A powder coating primer for metal fabricators

It’s a common story. A fabricator analyzes how much time parts spend in each process. Laser cutting, bending, welding—these and other in-house departments usually can be measured in minutes and hours. But send parts to outside subcontractors, and the time is usually measured in days, sometimes weeks. No wonder so many fabricators choose to bring many services in-house, including powder coating.

Still, not every fabricator has a powder coating line, and for good reason. If a shop has a close network of custom coaters in the area, making the leap into powder coating perhaps just doesn’t make sense. After all, launching a powder coating line is no simple feat. But without a close, responsive network of custom powder coaters, a fabricator sometimes just doesn’t have a choice. If it’s going to keep serving customers, a shop sometimes needs to take the plunge.

Fabricators need to learn the equipment basics, from pretreatment to the powder coating itself to the drying requirements. But shops shouldn’t forget a critical piece of the puzzle: the training requirements. Powder coating is deceptively simple. Damaging or otherwise scrapping a part in powder coating and finishing is an expensive mistake, considering all the value—from laser cutting to welding—that has been added before the coating process.

Considering this, The FABRICATOR spoke with Rodger Talbert of Grand Rapids, Mich.-based Talbert Consulting, which specializes in training for the powder coating industry. Turns out, this industry faces the same challenges as metal fabrication. The quality of a powder coat hinges on not just technology, but on the people managing and using that technology. It all starts with an operator, a spray gun, and the workpiece at hand.

On the First Day

Observing a novice powder coating operator on his first day, Talbert notices several things. First comes the misconception that it’s an easy job. “It’s odd, but it’s true. Powder coating can seem simple in the sense that the average person can figure out how to get powder on a part. But they soon find that they can’t achieve a uniform film, and they can’t cover certain areas of a part as they hoped.”

That classic inconsistency of coverage comes from a basic misunderstanding of exactly how powder coating works. Unlike wet paint, powder doesn’t have the surface tension that keeps it adhered to a surface. It is a powder, after all, not a liquid. A powder coat sticks only when there’s electrostatic action present, “and electricity has its own behavior according to Ohm’s law,” Talbert said. “It will follow the path of least resistance.”

When the novice starts spraying, everything seems to be fine at first. But soon enough, areas of certain parts, like tight corners of adjacent inside flanges or the point where two wires intersect, don’t have sufficient coating coverage, while flat areas have too much. That’s because the flat areas exhibit the least electrical resistance, while the corners have high electrical resistance.

The novice points the gun into the corners to try to attain enough coverage, only to find that a portion of the powder flows to those areas of least resistance, those flat areas. The piece ends up with some areas overcoated, other areas undercoated, and a general unevenness throughout. What’s the operator missing? It could be one or a number of many things, depending on the situation.

Equipment and Technique

Some variables are out of the operator’s immediate control during the operation, including pretreatment settings, line speed, part hanging and line density, as well as curing oven settings. As for the variables the operator can control, they can be grouped into two areas: equipment settings and technique.

Talbert suggests starting with the equipment. After all, to have good technique, an operator can’t be fighting poor equipment settings.

“You have two different subsystems that affect coverage,” he said. “First is the delivery component, including getting your air pressure right. This gives you the right amount of powder and prevents that powder from having excess velocity, giving you a nice, uniform pattern. It’s the pneumatic part of the process.”

Operators have two different air settings to consider: (1) percent of total volume and (2) velocity (atomizing or pattern control). Settings will vary depending on the powder, the part mix, and the person doing the coating. As Talbert explained, “A typical setting might be 40 percent [of total volume] and 4.0 Nm3/h (normal cubic meters/hour) for my secondary air supply ; that’s a good starting point.”

Second is the charging aspect, including the voltage and amperage—or, in powder coating parlance, “microamps.” Novices often use too much current. Just the right amount of current draws the powder to the metal surface in an even manner. Indeed, it’s that electrostatic attraction that causes the powder to adhere in the first place. But too much current can have a repelling effect.

“The draw of the current from the electrode of the powder to the part is so strong it interferes with the natural deposition of the powder,” Talbert said. “It’s just too much energy, and it can create a texture [in the coating], pinholing, and back ionization,” where high voltage collides with air molecules and splits them to create ions, which creates shapes and textures on the coating surface.

