Pretreatment, application, and material developments challenge traditional thinking
April 1, 2010
Customers are asking their metal fabricating suppliers to assume more production responsibilities as they look to reduce the size their vendor bases. As a result, more fabricators are becoming involved in powder coating, either doing it themselves or coordinating the finishing process with a nearby custom coater. Of course, metal fabricators are first and foremost in the business of cutting, forming, and joining metal, but they still need to stay on top of the latest developments in powder coating.
Complete coverage of powder coating is always a worthwhile goal, both in terms of application and knowledge about the subject.
More metal fabricators are finding themselves in a position where they are expected to take on responsibility for finishing metal parts. Either OEM customers want them to take on powder coating activities, or at the very least they want the shops to coordinate the finishing process in addition to the fabrication.
Of course, in this economy, many metal fabricators aren't in a position to say no to such requests. Fortunately, powder coating technology has advanced to the point where it's more user- and production-friendly than ever.
It's one of the younger metal finishing technologies on the marketplace, having appeared on the manufacturing scene in a large way in the 1970s. The technology grew rapidly in the 1980s as key industry segments, such as appliance and architectural, gravitated to it, and powder coating usage grew with international expansion in the 1990s. The growth of the technology is not as robust as in past decades, but this lull has put pressure on the material and application equipment sector to develop new products in an aggressive manner.
The basic premise for powder application remains the same. Dirt, oils, and lubricants have to be removed from the material with chemicals, which also help prepare the metal surface for powder application and improve powder adhesion. These pretreatment chemicals usually are applied in a spray process, but sometimes a submersion method with several tank stages is used. Once the parts are dried (see Figure 1), they enter a booth where an application gun shoots electrostatically charged particles at the grounded metal parts (see Figure 2). The powder adheres to the parts as long as electrostatic charge remains on the powder, which is usually more than enough time for the parts to travel to a curing oven. The oven provides the elevated temperature necessary to melt the powder and cause the material to flow out, creating a skin over the metal part.
The powder coating industry is not one marked by regular technological revolution, but that doesn't mean noteworthy advancements haven't taken place over the last five to 10 years. Here are five developments you should be aware of if your company is increasing its involvement in powder coating.
Advancements in guns for powder application ensure more of the product ends up on the metal and not on the booth floor.
Phosphates aren't toxic, but increasingly municipalities don't want to deal with them. The problem is that phosphates have been key ingredients of pretreatment formulas, more specifically iron and zinc phosphates, for years.
Why do municipal water treatment facilities have phosphates on their check list? Call it cultural eutrophication. Eutrophication is the word that describes the aging process for a body of water. Plant life eventually emerges in a body of water, and through a combination of dead plant matter and more sediment, the body of water grows more shallow over time, usually hundreds of years. Phosphates, which can be found in human and animal waste, household cleaning compounds, agricultural fertilizers, and industrial waste, hasten this aging process. The phosphates have a "steroid" effect on the aquatic vegetation, and the plant life chokes the life out of the body of water.
"So [municipalities] go to industry and say, 'We can't control it at the agricultural level, but we can control it at [the industrial] level," said Nick Liberto, president, Powder Coating Consultants, Bridgeport, Conn.
That leads to a push for phosphate-free pretreatment chemicals, and some of the corresponding developments have been promising.
Rodger Talbert, technical director, Powder Coating Institute, pointed out that zirconium metal pretreatment as a replacement for iron phosphate is a good example. The material does not produce as much sludge, eliminating some of the cleanup that's necessary with traditional phosphate products, and it can be run at lower temperatures than typical pretreatment compounds, producing some energy savings.
Some tradeoffs exist, however.
"You need stainless steel plumbing or, at least, corrosion-resistant plumbing, tubs, tanks, and pumps," Talbert said.
"The other thing is that most of these solutions are pretty sensitive to dirty water. They don't like excessive salts or well water. So most of those baths are charged with filter water, meaning either deionized or reverse-osmosis water."
Liberto has a perfect description of how changes in inventory management have affected the world of powder coating: "There is no inventory anymore. There are no warehouses with black office furniture."
Over the last 20 years, U.S. manufacturers learned that inventory, for the most part, was evil. It sat on the floor and cost them money.
That led to embracing just-in-time manufacturing and kanban-style replenishment of parts. Simply stated, products are fabricated when they are needed, not before and not later.
