Maintaining Lumber Quality in Press Drying by Manipulating Sawing Patterns
Keywords:
Drying, press drying, high-temperature drying, rapid drying, oak, drying defects, honeycombAbstract
Lumber is traditionally dried in kilns by processes that often take several weeks to complete. Considerable research has been conducted on more rapid processes. Press drying can dry common 25-mm-thick lumber in 1 to 2 hours instead of several weeks. However, little success has been achieved with this technique because intolerable drying defects usually accompany such rapid drying. This paper reports on a press-drying technique that offers considerable promise in avoiding honeycomb that usually develops. In this technique the sawing pattern of boards from logs is changed from the usual flatsawn pattern to quartersawn. The honeycomb that usually develops occurs at interfaces of ray tissue and the surrounding tissue and is caused by internal tensile forces that develop during drying. With flatsawn boards the wide face, which makes contact with the platens during drying, is parallel with the critical internal tensile forces. By using a quartersawn pattern, the direction of the critical internal tensile forces is effectively rotated by 90 degrees. The critical internal tension is then perpendicular to the wide face of the board.
The hypothesis of the study was that the compressive pressure of the press therefore opposes the internal stresses and prevents honeycomb. In this study honeycomb in press-dried quartersawn red oak was significantly less than in press-dried flatsawn red oak. Small compressive forces of the platens were less effective than large forces in reducing honeycomb in quartersawn oak, thus verifying the hypothesis.
References
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