Fiberboards 



2815 



140 



375 400 425 450 475 500 525 

 PRESSING TEMPERATURE (*F) 



Figure 23-50. — Effect of press temperature on press time for dry-formed hardboard. 

 (Drawing after Norberg and Back 1968.) 



distance from the press platen. This gives rise to density variation over the board 

 cross section. Some board properties such as surface hardness, bending stiff- 

 ness, and internal bond (tensile strength perpendicular to board plane) are very 

 sensitive to such density variation. In thicker medium density fiberboard and 

 particleboard this density variation can be controlled to some extent by changes 

 in the press cycle, in S IS hardboards density is near maximum at the smooth top 

 surface, and minimum at the screened back (fig. 23-53). This unsymmetrical 

 distribution results from water flow toward the screen. 



