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。     

熔体层厚度对L形异型材包覆共挤成型影响的数值模拟

以L形包覆共挤异型材为研究对象,采用Phan-Thien-Tanner(PTT) 本构方程,通过有限元方法分析了包覆共挤成型过程中的挤出胀大现象。结果表明,熔体层厚度的变化对整体挤出胀大率基本无影响,而对壳层、芯层胀大率及共挤界面L形内直角处沿X、Y方向的偏转均有较大的影响;当整体口模尺寸不变时,在口模出口处的最大剪切速率不受壳层、芯层厚度变化的影响。     

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。     

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。     

聚合物熔体在任意长径比圆锥口模的挤出胀大唯象研究

深入讨论了聚合物熔体在不同长径比、不同角度圆锥口模的挤出胀大现象及机理。对口模长径比较小的挤出胀大,由于熔体入口拉伸弹性变形来不及松弛,产生较大的挤出胀大;对长径比较大的口模,熔体在平直流道内停留时间较长,入口弹性形变逐渐松弛,这时主要是流动剪切应变引起的弹性变形,产生较弱的挤出胀大,比长径比小的挤出胀大来得小,并且聚合物熔体的挤出胀大随着长径比的增大而趋向一恒定值。结果还表明:聚合物熔体在圆锥口模的挤出胀大受到挤出口模入口角影响。当L/D较小时,挤出胀大与口模入口角有关;当L/D较大时,口模入口角对挤出胀大影响较小。     

壁面滑移对两种聚合物熔体共挤出影响的数值研究

在壁面滑移的边界条件下,利用聚合物流体计算软件包POLYFLOW对两种熔体的二维等温共挤出进行了数值模拟.在两侧壁面滑移系数相同和两侧壁面滑移系数不同这两种情形下分别计算了共挤出流动的速度场、压力场、黏度场及剪切速率场,讨论了壁面滑移对共挤出流场、界面形状和挤出胀大的影响.模拟结果表明：当两侧壁面滑移情况相同时,滑移系数越大,界面偏移越大,熔体胀大率增大;当两侧壁面滑移情况不同时,滑移系数相差越大,界面偏移越大,滑移系数小的一侧熔体挤出胀大显著.     

壁面滑移对塑料异型材共挤影响的数值模拟研究

在壁面滑移的边界条件下,利用流体有限元分析软件Polyflow,采用Giesekus本构方程,对L型塑料异型材进行了三维等温共挤出数值模拟。分析了滑移系数对共挤出胀大和粘性包围、口模出口面的剪切速率场及共挤出口模压力降的影响。研究表明,随着滑移系数的增大,共挤出胀大和粘性包围、口模出口面的最大剪切速率及口模压力降也逐渐增大,且滑移系数介于105和108之间时增速最大;滑移系数越小,共挤出制品质量越好。     

聚合物熔体挤出胀大的唯象研究进展

介绍了目前国内外对聚合物熔体挤出胀大的研究工作进展,评述了近50年来挤出胀大研究发展情况。从唯象理论出发,应用张量分析方法,研究了短口模的挤出胀大行为;引入了随流坐标系的随流协变度量张量,并利用张量的特性描述了聚合物熔体微元在挤出口模的流动状态;结合流变本构方程,探讨了聚合物熔体对称和非对称的挤出离模后的胀大过程,提出挤出胀大相关研究的发展趋势,为未来研究方向提出了一个新的研究方法。     

不同角度圆锥口模对聚合物熔体挤出胀大的影响

从理论上和实验上研究了圆锥挤出口模的几何结构对挤出胀大的影响,引入口模入口角、流动自然收敛角来计算圆锥型口模的挤出胀大比,从理论上和实验上指出聚合物熔体在圆锥口模的挤出胀大受到挤出口模入口角影响的一些规律.结果表明,不同角度圆锥口模挤出过程中,熔体在收敛流道受到拉伸流变,导致强烈的入口弹性效应,表现出熔体在不同角度圆锥口模挤出时有不同的挤出胀大比.当L/D较小时,挤出胀大与口模入口角有关,口模入口角θ为15°、30°时,挤出胀大较小,当口模入口角θ=45°～120°时,挤出胀大较大,但在这个圆锥口模入口角范围内,口模入口角的变化对挤出胀大影响很小;当L/D较大时,口模入口角对挤出胀大影响较小.     

