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Whether for highly specialized color grades or commodity newsprint grades, paper machine clothing suppliers are working with papermakers to address problems that ultimately affect the bottom line. Costly wire mark problems, edge-related breaks, and other quality issues such as smoothness, two sidedness, and edge wrinkles can all be addressed by the appropriate clothing application. Also, reduced chemical use and improved efficiencies and speed as a result of clothing choices can further impact profitability.
To determine the right clothing, suppliers continue to use a variety of high-tech analysis tools, as the case studies in this article describe. Such tools range from software that matches fabric hole geometry to fabric length, process simulation, and image analysis hardware and software to speed measurement devices, strobe photography, and gamma gauge technology. As more papermakers try to distinguish their products with value-added features, teamwork between the mill and clothing suppliers that incorporates tools, detailed analysis, and innovative fabric choices will play a significant part in creating and maintaining the required quality.
Detailed Analysis, Customized Fabric Improves Quality on SC-B Machine
Problem: A Canadian mill with a new Voith Duoformer CFD on its SC-B paper machine was having problems achieving satisfactory formation, and fabric life was also short.
Project: To determine the optimum forming fabric design for this application, the mill contacted AstenJohnson. The supplier chose to use its engineered approach, and, as part of this, an extensive survey was conducted. The survey looked at the paper machine operating conditions, the forming fabric structures in use, and stock furnish. It also performed a detailed study of existing sheet quality under specific operating conditions. Once all the facts surrounding the existing conditions were established, a plan was developed with the paper machine operating personnel to address the problem(s).
A sample of the headbox furnish was analyzed to determine the fiber length distribution of the headbox stock. This was necessary for understanding the relationship of the fiber lengths to the drainage hole sizes in the forming fabric structures in use. The geometry of the drainage holes was calculated using a computer software program developed by the clothing supplier. This software allows characterization of the forming fabric into quantifiable entities such as drainage area, frame length (hole size), CD fiber support, MD fiber support, etc. With this tool, the fabric hole geometry can be matched to the fiber length, and one fabric can be scientifically compared with another in order to understand their differences and apply t hem properly.
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| FIGURE 1. To address problems with smoothness on a printing grade, Weavexx suggested Huyperpunch-D, which employs a special needling technology to improve press felt uniformity. |
The Duoformer CFD is a roll-blade former. The sheet is initially set on a forming roll followed by an opposing blade section where shear is applied to the fibrous suspension on both sides of the slurry. This type of former is now common in the industry, providing good retention with the formation of a thin web early in the papermaking process, followed by the activity of the dual set of blades. The detailed study combined with mill observations indicated that excess drainage was occurring around the forming roll because of the very high wrap angle (57º). This resulted in the sheet being too dry entering the opposing blade section, which meant that the shear forces had little or no advantageous affect on sheet formation. Additionally, because the fabric was lacking in lubrication as it went through the opposing blade section, high levels of wear were present and the fabric life was cut short. The on-machine diagnostic analysis fully supported this conclusion.
A detailed split sheet image analysis was conducted using image analysis hardware and software programs that were assembled by AstenJohnson specifically for this type of application. This process digitizes the image of the sheet and can be used to calculate a two-dimensional fast Fourier transform on the micro-density differences within the sheet in order to highlight the repeating patterns. This, in essence, highlights the defects of the sheet and is essential in monitoring the progress (or lack of) accomplished through clothing changes.
Armed with a full understanding of the paper machine operation, the fiber lengths, the details of the drainage structure of the forming fabrics, and, finally, the detailed analysis of the sheet produced with these conditions, the clothing supplier met with the mill's technical and production team to discuss options. It was determined that a method was needed to "retard" drainage to bring more diluted slurry to the blades for both formation and forming fabric life. This had to be accomplished without losing CD support needed to support the very short fibers coming from the headbox. An intrinsic weft triple layer design InTegra forming fabric with very short frame length was selected for this application. This technology offers the known incremental papermaking properties of the triple layer fabric without the delamination propensity of the old, "stitched" technology, which is prone to delamination.
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FIGURE 2. Felt surface improvement with Huyperpunch-D offers a more uniform support structure for the sheet. |
Results: Significant improvements were seen after installing the first set of fabrics. Formation was significantly better and forming fabric life improved dramatically. The team felt that formation could be further improved if they found a way to further retard drainage. The supplier included members of its new product development team to "engineer" an InTegra forming fabric that could retard drainage further while maintaining high CD support. A new InTegra forming fabric, offering a frame length of 0.114 mm and a drainage area of 28%, was installed and is now the standard for this machine. Not only did the formation improve, but also the retention aid addition was reduced (for the same first pass retention), sheet quality (wire mark) improved significantly, and a lifetime speed record was obtained with these fabrics.
