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.

2/8/2016 - Q: I’ve been coating automotive wheels for years, mostly older steel wheels from classic car restorations but also newer wheels. I’ve been hearing lately that aluminum alloy wheels shouldn’t be powder-coated because the curing temperature and dwell time make the aluminum wheels brittle. I have also seen feature articles in the magazine about companies coating wheels but no mention of this problem. Is this true and are there other coating methods that can be used to overcome this problem? K.W., Westminster, Md.

A: Some aluminum wheels are made from tempered aluminum alloys. The most common type is 6061 T6 aluminum. The T6 represents the temper and is used to increase the hardness and strength of the aluminum alloy. Temper can be annealed (softened) under heat above 300°F. Therefore, great care must be exercised when powder coating aluminum wheels with tempered alloys. Always use powder coatings that have good cure characteristics at 300°F and don’t cure them above this temperature. —N.L.

2/1/2016 - Q: We’re powder coating 18-gauge Galvannealed material. We’re cleaning with a five-stage washer and powder coating with a black wrinkle epoxy. We’re seeing bubbling and fingerprints coming back through the powder. We use infrared heat and are manually spraying the powder. Is there something we can do to prevent this? This material has given us trouble in the past. K.M., Cedar Rapids, Iowa

A: The bubbling is a problem with galvanizing of any type. Often, a little preheat will drive out the gases, and then you can powder coat after cooling the part to ambient. The fingerprints? Well, check closely before washing to see if someone is handling the parts with heavy grease or oils on their hands or gloves to determine if the washer is having a problem removing the residual. Maybe the sprayer is turning the parts with his hand and leaving a print. If he has to do this, give him a wire hook to touch the parts so that he doesn’t have to use his hand. —G.T.

1/25/2016 - Q: Is it possible to apply a powder coating over satin chrome or high-polished chrome-plated parts? If we could, it would save us a lot of money in rework and scrap costs. However, some of our parts are hand-operated, and we don’t want our decorative finishes to start peeling after several years. R.C., Steeleville, Ill.

A: I don’t know of any way you can powder coat over chrome without a high risk of peeling in the immediate or near future unless you abrade the surface with steel wool, 3M-type pads, or similar materials. The hard chrome surface won’t allow the coating to get a grip. Satin chrome might extend the time before peeling commences. I’ve never tested adhesion to satin chrome, so I suggest you try some samples before you go to production pieces. Acid etching the chrome is ineffective unless the acid is very strong, and then you have a great risk of damaging the chrome. It’s very risky to expect the powder coating not to peel. You have no control over how the part may be used by the customer. —G.T.

1/18/2016 - Q: We powder coat in two facilities, and our safety director is directing us not to use compressed air to clean off ourselves after applying powder. I guess OSHA (Occupational Safety and Health Administration) will site us for this practice. I’m curious how others in our industry are cleaning off afterward. I have an air-operated vacuum, but it’s so powerful, I’m afraid it would be more dangerous than compressed air. M.G., Iron Mountain, Mich.

A: There is an OSHA-approved tip for an air hose that restricts output pressure and has relief holes that will divert the air in case the user gets the tip too close to something, namely the body. These are used just about everywhere and have been around for years. The automobile companies have been using these tips for many years, and they are extremely safety conscience. Your safety director should be made aware of these devices so that you can go about your business. Using a vacuum with the suction restricted is a possibility, but vacuuming clothes is awkward and time-consuming. I hope the safety person listens to you when you tell him about this. —G.T.

1/11/2016 - Q: We have an aluminum frame that has gold chromate conversion coating under the powder and masking lines. We have to re-powder coat the part. The part was powder-coated twice, and the customer doesn’t like the finish. The part came out with heavy texture and a rough finish. We had to sand and re-powder coat, and the gloss ended up being dull and sandy feeling. L.B., Springfield, Mass.

