Fiberboards 



2843 



I 



AVERAGE DENSITY 



FACE 



V 



CENTER BACK 

 BOARD THICKNESS ► 



Figure 23-72. — Examples of density distributions over board cross section at constant 

 average board density. (Drawing from Suchsland and Woodson 1985.) 



lower bending properties. Board A, with its higii-density contrast, would have a 

 relatively porous board edge, poor machinability of the edge, and poor edge 

 screw holding power; board B would have greatly improved edge properties. 



The stops or gage bars employed in pressing of all thick medium-density 

 fiberboard can be used to modify density profile. These stops determine density 

 independent of the applied pressure, as long as the applied pressure is sufficient 

 to close the press. Density contrast can be controlled, within certain limits, by 

 controlling the closing time, a function of the applied pressure. 



Density contrast in medium-density fiberboard is greatest at short closing time 

 (fig. 23-73), obtained by application of high pressure. Reducing the pressure 

 moderates density contrast until the pressure reached is too low to close the press 

 by the end of the press cycle (point "2"), which in turn is determined by the press 

 temperature and the curing characteristics of the resin. 



At short closing times only thin surface layers are heated and therefore 

 weakened before densification is complete, resulting in higher compression of 

 the faces. At long closing times, the entire mat is heated while still under full 

 pressure, and the various layers of the mat reach similar low compression 

 strength levels at one time or another, reaching a more uniform densification and 

 low density contrast. 



Increasing the pressure to extreme values causes instantaneous closing of the 

 press with no density contrast at all (point "1"). Here, no heat was transferred to 



