Fiberboards 2793 



enough water can be removed from it before it reaches the wet press, where it 

 may be "crushed".^ 



Secondary headboxes are located past the wet line. The overlay stock flows 

 over a dragging apron onto the substrate. Normally, insulation board is not 

 overlaid. 



Fourdriniers for SIS hardboard compared to those for insulation board 

 and S2S hardboard. — An SIS hardboard or medium-density board forming 

 line is normally coupled directly to the hot press, i.e., a forming line feeds one 

 hot press; press cycle and forming speed are synchronized for continuous oper- 

 ation. The width of the hot press and former is either 4 or 5 feet. Wider presses 

 require very substantial strengthing to resist bending stresses and would be less 

 cost efficient. 



In the manufacture of insulation board and S2S wet formed hardboard, the 

 forming line is coupled to a dryer rather than to a press. Drying a 12-foot- wide 

 mat is much more efficient than drying three 4-foot- wide mats, and increased 

 panel width does not call for extraordinary design measures. Widths of insula- 

 tion board and S2S hardboard forming machines are therefore equal to a multiple 

 of one standard panel module: 8, 12, or 15 feet. The dried mat, much more easily 

 handled than the wet mat, is cut to a standard width before fabrication or 

 pressing. 



Wet press. — A wet press further reduces water content of a mat after extrac- 

 tion by vacuum and gravity in the uncompressed mat has reached a practical 

 limit. This limit is about 80 percent water content (20 percent dry fiber) for 

 Fourdrinier forming machines and somewhat less water content for cylinder 

 forming machines. Most wet presses are continuous roller presses — independent 

 units in the case of cylinder forming, and an integral part of Fourdrinier forming 

 machines (fig. 23-32). The discontinuous Chapman process described earlier in 

 this section uses a cold platen press as wet press. The following discussion 

 pertains to the wet press as it is found on Fourdrinier machines; those for 

 cylinder forming are similar but simpler because cylinder-formed mats reach the 

 wet-press at lower water content. 



SIS hardboard differs from insulation board or S2S hardboard in requiring 

 less complete dewatering in the wet press. In both cases the final product is 

 practically dry, i.e. , all of the water must be removed, and some of the water is 

 removed at low cost in the wet press (fig. 23-35). Water remaining in the 

 insulation board and S2S mats must be converted into steam in the dryer at very 

 high energy cost. The cost of manufacturing insulation board and S2S hardboard 

 is, therefore, very sensitive to the moisture content of the mat at the dryer. At 

 $2.50/million Btu's, and 65 percent dryer efficiency, a reduction of tipple 

 moisture of 3 percent in a 350-ton/day machine would save annually more than 

 $165,000. 



In the manufacture of S 1 S , a very high pressure applied at the beginning of hot 

 pressing squeezes out most liquid water before heat from the platens converts it 

 to steam. As in the wet press, this uses little energy and the little water remaining 

 is essentially independent of the water content of the mat leaving the wet press. 



