22.214.171.124 Application and Function
126.96.36.199.1 Corrosion and Wear Protection
Covers for paper, wire or felt guide rolls have to protect the metallic roll body against corrosion or wear. In general, these covers are plain and relatively hard materials are used. In drive roll applications the cover has to transfer the torque to the driven wire or felt, depending on machine design, softer elastomeric covers are also applied.
188.8.131.52.2 Nip Design in the Press Section
In the press section the individual rolls are hard or soft covered. The press nip (Fig. 6.7) is created either by two profiled rolls or by a combination of one profiled and one plain roll. Covers with high open surface area, similar to tyre technology, offer the necessary storage capacity for the water squeezed out in the press nip. The various surface designs include grooves, blind drilled holes or suction holes as well as combinations of these.
The press nip geometry is mainly defined by the diameters of the press rolls, the deformation characteristics of their covers and the compressibility of the felts. The deformation characteristics depend on the dynamic Young’s Modulus of the cover material, the thickness of the cover and its surface design. The press nip geometry defines the nip pressure profile, which in turn affects dewatering in the press nip and thus the final paper sheet properties. Optimization of press dewatering means optimum press nip geometry as well as optimum water storage capacity and in¬cludes the rolls with their covers and surface geometries as well as the felts.
184.108.40.206.3 Release Properties
In some press designs with hard plain press rolls, the paper web comes into direct contact with a cover or coating. It needs to be pulled off the roll surface uniformly with preferably low force as it exhibits low strength due to its high water content. Web elongation in the machine direction also needs to be reduced to a minimum. Therefore, easy release of the paper web from the cover or coating is required for good paper quality and high machine runnability. This can be achieved by a special cover or coating material design, which has been optimized with regard to hydro-philic/hydrophobic areas and defined surface porosity.
Coating of drying cylinders in some positions aims to reduce the deposition of stickies or color on the roll surface. The release characteristics of the coating allow easier removal of the deposits by doctoring.