Measurement Device Helps Correct Edge Drop Offs in Newsprint Application
Problem: High speed, low grammages, and a closed water system with deinked pulp (DIP) stock tended to increase edge drop offs in the pick-up position of a newsprint machine. The amount of edge-related breaks were three per day.
Project: The mill focused on pick up roll penetration, edge trimming, and pick-up roll vacuums when trying to solve this problem. In addition, the mill contacted Tamfelt, which was not currently a clothing supplier to the mill, to provide further analysis. During the first service call, this supplier used its speed measurement device to help diagnose the edge drop off problem.
The speed measurement device allowed the mill to measure roll speeds and the speed of the fabric itself in different positions. It is well known that the fabric speed measurements differ between various positions due to stretching and tension releasing. For example, the fabric tension can vary from 6 kN/m up to 20 kN/m when measured at different positions. Also, the actual speed of the fabric behaves the same way when measuring different positions of the fabric loop. The measuring principle for this device is based on a fabric contacting running wheel that gives a pulse to the computer at every rotation while the speed is based on the contacting wheel rotational frequency. The same computer can also be connected to a trigger that measures the actual roll speeds in the traditional way.
Results: The result of the speed difference measurement was very surprising to the mill and to the supplier. The mill drive control system showed slightly higher speed of the pick-up roll compared to the forming drive roll, but the contacting wheel measurement result showed that the actual speed of the pick-up felt was 7 m/m lower than the forming fabric speed. The speed difference was corrected right away so that the supplier's measurements showed positive speed differences between pick-up felt and forming fabric. The mill considered this a significant discovery, which ultimately resulted in a reduction in the amount of breaks due to edge problems from three per day to three per month.
Tight Seam on Wet Press Felt Avoids Marking on Dark Paper Grades
Problem: A specialty mill was resigned to absorbing the downtime required by a non-seam wet press felt. The mill, which specializes in manufacturing fine grades, had all but given up on using seam felts for dark colors. While the mill preferred the limited downtime and relatively simple installation of a seam felt, none of its current suppliers could produce a seam felt that did not cause marking on dark color grades.
Dark color grades are especially susceptible to drainage differences along felt seams because these grades tend to use more dye. At this specialty mill, the excess scrap due to marking caused the machine superintendent to abandon seam felts altogether. While non-seam felts did not have marking problems, these felts were more difficult and dangerous to install, leading to increased machine downtime. Ideally, the machine superintendent wanted the cost savings and decreased downtime of a seam felt without the marking problems. The challenge was to design a seam that would drain like the rest of the felt and not leave an impression or drainage mark in the sheet.
Project: The mill began working closely with a new supplier, Orr Felt, to procure a felt that met its needs. The new supplier brought together a team of service, technical, design, and manufacturing personnel to create a solution that would provide the benefits of a seam felt without the marking problems. Working closely with the machine superintendent and his technical staff, the supplier's team created an innovative design for a seam felt that avoids marking even dark color grades. The felt featured a smaller loop construction resulting in a tight seam that could still be easily installed on a paper machine. The team made several changes to the loop setup in the weaving of the base fabric (called the FS Series) to create smaller and more uniform loops. The batt flap was also changed to improve its integrity so that marking would not become an issue as the felt wore from normal operation.
The supplier manufactured a trial felt using the new set up and worked with the customer to install it. Installation went well; despite the smaller loops, seaming time was only 15 minutes longer than normal. Using this new design, the mill was able to run dark colors without marking for a 32-day period. After 32 days, a slight seam mark began to appear on dark colors. The mill chose to back off the loading on the press, which allowed the sheet to return to specification. As a result, although the mill was able to successfully complete the run, it lost some production time. After the felt completed its run, the supplier analyzed samples of the felt and the sheet before and after the press was unloaded, determining that it had achieved its first goal: a seam felt that could produce dark color grades without marking.
The second task was to maintain this capacity through a run of more than 40 days, without forcing the mill to back off on press loading and lose production. The supplier team worked to construct a seam that would not stretch and a reinforced system that would eliminate any roll back or seam flap failure. By changing the support system at the seam (which was later called the RS Series), the supplier was able to achieve the desired results in lab tests. Based on the success of the lab tests, it manufactured a felt using the new RS Series system. This second trial felt also featured a different flap cut angle.
Results: The second trial felt was installed with no differences in seaming from the original trial. After completing a 40-day cycle, the mill removed the felt for a scheduled shutdown, with no signs of seam marking on any grade, including a number of runs on dark colors.
The mill then took the next step, running a set of these felts on another paper machine with a more critical application. Trials made with both the FS and RS Series from Orr Felt performed to expectations, with all felts running through their expected life without marking.
Angular Needling Technology Improves Smoothness for Printing Grades
Problem: For its machine producing coated and uncoated printing grades, a mill had attempted to improve sheet smoothness and two sidedness for some time. The machine ran at 4,010 fpm and had a shoe press. Although various conventional press felt designs from several vendors were used in trials, no significant results were achieved for this demanding grade.
