Viscous Dissipation Of The Filling Process In Micro Injection Molding

With the fast development of MEMS technology, the micro-parts made of single silicon-based material cannot meet the quality and performance requirements of products applied in MEMS of different fields. Polymer material, due to its light quantity, high corrosion resistance, improved mechanical properties and so on, is widely used in MEMS. But the flow behaviors of polymer melt in micro injection molding differ from that in traditional injection molding. Especially for the temperature rise of melt in micro-channels caused by viscous dissipation, it directly changes some rheological parameters, such as shear viscosity and velocity gradient, and then influences the flow behaviors of melt and molding quality of the plastic part.Based on the comprehensive analysis of current researches on micro scale flow theories, the impact of viscous dissipation on melt's filling flow was studied with the combination of theoretical analysis, numerical simulation and experimental measurement. The control equation and constitutive equation were established from reference to the research methods of conventional injection molding and the characteristics of micro injection molding. The mechanism of viscous dissipation and its influence on the filling behavior of melts were also studied.The shear viscosity of PMMA, POM, ABS, PP, and PS polymer melts flowing through the different micro channels was measured by double-bowl barrel capillary rheometer. The Cross-WLF viscosity model parameters were also fitted by Matlab, which supplied accurate model parameters for the simulation of viscous dissipation. The melt's temperature and viscosity distribution were simulated by Polyflow, and the impact of shear rate, inlet temperature and micro channel's dimension on the viscous dissipation was studied.A micro scale measurement method of viscous dissipation was proposed. The outlet temperature rises of melts in circular micro channels with the diameters of 350μm and 500μm, and the rectangular channels with the same equivalent diameters at different inlet temperatures and shear rates were measured under the same conditions of simulation.The simulation and experimental results show that, the viscous dissipation is weakened with the decrease of channel's diameter and aspect ratio (the ratio between the length and the diameter), strengthened with the decrease of inlet temperature; the viscous dissipation in rectangular channel is more significant than that in circular channel. Also, the comparison of the experimental results with those predicted from the simulation was also analyzed. The results indicate that the outlet temperature rise of melt in micro scale depends on the impact of the combination of viscous dissipation and heat transfer through channel walls. The actual heat generated by viscous dissipation is more than that measured in experiment.