That repelling effect on the inside corners (and other high-resistance areas of a workpiece) is something powder coaters call a Faraday cage. The effect is named after the English scientist Michael Faraday, who discovered the electromagnetic behavior.

Older powder guns have no limit on the current settings, which means that the amps can ramp up or down dramatically, depending on the electrical resistance between the part and the electrode in the powder gun. As the gun moves closer to the work, resistance goes down and, hence, the amperage goes up. Move the gun farther away and the opposite occurs.

The trick, especially for novice operators, is to limit the current. With a set limit, the guns can be moved in and out—that is, the gun-to-workpiece distance can change—and the operator will still achieve the desired part coverage.

Newer guns have current limits that can be set. So instead of an operator moving the gun several inches toward the part and causing the current to jump to, say, between 60 and 70 microamps (a range that can cause trouble in many applications), the gun can be limited to between 20 and 40 amps. So when the operator moves closer to the part, technology within the gun compensates electrically so that the amps never rise above 40 (or whatever is optimal for the equipment and application), allowing for smooth, consistent coverage.

Another equipment variable is the nozzle configuration. Like nozzles for laser or plasma cutting, powder coating gun nozzle choices abound, but two configurations pervade the coating business. First is the fan nozzle, which sprays powder in a fan shape and produces a coverage area resembling an elongated oval. The second is the conical defector, which produces a concentrated, doughnut-shaped coverage area. It can spray patterns as small as about 2 to 3 in. diameter, which can be good for certain shapes like tubes or perhaps a small box with an inside corner that needs complete coating. But, of course, such small spray patterns are very inefficient for large parts.

The fan configuration remains the most popular, simply because its spray gives the operator the best efficiency across a range of part shapes and types of part surfaces. An operator may overspray a little more on certain parts, but for a high-product-mix situation, the excess powder waste costs less than having to change out gun nozzles.

“In a lab setting you’d change nozzles frequently,” Talbert said, “but in a production setting, it’s really hard to do.”

The gun nozzle dictates an operator’s technique, as does the part shape and other factors like current settings and line speed. The technique is somewhat intuitive; a steady hand moves in a regular pattern with a consistent, straight stroke and a consistent coverage-area overlap between strokes. “You often see new operators moving guns in irregular patterns. You don’t want an operator moving the gun in an irregular pattern across the part,” Talbert said. “You want a consistent movement, stroking from left to right, drop the gun, overlap the pattern about 50 percent, and stroke right to left.”

Two critical variables when it comes to powder coating technique are the gun-to-part distance (also called the target distance) and the spray sequence. The target distance must be as consistent as possible. “If I spray 3 inches away for one part and then 6 inches away on another part, I’m going to see a difference in film build and the behavior of the powder,” Talbert said. “A powder coating operation needs to have a consistent target distance, from part to part and from person to person.”

Besides this, operators should start in hard-to-cover areas first, the nooks and crannies, the inside creases, all with high electrical resistance susceptible to Faraday cages and other powder coating headaches. Only then should operators move to the flat, low-resistance, easy-to-coat surfaces. “If you build up powder on the easy-to-coat surfaces first,” Talbert said, “you’re only going to enhance the overall electrical resistance. That is, the electrical resistance will grow, not diminish.” Coating the flat, open spaces first will make the nooks and crannies even more difficult to coat consistently and evenly.

Racking and Hanging

A gun operator cannot be successful unless the parts are racked and hung consistently and in the right orientation. If a part is being washed, it needs to be hung in a fashion that prevents water from becoming trapped and pooling in the corners. Parts need to be grouped close enough for good efficiency but not too close, or the spray gun operator will have trouble getting all the parts completely coated.

“A good racking system will ensure the operator has good access to all the areas of every part,” Talbert said.

The hung parts also need to be stable. Once in motion on the line, they shouldn’t swing or twist. This again will make it difficult for the operator to achieve a complete, consistent coat. This can be especially problematic for lightweight parts. Small parts can be so lightweight that the powder coat spray itself can cause them to swing. This might seem harmless, but that swinging is changing the gun-to-target distance, which in turn can cause inconsistent coverage. Good racking should provide support to prevent this.

