TPU流变性能测试及胶片挤出过程模拟仿真

利用高压毛细管流变仪对热塑性聚氨酯弹性体(TPU)熔体的流变性能进行测试分析,建立相应的熔体本构方程。利用polyflow软件对TPU熔体在经过优化的衣架型口模内的流动情况进行模拟分析,以检验所用口模的性能,同时采用广义Navier滑移模型分析不同滑移系数对流动状态的影响。结果表明,Bird-Carreau模型可以准确描述TPU熔体的流动行为。在不同温度下,TPU熔体在模拟所用的衣架型口模中均能实现均匀挤出,并且挤出过程接近壁面无滑移情况。     

减轻聚合物熔体挤出整体畸变和熔体破裂的辩证对策

根据对熔体挤出整体畸变和熔体破裂机理的分析,设计一系列方法减轻和消除畸变。采用原位成纤复合法和纤维流动取向效应改善无规共聚聚丙烯(PP-R)熔体在口模入口区的流动稳定性,消除环向次级流动,减轻螺纹畸变。添加一定质量的炭黑或白炭黑,利用炭黑形成三维网络结构改善溶聚丁苯橡胶(SSBR)的挤出畸变和熔体破裂行为。改变口模尺寸,适当增大口模长径比,有利于挤出过程稳定。     

胶管挤出模熔体流动行为的数值研究

基于待修正的直角胶管挤出模,根据流体力学定律建立虚拟熔体挤出系统。采用ANSYS有限元分析软件模拟在挤出预成型系统中橡胶熔体的流动行为,考察流道主要结构参数(分流锥宽度、阻流锥前端到入口距离及流道底部半径)对挤出模关键指标(速率相对平均偏差)的影响。结果表明,根据流体力学定律调节主要结构参数能够降低直角挤出模出口熔体的速率相对平均偏差。修正挤出模试验进一步验证模具结构设计的准确性,对优化挤出模设计有指导意义。     

C形口模共挤出胀大的三维黏弹数值研究

运用有限元方法,采用PTT本构方程和Arrhenius黏度对温度依赖方程,对C形共挤口模中的聚丙烯(PP)和聚苯乙烯(PS)两熔体进行了三维非等温黏弹数值研究,主要研究了口模入口端熔体层间界面位置（r）对挤出胀大率和界面位置稳定性的影响,研究表明,随着r的增大,口模出口处熔体的二次流动程度减弱,挤出胀大率减小;在两熔体入口流率相等的情况下,取使得两熔体入口面面积近似相等的r值,能保证口模内及口模出口端熔体层间界面位置稳定性较好。     

管壁厚度对微挤出成型的影响分析

基于Bird-Carreau黏度模型,运用有限元方法对三维等温微管挤出成型流动模型进行了数值分析,主要研究了管壁厚度对微管挤出成型过程中挤出胀大、速度分布、剪切速率和口模压降等重要指标的影响。结果表明,当熔体入口体积流率相等时,随着管壁厚度的增大,挤出物挤出胀大率和横截面尺寸变化量增大;口模出口端面上熔体的二次流动增强,但挤出速度和剪切速率减小;熔体在口模内的压力降明显下降;适当增加管壁厚度,有利于提高微管挤出质量。     

原位成纤复合法改善PP-R的挤出螺纹畸变

研究了聚丙烯熔体挤出螺纹畸变产生的机理,采用原位成纤复合法制备无规共聚聚丙烯(PP-R)/聚对苯二甲酸乙二醇酯(PET)复合材料来改善PP-R的挤出性能。结果表明:口模入口区的环向次级流动是PP-R熔体产生螺纹畸变的主要原因;采用"挤出-热拉伸-淬冷"工艺制得的复合样品中PET以微纤状分散于PP-R基体中,有效地推迟了PP-R熔体挤出螺纹畸变的发生。     

