2832 



80 



^ 60 



40 



I 



Uj 20 



Chapter 23 



- 1,000 



800 S 



600 fe 

 400 ^ 



it 



200 



60 120 135 180 



PRESS TIME (SECONDS) 



240 



Figure 23-62. — Single-phase press cycles for Vs- and y4-inch-thick, dry-formed hard- 

 board of three-layer construction. Press temperature, 455°F. (Drawing after Lampert 

 1967.) 



Klamath Falls, 60 grams of water per square foot is applied to the surface of the 

 siding mat, increasing the total moisture content of the mat considerably. The 

 press cycle starts with a low pressure phase (IVi minutes at 150 psi) followed by 

 a high pressure phase to stops. The paintability of this particular siding product is 

 very good. 



Process (density) control. — Board density is a basic property of hardboard 

 often used as a quality indicator. It affects most of the physical and mechanical 

 properties such as bending strength, modulus of elasticity, hardness, and thick- 

 ness swelling. It also affects cost since higher board density requires more wood, 

 additives, process water, and energy. 



Figure 23-65 (top) shows the basic relationship between board density and 

 bending strength as reported by Kumar (1958). The curve below is the derivative 

 of this relationship; it shows that for each 1 -percent change from an average 

 board density of 58 pounds/cu ft, the bending strength will change about 3 

 percent. A density variation of ±7 percent, either within or between boards, 

 would be reflected in bending strength variation of ±21 percent. 



Efforts to control board properties should concentrate on board density, or, 

 more precisely, on mat weight. The economic advantage of such process control 

 efforts is illustrated in figure 23-66. The hypothetical distributions of bending 

 strength show how closer board density tolerances would reduce the variation of 

 the bending strength about the average. Since the board now exceeds the re- 

 quired bending strength, the board density or the resin content — or both — can be 

 reduced. Other important board properties would be similarly affected. 



Controlling board density requires accurate measurement of the mat density or 

 mat weight. A density control system must include accurate moisture content 



