3040 



Chapter 24 



Stability across the grain can be modified by altering thickness of face veneers; 

 i.e., strength decreased and linear stability increased as veneer thickness de- 

 creased. Hse concluded that the 1/16-inch veneer thickness may be a good 

 compromise for commercial applications. 



Table 24-29. — Combination of face and core constructions yielding best properties for 

 composite panels with southern pine veneer faces and cores of southern hardwood flakes 



(Hse 1976) 



Review of available data suggests that long, thin, randomly oriented southern 

 hardwood flakes yield all-flake boards that have linear expansion of less than 

 0.25 percent when subjected to OD-VPS test (table 24-21) and less than 0. 12 

 percent when subjected to a change from 50 to 90 percent RH (figs. 24-42 and 

 24-55). Such boards with random flake placement also are strong and stiff at a 

 density of 44 to 46 Ib/cu ft (MOR of about 4,700 psi and MOE of about 650,000 

 psi; see table 24-12). 



In view of these more than adequate properties, it seems not necessary to 

 either orient flake layers or to overlay such flakeboard with veneer to alter linear 

 expansion and bending properties. Moreover, if thin flakeboard is faced with 

 thick veneer, linear expansion across the face grain may become excessive. To 

 obtain a smooth paintable surface that will stay smooth with changes in humid- 

 ity, perhaps very thin veneers or very thin impermeable paintable faces and 

 backs could be applied — rather than thick veneers. These thin faces would not 

 increase linear expansion across the panel, but could retard moisture movement 

 into the panel and preserve smooth surfaces under paint. 