A: As you probably know, at least by now, heavy powder coating will result in pronounced orange peel, which some people don't like. Determine the extent of the orange peel the customer will find satisfactory. That will become your standard. Then, you should show your customer what a second coat looks like to see if he objects. Your customer must be aware that if the second coat isn't to his liking, the part will probably be scrapped. I hasten to add, however, that if you have or have access to chemical stripping, you might be able to strip off the old powder and then reprocess the parts. Most likely, the chemical stripping will affect the conversion coating, and it will need to be reprocessed through the bath. You should discuss this with the conversion coating supplier. I'm not sure why the reworked part was dull and rough. That would depend on how you sanded it and how you cleaned it after sanding. If you sanded through the chromate treatment, then you set up the aluminum to begin its self-protecting patina, which could cause the rough finish--and will cause a loss of adhesion at some point. —G.T.

1/4/2016 - Q: We recently coated machined aluminum parts with many holes of different sizes with a flat, beige polyester powder. The surface immediately surrounding the plugs was bumpy. The second time we coated this same part the customer purchased the plugs from a different vendor and cut the plugs flush with the surface, but the results were the same. We cure at 400°F for 15 minutes; we've had the problem with other parts and think the plugs are made from silicone. Is there a reaction between the plugs and the powder? How can we correct the problem? Thank you in advance. P.L., Cleveland, Ohio

A: All masking devices (plugs, caps, tape) must withstand the cure temperatures for powder coatings. In most cases, the plugs are made from silicone materials for this purpose. Silicone in solid form isn’t a problem with any coating system. Airborne silicones are what caused fisheyes and other finishing problems.

The bumpy and rough surface around the plugged holes is due to low film build (coating thickness). The space between the end of the plug and the part is very narrow, making coating this area difficult. Trimming the plug to reduce the shielding from the masking device can help with this issue. Ensuring proper coverage in this area will also eliminate this problem.

Finally, the machined part may have some machining fluids/coolants inside the hole. If you mask the part before pretreatment/cleaning activities, this fluid/coolant will stay in the hole. When you cure the powder coating this liquid will vaporize and push this gas past the plug mask and blow off some of the powder coating in the area. You can try fixing this by cleaning and drying the part first, then inserting the plug for masking. —N.L.

12/28/2015 - Q: We are a manufacturer of industrial machines that we have powder-coated. From an electronics point of view, there are parts of the cabinet that I don't want powder coated so that they can make an electrical connection for grounding. The company that we have doing our powder coating used regular masking tape to cover these areas. After curing, the masking tape was extremely difficult to remove. I'm sure there is a better way, but they don't know what it is. What is the industry standard for masking off areas that you don't want coated? J.L., Nevada, Mo.

A: The most efficient way is to contact one of the masking companies listed in the magazine’s annual Reference & Buyer’s Resource Issue. You can find it by going to the magazine’s Web site and clicking Online Buyer’s Guide. You can waste a lot of time trying out different things, and I’ve found that there are almost as many masking requirements as there are needs. Masking tape is not a good way due to the heat for curing. Sometimes, you can use a plastic, heat-resistant film, which has a small amount of static charge. The film clings to the parts, yet it will peel away easily. There are also some new magnetic devices available. In very difficult masking situations, you can use a vacuum system. This means, usually, that you need a person to do the work, but a vacuum system with hose reductions to as little as ¼ inch can suck powder away from an area where you don't want any finish. Don't try compressed air. Blows powder everywhere. If the part position is consistent, you may be able to fixture a device that will draw away the powder. Again, check with masking suppliers. They do custom work as well. —G.T.

12/21/2015 - Q: I work for a company that applies powder coatings to industrial fans. The question has come up regarding how many square feet per hour, shift, and so on our line is capable of processing. Short of pulling all the prints and calculating square footage, I’m not aware of any system that can accomplish this. It was suggested that some type of camera system could take "shots" of parts that pass through the line, and it would be capable of providing our company with this information. I’m wondering if you’re aware of any kind of system that could automatically keep track of the square footage a paint line can process? Just so you know, we have a 37-year-old, three-stage system and all our spraying is done manually. Many of the parts we run are three dimensional with deep cavities. A.G., Schofield, Wis.

