Modeling the physical processes relevant during hot pressing of wood-based composites. Part I. Heat and mass transfer

A three-dimensional model based on fundamental principles to simulate heat and mass transfer and mat densification during batch or continuous hot pressing of wood-based composites is presented in this pair of papers. The present paper gives a comprehensive description and mathematical formulation of the relevant heat and mass transfer mechanisms, which are water vapor and air transfer by gas convection and molecular gas diffusion, conductive and convective heat transfer, including sorptive effects. The heat and mass transfer model consists of a set of constitutive flux equations that are coupled by local energy and mass balances. Material property data used as input parameters for the model are provided for medium density fibreboard (MDF). These properties are highly dependent on one or more of the internal mat conditions, being temperature, moisture content and density. The determination of mat permeability, obstruction factor for molecular diffusion, and thermal conductivity was part of the project described here, while the specific heat, heat of sorption, and hygroscopicity data were taken from the literature. In addition, physical property data for the water vapor-air mixture are listed. The model may be used as a tool to optimize the manufacture of existing natural fiber composites, to aid in the development of new products and technology for their production, and for training purposes.