芯棒式管材挤出机头的流动模拟分析

以实测高密度聚乙烯(HDPE)的性能参数为基础,提出了以挤出口模壁面附近的剪切速率、壁面附近的熔体流动速度与管中心熔体流动速度之比值来描述速度变化、用压力梯度描述压力变化的观点,采用Ansys、Matlab软件对芯棒式管材机头中挤出HDPE薄壁管的成型过程进行了模拟,并系统地描述、分析了成型段的长度及入口压力、入口速度发生变化时对成型段上的速度分布和压力分布的影响,得到了成型段长厚比与入口压力的优化值。     

PA6/EPDM复合材料共挤过程的三维非等温数值模拟

建立橡胶和塑料2种熔体在矩形流道中共挤流动的三维非等温有限元模型,采用有限元法数值模拟了PA6/EPDM的共挤流动过程,研究了橡塑共挤的流动规律以及入口体积流量比与复合材料层厚比的关系。结果表明:在共挤过程中,共挤界面会倾于向熔体入口体积流量相对小、熔体黏度低的一侧偏移,同时黏度低的熔体流动速度快;入口体积流量比的减小有利于减小界面偏移和界面波动。将模拟计算得到的层厚比与实验结果进行了对比,结果表明数值模拟与实验基本一致。     

入口速度振动对单管挤出口模熔体流动的影响

研究了塑料挤出过程中入口速度振动对挤出口模内熔体流变性能的影响规律。基于计算机流体力学理论,利用FLOTRAN软件对单管口模成型段熔体流动过程进行数值模拟,分析了振动力场作用下与稳态加工成型中口模成型段熔体压力场、速度场及温度场的分布情况。研究表明,沿熔体流动方向上叠加一个正弦振动的入口速度时,挤出压力、温度是周期性变化的;振动速度的引入可以提高挤出速度、降低挤出压力、加剧黏性生热,同时也能提高口模体积流率,而且随着振动幅值的增大,效果越来越明显。     

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。     

振动力场下圆管内聚合物熔体速度场的数值模拟

利用ANSYS-FLOTRAN有限元分析软件模拟了不同振动频率下圆管内不可压缩幂律流体非定常流动的速度场.结果表明:1)振动力场作用使熔体在入口附近出现速度突变,振动频率越高,突变越厉害,速度值越大,稳定时圆管的无量纲长度越大;2)熔体轴向速度在径向呈类抛物线分布,振动频率存在最佳值,大于或小于最佳振动频率,熔体轴向速度分布范围均会变宽,流动变得不稳定;3)出口处速度响应存在时间延迟和速度峰,随着振动频率的增加,速度峰值升高,稳定时熔体轴向响应速度波动范围变宽,波动平衡点速度值升高.     

ABS熔体挤出流变性质的研究

考察了毛细管挤出过程中温度和流动速率对ABS树脂熔体流变行为的影响。结果表明 ,当剪切速率大于 10 3s 1后 ,熔体的剪切流动不服从幂律 ;而剪切粘度对温度的依赖性符合Arrhenius方程 ;入口压力降和入口拉伸应力均随着剪切应力的增加而呈非线性函数形式增大     

直角机头流道结构参数对流动平衡的影响

直角机头由于入口和出口流向不同,导致熔体在机头流道中流动不平衡,所以需要选择和调整优化影响熔体流动平衡的各类流道结构参数。本文针对熔体流动平衡性问题总结以往计算研究中影响机头流动平衡的各类机头参数进行分类计算、逐步优化计算结果,得出一组优化参数。     

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。     

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。     

水下切粒模板流动场的数值分析及优化

为了完善水下切粒模板流道尺寸设计,对模板流动场进行了数值模拟和优化:首先进行塑料熔体在模板中的流体力学分析,以此为根据分析影响流道压力降的主要因素,最后对流道尺寸优化和ANSYS流动分析。结果表明:最小压力降△P=4964642(Pa);流道入口长度L1=10(mm);出口长度L2=5(mm);入口半径R1=9(mm);出口半径R2=2.9(mm);锥形角度θ为30°。流动分析显示流体在流道中的速度分布基本稳定。     

Elastic fracture of polystyrene solutions

Based on the assumption of a constant critical shear strain, an expression for the critical stress at the onset of entrance fracture as a function of polymer concentration has been developed. Experimental results with 10-25 percent narrow distribution, high molecular weight polystyrene-benzene solutions show the critical stress to be much lower than that for the polymer melt and in agreement with predicted values. This result is all the more impressive when the contrast in flow behavior at the capillary entrance for melts and solutions is observed. Instead of the rotating toroidal vortices surrounding a 90-deg material entrance cone observed with polystyrene melt, cine movies of the solution flow birefringence patterns in the capillary entrance region reveal only a stagnant zone surrounding a narrow cone less than 20 deg. At fracture, the cone axis moves in a rotary path circulating about the capillary axis without undergoing the flow discontinuities typical of melt behavior.     

