Fundamentals of Vertical Density Profile Formation in Wood Composites. Part II. Methodology of Vertical Density Formation Under Dynamic Conditions
Keywords:Density profile, in-situ measurement, oriented strandboard, pressing, radiation, consolidation, moisture, bonding, resin, unsteady state, compression
AbstractThe vertical density profile or density distribution through the panel thickness has been identified as one of the important panel characteristics that correlates well with strength and physical properties of wood-based composite panels. We have studied the fundamentals of oriented strandboard (OSB) vertical density profile formation during hot-pressing. Experimental results are from the in-situ density measuring system installed on our laboratory hot-press. Results indicate that the vertical density profile of OSB is formed from a combination of actions that occur both during consolidation and also after the press has reached final position (i.e., thickness). We propose a methodology to describe the formation of the density profile into two periods and five stages. The consolidation period is the time of consolidation until the press reaches final position and contains two stages. The adjusting period is the time after the press has reached final position and continues until the culmination of the cycle. The adjusting period contains three stages. The resulting density profile is influenced by both periods and all five stages. The vertical density profile results from the combined effects of many process variables, but basically occurs from the effects of furnish moisture conditions, mat structure and the pressing environment. During pressing the mat is always in an unsteady state, and internal mat temperature, moisture content distribution, vapor pressure, layer density, and compaction stress are all related to the pressing operation. The unsteady state of the mat during the early stages of pressing may result in poor bonding strength development throughout the mat.
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