2814 Chapter 23 



When pressing low density boards (less than 55 pounds/cu ft), phase three 

 may be eliminated; phase two would simply be extended to assure adequate 

 drying. At board densities of 60 pounds/cu ft and up, however, the consolidation 

 phase is necessary for final thickness adjustment. The total press cycle as 

 described above requires about 6 to 8 minutes for a Vs-inch-thick hardboard. A 

 6-minute cycle would allow the pressing of eight press loads per hour, assuming 

 IVi minutes per cycle for loading and unloading. 



Although figure 23-50 applies to dry formed hardboard, it clearly demon- 

 strates the important effect of press temperature on press time and therefore on 

 productivity. However, there are several practical limits to increasing the press 

 temperature in fiberboard manufacture: 



• Too rapid steam generation and subsequent blow-out danger, particularly 

 in high density board. 



• Beginning deterioration of wood at temperatures in excess of 420°F. 



• Limited heat resistance of printing ink on overlay papers applied to the 

 mat prior to hot pressing. This method is used by Abitibi Corp. and limits 

 platen temperature to 380°F (fig. 23-51). 



Pressing to stops. — Most fiberboards are pressed without stops. Final board 

 thickness is determined by the total accumulative response of the mat to the press 

 cycle. The pressure varies during the cycle according to the requirements of 

 efficient water removal, while the compressibility of the mat varies with chang- 

 ing mat temperature and moisture content. A given press cycle will compress 

 identical mats to identical final thickness. Uniform mats are therefore essential 

 to small thickness tolerances. 



Thickness control by gage bar or pressing to stops is used in the manufacture 

 of some relatively thick, low density siding products. This method is also 

 standard procedure in the manufacture of particleboard and medium-density 

 fiberboard. Metal strips of a thickness equal to the final board thickness are 

 inserted between press platens alongside the mat and limit the compression of 

 the mat. The pressure on the mat is determinate only until the spacers are 

 reached, when they carry part — and eventually most — of the total load (fig. 23- 

 52). 



In practice the press load is reduced constantly to the level required to just 

 keep the press closed. In such an operation the boards may be removed when the 

 mat pressure approaches zero, regardless of the moisture content; beyond this 

 point, the press is simply used as a dryer. Final moisture reduction could be 

 accomplished in heat treating ovens, if available. Gage bars are often replaced 

 by electronic control devices. 



Thickness tolerances. — Final board thickness is affected by springback, an 

 instantaneous recovery upon pressure release of the preceeding compression 

 deformation. Springback is greater along board edges than in the center, due to 

 lower edge temperatures there from heat losses as water flows out of the mat. 

 Prior injection of additional resin into board edges reduces this springback and 

 shortens the press cycle (U.S. Patent No. 4,168,200). 



Density profile. — Mat densification, in response to applied pressure, is af- 

 fected by its temperature, moisture content, and other factors which vary with 



