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As mills look to differentiate their products from competitors, paper machine clothing can become a way to achieve quality objectives. Many clothing suppliers team with mills to address these objectives through the use of custom-designed fabrics or unique applications of existing ones.
Suppliers are also providing a variety of tools and techniques to help determine the ultimate print quality for paper and paperboard. These tools include digitally enhanced imaging techniques, simulation software, multiple air leak smoothness tests, stylus profiling measurements, and low angle reflected light photographic studies.
On the No. 9 fine paper machine’s hybrid former at UPM-Kymmene Kuusanniemi, introduction of a fine bottom fabric and dense surface top fabric resulted in reduced basis weight variation, according to paper top machine superintendent Pentti Putkinen (shown).
In addition to a better quality product, mills are also experiencing lower costs as a result of new approaches to paper machine clothing. These reductions stem from improvements in various areas, including increased fiber efficiencies, better runnability, lower retention aid costs, and extended fabric life. In case study format, this article describes how paper machine clothing suppliers team with mills to address the competitive issues of quality, cost, and paper machine performance.
FORMING
Problem: UPM-Kymmene Kuusanniemi decided that it should increase the speed from 1,000 m/min to 1,200 m/min on its No. 9 fine paper machine. At the same time, the Finnish mill targeted an improvement of formation on the machine. However, as the speed increased, there was more bleeding, and fibers started to accumulate at the dewatering elements.
The No. 9 paper machine’s former is a Valmet SymFormer. "The running conditions have changed several times during the history of the No. 9 paper machine," says paper machine superintendent Pentti Putkinen. "Among other things, the fillers and retention chemicals have changed, the machine speed has been increased, and the forming fabric style has been updated in pace with the changing conditions."
Project: Tamfelt has been the main supplier of forming fabrics to UPM-Kymmene Kuusanniemi’s No. 9 fine paper machine since startup, and the supplier was invited to help solve the problems. The matter was discussed widely and openly in mutual meetings. The dewatering and turbulence levels in the forming section were measured and optimized. Analyses of paper samples at the Tamfelt laboratory gave additional information.
Two years ago, the No. 9 paper machine ran an Optistar 2860 double layer fabric as the bottom fabric, and a Hifi Open 2868 double layer fabric as the top fabric. As machine speed increased, the operation of the dewatering foils clearly became more active in the fourdrinier section of the hybrid former. To prevent bleeding, the bottom fabric was changed into a finer and denser fabric with better retention capability—the Optistar 2865. Instead of the previous 6,000 m3/m2h permeability, the new permeability was determined to 4,700 m3/m2h.
The slower initial dewatering with the denser fabric has also contributed to improving formation. "The running life of the bottom fabric improved slightly after the new finer style was taken into use," says Putkinen.
In a discussion concerning the development of the top fabric, the mill expressed the wish that the top fabric should remove water as slowly as possible in order to improve formation. Tamfelt’s multilayer fabric Multistar 5165, with a very dense surface, was chosen as the top fabric. "Already the first fabric has worked as planned, resulting in improved formation," Putkinen remarks.
Results: At present, the combination of fine Optistar 2865 bottom fabric and dense surface Multistar 5165 top fabric is a standard on the No. 9 paper machine. Machine speed is now at the targeted 1,200 m/min, and paper quality is very good for a hybrid former. On 80 g/m2 paper, Beta formation (basis weight variation) has improved 1.0 g/m2 and is now at 4.5 g/m2.
Problem: A Scandinavian newsprint mill committed to supplying a high quality product began looking for ways to improve sheet quality and printing characteristics for its customers. The mill specifically wanted to determine the optimum forming fabric design for achieving these quality objectives on its No. 9 paper machine.
Project:The mill sought help from AstenJohnson, and the supplier suggested using its Engineered Approach to determine a forming fabric design. Prior to the design change, detailed studies were performed on paper samples.
First, the team conducted an analysis of the headbox stock to understand the relationship of fiber lengths to the drainage hole sizes in the forming fabrics. To do this, they had to define the drainage holes’ physical dimensions as well as the support provided to the fiber exiting the headbox by yarns in the fabric's woven structure.
All of the data was put into a Microsoft Excel-based program developed by AstenJohnson and Heimbach, its European partner. Through the calculations of the computer program, it was determined that the existing extra-support fabric had a significantly higher drainage area than the triple layer. It was also determined that, because of the number of holes and their dimensions, the extra-support fabric would retard the drainage through “sheet sealing.” A triple layer fabric would cause little or no sheet sealing, thereby enhancing drainage.
