Fiberboards 2809 



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TOASTING" CYCLE 

 BREATHING" CYCLE 



PRESS TIME 



Figure 23-46. — Press cycles for S2S hardboard mats that have not been pre-dried. 

 (Drawing from Suchsland and Woodson 1985.) 



Some thickness variation develops in the finished board after the press opens. 

 Some pressed boards expand (springback) as the pressure is released. Generally, 

 the higher the press temperature, the less springback. Higher press temperature 

 tends to plasticize fibers and relax internal stresses. Temperature variations 

 within or between press platens can cause board thickness variations. The center 

 of the board is generally thinner than the edges, and the top and bottom boards in 

 a multi-opening press are often thinner because the heat losses are smaller at 

 these locations. 



Typical thickness variation in a Vs-inch-thick, 4-foot- wide board pressed in a 

 20-opening press to a density of 65 pounds/cu ft could be 0.015 to 0.020 inch 

 within boards and 0.025 to 0.030 inch between boards. Within-board variations 

 can be greatly reduced by pressing 4-foot- wide boards in a 5-foot press. ^ 



Another phenomenon in S2S hardboard associated with springback is so- 

 called chip pop, a very localized thickness variation due to springback of a 

 highly compressed sliver of wood or fiber bundle. In this state, the compression 

 deformation is apparently not as plastic as it is in completely defiberized ma- 

 terial, or is not sufficiently arrested by fiber bonds. The closer the "chip" is to the 

 surface, the more severe the distortion. Slush overlays will obscure these chip 

 pops to some extent. Chip pops are not as common in SIS boards, because the 

 presence of water apparently contributes substantially to plasticizing wood un- 

 der pressure. 



Density distribution. — Surface layers of S2S hardboard are typically more 

 dense than interior portions (fig. 23-47). The symmetry of this density accounts 

 for the bending stiffness of S2S hardboard. 



