PAPER MACHINE CLOTHING

As customers demand higher quality sheets, mills seek quality and production improvements from paper machine clothing suppliers to remain competitive

Clothing Suppliers Offer Quality, Efficiency with Clothing Design and Analysis

In addition to addressing quality-related competitive issues, paper machine clothing suppliers are also working with mills to help increase production efficiency and cut costs.

Partnering with mills—and sometimes paper machine manufacturers, as well as end-customers such as printers—clothing suppliers are suggesting fabric, as well as process changes, to help bring about the required improvements. In case-study format, this article shares information on how paper machine clothing suppliers use product and process analysis to help mills stay competitive.

FORMING

New Fabric, Headbox Changes Boost Efficiency and Reduce Streaking

Problem: A northeastern mill that produces printing and publishing papers was experiencing efficiency problems with its No. 2 paper machine, as well as streaky sheet formation at the headbox. To address these problems and additional ones on its other paper machines, the mill’s productivity improvement team asked for help from its approved paper machine clothing suppliers.

Project: As part of the mill’s efficiency improvement program that started in mid-December 1997, Appleton Mills was assigned to work on problems with the No. 2 paper machine. The mill’s specific goal was to boost efficiency by 3% in 60 days. To improve the No. 2 machine’s efficiency rate, Appleton Mills suggested the following changes and worked with the mill to implement them:

• Change breast roll shower position

• Eliminate press section pluggage problem

• Install camera system to detect hole/break locations and causes

• Improve machine cleanliness

• Install a new single-layer forming fabric— Design 952

• Conduct extensive analysis of operational problems in the dryer section.

To address streaky sheet formation at the headbox of the No. 2 machine, Appleton Mills partnered with Voith Sulzer. Voith replaced seals, added packing to rectifier rolls, and reduced clearance between the cheeking piece and apron.

Results: After the first 60 days of the program, the No. 2 paper machine showed a 4.1% increase in efficiency, exceeding the mill’s initial goals. And, from the December 1997 project startup until February 1998, a 13.4% total efficiency increase was achieved (Figure 1).

In addition, other operational improvements were noted. The new single-layer forming fabric decreased draw from 39 fpm to 23 fpm—a 41% reduction. Also, improvements in the overall dryer section operation were a result of the project, and it was decided that two new dryer fabrics for new positions would further improve operations in this area. Furthermore, as a result of the headbox changes, streaking was significantly reduced.

Fabric Weave Changes Improve Print Surface Quality, Sheet Formation

Problem: An ongoing issue for mills that produce coated papers is how to aggressively pursue improvements in base sheet formation and print surface quality. At two mills producing fine papers, questions existed about the real impacts of fabric shed variation on coated paper formation and print quality. The mills eventually teamed with their paper machine clothing supplier to determine these impacts.

Project: To identify joint objectives for the teams, Asten worked with mill personnel to create detailed, written test plans. After these plans were completed, Asten began working with mills to conduct the trials.

At the first mill, the objective of the trial was to determine if sheet formation and printing surface could be improved by changing fabric weave construction on a Symformer base fourdrinier position. The decision was made to evaluate the impact of moving from a 7-shed product to an 8-shed surface with UltraLife construction. Asten decided to measure the impact of this change on 20-lb copy paper, with formation improvement as the primary goal. Formation measurements were taken over several weeks using mill laboratory equipment.

In this trial, top wire construction was not changed. In addition, the number of machine direction and cross direction strands in the fabric remained constant at 162 per inch so as not to complicate the trial.

In trials at the second mill, the Asten/mill team sought to determine if a change in mesh quality would have a real impact on sheet formation and print quality. In these trials, the machine direction yarn count was increased from 144 to 162—a change of 12.5%—while keeping cross direction count constant in the 8-shed UltraLife weave pattern. Measurements were made on 35-lb to 50-lb coated sheets on a single wire fourdrinier.

