Tips on maintenance and energy efficiency
by Brynna Leslie
September 3, 2009 (Originally published in Coatings Magazine)
Curing is an important, but also costly, part of the finishing operation. Using the right equipment and using it properly is essential to achieving optimal results. Coatings Magazine recently spoke to a number of industrial oven suppliers about how to choose the right oven, and how to maintain equipment to ensure you’re getting the best finish in the most energy-efficient way.
MAKE THE RIGHT INVESTMENT
"The biggest mistake clients make is not doing some testing before they purchase," says Allan McKellar, Vice President of Sales for CCI Thermal Technologies Inc., Edmonton, AB. Next door to the company's Greensburg, Indiana manufacturing facility, there is a 6,000 sq ft research and testing centre, where CCI Thermal develops new products and allows clients to try various ovens in its DriQuik™ range.
"We have potential clients who show up for testing with their coatings reps attached to them. And we have this great opportunity to prove or disprove the technology for them in a touchy-feely way," says McKellar.
The size and shape of the parts on the finishing line, the type of coating to be cured, and even the amount of floor space available on the shop floor are all considerations before making the capital investment in an oven.
"Some people don't understand that there's a temperature range for optimum curing results," says McKellar. "The higher the temperature, the less time it's going to take. Inside a convection oven, you can only move around so much air. How well the coating transfers to the part depends on how hard you bang it. In a convection oven, you can only impinge the air so fast or you'll blow the powder off."
CCI Thermal only deals in infrared ovens. McKellar says the decades-old technology has developed extensively over the past 20 years, making infrared a viable and energy-efficient option for any size or shape of part.
"There is still a belief out there that if the infrared can't see the entire part then it can't be used, that if a part is geometrically complex, or if it's oversized, only a convection oven can be used. Nothing could be further from the truth.
"The longer the wavelength, the less it cares about line of vision and it is not as range-to-object sensitive either. Longer wavelengths are way less sensitive to these variances than the short wavelength infrared that have been around much longer that have contributed to this myth."
However, the capabilities of infrared are still being debated within the industry. Most oven manufacturers agree that infrared is up to 75 per cent more energy efficient than convection ovens, but not everyone is ready to render convection ovens obsolete just yet.
Fostoria Process Equipment Division, Gray, TN, (formerly Fostoria Industries), pioneered infrared curing on automobile parts in conjunction with the Ford Motor Company in the late 1930s. Despite this long and successful history using infrared curing technology, the company still manufactures both infrared and convection ovens, suggesting there are benefits and drawbacks to both.
"Basically, infrared is going to cure it if it sees it, so if you have a line of sight, to me it's a no brainer," says Steve Fruth, vice-president of Fostoria Process Equipment Division. "You can have gas or electric infrared, the equipment cost is going to be less, it makes less of a footprint and there's a lot faster response."
Fruth says the difference between short and long wavelengths has been overblown by some in the industry.
"Yes, the coating will still get cured in those hidden pockets," Fruth says, "but it's not going to be as effective as if it's done with, or in combination with, a convection oven."
Infrared is generally considered the best option for small, symmetrical parts that run on a chain and edge conveyor. Infrared ovens can be powered up quickly and can cure coatings on these small parts at higher temperatures in a fraction of the time used by a convection oven. But convection ovens are still the favourite for large and irregularly shaped parts. Most advocates of convection ovens say the warm air circulation inside the oven ensures coatings in every nook and cranny are heated equally.
"You really have to understand the science behind the ovens to understand where you're going to gain efficiency," says Doug Canfield, president of Casso-Solar Corp., Pomona, NY. "Infrared heats the part directly. So the parts closest to the infrared energy will get more heat. Convection ovens heat the air in the oven, and the air then heats the part evenly, so it works really well for large pieces of sheet metal, or parts that have an irregular configuration."
Canfield says many of his clients have opted for an infrared booster inside a convection oven to get the best of both worlds.
