Powder coating: Good contamination control prevents defects from start to finish
08/01/2005
With its durability, resistance to corrosion and high quality finish, powder coating has become a preferred finishing choice among both consumers and manufacturers, and good contamination-control techniques can ensure that a powder coating operation is producing the highest quality, defect-free finish possible.
By Angela Godwin
In the late 1960s, powder coating emerged on the horizon as an attractive alternative to other methods of surface coating. With its durability, resistance to corrosion and high quality finish, powder coating quickly became a preferred finishing choice among both consumers and manufacturers. In addition, powder coating doesn’t produce the hazardous by-products-overspray waste, wastewater sludge, and volatile organic compounds (VOCs)-that are a consequence of more traditional surface-coating techniques, making powder coating a “green” alternative in a market where environmental regulations are becoming ever more stringent. Today, powder coating is used in nearly every industry-furniture, appliances, medical devices, and automotive are just a few.
The powder used in the finishing process is made up of a combination of resin and pigment. It’s applied to a surface, or substrate, with a spray gun that puts a positive charge on the powder particles as they pass through an electrostatic field on their way out of the nozzle. The parts to be painted are typically moved along via a grounded overhead conveyor, which imparts a negative charge to the parts (see Fig. 1). The opposite charges attract, causing powder particles to cling to the substrate surface. The parts are then moved along to an oven where the powder melts and cures into a smooth coating.
 Figure 1. Parts are moved along via a grounded overhead conveyor, which puts a negative charge on the parts. Photo courtesy of ITW Gema. |
In order to ensure that a powder coating operation is producing the highest quality, defect-free finish, however, certain steps must be taken to incorporate basic contamination-control techniques into the finishing process.
Keep it clean
The first line of defense against a defective powder-coated finish is the pretreatment process. “In powder coating, your finish is only going to be as good as your pretreatment process,” says Jeff Hale, manager of distribution and marketing for powder-equipment manufacturer ITW Gema (Indianapolis, Ind.; www.itwgema.com).
The type of pretreatment process used to clean and treat a substrate can vary from application to application. “It’s dictated by whatever your performance requirements are for the product you’re coating,” notes Hale. In any case, the first step is to clean the part, usually with a spray washer and an alkaline cleaner, to remove any organic soils-dirt, waxes, or rust inhibitors-that may be present on the substrate.
Inorganic soils, like rust, oxidation, mill scale, or heavy mildew, may be difficult to remove with a spray washer, notes Rodger Talbert, president of R. Talbert Consulting (Grand Rapids, Mich.; www.talbertconsulting.com). “If you have problems with those kinds of soils, you can either chemically treat the surface with something more aggressive than the typical spray-washer solutions, or you can mechanically remove the soils by grinding, polishing or blasting.”
If the substrate has not been cleaned well, ITW’s Hale explains, then the powder coating may not adhere to the surface. “You can still put powder on a part that’s covered in oil; the question is, once it’s been heated and cured, will it stay on?” Any contaminants-smut, dirt, or oil-left on the substrate, he warns, can cause the powder to flake or peel off later.
Once the part has been cleaned, it is rinsed in water or, oftentimes, deionized water. In some applications, where corrosion resistance is not a concern, this is the extent of the pretreatment process. “If you’re coating fence posts, for example,” says Hale, “you’re probably not too concerned about the amount of corrosion protection in the coating on the bottom of the stake; as long as it looks green when it’s sticking out of the ground, that’s fine.”
In more particular applications, however, corrosion resistance may be a very big concern. “A good example is your washing machine or dryer,” states Hale. The household chemicals and detergents that will come in contact with the powder-coated surface are basic in pH, he explains, while other cleaners that might be used to wipe up dust and spills on the appliances can be more acidic. The powder coating must be able to resist that. “Let’s say there’s a crack in the coating,” he suggests. “There needs to be a level of pretreatment underneath the coating to protect the metal from rusting or corroding.”
In such cases, the substrate will undergo a phosphatizing process-called a conversion coating-whereby an acid solution is used to attack, or etch, the surface of the base metal. The solution-usually iron phosphate or zinc phosphate-provides a layer of improved corrosion resistance and strengthens the adherence of the powder coating to the substrate. As Hale explains, “Phosphatizing is a way to lay a conversion coating over the substrate-a barrier between the raw metal and the powder. [This] gives the powder coating something to ‘dig its fingers into,’ if you will.”
It’s gettin’ hot in here
Beyond the cleanliness of the substrate, the two biggest threats to a perfect finish are ones that contamination-control professionals know well: temperature and humidity. Extreme swings in either, or both, of these variables can cause an assortment of defects ranging from uneven coating to bumps or blemishes in the finish. In some industries, this can translate into a high loss of yield if a batch of product must be stripped and recoated because of obvious imperfections in the finish.