壁面滑移条件下聚合物微共挤成型机理探究

基于壁面滑移条件，建立了微小流道内聚合物黏弹三维共挤出流动模型，并运用有限元方法对流动模型进行了模拟计算。为探究壁面滑移条件下聚合物熔体工艺条件和物性参数对成型的影响，分别设置芯、壳层熔体不同的流率比和黏度比，通过分析流道内外的熔体速度分布及层间界面形貌，探究了无滑移和完全滑移两种壁面条件下，熔体流率和黏度对聚合物微共挤成型层间界面的影响规律。结果表明，无滑移壁面条件下，熔体层间界面不稳定，口模内和口模外界面均发生偏移，且在口模出口处发生突变，熔体离开口后存在胀大和变形现象，其胀大和变形程度随着熔体入口流率比和黏度比的变化而变化；完全滑移壁面条件下，口模内熔体层间界面发生偏移，但口模外界面稳定，不存在挤出胀大和变形，且不受熔体入口流率比和黏度比的影响，这对实现聚合物微复合制品的精密成型具有重要意义。     

振动力场对PP/PS共混物流变特性的影响

利用自制的聚合物流变测试装置,研究了不同配比的聚丙烯(PP)/聚苯乙烯(PS)共混物在振动力场作用下的流变特性。结果表明:与稳态相比,振动力场的引入明显降低了共混物的表观黏度和挤出胀大比;随着PP含量的增加,共混物的表观黏度在PP/PS=40/60时出现极小值,而挤出胀大比逐渐降低。在PP/PS=60/40时,当振幅为0.5mm,随着频率的增加,共混物的表观黏度呈现无规律的波动状态;当频率为20Hz时,随着振幅的增加,共混物的表观黏度逐渐降低。     

Numerical simulation of coextrusion from a circular die

A numerical simulation of coextrusion flow in a capillary die has been undertaken for polymer melts used in a previously reported experimental study of coextrusion. Viscosity data are used for PS, HDPE, and LDPE melts. A Newton–Raphson scheme is employed to solve the equations for a fully developed pressure-driven flow of two concentric layers in a capillary. A finite element method is used to simulate the full flow field behavior, including determination of the interface and free surface of the exiting stream. Double nodes have been used at the interface to ensure continuity of velocities and stresses and to capture the pressure discontinuities. Pressure gradients, extrudate swell, interface swell, and other relevent flow characteristics are presented and compared with the experimental findings. The finite element analysis revealed that satisfactory convergence of the interface location is found for the cases when the less viscous material wets the capillary walls, which is also the preferential configuration in coextrusion. For the opposite configuration, convergence proved either difficult or impossible depending upon the viscosity ratio. Discrepancies were found to exist between the theoretically predicted and experimentally measured pressure gradients.     

A parallel coextrusion technique for simultaneous measurements of radial die swell and velocity profiles of a polymer melt in a capillary rheometer

