Abstract:

In co-extrusion, the interaction between the polymer melt viscoelastic nature and unavoidable fluctuations of the operating conditions results in fluctuating die swell, which makes the design of co-extrusion die according to the shape of product a technical challenge.To address the technical difficulties, the influence of process parameters and viscoelastic rheological parameters on polymer co-extrusion die swell was investigated with the stable state finite element numerical algorithm established by the authors and the mechanism was studied.Research results showed that co-extrusion die swell was caused by the secondary flow of viscoelastic melt，and depended on the direction and strength of melt secondary flow.The direction of melt secondary flow depended on the sign of difference of secondary normal stress, and the strength of melt secondary flow was proportional to the difference of secondary normal stress.The core melt die swell was proportional to the strength of the outward secondary flow in the exit of die and the inlet of mixing zone, and the skin melt die swell depended on the outward secondary flow relative intensity of its inner surface and outer surface.Moreover, research results also showed that core, skin melt and holistic die swell increased with increasing skin melt viscosity, but decreased with increasing skin melt inlet volumetric flow rate.