Moisture-induced distortion of laminated veneer products

Laminated veneer products consist of veneers bonded together with adhesive into apredetermined shape. Since wood is a hygroscopic material and also anisotropic bynature, laminated veneer products are especially shape-sensitive to changes in moisturecontent. A deviation from the intended shape is a problem for both the manufacturersand users of the final products and annually such deviations cause great economic lossesin the manufacturing industry.To illustrate the influence of moisture on distortion and shape stability, studies havebeen performed in industrial conditions and in a laboratory environment. Veneers ofbeech and birch and a seat shell moulded from these veneers were used in the study.Distortion, i.e. spring-back, position and twist, has been determined directly aftermoulding and during subsequent moisture and drying cycles.The distortion follows more or less slavishly the changes in relative humidity around theproduct. The distortion is generally small directly after moulding but, after the laminateshave been exposed to a variation in relative humidity, the distortion increases. Some ofthe problems of poor shape stability that may arise later in the bending process can bereduced if attention is paid to moisture content and fibre orientation already in theproduction of the veneer.To achieve good shape stability of laminated veneer products in practice, the followingshould be followed by the manufacture industry:• develop cooperation with suppliers of veneer and set requirements of veneerwith regard to deviation of the fibre orientation, and require that the veneer bedried and conditioned to a moisture content consistent with production,• control incoming veneers with respect to fibre orientation and moisture content,• plan warehousing of veneers and ensure adequate conditioning, and• consider the orientation of the veneers and the species.
Godkänd; 2013; 20131112 (ysko)