Causes of shrinkage and curling of paper sheets produced by paper mills

The fundamental reason for the shrinkage and curling phenomena that appear after the paper sheet is finalized is that the speed ratio of the pulp to the mesh is inappropriately selected when the slurry is fed to the mesh, and the proportion of fibers arranged in the longitudinal direction is too high.

Because as far as a single fiber is concerned, it's transverse expansion rate is several times higher than the longitudinal expansion rate. Due to the pattern characteristics of the paper machine during paper making, the fiber arrangement of the paper is in the longitudinal direction (that is, the running direction of the paper machine). Most of them are arranged. If there is no reasonable pulp-to-web speed ratio, the fibers will be basically arranged longitudinally in the paper. In the end, the paper will shrink in the width direction of the paper due to the sharp shrinkage of the fibers during the drying process or curled.

The following methods can be used to alleviate and eliminate the above problems: (1) appropriately adjust the water level of the head tank to keep the pulp-net speed ratio within a reasonable range as much as possible. (2) Reduce the traction tension in the wet paper state, especially the traction tension in the open paper-feeding parts of the wire section and press section. The tension should be reduced as much as possible while ensuring the normal operation of the paper mesh. (3) Tension the dryer felt or dryer fabric in the dryer. Moderately tighten the dryer felt or dryer fabric to prevent the paper web from shrinking freely during the drying process in a loose state. If some felts have local inconsistencies in tightness, they must be adjusted or replaced in time, otherwise paper defects such as local shrinkage and unevenness of the paper sheets will occur. (4) Check and adjust the drying cylinder temperature curve to keep the drying cylinder temperature curve smooth and avoid excessive temperature difference between the upper and lower drying cylinders. Otherwise, the resulting drying on both sides of the paper web will be inconsistent, which is also the main cause of paper curl.