Varying Moisture Content of Mechanically Dried Veneers

Veneers dried in mechanical dryers exhibit wide variation in moisture content both within and between individual sheets. Experience developed over a number of years at the Forest Products Research Laboratory, Princes Risborough, England, have permitted classification of species into three broad classes of freshly dried veneer including those with less than normal moisture content variation, those with a normal variation, and those with greater than normal variation. It was found that moisture content variation was not affected by veneer thickness, drying time or by machine settings, and the differences due to species were small. The factor contributing greatest to moisture variation was reported to be the average moisture content to which the veneers were dried. While the variation decreases as the average moisture content is lowered, wet patches of more than 20 percent moisture content can occur even with veneers dried to an average of 6 percent. Because of within-sheet conditioning, wet spots were not found to cause mold. On the other hand, in hot pressing at temperatures above 100?C, wet spots can form steam pockets and cause blisters in the plywood, making it necessary to condition veneers before gluing. Conditioning in the stack was found to be effective, except that periods of weeks were necessary to attain uniform moisture distribution. An alternative to natural conditioning was suggested in a forced conditioning of individual veneers in a specially designed chamber which introduced high-temperature air at high rates of circulation. Small-scale tests of such a process indicated that uniform moisture content could be obtained with 0.064-inch beech (Fagus sylvatica) veneers in about 15 minutes.

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