Moisture-induced Distortion of Laminated Veneer Products

Laminated veneer products consist of veneers bonded together with adhesive into a predetermined shape. Since wood is a hygroscopic material and also anisotropic by nature, laminated veneer products are especially shape-sensitive to changes in moisture content. A deviation from the intended shape is a problem for both the manufacturers and users of the final products and annually such deviations cause great economic losses in the manufacturing industry. To illustrate the influence of moisture on distortion and shape stability, studies have been performed in industrial conditions and in a laboratory environment. Veneers of beech 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 after moulding and during subsequent moisture and drying cycles. The distortion follows more or less slavishly the changes in relative humidity around the product. The distortion is generally small directly after moulding but, after the laminates have been exposed to a variation in relative humidity, the distortion increases. Some of the problems of poor shape stability that may arise later in the bending process can be reduced if attention is paid to moisture content and fibre orientation already in the production of the veneer. To achieve good shape stability of laminated veneer products in practice, the following should be followed by the manufacture industry: • develop cooperation with suppliers of veneer and set requirements of veneer with regard to deviation of the fibre orientation, and require that the veneer be dried 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.