2860 

 130 



Chapter 23 



320°F 



I I I I L 



2 3 4 5 6 



TREATING TIME (HRS.) 



Figure 23-88. — Tensile strength of Vs-inch hardboard as affected by temperature and 

 treating time. (Drawing after Brauns and Strand 1958.) 



Today oil is applied to one or both sides of the board by direct roll coaters or, 

 in certain cases, precision roll coaters. In an ordinary direct roll coater (fig. 

 23-90) oil is fed into the nip between a rubber contact roll and a steel doctor roll, 

 the amount applied being controlled by the gap between them. The contact roll 

 also feeds the board through the machine. 



The precision roll coater works like an off-set printer. An embossed steel roll 

 transfers to the rubber contact roll a precise amount of oil, the amount being 

 determined by the depth of the embossed pattern. Precision roll coaters can 

 apply as little as 1 Vi ounce per 4- by 8-foot sheet. They are also used to temper 

 embossed panels, where they will cover only the top of the embossed pattern 

 without applying oil to the low spots. 



The oil, whether applied by soaking or coating, is immediately oxidized in 

 heating ovens which are also used to bake oil -treated boards and for heat treating 

 boards without oil treatment. 



Other uses of oil include the addition of drying oil to the furnish as described 

 in section 23-7. The oil serves the same purpose as it does in the regular 

 tempering process; it dries by oxidation and forms a thin hard layer on fiber 

 surfaces, thereby improving inter-fiber bonding. Tempering oil is also used in 



