Powder Coating 1998.


United Lighting Standards makes steel and aluminum lighting poles and related products at its 57,000-square-foot plant in Warren, Michigan. The company, founded in 1971 as Wolmac Engineering, originally hand sprayed its products with a primer and then applied an acrylic enamel finish. Each coat took from 2 to 4 hours to dry depending on the weather. Production was about 30 poles daily. In 1987, the company replaced liquid paint finishing with powder coating, based on the superior appearance and abrasion resistance powder offered. However, the powder line, which included an electric infrared oven, wasn't quite living up to its promise, according to Robert Wesch, company president. "Our capability was limited by the productivity of our existing oven," he said. The oven failed to completely cure large, light-colored poles, causing slowdowns on the line. In addition, operators frequently had to shut down the line for maintenance work and element replacement.

The search for a better curing system, headed by Bernie Jenkins, general manager, focused on three options: A new electric infrared oven, a catalytic system, and a combination gas-fired infrared and convection curing system. The company contacted vendors it was familiar with to analyze the three types of oven systems. Vendors took three criteria into account - capacity, energy costs, and maintenance and replacement costs.

In analyzing the needs of the lighting company, one vendor determined that the company's existing electric IR oven yielded the kilowatt (kW) equivalency of 1.18 million British thermal units per hour (Btuh). A vendor for a new electric infrared system recommended a 1.4-million Btuh system; a catalytic system vendor recommended 1 million Btuh. As for the combination gas-fired infrared and convection system, the burner capacity for the infrared section had to be 1.6 million Btuh to provide for the 30 percent increase in productivity that United Lighting sought. Taking into account engineering safety and rapid heat-up capability, the vendor with the combination oven recommended a system with a capacity of 2 million Btuh to allow for growth in productivity. "We realized we would need this additional capacity to reach our increased production goals and to provide for future needs," Jenkins said.

Operating expenses were another factor. The engineers with the vendor offering the gas-fired, infrared convection oven compared the operating costs of an electric infrared oven with the costs of the combination gas-fired infrared convection system. First, they estimated the operating costs of an electric infrared system with a capacity of 392 kW of demand and an average usage level of 300 kW. When operated 8 hours a day, 22 days each month, the estimated monthly electrical energy cost was $7,168, of which about 60 percent was demand charge - a surcharge on energy consumed during peak demand. The remaining 40 percent was energy used during periods of normal demand.

Then, engineers considered a gas-fired infrared convection system with a burner capacity of 1.6 million Btuh. With the same amount of use per month as the electric infrared system, engineers estimated that the gas-fired infrared system would cost about $1,048 to operate. (There are no utility demand charges when using gas.) Broken down further, engineers figured that operating the gas-fired infrared system would amount to a cost of $6 per hour, compared with $41 per hour for the electric infrared system. Working with an 8-hour-a-day production schedule, the company could save up to $82,000 a year with the gas-fired infrared system, according to the vendor's figures, and $87,200 a year on a 10-hour-per-day production schedule.

According to Jenkins, replacing electrical heating elements costs the company nearly $10,000 annually. Jenkins rated the longevity of the elements at about 5,000 hours under the best conditions.

The vendor of the gas-fired infrared convection oven advised using heavy duty cast iron burners designed for long service life under demanding operating circumstances. According to the vendor, the burners last longer than electric elements and less rugged formed-sheet metal burners typically found in space heaters. The vendor noted that the design of the burners permits them to run on a premixed volume of air and gas, rather than relying on atmospheric air for combustion. The premix burners allow operators to control the oven temperature and modulate heat input, even when work loads fluctuate. With atmospheric burners, operators can only turn the burners on or off.

Said Jenkins: "When we looked at these design elements and cost estimates, we realized that the additional capital investment for the combination gas infrared convection system was insignificant in comparison with the operating and replacement costs of the electric infrared oven." The company ordered the system, convinced it could effectively and efficiently accommodate an assortment of parts, substrates, powders, and line speeds.

