Measuring and simulating the effects of the pressing schedule on the density profile development in wood-based composites

The cross-sectional density profile of wood-based panels has a strong impact on many of its end-use properties. In modern panel manufacture, not only press closure but also changes in mat thickness later in the pressing cycle are of importance for density profile development, particularly in the MDF process. A full-factorial analysis of the effects of the pressing cycle on the density profile is presented, with the factors considered being press-closing time, mat thickness level after first densification, and time of secondary densification. It was found that the mat thickness level after first densification dictates the density difference between surface and core regions. For pressing schedules that include a second densification step, intermediate density maxima appear, with magnitude and position of the intermediate density peaks being determined mainly by the first density level and the time span between press closure and final densification. Computer simulations are used to link the density variations to the conditions of local temperature and moisture content during pressing.