2808 



Chapter 23 



10 SEC 



— 60 SEC. 



1/8 IN., 65 LBS./FT.'^ 

 1/4 IN., 53 LBS./FT.^ 



PRESS TIME (MIN.) 



Figure 23-45.— Press cycles for Vs- and Va-inch-thick S2S hardboard from hardwood 

 fibers; press temperature 450°F. (Drawing from Suchsland and Woodson 1985.) 



Figure 23-46 shows two types of press cycles used when mats are not pre- 

 dried. The board is then either "toasted", i.e. , dried under very low pressure and 

 then densified during a second high pressure phase, or the water vapor is vented 

 by "breathing" the press once or several times. 



When S2S board leaves the press it is right at the threshold of combustion. If 

 not immediately cooled, it will ignite spontaneously. It is cooled by rapidly 

 moving air immediately after discharge from the press and before it is lowered to 

 a conveyor line by the unloading elevator. The boards must be kept moving to 

 prevent condensation of fumes from blemishing board surfaces. Such fumes will 

 also condense on any steel members of the cooling racks that are allowed to cool 

 below 300°F. 



Thickness tolerances. — S2S hardboard is often used as a substrate for high 

 quality finishes involving precision printers, which require close thickness toler- 

 ances. Thickness variations of the finished board can be caused by the mechani- 

 cal limitations of the press and associated equipment, such as thickness 

 variations in the caul plates. They can also be caused by a non-uniform distribu- 

 tion of furnish in the mat, or by non-uniform mat thickness, resulting from roll 

 deflection in the forming machine and wet press. Such mats coming out of the 

 dryer could be 1/16-inch thicker in the middle than at the edges. The trimmer 

 would therefore produce one thick mat from the center and two wedge-shaped 

 mats from the outside. These mats are not easily pressed to uniform board 

 thickness, particularly when they are used for the manufacture of low density 

 boards. 



