| With the "fourteenth Five-year plan" "double carbon" target approaching and "Made in China 2025" plan outline proposed,electronic fuel injection technology has become an important way to achieve energy conservation and emission reduction in the automotive field,among them,alumina ceramics because of high melting point,small dielectric constant,large volume resistance,heat resistance and strong impact performance advantages,it is considered as an ideal high temperature resistant insulating material for the outer layer of engine temperature sensor used in electronic fuel injection system of automobile.But the characteristics of high hardness and toughness of low alumina ceramic itself make it not to the traditional deformation processing and mechanical processing,injection molding process for can direct the preparation of various products,complex geometry and high precision size without the characteristics of mechanical processing,easy to implement mass production,meet the demand.In the alumina ceramic injection molding,to save costs,improve production efficiency,often using more than one mold cavity mode of production,however,the unreasonable design of the gating system and cooling system easily leads to the residual stress in the parts,and then leads to the slippage phenomenon in the pores of the grain boundary in the parts and induces the micro-cracks,which seriously affects the forming quality of alumina ceramics.Therefore,how to reduce the residual stress in alumina ceramics has become an urgent problem to be solved in the field of high temperature resistant alumina ceramics injection molding for automobile engines.To solve the above problems,with the support of the National Natural Science Foundation of China,the design work of automatic hot die casting machine and supporting mold for high temperature resistant alumina ceramics used in automobile engine was carried out.The optimization of geometric parameters and related process parameters of casting system and cooling system was carried out from two aspects of reducing residual stress in forming and cooling stage.The main research contents are as follows:(1)The advantages and disadvantages of the main molding processes of alumina ceramics(dry pressing,isostatic pressing,flow casting,injection molding)and the optimization research status of injection molding at home and abroad were analyzed.Combined with the major application requirements of high-temperature resistant alumina ceramics used in automobile engines,the research contents and technical route of this paper were determined.(2)Aiming at the disadvantages of low automation and poor product consistency of traditional hot die casting machine,alumina ceramic automatic hot die casting machine and its die were designed.First of all,the main channel,shunting channel and gate design of alumina ceramic hot die casting mold were carried out,and combined with the influence of alumina feeding special shear thinning effect on molding quality,the shear rate range of the design parameters were checked;Then,the parting surface,forming parts,guiding and positioning mechanism,demoulding mechanism,cooling system,exhaust system,hot die casting device were designed.(3)Based on the laws of conservation of mass,momentum and energy,three fluid governing equations of suspension were established.Einstein model for thin suspension,Krieger-Dougherty model,Quemada model,Sisco model for high concentration suspension were established according to the characteristics of different suspensions.Power rate model for pseudoplastic fluid and dilatant fluid and Cross model for pseudoplastic fluid.(4)Aiming at the residual stress caused by unbalance filling,the optimization of alumina ceramic mold pouring system was studied.Based on the orthogonal test method,the gate structure was optimized to increase the shear rate and reduce the flow viscosity,and the optimal gate geometric design parameters were obtained when the shear rate at the gate was the maximum.Based on the geometric parameter balance method,the influence law of the diameter difference of the inner tangent circle of the four-stage shunt passage on the unbalance rate of the inner and outer cavity was analyzed,and the optimal geometric design parameters of the flow passage were obtained.Analyzed the molding process parameters(mold temperature,melt temperature,injection rate)inside the influence law of the outer cavity filling imbalance rate and cubic design based on response surface(CCF)was established between unbalanced filling rate and molding process parameters of the second order response surface model,the optimum parameters of filling uneven when the lowest rates.Finally,the reliability of the optimization scheme of pouring system was proved by the verification test.(5)Aiming at the residual stress caused by uneven cooling,the optimization of cooling system of alumina ceramic mold was studied.Based on single factor experiment,the influences of flow direction of cooling medium,geometrical parameters of cooling loop(channel diameter),physical parameters of cooling medium(coolant inlet flow rate,inlet temperature)on cooling effect(temperature rise of inlet and outlet of cooling loop,pressure loss,average temperature of die surface,standard deviation of die surface temperature)were studied.The geometric parameters of cooling loop and physical parameters of cooling medium were orthogonal tested with three factors and three levels,and the effects of them on the average temperature and standard deviation of die surface and the optimal combination level were studied by using single-objective range analysis.Comprehensive weighted scoring method and multi-objective range analysis method were used to comprehensively optimize the experimental factors,and the optimal combination level and influence sequence of experimental factors were obtained under the objective of minimizing the mean temperature and standard deviation of die surface temperature simultaneously.Finally,the reliability of the cooling system optimization scheme was proved by verification test. |
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