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What else is available in powder coating finishes?

There’s more to this coating metal parts than durability and a glossy finish

Powder coatings can deliver different types of finishes.

Powder coatings are not limited to simple glossy finishes. Several specialty finishes are available for manufacturers desiring added performance features or a different look. Getty Images

The reasons that a product designer recommends a powder-coat finish are numerous. This dry finishing process is proven after being used in North American industry since the 1960s. If applied correctly, the finish is very durable, much more so than a typical liquid coating. The finishing process is considered to be environmentally friendly because the powder coatings contain no solvents and the application process releases negligible, if any, volatile organic compounds into the atmosphere. Also, any powder coating overspray can be retrieved and reused.

That’s a pretty impressive resume, but there’s even more to powder-coat finishes, which now account for 15% of all industrial finishes, according to the Powder Coating Institute. When polymer resin systems are combined with the right curatives, pigments, leveling agents, flow modifiers, and other additives that make up the powder, these coatings can deliver special looks and performance attributes that expand upon powder coating’s reputation for durability and environmental friendliness. Are metal fabricators aware of some of these special attributes? Those that already powder coat in their own facilities might know of some of them, but this discussion might prove to be enlightening for those metal fabricators that are more focused on cutting, bending, and welding metal. There’s a lot to learn about powder coatings.

A Coating to Fight Virus Transfer

Antimicrobial powder coatings are nothing new, but a pandemic might make them more interesting to manufacturers looking for another layer of protection from the coronavirus and other contagious viruses. G.R. Kearney, president, Acme Finishing, Elk Grove Village, Ill., said that a new customer approached the company recently with a request for this type of coating for a new sanitization product, and another longtime customer asked that its home fixtures start receiving the antimicrobial coatings. So some interest is there even if it only represents “a small uptick” of Acme Finishing’s overall business.

How does it work? Silver ions are the important ingredient in the powder coating formula, according to Marty Korecky, AkzoNobel Coatings. You might not realize it, but silver ions also are used in activewear fabric to stop stink-causing bacteria from reproducing when you sweat. The same thing happens in the powder coating: The silver ions prevent the bacteria that land on the surface from reproducing.

Even though antimicrobial powder coatings demonstrate a distinct performance characteristic, they are applied just like any other powder, Korecky said. Powder coatings most often are applied via electrostatic deposition. A spray gun applies an electrostatic charge to the powder particles, which then are attracted to the grounded metal part to be coated. This can make applying the coating somewhat easy for the sprayer as the charged particles naturally gravitate to the grounded part, but the spray technician has to be careful not to apply too little or too much. Also, technicians have to be aware of the Faraday cage effect: Charged powders don’t necessarily flow easily into tight angles or corners.

Kearney said that because antimicrobial coatings are considered specialty products, they cost more than other widely sold powder coatings. That may contribute to some of the reluctance to embrace this type of coating more widely.

A Coating to Provide Protection

The use of textured powder coatings suggests special cosmetic treatments, such as veining, or safety applications, such as nonslip surfaces. Both are possible and widely available.

The truth of the matter is that most OEMs specify textured coatings because they hide the imperfections of the materials that are being coated. Of course, metal fabricators are all too familiar with the inconsistency of sheet metal and plate that they get from their distributors. It’s a part of life in the metal business, and textured powder coatings are there to help.

“When the substrate isn’t real smooth and you put the smooth coating on there, the coating is going to look the same as what the substrate is,” Korecky said. “One way to hide the defects in the coating is to put a textured finish on there.”

Kearney said that while high-gloss finishes look really sharp, they also can be easily damaged. “Our production guys joke about being able to see a single piece of dust on the coating, which will lead a part to be rejected,” he added. A scratch on such a surface looks that much worse.

Antimicrobial powder coatings applied to panels can help to prevent the spread of bacteria.

Panels used in medical office cabinetry are candidates for antimicrobial powder coatings because they help to prevent the spread of bacteria. Getty Images

Textured surfaces, on the other hand, are able to absorb a little punishment and conceal the damage to a point. Kearney offered the example of black consumer electronics products, which have slightly textured coatings that not only are durable but also have become synonymous with these products.

The textured powder coatings are so effective, according to Kearney, that they are used on die-cast parts made of zinc to battle outgassing issues. During die casting, gas can get trapped inside the part. The curing process for the powder particles, during which the material melts in ovens to create the coating, also tends to release gases that might have been trapped in the die-cast zinc parts. As the gas escapes, small holes are created. Textured powders do a good job of hiding these pinholes.

