EFFECT OF THICKNESS VARIATION ON WARP IN HIGH-TEMPERATURE DRYING PLANTATION-GROWN LOBLOLLY-PINE 2 BY 4S

Currently, an increasing proportion of southern pine dimension lumber comes from plantations; therefore, an increase in grade, value, and vo lume loss from warp is expected. One factor that has not been fully ex plored is the effect of lumber thickness variation on warp. The primar y objective of this study was to increase quantitative understanding o f the effects of thickness variation on crook, bow, and twist during h igh-temperature kiln-drying of plantation-grown loblolly pine to deter mine the importance of its control on the development of warp. Plantat ion-grown, 2 by 4 (nominal 50- by 100-mm) loblolly pine were kiln-drie d 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 thi rds, and each third surfaced to a different thickness. In this group, boards of the same thickness were stacked in vertical alignment to exa ggerate the effect of the thickness variation. The fourth group differ ed from the third group in that the three thicknesses were randomly pl aced in the package. The extreme thickness variations did aggravate wa rp, especially twist. As a result of better sticker contact, thick boa rds warped less than did thin boards. However, even with perfect stick er contact, a substantial amount of warp developed, indicating that co ntrol of thickness variation can reduce but will not eliminate warp. C orrelation of warp with board characteristics suggests that boards con taining pith warp more than ones without pith, and boards sawn from ne ar the center of the tree warp more than boards farther from the cente r of the tree.