Fiberboards 



2805 



PLATENS- 



? CROWN 2 



MECHANISM FOR 

 SIMULTANEOUS CLOSING 



PRESS COLUMNS 



RAMS 



Figure 23-44. — Eight-opening press with simultaneous closing arrangement. (Drawing 

 after van Hullen 1966.) 



The hydraulic medium in fiberboard presses is generally water, to which 

 lubricants and other additives have been introduced. Pure oil reduces wear and is 

 less costly to pump, but can easily be contaminated by water squeezed from wet 

 mats during initial phases of press cycles. For this reason water is preferred. 



Demand for fast closing and pressure build-up makes it necessary to provide 

 hydraulic fluid at very high flow rates which may exceed pump capacity. Either 

 jack rams or low pressure accumulators are used to overcome this difficulty. 

 Jack rams have small diameter and can close the press quickly with a small 

 quantity of hydraulic fluid. The larger main cylinders are filled by gravity 

 without pumping. An accumulator is a low pressure storage device, which can 

 deliver large quantities of hydraulic fluid in a very short time. During periods of 

 low fluid demand (after press has closed) the pumps recharge the accumulator 

 against a cushion of air or nitrogen. 



It is advantageous to close all openings simultaneously rather than succes- 

 sively as the rising rams push the platens upwards. Simultaneous closing pro- 

 vides more uniform heat transfer on both surfaces of each board and assures that 

 the boards in all openings are subjected to identical pressing and heating condi- 

 tions. However, the closing speed of the individual openings is then only a 

 fraction of the ram speed. Simultaneous closing therefore requires high ram 

 speeds. Figure 23-44 illustrates the principle; as the press table moves upward at 

 speed Vr, the tie rods lift all platens simultaneously at speed V^ = V^ /number of 

 platens. 



