Heat Balance Optimization Of The Mica-type-heater Nozzles

Hot runner injection molding has many advantages over traditional cold runner injection molding,which is the development direction of mold in our country.The hot nozzle is the core component of the hot runner system and is the part closest to the mold cavity.The equilibrium of its temperature field directly affects the quality of the product.The use and research of mica-type-heater thermal nozzles in hot runner are rare.In actual production,lacks of standards and depends on experience often leading to product quality defects.In this paper,the mica-type-heater thermal nozzle is researched for the need of using a single thermocouple to control the temperature.The key process parameters affecting the thermal balance of the hot nozzle were studied by the experiment of the mica-type-heater hot nozzle.And by solving the exact thermal boundary conditions,these key process parameters can be optimized by using precise CAE temperature field analysis.So the hot nozzle can achieve thermal balance and meet the quality requirements of products.(1)The temperature field of the hot nozzle is analyzed and the key process parameters that influence the temperature field balance of the hot nozzle are found out.The thermal nozzle uses a single thermocouple feedback heating method.The study of the temperature field of the whole hot nozzle is simplified to the study of the temperature field of the hot nozzle region where a single mica heater is located,that is,to study the “heating unit”.An orthogonal experiment table was designed to test the five factors affecting the thermal balance of the hot nozzles at four levels.The test data was analyzed by range analysis and variance analysis.It was found that the impact of the influencing factors from large to small were: Heater length,heater power,heater spacing,hot nozzle internal diameter,mold temperature.In the range of selected factors,the longer the length of the heater is,the smaller the temperature difference of the hot nozzle is;The greater the heater power,the greater the temperature difference;The heater spacing has an extreme value at 30 mm,and the temperature difference appears to decrease first and then increase.The greater the inner diameter of the hot nozzle is,the smaller the temperature difference is;With the increase of the mold temperature,the temperature difference of the hot nozzle drops first and then rises,but the amplitude is very small,only 2 °C ~ 3 °C.(2)The hot nozzle thermal boundary conditions were experimentally studied,the accurate thermal boundary conditions required for temperature field simulation of thermal nozzle are solved.An experiment was designed to prove the importance of accurate thermal boundary conditions for simulation accuracy.The maximum deviation between the simulation results and the actual values was 46.67% when the middle range of the thermal boundary conditions was selected according to the literature.Then combining ANSYS Workbench with inverse method and golden section method to fit the simulation data and experimental data,the convection heat transfer coefficient and emissivity of the hot nozzle body H13 steel and air are obtained,and the empirical formula of the thermal boundary condition is calculated by the Lagrangian interpolation method.The error between the temperature field and the experimental value simulated by this boundary condition is within 5%.A set of comparative experiments was designed to examining the influence of the thermal nozzle head material tungsten steel,beryllium copper and their combined thermal conductivity on the gate temperature,the results show that the difference in thermal conductivity below 250°C does not affect the gate temperature.(3)According to the obtained thermal boundary conditions,the thermal nozzle heating unit is parameterized in with the help of ANSYS Workbench software.The design variables are heater length,heater spacing,and heater power.Useing experimental design DOE and direct optimization tools to optimize the maximum and minimum temperature of the heating unit.Recording the sample point parameters of the conformity range,round it according to the specifications of multiple heaters,and import it into ANSYS Workbench for secondary calculation.Record the final sample point of the conformity range and make heater selection and assembly tables.The mica-type-heater hot nozzles selected according to the type selection and assembly table can meet the molding quality requirements of the products.The study found that under the premise of a single thermocouple feedback temperature control,the hot nozzle thermal balance can be achieved by ensuring a reasonable key parameters of the mica heater-type thermal nozzle,i.e.the heater length,power,and spacing.And the key parameters above have a suitable parameter range in the range of plastic injection molding temperature,which can meet the needs of different lengths of the hot nozzles and ensure the quality of the products.