Finite element modelling of calendering — some aspects of the effects of temperature gradients and structure inhomogeneities

The compression of a paper web in a calender nip has been simulated using the finite element method. The mechanical properties of the paper were allowed to vary in the thickness and machine directions of the web. This was done in order to model the influence of temperature gradients as well as density variations (due to the presence of fibre floes) on the deformation behaviour in the nip. Paper was assumed to be an elastic-plastic material exhibiting strain hardening. The yielding behaviour was governed by the Drucker-Prager yield condition. Simulations of the deformation behaviour when paper was subjected to a temperature gradient, clearly revealed that the deformation gradually became more concentrated towards the surface layers as the temperature of the surface increased. This is in accordance with experimental results which indicate that temperature-gradient calendering promotes the surface properties, whereas the bulk of the structure is preserved. Modelling the deformation behaviour of a structure containing density variations reveals that the paper may contain an inhomogeneous strain distribution after unloading, i.e. after passage through the nip.