DIFFERENTIAL CD SHRINKAGE OF PAPER WEBS

In the drying section paper shrinks in cross direction (CD), whereas t he opposite effect is observed in machine direction (MD), i.e. paper e longation. By means of deterministic wire marks imprinted on paper sur faces differential CD shrinkage of paper can be determined, when corre ctions are applied to account for the distortion of the fabrics in the paper machine. Shrinkage is small and roughly constant in the central part of the web and increases significantly within 1 m-1,5 m at the e dges. This shrinkage shape is independent of the type of the drying se ction. Newsprint dried on single - tier drying section leads partly to a higher shrinkage close to the edges than papers manufactured on pap er machines with two-tier driers. An advantage of papers of single-tie r drying sections is the low total shrinkage and a smoother shrinkage profile. It turned out that the shrinkage profile is constant over a l ong period of time. An analytical method based on the measurement of t he geometry of wire mark patterns has to consider the deformation of w ires affected by their manufacturing process and the wire section. Fur thermore, it becomes evident that the top wire of twin-formers shows s ometimes considerable deformations. It is unknown to which extent thes e deformations affect the uniformity of drainage in the wire section.