LDPE熔体在圆锥型短口模挤出过程的粘弹行为研究

主要研究不同入口圆锥角短口模流道挤出流动过程中聚合物熔体的粘弹特性,以及在口模流动过程压力损失,入口弹性贮能和挤出胀大比之间的关系。对于不同的口模入口角,有不同的剪切速率与剪切应力的规律,流变曲线各自不同。同时,不同的圆锥入口角,表现出不同的Bagley校正因子对应不同的挤出胀大值,反映了聚合物熔体在不同圆锥入口角短口模挤出过程拉伸弹性形变特性的差异。聚合物熔体在不同入口圆锥角短口模挤出流动过程的压力降,依赖口模流道的几何形状(入口角、长径比)、温度、流动速率等,入口损失主要归因于拉伸形变的弹性贮能。     

超高摩尔质量聚乙烯薄膜挤出成型有限元模拟

建立了超高摩尔质量聚乙烯(UHMWPE)薄膜熔体挤出成型有限元模型,采用鱼尾式机头设计了厚度为1mm、幅度为100 mm的UHMWPE薄膜挤出模具,基于有限元软件Polyflow对挤出流道内熔体的流动开展了有限元模拟,采用不同几何模型分别获得了流道内熔体速度和压力的分布规律。分析了有无阻流块、有无稳流区和不同长度成型区对流动的影响,获得了阻流块高度、稳流区长度、成型区长度对挤出成型的影响规律和影响效果,对UHMWPE薄膜挤出口模设计给出了建议。     

马鞍型异型材挤出成型过程三维等温粘弹性的数值模拟

基于PTT粘弹性本构模型,通过马鞍型异型材挤出成型过程的全三维稳态等温有限元数值模拟,系统研究了聚合物粘弹性流变性能参数和成型工艺参数对异型材口模挤出成型过程的影响规律,并揭示了其影响机理.研究结果表明,聚合物异型材口模挤出离模膨胀是由口模出口处的二次流动引起,离模膨胀比随着口模出口处的二次流动强度增加而增大.聚合物异型材口模挤出离模膨胀随着进口流量和聚合物熔体松弛时间的增加而增加,而随着聚合物熔体材料常数和粘度比的增大而减小.     

基于Polyfow的吹膜模头熔体流动二维数值模拟

对共挤复合吹膜工艺所采用的螺旋芯棒式模头内部熔体流动进行二维数值模拟,简化分析模型并得到挤出过程中熔体流动的流速及压力变化的相对趋势。模拟结果表明:熔体进入模头螺旋部分后具有一定的环向速度,随着螺旋槽深度变浅,流动的方向逐渐由螺旋环向变为挤出方向的轴向,且流速变得均匀;熔体在螺旋槽旋转处静压力较大,接近挤出方向后压力逐渐变小,口模出口处熔体流动速度仍大于流入模头的入口速度。     

异型材挤出口模内的流动模拟

本文根据挤出机理，对口模内的流动行为进行适当的简化和假定，建立了挤出口模内流动分析的数学模型，并采用横截面／假想区域法实现了复杂异型材挤出口模内的数值模拟，数值分析结果与三维有限元解和解析解的对比，取得了满意的效果。     

熔体挤出速度对共挤吹塑型坯离模膨胀影响的数值模拟

基于三维非等温黏弹性熔体多相分层流动有限元数值模拟技术,模拟研究了熔体挤出速度对多层共挤吹塑成型环坯离模膨胀和初始温度场的影响规律,揭示了型坯离模膨胀的产生机理。结果表明,多层共挤吹塑成型环坯离模膨胀是由熔体的二次流动诱发而产生,与熔体流出机头进入自由膨胀段的二次流动强度成正比,而其二次流动强度随着熔体挤出速度的增大而增强,因而导致环坯离模膨胀随着熔体挤出速度的增加而增大;多层共挤吹塑成型熔体的二次流动强度与其第二法向应力差成正比关联关系,这与Debbaut的试验研究结论完全吻合,表明二次流动是由第二法向应力差驱动而产生。     