“When you think of an automobile, the hood has a really smooth finish,” Korecky said. “If you look at the bottom 6 in. of the car door, you’ll see a textured coating. That’s where a lot of the rocks and debris from the road hit the car.”

A powder coater does need to pay attention to minimum film thicknesses when working with these coatings. On a textured coating there is no flat surface on which to determine mil thickness; a textured coating is a collection of peaks and valleys. A customer spec requiring a 3-mil thickness for the coating might result in a coating that is 3-mil thick at its peaks, but not necessarily in the valleys, which in some cases might be a third of that maximum mil thickness.

“We have to be careful that we make sure we put on enough film thickness so that we get the corrosion resistance that we want,” Korecky said.

When it comes to applying textured coatings, the powder material is not that much different from applying standard powder coatings, unless larger particles, such as those used in nonslip applications, are part of the mix. When the larger particles are charged, they have more of a tendency to bounce off the grounded metal part than finer powders.

A Coating That Catches the Eye

Metallics grab attention. That’s why you see them on luxury items like custom motorcycles or high-end wheel rims.

Making powder coatings with metallic features is possible. Given how they are made makes application tricky sometimes.

“Let’s go back to high school science,” Korecky said. “So we’re in class, and we get a bunch of iron fillings in a bag and a magnet. We put the iron fillings on a piece of paper, and we run the magnet under the piece of paper. The iron fillings want to stand up and follow the magnet around. When the magnet is pulled away, they lay back down, and they’re just in a pile.

“Well, the problem with metallics [when they are just mixed with the powder] is when we apply them, they want to do the same thing because we’re using an electrostatic gun.”

Textured finishes help to hide material imperfections and scratches.

Textured finishes are popular with manufacturers because they can help hide material imperfections and scratches, something that glossy finishes can’t do. Getty Images

When applied via the electrostatic process, the metallics, usually aluminum, are going to act in a different way from the charged powder particles, Korecky said. A nice-looking metallic finish is possible, but it’s difficult to get a uniform look over a large area. In addition, a powder coater can’t reuse any of this overspray because the balance between metallic particles and powder is unknown.

Metallics can be included in the powder in some other ways as well. They can be put through the extruder with the powder to get a much more uniform mix, but when applied and cured the effect is somewhat muted. More of a metallic “sheen” is present as the colored pigment in the powder coating dominates in this type of formulation, Korecky said.

Metallics can be mixed with the powder and then heated to create strong bonds between both particles. Korecky said that this provides the best metallic look possible. Powder coatings still can’t deliver a finish that looks just like chrome, but they can get close, he added.

Kearney acknowledged the challenge of working with metallics in a powder coating. He said that his company worked with a large OEM of outdoor products for almost two months before finding the right metallic look for their powder coating finish.

“When you talk about something like antimicrobial coatings, there’s effectively no difference between that and a stock powder coating,” Kearney said. “When you get into metallics, you ought to plan for some research and development. No one should take a production order requiring metallic powder coatings with the due date being three days later.”

A Coating That Looks Like Wood

Dye sublimation doesn’t sound like it would involve powder coating, but it does. The process is especially popular in the window and door industry where people want the look of wood, but also the affordability of metal and the durability that comes with a powder coating finish.

Carl Troiano, president of Trojan Powder Coating, with locations in New York and Florida, has been involved with dye sublimation for 15 years. He said that the process has two major steps: the powder coating and the shrink-wrap.

The powder is formulated to accept dyes. It’s applied like a regular powder coating, and just the right thickness, usually 3 to 4 mils, is needed for the shrink-wrap to work. After a trip through a 400-degrees-F oven, the powder basically turns transluscent. It’s also typically textured.

The shrink-wrap with an ink design in it then is pulled over the substrate and vacuum-formed to it. The powder coating accepts the inks, which penetrate deep into the finish. After a set amount of time, the film is pulled off, and the ink pattern remains on the powder coating finish.

“We are primarily doing aluminum extrusions with this,” Troiano said. “They come here, and we put them through the process. We send it to a window factory, they chop the sticks up, and they put them together and make windows. They look like wood, but they’re made out of aluminum.”

About the Author
The Fabricator

Dan Davis

Editor-in-Chief

2135 Point Blvd.

Elgin, IL 60123

815-227-8281

Dan Davis is editor-in-chief of The Fabricator, the industry's most widely circulated metal fabricating magazine, and its sister publications, The Tube & Pipe Journal and The Welder. He has been with the publications since April 2002.