Fiberboards 



2861 





0.5 



3 6 12 



CURING TIME (HRS) 



24 



Figure 23-89. — Weight loss of Vs-inch hordboard during heat (curing) treatment at 

 various temperatures. (Drawing after Back and Klinga 1963.) 



the manufacture of paper overlaid hardboard as described in section 23-8. The 

 oil is applied to the back of the printed paper after which the paper is positioned 

 on the wet mat. The oil reinforces the paper structure and bonds the paper to the 

 mat. 



Heat treating. — Heat treating of hardboard, oil treated or not, requires a 

 board temperature of about 300°F. Close temperature and exposure time control 

 is critical for two reasons. First, hardboard and especially the lower-density 

 boards can ignite at temperatures of 300°F; second, heat treatment causes exoth- 

 ermic reactions, particularly in tempered hardboard. These reactions may raise 

 the board temperature above oven temperature greatly increasing the fire danger. 



Hot boards direct from the press are air-cooled prior to entering the heat 

 treatment oven to limit board temperatures in the oven. Board temperatures are 

 brought up to 300°F slowly and the heat of reaction is removed from the boards 

 by circulating the air at high velocities (750 to 1,000 feet/minute). 



Heat-treating ovens may be continuous, progressive, or of batch type. Con- 

 tinuous ovens provide a means of transporting the boards through the oven at a 

 uniform speed. Boards can be suspended, supported on edge by pickets mounted 



