Flow Characteristics Of The Filling Process,Morphology And Nano-indentation Mechanical Properties Of Isotactic Polypropylene Micro Injection Molded Parts

In recent years,the application of thin-wall micro injection molded parts have been more and more widely used in the fields of micro electrical-mechanic,aerospace and biological medicine etc.At present,the systematic theories for the study of the flowing characteristics,the morphology and the mechanical properties of micro injection molded parts has not been formed.Single-factor experiment method was used in this paper to investigate the influence of the injection speed,melt temperature and mold temperature on the filling process,the morphology and structure,and the mechanical properties of the six-wings thin wall micro injection molded parts with different thickness.The research works and the conclusions are as follows:1.Flow analysis of the filling stage of thin wall micro injection molded partsThe 3D solid of the 6-wing micro injection molded parts with the thicknesses of 1.0mm,0.5mm and 0.2mm were molded using UG and MOLDFLOW software,The influence of the processing parameters on the filling process was analyzed.The results showed as following:The flow of the melt in the cavities of the micro injection molded parts is imbalance due to the sub-runner effect;The impact of the injection speed on the melt velocity,shear rate is relatively larger than the impact of the melt temperature and mold temperature.The velocity of the melt in the cavity was the largest in the center and the smallest at the wall,while the shear rate of the melt was the largest at the wall and the smallest in the center.The velocity and the shear rate of the melt in the cavities also increase with the injection speed increasing.2.The morphology of the micro injection molded partsThe morphology of thin-wall micro injection molded parts with different thicknesses molded under different processing parameters was observed by the polarizing microscope.The influence of various process parameters on the morphology of micro injection molded parts was analyzed.The influence mechanism was analyzed according to the numerical simulation results.The results showed:The fraction of the shear layer of the micro parts increased with the injection speed increasing,and the fraction of the core layer decreased.When the injection speed reaches a certain value,the core layer with spherulites in the micro injection molded parts disappeared,and the part showed a non-core morphology with the shear layer on the entire thickness.The micro injection molded parts with the thickness of 0.2mm showed a non-core morphology even under very low injection speed due to the micro scale effect;With the increase of the melt temperature,the proportion of core layer of micro-products gradually increased,and the proportion of shear layer gradually reduced;With the increase of the mold temperature,the proportion of the core layer increased,and the proportion of the shear layer decreased.3.The influence of the injection speed on the nano-indentation mechanical propertiesThe nano-indentation experiments of the micro-injection parts molded under different injection speed were carried out in the directions perpendicular to the flow direction and parallel to the flow direction.The influence of injection speed on the nano-indentation mechanical properties of the parts was analyzed based on the results of numerical simulations of the filling stage and morphology of the micros parts.The results showed:The nano-indentation process of the micro injection molded parts is continuous plastic deformation.The mechanical properties of different structural layers are different in the thickness direction of the micro injection molded parts.The elastic modulus and hardness of the core layer are the largest and the ice layer`s is the smallest;The injection speed had the familiar influence on the modulus and hardness of different structural layers,and the trend of different structural layers were familiar.The modulus and hardness of different structural layers had the same trend with the change of injection speed.In the nano-indentation direction perpendicular to the flow direction,the modulus and hardness of different structural layers increased first and then decreased with the injection speed increasing under the melt temperature of 240?;In the nano-indentation direction perpendicular to the flow direction and the nano-indentation direction parallel to the flow direction,the modulus and hardness of different structural layers decreased first and then increased with injection speed increasing under the melt temperature of 220?.The change of injection speed had large influence on the mechanical properties of nano-indentation along the direction perpendicular to the flow direction,but had little effect on the mechanical properties of nano-indentation along the direction parallel to the flow direction.