Talbert added that identical parts should be hung in the same orientation and height so that the operator can coat each piece in a consistent, repeatable manner—from part to part, shift to shift, and day to day.

“Racks should be clean, and the contact area [where the part is hung and secured] needs to be free of powder,” Talbert said. “This ensures you get a good electrostatic attraction by virtue of earth-grounding.”

Books could be written about part-hanging density on powder coat lines. At custom coaters, finding the optimal line density can be a strategic business advantage, the difference between being truly competitive or not.

Line density might not be as critical at fabricators, but as Talbert cautioned, shops that ignore line density do so at their own peril. After all, fabricators that bring powder coating in-house often do so to relieve a powder coating bottleneck. The last thing they want is to make a big investment in powder coating and end up running that investment inefficiently.

“The goal of racking is to ensure I can easily coat it, the part is dry when I coat it, and I get good volume off the line,” Talbert said. “It’s about productivity, consistency, ease of coating, and low waste. A lot of shops waste massive amounts of money by not racking and hanging parts properly.”

Considering this, the person assigned to rack and hang parts needs to know how critical his job is, and what happens if he doesn’t do it properly. If he doesn’t pay attention, every finishing process thereafter can suffer.

Leadership in a High-product-mix Environment

Operators need a working knowledge of how powder adheres to the surface. Supervisors and other department leads need to know more. They need to know how a powder coating line operates best. And in the high-product-mix world of custom fabrication, finding the best operating conditions can be quite the balancing act.

A powder coat line works most smoothly as a batch manufacturing system. “It’s ideal to batch things by size and style rather than by kit. It just makes it easier to set up your guns, the oven, the racking,” Talbert said. “Altogether, it’s a preferred way to coat.”

This is not to say a fabricator needs to run a single color for hours on end. After all, a big reason custom fabricators bring coating in-house is to take advantage of quick color changeovers. Because fabricators don’t focus solely on powder coating, and don’t consume massive amounts of powder, reclamation often isn’t needed.

All the same, a coating line is designed to a specific speed, which hinges on volumes, part mix, pretreatment requirements, line density (how many parts can be hung within a certain space), and curing time.

Pretreatment usually is somewhat flexible to kit-based processing. Pretreatment in powder coating has two essential teps: (1) cleaning and (2) the application of a conversion coating, which can protect against corrosion, promote good powder adhesion, and improve the coating life. The pretreatment process may include a number of steps to prepare and otherwise treat the part, but the goal is to get a clean surface and one that is receptive to bonding.

Regarding the washing step, I’ve worked with many lines where they run quite a variety of material types and manage them quite well,” Talbert said, adding that issues sometimes arise with conversion coatings, especially if a variety of parts of different material types need corrosion protection for outdoor use—say, a job that has both aluminum and steel components. Conversion coatings that work effectively for aluminum tend to not work well with steel. (Though certain coatings with the so-called “transitional metal” products like zirconium oxide make pretreatment systems more adaptable to different materials.)

“But generally speaking, for kit-based processing, pretreatment is less of a challenge,” Talbert continued. “What does present a challenge, however, is curing. When curing thicker parts, it takes more time for the core of the substrate to reach the temperature required for cross-linking.”

Most powder coating uses thermoset powder, which requires a certain amount of energy and time to create a chemical reaction within the powder for it to melt and fuse as a film. “Cross-linking” happens when the molecular structure changes as the powder transforms from a group of discrete particles into a consistent film. This takes a certain amount of time depending on the part the oven is curing, though line speed through curing can be tweaked to some happy medium for all parts hanging on the line during a particular run. Using hypothetical numbers, Talbert explained, “A line may run best at, say, 10 feet a minute, and it can run well even at 8 or 12 FPM. But run it at only 5 FPM or 14 FPM and you may start to run into a problem.

“This does not mean people do not run parts in kits,” Talbert continued. There are limits, but a knowledgeable line designer should be able to compensate for many kit-based approaches, as long as coating requirements are properly understood during the design stage and racking is efficient.

“It’s more challenging, and you need to limit the amount of range of mass from part to part, like hanging a light piece of steel next to a thick piece of steel, which could be problematic to cure. But you can compensate to some degree inside the oven,” he said. “You might use an infrared oven, which is controlled by a PLC and reacts differently to those masses and shapes going through and radiates higher or lower, depending on what’s passing through. Then you’d go into a convection style oven to finish the cure process.” Again, there are limits; sometimes the finishing requirements between parts in the same kit are just too different to flow together in the same run.