Interestingly, the biggest contributors of heat in the powder coating process are often the parts themselves. When the parts exit the cleaning and/or pretreatment stages, they are wet and must be dried in a drying oven. Temperatures in the drying oven can exceed 215°F, meaning that parts coming out of the oven area are extremely hot.
“Hot parts attract powder more readily,” says Hale. “The hotter the part is, the more likely you are to apply excess powder.” Excessive temperatures, he explains, can cause a premature melting of powder particles on the substrate, causing the powder to become tacky. As more powder is sprayed onto the part, it adheres to the tacky spots and builds up, resulting in uneven coating thickness. This can affect not only the aesthetics of the part, but the ability to assemble it. More powder also means wasted product, and can result in a significant increase in materials cost.
Ideally, these parts need to be cooled down to below 100°F in order to effectively control the thickness of the powder coating. “We would prefer the temperature to be in the range of 80 to 90°F. It won’t be difficult to coat something at that temperature, and it certainly won’t hurt to touch it,” Hale explains.
In many paint-spray applications, parts are allowed to cool in ambient temperatures before continuing to the paint-spray booth. This is only effective, of course, if the ambient temperatures in the plant are reasonably cool. “If the ambient temperature is hot, then product won’t get down to a reasonable coating temperature,” notes Hale.
In more sophisticated systems, however, a cooling tunnel is used to quickly and effectively cool parts. A cooling tunnel is simply an uninsulated sheet-metal tunnel in which filtered outside air is directed across the parts with large fans. The hot air coming off the parts is then directed outside the plant. Once the parts have cooled to an acceptable temperature, they can be safely powder coated.
Humidity headaches
Fluctuations in humidity can also contribute to defects in a powder-coated finish. Powder is extremely sensitive to moisture; excessive humidity can cause the powder to agglomerate, or clump. Consequently, the ability to spray the powder consistently and fluidly is compromised, causing little spits or balls of powder that can clog the pump and other equipment. According to Mike Preston, president of MaxAir Environmental Systems (Burlington, Ontario; www.maxairev.com), “Powder loves 72 to 75°F and 50 percent humidity. If the humidity is too high or too low, sometimes the powder goes on clumpy, or too thin, or it won’t stay on the part long enough to make it to the oven.”
Excessive humidity, as well as excessive dryness, can also cause problems with electrostatics, making it difficult or impossible to impart a charge to the powder particles as they are sprayed. “With electrostatics, we’re discharging energy off of a small wire at the end of the spray gun,” explains Hale. “This energy is actually ionizing the air molecules. There’s a certain range of moisture in the air that helps us to be able to do that: If there’s an excessive amount of moisture in the air, it doesn’t allow us to conduct the charge onto the paint particles as they pass through the ionized airfield. If it’s too dry, there’s not enough moisture for the process to work either.” Without the proper electrostatic charge, the powder will not adhere properly to the substrate.
Airborne contamination
Airborne contamination can be another cause for concern in the powder coating industry. Shop dirt, fibers, grit, or metal particulate from other facility processes can cause protrusions or bumps in the finish if allowed to infiltrate the powder system or paint-spray booth. “Airborne contamination can hurt the aesthetic appearance of the part,” explains Talbert. “There are different degrees of concern in different industries, but most won’t accept very much particulate in the coating.” Depending upon the particular application-office furniture, bicycles, or home appliances, for example-the presence of visible particulate on the surface is often unacceptable and can result in a dissatisfied customer at the very least.
One of the beauties of powder coating is that there is very little wasted material-whatever powder doesn’t adhere to the part falls to the floor of the spray booth where it can be collected and reused. Unfortunately, particulate or dirt can be collected along with the powder, often building up within the reclaim system. “If the powder is going to be recycled for reuse,” Talbert explains, “the reclaim system will route the powder back through a sieve or screen designed to remove any built-up fibers or particulate and deliver clean powder to the feed hopper.”
Typically, an 80-mesh screen (meaning 80 holes per square inch) is used to filter particulate from the powder. Powder particles are typically 35 to 45 microns in size, but can be as large as 100 microns. A higher mesh screen, Talbert cautions, could become clogged or plugged up, since the powder particles won’t be able to pass through. “You’ll waste a lot of powder; the screen will take it out [of use] and you’ll have to throw it away.”
“Some manufacturers produce industrial-use products, or they use a coating that has a texture to it anyway,” says Nick Liberto, P.E., president of Powder Coating Consultants (Bridgeport, Conn.; www.powdercc.com). “They don’t have that level of contamination. Or, they just don’t use reclaim powder.”
