聚合物气辅共挤成型中挤出胀大的数值模拟

以一矩形截面共挤型材为例,采用Giesekus本构方程和Navier滑移模型建立数值模型,使用EVSS、SU等有限元方法对气辅共挤和传统共挤时两种聚合物熔体在口模内外的等温粘弹流动做了三维数值模拟,得到了气辅共挤和传统共挤时的挤出胀大率、速度场、应力场及剪切速率分布。对模拟结果进行了分析和对比,结果表明,气辅共挤能消除挤出胀大和模外熔体偏转流动现象;气辅共挤时两相熔体的速度场均匀一致,熔体流动稳定,呈柱塞状挤出;熔体表面的切向和法向应力为零,因而可有效提高挤出速率,并防止制品表面"鲨鱼皮"现象的出现。

Numerical Simulation of the Die Swell in Gas-Assisted Coextrusion

Taking a rectangular shaped coextrusion polymer profile as an example,a strict three-dimensional numerical simulation of two polymer melts’ isothermal viscoelastic flow in gas-assisted and conventional coextrusion process was performed separately by means of finite element methods such as EVSS,SU method. The die swell ratio,velocity field,stress field,and shear rate distribution were worked out. The simulation results were compared and analyzed. The study shows that for gas-assisted coextrusion,die swell and deflection flow outside the mould can be eliminated,the velocity field is well-distributed and uniform,the melts flow steadily and are coextruded as a plunger; also,the extrusion speed can be raised and the sharkskin phenomenon can be prevented effectively due to the tangential and normal stresses on the surfaces of the melts vanish.