High frequency heating of medium density fiberboard (MDF): theory and experiment

Medium-density fiberboard (MDF) is a wood-based panel manufactured from wood fibers with a synthetic adhesive through the conduction of heat from the hot platens. One alternative is the use of high-frequency (HF) heating, which has the advantage of reducing the press cycle, the platens temperature and the post-curing time, with constancy of resin formulation. For this approach, an electromagnetic (EM) heating model was developed and coupled with a three-dimensional model for heat and mass transfer and resin polymerization already existent. A solution of the Maxwell equations is used to determine the spatial distribution of the electric field inside the material and the absorbed power. MDF mat was assumed as a porous and homogeneous material at macroscopic level and a dielectric mixture at microscopic level (volume of control). The mat dielectric properties (wood fibers/particles and UF resin) were estimated using a “rule of mixture” and depend on local temperature and moisture content during the heating process. This dynamic model was used to predict the evolution of the local variables related to heat and mass transfer (temperature and moisture content), as well as the variable connected to the EM behavior (dielectric properties of the mat). The model performance was analyzed using the experimental results of HF ( 13.56 MHz ) MDF heating. It was concluded that the model could suitably predict the evolution of the internal mat temperature during heating.