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1.
ABSTRACTGas permeability and structure of fiberboard mats are essential properties because of their impact on mat internal gas pressure, moisture content and temperature evolution during the hot-pressing process. The objectives of this work were to determine the effect of fiber size and mat density on the intrinsic gas permeability of the mat. For the study of these mat properties, panels with a homogeneous density profile through the thickness were manufactured at five density levels (200, 400, 600, 800 and 1000?kg m?3) for three different fiber sizes. Fiber refining was performed in a disk refiner at three plate spacings. Gas permeability was measured with an in-house built apparatus. The results showed that the fiber sizes studied had no significant effect on the intrinsic permeability. This was likely due to a more significant impact of the internal porous structure of the mat compared to fiber size. Besides, the intrinsic permeability decreased significantly when the panel density increased from 198 to 810?kg m?3. This suggests that the decrease of the intrinsic gas permeability during the last seconds of press closure plays an essential role in the bulk moisture mass transfer through the fiberboard mat. 相似文献
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研究刨花板板坯表面增湿处理对其热压过程中表面、中心层温度变化的影响,比较分析不同板材密度和厚度、板坯含水率及热压温度等工艺条件下,板坯中心层的升温速度随其坯表面喷水量的变化曲线,总结出板坯表面增湿处理对其中心层温度的影响规律. 相似文献
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采用常规热压法对刨花板板坯进行热压,探讨热压时中心层温度变化规律与板坯含水率、板厚、板材密度及热压温度等的关系.结果表明:在快速升温段,升温速度随板厚的增加而明显减小,随热压温度的提高而加快;在慢速升温段,升温速度随板厚的增大而显著加快,随热压温度的升高而明显加速,升温速度受目标密度和板坯含水率影响很小;板坯内水分蒸发所需时间随板厚、板坯含水率、热压温度、板材密度的增长而增加;板坯内水分蒸发温度随板材密度的增加而升高,随板厚的减少而升高,热压温度和板坯含水率对其几乎没有影响;加入胶粘剂会使快速升温段的升温速度有所加快,而使恒温段的水分蒸发温度有所降低. 相似文献
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以麦秸碎料为主要原料,采用漆酶水浴与干法两种不同处理方法压制麦秸无胶碎料板,研究热压温度、热压压力和热压时间对板材物理力学性能的影响。试验结果表明:影响麦秸无胶碎料板物理力学性能的主要因素是热压温度;水浴处理方法压制碎料板的物理力学性能优于干法压制的碎料板;漆酶水浴处理方法较优的热压工艺参数为漆酶用量43.6U/g,含水率10%,热压温度170℃,热压压力3~4MPa,热压时间20~25min。 相似文献
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微波预热MDF板坯的实验研究 总被引：2，自引：0，他引：2
利用微波对MDF板坯进行预热处理的研究结果表明,微波预热处理不仅能够在很短时间内提升板坯的温度,还能使板坯内的水分重新分布。通过处理,板坯表层的水分得到了提高,芯层水分被降低。这一结果有利于板坯在热压时热量从表层向芯层传递,从而缩短热压时间,提高生产效率。 相似文献
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A global model for the hot-pressing of MDF 总被引：2，自引：0，他引：2
The hot-pressing operation is the final stage in medium-density fiberboard (MDF) manufacture, where the mattress of fibers is compressed and heated to promote the cure of the resin. In MDF hot-pressing, many physical, chemical and mechanical processes are involved; the complexity of this operation arises from the fact that they are coupled. A global model is presented for this operation that integrates all mechanisms involved in the panel formation (heat and mass transfer, chemical reaction and mechanical behavior). This approach results in a two-dimensional unsteady state problem, which involves the knowledge of the polymerization kinetics of the resin, the transport properties and material properties, which are position and time dependent. This dynamic model was used to predict the evolution of the variables relating to heat and mass transfer (temperature, moisture content, gas pressure and relative humidity), as well as the variables relating to mechanical behavior (pressing pressure, strain, modulus of elasticity and density). The model performance was analyzed using the typical operating conditions for the hot-pressing of MDF and the results were compared to the experimental data from an industrial MDF press. We concluded that the model could predict in an acceptable way the behavior of the key variables for the control of the pressing cycle, as well as some physico–mechanical properties of the final product. The improvement of this model will permit the scheduling of the press cycle to fulfill objectives of minimization of energy consumption, better quality of the board and increased process flexibility. 相似文献
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重组竹热压板坯内部温度变化的研究 总被引：1，自引：1，他引：0
利用热电偶测试重组竹压制过程中板坯内部的温度变化,探讨了板坯厚度、板坯含水率、热压温度等因素对重组竹传热的影响。结果表明:①利用检测结果绘制的散点图趋势线有很好的线性关系。②重组竹热压过程中中心层温度的变化曲线可分温度缓慢传递阶段、快速升温阶段和慢速升温阶段。③板坯内含水率越高其传热效果越好。④酚醛树脂在高温下固化形成的隔热层对温度传导不利。 相似文献
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Hot-pressing is the main process in flakeboard manufacture. Studies in this field also emphasize the effect on the heat-transfer
process of the following factors: hot-pressing temperature, original moisture content (MC), target thickness and target density.
In this experiment, dynamic data of changes in temperature in mats can be logged through temperature transducers and a computer
data log system. The results of the experiment indicate that the core temperature-time curve can be divided into three stages:
a stage of a rapidly rising temperature, a stage of moisture vaporizing and a stage of a slowly increasing temperature. If
the hot-pressing temperature or the original MC increases during the first stage, the temperature will increase at an accelerated
rate. This rate of acceleration in thin or low-density boards is very high. During the second stage, increasing the hot-pressing
temperature or decreasing the original MC can shorten the time used to vaporize moisture. In thin or low-density board, this
time period is short. In the third stage the original MC does not affect the rate of temperature increase, while the effect
of the other factors is the same as that at the first stage. Given different conditions, vaporization temperature in the mat
changes.
