Factors influencing the mat forming process via aerodynamic spreading in cement-bonded particleboard production

Abstract

AbstractThe production of three-layered cement-bonded particleboards (CBPB) differs in most cases from that of adhesively bonded particleboards. However, one production step is quite similar: mat forming. A homogeneous mat is formed by aerodynamic or by mechanical spreading or by a combination of both methods. The distribution of the particles is influenced by their separation and flight characteristics. Those characteristics are in turn influenced by particle size and the combination of the particle’s surface area and weight. Because density of cement is higher than that of wood, the cement/wood (c/w) ratio will influence the particles’ separation and flight characteristics significantly. In the current study, laboratory-made particles of different sizes were mixed with different amounts of cement. An aerodynamic former was then used to help determine the influence of the different particle types and the c/w ratio of the mix on mat forming. The strewn particles were analysed for their distribution of mass, size and cement content as a function of spreading distance. The particle size decreased with increasing spreading distance. In addition, the cement content and spreading distance were positively correlated. For a c/w ratio of 1.9, increasing cement content correlated with increasing distance. This means, large particles and less cement tended to fall earlier than smaller ones and more cement. This corresponds to the observation that the specific surface area of the particles increases with increasing spreading distance. Vice versa, the density of the particles decreases with increasing spreading distance. The effect is more apparent when the particle mixture is heterogeneous in size at constant process parameters (e.g., air velocity). Looking at the density profile of a CBPB board, the surface layers consist of small particles with a higher cement fraction, and the core layer consists of large particles with smaller cement fraction. Thus, the production of CBPB requires two aerodynamic formers working in opposite directions. With the results of this study, it is possible to optimise production parameters, such as the speed of the transport caul and the air velocity with regard to the infeed raw material parameters in order to create a symmetrical particle distribution from top to bottom of the mat, resulting in an optimized and application-oriented density profile.