Results: At the first mill, the 8-shed fabric with UltraLife construction proved that it could improve formation quality for the mill. Trials showed that a significant shift occurred on fabric installation. A 20% to 25% improvement in the formation number also continued over the life of the fabric. In addition, dimensional stability relating to width and length was reported as very good with the new fine-mesh 8-shed weave design. The outcome of these trials significantly improved paper formation quality and set a new benchmark for evaluating future improvement at the mill.

At the second mill, the formation number at the lighter-coated sheet weights improved by 42% and Parker Smoothness improved by 0.3 units. This change provided better machine productivity, as well as improved converting properties.

Parker Smoothness test numbers have been known not to change even when beneficial sheet printing characteristics are evident. In this situation, surface image analysis is an effective tool for verifying and measuring real changes. In both tests, improvements were seen in the paper surface by image analysis techniques confirming other mill data.

Triple-Layer Forming Fabric Removes Wire Mark in Newsprint

Problem: A North American newsprint supplier needed to improve print quality and reduce the two-sidedness of its sheet. The mill had made changes to forming fabric designs, but with little success.

The initial fabric used by the mill had high drainage and non-uniform fiber support with irregular drainage holes. Though trials on other fabrics saw better fiber support, low drainage capacity combined with high drainage velocities were forcing the short fibers into the fabric structure. The results from trials with other fabrics showed wire marks, a high number of embedded fibers in the fabric body, a wet sheet off the couch, and poor paper machine efficiency.

Project: After conducting its own trials, the mill approached JWI to assist with a solution. JWI technologists then performed a detailed engineering analysis on a number of printed and unprinted sheets using its Image Analysis equipment and software programs.

To find the source of the problems, a fiber-length analysis was taken of the headbox stock and a complete machine audit of the forming, pressing, and drying sections was performed to determine the impact of each section on the quality of the final product. It was discovered that the combination of short fiber length, high drainage velocities, and turbulence resulted in significant micro-density variations and a rough surface on the sheet. These elements directly affected both the uniformity of ink laydown and uniformity of ink absorption into the sheet. The final printed product had a mottled appearance on the surface.

The solution recommended by JWI’s engineered approach was to select a fabric design with a high drainage area and with high uniform fiber support. The fabric selected was Integra—JWI’s new triple-layer forming fabric.

Results: After the Integra installation, the mill began producing a sheet that was uniform in both micro-density and smoothness. An added benefit of the new fabric was improved operating efficiency and drainage, since the fiber was not forced into the body of the fabric.

In addition, a controlled experiment was conducted by the mill in five press rooms using letterpress, offset, and flexographic printing processes. Rolls from the original trials were run immediately prior to the rolls from JWI’s engineered approach. Every press room reported considerable improvement in print quality with the Integra-produced paper. As shown by the printing samples, wire mark was eliminated (Figure 2).

PRESSING

Grooved Belt Technology Breaks Production Records at Containerboard Mill

Problem: As part of a cost-reduction program at its Big Island, Va., containerboard mill, Georgia-Pacific (G-P) analyzed several areas to find the largest benefits at the lowest costs. One of the most obvious ways to achieve such an improvement was to remove more water from the sheet as it passed through the presses, leading G-P to examine the latest in grooved belt technology for its No. 3 paper machine.

The No. 3 paper machine was designed to utilize Beloit’s ENP-O in the press section, which uses a polyurethane-coated belt developed by Albany International. This type of press offers improvement over conventional presses and removes a significant amount of water with its wide nip and high loads. However, the press was removing all the water it possibly could with its current configuration and belt design.

Project: To determine the appropriate technology for the problem, G-P consulted with Albany International’s Process Belt Group. Albany International’s solution was the Ventabelt. This grooved belt product offered more void volume in the nip, thereby increasing dewatering capacity and yielding a drier sheet.

