Heat And Mass Transfer Hot-pressing Model Of Medium Density Fiberboard

Hot-pressing is an important process, which both affects the quality and the quantity of the products. There are a lot of factors that affect the hot pressing, and they are relative to each other. Therefore, it is very difficult to understand this process only through experiments. However, simplifying this process properly and establishing the model makes understanding the hot-pressing possible. The comprehensive hot-pressing models published these years which the adhesive was took account into was analyzed in this research. The aim of this research is to improve the sensitivity of the heat and mass transfer hot-pressing model and make some experimental exploration on the effect of adhesive to the permeability coefficient of the mat. The research results would have an important value for the further study.A two-dimensional theoretical model for heat and mass transfer in medium density board (MDF) mats during hot-pressing was developed. Mechanisms included in the analysis are water vapor and air transfer by gas convection and molecular gas diffusion, conductive and convective heat transfer, and sorptive effects. The model mainly consists of vapor and air constitutive flux equations and energy conservation equations. A finite difference method was used to get the numerical results of the equations. The upwind scheme is applied to the flow part of the differential equations to ensure the stability of the numerical process. Finally, the results are obtained by the execution of the program developed with the software Matlab 7.0. The variables the model could output are temperature, whole gas pressure, partial pressure of the vapor and the air, the density of the vapor and the air, relative humidity, moisture content, water evaporation velocity, the gas flow velocity in the directions that are parallel to the plane and perpendicular to the plane.The results of the model agree with those of experiments well. It indicates that it is possible to use the established model to analyze the heat and mass transfer process happened in the hot-pressing and discuss the internal environment development in the mat.After simulation of several different vertical density formation profile, it is discovered that the vertical density formation profile plays an important role in controlling the initial time and rate of the rapid increasing process of the gas pressure curve.The method, viewing the temperature curve of the center in the mat as a base line and then dividing the curve into five period, proposed by Kavvouras(1977) was used to explain the output results of the model.The result of the experiment indicates that the gas pressure in the mat during the hot-pressing is affected by a lot of factors. Therefore, it is very difficult to evaluate the obstacle of the adhesive to flow of the gas in the mat by measuring the gas pressure in the mat. The rapid decease in the gas pressure curve of the later period of the hot-pressing is very special, which is never reported in the literature. The possibility of the effects of adhesive and the development of the vertical density during the hot-pressing is eliminated. An explanation of the rapid movement of the vapor in the direction that is perpendicular to the plane is proposed. However, the exact reason of this phenomenon is unclear, which needs further investigations.