Fiberboards 280 1 



Fires in board dryers are almost always caused by overdrying, which lets 

 board temperatures rise to danger levels. Overdrying also causes thermal break- 

 down of wood substance and formation of pyrophoric carbon at board surfaces. 

 This carbon is very reactive, oxidizes exothermically, and can cause fire or 

 charring in stacks of overdried insulation board. 



HOT PRESSES, TYPES AND CONSTRUCTION 



Because of the large forces involved, the difficult problems associated with 

 the rapid transfer to the mat of large quantities of heat, and because of the need to 

 precisely adjust and modify press cycles, continuous hot presses are not 

 practical. 



The hot press is thus a batch operation in an otherwise continuous process. 

 Since pressing time is generally constant for a given board type and thickness, 

 the press capacity and the capacity of the entire production line is a function of 

 platen size and number of press openings. The multi-opening press used univer- 

 sally in the fiberboard industry has the advantage, that, by virtue of the series 

 combination of mats and press platens, any number of boards can be compressed 

 with the same total force (fig. 23-41). The important relationship between 

 forming line speed, press time, and press size is illustrated in figure 23-42. An 

 18-foot-long, 20-opening hot press, for example, with a total press time includ- 

 ing loading and unloading, of 6 minutes, could accommodate a forming line 

 speed of 60 feet/minute. If this same line were used for the manufacture of a 

 thicker board, requiring a press time of 8 minutes, the line speed would have to 

 be reduced to 45 feet/minute. Given data on width of forming line, number of 

 press openings, platen dimensions, and press time, the line capacity in square 

 feet per minute can be readily determined. 



Almost every part of a press is subjected to very large forces, which are 

 generated by the action of hydraulic rams compressing the boards between 

 platens. Given a specific pressure on a fiberboard mat of 1 ,000 psi, for example, 

 the total force on a 5- by 18-foot press platen is almost 13 million pounds. This 

 total force is resisted by the structural members of the press, causing stresses and 

 bending moments. No reaction forces other than the press weight are transmitted 

 to the foundation (fig. 23-43). 



Frame members of the press may consist of press crown and base bolted 

 together on each side by a series of cylindrical columns to form a column press 

 (fig. 23-41). In a frame press the structural members consist of solid steel 

 frames, each cut from a single piece of steel plate; crown and base are mounted 

 inside of the frames. Both types are found in the fiberboard industry. Newer 

 presses seem to favor the frame design. 



Platens provide the plane, smooth surfaces against which the fiberboard 

 surfaces are pressed and molded, and they are carrier of the heating medium, 

 either saturated steam or hot water. A system of interior channels provides 

 passage and distribution within the platens of sufficient heating medium to 

 assure quick and uniform heating of the mat over its entire area. Most fiberboard 

 presses in the United States are heated by saturated steam. The European indus- 

 try has turned to hot water systems. Greater uniformity, lower accumulator 



