聚合物多层共挤成型离模膨胀过程的三维黏弹性数值模拟

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。     

异形医用双腔导管挤出离模膨胀变形机理与调控

如何准确预测和精密调控异形医用双腔导管的离模膨胀变形是实现其精密成型的关键技术。通过传统和气辅精密成型的对比分析，研究表明离模膨胀变形是由熔体的第二法向应力差驱动的径向二次流动所诱发，传统挤出成型会产生较大的第二法向应力差，第二法向应力差驱动诱发的径向二次流动是离模膨胀变形的直接驱动力，从而导致传统挤出成型的异形医用双腔导管不仅产生离模膨胀，而且还产生椭圆度误差，其最大离模膨胀比和椭圆度误差分别为1.86和6.3 %。异形医用双腔导管的气辅精密挤出成型基本可以消除熔体的第二法向应力差，必然消除了挤出成型过程的径向二次流动，从而实现了异形医用双腔导管的精密控形。     

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。     

挤出吹塑型坯的膨胀

本文系统地分析了挤出吹塑型坯的膨胀。介绍了两种测量型坯膨胀的方法,分析了机头结构(平直机头,发散机头与收敛机头)、树脂的流变性能与分子量分布及挤出条件(熔体温度、流动速率及剪切速率)对膨胀的影响。本文认为,聚合物熔体的离模膨胀主要由其分子的取向效应引起。     

黏度对Y形包覆共挤成型影响的数值模拟

采用Phan-Thien-Tanner(PTT)本构方程,建立Y型包覆共挤模型,通过有限元数值算法系统模拟黏度对包覆共挤成型离模膨胀的影响规律。结果表明:随着熔体处黏度的变化,熔体胀大率及包覆共挤变形均有较大的变化,当内层处熔体黏度增大,而外层处黏度减小时,包覆共挤出挤出胀大率减小。     

PTT熔体在塑料异型材挤出口模内黏弹流动的数值模拟

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.     

熔体层厚对聚合物共挤出胀大的影响

基于流变学理论和Phan-Thien-Tanne本构方程,建立了三维等温黏弹共挤成型流动过程的数值模型。运用有限元方法对数值模型进行了模拟计算,研究了熔体层厚对共挤成型的影响,分析了熔体层厚对挤出胀大率、偏转变形及界面形貌的影响。结果表明:熔体层厚对挤出胀大率和界面形貌的影响较大,随着熔体层厚的增加,挤出胀大率减小;随着共挤出熔体层厚差值的增大,界面形状趋于不稳定;熔体离开口模后产生低黏度熔体向高黏度熔体一侧偏转变形,但变形程度受熔体层厚的影响不大。     

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。     

圆环和锥形口模挤出吹塑的数值研究

用Polyflow软件的Carreau本构模型,数值模拟了用于管材挤出吹塑和型坯成型的圆环口模和锥形口模内熔体及其型坯的三维等温流场,考虑了挤出胀大和垂伸效应的综合影响,分析了两种口模的差异.数值计算结果表明,对于同一直径和口模间隙的环形口模和锥形口模,在相同入口流量下,环形口模内熔体的压力梯度大于锥形口模,环形口模成型型坯的厚度小于锥形口模,环形口模成型型坯的直径大于锥形口模;环形和锥形口模内熔体的压力梯度随着入口流量的增加而增加,在环形和锥形口模成型型坯的底端,型坯的厚度和外半径随着入口流量的增加而增加;锥形口模成型型坯厚度较环形口模对流量更敏感.     

Numerical simulation of blow molding—Viscoelastic flow analysis of parison formation

The simulation of the parison formation process in blow molding has been studied. The flow field was divided into two regions, namely, the extrudate swell region near the die lip and the parison formation region after the exit swell. In the swell region, we predicted the swelling ratio and residual stress distribution for high Weissenberg numbers for steady planar well using the 1-mode Giesekus model. In the parison formation region, the flow is assumed to be an unsteady unaxial elongational flow including drawdown and recoverable swell and is modeled using the 10-mode Giesekus model. We calculated the time course of parison length and thickness distribution, and compare the calculation results of parison length with experimental data. It was found that the predicted values agreed rather well with the experimental values. The calculation results could especially predict the shrink-back, which is the phenomenon where the parison length becomes shorter after the cessation of extrusion, and it was found tat this was caused by the recoverable swell of the parison, which depends on the tensile stress generation in the die. Various flow rates and die geometries were studied and confirmed the reliability and usefulness of the method.     

