Fiberboards 



2831 



Such vacuum felters yield more uniform mats than gravity felters, and are the 

 standard dry-process forming devices, despite their considerable electric power 

 requirement. In a more recent four-stage vacuum former (fig. 23-60), the furnish 

 is distributed across the machine width by means of swing spouts oscillating at 

 about 120 cycles per minute. The furnish is metered to the spouts at air speeds of 

 3,600 to 5,400 feet/minute. To improve mat uniformity, furnish is applied in 

 excess thickness and then reduced to required mat thickness by equalizer rolls 

 and vacuum devices. 



BELT 



BELT 

 FOURDRINIER WIRE 



Figure 23-61. — Roll pre-press. (Drawing after Peters 1968.) 



Pressing. — The dry-formed mat has very low density, and for 1/4-inch board 

 could be 8 inches thick. To reduce daylight requirements and closing times in the 

 hot press, and to improve handling quality of the mat and surface quality of the 

 finished board, the mat is prepressed in a continuous band press (fig. 23-61). 

 Roll pressures are about 1,000 pounds/inch. Mat density is thereby doubled or 

 tripled (Peters 1968). In some cases a surface layer of fine fiber material is 

 applied to the precompressed mat. 



The travelling densified mat is then trimmed by disk cutters and transferred to 

 caul plates which carry the mat through the pressing operation. Dry formed 

 board is hot-pressed in multi-opening presses very similar to those for wet- 

 formed S2S board. Presses allow rapid, simultaneous closing. Platen tempera- 

 tures are high (400°F -h) and press cycles are short. 



At low mat moisture contents (5 percent) single-phase press cycles are used 

 (fig. 23-62). Higher moisture contents require 2 or 3 phases (fig. 23-63). Two 

 phase press cycles work particularly well with hardwood furnish. Three phase 

 cycles require precise control; the total pressure release after phase one can 

 reduce surface quality. 



Quick drying of surface layers of the loose mat during closing of the press 

 increases their compressive strength and leads to a densified core layer (fig. 23- 

 64). Surface densification, at least in the top surfaces can be increased by adding 

 water spray to the mat surface prior to hot pressing. At the Weyerhaeuser plant in 



