Analysis of crescent former paper manufacturing.

Abstract

The mechanics of paper formation in a crescent forming machine is studied using two models. These models incorporate four physical mechanisms including (1) the fluid mechanics of the fiber/fluid suspension, (2) the drainage of water through the mat and wire, (3) the structural mechanics of the mat and wire, and (4) the contact of the mat/wire on the roll former. A one-dimensional model is firstly developed to capture the dynamic and steady-state forming process in the machine direction. This one-dimensional model follows from an earlier model by Turnbull et al. (6), but includes fluid viscosity for the fluid layer between the roll and the mat. A new treatment of the dynamic contact of the mat/wire on the form roll at the nip is introduced and represents a significant simplification of the treatment by Turnbull et al. (6). Numerical solutions of the improved model are used to highlight the influence of fluid viscosity for both steady-state forming conditions and for dynamic forming conditions created by process disturbances. In addition, a simple and accurate closed-form estimate of the steady-state forming length is introduced. Following the one-dimensional viscous model, a two-dimensional viscous model is developed to examine the variations of mat thickness in the machine direction (MD) and the cross machine direction (CD) in the forming region. This study examines the CD variations of mat thickness due to the influences of major classes of steady disturbances, and the coupled MD and CD variations due to dynamic process disturbances in the fluid jet. Among the steady disturbances considered, nonuniform fluid velocity in the CD generated at the entrance to the forming region significantly amplifies the CD variations in the final mat thickness. Consideration of unsteady disturbances reveals that the coupled MD and CD variations in mat thickness are magnified near the fundamental natural frequency of a translating wire for disturbances having wavelengths longer than the forming length.