Fiberboards 



2791 



At some point on the forming table, the stock will change from a watery 

 suspension of fibers to a fiber mat. Past this point, the wet line (fig. 23-34) 

 where light reflected from the liquid water surface disappears, the mat surface 

 appears dull. In the arrangement depicted in figue 23-32, a rota-belt suction 

 unit is located past the wet line. It carries a slotted rubber belt which travels with 

 the screen to reduce friction and wear; pumps apply a vacuum of up to 10 inches 

 Hg to boxes below the belt, further de watering the mat. 



Vacuum should be applied gradually along the length of the wire. Too rapid 

 drainage in the beginning of the forming process may tighten the sheet, slowing 

 subsequent drainage. Springy stock resists compaction better and can tolerate 

 more suction. Past the wet line the vacuum can be increased, but if increased too 

 abruptly, may suck the sheet down so forcefully that the surface may crack. 



One of the biggest changes made to Fourdrinier machines in recent years is 

 elimination of table rolls and acceptance of polyethylene-topped suction drain- 

 age boxes. Because these boxes of ultra-high-molecular- weight material are 

 structually more rigid than table rolls, they provide a flat surface for formation. 

 Under some conditions a normal table roll may deflect as much as 0.200 inch, 

 while the suction box will deflect less than 0.040 inch.'* 



A modem Fourdrinier without table rolls normally has 10 suction boxes, each 

 box 3 feet long (in direction of screen travel). The first three boxes after the 

 headbox are operated through a drop leg to a seal box, 6 or 7 feet below the wire. 



Figure 23-33. — Dewatering action of a Fourdrinier table roll on a paper machine is 

 attributable to the vacuum developed at the outgoing nip below the mat. Action on a 

 fiberboard former is similar, but due to lower speeds (100 feet/min.) vacuum is 

 smaller. (Drawing after Strauss 1970.) 



