Moisture Leveling in Veneer By Microwaves and Hot Air

A microwave hot air drying system was developed to selectively dry and level the moisture content in veneer of Douglas-fir, western larch, western hemlock, white fir, Engelmann spruce, ponderosa pine, and southern yellow pine. Two magnetrons produced 50 kW microwave power of 915 MHz frequency and supplied it to a meander type wave guide applicator. This applicator was combined with a direct gas-fired jet dryer. In one phase of the study, veneer that had remained too wet after drying in a production hot air dryer was redried at a number of constant feed rates and air temperatures up to 300?F. In another phase, the feed rate was controlled by a newly designed speed control system and was varied according to the initial moisture content of veneer. Moisture leveling in veneer could be achieved. The drying selectivity was due to a greater microwave absorption in wetter areas. Drying times varied between 45 and 463 sq. ft./min., depending on species, thickness, and initial moisture content. Differences in microwave drying characteristics of tested species were due to differences in specific gravity and, moisture distribution. A comparison was made of glue-line quality in panels manufactured from once-dried veneer, microwave redried veneer, and veneer redried in hot air only. Results obtained with exterior and interior type adhesives showed the gluability of microwave redried veneer to be equal to or better than of veneer of the other categories. Microwave redrying is believed to represent marked improvement over conventional redry methods where surface inactivation is observed. It prevented over-drying and also kept the veneer flexible and less likely to break and split in handling.

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