Powder coating basics for metal fabricators

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Editor’s Note: The following is based on the second half of “Batch versus Automated Finishing Systems,” presented at FABTECH 2023 in Chicago by Frank Mohar, Northeast sales manager at Nordson Corp. The first half, covered last month, was presented by Nick Dawson, sales account manager at George Koch Sons LLC. nitriding furnace

Click here for Part I in this series.

So, you’ve decided to bring powder coating in-house. You’ve analyzed whether you’ll be integrating a batch or automated in-line system. You’ve determined the space you’ll use and the technology you’ll need before and after the actual powder coating process, including pretreatment and curing. You’ve also determined your masking strategy. Some parts are masked before entering the powder coating system, but parts with oil on them are masked only after pretreatment, washing, and drying, just before powder coating.

Now it’s time to dive into the coating process itself, including the booths, color changes, the guns, the hoppers, and how automated you want your powder coating department to be.

Booth design depends on the system type (batch or in-line), the largest part sizes to be coated, and the color changes required. The most basic booths are typically spray-to-waste, or nonreclaim, systems, while more advanced booths have collection systems designed to recover unused powder media. Booths range in types and styles, from manual to automated, and many incorporate the ability to change colors quickly.

In some booths, workers simply hang parts onto a rack cart and roll it into the booth. Other systems have manual conveyor lines that allow workers to push a button or physically move the work into the booth, entering and exiting through the same door. Other booths have a conveyor line running through them, entering on one side and exiting out the other.

Color change technologies range from simple manual interaction to completely automated. In some setups, operators spray directly out of boxes of powder placed on a vibrating plate. When they’re ready for a color change, they blow compressed air through the tubing that was carrying the powder out of the box and into the spray gun. This color change usually takes less than five minutes.

Some automated systems change out colors in as little as 30 seconds. With this system, operators have all of the colors they’ll be spraying lined up for the day in fluidized hoppers. When they need to change colors, they simply press a button on an interface panel, and the system will purge out all the lines and draw the next color out (see Figure 1).

Which system suits best depends on the part mix, color change frequency, and time allotted for color changes. If you change colors only a few times a day, a five-minute color change time will probably suffice. If you do 100 to 300 color changes or more a day, a 30-second color change could be well worth the investment.

Powder booths are necessary to collect overspray of powder and potentially reuse that excess. If every particle of powder sprayed from the gun adhered to the workpiece surface, booths wouldn’t be needed. Powder guns cannot be 100% efficient, which makes booths a necessary piece of equipment within a powder system.

FIGURE 1. This arrangement features a button-actuated color change that reduces the changeover time to about 30 seconds. Images: Nordson Corp.

Operators can adjust the spray tips and application technique to maximize efficiency, but some waste is simply unavoidable. This is where booth design comes into play.

Every booth has a certain fan size and a specific number of primary and secondary (final) filters. Typically, the larger the fan size, the more filter media the booth will have. These work together to create a certain airflow within the booth space.

The more square footage of openings in a booth, the more airflow it needs. A small-batch booth with just one opening will use a smaller fan, where an automated booth with a conveyor running through it—comprising automated powder spray and manual touch-up—will require a larger fan to contain the overspray powder (see Figure 2).

The idea is to “tune” the airflow to keep as much powder inside the booth as possible. This results in an optimal booth draw that directs filtered air into the booth. It’s all designed to prevent excess powder from escaping without affecting the integrity of the powder coating process.

Excessive powder escaping adds an uncontrolled variable to powder coating and can cause an unsafe working environment. A properly designed booth needs to have an average airflow of 100 to 150 feet per minute (FPM) throughout all the openings, depending on the booth style and application. Ideally, the booth will have dead zones where the air movement is very minimal, so that the powder being sprayed from the gun flows with minimal outside turbulence affecting its path. The only real airflow should be near the booth openings and other areas far away from the part itself.

If powder is escaping, the booth could be undersized for the application, or the fan could be undersized for the booth itself. Sometimes, the booth, fan, and filter media could be right-sized, but environmental factors like cross drafts cause issues. A fan (or an open garage door or air conditioning duct) blowing a breeze near a booth opening could pull the powder from its intended path.

Any external airflow source that blows air more than 60 FPM will overtake booth draw and pull powder out of the booth. It can be helpful to have the booth in a room in which temperature and humidity can be controlled.

Booths with conveyors running through them have certain design criteria. There needs to be at least 27 in. from the hang point of the conveyor to the top of the part, as well as 6 in. of clearance all the way around, just like in the ovens and washing stations. This prevents swaying parts from crashing into walls, of course, but it also allows space for the required airflow. If a part has only a ½ in. of clearance all the way around, the booth won’t be able to “breathe,” or pull air into the booth.

Color change requirements also could play a critical role in booth design. The more frequent and quickly the color changes occur, the greater the booth draw required. After all, a 30-second color change isn’t very effective if particles of the previous powder color are still floating in the air. Whatever powder doesn’t attach to the part needs to get drawn beyond the part quickly and go right into the filter.

The use of robotics and automation will affect booth design and airflow requirements, and in manual setups, the number of operators spraying at once does factor into the booth design due to the square footage of openings that would be required. But the exact equipment they’re using—including the guns and hopper types—usually does not enter the equation yet.

FIGURE 2. In-line powder coating with automation can have a number of openings for mechanized guns or robotics. This affects airflow requirements for adequate booth draw.

