Modelling of hot pressing of MDF
The vertical density profile (VDP) has long been recognised as a critical determining factor for the strength and quality of MDF panels. This has led several previous workers to develop phenomenological models to predict the VDP during hot pressing. The models have included the processes of heat and mass transfer within the mat; rheology of the mat during pressing, including creep; kinetics of resin curing. The objective of this, as with previous models, is to assist in obtaining lower energy consumption, better quality pressed boards and more flexible operation in commercial plants.
This paper attempts a more complete integration than hitherto of all these component processes in a one-dimensional model of pressing. The heat and mass transfer part of the model predicts the moisture content changes, temperature profile, partial vapour pressure and total gas pressure across the thickness during closing of the press as well as after the final platen position is reached. The correlation used for the calculation of equilibrium moisture content is an improvement over that used in previous models.
This model includes the mat mechanical and rheological properties, varying with moisture content and temperature under pressing force. The simulation will predict pressing pressure, strain and density across the thickness during pressing, using a Maxwell-element model for stress relaxation and visco-elastic properties. The rate of resin cure at the various temperatures in the panel dictates the relative compression at different points during the press closing. The polymerization kinetics of phenol-formaldehyde resin is included in the model to allow prediction of the rate of resin curing, amount of polymerization during the hot pressing of MDF boards. The use of Matlab to solve the model equations gives a fast solution while providing a very convenient platform for producing graphical results.
The simulations results were validated by experimental measurements in a pilot press. Twenty two MDF boards were made with different pressing parameters and the data collected were compared with the simulation results from the model. The model could predict in an acceptable way the main variables that control the manufacturing of MDF boards. The simulation results for steam injection pressing and new cooling technology in continuous presses is also generated to increase the understanding of internal processes.