Fiberboards 2789 



Production rates of cylinder forming machines depend mainly on stock free- 

 ness and on the required thickness and density of the mat. For a given pulp the 

 build-up of the mat is determined by the cylinders' peripheral speed. The slower 

 the speed, the thicker will be the mat on the screen. The relationship between 

 mat thickness and forming speed is not linear, however, because as the mat 

 builds up, the vacuum or the pressure differential becomes less effective in 

 forcing water through the screens. The forming speed for very thick mats is, 

 thus, greatly reduced. 



Forming speeds of mat for !/2-inch-thick insulation board on double cylinder 

 machines are about as follows (Lyall 1969): 



Cylinder diameter Cylinder speed Forming speed 



Fourdrinier forming machines. — The Fourdrinier fiberboard forming ma- 

 chine (fig. 23-32), modified from the paper forming machine invented by Henry 

 Fourdrinier in England, forms the sheet continuously on a wire screen travelling 

 in a horizontal plane. Water is removed initially by gravity and vacuum, and in 

 the press section of the machine by hydraulic pressure. Fourdriniers can accom- 

 modate a wide range of pulp freeness and are used to manufacture SIS and S2S 

 hardboards as well as insulation board. 



The main element in a Fourdrinier forming machine (fig. 23-32) is an endless 

 wire screen, or simply the wire. It is driven by the couch roll, the last bottom 

 roll of the machine, and serves as both the forming surface and conveyor which 

 carries the mat through the continuous wet press, the final section of the ma- 

 chine. The wire has a weave of 14 to 32 mesh using phosphor bronze for the 

 warp wires (running along the machine direction) and brass for the shute wires 

 (running across the machine direction). Newer machines use plastic wires. "^ 



The wire is joined to form a loop, either by a seam produced at the factory, or 

 by a pin seam secured on the machine. The seamed wire requires a cantilever 

 frame for installation so that the loop can be slipped over the rolls from one side 

 of the machine. The pin seam can be used on any machine but does leave an 

 impression on the backside of the board. Pin seams on plastic wires, however, 

 leave virtually no marks and have made the cantilever design unnecessary."^ 



The pulp is supplied at the appropriate consistency (1 to 2 percent) to the 

 headbox which is preceded by a manifold which assures uniform distribution of 

 the pulp over the width of the machine. The headbox releases the pulp onto an 

 apron, a piece of plastic-covered cloth attached to the lip of the headbox and 

 extending out over the forming surface, from which it flows onto the forming 

 table. A fence (deckle) on both sides of the wire prevents the stock from flowing 

 over the sides of the screen and establishes the board edges. 



The forming table consists of supportive frame work, screen supports, and 

 dewatering devices. On it the sheet is formed, transforming the dilute pulp slurry 



'^Private communication (1981) from Edge Wallboard Machinery Company, Downington, Penn. 



