2812 Chapter 23 



kamp Corporation in a number of plants in the United States, can be used to 

 illustrate typical procedures. The main features of this system are a simulta- 

 neously-closing, 24-opening, 4- by 16-foot press, and a vertical wire-screen 

 orbit which returns the carrier screens under the press to the mat conveyor. Here, 

 each mat, cut to press size, is placed on a wire screen, which in the hot press will 

 allow water and steam to escape from the densified mat and which gives one side 

 of the board the characteristic screened appearance (fig. 23-48 top). The screens 

 are not continuous, but are sized to accommodate one mat which has been 

 previously cut to hot press size. In the hot press the top board surface is made 

 smooth by a smooth top platen, and the bottom board surface shows a screen 

 pattern. See Suchsland and Woodson (1985) for a more complete illustrated 

 description of this system. 



Other press lines use steel cauls (rigid steel sheets) as screen and mat support. 

 These cauls carry screen and mat, and the entire assembly proceeds through the 

 press. Cauls and screens are returned on a horizontal conveyor system. 



In time, carbon deposits appear on caul plates and wire screens, which if not 

 removed (by cleaning in sodium hydroxide solution), will cause carbon particles 

 to appear on board surfaces. 



Press cycle. — Water content of mats entering the hot press is typically 65 to 

 75 percent, yielding a ratio of water to dry fiber of about 2 to 1 . Press cycles are 

 designed to remove the water from the mat at minimal cost, while developing 

 optimal physical and mechanical properties in the board. While press cycles 

 vary, most manufacturers of SIS hardboard approximate the pressure-time pat- 

 tern shown in figure 23-49. The platen temperature is constant. Pressure-time 

 functions clearly divide the press cycle into three phases: 



1. High pressure squeeze phase. 



2. Low pressure drying phase or dwell phase. 



3. Consolidation phase. 



The first phase is designed to remove as much water as possible from the mat as 

 quickly as possible without transferring unnecessary heat to the water. The high 

 pressure level, normally between 800 and 1,000 psi, is therefore established as 

 rapidly as the hydraulic system of the press allows. Some lower density boards 

 may be squeezed at pressures as low as 400 psi. Pressures in excess of 1 ,000 psi 

 do not result in appreciable improvements. 



On application of this pressure the water is forced downward through the mat 

 and then escapes laterally through voids in the wire screen. The process is aided 

 by reduced viscosity as water temperature rises. 



The large temperature differential between mat and press platen and the great 

 heat capacity of the wet mat cause large quantities of heat to be transferred from 

 the platen to the mat, placing a heavy burden on the boiler. Steam demand of a 

 20-opening press may jump to 20,000 to 30,000 pounds/hour for a short time 

 when the press is first closed. Steam accumulators are used to ease this burden. 



Ideally, the first phase ends when all the squeezable water has been removed 

 and the remaining water has reached a temperature of 212°F. Phase one takes 

 about 1.5 minutes of which 40 to 45 seconds are required for the initial pressure 



