Modeling the physical processes relevant during hot pressing of wood-based composites. Part II. Rheology

A rheological model to describe the development of the vertical density profile and of internal stresses within wood-furnish mats during hot pressing is presented in this paper. The rheological model is part of a comprehensive three-dimensional simulation model that accounts for those mechanisms most important during the pressing process, including heat and mass transfer inside the mat and adhesive cure. To model the rheological behavior of the mat, the four-element Burgers model commonly used to describe visco-elastic material behavior has been expanded with the addition of a fifth element that represents plastic and micro-fracture related deformation. The coefficients of the non-linear model are highly dependent on the material conditions. Equations of the coefficients as a function of temperature, moisture content and density, as well as a mathematical formulation of the five-element model is presented in this paper. Furthermore, model predictions for both a batch and a continuous press are given. A comparison with experimental results shows that the expanded Burgers model is suitable to predict typical features of the vertical density profile, such as the development of density maxima near the surfaces, shoulders or side maxima as a consequence of intermediate or final densification steps, and differences in the density profile between the mat center and the edges in the horizontal plane. Such agreement provides the basis for a wide range of industrial and research applications.