Results: Since the Ventabelt was installed, the No. 3 paper machine, with its tandem fabric press, has broken 11 production records previously established in 1991. In addition, speeds have increased across all paper grades.

The Ventabelt has also reduced the amount of water in the sheet before the dryer section, and steam consumption has decreased from 1.8-lb to 1.6-lb per pound of paper. Further improvements include an increase in press solids by an average of 2.5% (Figure 3).

G-P has increased average daily production at the Big Island mill by 83 tons, and 48 tpd can be directly attributed to the installation of the Ventabelt—a bonus of almost $19,000 daily. At the same time, a new forming board accounted for an additional 10 tpd, while a Lazy steam shower provided approximately 15 tpd.

Double-Layer Laminated Fabric Reduces Blowing Problems in First Press

Problem: A mill in the Midwest was having problems with “blowing” in the first press on one of its paper machines. This machine produces a wide basis-weight range of fine paper at speeds up to 1,200 fpm. The two presses of this machine are straight through, with the first press bottom felted and the second press top felted.

Project: Since the paper mill superintendent believed that the blowing problem was related to the press fabric designs in use at the time, a paper machine clothing supplier was contacted. Cristini North America was given an opportunity to supply a first press fabric on the basis that it could eliminate or drastically reduce the blowing problem.

Following an in-depth study of the press section by Cristini’s design and application team, combined with papermaking input from the mill, a decision was made to try Cristini’s patented Hydrosmooth fabric on the first press. The Hydrosmooth design supplied was a conventional double-layer laminated fabric with the batt made of Cristini’s Dynabelt machine direction oriented fibers. Prior to the trial, the machine was running double layer, pin-seamed fabrics made with typical cross-lapped batt.

Results: In early December 1997, the Hydrosmooth trial fabric was installed on the first press. To the satisfaction of mill supervision, the blowing problem was immediately eliminated.

In addition, the first Hydrosmooth trial fabric ran a record of more than 50 days vs a normal life of 30 days. Based on this initial trial, the mill ordered additional first press fabrics, as well as second press fabrics.

Process Changes Increase Press Fabric Life

Problem: A paper mill providing copy/bond grades made modifications on its pickup position to increase production by 7%. Among other modifications, a steam box was installed in the press section to improve water removal. This modification was successful in improving water removal and increasing machine speed, but fabric life was reduced from an average 44 days to less than 21 days, as shown in Figure 4.

In addition, analysis of new and used fabric samples showed a significant increase in fabric weight loss, as Figure 4 also shows. This indicated that all fabrics were simply wearing out at an accelerated rate that was far faster than a simple 7% increase in speed should produce.

Project: As a major supplier for the pickup fabric, Wangner Systems (WSC) was contacted to join a task force for identifying problems. After an extensive investigation, WSC’s technical support team identified five major possible causes of problems:

• An increased number of nip cycles due to increased machine speed

• Increased chlorine carryover in the stock

• Use of chlorine dioxide as a biocide in the water

• Increased fabric and sheet temperature accelerating the degradation of the fabric

• Abrasive action occurring between the inorganic fillers and the fibers.

To eliminate the problems, WSC made the following suggestions:

• Discontinue using chlorine dioxide in the water system and, in its place, use hypobromous acid to reduce the degradation of the nylon fibers in the fabrics.

• Use a proprietary anti-chlorine agent in the blend chest to neutralize the carryover chlorine.

• Monitor the chlorine level in the stock and water to less than 0.5 ppm.

Results: The corrective actions suggested by WSC increased fabric life significantly, as seen in Figure 4. The mill is currently using scheduled maintenance downtime every 28 days to change the fabrics. However, the weight-loss reduction indicates the fabric could run longer on the machine if needed.

Laminated Pickup Fabric Reduces Downtime for Fabric Changes

Problem: A mill that produces printing, writing, and copy paper grades needed to increase production on one of its paper machines without affecting the paper quality. In addition, the mill was having problems with filled, compacted fabrics in the pickup position.

