The Right Combination for Curing a Powder Coating
Often used to cure powder coatings, combination ovens -- ovens that employ both infrared and convection -- usually connect infrared and convection heating zones together in series: that is, a zone of infrared followed by a zone of convection. This is logical given that when curing powder coatings, the objective is first to fuse the coating, then cure it. But an innovative combination oven, installed in Michigan one year ago, actually utilizes infrared and convection simultaneously in each zone, creating a fast yet flexible powder coating curing oven.
United Lighting Standards manufactures steel and aluminum light poles at its plant located just outside Detroit. When the company began operations in 1971, the poles were hand sprayed with a liquid primer, then painted with a liquid acrylic enamel finish. The paint drying process took two to four hours, limiting production to only 30 poles per day.
To take advantage of the superior appearance and abrasion resistance provided by powder coating, the company replaced its liquid primer and paint line with a powder coating line in 1987. But the powder line never quite lived up to its promise, according to United Lighting Standards' president, Robert Wesch. "Our capability was limited by the infrared oven," he said.
The problems with the electric infrared oven were many. Foremost, curing poles coated with light-colored powder was extremely slow because the infrared intensity necessary to accelerate curing discolored the powder. To prevent this, lamp intensity was reduced, and the entire line was slowed when light-colored poles were being cured.
Additionally, the high intensity heat, combined with the necessary curing time for the poles, would cause light-gauge housings assembled to the poles to discolor. United Lighting switched to painting the housings separately from the poles and attaching them after curing, but this added another process step. Finally, the electric infrared elements were expensive to operate and required significant maintenance to replace and repair burned-out or broken elements.
Finding a SystemThe square and round light poles produced at United Lighting Standards vary in length from 10 to 40' with wall thicknesses from 0.125 to 0.5". Typically tapered, the poles are welded to heavy, 0.5 to 2" thick solid bases that range from 10 to 17" in diameter. Pole weight ranges from a 30 lb, 10' aluminum pole to a 1,100 lb, 40' steel pole. A continuous overhead conveyor transports the poles, base plate at the rear, through the powder application line and curing system.
United Lighting coats the poles with polyester TGIC powder purchased from several suppliers. More than 70% of the poles are coated bronze while black accounts for another 10%, and about 8% are coated white. The remainder are coated assorted colors depending on the intended use. In addition to the poles, United Lighting also powder coats the poles' associated flat, formed and tubular hardware.
The search for an improved curing system, headed by then general manager Bernie Jenkins, focused on three options: A new electric infrared oven, a catalytic infrared oven or a combination gas infrared and convection curing oven recommended by Thermovation Engineering Inc., Cleveland.
"We exhaustively evaluated all three approaches and decided to go with the combination oven from Thermovation Engineering," Jenkins said. Using both gas infrared and convection together in each zone appeared to provide the flexibility and controllability needed to accommodate the broad range of parts, substrates, powders and line speeds effectively and efficiently.
With the new combination oven, poles first are cleaned by shot blasting, then conveyed through the powder-application unit. Once coated, the poles enter the oven designed and built by Thermovation Engineering. The combination curing system is a free-standing 32' long, 8' high and 5' wide structural steel assembly with 4" thick, double-insulated sheet metal floor, roof and wall panels. In the first zone, powder fusion is achieved by applying high-intensity gas infrared and low-velocity convection. This combination rapidly brings the substrate and powder to fusion temperature without disturbing the powder. Once the coating has fused, the cure is completed with the continued application of infrared and moderate-velocity convection, which hold the substrate and coating at the curing temperature. After exiting the oven, the overhead conveyor carries the poles laterally to where the poles are stacked and packaged for shipment.
Electric or Gas? Why It MatteredFor this application, using gas infrared provided two significant advantages over electric infrared: Increased productivity and lower operating and maintenance costs.
Productivity. In analyzing the needs of United Lighting Standards, Thermovation engineers determined that the existing electric infrared oven yielded the kW equivalent of 1.18 million BTU/hr. Therefore, the burner capacity for the gas infrared section of the combination system had to be at least 1.6 million BTU/hr to provide the desired 30% increase in productivity. In fact, the curing system was designed with a 2 million BTU/hr max. output to provide an engineering safety factor, allow for increased production and ensure rapid heat up from a cold start.
