Hygro-viscoelastic stress analysis in paper web offset printing

Abstract

A model, using the finite element method (FEM), was developed for the analysis of the time and humidity-dependent stresses of paper webs at offset printing press conditions. The offset press was simplified to a flat thin moving web and its unit process steps were described by combinations of prescribed displacements, tractions and moisture ratios. Based on previous modelling and experimental findings, a convective hygro-viscoelastic model of the paper web was derived. The constitutive law and some of the boundary conditions were implemented as user-defined subroutines into commercial FEM software in a displacement-based framework. Some simulation examples were used to study the applicability of the proposed formulation in the analysis of web-fed offset printing at stationary conditions. The numerical stationary solutions converged rapidly and the model was robust for the investigated conditions. Incorporating effects from moisture, the numerical results show web tension profiles along the analysed model press comparable to those found by others from full-scale newsprint printing press trials. Effects of altered levels of the peripheral velocities of rollers, friction, web velocity, moisture and web properties on in-plane stresses and strains are presented. In order to exemplify the practical use of the model for runnability investigations, different analysed quantities obtained from the numerical solutions were related to web tension, web widening, misregister in multi-colour printing, sheet size analysis, conditions for web stability and traction conditions at drag rollers.