Effect of wall slip on the uniformity of flow from sheeting extrusion dies

A theoretical study for analyzing the uniformity of flow from sheeting extrusion dies is presented. In this study it is assume that a slip condition exists at the wall of the die, the magnitude of slip velocity is proportional to the shear stress at the wall, the flow is isothermal and steady state, and a power law model is valid for viscosity. Two extrusion dies, T-dies and coat-hanger dies, are examined. The flow uniformity at the exit of the die is calculated and compared with that for a nonslip analysis. The discrepancies between the slip and nonslip models imply that the wall slip condition induces a significant nonuniform flow distribution. Traditional design criticism based on the nonslip model are invalid for flow with the wall slip condition, and it is necessary to increase the length of the die land to even the flow distribution at the exit of the die.     

Extrusion die design for slowly reacting materials

The flow distribution of slowly reacting polymeric materials inside an extrusion die was analyzed by lubrication theory. The viscosity of the polymeric liquid was assumed to be time-dependent owing to chemical reaction. Three types of extrusion dies were considered: a T-die, a linearly tapered coat-hanger die with a choker bar, and a curvilinearly tapered coat-hanger die. It was found that the T-die is not suitable to deliver the polymeric liquid films, whereas the two coat-hanger dies can deliver the polymeric liquid films with acceptable flow uniformity and residence time distribution.     

塑料异型材挤塑模模头校核

通过对塑料异型材挤出模模头结构进行分析,以聚合物流变学为理论基础,对机头流道的不同区域分别建立压力降计算公式和熔体停留时间计算公式,同时,通过对分流支架及型芯连接螺相进行受力分析,从而建立强度校核公式,最后介绍了以PBA为工具对AutoCAD进行二次开发,并编制了塑料异型材挤出模的分析验算程序。     

塑料异型材挤出口模的三维罚有限元设计

讨论了现有塑料异型材挤出口模研究中的流变学分析方法,指出了前人运用流变学方法设计挤出口模的不足。本文采用三维罚有限元方法,对幂律流体在塑料异型材口模中的流动进行了计算机模拟研究,阐述了采用三维罚有限元方法进行塑料异型材挤出口模设计的重要意义     

Simulation of polymer flow through a coat-hanger die: A comparison of two numerical approaches

Coat-hanger dies are commonly used for the extrusion of plastic sheets and films. To describe the flow of a molten polymer through a coat-hanger die, a two-dimensional approach is necessary. Moreover, the thermal effects, which play an important role in the flow distribution, have to be taken into account. In this paper, two numerical models for the simulation of coat-hanger dies are described and compared. These models differ mainly in the simplifying assumptions used and in the treatment of the thermal problem. The simulations obtained with the two models were compared with each other and with experimental data. The discrepancies between the two models can be explained by the different theoretical treatments.     

Three-dimensional finite element analysis of polymeric fluid flow in an extrusion die. Part I: Entrance effect

The Galerkin finite element method has been applied to study the three-dimensional flow field of power-law fluids inside an extrusion die. Two inlet designs, i.e., center-fed and end-fed, have been considered. The effects of inertial force as represented by the Reynolds number Re, inlet geometry, and the power-law index n on lateral flow uniformity and vortex formation in the entrance region have been examined. A flow visualization technique has been carried out to experimentally verify the theoretical prediction of the three-dimensional flow field inside a die. It has been found that increasing Re or decreasing n will deteriorate flow uniformity. Depending on the direction of the inlet jet stream, the inertial force may create a flow peak in the central region of a center-fed die, or the maximum flow rate will appear close to the end of the die for an end-fed die. For highly shear-thinning fluids, lower flow rates are always observed close to the end of the dies. It is concluded that creating a plug flow in the inlet tube of the extrusion die is advantageous for both center-fed and end-fed designs.