Factors to Be Considered in Force Drying Wood Finishes

In force drying finishes, the most important property of the wood to be considered is its hygroscopicity; its hollow cell structure allows it to absorb and release moisture readily. The relative humidity in ovens must be at a level which will prevent sudden and severe loss of moisture from wood. Temperatures above 150?F cause such rapid moisture change that checking, cracking, delamination, and loosening of glue joints can occur. In drying, finishes undergo evaporation of the solvent, oxidation of oils in finishes such as oleoresinous and alkyd-type varnishes, and polymerization by heat or chemical catalysts. Force drying equipment must thus provide for the evaporation of the solvent, a supply of oxygen, and thermal molecular excitation, while maintaining controlled humidity. Enough time must be provided for a solvent to evaporate to avoid defects such as casehardening. In general, the temperature in the oven should rise gradually, reach a specified level, and remain at that level for a specified time. Also of importance is the distribution of heat in the oven. The flow of air through a conveyorized oven should be counter to the direction of movement of the finished pieces. Three parts of recirculated air to one part of fresh air is a generally accepted ratio.

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