2792 



Chapter 23 



Figure 23-34. — Wet-line on Fourdrinier forming machine; puddler board is located 

 above the mat just ahead of the wet line. (Photo courtesy of Edge Wallboard Machin- 

 ery Company.) 



The next four boxes are piped to a deeper seal pit and a low-vacuum manifold (3 

 to 5 inches Hg). Then by valving, the vacuum level on these boxes is graduated 

 to control the rate of dewatering. The eighth box is piped to a high vacuum 

 manifold (15 inches Hg). The last two boxes are piped to individual separators 

 and the high vacuum manifold. The polyethylene topped suction box has also 

 replaced the rota-belt on modem machines."^ 



The amount of water that can be extracted from an uncompressed mat is 

 limited. At some point (about 80 percent water content) the vacuum will suck air 

 and very little water. To extract any more water requires compressing the mat to 

 saturation. This takes place in the wet press. 



Board surface modification on a Fourdrinier. — All modem Fourdrinier 

 machines are equipped with a puddler (fig. 23-34). This flat board, spanning 

 the wire and located ahead of the wet line, oscillates up and down to just dip into 

 the top layer of stock. This puddling action brings water and fines to the surface 

 and develops a superior surface finish. 



Most SIS hardboard siding machines and some S2S hardboard machines are 

 equipped with a secondary headbox, which applies a surface layer of fine pulp 

 on top of the main sheet. The overlay is generally a much slower stock, applied 

 to upgrade the surface quality of an otherwise fast and relatively coarse stock. 

 Secondary headboxes apply up to 10 percent overlay based on the weight of the 

 substrate. The drainage time of the overlay may be twice as long as that of the 

 substrate. For instance, an 18- to 20-second stock (TAPPI Standard SFMC) may 

 receive an overlay of 50-second stock. If the overlay stock is too slow, not 



