Fiberboards 2797 



produce stronger boards. Higher stock temperatures, because of their effect on 

 water viscosity and thus on stock drainage, increase the water flow according to 

 equation 23-1. However, depending on stock consistency, some or all of the 

 energy saved in the dryer must be expended to heat the stock going into the 

 headbox. Thus, higher consistencies favor net energy savings. All these consid- 

 erations are much more important in the manufacture of insulation board and 

 S2S hardboard than in SIS manufacture. The wet press illustrated in figure 23- 

 36 (top) uses four suction rolls on top and three perforated rolls on the bottom. 

 The fourth bottom roll is the main wire drive roll and is rubber covered to 

 increase traction. Perforated bottom rolls have proven as effective as suction 

 rolls, yet do not require the maintenance and the vacuum and separation system 

 demanded by suction rolls. Insulation board may be de watered to 45 to 65 

 percent water content in such a wet press. 



The insulation board wet press not only dewaters but also presses the mat to a 

 thickness equal to the final board thickness plus an allowance for shrinkage in 

 the tunnel dryer. Deflection of press rolls sometimes causes difficulties in 

 thickness and moisture content control, moisture content along mat edges being 

 lower than in the center. Increasing stiffness of the rolls by using stainless steel 

 shells, or by use of crowned rolls are two measures to combat roll deflection. 

 Crowned rolls, however, can cause distortion of the wire from small peripheral 

 speed differences. 



In the hardboard machine (fig. 23-36 bottom) solid rolls are used instead of 

 the expensive suction rolls. The squeezed-out water is removed from the nip by 

 suction slices, which are vacuum devices with narrow slots extending across the 

 width of the machine and located as close to the nip as possible. Initial compac- 

 tion is accomplished by a series of so-called "baby rolls", also equipped with 

 slices. Dewatering is not as efficient as in the insulation board machines, output 

 water content ranging from 65 to 75 percent. 



Modem board machines are driven by sophisticated DC or variable-frequency 

 AC drive systems, with individual motors for each press roll and various auxil- 

 iary drive points. They run at speeds of 20 to 100 feet/minute, 60 feet/minute 

 being average. Both hardboard and insulation board machines are about 60 feet 

 long including the forming section (27 to 30 feet) and wet press. SIS machines 

 are generally 5 feet wide; insulation board and S2S machines are 165 inches 

 wide (nominal width, 12 feet). 



Trimming of wet fiber mat, — All forming machines with the exception of the 

 Chapman former produce a continuous mat, which has to be subdivided into 

 individual sheets that conform to press size or multiple final board sizes. 



End trimming is accomplished by rotating steel disks while the mat is travel- 

 ing on roller conveyors (fig. 23-38). Simultaneously mat edges are trimmed by 

 steel disks or water jets. The trim waste from the edges as well as entire defective 

 mats can be detoured and returned to the process via a repulper in which the 

 waste is broken down and diluted. 



