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Chapter 23 



PLAIN 

 ROLL 



PERFORATED 

 ROLL 



DISTANCE 



PRESSURE GRADIENT 

 PLAIN PRESS 



PRESSURE GRADIENT 

 ONE ROLL VENTED 



PRESSURE GRADIENT 

 BOTH ROLLS VENTED 



Figure 23-37. — Internal mat pressure in wet-press nip rolls. (Top) Components. (Bottom) 

 Mat pressure distribution between press rolls of various designs. (Drawings after 

 Strauss 1970.) 



by increasing the press loading. However, excessive nip pressures will cause the 

 hydraulic pressure to exceed the resistive pressure of the mat, at which point 

 fibers will be dislocated and the mat structure destroyed. This condition is 

 known as crush and is evidenced by the appearance of wrinkles in the sheet. 

 Modem machines are run "crush limited", i.e., pressure is increased until crush 

 occurs and then the rolls are backed off. Even then, fiber dislocation can occur in 

 the center of the sheet, because there the hydraulic pressure is higher than in the 

 outside layers (fig. 23-37 bottom). 



Edge Wallboard Machinery Company (n.d.) found that perforated rolls or 

 suction rolls reduce the pressure gradient, allow higher press loading, and 



