Simulation of the Internal Conditions During The Hot-Pressing Process

The development of a two-dimensional mathematical model to describe the internal conditions during the hot-compression of wood-based composite panels is discussed. Five primary variables were considered during the model development: air content, vapor content, bound water content, and temperature within the mat, and the extent of the cure of the adhesive system characterized by the cure index. Different heat and mass transfer processes were identified for the transport of the heat and of the moisture phases. The heat was transported by conduction and convection due to a temperature gradient, while the water phases were transported by bulk flow and diffusion due to total pressure and partial pressure gradients. The resulting differential-algebraic equation system was solved by the method of lines. The spatial derivatives of the conduction terms were discretized by central differences, while the spatial derivatives of the convection terms were discretized according to an upwind scheme. The resulting ordinary differential equations in the time variable were solved by a freely available differential-algebraic system solver (DDASSL). The mathematical model predicted temperature, moisture content, partial air and vapor pressures, total pressure, relative humidity, and extent of adhesive cure within the mat structure under a typical hot-compression process. A set of three-dimensional profiles describes the evolution of these variables with time, in the thickness and width dimensions of the mat. The model results allow a better understanding of the interacting mechanisms involved in a complex production process. The model also supports optimization of the hot-pressing parameters for improved quality of wood-based panel products, while reducing pressing time.