Modelling of Vertical Density Profile of MDF in Hot Pressing
The hot pressing operation is one of the most important operations in medium density fibreboard (MDF) manufacture. Complicated dynamic interactions occur during pressing, including heat transfer, moisture movement, development of gas pressure, internal stress development and relaxation, wood consolidation, resin curing, bonding between particles and eventual development of a nonuniform density distribution through the panel thickness. Consequently the mat experiences continuously changing internal conditions (temperature and moisture content) as the pressing operation proceeds.
The vertical density profile (VDP) is one of the most important properties that determine MDF strength and physical properties. The influence of the VDP on the board properties is generally recognised, but the formation of the density profile and their specific effects on the board performance have proved difficult to quantify.
A mathematical model based on theoretical analysis and experimental information is being developed. In the model, the mat is divided into a number of thin parallel layers. The deformation of each layer is a function of stress, temperature and moisture content of the layer. The model considers the variation of the mat mechanical and rheological properties with moisture content and temperature. The changes in temperature and moisture content are provided by a separate heat and mass transfer model. The press model can predict internal pressure, layer deformation and density across the thickness during pressing.
The performance of the model was validated by experiments conducted in a pilot-scale press. Twenty two MDF boards were made with different pressing parameters, and the VDP were measured and compared with the simulation results from the model. The model could predict the density profile with an acceptable accuracy for the main variables that control the manufacturing of MDF boards.