E-rooms
An environmental room-the cleanroom of the powder coating industry-is one way to combat contamination challenges. An environmental room-or E-room-is a controlled environment that houses the actual paint-spray booth, allowing manufacturers to reduce or even eliminate defects in the powder coating finish by controlling the variables that can cause problems (see Fig. 2). “Environmental rooms are very popular,” states Talbert. “They cut down on the defects substantially.”
“The environmental room is probably the number one safeguard against airborne particulate,” affirms Talbert. By enclosing the paint-spray booth within the E-room and positively pressurizing the air, he explains, dirt and contamination can’t infiltrate.
Although an E-room is not a prerequisite for a successful powder coating operation, if the ambient conditions cause prolonged, extreme swings in temperature and humidity, most industry observers believe an E-room is worth considering. ITW’s Hale agrees: “As we’ve told customers, if they want to minimize the headaches they have in terms of material handling or sprayability issues, we strongly advise that they look at an environmental room.” Hale acknowledges that if a customer does not invest in an E-room, it doesn’t mean that they can’t powder coat; it just means that, during hot, humid summer months for example, they must be prepared for increased material-handling challenges.
Liberto echoes this sentiment. “Not everyone needs an E-room. The determining factors would be the quality of finish that the applicator is trying to attain. If you have a product that requires a contaminant-free finish, and you have a preponderance of contaminants in your process over time, then you would look at an E-room.” In some parts of the country, he explains, temperature and humidity may only be an issue for a month out of the year. In this case, when challenges can be overcome simply by paying closer attention to the application system and making manual adjustments, investing in an E-room may not make sense.
And it can be quite an investment. Depending upon the requirements of the operation, an E-room can be as simple as a stick-built, two-by-four construction with drywall and a residential air conditioner; or it can be much more complex, with air curtains and sophisticated filtration and HVAC systems. A properly built E-room, Liberto explains, can total a third or more of the customer’s overall equipment purchase-that can be upwards of half a million dollars.
If a company opts for an E-room, however, Liberto advises that it be built according to the guidelines set forth in the Powder Coating Institute’s (PCI; Alexendria, Va.; www.powdercoating.org) Tech Brief #2, which he authored.1 “I’ve seen some companies just put up a few two-by-fours and a window air conditioner and call it good. But that just doesn’t work.”
Ideally, Liberto says, an environmental room should have smooth, easily cleanable walls; positively pressurized airflow to keep dirt and contamination out; and controlled temperature and humidity. “To do that,” he explains, “you need some sophisticated air conditioning systems, because the heat load is more than just the personnel and motors in the room.”
According to Tech Brief #2, discharge vents should be appropriately sized for the room, and uniformly distributed throughout the room. The guidelines state that air velocities should be “50 FPM anywhere in the room or a maximum of 200 FPM at the diffuser face.”
Filtration is also necessary, Liberto says, “for no other purpose than to protect that 150-ton air conditioner from stray powder that might be in the room.” Stray powder, from a spill for example, can build up on the air conditioner’s condensing coil; the heat from the coil will cause the powder to cure, insulating the coil and causing it to stop working. “We recommend 2-micron or finer filters, just to protect the return air going to the air conditioner,” he says. The guidelines also recommend that the return air velocity be less than 200 FPM.
“It’s like anything,” Hale adds. “It depends on how much money you’re willing to put into it.” If a manufacturer feels that the additional expense isn’t warranted, he suggests considering a few simple questions, such as: How many rejects are being created? How much excess powder is being sprayed? “All those issues are usually related to the temperature and humidity of the environment,” Hale notes.
MaxAir’s Preston believes the benefits of an E-room justify the cost. “For top-quality, ‘Class A’ finishes, like those required by the automotive and appliance industries, there’s no way to do it without having a proper environmentally controlled room.” Although a company may be able to get away with a less expensive version, Preston says, certain industries require a specific paint coverage, thickness, and a very low rejection rate-specifications that can only be achieved with a proper E-room.
Preston’s company, in conjunction with TLR Consulting (Chicago, Ill.), have developed a patented vestibule system whereby the conveyor entrance and exit areas are filtered 100 percent to 2 microns. All of the return air for the HVAC system comes from the vestibule. The air velocity, which is tightly controlled, creates an air curtain that prevents air from leaking out of the room, and the filtration equipment keeps dust and dirt from entering the air conditioning system, drastically reducing the likelihood that airborne particulate will contaminate the powder coating process. “It has to be heavily engineered,” Preston explains, “or you’ll blow the powder right off the product.”
A high-tech control system constantly monitors the temperature and humidity in the room and triggers the HVAC system to maintain the required levels. “With our control room, and our process and experience in the industry, we can maintain the temperature to +/-2°F, and the humidity to +/-5 percent,” says Preston.
