The Compression of Douglas-Fir Veneer During Pressing

Several factors influence the compression of veneer: surface characteristics, specific gravity, moisture content, adhesive spread, pressure, pressing time, and final conditioning treatment. Compression was shown to increase with increasing temperature. Likewise, compression increased with increasing pressure, with compression approaching a maximum after about 10 minutes of pressing time for all pressure and temperature combinations. The major amount of compression occurred during the first 5 minutes. The effects of compression could be overcome by conditioning the samples to a 12 percent EMC. In terms of strength, both wood failure and shear strength variation were limited within the range of conditions (100- 260 psi pressure and 220-320?F. temperature). The control of compression was shown through the use of limit-control instrumentation and remote by-pass valves located in the hydraulic pressure lines. The economic advantages of controlling compression were cost reduction through material savings, power consumption reduction, increased press output, and reduced mechanical workload. Experimental results on 3-ply Douglas-fir plywood using 1/8-inch veneers subjected to controlled compression, indicated a plywood that was at least equivalent, if not better, to those produced under uncontrolled conditions. The use of radio frequency heating yielded panels of comparable quality.

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