不同工艺参数时快速降压口模的数值模拟

在微孔塑料连续挤出成型中,运用Ployflow软件对快速降压口模内熔体流动进行模拟分析,研究了不同CO2浓度、不同熔体体积流量对口模内熔体压力、速度分布及挤出胀大影响。结果表明,口模内熔体压力降在一定的范围内随着熔体体积流量的增大而增大,随着CO2浓度的升高而降低;一定范围内,CO2浓度对口模出口处熔体平均速度的影响不明显,而熔体体积流量对口模出口处熔体平均速度的影响很明显;对于挤出胀大的影响,CO2浓度不宜过高或者过低,2 %（质量分数,下同）时表现最佳;在一定熔体体积流量的范围内,熔体体积流量越高,挤出胀大表现得越不明显。     

A comprehensive experimental study and numerical modeling of parison formation in extrusion blow molding

Parison dimensions in extrusion blow molding are affected by two phenomena, swell due to stress relaxation and sag drawdown due to gravity. It is well established that the parison swell and sag are strongly dependent on the die geometry and the operating conditions. The availability of a modeling technique ensures a more accurate prediction of the entire blow molding process, as the proper prediction of the parison formation is the input for the remaining process phases. This study considers both the simulated and the experimental effects of the die geometry, the operating conditions, and the resin properties on the parison dimensions using high density polyethylene. Parison programming with a moving mandrel and the flow rate evolution in intermittent extrusion are also considered. The parison dimensions are measured experimentally by using the pinch-off mold technique on two industrial scale machines. The finite element software BlowParison® developed at IMI is used to predict the parison formation, taking into account the swell, sag, and nonisothermal effects. The comparison between the predicted parison/part dimensions and the corresponding experimental data demonstrates the efficiency of numerical tools in the prediction of the final part thickness and weight distributions. POLYM. ENG. SCI., 47:1–13, 2007. © 2006 Society of Plastics Engineers     

Overall numerical simulation of extrusion blow molding process

This paper focuses on the overall numerical simulation of the parison formation and inflation process of extrusion blow molding. The competing effects due to swell and drawdown in the parison formation process were analyzed by a Lagrangian Eulerian (LE) finite element method (FEM) using an automatic remeshing technique. The parison extruded through an annular die was modeled as an axisymmetric unsteady nonisothermal flow with free surfaces and its viscoelastic properties were described by a K‐BKZ integral constitutive equation. An unsteady die‐swell simulation was performed to predict the time course of the extrudate parison shape under the influence of gravity and the parison controller. In addition, an unsteady large deformation analysis of the parison inflation process was also carried out using a three‐dimensional membrane FEM for viscoelastic material. The inflation sequence for the parison molded into a complex‐shaped mold cavity was analyzed. The numerical results were verified using experimental data from each of the sub‐processes. The greatest advantage of the overall simulation is that the variation in the parison dimension caused by the swell and drawdown effect can be incorporated into the inflation analysis, and consequently, the accuracy of the numerical prediction can be enhanced. The overall simulation technique provides a rational means to assist the mold design and the determination of the optimal process conditions.     

超高分子量聚乙烯气辅挤出影响因素的数值模拟及分析

以超高分子量聚乙烯的圆形轴对称气辅口模挤出为研究对象,在采用Polyflow软件对气辅口模挤出时的等温流动进行数值模拟之后,就入口流率、松弛时间以及零剪切黏度等物性和工艺参数对挤出胀大、速度分布、口模压降和熔体外表面上剪切速率的影响进行了数值模拟和分析。分析表明:气辅挤出是克服超高分子量聚乙烯传统挤出时面临一系列困难的有效加工方式。     

塑料挤出吹塑的机理问题

采用不同的方法对挤出吹塑过程的型坯成型、型坯吹胀与制品冷却三阶段的机理问题进行了研究.采用人工神经网络方法预测了受模口温度和挤出流率影响的型坯成型阶段的膨胀.利用建立起来的神经网络模型预示的膨胀与实验结果很吻合,且可在一定范围内,预示不同工艺条件下型坯的直径膨胀和壁厚膨胀,为型坯的直径和壁厚的在线控制提供了理论依据.基于薄膜近似和neo-Hookean本构关系,建立了描述型坯自由吹胀的数学模型,并通过实验方法获得了型坯吹胀的瞬态图象.