The vendor ran tests and troubleshooting exercises on the oven before the system left the factory. For each oven zone, the vendor mounted a complete gas-valve train. The vendor's employees piped and wired the system before delivery, which was done in three complete sections to simplify installation. As a result, downtime for oven installation at United Lighting Standards lasted only 6 days.

The company's light poles vary in length from 10 to 40 feet, are round or square in cross section, and are usually tapered. Wall thicknesses vary from 1/8 to 1/2 inch. Employees weld the poles to heavy, 1/2- to 2-inch-thick solid base plates that range in diameter from 10 to 17 inches. Poles vary in weight from a 30-pound, 10-foot aluminum pole to a 1,100-pound, 40-foot steel pole. The company also powder coats a range of associated flat, formed, and tubular hardware.

Operators clean poles and parts with steel shot before loading the items on a continuous overhead conveyor. The conveyor moves lighting poles, with base plate at the rear, through the application booth, where four coronacharging guns spray TGIC-based polyester powders. Manual guns are also available for touch-ups.

Operators spray bronze-colored powder on more than 70 percent of the poles, black on about 10 percent, white on about 8 percent, and a number of other colors on the remainder of the poles. Minimum film thickness is 2 mils. Maximum film thickness depends on several factors. For example, one major factor is whether or not the lighting pole will be placed in an area where it will be exposed to atmospheric corosion, such as salt spray at a seashore location.

Parts then enter the cure oven, which is 32 feet long, 8 feet tall, and 5 feet wide. Assembled of structural steel, the freestanding system includes 4-inch double-insulated sheet metal wall, floor, and roof panels. High-intensity, or long wavelength, gas-fired infrared and low velocity convection rapidly fuses the powder to the substrate. After the powder fuses to the part, IR and convection elements continue heat application, maintaining the substrate and powder at the curing temperature, usually 450°F. Cure times range between 2 and 7 minutes, depending on base materials and coating thickness. For example, steel poles take more time to cure than aluminum ones because heat penetrates aluminum faster than steel.

Convection heating acts as a supplement to the IR elements, according to the company. The convection section of the oven transfers heat accurately and uniformly along and across the parts. This is especially important for curing the underside of lighting poles. Operators have the capability to control and direct airflow so that heat reaches all areas of the part.

The oven features a prewired main control panel that includes all the necessary temperature, controllers, combustion safeguards, push buttons, and lights for operation of the curing system. Digital temperature controllers in each zone modulate fuel input to the burners to maintain desired setpoint temperatures. A digital display provides a continuous reading. A noncontact sensor measures pole temperature at the oven exit.

After poles exit the oven, the overhead carries them laterally. This moves the base plates through a supplemental electric infrared section adjacent to the main oven, completing base-plate curing. Operators then stack and band the poles for shipment.

After a mere 3 months of operation, company employees were impressed with the gas-fired infrared convection oven. Production was up from 30 poles to nearly 40 a day. "We're confident we can easily raise that to another 50 or 60 percent," Jenkins said. This allows operators to coat heavy, cast transformer bases, which were previously farmed out. The oven allows for excess capacity of the powder coating system, which means the company can raise production rates with minimal renovations.

Using natural and supplemental convection heating enhances product coating quality and curing system efficiency. Oven efficiency - estimated as the ratio of the heat input to the product, to the energy consumed by the oven - has increased. Electric radiant elements typically have a radiant efficiency of 60 to 80 percent; gas-fired infrared burners, 40 to 60 percent. In each case, the remainder of the energy input not converted directly to infrared radiation appears as heated air within the oven.

The company's combination oven makes use of this heated air, using it as additional heat for parts, and offsetting the unavoidable losses through the oven enclosure and to the exhaust. This type of convection heating system heats the poles faster and more consistently than radiant heating alone.

"We can expect to reduce long-term operating costs by more than 60 percent," Jenkins said. "And maintenance and replacement costs should drop by better than 50 percent."