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采用PTT黏弹模型,对聚合物熔体在T型异型材挤出口模内的三维等温流动进行数值模拟,得出口模内外速度、剪切速率、压力和应力分布。结果表明:由于出口效应,在口模出口处,速度、剪切速率、压力分布均发生突变。这种突变有可能造成负压,从而引发熔体破裂。口模内二次流动产生的剪切应力甚至会超过因挤出流动产生的剪切应力。应力易集中发生在口模截面的拐角处,随着挤出流量的增加,口模内最大剪切应力和最大第一法向应力差几乎线性地增加,而不会急剧增大。
The PTT viscoelastic model was used to simulate the three-dimensional isothermal flow of polymer melt in the extrusion die of T-shaped profile. The velocity, shear rate, pressure and stress distribution inside and outside the die were obtained. The results show that due to the exit effect, the velocity, the shear rate and the pressure distribution all change suddenly at the die exit. This sudden change can cause negative pressure, causing the melt to rupture. Shear stress caused by secondary flow in the die exceeds even the shear stress caused by the extrusion flow. With the increase of extrusion flow, the maximum shear stress in the die and the maximum first normal stress difference almost linearly increase without sharp increase.