Trends and Developments in Size Press Technology, 1990 Papermakers Conference Proceedings

Charles P. Klass--Hydrodynamic forces in the ingoing nip, which cause pond turbulence and nip rejection, limit the practical application range of the conventional two roll size press; these forces increase with machine speed and viscosity. Resistance to pressure induced migration and/or wet tensile strength are needed for runnability and to control pickup. Film splitting limits the practical use of the size press to apply pigmented coatings. Modifications in size press design, such as larger roll diameters and plastic aprons can re-duce the pond turbulence problems and provide better runnability. Recently, several modified size press designs have been introduced. These premetering size presses use short dwell coater heads to apply size to the rolls for transfer to the sheet. The choice of metering element, blade or rod, has a major impact on sheet quality. Recirculation rates in these modified size presses is high. Use of a perforated sealing blade ahead of the metering element can reduce recirculation rate. A wire wound roll can also be used to get high solids application at low recirculation rates. The new modified size presses make it practical to control surface size solids and pickup as independent variables. The depth of penetration of size into the sheet can be controlled. These factors contribute to improved sheet quality. Surface sizing at higher solids decreases after dryer load and can allow higher machine speeds.