The paper machine, a fourdrinier with a Tri-nip press, traditionally scheduled fabrics off after 30 days of operation to avoid having to shut down at unscheduled times. However, the pickup fabric often filled up or became plugged as the fabric compacted, which could happen at any time during the 30 days. The filled, compacted fabrics were requiring increased drying, reducing machine speed, increasing paper breaks, and often causing uneven moisture profiles that are unacceptable in sensitive copy paper grades.

Project: To correct these problems and increase production, Weavexx technicians and design engineers convinced the mill to trial a laminated pickup fabric that had been successful on similar machines and grades. The Profiler laminated fabric provides improved stability over standard double-layer fabrics. Its design also offers a free-flowing structure to provide improved water removal and effective sheet pickup characteristics. In addition, this laminated fabric’s open design resists plugging and filling and is therefore easier to clean.

Results: The Profiler pickup fabric ran for over 80 days until it was scheduled off. In addition, the fabric broke in well and showed little degradation in caliper and operating properties over its life.

The new pickup fabric was also an important factor in record-breaking production during its life. The mill now exclusively runs this fabric in the pickup position, resulting in reduced shutdown time for fabric changes, as well reduced downtime for paper breaks.

DRYING

Fabric with Hydrophilic Coating Lets Mill Run Lower Draws

Problem: A high-speed, lightweight paper machine consistently ran a 2.5% draw between the press section and first single run in order to maintain good threading and consistent sheet edge control. In addition, draw was used to set the sheet width, and, if it was less than 2.5%, sheet width increased. Though this effect is a benefit on most machines, a wide sheet can be a problem because existing blow boxes have an effective slot width inside the dryer fabric. Mill personnel wanted to run lower draws, but the wide sheet would flip excessively because the sheet edge was outside the holding zone of the blow box.

Project: To correct the problems, the mill contacted Albany International. Albany International recommended that the mill install a Bel-Plane dryer fabric enhanced with Aerogrip fabric coating. Bel-Plane met the requirements of a single run fabric for caliper, surface, seam uniformity, and cleanability. Aerogrip further improved Bel-Plane by making the fabric surface more like the sheet. The shift from hydrophobic polyester, which is the material of construction for Bel-Plane, to hydrophilic Aerogrip made the fabric more compatible with the sheet for better fabric holding capability, improved runnability, and increased speed.

Results: After installation of the Bel-Plane fabric, the desired lower draws were set. The sheet remained calm and flutter-free even at the edges, which were outside the blow box. In addition, the new, enhanced fabric held the sheet outside of the box width. Because of the changes, the mill was able to take advantage of lower draws and realize a wider sheet, better profile, and fewer breaks for higher uptime efficiency.

Smooth Fabric Weave Removes Sheet Surface Imprint

Problem: A mill that produces heavyweight brown paper had increased paper machine speed beyond the limits of the press section, causing the sheet that entered the dryer section to become very wet—less than 38% solids. This increased wetness into the dryers left the sheet very impressionable, resulting in a dryer fabric imprint on the sheet’s printing surface from the unirun and second top dryer fabrics. This dryer fabric imprint could be matched on the sheet surface without visual aids.

Though the fabric designs used on these positions had a fairly smooth surface, they were not smooth enough to overcome the marking problem. Since the mill’s product required a good surface for color printing, it was crucial to eliminate these marks.

Project: To correct the problem, the mill contacted Weavexx, who began a trial at the mill with its SST R dryer fabric. This fabric has a high machine direction mesh of thin, round yarn, and its weave pattern creates a surface in which the machine direction yarn is in sheet contact for 66% of its travel through the fabric.

Results: The SST R dryer fabric was first installed in the unirun section, where it eliminated most of the mark. After adding the SST R dryer fabric to the second top position, the marking problem was completely eliminated, allowing the sheet to harden smoothly before going on to more conventional surfaces.