2846 



Chapter 23 



any part of the mat before it was completely densified. This is equivalent to cold 

 pressing, which produces no density contrast. 



High press temperatures combined with short closing times yield medium- 

 density fiberboards of high modulus of elasticity (fig. 23-74). Density profiles of 

 some of the boards produced to obtain data for figure 23-74 are plotted in figure 

 23-75. Cold-pressed boards and those made at longest closing time had lowest 

 densities in surface layers. To achieve reasonable closing times (0.5 to 1.5 

 minutes), practical pressures on the mat are between 500 and 750 psi. Total press 

 time for a y4-inch board may be 8 to 10 minutes. 



The first medium density fiberboard plant (Deposit, N. Y.) was equipped with 

 a high-frequency press. One of the remarkable properties of this board was its 

 solid edge, ascribed to a lack of density contrast due to high-frequency heating. 

 The uniform temperature rise over the board thickness produced by high- 

 frequency heating favors more even moisture removal and uniform mat 

 compression. 



Subsequent investigations (Suchsland and Woodson 1974; Suchsland 1978) 

 and practical experience have indicated that for boards y4-inch thick or less, 

 these differences are too slight to make the use of high frequency mandatory. 

 There is little doubt, however, about the advantages of high-frequency heating 

 for thicker boards (1 inch or more). About half the present medium-density 

 fiberboard plants use high-frequency heating, while the other half uses either 

 steam or hot water. 



In a high-frequency-heated press (fig. 23-76), the electrodes, which can be 

 thin sheets of copper or other conductive material are placed between press 

 platens and mat. An electric field with nominal frequency of ten megacycles is 

 established between the two electrodes causing the mat to heat up. The press 



Electrode 



ELECTRODE 



RADIO FREQUENCY 



GENERATOR 



(-^10 mega -cycle ) 



Figure 23-76. — Principle of radio-frequency-heated board press. (Drawing from Suchs- 

 land 1978.) 



