Effect of Thickness Variation on Warp in High-Temperature Drying Plantation-Grown Loblolly Pine 2 by 4'S

Abstract

Currently, an increasing proportion of southern pine dimension lumber comes from plantations; therefore, an increase in grade, value, and volume loss from warp is expected. One factor that has not been fully explored is the effect of lumber thickness variation on warp. The primary objective of this study was to increase quantitative understanding of the effects of thickness variation on crook, bow, and twist during high-temperature kiln-drying of plantation-grown loblolly pine to determine the importance of its control on the development of warp. Plantation-grown, 2 by 4 (nominal 50- by 100-mm) loblolly pine were kiln-dried at high temperature after surfacing them in such a way as to produce certain patterns of thickness variation. One group was not surfaced, i.e., left as mill run. All boards in a second group were surfaced to the same thickness. In a third group, the boards were divided into thirds, and each third surfaced to a different thickness. In this group, boards of the same thickness were stacked in vertical alignment to exaggerate the effect of the thickness variation. The fourth group differed from the third group in that the three thicknesses were randomly placed in the package. The extreme thickness variations did aggravate warp, especially twist. As a result of better sticker contact, thick boards warped less than did thin boards. However, even with perfect sticker contact, a substantial amount of warp developed, indicating that control of thickness variation can reduce but will not eliminate warp. Correlation of warp with board characteristics suggests that boards containing pith warp more than ones without pith, and boards sawn from near the center of the tree warp more than boards farther from the center of the tree.