Project: Machine management at the mill formed a formed a continuous improvement team (C.I.T.) with Weavexx personnel. The team's focus was to develop a strategy for improving sheet quality without sacrificing machine productivity or runnability.
After careful evaluation of existing key performance indicators and machine parameters, the C.I.T. analyzed the performance of the press section. Strobe photography, press section gamma gauge technology, PressManager press simulation, and sheet splitting techniques were employed to better understand the dynamics of the water removal process in the press section.
Upon the final analysis, a totally different press felt design (Capillaris with Huyperpunch-D technology) was recommended. This recommendation was based partly upon previous successful machine trials in Europe where sheet surface quality and uniformity had been optimized under similar machine conditions making the same grades.
Huyperpunch-D uses an exclusive angular needling technology to apply the fiber batt to the sheet side of the felt (Figure 1). In addition, the needle board moves in a synchronous motion travelling with the felt. This new technology improves the uniformity of the press felt (Figure 2) providing a finer, more uniform support structure for the paper sheet.
Results: The new felt design was installed and immediately improved surface characteristics of the sheet. The mill is pleased with the high quality of the sheet (Figure 3) and improvement in smoothness. Also, it has made this new felt technology standard on the other machines at the mill.
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| FIGURE 3. Before (left) and after (right) Huyperpunch-D on uncoated, uncalendered samples. |
New Fabric on Third, Fourth Press Reduces Vibration for LWC Paper Machine
Problem: For a high-speed lightweight coated (LWC) paper machine in the U.K., vibration had been an ongoing issue on the fourth press. The machine clothing that had been applied to the third and fourth presses did not significantly overcome the vibration problem.
Project: Some months before, Voith Fabrics had successfully solved a similar problem for a mill in Canada by applying its patented Spectra fabric. Although, at the time, this fabric had never been used in the U.K., the supplier team decided to apply the Spectra fabric to this LWC paper machine in an attempt to solve the vibration problem on the fourth press.
Results: The first full trial of the new fabric at the mill was extremely successful, with the fabric running 47 days without a break. In subsequent trials, and now as the standard design on this position, the Spectra fabric runs an average of 56 days. Also, within the first trial period, the mill achieved a record monthly efficiency of 95.7%.
The mill advised the supplier team that during one run, the paper machine ran with the Spectra fabric on the fourth press for a record 166 hours without a sheet break (previous best was 104 hours). During the same run, the mill set a new 24-hour speed record of 4,626 fpm. In addition, vibration levels have been significantly reduced throughout the running period of the new fabric.
As a direct result of the positive performance on the fourth press, the mill successfully tested the Spectra fabric on the third press. Today, Voith Fabrics supplies 100% of the fabric for the third and fourth presses.
Newsprint Mill Solves Edge Wrinkle Problem with Moisture-Attracting Fabric Enhancer
Problem: A newsprint mill was trying to increase sheet width on certain runs of its 3,800-fpm newsprint machine to prevent the production of unsalable edge rolls. The main obstacle was a problem with edge wrinkles in the first single run dryer section. These edge wrinkles caused a trim loss, which resulted in edge paper rolls too narrow to sell to its customer. The wrinkles were a result of blow boxes too narrow for the wide deckle width used for certain customer runs. Due to the inability of the blow box to help hold the sheet against the dryer fabric on the very edges, the sheet would flutter and end up in wrinkles. Although the mill only ran at this wider width a few days each month, it was very costly.
Project: The mill asked Albany International to help eliminate the sheet wrinkling so they could sell the edge rolls. After technical study and analysis, it was determined that the best course of action would be to trial a dryer fabric that would actually hold the sheet more effectively around the single run section.
Albany chose to apply Aerogrip, a proprietary enhancement that actually changes the surface of a dryer fabric from a hydrophobic nature (repels water) to a hydrophilic nature (attracts water). Standard dryer fabrics are made from petroleum-based materials, so they actually have no attraction to a wet sheet of paper. Aerogrip changes that by providing an attraction to the moisture in the sheet. Along with hydrophilic surface properties, these fabrics also have a significantly higher coefficient of friction than a normal dryer fabric does, helping to ensure that the sheet will stay with the fabric. The mill agreed to attempt the low risk trial.
Results: After the new dryer fabric was installed, the wrinkles were successfully eliminated and the mill was able to sell its edge rolls. The sheet stayed with the dryer fabric throughout the entire run, eliminating the flutter that led to wrinkles. Based on these results, the mill estimates cost savings of about $1.2 million per year. The savings are substantial even though the mill only runs this wide deckle width a few days each month. Along with the cost savings, the mill saw better machine runnability, increased sheet stability, and higher sheet quality as a result of the partnership with its supplier.
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