The Influence Of Injection Speed On The Filling Stage Of Micro-injection Molding And Morphology Of The Parts

Micro-injection molding process optimization is one of the most important parts in the process of micro-injection molding. However, there are many behaviours of plastic melt in the process of micro-injection molding which can’t be explained by the conventional theories of filling and the setting of process parameters is not suitable for it as well. As the injection speed is rising, there may exist wall slip that caused by the higher shear rate in the filling runner, at the same time, the shear condition of melt in the mold is very different from conventional injection. But the shear state has important influence on the crystallization of melt, which make a great difference in the products contrasted with the conventional injection. It is necessary to proceed with the research on the law of slip condition and morphology evolution of the micro-parts arising in micro-injection molding.Based on the traditional polymer rheology theories and flow characteristics of micro injection molding, the relationship of wall shear stress and injection speed was investigated by combining the micro injection molding experiments and MOLDFLOW simulations, and the3D simulation of filling stage in micro injection molding was implemented on ANSYS CFX platform. The influence of injection speed on morphology of the micro injection molded parts was investigated by experiments. The work and achievements are as followings,1. Base on the fundamental equations of viscous fluid dynamics and flow characteristics of the filling stage of micro injection molding, the viscosity model and wall slip model suitable for the micro-injection molding were adopted.2. The single factor experiments of injection speed were conducted on the micro-injection machine and some micro parts were made. At the same time, the data of injection pressure in the experiment were recorded which was important for the analysis of pressure and flowing rate of melt. And the shear stress from the simulation result was compared with critical shear stress of iPP material. The comparison results indicated that the wall slip will exist in the filling stage in the condition of higher shear rate. 3. The secondary development of ANSYS CFX software was implemented and the factor of wall slip was considered in the simulation of micro-injection molding. The micro-scale parts (1000μm) were adopted to investigate the influence of injection speed. The simulation results indicated that the melt velocity as well as the pressure in the cavity increased with the injection speed increasing. But after considering the factor of wall slip into the simulation, the velocity of melt increased but the pressure decreased. So the influence of wall slip was very obviously, and couldn’t be neglected in the process of micro-injection molding.4. Polarized light microscopy (PLM) experiments were conducted to investigate the influence of injection speed on morphology of micro parts. The results indicated that the core thickness reduced as the injection speed increased and the core layer disappeared at high rate compared with the conventional injection.