By using image analysis hardware and software technology, the supplier and its partner provided a state-of-the-art computer-simulated paper and print quality evaluation. Based on the team’s experience, they knew that print quality for any printing process is directly connected to the micro- (wire mark) and macro- (large flocs) density variations on the surface on which the printing ink falls. The ideal print surface is one that is uniformly dense.
AstenJohnson and Heimbach research groups have developed a test that will simulate, through computer software, the final print quality. This computer program was used on the mill’s split sheets to evaluate the printing characteristics of the sheets produced before the design change was introduced. It was also used to evaluate the results of that change.
Using Fast Fourier Transform (FFT), it was determined that the micro-density difference pattern corresponded directly with the pattern on the forming fabric’s top surface. This then showed the relationship of fiber length to drainage hole size of the fabric surfaces.
Once the team had a detailed analysis of the headbox fiber lengths and the forming fabric used on the No. 9 paper machine, they had a good understanding of the interrelationship of the No. 9 machine’s headbox fiber lengths and drainage forces.
The team met with the paper machine superintendent to discuss and compare various alternatives. The Engineered Approach was used to select the two new intrinsic weft fabric designs and to modify them to improve both sheet properties and the paper machine’s efficiency, given the mechanical forces caused by the physical design of the No. 9 paper machine.
Results: By selecting the appropriate intrinsic weft fabric structures for each position, the papermaker achieved good machine performance and improved printability while improving fiber efficiencies through increased first-pass retention. Having concluded that micro-density differences in the sheet surface will directly affect print quality, the choice of intrinsic weft improved sheet uniformity. The resulting simulated print quality on both sides of the sheet was predicted by the latest computer technology without a printing press run. It has subsequently been confirmed on commercial printing presses.
In addition to the paper and print quality improvements, the following improvements were also observed:
• Mechanical stability. Both fabrics exhibited excellent stability and ran flat in the return run, particularly in the areas directly after the power-driven roll where fabric rippling was usually observed.
• Retention. The retention levels increased with the installation of the intrinsic weft fabrics from a normal operating range of between 60% to 65% to a level of between 66% to 70%. Another key point is that, because of the fabrics’ increased mechanical retention, the use of expensive retention aids was reduced. This reduction resulted in a considerable cost savings for the mill.
Problem: A fine paper mill was having serious wire life and performance issues relating to traditional forming fabric designs. With an average life of only 43 days, these fabrics would prematurely develop ridges and holes, making clothing expenditures excessive for this position and paper machine.
FIGURE 1: Results of stylus profiling measurements (Emveco) improved in excess of 50% after introducing multiaxial press fabrics on a linerboard machine.
In addition, the high quality grade demanded fine surface, high mesh forming fabric, but due to demands of the former itself, the fine fabric designs were not mechanically stable enough.
The paper machine produced freesheet, envelope, offset, reprographic, and postcard grades at speeds of up to 2,300 fpm. Its design included a hydraulic headbox with a top former.
Project: To apply the correct forming fabric design, the mill teamed with Weavexx. The supplier also assisted in developing a machine setup that would improve wire life without sacrificing sheet quality and runnability. It was ultimately decided that Weavexx should apply its patented Huytexx forming fabric.
Results: The new fabric achieved a 90-day life compared with an average 42.5-day life for the last fabric. Removed due to the mill’s shutdown schedule, the returned fabric was analyzed and showed that it had 40 to 60 more days of residual life, which would have tripled the average life of the last fabric.
The new forming fabric ran extremely flat and stayed flat due to the structure of its base design, which helped improve stability, and the mill reported no runnability problems. The stretch roll remained in one place, tension was the same, the hand guide was never moved, and there were no guiding problems overall.
In addition, operators were able to get more water in the sheet with the new fabric. Since it was a triple layer, there were concerns about the amount of water this fabric would carry. However, the mill found that the fabric did not carry water and that sheet quality remained on the upper limits of test results.
The new fabric also remained clean due to the degree of openness in its triple layer design. Because of its positive life, runnability, and cleanness aspects, the mill reordered Huytexx fabrics for both former positions.
PRESSING
Problem: A linerboard mill in North America sought to improve sheet quality issues related to topside smoothness and printability of its linerboard grades. The mill produces 35 lb/1,000ft2 to 42 lb/1,000ft2 linerboard from 100% OCC waste furnish. The press section utilizes a tandem bottom felt configuration with two top felts and a shoe press in the second press capable of loads up to 7,500 pli. It was the mill’s desire to work with a supplier to investigate possible sheet quality improvements through the use of new press fabric technologies and/or machine operating practices.