According to Preston, the vestibule design can also translate into a measurable cost savings because the vestibule’s airlock helps increase the efficiency of the air conditioning system, significantly reducing the size of the air conditioner needed. “To maintain those levels without the vestibule, 100 tons of air conditioning might be required. We can do it with 60 tons,” says Preston. Long-term operating costs are reduced, he explains, and the initial capital expenditure is less.
Other contamination-control measures
In addition to utilizing an environmental room, some manufacturers of high-quality products-as in the automotive industry-may also employ other traditional cleanroom procedures and protocols. “You walk into an automotive paint booth and you have booties on your feet, you’re wearing a hair net, special garments, and you pass through an airwash vestibule where they blow you off with ionized air,” says Liberto. “That’s an absolute cleanroom environment because their quality requirements are so stringent.”
The overhead conveyors that transport parts through the powder coating system can also be a potential source of contamination. Drips of lubricant or metal shavings from the chain, hooks and track can fall from the conveyor onto the parts, resulting in staining or visible defects in the finish. To eliminate this problem, some manufacturers will install a sanitary pan between the conveyor track and the hanger, using a C-hook (see Fig. 3). In this way, any drips from the chain will land in the sanitary pan before they can land on the parts. “It’s something that I always recommend to my customers,” says Talbert. “I don’t like to install a system without it.”
Powder storage
Properly stored powder can prolong its shelf life, and possibly reduce contamination issues further down in the process. The Powder Coating Institute recommends storing powder at less than 80°F and in 50 to 60 percent relative humidity. In addition, powder should be stored away from a heat source, such as an oven, washer, furnace, or space heater to keep it from prematurely melting. PCI also advises that boxes of powder should never be stacked on top of one another; this can result in lumps that are not easily broken.2
Fluidization of the powder-adding compressed air to the powder to give it better flow characteristics in the spray application-can be compromised if powder is not carefully stored. “It won’t flow very well,” explains Talbert. “It doesn’t charge well, and it can create blemishes on the part surface from the agglomerations.”
Talbert adds that it’s also important to protect the powder from dirt. “So you wouldn’t leave an open container sitting around in the plant. It should be carefully closed and put away to prevent dirt from getting inside the container,” he advises.
Certain powders are particularly susceptible to contamination from adverse storage conditions. “Low-temperature-cure powders, for example,” says Talbert, “are less stable in storage.” Such powders are used when coating a substrate-like plastic or wood-that cannot tolerate the normal high-bake temperatures used when curing coated metals. These powders are extremely useful, he explains, “however, they are more sensitive to atmospheric conditions when they are in storage.”
Some manufacturers find that simply storing their powder in an environmentally controlled room is sufficient for avoiding difficulties later on. According to Liberto, “Many manufacturers will store their material long-term in an environmentally controlled space, but not spray in one.” That’s acceptable, he adds, in a situation where the plant conditions in and around the spray booth don’t necessitate putting in an environmental room.
Industry trends
Since the introduction of powder coating methods more than thirty years ago, the process has gained increased popularity. Its cost-effectiveness and extremely durable finish, combined with its environmentally friendly nature, have convinced many manufacturers to abandon their wet-paint lines and convert to powder. Bernie Gaon, director of engineering for Terra Universal (Anaheim, Calif.; www.terrauniversal.com), is one such convert. “All our products are powder coated. We stopped painting about five years ago,” he explains. “There are a lot of reasons why: it has a more durable finish, and the problems with fumes aren’t there. So as far as OSHA goes, this was much easier for us to handle.”
And it’s much faster, he adds. Unlike paint, powder doesn’t require any extra drying time; it’s immediately cured in an oven. “With regular [wet] paint, you have to let it dry, even if it’s been cured in an oven,” Gaon says. “If you wrap it [before it’s dry], especially with plastic, it will adhere to the surface and peel off the paint.” This is not an issue with powder coating; once it’s cured, it is completely dry to the touch.
Faster production time translates into significant cost savings over the long run. “A lot of huge manufacturers,” observes Preston, “are getting rid of their wet-paint systems for powder. They are recognizing-with production lines cranking out millions of pieces of product-that it’s important to be fiscally responsible for how their products are manufactured.”
Preston predicts that, in the future, the quality, safety, and cost benefits of powder coating will continue to impress the finishing industry: “The whole industry will be moving so much more toward powder.”
Overall, powder coating has proven itself an appealing method of surface finishing with tremendous benefits for the manufacturer and consumer alike. Although contamination can threaten a perfect finish, incorporating some basic contamination-control techniques into the manufacturing process can drastically reduce, or even eliminate, coating defects.
References
- Powder Coating Institute. “Technical Brief #2.” Alexandria, Va. September 1996.
- Powder Coating Institute. “Frequently Asked Questions,” http://www.powdercoating.org/industry/faq.htm (accessed June 28, 2005).