Structural Flakeboards and Composites 2959 



Maximum pressure on the mat, and closed press time varies with panel 

 thickness. Some typical times and pressures for southern hardwood flakeboards 

 pressed to about 45 pounds per cubic foot (ovendry weight basis) are as follows: 



Closed press times Maximum pressure 

 Panel thickness (after reaching stops) on mat 



Inch Minutes Psi 



3/8 3.5 600 



7/16 4.5 650 



1/2 5.0 700 



5/8 6.0 700 



3/4 8.0 750 



7/8 10.0 750 



1 12.0 750 



Press cycles can be significantly shortened and panel stability increased by 

 steam injection into the mat during hot pressing, but panel strength properties 

 may be reduced by this procedure (Heebink and Hefty 1969; Shen 1973; and 

 Thoman and Pearson 1976). Catalysts can also be added to some resins to speed 

 press cycles, but results with phenol-formaldehyde resin formulations have not 

 been promising (Lehmann et al. 1973). Application of radio-frequency energy to 

 flakeboard hot presses is technically possible, but has not proven to be an 

 economic way to speed press cycles. 



24-8 POST-PRESSING OPERATIONS 



Structural hardwood flakeboard for use as sheathing or floor decking is 

 brushed to remove loose flakes as it emerges from the hot press, scanned by an 

 ultrasonic device (see Albright and McCarthy's 1975 description of the technol- 

 ogy) to detect internal delamination or blows, and then trimmed on panel sizing 

 machines resembling two double-end tenoners coupled at 90° to each other so 

 that large panels (8 by 24 feet, for example) can be reduced to smaller size (4 by 

 8 feet) in one pass. If the smaller panels are to be used as single-layer floor 

 decking, edges and ends may be tongue and grooved on similar machines (see 

 sec. 1 8-20). Dulling of saws and grooving heads may be a problem when sizing 

 and edge machining flakeboard. Readers interested in information on the subject 

 beyond that given in section 18-29 are referred to Davis (1957), Theien (1970), 

 Neusser and Schall (1970), Bridges (1971), and Stevens and Fairbanks (1977). 



Sanding is not usually necessary for sheathing panels, but may be required for 

 accurate thickness control of single-layer floor decking. Figure 18-168 and 

 accompanying text describe this technology. 



24-9 MECHANICAL PROPERTIES 



Performance of structural hardwood flakeboard as sheathing and floor deck- 

 ing is linked to its density, modulus of elasticity, modulus of rupture, internal 

 bond strength, plate shear strength, and impact strength. These properties can be 