Talbert added that this balancing act is a common challenge for many manufacturers that bring powder coating in-house, and it’s for this reason that hiring a powder coating supervisor with experience can be very valuable. That supervisor can then train operators who will have to react to different parts hanging on the line, change their gun-to-target distances, adjust their power and flow settings, perhaps even change nozzles. Should an operator keep the same flow rate but simply move the gun farther away from the target? Or perhaps keep the same flow rate and just make fewer passes? An operator needs to think about these things on-the-fly. It’s not mindless work.

How Automated?

Automated powder coat lines have gotten much more flexible in recent years not only because of PLC-based infrared curing technology, but also because of intelligent gun automation. Mechanized or robotic guns can be set up to change flow rates and gun-to-target distances to match the product that’s passing in front of them. Depending on a shop’s part mix, full-on automation may indeed be the way to go.

Still, these mechanized systems may have trouble reaching all areas of the part, including those hard-to-coat Faraday cages. Articulated robot arms can reach more places with a spray gun, but even here there are trade-offs.

“Most hang their parts on simple hooks,” Talbert said. “But if you’re going to use an articulated-arm robot, that hook had better be a really good hook, hung in a good position all day long. Otherwise, the robot will coat only what it’s programmed to coat, and the part may not be positioned correctly. A manual operator has eyes. Can you automate fully? Yes, but often the process complexity moves the argument toward using the manual operator.”

Training: At the Heart of It All

A fabricator’s powder coating department needs parts designed with powder coating in mind. As Talbert explained, tight corners can be coated, as can some extraordinarily complex geometries, but they also add a lot of coating variables and increase the chance for error, rework, and scrap.

That said, training remains essential, and it starts with the department supervisor. “The department supervisor needs to have a great overall knowledge of the line and powder coating process,” Talbert said. “They know pretreatment. They understand what’s going on in the washer. They know why and how you rack properly. They know settings for the powder guns, for the curing ovens, and they must understand electrostatics. They know how to troubleshoot. They need to walk through the line, look at something, and know instantly that it’s not right.”

Consider pretreatment. A shop employee from another department might just see parts getting cleaned. The supervisor should look at the wash, know it will have so many milligrams of conversion coating applied to it, know that it will be sprayed with deionized water, and dried in a certain fashion.

Where do people get this knowledge? They get it through experience, but they can get training from outside sources, their equipment vendors, industry associations, and other sources of outside training. Good sources include the Powder Coating Institute (www.powdercoating.org) and the Chemical Coaters Association Intl. (www.ccaiweb.com).

The supervisor in turn needs to determine the level of training operators need to be effective and efficient. As Talbert said, “A supervisor really needs to coach them into being successful powder coaters.”

Here’s the rub. Like at a fabrication plant’s upstream processes, powder coaters can’t be mere button-pushers (or trigger-pullers). Talbert added that it’s a problem that often goes unnoticed, because a poor powder coat operator still can get a job out the door. “They just often can’t coat a part efficiently or effectively.”

They may use a full-on amp-potential setting, for instance. But if they knew more about the equipment and how different microamp settings affected coverage, they could coat parts much more effectively.

True, some controllers do have “recipes” that operators can use. They press a button, and all the air pressure and electrostatic variables are automatically adjusted to suit the part run at hand. But, Talbert added, powder coaters will be much more successful if they understood why those settings worked so well.

Say you ask an operator to set up a gun he usually doesn’t spray with and he says he doesn’t know how. “He tells you, ‘Well, that’s Joe’s gun, not mine, and Joe’s not here today.’ That’s a problem,” Talbert said. “Joe should have taught everybody how to set that gun up. And it should have been documented. It doesn’t have to be a powder coating encyclopedia, just basic instructions.”

Turns out what is true in upstream fabrication processes is also true in finishing. Lack of formal training leads to lack of engagement, turnover, and operational turmoil. Process knowledge, in coating as in everywhere else on the fab shop floor, lies at the heart of it all.

Talbert Consulting, 616-915-2769, rodgertalbert12@gmail.com