Experimental Study On Convective Heat Transefr Of Filling Flow In Micro Injection Molding

Due to the micro size of plastic parts and the large surface-to-volume ratio of the micro cavity, the heat of melt penetrated into the mold is little and the heat exchange between the melt and the micro cavity is accelerated, resulting in the rapid decrease of the melt temperature, the increase of the viscosity and the flow resistance, which fanially influences the filling flow of melt and the quality of molded parts.Based on the comprehensive analysis of the present research of the micro injection molding theory and technology, the convection heat transfer, behavior in micro injection molding was studied with the combiantion of theoretical analysis and experimental measurement.Firstly, the fundamental equations and convection heat transfer coefficient model of the filling flow in micro injection molding were established by introducing some simplifications and assumptions for the basic equations of viscous fluid dynamics.Secondly, the filling length of the polymer melt in the micro cavity with different cavity thicknesses was measured to study the influence of convection heat transfer behavior on the filling flow process. The results show that, the convection heat transfer behavior has a significant impact on the filling flow of the melt in micro scale. Then, the filling length was simulated with different constants and the theoretical models of convection heat transfer coefficient which are widely used at home and abroad under the same condition of experiment. The simulation differs a lot from the experimental result. It is indicated that the ignorance of wall slip, surface tension and other micro-scale effects in simulation is the main cause of the great error between the numerical simulation and experimental result and the reason why the simulation can not reflect the convection heat transfer behavior in micro injection molding accurately.With application of the convective heat transfer coefficient measurement system which consists of the self-designed micro mold, temperature transducer and other devices, the temperature distribution of different polymer melts filling through the micro cavity with different cavity thicknesses and surface roughnesses was measured, the convective heat transfer coefficient between the melt and the wall of micro cavity was calculated, and the influence of the injection rate, micro cavity thickness, surface roughness and thermal properties of polymer on convection heat transfer coefficient in micro scale was studied. The results indicated that, the convective heat transfer coefficient in microscale increases significantly with the increase of injection rate, surface roughness of the cavity and the decrease of the thickness; but it decreases with the increase of the distance between the measurement section and the entrance section of cavity in the development stage of the thermal boundary layer; Also, the convective heat transfer coefficient is closely related to the thermophysical properties of polymer, for the material with higher values of thermal properties, the convective heat transfer coefficient is larger.On this basis, according to the research methods of the convection heat transfer of Newtonian fluid, an appropriate experimental correlation of Nu which is suitable for the convection heat transfer in micro injection molding was established by fitting the experimental datas with the Matlab software. Compared with the experimental results, the values calculated by the Nu correlation agreed well with the experimental datas, indicating that the convection heat transfer behavior in micro injection molding can be well characterized by the correlation of Nu set here.