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Chapter 24 



Figure 24-26. — (Top) Cross-sectional views of 1/2-inch-thick, three-layer, structural 

 flakeboard comprised of 20 percent each of white oak, southern red oak, hickory, 

 sweetgum, and southern pine pressed to a density of 45 pounds/cu ft , OD-weight 

 basis. (Bottom left and right) Voids result from flake placement and variable 

 compressibility of the species; flakes may fracture from deflections during pressing. 

 Flakes are about 0.020-inch thick. (Photos from files of C.-Y. Hse and W. A. Cote; see 

 text footnote^.) 



Press closing time also significantly alters the density profile of flakeboard 

 panels. As closing time decreases, face density increases and core density 

 decreases, resulting in higher bending strength but lower internal bond strength. 



To summarize (fig. 24-27 and 24-28), hot pressing of 1/2-inch structural 

 flakeboards bonded with liquid phenol-formaldehyde binder and fabricated with 

 50 percent or more of dense oak and hickory flakes calls for platen temperatures 

 of about 350°F, closing time — with no hesitation or dwell — of 30 to 60 seconds, 

 and mat moisture content in face layers of about 1 1 percent and in core layers of 

 about 9 percent. This regime will yield densified face layers with resultant high 

 bending strength and cores sufficiently densified for adequate internal bond 

 strength. Readers interested in a more detailed analysis of factors controlling 

 press time are referred to Heebink and Hefty (1972). Suchsland's (1967) expla- 

 nation of behavior of a particleboard mat during the press cycle is also 

 instructive. 



