Waferboard Press Closing Strategies

During pressing of aspen waferboard, the wood wafers are compressed to an average density 1.75 times as great as the original wood density. The pressures required to accomplish this densification are related to the press closing time (PCT) and are substantially lower than that required to compress solid wood to the same extent. The mechanism by which the wafer mat is compressed involves stress concentration due to packing density variation and wood plasticization caused by heat and moisture. The PCT influences the shape and range of the density profile through the board thickness. With very short PCT the density profile is U-shaped. As PCT lengthens, the density profile takes on various M-shapes. The shape of the density profile strongly influences board strength properties and also edge permeability. The board edge permeability characteristic is indicative of the ease with which steam can escape from the board during pressing. As this value decreases, the board becomes more susceptible to delamination or “blows.” Waferboard is best made with as short a PCT as possible since this provides highest strength properties, highest edge permeability, and fastest heat transfer.

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