The Numerical Simulation On Influence Of Interfacial Slipping On Polymer Co-extrusion Molding Process

Interfacial instability and die swell are two important factors of co-extrusion molded part's performance. are worth to study both in practical and theoretical. The interaction of the stresses on the interfaces between adjacent melts in the multi-phase flow of the three dimensional co-extrusion molding makes the transporting rule and the dynamic character of the co-extrusion molding process special. Thus, the influencing laws and mechanisms of process parameters and rheological parameters on interfacial instability and die swell are very complicated and the related mechanisms are still unknown. In this paper, based on the theories of rheology and hydrokinetics, the full 3D, isothermal viscoelastic theoretical model of co-extrusion molding process and the adapted highly effective finite element model are established and through numerical simulations the influencing rules and mechanism of process parameters , rheological parameters and interfacial slipping on interfacial instability and die swell are systematically studied. The following main achievement are yielded:(1) Based on technique characteristics of the traditional co-extrusion molding process and polymer rheology, thermodynamics and hydrokinetics, the full 3D, steady isothermal viscoelastic theoretical model describing the traditional co-extrusion molding process is established by means of some reasonable assumptions.And on the basis of the mixed finite element stable discrete techniques of Galerkin method, penalty function method, EVSS, SU and Mini-Element method etc., the corresponding highly effective stable finite element model is put forward. The three dimensional large-scale, high non-linear finite element simulation on traditional co-extrusion molding process was implemented in common personal computer.(2) Through numerical simulation on traditional co-extrusion molding process, the influence laws and mechanisms of the process parameters, reological parameters and interfacial slipping on interfacial instability and die swell is studied systematically. The result shows: the interfacial instability increase with inflow-velocity and viscosity of upper-layer melt, and decrease with the slipping coefficient. The interfacial instability is mainly determined by the wavy degree of first and second normal stress difference. The interfacial instability increases with the wavy degree of normal stress. Or else, interface gets steady.(3) The die swell of core, shell and whole melt increase with slipping coefficient. That's because the first and second normal stress difference increase with the slipping coefficient (the slipping degree increasing). So the secondary flow of core on inlet and the secondary flow of shell on outlet in mixed field of die increase , and die swell of melt is directly proportional to of strength of secondary flow.