Fiberboards 2807 



Press loading. — S2S mats, rigid enough to be handled without supports, are 

 charged into the press loader, where they are supported along the edges only. As 

 the press opens, a hydraulically operated charging ram pushes the mats into the 

 press simultaneously by means of a series of lugs, which engage the end of the 

 mats, accelerate them, push them into the press opening, slow them down, and 

 retract, leaving the mat exactly positioned on the press platen. Edges of all mats 

 in the press must be in exactly the same position relative to the edge of press 

 platens to assure maximum control of thickness tolerances. 



As the mats enter the press openings, they encounter and push forward the 

 pressed boards into pinch rolls, which remove them into the unloading cage. 

 These boards are emitting gas and thus are not actually resting on the platens, but 

 are supported by a layer of gas, which practically eliminates the friction between 

 board and platen surfaces, allowing boards to slide out of position at the slightest 

 touch. If the press is not leveled properly, the force of gravity may dislocate the 

 boards. Positioning devices are often used to align the pressed boards just before 

 discharge and the mats on the platens before press closure. 



S2S presses may or may not use caul plates. When plates are used wear of the 

 press platens is reduced but the press daylight, i.e., the maximum clearance 

 between platens when the press is fully opened, must be increased because the 

 cauls don't stay flat. The upper caul will have considerable center deflection and 

 both cauls will show distortions due to thermal stresses. 



When embossed boards are made, the embossing plate — made of mild steel — 

 is the top caul. For embossed board, proper alignment of the mat in the press is 

 particularly important, because the final trimming must match witness marks left 

 by the embossed plate. Removal and replacement of caul plates is made easier by 

 quick release fasteners. 



Press cycle. — Press cycles vary depending on species, board thickness, board 

 density and press temperature. Hardwoods require less press time than 

 softwoods. Thin boards, low board densities, and high press temperatures also 

 favor short press cycles. 



Figure 23-45 shows S2S press cycles for Vs-inch and !/4-inch boards made 

 from hardwood fibers at press temperatures of 450°F. In both cases the pressure 

 is built up as fast as the press will allow, held for the required time — 60 seconds 

 for the Vs-inch board and 75 seconds for the 'A-inch board — then released as fast 

 as the escaping gas will allow without causing blisters. Kept under pressure too 

 long, excessive gas may develop from destructive distillation and blow the board 

 out of the press. As closed white water systems retain more dissolved solids in 

 the board, these difficulties increase because the pollutants are volatilized in the 

 hot press and discharged into the air. 



These dissolved solids (hemicelluloses) also increase the danger of boards 

 sticking or clinging to the top caul or the top press platen. The boards don't 

 adhere strongly, but require release by compressed air. Release agents applied to 

 the board before pressing eliminate this problem. 



Volatiles escaping from the press are vented outside. Substantial quantities, 

 however, condense in the vent stacks as their temperature drops below 300°F, 

 where they pose a fire hazard unless burned off periodically. 



