Research On The Mechanism Of In-Mold Micro Assembly Under Multi-Physical Field

In-Mold Micro Assembly molding technology is expected to be cost-efficient and powerful technology of large-scale industrialization of polymer micro-mechanical systems manufacturing.Polymer material due to light weight,corrosion resistance,good insulation,high forming efficiency,it has became the first choice for in-mold micro assembly forming process.Polymer processing melt involves not only is viscous,showing the characteristics of fluid,but also is elastic,exhibiting the characteristics of solid.That is so-called viscoelastic.In the process of in-mold micro assembly,preformed micro solid part induces thermal-fluid-solid coupling deformation during the filling flow process of subsequent molding high temperature viscoelastic melt,which affects the quality and precision of assembly.By searching literature,the related research reports about deformation behavior of micro solid part in the environment of viscoelastic melt filling has not been found.Based on the theoretical premise describing scientifically and accurately the viscous friction constraint boundary conditions and the elastic support constraint boundary conditions between tiny solid structure and viscoelastic melt,the thermal-fluid-solid multi-physical field coupling mechanism of in-mold micro assembly molding process was revealed,and finally implement the leap transformation of polymer micro mechanical system in-mold micro assembly molding technology from recent groping manufacturing in which one success is searched in many accidental opportunities to seeking to the whole process integrated control industrialized and scientific manufacturing in which quality was guaranteed by science and success was guaranteed by technology.The advanced in-Mold Micro Assembly molding process was systematically studied by numerical simulation and experiment in this paper.The main results are as following:The theoretical model describing viscoelastic thermal-fluid-solid multi-physical field coupling of viscoelastic melt filling flow and preformed micro solid part in-mold micro assembly molding process has firstly been established in this paper.In this theoretical model,elastic normal stress and viscous shear stress of the extra stress tensor can be a true reflection of the effect of viscoelastic melt elastic supporting boundary constraints role and viscous friction drag clipping boundary constraints effect on micro assembly interface of preformed solid micro part,the theory can not only accurately predicts the secondary molding viscoelastic melt filling flow induced thermal-fluid-solid coupling deformation of preformed micro solid shaft,and can accurately predicts preformed micro solid shaft near the surface melting locally induced micro assembly interface failure;By the establishment of digitized virtual research platform of micro mechanical systems in-mold micro assembly molding manufacturing process simulation,the influence rheological parameters of viscoelastic polymer melt and process parameters on thermal-fluid-solid coupling deformation and micro assembly interfacial failure in in-mold micro assembly forming process has been studied systematically,and the influencing mechanism was revealed and the key regulatory parameters was clarified;Research results show that the thermal-fluid-solid coupling deformation of preformed micro shaft and viscoelastic support normal stress has negatively related relationship,viscoelastic support normal stress plays an inhibitory effect on the deformation,and the thermal-fluid-solid coupling deformation of preformed micro shaft and viscous friction drag shear stress has positive related relationship,viscous friction drag shear stress plays an enhancement effect on the deformation,and the thermal-fluid-solid coupling deformation of preformed micro shaft;The increasing of melt injection temperature make near the surface temperature of performed PMMA micro shaft across the glassy phase change completely(micro shaft near-surface temperatures in excess of 393K),which induces localized melted layer near the surface.while the thickness of partial melting layer that across the glassy phase change completely exceeds the critical thickness,the melt in the partial melting layer region crossed the glassy phase change completely near the surface of micro shaft will loss resistance to deformation,finally causing crushed failure at the micro assembly interface is induced by fluid-solid coupling pressure difference and viscoelastic support normal stress.On the other side,the viscous friction drag shear stress in the micro assembly interface will cause flashing failure of the melt in the partial melting layer region near the micro shaft surface.Finally preformed micro shaft micro assembly interface lose their original smooth and flat surfaces,causing micro assembly interfacial failure.