Study On Radio-Frequency Vacuum Drying Technique And Strain Distribution Of 5mm Thick Mongolian Oak Veneer

Solid wood composite flooring combines the stability of laminate flooring with the beauty of solid wood flooring,and has the advantage of environmental protection.However,the difference in moisture content between the surface layer,core layer or the bottom layer must be strictly limited during processing to ensure that it has good dimensional stability in use.and therefore high quality drying of the substrates used,especially precious surfacing veneers,is crucial.The use of high-frequency vacuum drying technology for veneer drying has the advantages of drying capacity,speed,and easy installation,but in the industrial production,the moisture content,temperature parameters are difficult to detect,and the drying process is not mature with the serious drying defects of dried veneers.In addition,there are few studies on high-frequency vacuum drying of veneers.Based on the above reasons,the 5mm thick mongolian oak veneer was as the research object in this paper.First of all,we experimentally determined its appropriate high-frequency vacuum drying process(high frequency vibration and stop time,wood control temperature,environmental pressure),aiming to provide reference for high-frequency vacuum drying production of veneers;Then,we measured the temperature of the veneers in different drying stages and the moisture content distribution after drying under the abtained appropriate drying process,and the relationship between the moisture content distribution and the temperature distribution was analyzed to provide basic data for further research on the heat and mass transfer mechanism during the drying process of the veneers;Finally,the digital image correlation method(DIC)was applied to abtain the change of the strain distribution in the drying process of the veneers,which to provide the basis for further research on the development rule of the drying stress of the veneer,the mechanism of the suppression of the drying defects,and the establishment of the suitable drying process.The specific results of the research and the important conclusions are as follows:1)The mongolian oak veneers were performed different high-frequency vacuum drying experiments with parameters including wood temperature,drying rate,final moisture content and its standard deviation,cracking and warp measured,then through the analysis of these parameters to determine the suitable drying process.The results showed that:(1)The quality of veneers after drying depended on the temperature and drying process.Although the increasing temperature can accelebrate the drying speed,the veneers were easy to crack;Among them,there were 11 cracks(Tc=64.5?)for the veneer under process 1,and only 1(Tc=54.5?)for process 6.(2)The distribution uniformity of the final moisture content of the veneers was mainly affected by the temperature distribution uniformity determined by the equipment andthe drying process.The thermal insulation material between the veneers and the electrode plates can reduce the temperature gradient in the height direction of the veneers stack and lessen the difference in moisture content among veneers;The maximum and minimum moisture contents of the veneer under process 6 were 10.31%and 7.90%,respectively.The standard deviations of the maximum and minimum final moisture contents were 0.79%and 0.35%,respectively.The average final moisture content was 8.60%.and the mean square deviation was 1.06%.(3)The warpage of the veneers under process 6 were the lowest,all less than 1.16%;(4)The more suitable drying process for the veneers obtained from the experiments was that the high-frequency vibration was 7 min/stop 1 min,the wood control temperature was 54.5?,and the ambient pressure was 6.5kPa.2)Based on the suitable drying process determined in the above study,the temperature distributions of the veneering during the different drying stages were measured by the optical fiber temperature sensors and the shielded Pt100 and the water content distribution after drying was measured by the weighing method,and then the distribution of temperature,moisture content,and their relationships were explored.The results showed that:(1)The temperature distribution,along length direction,were that the near end of the drying stage was slightly higher,and the uniformity in the middle and later stages increased;Along the width direction,the uniformity in the initial stage of drying was large,and center of the veneer was high,the near side was low in the middle stage,and in the later stage the moisture gradient had a slight increase;In the height direction,the center was high and the near electrode plates was low.especially near the ground plate;(2)Distribution of moisture content after drying,along length direction,due to the near initial moisture content in the length direction of the veneers,the temperature distribution in the drying process was also close,so that the moisture content distribution was also relatively uniform;In the width direction,the moisture content was mainly affected by the temperature distribution so that the center was low and the proximal end face was high;And the moisture content in the height direction was also affected by the temperature distribution,the center was low,and the near electrode plates was high,especially near the ground plate.3)For the veneer surface after spraying with different painting colors,using the average gray gradient as the quality evaluation parameter,to have determined the color of black and silver as the spot color combination.And then we also collected the digital image of the veneer drying process and calculated displacement and strain distribution changes with the DIC technique.The results showed that:(1)The strain distribution in the longitudinal direction during the drying process was not obvious in the initial stage of drying,and the strain in the center was close to zero,the closer to the end surface and the larger the strain was,and in the later stage the strain even more increased;(2)The strain distribution in the width direction had no obvious regularity in the initial stage of drying,and in the middle stage the compressive strain and the tensile strain were showed up in the proximal end surface,and in the later stage the whole field on the veneer surface were the compressive strain after the late tensile strain was transformed to compressive strain;(3)As for the shear straindistribution,the initial stage of drying had no obvious regularity,in the mid-stage and late-stage the centers were close to zero,and the strain value from the center to the surrounding areas gradually increased.However,due to the wood properties,whether the strain value was positive or negative was not certain.