"The catalytic system supplier recommended a system delivering 1 million BTU/hr, and the electric infrared supplier recommended a 1.4 million BTU/hr system," Jenkins noted. "Thermovation engineers recommended a combination curing system with 2 million BTU/hr capacity. We realized that the additional capacity would allow us to reach our increased production goals while providing for future needs."
Operating Costs. Gas infrared is significantly less expensive to operate than electric infrared. A significant portion of electric energy costs for the previous oven derived from the monthly demand charges imposed on energy consumed during periods of high demand. For purposes of comparison, Thermovation engineers analyzed the energy costs of an electric infrared system with a demand capacity of 392 kW and a 300 kW average usage level operating eight hours a day, 22 days per month. With these figures, estimated monthly electrical energy cost was $7,168.24 -- of which almost 60% was attributable to demand charges.
These operating costs were compared with those of the proposed 1.6 million BTU/hr gas infrared system. With the same usage per month, gas charges were estimated at $1,047.55. The significant savings were possible because there are no utility demand charges for gas usage. Thus, energy-related operating costs for the proposed larger system were estimated at about $6 per hour vs. almost $41 per hour for the previous system.
Maintenance Costs. Rated at 5,000 hr under the best of conditions, the glass lamps used in the electric infrared were no match for the light poles. Replacement costs were estimated at approximately $12,000 per year.
Thermovation Engineering recommended using heavy-duty cast-iron burners designed for long life under tough operating conditions. These burners far outlast any electric element and have three times the expected life of less rugged formed-sheet metal burners. Experience has proved this decision right: After one year of continuous operation, just two of the 90 burners in the oven have been replaced at a combined cost of $350.
The cast-iron burners are designed to operate on a premixed volume of air and gas rather than relying on atmospheric air for combustion. With premix burners, the oven temperature can be held constant while product loads fluctuate. Heat input is modulated to match the control.
"When we looked at these design elements and cost estimates, we realized that the additional capital investment for the combination oven was insignificant in comparison to the operating and replacement costs of the electric infrared oven," Jenkins said.
Convection CuringIncorporating convection heat transfer in this oven also provided two process advantages: improved quality and enhanced efficiency.
Quality. Used in combination with infrared, convection heating significantly improved coating quality for United Lighting. By precisely controlling airflow and velocity, convection provides efficient, effective heat transfer that ensures accurate and uniform temperatures along and across the parts. Adding convection particularly improved the cure for the poles coated in light colors, and its addition allowed the less-visible undersides of the poles, light-gauge housings and miscellaneous hardware to be cured more effectively.
Efficiency. Oven efficiency is the ratio of the heat input into the product vs. the energy consumed by the oven. Electric radiant elements typically have a radiant efficiency (the ratio of radiant energy emitted vs. energy consumed) of 60 to 90%. Gas infrared burners typically have radiant efficiencies of 40% to 60%. In each case, the remainder of the energy input (that which is not converted directly to radiation) becomes heated air within the oven.
Thermovation engineers designed the oven to use this heated air to provide additional heat to the product and offset losses that typically occur through the exhaust and enclosure. The moving air improves overall oven efficiency, ameliorating the inherent radiant inefficiency of gas infrared (when compared to electric infrared). The additional convection heating system supplements the preheated air, helping to heat the poles more rapidly and uniformly than is possible with radiant heating alone.
The Right CombinationOven testing and troubleshooting was performed before the assembled system was shipped, so downtime at United Lighting Standards was reduced to six days.
"We've met and exceeded our productivity, quality and cost-reduction goals. Number of poles per shift through the system is up more than 30%, and we're confident we can easily raise this to 50% to 60%," company president Wesch noted. "Light-colored poles cure as fast as dark colored poles. And, we have reduced operating costs by more than 60% while maintenance and replacement have been reduced to almost nothing."
For United Lighting, infrared and convection together was the right combination.