Shrinkage of Douglas-Fir Veneer Dried By Air Impingement

The shrinkage of Douglas-fir veneer dried in an air impingement dryer was studied. The effects of three dryer temperatures, three air velocities, two veneer thicknesses, two veneer qualities, two wood types, and MC were analyzed with respect to length, width, and thickness shrinkage. Length shrinkage was unaffected by any of these factors. Width and thickness shrinkage were found to be a complex phenomenon involving all of these factors; statistical analysis revealed significant effects and interactions in many cases. Approximately 40 percent of the variation in thickness and width shrinkage was unexplained by any of these factors, probably because of difficulty in measuring veneer dimensions due to surface roughness and grain orientation. All factors except MC were considered of little practical importance in minimizing veneer shrinkage; however, dryer temperature was found to have a fundamental effect on width and thickness shrinkage. Shrinkage generally increased with increasing temperature. In thickness there was apparently a critical point, after which shrinkage decreased with increasing temperature. The overall average shrinkage of Douglas-fir heartwood veneer dried from a green condition to approximately 8 percent MC was 4.0 percent in width, 3.1 percent in thickness, and 0.1 percent in length.

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