The amount of overspray will vary depending on operator training, the part geometry, and the equipment they’re using, but that variance shouldn’t affect how the overall booth performs. A well-designed booth should be able to handle a range of manual spray equipment and operators with varying levels of training and experience.

Booth walls and ceilings (known as canopy materials) historically have been stainless steel. The problem, of course, is that charged particles of powder are attracted to metal. If they don’t land on the part, they will land on the wall, and cleaning them off can be challenging.

For this reason, booths with white polyurethane walls and ceilings have been growing more popular. Powder isn’t attracted to them, and the powder that does land on them can be cleaned off easily. The white wall material also reflects light, so it helps make the booth much brighter. A clear polyurethane ceiling allows overhead light to shine through. The result: Operators have a bright, clear view of the parts they’re coating (see Figure 3).

Notice excessive amounts of powder start escaping from the booth? In this case, filters might be plugged up. Check the cartridge filter pressure gauge to see if the pressure is excessive. On a Magnehelic gauge with a range of 0 to 10, typical readings will be about 5 in., which means everything is sealed and working as intended. A reading of 1 in. or 2 in. could mean the filter has a hole in it, because there’s little back pressure. Once the gauge reaches 7 to 9 in., the filter is starting to plug up and needs to be changed.

Changing filters out on a regular preventive maintenance (PM) schedule is the ideal approach. Most single-shift operations change cartridge filters once a year. In a booth operating multiple shifts with several operators spraying simultaneously, cartridges will likely need to be changed every six to nine months. Secondary (or “final” filters) on top of the booth need to be changed out less frequently. (Of course, always follow the recommended PM schedule from the manufacturer.)

As mentioned previously, escaping powder also might mean that cross drafts are present. In rare cases, the fan blower might be running the wrong direction. This can happen after a maintenance tech or installer works on the system and mistakenly reconnects the wire leads. The fan usually has an arrow showing the direction of rotation. If it’s rotating the opposite way, simply switch the leads.

Recycling techniques vary, but in a typical reclaim booth, quick pulses of air purge the filter, after which the powder falls into a fluid bed, where the powder is extracted and recycled (see Figure 4).

Is recycling powder worth it? It might seem like a no-brainer, but answering that question isn’t always simple. Many variables factor into the decision, two principal ones being color change requirements and the amount overspray the coating operation has. The more overspray there is, the more powder recycling makes sense.

Parts with large cutouts and wire goods will naturally have more overspray, as will some automated operations. Robots can be programmed to minimize overspray on specific parts, but stacked vertical or horizontal guns that move back and forth usually aren’t aiming at anything, so will likely overspray much more than an experienced operator. In many cases, operations with automated guns choose to recycle while systems with manual powder coating guns do not.

All this must be put in context, especially when it comes to color changes. A color change without reclamation requires cleaning on the front end—that is, just the virgin powder. Add reclamation, and the recycling system also needs to be cleaned. This essentially doubles the color change time. Reclamation still might be worth it, especially if color changes are limited or the application has significant overspray, but the color change requirements, combined with other factors, also might steer an operation away from recycling. Again, context matters.

FIGURE 3. White canopy walls and a clear ceiling create a well-lit environment for operators.

Manual spray systems usually include a gun connected to a box feed or fluidized hopper. Fluidized systems work best for single-color operations or less frequent color changes, while box feed systems excel at faster color changes in manual settings. Fluidized hoppers also tend to work best when spraying a metallic powder that has different particle sizes.

If using a box feed system, the heavy particles sink to the bottom of the box as it is being vibrated. This can produce a color shift on the parts from the beginning of the run to the end. The metallic powder, accumulated at the bottom where the powder is being picked up, would be sprayed first. A fluidized system mixes those different particle sizes together to produce a better result.

Attached to the hopper is a pump that connects to the hose and gun. Venturi pumps have plastic wear sleaves that are sacrificial; they’re not designed to last years. Changing venturi wear sleaves is like changing the oil on a car; most do it every three or four months.

It’s also a good idea for the operator, at the end of every shift, to remove the venturi sleave, blow it, and reinsert it. Blowing can remove some debris, but the greatest benefit is the fact that the operator reinserts them in a slightly different rotation. This helps them wear evenly and extends their life (see Figure 5).

Different nozzles create different spray patterns. Large panels benefit from large, flat spray patterns, while small pieces and parts with a lot of corners can benefit from small, precise spray patterns (see Figure 6).

Some nozzles spray in a conical pattern, which can help coat parts with cutouts as well as wire goods—any pieces of sheet metal with a lot of air between the surfaces to be coated. The conical nozzles emit a very soft pattern that doesn’t “blow past” a narrow piece of metal, allowing the charged particles to adhere to the surface.

All powder coating guns have electrodes and nozzles that need to be cleaned per the manufacturer’s PM schedule, though the schedule can change depending on the volume of powder being used. A typical cleaning occurs at the end of every shift.

As technology changes, so does the world of powder coating. Regardless of what level of automation a system has, though, physics cannot be escaped. Powder will always need to be cured. Metal surfaces need to be clean and dry to promote good adhesion. And a little overspray is almost always unavoidable.

This review of the basics should give you a sense of what to expect, how to move forward, and, ultimately, add more value for your customers.

Powder coating basics for metal fabricators—Part II