Hot-pressing stress graded aspen veneer for laminated veneer lumber (LVL)

Abstract Aspen (Populus tremuloides) is emerging as an important species for laminated veneer lumber (LVL) products in North America. During LVL manufacturing, both veneer stress grades and hot-pressing schedules are vital to product performance. In this study, an experimental design with four three-level variables comprising veneer moisture content (MC), veneer stress grade, platen pressure, and glue spread level was employed to investigate their effects on pressing behavior and stiffness and strength properties of LVL panels. The results show that, within the ranges studied, glue spread level and platen pressure were the two most important variables affecting the hot-pressing time needed for the innermost glueline to reach a target temperature of 105°C. The MC from the glue (mainly in the glueline) affected the rise of the innermost temperature more significantly than the MC in the veneer. Among the four variables studied, veneer stress grade and veneer MC were the two dominant variables that affect LVL stiffness and strength properties. Flat-wise and edge-wise bending stiffness (MOE) and strength (MOR) of LVL panels made from higher stress grade veneers were higher compared to those made from lower stress grade veneers, but there was no direct correlation between LVL shear strength and veneer stress grade. In addition, LVL edge-wise bending stiffness had the highest veneer MC tolerance among all LVL stiffness and strength properties. Further, LVL stiffness enhancement (the ratio of LVL MOE over veneer MOE) was lower with higher stress grade veneers than with lower stress grade veneers. These findings are useful for manufacturing high-stiffness LVL for engineered applications.