Study On The Structure And Hygroscopic Deformation Of Plywood

Along with the change of the raw material strategy, the plywood industry has already faced a problem of matching many kinds of trees and different veneers together. The calculating model and program can be used for guiding the structure design of plywood, which has the great advantages of high production efficiency and saving production resources. The warping of the wood composite such as plywood and so on is a serious problem, which effects the production quality. Thus, it is very important to develop mathematics model to calculate and reduce the warp deformation of wood composite.Based on classical laminated plate theory(CLPT), and wood composite characteristics, the stiffness system of the wood composite was built in this thesis, and the stiffness system was verified by testing the strain of face and back plies through strain gage, when the test specimens were under single axial tensile load or pure bending moment load. The results from experiments showed that, the stiffness system could predict the stiffness of plywood well.In the application of stiffness system, four different kinds of unsymmetrical wood based panels were designed, and the qualitative relationship of warp deformation and coupling stiffness matrix were discussed. The results from experiments showed that, the bigger absolute value of coupling stiffness coefficients( Bi j) , the more panel warping ; the coupling coefficients—B1 1 and B2 2 had the major effect on panels warping. In addition, the structure of fir construction plywood was designed through puting the compressed veneers on the surface and under surface layers, and the MOE of plywood was increased obviously. The results from experiments showed the prediction and trial values were accordant, and the plywood elastic property had meet the design requirement. The content of this part has been already applied for the patent.In the environment of invariablenes temperature and increasing humidity, the hygroscopic deformation on the longitudinal, transversal and thickness directions of the poplar veneer, which treated by different glue spreading and hot pressing technologies, were tested. The results from experiments showed in the range of the moisture content from 0% to 25%, the relationship of hygroscopic deformation on the longitudinal, transversal and thickness directions and moisture content was nearly linear, the hygroscopic expansion coefficient coulde be seen to be a constant. Besides, the technology factors including glue spreading, pressure and adhesive type effect the transversal and thickness hygroscopic expansion coefficients of veneer greatly. Because of the technology characteristic of normal plywood, the Urea-formaldehyde(UF) between veneers effected the hygroscopic expansion property lower than Phenol-formaldehyde( PF),which has lower molecular weight, in addition, the addition of flour will increase the hygroscopic expansion property of veneer. On the one hand, the compressed veneer coulde increase the hygroscopic expansion property on the direction of thickness, on the other hand, the veneer compressed with higher density will reduce the hygroscopic expansion property on the longitudinal and transversal directions of veneer.According some related theory, the calculate model of wood compositehygroscopic deformation in and out plane was build in this thesis. The in-planehygroscopic deformation and warping of different structure plywood were tesyed, and the model ofhygroscopic deformation was validated, after the plywood treated in the environment of invariablenes temperature and increasing humidity. The thesis analyzed the orthography structure of plywood could reduce the hygroscopic expand property, and also brought the residual stress and strain. And it was point out that the panel warping could reduced by using the model of hygroscopic deformation to design the panel structure.In the last, structure design program for wood composite was developed with the Visual Basic and Access program, based the CLPT and the stiffness system of the wood composite.