This article proposes a new experimental technique to simultaneously measure radial die swell and velocity profiles of polystyrene melt flowing in the capillary die of a constant shear rate rheometer. The proposed technique was based on parallel coextrusion of colored melt‐layers into uncolored melt‐stream from the barrel into and out of the capillary die. The size (thickness) ratio of the generated melt layers flowing in and out of the die was monitored to produce the extrudate swell ratio for any given radial position across the die diameter. The radial velocity profiles of the melt were measured by introducing relatively light and small particles into the melt layers, and the times taken for the particles to travel for a given distance were measured. The proposed experimental technique was found to be both very simple and useful for the simultaneous and accurate measurement of radial die swell and velocity profiles of highly viscous fluids in an extrusion process. The variations in radial die swell profiles were explained in terms of changes in melt velocity, shear rate, and residence time at radial positions across the die. The radial die swell and velocity profiles for PS melt determined experimentally in this work were accurate to 92.2% and 90.8%, respectively. The overall die swell ratio of the melt ranged from 1.25 to 1.38. The overall die swell ratio was found to increase with increasing piston speed (shear rate). The radial extrudate swell profiles could not be reasoned by the shear rate change, but were closely linked with the development of the velocity profiles of the melt in the die. The die swell ratio was high at the center (～1.9) and low (～0.9) near the die wall. The die swell ratio at the center of the die reduced slightly as the piston speed was increased. Polym. Eng. Sci. 44:1960–1969, 2004. © 2004 Society of Plastics Engineers.     

Die swell from capillary die and slit die: A theoretical study

Assuming polymer fluids obey the CEF equation, equations concerning die swell from capillary die and slit die were derived. The die swell of polymer increased with increasing shear rate and recoverable shear. Our theoretical predictions conformed well with the experimental data of die swell for PP and PS.     

Experimental studies on radial extrudate swell and velocity profiles of flowing PS melt in an electro‐magnetized die of an extrusion rheometer

This article investigates the radial extrudate swell and velocity profiles of polystyrene melt in a capillary die of a constant shear‐rate extrusion rheometer, using a parallel coextrusion technique. An electro‐magnetized capillary die was used to monitor the changes in the radial extrudate swell profiles of the melt, which is relatively novel in polymer processing. The magnetic flux density applied to the capillary die was varied in a parallel direction to the melt flow, and all tests were performed under the critical condition at which sharkskin and melt fracture did not occur in the normal die. The experimental results suggest that the overall extrudate swell for all shear rates increased with increasing magnetic flux density to a maximum value and then decreased at higher densities. The maximum swelling peak of the melt appeared to shift to higher magnetic flux density, and the value of the maximum swell decreased with increasing wall shear rate and die temperature. The effect of magnetic torque on the extrudate swell ratio of PS melt was more pronounced when extruding the melt at low shear rates and low die temperatures. For radial extrudate swell and velocity profiles, the radial swell ratio for a given shear rate decreased with increasing r/R position. There were two regions where the changes in the extrudate swell ratio across the die diameter were obvious with changing magnetic torque and shear rate, one around the duct center and the other around r/R of 0.65–0.85. The changes in the extrudate swell profiles across the die diameter were associated with, and can be explained using, the melt velocity profiles generated during the flow. In summary, the changes in the overall extrudate swell ratio of PS melt in a capillary die were influenced more by the swelling of the melt around the center of the die. Polym. Eng. Sci. 44:2298–2307, 2004. © 2004 Society of Plastics Engineers.     

Effect of processing history on the morphology and properties of polypropylene/thermotropic liquid crystalline polymer blends

