2954 



Chapter 24 



Multiple-opening hot presses capable of delivering the high pressures re- 

 quired when pressing flakes of dense woods, are very expensive and plant 

 capacity is usually determined by press output. This output increases as press 

 cycle time is decreased. A 1/2-inch southern hardwood flakeboard might require 

 a closed press time of 5-1/2 minutes at 350°F if bonded with liquid phenol- 

 formaldehyde resin. Pressures available to close the press must be high (600-750 

 psi specific platen pressure) to achieve closure to desired thickness in 30 to 60 

 seconds. A 16-opening press with platens measuring 8 by 24 feet (fig. 24-24) 

 requires 800-1 ,200 hp to drive hydraulic pumps and pistons adequate to close the 

 press in 30 to 60 seconds on dense hardwood mats that will yield 1/2-inch-thick 

 panels. 



Platen temperatures are generally in the range from 340°F (liquid resin) to 

 about 420°F (powdered resin). While it is quite possible to heat platens to this 

 temperature with high-pressure steam, most plants use presses with oil-heated 

 platens, the oil being heated with energy from combustion of wood residues. 

 Typically, a wood-fired oil heater (fig. 24-25) might burn 2 tons per hour of 

 green hardwood bark or sawdust to heat the press illustrated in figure 24-24 

 when pressing 7/16- or 1/2-inch panels. 



Figure 24-24. — Multi-opening, simultaneous-closing hot press. Hydraulic pumps and 

 pressure accumulators are in foreground. (Drawing from A. B. Motalla Verkstad.) 



