Research On The Laws Of Hot-Pressing Heat Transfer And Process Optimization Of Bamboo Scrimber

Bamboo-based fiber composite material featured by controllable performance,adjustable specification and design structure is a new type of high performance composite material.In recent years,it has been widely used in indoor furniture,landscape,wind power blades,building structures,traffic guardrail and other places are favored by consumers from all walks of life.It enjoys great development prospect for development at home and abroad.The material,with fibrotic bamboo veneer as the basic manufacturing unit,phenolic resin as the adhesive,is a new bamboo recombination material made through the line group blank,cold pressing or hot pressing.Hot pressing process is one of the important production processes of bamboo based fiber composite material.The selection of different controllable variables and process parameters can play a vital role to the efficiency and product quality of hot pressing production.This paper takes fibrous bamboo veneer as the research object,studies the influence of density and moisture content on the heat transfer performance of bamboo-based fiber composites and the three-dimensional temperature field change inside the slab,puts forward the "boiler effect" inside the slab during the hot pressing process,and optimizes the hot pressing equipment through the process and electromechanical control,and obtains the research contents.Through experimental analysis and mathematical modeling,the effect of water content on the heat transfer during hot pressing of bamboo based fiber composite materials with different densities was studied.The temperature change curve inside the bamboo based fiber composite slab was divided into four parts: constant temperature section,heating section,pressure maintaining section,and cooling section.The effect of water content on the change rate of each section was studied separately.At the same time,based on experimental data,a mathematical model of the effect of density and moisture content on the internal temperature rise of the slab is established to explore the thermodynamic phenomena within the slab during the hot pressing process.Combining the thermodynamic phenomena and prediction models,effective suggestions are proposed for the actual production of bamboo based fiber composites.In the hot pressing process,because internal pores of the plate blank are compressed and channels are blocked by the adhesive,water inside the plate blank vaporizes and expands,forming high pressure steam in the micro cavity.This is similar to the formation of a small“boiler”inside the plate blank.At this time,due to the increase of boundary moisture content,it will be not that wet.The superheated steam forms boiling bubbles in the micro-cavity of the plate blank.At this time,if the moisture content is higher or the temperature continues to rise,multiple boiling bubbles will gradually increase and connect to form larger boiling bubbles.Then,the vapor pressure close to 0.5MPa will be formed inside the plate blank.The phenomenon is called as“boiler effect”during the hot pressing procedure.“Boiler effect”,which can have a great influence on changes of temperature fields inside the plate blank,is the main cause of plate explosion.Based on the experimental data,the three-dimensional temperature field distribution model and moisture migration model of bamboo matrix fiber composites during hot pressing process are established.The results show that the temperature and moisture mainly migrate along the longitudinal direction of the fiber,and the transverse migration rate of the fiber is far less than the longitudinal direction of the fiber;The influence of paving width on the change of temperature field inside slab and "boiler effect" is determined by changing the paving length;The existence of "boiler effect" will significantly affect the water transfer law inside the slab.The water distribution of the slab during unloading is analyzed and compared through the test.Since the moisture content in the plate during the hot pressing process will greatly affect the heat transfer characteristics and "boiler effect" of the slab,a moisture content gradient paving process method is proposed,and the thermal and mechanical indexes of the gradient paving are compared.It is found that when the uniform moisture content paving is used,the higher the moisture content is,the stronger the "boiler effect" formed in the pressure maintaining stage is,and the higher the maximum temperature and steam pressure of the slab core is;When the gradient moisture content pavement is adopted,the internal moisture cannot form saturation,so the strong "boiler effect" will not be formed in the pressure maintaining stage,which reduces the risk of plate explosion to a certain extent.At the same time,gradient pavement does not significantly reduce the mechanical properties of the plate,which is a scientific and effective process method.In actual production process,it is difficult to pave moisture gradient,which may affected the production efficiency.Therefore,the water spraying method is used to optimize the process and replace moisture gradient pavement so as to realize the efficient utilization of steam shock heat transfer.Moderate control of water spraying volume and implementation of water spraying and pressing on plate blanks can ensure efficient heat transfer and reduce steam pressure inside the plate blank during the pressure holding stage.This can ensure the convenience and feasibility of the hot pressing process.At the same time,structures of the thermocompressor are optimized.The circumferential water flow pipes are drawn from the cooling water channels of the hot press plates and four atomizing nozzles are set on the water flow pipes to spray water on the plate blank surface according to the weight to optimize the hot pressing process.Regard to the need that enterprises cannot interfere with production,beauty,speed and efficiency,based on the temperature field change mechanism and production experience inside the slab,a multi-layer hot press PLC intelligent control platform has been built,which uses PLC controller and neural network algorithm to realize intelligent control of hot press equipment,and uses real-time monitoring automatic intelligent hot press equipment control system software to achieve better human-computer interaction experience,It is convenient for users to monitor and control the multi-layer press simply and efficiently to ensure the safety and reliability of the production process.