Preparation, morphology, and mechanical properties were studied of blends of a thermotropic liquid crystalline polymer (TLCP) with two different grades of polypropylene, one with and one without overlap in processing temperatures, using two different blending methods. The highly viscous grade (PP-1) was of sufficient thermal stability to be blended with the TLCP (Vectra A950) in a single-screw extruder with an Egan mixing section on the screw. The low viscous grade (PP-2) could not be processed at the same temperature as the TLCP because of degradation. Its blends were, therefore, prepared by a special coextrusion technique, i.e. feeding the two components from two separate extruders to a Ross static mixer. In both methods drawing of the extrudate is necessary to obtain satisfactory mechanical properties. The PP-1/TLCP blends had to be extruded twice in order to obtain proper mixing. The morphology of these blends ranges from a pronounced skin-core morphology at low extrudate draw ratio (DR = 3) to a high-aspect ratio fiber/matrix morphology at high draw ratio (DR = 15). The morphology of the PP-2/TLCP blends was always a high-aspect ratio fiber/matrix morphology even at low draw ratios. The TLCP fibers were generated in this coextrusion process under conditions where the viscosity of the dispersed phase was higher than the viscosity of the matrix. Breakup experiments demonstrate that fibers of a thickness of approximately 1 μm disintegrate into droplets within a few seconds at temperatures above the melting point of the TLCP. This is probably the cause of the skin-core morphology obtained with single-screw extrusion. Tensile modulus and strength of all blends increase with extrudate draw ratio. The deformation of the TLCP phase in the drawn blends is less than affine, probably because of slip between the phases. The moduli of the PP-1/TLCP blends as a function of the draw ratio can be described well by a modified Halpin-Tsai equation taking into account both changes in aspect ratio and molecular orientation of the TLCP fibers. The level of reinforcement in the PP-2/TLCP blends is lower than expected, probably because of the low temperature of drawing. This demonstrates a limitation of the coextrusion process: blending at temperatures that are too low reduces mechanical properties.     

用毛细管流变仪研究发泡聚丙烯(PP)挤出胀大行为

利用毛细管流变仪研究发泡聚丙烯(PP)挤出胀大行为,观察了压力、温度等参数与膨胀率的关系,分析加入不同的AC发泡剂和CaCO3后其膨胀率不同的变化情况,指出这不仅仅是因为发泡体系弹性的变化,气泡的存在也对其膨胀率影响很大。     

聚丙烯熔体强度对加工性能的影响

通过对均聚、共聚和高熔体强度PP的离模膨胀，不同温度下的熔体强度，结晶温度和拉伸粘度进行了测量，比较和表征，揭示了熔体强度对PP加工性能的影响因素，以对PP的加工和应用提供必要的参考。     

Effect of molecular structure on extrudate swell behavior for different thermoplastic melts in an electro‐magnetized die

The extrudate swell ratio of five different thermoplastic melts flowing in a constant shear rate rheometer having a capillary die with and without application of magnetic field was studied. The effects of the magnetic flux direction and density, die temperature, and wall shear rate on the extrudate swell and flow properties were investigated. The experimental results suggested that an increasing wall shear rate increased the swelling ratio for the polystyrene (PS), LLDPE, and PVC melts, but the opposite effect was observed for the ABS and PC melts. The extrudate swell ratio for the PS, ABS, PC, and LLDPE melts decreased with increasing die temperature, the effect being reversed for the PVC melt. Thermoplastic melts having high benzene content in the side‐chain and exhibiting anisotropic character were apparently affected by the magnetic field, the extrudate swell ratio increasing with magnetic flux density. The effect of the magnetic field on the extrudate swell ratio decreased in the order of PS → ABS → PC. The extrudate swell ratio for the co‐parallel magnetic field system was slightly higher than that for the counter‐parallel magnetic field system at a high magnetic flux density. POLYM. ENG. SCI., 47:270–280, 2007. © 2007 Society of Plastics Engineers.     

Deformation mechanisms of polypropylene/polystyrene multilayered films

Multilayered composites of polypropylene (PP) and polystyrene (PS) fabricated by a layer‐multiplying coextrusion technique are described. The aim of this investigation was to find a correlation between the morphology and the mechanical and micromechanical deformation behavior. The multilayered films had primarily continuous layers, exhibiting only few defects in layer construction and turning into an irregularly layered system when the calculated layer thickness was only 5 nm. The morphology and layer thickness of both the PP and PS layers affected the mechanical and the micromechanical behavior, which was brittle for the films having PS layers thicker than 75 nm and ductile when the PS layers were 50 nm and thinner. Transmission electron microscopy showed crazes in the thicker PS layers and homogeneous deformation in the thinner ones. The molecular orientation during deformation of the ductile films was calculated from rheo‐optical measurements with Fourier transform infrared spectroscopy. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012