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

2. 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

3. 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.

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

5. 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)

6. 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