The management AT Guilford Mills, a textile company in Greensboro, N.C., that manufactures both wrap-knit fabrics and circular knits for upholstery, was faced with increasing customer demand for quicker production of high-quality printed fabrics. Only two options were available: closing the existing transfer-printing line and spending $500,000 on building a new one; or significantly increasing the speed of the existing transfer-printing line. With the assistance of local power provider Duke Energy Corp. in Charlotte, N.C., Guilford Mills tripled the line's production speed and rolled out brighter fabrics with consistent quality.

Preprinted paper comes in contact with a steel roll, warmed to 425°F through induction heating, to transfer a pattern to wrap-knit fabrics and circular knits for upholstery.

The transfer-printing process at the Greensboro plant involves paper preprinted with the desired design. The paper travels with the fabric as it is pressed into contact with a hollow steel roll heated to 425—F or more. The high temperature vaporizes the dye, transferring it to the upholstery fabric, while the engraved roll embosses the distinctive pattern onto the fabric. An electric infrared heater is used to set the dye.

Uniformly high temperatures must be maintained across the full 6- to 8-foot-long, 12-inch-diameter roll to ensure a high-quality print. Plant managers tried piping steam through the roll and other technologies to impart uniform roll temperatures. Process control was insufficient, however, causing large amounts of fabric to be rejected.

Guilford Mills management then tried to run electrically heated oil through the roll, but the hazardous conditions, unpleasant smell, and inconsistent dye transfer resulting from that technique made it unusable.

In February 1994, the managers approached Duke for assistance. "We decided to examine using induction heating because it is a very fast, uniform, and highly controllable heating process," said John Millard, an electrical engineer and process applications engineer at Duke who specializes in electrotechnologies.

Induction heating involves sending electricity into a coil to generate a magnetic field that will jump to a metal roll. This creates eddy currents in the roll, generating heat for the process. Induction heating has been used for years in metalworking but not in textile printing, to the best of Millard's knowledge. It "was a natural fit because the roll need not contact the coil," he said. "This facilitates changing rolls to print different patterns."

Duke contacted Pillar Industries Inc., a mechanical engineering firm in Menomonee Falls, Wis., to develop an induction-heating system for transfer printing. Most of Pillar's induction-heating systems are used in automotive, foundry, machine-tool, and parts-manufacturing applications.

Studies conducted by Pillar engineers on the most commonly used pattern roll at Guilford Mills revealed that the roll's mass could not retain the heat needed for printing, because the heat dissipated too quickly. The designers then thickened the wall of the roll by about 1/2 inch; the change resulted in sufficient thermal mass to hold the temperature needed.

"Another concern involved the electromagnetic field itself, which would be somewhat stronger in the middle of the roll than at the ends," Millard said. "It was believed that this would cause a temperature differential great enough to compromise print quality." Pillar engineers worked with Acrolab in Windsor, Ontario, to install Acrolab's Isobar rods, which contain a thermal fluid, into longitudinal holes drilled through the length of the roll. The holes were then sealed. The fluid in the Isobar rods evenly distributed heat through the roll.

The designers incorporated an infrared pyrometer to monitor the roll's surface temperature. This noncontact sensor provides feedback to the transfer-printing line's computer so that it can vary power accordingly, saving energy costs and ensuring printing quality.

While Pillar conducted the calculations and studies, the induction system was tested at the mill at Greensboro prior to installation in early 1995. The system has provided dramatic results since then. In terms of scrap rate, the mill had previously lost between 12 to 15 percent of its output because of unacceptable printing. That figure has dropped to less than 1 percent, even though production speed has increased from 3 to 6 yards per minute the old way to 12 to 15 yards per minute, depending on fabric and style.

Guilford Mills has also realized less-apparent benefits with the new induction-heating system, according to Phil McCartney, vice president of technical operations at the mill. "Induction heating enables us to raise the temperature of the printing rolls 40 percent faster than previous methods," he said. "Because the induction coils do not actually contact the printing rolls, we can change over to new patterns about 40 percent faster than before."

These benefits returned Guilford Mills' investment in the new system within a year. The capital costs of similar induction-heating systems range from $75,000 to $100,000 plus roll modifications, a fraction of the $500,000 price tag of a new printing line. In addition to saving on its capital investment, the textile firm avoided the added floor space, labor costs, and maintenance costs of a new print line by modifying its line to induction heating.

Since the installation at Guilford Mills, Pillar has installed an induction-heating system for textile printing at Malden Mills in Lawrence, Mass. The newer system is not equipped with Isobars, because Pillar engineers found that the temperature differential between the middle and ends of the rolls in this case was not enough to harm print quality, according to Frank Wilson, vice president of sales at Pillar.

"We also improved both the control and power supply originally developed for the Guilford Mills project," said Donald Gibeaut, a mechanical engineer and southeast regional manager at Pillar. Gibeaut incorporated an Allen-Bradley programmable logic controller into the printing process so that the entire printing process, rather than just the roller temperature, can be monitored and provided with self-diagnostics. "We also upgraded the power supply with insulated-gate-bipolar-transistor components," Gibeaut said.

Duke is now looking for further textile applications for induction-heating systems. "One that comes to mind is replacing the big steam cans used to heat and dry yarns and fabrics," Millard said. "An induction-heating system could free up the floor space these big cylinders take up, and would use less energy besides."

Gibeaut added that some textile companies are already inquiring about using the induction-heating systems to replace steam cans in drying applications. "Some plastics manufacturers are asking us to design induction-heating systems to extract polypropylene," he said.

McCartney of Guilford Mills said that virtually any company interested in improving an energy-intensive process would benefit from approaching its utility for assistance. "In a world of spiraling costs, it makes sense to go to the experts," he said. "Why go through a complete learning curve on your own when you can get the appropriate resources through the power company?"


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