Summary
In order to establish a suitable pressing schedule in the dry process hardboard manufacturing, 35 cm square of 3.5 mm thickness boards were experimentally made, utilizing air-felted oak fiber mats of approximately 8% moisture content, containing 2% phenolic resin and 1.5% wax, hot-pressed at 210°C under 50kg/cm² of the first stage pressure on mat, employing a two-stage pressing diagram, on which the effect of the first stage pressure duration time, the second stage pressure duration time and its pressure on the board characteristics, were analyzed according to the experimental design of three-way lay-out in triple repetition (Fig. 1 & Table 1).
Dry process 525 hardboards having modulus of rupture of 480 kg/cm²; air-dry density of 1.10 g/cm³; water absorption of 22%, were obtained, under 15 seconds of the first stage duration and 15kg/cm² pressure with 30 seconds duration at the second stage pressing, in total pressing duration of 120 seconds. The defects such as punctures or blown outs were not found in all boards obtained, because there were sufficient differences between steam pressure generated in mat, equivalent to pressing temperature, and the second stage pressure. However boards were not satisfactory when the second stage pressure were below 10kg/cm², because of insufficient density.
Considering these results, two-stage pressing diagram was suitable when the hot-platen temperatures were higher than 206°C, equivalent saturated steam pressure of 17 atg. In case of lower temperatures, it would be necessary to set a breathing period of lower holding pressure than 10kg/cm², which would need the third stage pressing at more than 20kg/cm² after the breathing stage to make a board density higher.
The first stage duration and the second stage pressure were both effective for improving board characteristics, although the second stage duration was effective only to improve water absorption (Tables 24). This would indicate the presence of thermal decomposition of the boards at a prolonged pressing duration.
The variation of board characteristics in this experiment seemed to be caused by the minor variation of mat moisture contents, which were more effective than other factors in hot-pressing. Another cause of variation was a fluctuation of board moisture contents at test. Other factors affecting to the board qualities, but not involved in this experiment, were phenolic resin, wax and pressing temperature, as summarized on Table 7 from the previous works.
A hyperbolic correlation between mat moisture contents and the first stage pressing duration time to make equivalent strength board without defects was found as Fig. 9. Also it would be expected that the total pressing duration would be shortened with higher pressing temperatures. These factors would be important in determining an optimum pressing schedule to obtain economically sound boards.

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