Study On Mechanism And Optimal Control Of Powder Conveying And Pressing Process

The powder conveying efficiency is affected by the clogging of the silo and the particle segregation,brick defects have a great impact on the quality of powder products.In order to ensure the continuity of powder conveying and the uniformity of powder distribution,and improve the efficiency of powder conveying and the quality of powder forming,this paper mainly studies the discharge characteristics of the silo,analyzes the mechanism of powder conveying and pressing process,optimizes the related control parameters.The major contents of this dissertation include three aspects: 1)Based on the theoretical analysis of the formation mechanism of hopper arching,pneumatic arch-breaking system ultilizing air jets is designed,whose validity and rapidity of of arch-breaking is proved by pre-experiment.The influences of air jet injection position height,jet number,injection speed,injection angle and injection time on the arch-breaking effect are analyzed by means of numerical simulation and experimental investigation.The impinging zone of air jets is simulated by Fluent on the basis of 3D gas-solid two-phase flow model.The optimal jet injection parameters are obtained after the analysis of simulation and experiment results.2)The feeding system of ceramic press machine,which is mainly consists of conical silo,wedge hopper and feed frame,is the main research object.Based on the dynamic arch theory,the discharge rate prediction model of the conical silo and wedge hopper are derived.The accuracy of the prediction model is verified by the unloading experiment.The wedge hopper is optimized to the curved hopper,and it is proved that curved hopper can improve the discharge efficiency and powder distribution uniformity by theoretical and experimental analysis of discharge resistance and powder pressure.Subsequently,in order to ensure the sufficient amount of powder conveying,the relationship between the sliding speed of feed frame and hopper discharge rate is derived.The mechanism of particle segregation during the sliding process of feed frame is analyzed.The effects of feed frame speed,sliding surface roughness and lattice size on the particle segregation degree are studied by experiments.To improve the uniformity of powder distribution,the optimal control measures are proposed to reduce the particle segregation degree.3)The stress state of ceramic powder during the pressing process is analyzed and the causes of the defects of ceramic brick are summarized.The effects of pressure loading rate,pressing force,holding time and depressurization rate on the forming quality of ceramic bricks are studied by single factor experiments.In addition,through orthogonal test,the primary and secondary order of compression parameters that affect the forming quality of ceramic brick is pressure loading rate,pressing force,depressurization rate,holding time.In order to improve the compression productivity and the forming quality of brick,the optimal combination of pressing process parameters is obtained according to orthogonal test results.Finally,the pressing equation between ceramic body density and forming pressure is obtained,which is fitted by exponent function.The pressure distribution gradient and air exhaust path during the pressing process are also obtained.