Project: The mill contacted Albany International, and a plan was formalized to initially define the existing sheet conditions with the use of various techniques available through the clothing supplier’s sheet analysis resources. The mill was able to collect samples over the entire grade mix range at various points in press fabric age to determine if the potential existed for improvements within the scope of press fabric structure developments. Digitally enhanced imaging techniques, multiple air leak smoothness tests, stylus profiling measurements (Emveco), and low angle reflected light photographic studies were all used to help characterize the sheet surface and provide baseline data for future comparisons.
Upon completion of the initial sheet characterizations, it was apparent that the potential did exist for improvements to the topside sheet smoothness. The next phase of the project would entail development efforts to produce a press fabric structure with the required improvements to pressing uniformity without sacrifice to water handling capabilities, machine productivity, and press fabric life. In addition, analysis of the mill’s standard press fabrics indicated that conventionally woven structures currently available would probably not yield the degree of improvement to pressing uniformity required to provide the mill with the desired result.
After discussions with the mill, it was decided that multiaxial press fabric technology would be utilized. This relatively new technology, which incorporates "flat" weaving techniques, would bring forming fabric quality, sheet support, and pressing uniformity to the structure which were unavailable in current laminated products using conventionally (endless) woven base fabrics. Also, the nature of multiaxial laminated products had already been proven to enhance compaction resistance and water handling capacity over the life of the product for potential further gains in machine productivity and press fabric life.
FIGURE 2:Low angle reflected light photographic analysis results after introducing multiaxial press fabrics on a linerboard machine. Results: Two complete sets of the multiaxial press fabrics have been run to date with promising results for the mill. Virtually all linerboard grades produced by the mill have thus far shown dramatic improvements in topside smoothness and sheet appearance as measured by air leak smoothness tests, stylus profiling measurements (Emveco), and low angle reflected light photographs.
Sheffield values, as measured by the mill, improved by approximately 9% to 10% on average across the range of grades produced. In addition, Emveco microdeviation measurements of samples collected (Figure 1) indicate improvements in excess of 50% since introducing the multiaxial products. Finally, low angle reflected light photographic studies with digitally enhanced imaging techniques (Figure 2) confirm the step change improvement to topside sheet smoothness as indicated by the significant reduction of repeatable/recognizable patterns found and the reduced intensity of those that were identified.
The end result for the mill has been improved sheet quality, a more saleable product, and the resulting expanded potential customer base.
Problem: A Southern corrugating medium producer was interested in methods for increasing press solids exiting a tandem ENP arrangement. The mill was subject to upwards of 4% rewet because of the sheet transfer to its single tier drying section.
Before looking at new press fabric concepts, the mill made the decision to transfer the sheet into the drying section from the bottom fabric. It believed that transferring from this fabric would help runnability. However, the transfer from the top fabric gave a drier sheet, but also led to more breaks at the transfer point. Immediately after the conversion, the drying rate on the paper machine decreased by up to 4%.
Project: At that point, Scapa Paper Machine Clothing was asked to come up with new or innovative press fabrics to help increase solids exiting the press. The initial recommendation to the mill was the patented Spectra press fabric. The use of an elastomeric composite has led to increased solids in many applications, and it only made sense to apply this design first. The immediate recovery of the composite fabric exiting the nip was also believed to move the water away from the fabric's surface at this critical rewet point. The clothing supplier also believed that the composite fabric would stay more open, thus increasing water removal throughout the life of the press fabric. The initial trial was installed in late February 1999.
Results: Results from the initial trial were excellent—an immediate 1.5% increase in press solids. The mill was pleased with this result, but urged its supplier to achieve 48% solids exiting the press. The initial trial improved drying, but not to the desired level. After running several repeats of the same design, it was decided that an improvement had been made, but something more was needed.
With the formation of Voith Fabrics from the merger of Scapa Paper Machine Clothing and Appleton Mills in August 1999, engineers could now add to the Spectra design by choosing from a variety of proven patented materials for reducing rewet. Voith Fabrics recommended Spectra with Flow Control as the next step to help the mill increase solids. This fabric was installed in early October 1999, and again the mill realized an increase in sheet solids exiting the press. The machine was able to run in excess of 3,000 fpm on several grades throughout the life of the fabric, resulting in one of the best production months ever. In November 1999, the supplier installed the follow up trial. Again, the mill recognized solids increases.
At this point, the mill is pleased with the progress its supplier is making. Though it has not yet reached its goal, the mill is closer than it was 10 months ago. The mill’s clothing supplier is now submitting design improvements for the tandem and other press positions. This will be another step toward reaching the final goal.
Obviously, a press fabric cannot be credited with all the improvements. The mill has made aggressive changes in many other areas to improve the press solids, and a very important aspect of this has been the blending of mill/supplier technology and teamwork to solve problems.
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