Holes (Pick-outs, Slime)
Some detective work may be required to determine the cause of holes in a web of paper coming from a paper machine. Even more detective work may be needed if the holes are causing breaks, so that the web doesn't make it to the reel. Besides causing web breaks on the paper machine, holes also cause breaks during subsequent coating. Customers usually will reject product that has more than a very rare incidence of holes, and papermakers spend a lot of time and effort splicing paper in order to remove and holes that have been automatically detected during production.
One of the first things that you will want to determine is the general category of the hole problem. Three primary causes of holes in paper are (a) tacky materials on rolls or other rotating elements within forming, pressing, or drying equipment on paper machines, (b) slime, and (c) equipment defects. Tacky materials on a roll can cause holes by "picking out" the fibers from the part of the paper that comes into contact with them. Consequences of this effect can range from a mild roughening of the paper surface, to the accumulation of dust or "crumbs" of fibers in the system, to partial delamination, to the development of holes in the paper by a pick-out mechanism, and ultimately to web-breaks. Bacterial slime can cause holes in quite a different way. It is useful to keep in mind that bacterial cells are composed mostly of water. When an agglomerated mass of such cells (often with inclusion of other materials) passes from the headbox and gets into the paper sheet, the area occupied by the slime tends to exclude any fibers that could provide strength to the paper. As a consequence, that slime spot tends to fall out of the web, so that the slime does not remain in the paper as a spot that can be analyzed.
A key type of evidence, to narrow down the cause of holes, is whether or not there is a pattern. A repeating hole, always in the same location, often can be traced back to its point of origin. Sometimes the spacing of such a hole or tear can be matched to the diameter of a roll in the press section or the circumference of the forming fabric, etc. In addition to pick-outs caused by tacky deposits, regularly recurring holes also can be caused by mechanical imperfections in the surfaces or rolls or fabrics to which the web of paper comes into contact. By contrast, if the incidences of holes seem to be totally random, then such evidence would suggest a slime problem is likely. In paper machine systems where holes have a significant impact on profitability, it is recommended to install video cameras at key points within the system in order to be able to narrow down where the holes are occurring, especially if they result in web breaks. Online detectors for holes and breaks are widely used.
Even though a hole is, by definition, empty space, sometimes papermakers can learn about their origins by observing their shapes and having analyses performed on the material around their edges. Stain tests are available that can reveal the presence of high levels of bacterial slime. On the other hand, if tacky materials are causing pick-outs, one also should look to see if some of the same material is transferring to the paper surface as spots.
Strategies to deal with slime and other issues related to bacteria and fungal growth in the wet end are suggested elsewhere in this guide. Sometimes gel-balls or "fish-eyes" of poorly dispersed retention aid polymers can mimic the behavior of slime. The retention aid make-down system and associated canister filters should be checked. Also, you may wish to review some of the material related to deposits and stickies in order to overcome problems with holes in a web of paper.
Some other possible causes of holes in paper include water drops from the ceiling, fiber bundles or lumps breaking away from the sheet, or a hole in the forming fabric.
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PLEASE NOTE: The information in this Guide is provided as a public service by Dr. Martin A. Hubbe of the Department of Wood and Paper Science at North Carolina State University (email@example.com). Users of the information contained on these pages assume complete responsibility to make sure that their practices are safe and do not infringe upon an existing patent. There has been no attempt here to give full safety instructions or to make note of all relevant patents governing the use of additives. Please send corrections if you find errors or points that need better clarification. Go to top of this page.