Shrinkage and the Development of Defects in Veneer Drying

Tangential shrinkage (green to ovendry) measurements were made on 144 heartwood specimens from two different yellow poplar (Liriodendron tulipifera L.) logs. The specimens were 3 inches wide (tangential direction) by 6 inches long (longitudinal direction) and 1/32, 1/16, 1/8, and 1/4 inch thick. They were dried using three levels of temperature (150?, 250?, and 350?F), three levels of air velocity (200, 600, and 1000 fpm) and two levels of humidity (very low and very high). Additional 3 by 6 inch heartwood yellow poplar and sweet gum (Liquidambar styraciflua) specimens, 1/8 and 1/4 inch thick, were dried at 150?, 250?, and 350?F using an air velocity of 600 fpm and a low humidity condition. After drying, specimens were conditioned to constant weight at 30 percent RH and 80?F, and strips 1/2 inch along the grain were sawn from the middle of each specimen. These were set on edge, split in half, and the amount of bowing used as a measure of residual stress. Analysis of variance showed that drying temperature, air velocity, specimen thickness and log differences had significant effects (5 percent level) on shrinkage. Tangential shrinkage tended to decrease with increases in temperature, thickness, and air velocity. Based on temperature and average moisture content determination of specimens during drying, it was concluded that drying stresses in veneer originate shortly before the average moisture content drops below the fiber saturation point. In yellow poplar residual stresses were essentially absent at 150?F but became progressively more pronounced as drying temperature increased. On sweet gum this effect appeared to be reversed. Bowing stresses were greatest at 150?F and apparently decreased with increases in drying temperature. There was, however, considerable honeycombing in the specimens dried at the higher temperatures.

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