Fiberboards 



2813 



PHASE I ►♦ 



PHASE II 



TIME 



♦ < PHASE III 



Figure 23-49. — Typical press cycle for wet-process SIS hardboard. (Drawing from 

 Suchsland and Woodson 1985.) 



build-up. About 50 percent of the water present in the wet mat is removed, 

 bringing the ratio of water to fiber to about 1:1. 



In the second (drying) phase, most of the remaining water is removed as 

 steam. Practical pressure levels are between 80 to 100 psi, or about 1/10 of the 

 initial high pressure. If pressures during this phase are too low, steep steam 

 pressure gradients may structually damage the mat. The second phase ends in 3 

 or 4 minutes when steam ceases to exit visibly but before the board has dried to a 

 moisture content of less than 8 percent. 



In phase three, pressure is increased again (to 400 to 500 psi), densifying the 

 board to desired thickness and developing fiber bonds. Plasticity of the mat, 

 afforded by high mat temperature in combination with sufficient fiber moisture 

 content, favors board consolidation. Compression deformation under these con- 

 ditions is essentially permanent. If the third phase is initiated too late, i.e. , when 

 the moisture content has fallen below 8 percent, then compression of the mat is 

 much more elastic and it swells to a much larger degree upon pressure release. If 

 phase three is started too early, higher moisture contents cause staining of board 

 surfaces. 



Phase three, requiring 2 or 3 minutes, ends with opening of the press when the 

 moisture content has been reduced to about 0.5 to 1.0 percent. Terminating the 

 press cycle at higher moisture content risks incomplete fiber bonding, splits 

 along the center plane of the board, and boards sticking to the top caul plates. 

 Final moisture in excess of 3 percent may cause permanent distortions (sagging) 

 during subsequent handling and transportation, and lead to misalignment of saw 

 cuts relative to embossed plant scores.^ 



