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.

摘要：

共挤成型中，聚合物黏弹特性与过程参数波动的耦合作用会产生波动的离模膨胀，使得根据共挤制品的形状设计相应的共挤定型口模在工程上仍是一项技术挑战。基于这一技术问题，通过建立的稳态有限元数值算法，系统研究了过程参数和黏弹性流变性能参数对共挤成型离模膨胀的影响规律和机理。研究结果表明，多层共挤口模芯壳层熔体离模膨胀是由熔体的二次流动引起，主要取决于芯壳层熔体二次流动的方向与强度。熔体二次流动的方向与第二法向应力差的正负号有关，而熔体二次流动的强度则与第二法向应力差大小呈正比。芯层熔体的离模膨胀与口模出口和混合区进口处芯层熔体向外的二次流动强度呈正比，而壳层熔体的离模膨胀取决于壳层熔体内外界面向外的二次流动的相对强度。研究还表明芯、壳层熔体及口模整体的离模膨胀随着壳层熔体黏度的增大而增加，而随着壳层熔体进口流量的增大而减小。