"With convection ovens, the most energy is required to bring the oven and then the part to temperature," Canfield says. "If you take a convection oven and replace portions with infrared, you'll gain efficiency because the majority of the work is being done at 60 to 70 per cent higher efficiency."
Convection ovens are certainly a dirtier breed than their radiant counterparts. Much like any residential furnace, convection ovens constantly circulate air, leading to heavy powder migration and residue collection on the interior walls, fans and filters. A lot of time, money and quality can be lost if the ovens aren't cleaned on a regular basis.
"Depending on how the system is designed, you should go in and manually scrape the inside of the ovens down," says Barrie Gillison, general manager of sales at R.F. Mote, Richmond Hill, ON. "If there are any filters attached to the system, make sure they're kept clean and changed often. Check the recirculation fans and exhaust fans and make sure there's no buildup there."
You also may not be getting the best bang for your buck if burners aren't running at full capacity, says Gillison. He recommends taking pressure readings on a regular basis to make sure the burners are running properly. If the quality of your finish is diminishing and you don't have time to do the readings, Gillison says a quick visual check will suffice.
"Most guys just look at the flames. If the flames are orange, instead of a nice blue, they are weaker than they should be and you'll find the oven is using more gas or the coating is not coming out the way it should be."
The lack of air circulation involved in the infrared process means the ovens require less frequent maintenance than convection ovens. There are no filters or fans to keep clean and there is less migration of coatings to the interior of the oven.
"Because there are very few moving parts, some electric heaters will run 30-40,000 hours before you even need to look at them," said Canfield at Casso-Solar.
But he warns that there can still be residue buildup, albeit minimal. For the infrared to reflect well, all reflective surfaces inside the inner chamber should stay clean and highly-polished. This includes the reflectors themselves, as well as the stainless steel interior walls, and even the bulbs. Generally a solvent wipe or a sponge-down with hot, soapy water is sufficient. But resist the urge to spray the interior with a pressure washer.
"There are fibrous pads in some of these ovens, which is a surface coated with a metal catalyst where the gas feeds in at the back," says CCI Thermal’s McKellar. "What you don't want to do is blow them off with the pressure washer and blow the fibrous pad right off. If you blow the catalyst out of it, the temperature in the oven won't rise above 250°F because it won't let you turn the gas on." McKellar says one of the easiest ways to keep your infrared oven clean is to run the emitters at full input with the oven empty for an hour every couple of weeks.
"Depending on how much powder migration you get from your plant into the oven, the powder can wander into the oven and into the emitters," McKellar said. "If you run the emitters on full, it basically works like your self-cleaning oven at home."
REDUCE AND REUSE
Besides good maintenance, suppliers point to a number of novel ways to save some money in the curing process by improving energy efficiency. R.F. Mote has come up with an innovative way to recycle some of the heat from the curing oven and turn it into a booster for another part of the finishing process.
"If there is an exhaust involved, you are basically pouring that excess heat outside," says Gillison. "When we design an oven, to try to minimize the cost losses, we minimize the exhaust. For example, we try to capture the exhaust from a curing oven and put it into a drying oven, where the heat doesn't have to be as precise or clean."
Similarly, one of the many benefits of infrared technology is the precision involved. The machines power up and down quickly, and they can be adapted to cure coatings on various sizes of substrates, says Fruth at Fostoria.
"Only have the emitters on when you need to have them on. If you only need to cure part of the product, you can zone infared, you can aim it onto a small section of the product that needs to be cured."
In addition, some finishers are using outdated techniques on the assembly line that the latest infrared technology renders unnecessary, says McKellar at CCI Thermal.
For example, "there's a misconception that everything needs a five-minute flash off after a paint coating to allow solvents to evaporate," says McKellar. "But with infrared – on metal or nonabsorbant substrates – the flash time is zero. Because the infrared is heating the entire thickness of the coating, it drives the solvents out before they have a chance to skin over."
Brynna Leslie is a freelance writer based in Ottawa, ON.
To view the article as it originally appeared in Coatings Magazine, click here for the article in PDF format.