Study on airflow field and fiber motion with new melt blowing die

In this study, a new melt‐blowing die was studied with the computational fluid dynamic approach. A bead‐viscoelastic element fiber model was established to model three‐dimensional paths of the fiber motion with the standard linear solid (SLS) constitutive equation in different airflow fields. The effects of this newly designed die on the velocity field, temperature field, and turbulence fluctuation field at the centerline were studied and compared with the traditional melt blowing die. The fiber motion was simulated and compared with the airflow field of different dies. The simulations results demonstrated that the new die was able to reduce the velocity fluctuations of the air flow near the outlet of the polymer capillary and generate the higher centerline air velocity and temperature. The fiber attenuation and motion were related to the centerline air velocity, temperature, and turbulent fluctuation in the melt blowing process. POLYM. ENG. SCI., 59:1182–1189 2019. © 2019 Society of Plastics Engineers     

Gross melt fracture mitigation in converging dies: A singular behavior due to polymer wall slip

This work focuses on mitigating the gross melt fracture defect of polymer flowing through axisymmetrical and two‐dimensional dies. The die entrance angle is considered as well as the influence of the converging wall roughness. Singular results are obtained with a random styrene butadiene rubber (SBR) copolymer, as the gross melt fracture defect cannot be eliminated or mitigated by reducing the die entrance angle. Other experiments carried out with rough converging dies do not give better results. Indeed, the polymer essentially slips along the walls, as shown from capillary rheometer and birefringence experiments. Thus, these results point out the importance of elongational stresses and interfacial conditions in the die entrance region on flow instabilities and the gross melt fracture defect.     

Finite element analysis of flow of thermoplastic elastomer melt through axisymmetric die with slip boundary condition

Abstract

A finite element model for the flow of thermoplastic elastomers in extrusion dies has been developed. The rheological behaviour of the polymer melt is assumed to be described by the generalised Newtonian models and as a special case, the well known, power law equation was selected. Owing to the very low variation of the temperature field, the flow regime was considered to be isothermal. The set of governing equations are solved using the finite element method in a cylindrical (r, z) coordinate system. Slip–stick of the polymer melt on the solid wall, encountered in the flow of highly viscous fluids, is incorporated into the model by the use of Navier's slip condition. A new method based on a technique developed previously is described for the inclusion of this condition in the working equations. The applicability of the model was verified by a comparison between the results of the simulation of a polypropylene–nitrile/butadiene rubber thermoplastic elastomer with experimentally measured data. These comparisons show that there are very good agreements between the model predictions and actual data, provided that the slip of the polymer melt during the flow in extrusion die has been taken into consideration.     

Effect of die temperature on the flow of polymer melts. Part I: Flow inside the die

The effect of die wall temperature on the flow of polymer melts in circular capillary dies was studied. At constant flow rates, it was found that die wall temperature had a greater effect on the pressure drop than melt temperature. A capillary die with two circular channels with different diameters was designed to simulate the profile extrusion. Changes of wall temperature varied the flow rate ratio between the two channels. An implicit finite difference method was used to simulate the velocity and temperature profiles inside the die. Values predicted by this model matched well with experimental data for both dies.     

In process measurement of apparent extensional viscosity of low density polyethylene melts using flow visualisation

Abstract

Flow visualisation has been used to study the in process flow behaviour of a low density polyethylene melt as it is processed through planar hyperbolic and abrupt entry slit dies on a commercial scale extruder. The former die profile consisted of a planar hyperbolic section that gradually merged with a parallel slit and was designed to promote constant extensional strain rates at the centreline of flow. The melt was processed through these dies at several flowrates. Extensional strain rates were determined by performing particle velocimetry at the centreline of melt flow in the contraction regions of each die. Constant extensional strain rate conditions were approached at low flowrates in the hyperbolic die. Constant strain rates were not attained for the hyperbolic die at high flowrates nor, as expected, for the abrupt entry die. Analysis of flows using birefringence showed significant shear boundaries developed at the wall of the hyperbolic die at high flowrates. Such boundaries, in combination with the non-Newtonian behaviour of the viscoelastic polymer melt, lead to non-constant strain rates along the centreline of the die at higher flowrates. Stress, strain, and strain rate data for the low density polyethylene melt are presented which, although derived under flow conditions that are not strictly steady in the Lagrangian sense, are experimentally accessible and informative. Stresses and strains derived from the flow visualisation technique are compared with constant strain rate data obtained from a Rheometrics RME elongation rheometer. Close agreement was found between data from the two techniques.     

Stratified two-phase flog of molten polymers in circular dies

A theoretical study has been carried out on the steady, axisymmetric two-phase flow of molten polymers in circular dies. For both fluids, the shear rate dependence of the viscosity is described by a power law and the temperature dependence by an exponential function. Taking into account viscous dissipation, a numerical program has been developed to predict the radial position of the interface between the two fluids, the developing temperature and velocity fields, the shear rates, the shear stresses, the pressure drop as well as the field of shear deformation and the residence time distribution for different thermal boundary conditions. The numerical program is applied to the flow of a high-viscous polymer melt, (high density polyethylene), surrounded by a small annulus of less viscous polymer ni lt, (low density polyethylene). Computed results are shown graphically.     

一种新型具有均匀流动分布的多流道片材机头的几何设计

提出了一种新型的全部流道由平直板条组成的片材机头结构。全部流道中在熔体特征黏度相等的假定条件下，给出了确定构成流道的栅条尺寸的算法及相关计算公式。在幅宽为0．87m、口模间隙为1mm的实验机头上进行了PP片材的挤出实验以证实机头结构的可行性和设计方法的正确性。结果表明，用这种形式的机头可以获得厚度均匀的挤出片材(沿幅宽最小厚度与最大厚度的比值为0．97)。熔体沿幅宽的平均停留时间分布分析结果表明，平均停留时间分布在幅宽上是不均匀的，但其不均匀性远低于T形机头。     

塑料异型材挤出成型的流动平衡分析

介绍了一种基于横截面法和流动路径法混合的异型材挤出成型中熔体流动平衡的分析方法，可用于确定具有多个分支流道、截面复杂的异型材挤出模头出口处物料的平均流速，帮助设计者判断模头流道设计是否合理，且计算量较小，适于工程应用。