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On-line gauging improves spunbonded fabric quality.

The increasing use of spunbonded nonwovens by both branded and private label manufacturers as a component in diaper production has signaled a coming of age for the material. This growth represents a challenge as well as an opportunity for suppliers. Spunbonded roll goods producer Poly-Bond Inc., Charlottesville, VA, the 19th largest manufacturer of nonwovens and part of the Dominion Nonwovens Group of Dominion Textile, Montreal, Canada, has been heavily involved in this market for several years and is addressing these challenges and opportunities.

"Hygiene markets are increasingly demanding," said Stephen Chester, Poly-Bond's technical services manager. "Customers require product consistency and conformance to targets as well as competitive pricing." Having the best production equipment is no longer enough to meet its customers' needs, according to Poly-Bond. "There is variability within every piece of equipment," said Michael Van Kay, technical development engineer. "Poly-Bond's mission is to eliminate variability as much as possible, insuring that its customers receive a uniform product they can count on."

Poly-Bond's first area of concern was the basis weight profile, which has an impact on strength, thickness and processing at the customer level. Buyers are demanding lighter, stronger materials; while not long ago an average basis weight of one ounce per square yard was acceptable, currently less that half an ounce per square yard is the target.

Traditionally, Poly-Bond conducted end of roll samplings to test the basis weight profile. That approach, however, has several limitations. "Checking the weight in the lab is too late," said Mr. Van Kay. "You already have product. If you test it and find out it is bad, it is all waste." Furthermore, from time to time it would take two to three production doffs to identify the cause of a variation. "We could spend six to eight hours solving the problem," he said.

To improve the process, Poly-Bond added an on-line basisweight gauging and control system supplied by NDC Systems, Monrovia, CA. NDC's Model 8000 series are top-of-the-line gamma backscatter (GBS) scanning systems with 19-inch high resolution color monitors with split screen displays and touchscreen controls. They offer versatility, ease of use and understanding and two-way communication with a host computer or CIM system. Poly-Bond opted to add the Model 8000's statistical process control (SPC) module and a Fast Fourier Transform (FFT) module to the system as well. "We can now get instant feedback, giving us the best information to make the best quality product," said Mr. Van Kay.

Since the gauge became operational, Poly-Bond has cut the variability of the basis weight profile across the web by half. The amount of scrap associated with the production process has been dramatically reduced as well. "As soon as a disturbance is identified," said Van Kay, "the operators can get instant feedback on their corrective action." The key to this improvement is statistical process control (SPC), which establishes minimum and maximum specifications for different product attributes and alerts operators when the production process is not operating within those specifications.

The ability to measure production process performance in real time has been invaluable for Poly-Bond. Not only are out-of-specification conditions immediately identified, SPC information has helped reduce the number of unnecessary adjustments operators make. "Operators can make more educated decisions," said Mr. Chester. "You can look at the X-Bar chart and not have to guess what to do." Furthermore, SPC has enabled Poly-Bond to trim the start up time for each production run. "That is very important, considering the trend towards shorter runs and custom runs," said Mr. Chester.

The Fast Fourier Transform module represents an advancement in process control for spunbonded producers by offering machine directional control in addition to cross web directional control. FFT is a mathematical method for determining the frequency components of a signal, showing which frequencies are dominant (i.e. large in amplitude) and which are not significant. Controlling the amplitude of the dominant frequencies reduces the variability in a production process.

For Poly-Bond, FFT means that for, the first time, operators can identify problems that may occur only in a single diaper by graphing the time and distance at which amplitude spikes occur. Using FFT information, operators can find variations that occur at regular intervals in the production process and correct the problem. In one example, a seam on a forming apron was to blame for a variation that occurred every 21 feet. Without FFT information, that kind of variation would probably never be found at all.

Poly-Bond selected gamma-backscatter technology after a systematic search of the available options including beta ray devices. "We change additives, colors and thickness frequently and gamma-backscatter gave us more flexibility," said Mr. Van Kay. "And on the production line, space is at a premium. Because it is a onesided gauging method, it is easier to install, particularly on wider lines." Poly-Bond has production lines running from 2.4 to 4.2 meters wide. Gamma-backscatter technology is also significantly less expensive than beta and NDC Systems has an excellent track record working with Reifenhauser technology.

In the final analysis, Poly-Bond believes that by adding on-line weight gauging, it is sending a specific message to its customers. "We are at the forefront using the latest technology, " said Mr. Van Kay. "In that way, we can offer added value without added cost."
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Title Annotation:Poly-Bond Inc.
Publication:Nonwovens Industry
Date:Nov 1, 1993
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