气体流量对管材气辅挤出的影响

Effect of Air Flow on Pipe During the Gas-Assisted Extrusion

将气体简化为广义牛顿流体,并作为单独的内外两层,针对管材型气辅挤出口模,建立了二维有限元模型,并进行了数值模拟,研究了气体流量对口模出口处剪切速率、熔体速度及口模流道中心处熔体压力的影响。研究表明,当气体流入口模流道内,聚合物熔体在口模出口处厚度为-0.85~0.85 mm剪切速率为零,为促使聚合物熔体既满足表面质量不因剪切速率不均引起表面质量缺陷,又满足在厚度方向的尺寸要求(即2 mm),故需将聚合物熔体与气体相汇处的流道沿厚度方向扩大0.3 mm;同时也为了进一步降低因速度梯度大而引起的表面质量缺陷,文中进气体流量为2.512×10-3m/s最佳。因此,当气体作为一种辅助工艺引入到聚合物挤出成型当中,需在两相流相汇处流道沿厚度方向预留足够的空间(本文是预留制品厚度的20%的尺寸)以满足制品的尺寸及表面质量要求。

A two-dimensional numerical simulation was developed for gas-assisted extrusion through a pipe profile using the finite element method. The model was considered as viscous constitutive equations and the gas was simplified as two generalized Newtonian fluid. The effects of the gas flow on the outlet of the die shear rate field, the velocity field at the outlet of the die and die runner melt pressure at the center of the field were studied. The results show that when the gas flows into the inlet die within the flow channel, the polymer melt - 0.85-- 0.85 mm in thickness shear rate is zero, the polymer melt is prompted not only to ~tisfy the surface quality eau^d due to the uneven surface quality shear rate defects, but also to meet the dimensional requirements in the thickness direction, so it is necessary to expand the runner of the polymer melt and the gas phase 0.3 mm along the direction of thickness; in order to further reduce the surface quality defects caused due to the large velocity gradient, the intake flow of 2.512x10-3 m/s is the best. Therefore, when the gas as a supplementary process is introduced to the polymer extrusion, which is required to reserve enough space（in this paper, reserve the size of 20 % of the thickness of the products）to satisfy the requirement of the product size and surface quality in two-phase flow exchange along the thickness direction of the flow channel.