2920 Chapter 24 



24-4 FLAKE MANUFACTURE, DRYING, AND STORAGE 



Researchers agree that careful control of flake cutting, drying, classifying, 

 and storing is essential to the manufacture of strong, stiff flakeboard panels. 



FLAKE CUTTING 



Optimum flakes for structural panels must be cut accurately in the 0-90 

 direction (fig. 18-1) to precisely prescribed thickness and length. Width is less 

 critical; in waferboards flake width may approximate length, while in oriented- 

 strand boards flake length is generally three or four times flake width. 



Wafers are illustrated in the upper and lower left corners of figure 18-274A, 

 and strands are shown in figures 18-264 and 18-274B; semi-flakes or chip 

 flakes, which usually contain considerable cross grain, may be variable in length 

 and width (see ring-flaked wood in figures 18-274ABC). 



Slope of grain in flakes. — Flakeboards can be no stronger than the flakes 

 from which they are made. Crossgrain in flakes significantly weakens them as 

 shown by the following data from Price (1976) on tensile properties of 0.0 15- 

 inch-thick sweetgum flakes tested at 7-percent moisture content over a 3/4-inch 

 gauge length: 



Property and 



slope of grain Value 



Degrees psi ' 

 Tensile strength 



12,331 



10 6,340 



20 349 



Modulus of elasticity 



624,500 



10 521,500 



20 418,500 



Price's (1976) study also showed that the hot pressing operation, compacting 

 flake mats to board thickness, increases face-flake density by 13.9 percent, 

 tensile strength by 7.6 percent, and modulus of elasticity by 9.8 percent, but 

 reduces both tensile strength and modulus of elasticity of core flakes by about 7 

 percent. 



Surface smoothness of flakes. — As flakes are cut, they develop micro- 

 checks similar to lathe checks in veneer (fig. 24-3 top). These checks, and the 

 discontinuities in flakes arising from large vessels (fig. 24-3 bottom left), weak- 

 en flakes and contribute to large variations in their strength. Readers interested 

 in further discussion of these effects are referred to work by Furuno et al.^ 



Southern hardwoods, e.g., the oaks, hickories, ashes, elms, red maple, 

 sweetgum, and black tupelo all yield smoothest flakes if cut from wood heated a 

 few hours in water held at about 150°F. See figure 18-238 for the most favorable 

 temperature of individual hardwood species. Dry hardwood yields rough, splin- 

 tery flakes poorly suited for strong panels. 