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Translated from Journal of Beijing Forestry University, 2005, 27(2): 92–95 [译自: 北京林业大学学报, 2005, 27(2): 92–95] 相似文献
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通过模拟实际生产,研究16 mm刨花板在不同含水率和密度下板坯的反弹力,并与引进刨花板生产线的热压工艺进行对比分析,提出生产中制定热压工艺时应注意的问题. 相似文献
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Dr. S. Avramidis Dr. S. G. Hatzikiriakos Dr. J. F. Siau 《Wood Science and Technology》1994,28(5):349-358
Summary A model that predicts heat and moisture transfer through wood in the hygroscopic range and which is based on the principles of irreversible thermodynamics, was evaluated with unsteady-state nonisothermal moisture desorption experimental data. The model predicted the phenomenon of thermal diffusion during the initial stages of desorption and results in a very good simulation of the desorption curve and the center's temperature change with time.Symbols Cp
specific heat of air (= 0.24 cal/g K @ 70 °C)
- CT
specific heat of wood, cal/g K
- D
transverse diffusion coefficient, cm2/s
- Eb
activation energy, cal/mol
- Eo
heat of vaporization, cal/mol
- EL
differential heat of sorption, cal/mol
- G
specific gravity of wood
- H
relative humidity, %
- hT
convective heat transfer coefficient, cal/cm2 s K
- hc
convective mass transfer coefficient based on the concentration of moisture in wood, cm/s
- hv
convective mass transfer coefficient based on the concentration of moisture in the air in equilibrium with the wood surface, cm/s
- KM
coefficient for diffusion due to moisture gradient, g/cm s %
- KT
transverse thermal conductivity coefficient, cal/cm K s
- M
moisture content, %
- Po
saturated vapor pressure, atm
- R
universal gas constant, cal/mol K (= 82.056 cm3 atm/mol K)
- t
time, s
- T
temperature, K
- x
distance, cm
Greek Letters
evaporation or condensation criterion
-
wood density, g/cm3
- W
water density (=1), g/cm3
- a
air density, g/cm3
Department of Wood Science and Forest Products Virginia Polytechnic Institute and State University Blackburg, Virginia 24061-0503 相似文献
16.
Ramunas Tupciauskas Ilze Irbe Anna Janberga Edgars Buksans 《Wood material science & engineering》2017,12(2):63-71
A study of self-binding fibreboards pressed without synthetic adhesives is reported. The board samples of steam-exploded grey alder (Alnus incana L. Moench) particles hot-pressed under a variety of conditions including moisture content of the mat (8% and 12%), temperature (150°C, 160°C, 170°C and 180°C) and pressure (0.2–8 MPa) are tested for moisture resistance (standard EN 1087-1), decay resistance (standard ENV 12038) and reaction to fire performance (ISO 5660-1). The moisture resistance of the board samples is found to satisfy the standard EN 622-5 for load-bearing medium-density fibreboard for use in humid conditions (MDF.HLS). The tested board samples are not fully resistant to the Trametes versicolor fungus, the mass loss of specimens exceeding 3%. Reaction to fire performance of the self-binding board evaluated by the average rate of heat emission being by about 25% higher is worse compared with MDF.HLS. 相似文献
17.
A model to predict the bending modulus of elasticity (MOE) of oriented strand board (OSB) panels produced by batch processing
is presented. The approach developed herein is unique in its comprehensiveness since the MOE is determined from information
on the panel structure, temperature and moisture profiles and vertical density profiles obtained from the mat formation and
compression models presented in Part 1. Comparison of predicted MOE values with those measured from 24 commercially produced
panels shows good agreement considering some of the uncertainties involved. Simulations show that the MOE can be increased
by any of the following changes: reduced fines content, increased panel density, better flake alignment in each of the three
layers within a panel, increased flake length and a larger difference between the density of the face and core layers. The
model was also used in a genetic algorithm to carry out an optimization study of batch OSB manufacturing. This analysis showed
that by combining the appropriate reduction in the amount of flakes used, increase in fines content, improvement in flake
alignment within each of the face and core layers and shortening of the batch time, a significant theoretical profit increase
from the base case scenario can be obtained. 相似文献
18.
R. M. Nelson Jr. 《Wood Science and Technology》1986,20(4):309-328
Summary Choong's (1963) data for isothermal sorption of water vapor by wood are used to compute pressures, chemical potentials, and entropies of water in the wood specimens of his nonisothermal mass equilibrium experiments. Entropies of both the bound water and water vapor were reasonably constant. A balance existed between thermal diffusion and mass diffusion, as indicated by gradients in temperature and chemical potential. This balance also is suggested by opposing gradients in spreading pressure and vapor pressure. Equal chemical potentials showed that the vapor and bound water were in equilibrium. The model proposed by Siau (1980) for nonisothermal diffusion is consistent with these results. Expressions are given for the two unknown parameters in this model: moisture conductivity and heat of transfer. The constant entropy of water vapor is used to show that the heat of transfer exceeds the activation energy for bound water diffusion by about 25 percent.The author wishes to thank Dr. Christen Skaar for his helpful comments during preparation of this paper for publication 相似文献
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采用非稳态法测定龙竹竹材热压干燥过程中的水分扩散系数,并探讨了温度对水分扩散系数的影响.结果表明:干燥温度越高,干燥各阶段水分扩散系数及平均水分扩散系数也越大;初始高含水率阶段,随含水率逐渐降低,水分扩散系数呈逐步增加趋势,在纤维饱和点附近时达最大值;随后,随含水率逐渐降低呈逐步减少趋势. 相似文献