For. 485: Lignocellulosic Composite Materials Lecture 1-6: Press Cycles/Development of Vertical Density Profile

VDP A function of raw material and process variables Consider a mat consolidated in a cold-press with a cold-setting adhesive: Theoretical constant compressive stress throughout the mat Then, df=di=dc, where df is density of face, di is density of an “intermediate” layer and dc is density of core

Hot-Pressing Temp and mc affect wood particle compression strength (perpendicular to grain) and bending modulus As press closes: Upper layers of mat are weakened, thus they compress and densify. Steam is generated and migrates toward the center of the mat, increasing temp and mc of core.

Hot-Pressing, continued Sub-layers (intermediate and core) begin to weaken and densify. General result: df>di>dc

Important Variables Moisture condition: Initial mat mc; variations of mc between face and core furnish; transient mc conditions during pressing Mat structure (raw material variables) Particle geometry Particle distribution (mat formation) Particle density (wood density)

Important Variables, continued Pressing Environment Temperature Pressure; pressure cycle (press schedule) Time: Total and Press Closing Time (PCT as discussed in reading 1-6a) Short closing time: Upper layers of mat are densified to a greater extent. Long closing time: Faces and core are densified to a more uniform extent. Consider effect on panel properties! Compaction ratio=Density of panel/density of raw furnish