A: So many things impact line production (output), and there have been many articles written about the subject. (See the Article Index on the magazine’s Web site.) Every detail would be too much for a column of this type. However, here are typical things to investigate. How many parts can you place on a hanger and still present an effective surface to the application equipment? How many hangers can you get on the line with spacing that allows for turns and inclines? What are the times in each stage of the washer? The first stage should be at least a full minute. The next two may be 30 seconds each, not counting drip time. This and the oven can be the limiting factors in output. The oven can be increased in temperature, and of the two, is the lesser influence. And you are absolutely correct. Obtaining the footage of each part from the prints would be very time-consuming but also the only effective way of knowing what you have. I seriously doubt a camera would tell you anything worthwhile. And I’ve never heard of anyone trying to achieve what it is you need without seeming to be spying on the workers. Obviously, this would get you all kinds of labor problems. Your description of the system and of the parts in general would imply that an automatic system couldn’t effectively get the powder coating where it has to go, and it would be expensive because you would need automatic guns and manual reinforcement stations. A 37-year-old washer, from my experience, is likely in need of much upgrading or replacement and may be part of the problem involving less than necessary production levels. —G.T.

12/14/2015 - Q: I’m a newly appointed engineer for a company that manufactures gray steel cages. We are having issues with our curing process. We use a gas-fired oven for curing the parts, which have a grainy surface. When methane is used as fuel, the parts turn out perfect. However, when we use liquefied petroleum gas (LPG), the parts turn darker and a black distorted web-like paint appears. I think this is due to carbon in the flue gases and that maybe the airflow rate is too low. I’m just not sure why it only happens when we use LPG. Any guidance would be much appreciated. E.A., Karachi, Pakistan

A: The problems you describe happen when using sooty fuels in a direct fired oven. These fuels do not burn as cleanly as natural gas or propane and therefore produce more byproducts of combustion (soot). To overcome this issue, the air “turnover” rate of the exhaust fan need to be increased to at least 12 changes per hour. This means the exhaust fan must change the air within the oven at least 12 times per hour to remove the byproducts of combustion. If the fuel source is particularly dirty, this value may need to be increased even higher. In the worst case, the oven heat source will need to change from direct fired to indirect fired where a heat exchanger is used to isolate combustion gasses from the oven interior. You may find that the size of the burner may need to be significantly increased to handle the amount of cold air that will be pulled into the cure oven to replace this higher exhaust rate. —N.L.

12/7/2015 - Q: We fabricate some aluminum parts that are then powder-coated off site and delivered to our customer. Many of the parts look very similar, and our customer sometimes has trouble identifying them. There are areas of the parts that aren’t cosmetic and could be used as a location to place a part number. I would like to avoid having to buy a costly stamping device, which might also change the part dimension or crush the thin-walled tubes. Is there some type of ink or marker that we could use to put a number on the parts before powder coating that will either bubble up or bleed through the powder? Most of the colors we do for our customer are either a texture or a gloss metallic, and all are various shades of silver and earth tones. S.M., Woodinville, Wash.

A: There are suppliers of materials that will withstand the heat of the oven and yet be removable after cure. These are typically available from masking companies. You can find them listed at the magazine’s Web site [www.pcoating.com]. Click on Online Buyer’s Guide. Black permanent marker won’t come off in the washer and will bleed through on lighter colors. Of course this doesn't work on dark colors; although it's there, you just can't see it. I would try anything to avoid the stamping method. Here are some ideas, which you may have tried: wire on a tag, attach magnetic markers, segregate the parts in containers and mark the container, or use an ink part number stamp, and make your coater responsible for the correct part segregation. See if any of these ideas work. —G.T.

11/30/2015 - Q: We’re currently trying to reduce the number of hooks we’re leaving between color breaks. We have a powder coating system that can change over in 20 seconds. The problem is that we have a metal hood over the conveyor chain that we have to clean (blow off with air) between color changes to avoid contamination of the new color. Any suggestions on how we can solve this problem? S.W., Asheville, N.C.

A: What is the hood used for? Is it directly over the spray booth because powder was drifting out of the conveyor opening? If that’s the case, then either the blower isn't doing its job or a gun(s) is too close to the opening. Correct these conditions and you should be able to eliminate the conveyor enclosure. You can pressurize the "tunnel" with a small blower, but the air must be gentle enough not to disturb the gun pattern, yet strong enough to keep powder from drifting up into the cover. If I'm missing something, please send me details of the cover and its location. By the way, there is no magic way to remove powder from inside the cover. Not that I know of anyway. —G.T.

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.

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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