That doesn't lend itself to long runs of the same color, so powder coaters needed to be able to change colors quickly and frequently. That wasn't the case for powder coaters that wanted to recapture overspray and employed the older cartridge-style systems that required a separate cartridge for each color that was sprayed. With 30 to 40 minutes needed for changing of the cartridge, booth technicians didn't want to change colors on a regular basis.
The emergence of cyclone technology changed that, however. These systems capture overspray, which rides into the recovery system on a high-velocity air stream, moving as fast as 5,000 FPM. Heavier powder particles are removed from the air stream as it circulates in a cyclonic motion within the enclosure and come to rest at the bottom of the unit. Newer units are designed to be cleaned quickly so powder coaters can move to the next color without worry of cross-color contamination. Multicyclone recovery units, lined up one beside the other, allow the powder technician to skip cleanup and switch to another color with simple line adjustments.
Talbert also mentioned the emergence of pneumatic color selection devices, which are also useful for quick color changes. These are manifold systems hooked up to multiple feed hoppers.
"You purge the gun out very quickly, throw a switch, and you are into a new color," Talbert said.
Some caution has to be used when switching between colors. A powder coater can't spray one part white and then follow that up with a black powder coat finish on the part just seconds behind. Several seconds are needed between distinct color changes such as that.
For many years stainless steel was the material of choice for constructing powder application booths. It was rugged and attractive. Some might argue too attractive.
The simple fact is that the stainless steel booths acted as great grounds, and the electrostatically charged particles stuck to the material aggressively. If a booth operator had to go in and clean the stainless steel booth after a long job, he more than likely had to rake as much as 0.50 inches off the walls and perhaps another 2 in. off the floor.
Today's booths are made of plastic or composite materials to reduce the amount of powder overspray sticking to the walls. They often feature rounded corners and a limited number of panel joints, which prevents excess powder from becoming lodged and difficult to remove. The booths also have downdraft airflow designs to guide the powder overspray to the floor.
Plastic or composite construction doesn't guarantee that powder overspray won't stick to it, but it should perform considerably better than stainless steel counterparts. Those looking to invest in a new powder coating line or upgrading an older booth should actually find a booth made of the same plastic or composite material to learn how much material sticks and whether compressed air or a squeegee is needed to remove the powder during cleanup.
The best way to control powder overspray cleanup is to reduce the amount of overspray. Liberto said equipment manufacturers have developed technology that does just that.
These new-generation pumps send less compressed air through the guns, leading to more powder being thrown toward and sticking to the grounded metal part. These pumps are much more efficient in powder application than Venturi-style pumps.
In addition to promoting material savings, this technology usually can work with smaller transport tubing and doesn't consume as much energy because it doesn't need as much compressed air.
Powder coatings are able to replicate finishes that 10 years ago weren't possible. For example, antique and distressed looks can be attained with the application of a dark base powder coat and a top coat of a metallic pigment. Material developers have even created a powder coating finish that can replicate chrome, which is derived from a heavily regulated plating process.
Sublimation is another coating innovation that has led to exotic finishes that become part of the powder coating finish. A powder coat base is applied to the metal and a sleeve of film with a design on it is slipped over the part. A vacuum is used to remove the air between the sleeve and the part, which results in the film hugging the metal part tightly.
"When you put it in the oven and cure it, it comes out and you have a camouflage, a wood grain, or whatever decorative look you want," Talbert said. "It bakes right into the powder. It's very durable."
The finishing field is not immune to the pressures of increasing production efficiency. Fortunately, technological developments have emerged that are helping those in the metal supply chain do more with fewer materials, using less energy, and working within very tight time frames.
In late 2009 the Powder Coating Institute (PCI), located near Houston, rolled out its first-ever certification programs. PCI 4000 certification is targeted at OEMs, and PCI 3000 certification is offered to custom coaters.
Rodger Talbert, PCI's technical director, described the certification as following the same tenants of ISO 9000, but striving to deliver an "outcome-based" focus. For example, when a company achieves ISO certification, the company has proven that it has documented processes that can be followed accurately and thoroughly on a regular basis by any number of people. This PCI certification looks to take that another step by suggesting that a PCI-certified coater can deliver a consistent powder coating process and, as a result, a quality finished product at the end of the process.
Talbert helps those powder coaters seeking the certification to create a methodology that can be used to improve the finishing process even after the PCI technical staff leaves. Like ISO 9000, the PCI certification becomes a tool in the battle to correct poor production habits or eliminate waste.