Study On Mechanism Of Interfacial Instability Of Polymer Multiphase-multilayer Flow Molding

Polymer multiphase-multilayer flow molding is the typical representative of the advanced new-style polymer molding technology, but interfacial instabilities have direct influences on geometric arrangement and interface shape, and then influence mechanical, chemical, optical properties and other performances of the final products seriously. However, the mechanism of interfacial instability is quite complicated, the study on the onset and evolution mechanism of interfacial instability is the important front of the advanced polymer molding technology. In this paper, the interfacial instabilities of polymer multiphase-multilayer flow molding have been investigated systemically, which yields the following main achievements:(1) To the flowing characteristics of polymer multiphase-multilayer flow molding process and on the basis of polymer rheology, fluid dynamics and thermodynamics, reasonable assumptions were put forward, theoretical models were established to describe non-Newtonian viscous and viscoelastic polymer melt multiphase-multilayer flow molding process respectively, and the corresponding accurate and stable numerical algorithms with fast convergence were proposed, and combining the programming techniques of the computer, the simulations of non-Newtonian viscous and viscoelastic instability of interface were implemented. Through a series of simulations, the effects of polymer rheological properties and several important process parameters on interfacial instabilities were investigated, and the mechanisms of interfacial instabilities were revealed.(2) As to the full three-dimensional, transient, non-isothermal, non-Newtonian viscous polymer melt multiphase-multilayer flow molding, based on Volume-of-Fluid method, the piecewise-linear interfacial reconstruction approach was adopted, and the free-surface and moving interface were tracked. On the basis of the mixed finite element stable discrete techniques of penalty function method and SUPG etc., the simulations of non-Newtonian viscous instability of interface were implemented and the interfacial instabilities of three layers symmetric flow systems were researched, results showed that with the flow rate of more viscous layer(core layer) increased, the unstable three-dimension fluctuated interface became placid("thin layer effect"), and with the viscosity ratio increased, the more viscous layer thickened and the interface became more unstable.(3) Mechanisms of non-Newtonian viscous interfacial instability were revealed: due to the differences of rheological properties across the interface and the incompatible geometric arrangement, the distributions of pressure field, temperature field, velocity field in molding process were nonuniform spatially and time-dependent, which caused the viscosity and shear rate nonuniform and time-dependent, and the internal stress of polymer melts correlated with pressure , viscosity and shear rate directly, the secondary flows would be induced by the nonuniform and time-dependent stress, therefore, the polymer melts in the mold cavity rearranged, the interfacial instabilities occurred and the three-dimension wave interface developed and evolved constantly.(4) As to two-dimensional, transient, isothermal, viscoelastic polymer melt multiphase-multilayer flow molding, the flows discretized based on moving mesh scheme and interfaces were fitted.Using the Giesekus constitutive model and the mixed finite element stable discrete techniques of EVSS, SU, etc., the simulations of viscoelastic instabilities of interface were implemented, results showed that the layer thickness rearranged with the change of flow rate, the higher the flow rate the thicker the layer, and the increase of flow rate of less viscous layer would make the interface more unstable while the rheological properties were not matching well; with the viscous differences enhanced, the thickness of each layer redistributed, interface deflected to less viscous layer, and the interfacial instability increased; with the elastic differences(relax time) enhanced, interface deflected to more elastic(higher relax time) layer, and the increase of relax time of thin layer would make the interface more unstable.(5) Mechanisms of viscoelastic interfacial instability were revealed: the distributions of pressure field, velocity field, stress field in molding process were nonuniform spatially and time-dependent because of the differences of rheological properties across the interface and the incompatible geometric arrangement, which produced a jump in the first normal stresses difference across the interface. The interface would be stable when the jump in the first normal stresses difference vanished. With the jump in the first normal stresses difference enhanced, interface deflected and the thicknesses of layers rearranged, the interfacial instability increased; the jumps in the first normal stresses difference across the interface were the radical cause of interfacial instability.(6) In the engineering process of polymer multiphase-multilayer flow molding, in order to circumvent the interfacial instability, suitable molding materials should be chosen and avoid excessive differences in viscosity and relax time, and flow rate should be set properly to get appropriate layer arrangement.