don't use a flashlight for highlight. You can view the part under standard lighting, move the part for a good view, and determine if there is a reject. Specifications can be written for what size defect is allowed and how many are allowed in an established dimension. Your customer should create this list. The customer should also be reminded that he can demand a part that is so perfect that he can't afford to have it painted. Your customer has to come to grips with his "standards" versus how much he will pay for meeting these standards. At the same time, you should review how much work there is available to justify installing a clean room. Having a clean room isn't a guarantee that all parts will be dirt-free however. —G.T.

9/21/2015 - Q: Is there a company that will come out to facilities and safety-inspect conveyor lines for stress fractures? I’m worried that as time goes on the conveyor line, which is welded to the trusses throughout the ceiling, may fail or even the ceiling itself may fail—or both. J.O., Des Plaines, Ill.

A: You could contact any of the better known conveyor companies to perform an inspection. (See the magazine’s Online Buyer’s Guide.) It will cost you whatever rate they charge for field engineering work. You could also contact a civil engineering company to do the same thing. They should be capable of determining what or if any damage has been done. Your issue is why I always strongly recommend that nothing, especially a conveyor, is ever welded to the building structure. You should use clips that bolt to columns or rafters or whatever piece of building structure you need to use.

The problem with welding to the building columns or trusses is that stress will occur from movement of the building and the conveyor. You should consider having the inspection and then modifying the connection points. While you’re at it, if you haven’t checked the conveyor rail, you should inspect all wear points and replace rail sections as necessary. Check incline and turn radii through the washer and any other places that stress may occur. —G.T.

9/14/2015 - Q: I’m interested in powder coating torsion springs in which the coils of the springs touch each other. I have two questions: 1. Will powder penetrate between the coils that are touching each other? 2. Will the powder chip off of the coils if the coated coils are rubbing against each other? J.R., Millbury, Ohio

A: Maybe. The powder may migrate between the two springs IF they are barely touching. However, even if they do touch, the motion of the springs will rub a bare spot where they touch. It doesn’t matter what type of coating you apply to the parts. When the two pieces touch, especially when driven by the weight of some type of vehicle, you’ll get only bare metal. What difference does it make anyway? The motion will always keep the metal shiny, and torsion springs as I know them will be too thick to ever rust to the breaking point. Rust would only occur if the vehicle sits idle for an extreme period of time, and the first few times the two springs move against one another, the rust will be abraded away. If you want the pieces to look pretty for a short time, you should be able to do that, but the coating won't last very long. —G.T.

9/8/2015 - Q: I need to use a palette of more than 200 customized colors on relatively large MDF (medium-density fiberboard) products. Can powder coatings be mixed to create new colors? How controllable is this? What is the minimum area of a certain color batch to make it convenient and easy to change colors? I'm afraid changing colors will take a lot of time and hence money. Thank you in advance for your reply. A.L., Montevideo, Uruguay

A: You can’t mix colors in powder coatings and achieve the kind of blended color that you can mix with liquid coatings. The powder particles don't flow together to blend into another color. Instead, you’ll get a salt-and-pepper look. For example, if you mix a black and a white, you won't get gray, you’ll get black and white. I'm not quite sure what you mean by a "certain color batch." If you mean purchasing different colors, then it would depend on the powder supplier and what standard colors are in inventory. If you want something special, it will cost you to have a color made up. If standard, off-the-shelf powder coatings would work, then it depends on availability. If you reclaim powder in your existing booth, then color-changing will be a bit time-consuming, and you’ll need a separate color module (filter section). If you’re spraying-to-waste, then it won't make any difference except for the time it’ll take to clean out the hopper, hoses, pumps, and guns. A great deal depends on the type of spray booth you now have. Time equals money, and you’ll definitely spend time cleaning up the previous color before you make a change. As a result, you’ll have lost line time while you were making the change. And if you reclaim, you’ll need another color module, as I mentioned. If you only have a few pieces, it won't be cost-effective. —G.T.

8/31/2015 - Q: I would like to know if there is an effective way to specify and accurately measure powder coating textures? I’m currently examining establishment of a worldwide powder coating standard specification for our company. How do multiple powder coating vendors worldwide control texture? S.H., Greensboro, N.C.

A: There is no standard for texture in powder coatings. There are standards for smoothness (Powder Coating Institute Smoothness Standards), but these don’t cover textures, only different degrees of orange peel. We have even tried to establish texture standards for clients by using surface roughness measurements (that is, media blast profile methods). However, these only provided the height of the texture and not the shape (sharpness or roundness) of the texture. Therefore, the only accurate way to specify the amount of texture is to use color-panel standards. This means you maintain and supply actual coated test panels that show the color and texture you require for your coating.

These standard panels are attached to the purchasing document, along with any other coating performance standards, when you’re selecting coatings or contracting for coating services. These standard panels will provide the color, gloss, and texture for your product and can be matched by the coating formulator or the coating contractor. They should be communicated to both the coating supplier and outside coating contractor, as application variables will affect the texture on your actual product.

For instance, the sharpness or roundness of a particular texture can be influenced by the applicator of the coating. Applicators can apply the coating at thicknesses that are outside the range provided by the coating formulator, causing the texture to be remarkably different than expected or formulated. Therefore, even if you have a powder that is properly formulated to provide the amount and style of texture you need, you must be sure that the applicator coats your product within stated limits to attain this final texture on your actual product. —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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