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。     

气辅挤出口模中滑移段长度对挤出胀大比的影响研究

采用壁面滑移方程和Thompson变换,对黏弹性高分子熔体在不同气体辅助挤出口模内的流动进行了数值模拟研究.考察了剪切速率、松弛时间和滑移段长度对挤出物挤出胀大比的影响.研究表明:随着剪切速率和松弛时间的增大,挤出胀大比增大;同时发现当剪切速率或松弛时间增大到某一数值时,挤出物出现先收缩后胀大现象,并伴有熔体破裂现象.模拟还表明:延长气辅挤出口模中滑移段的长度可以避免挤出物发生熔体破裂,并可有效降低挤出胀大比.     

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。     

流变性对两种熔体共挤出的影响

利用有限元软件包POLYFLOW对两种熔体的二维等温共挤出进行了数值模拟，讨论了两种熔体的流变性对共挤出的流动情况，界面形状和挤出胀大的影响。     

L形异型材共挤胀大及变形的三维非等温数值模拟

采用PTT本构方程,应用Arrhenius方程来描述温度对黏度的影响,建立了L形双层共挤模型,通过有限元方法分析了聚丙烯（PP）和聚苯乙烯（PS）熔体的三维非等温黏弹流动过程,对比分析了2种材料在不同组合情况下口模出口面的速度场、剪切速率场以及共挤出胀大和变形情况。结果表明,L形分层共挤的胀大和变形不仅与2种熔体黏度差异有关,还与口模截面形状有关;黏度较低的PP会向黏度较高的PS一侧偏转,且PP有包覆PS的趋势;黏度较低的PP位于L形内侧时共挤出胀大和变形程度大于其位于外侧时,且两熔体黏度差异越大,两方案的共挤出胀大和变形程度的差异越大。     

轮胎胶料共挤出成型过程的数值分析

利用有限元方法对轮胎胶料(TWD40/FS10)共挤出成型过程进行三维数值模拟,分析入口体积流量比和牵引速度对口型出口处速度分布、界面偏移以及挤出胀大比的影响。结果表明:两熔体共挤出时,入口流量比对口型出口处速度分布、界面偏移影响较大,对挤出总胀大比影响较小;入口流量比接近于口型入口面积之比时,口型出口处速度分布较均匀,界面偏移量较小,且宽度方向界面总体偏向粘度较小的一侧;口型出口处垂直于挤出方向速度较大值出现在界面较小区域会引起界面局部尖角;牵引速度提高,沿挤出方向界面末端平直段逐渐增加,同时两熔体挤出胀大比迅速变小,口型出口处速度分布和沿宽度方向界面偏移量变化很小。     

壁面滑移条件下UHMWPE单螺杆挤出过程有限元模拟

在壁面滑移条件下建立了超高分子量聚乙烯(UHMWPE)单螺杆挤出过程有限元模型,模拟了不同滑移系数对挤出成型过程的影响。由Navier线性滑移定律建立了螺杆壁面熔体所受剪切应力与熔体滑移速度之间的关系,基于Polyflow软件,采用不同滑移系数对单螺杆计量段内UHMWPE熔体的三维等温流动开展了有限元模拟,获得了熔体的速度、压力的分布规律。结果表明:滑移系数增大,沿着螺槽方向的速度和垂直于螺槽方向的速度明显增大,螺杆的均化质量和挤出产量明显提高;但熔体在螺槽间的压力降增大,易导致挤出过程不稳定。     

UHMWPE薄膜熔体挤出壁面滑移的研究

在壁面滑移条件下建立了超高相对分子质量聚乙烯(UHMWPE)鱼尾形流道挤出过程有限元模型,模拟了不同滑移系数对挤出成型过程的影响.由Navier线性滑移定律建立了挤出模具壁面熔体所受剪切应力与熔体滑移速度之间的关系,基于Polyflow软件,采用不同滑移系数对鱼尾形流道中UHMWPE熔体的三维等温流动开展了有限元模拟,...     

气辅挤出成型中挤出物直径的理论预测和实验研究

应用Polyflow软件对粘弹性聚合物熔体在气辅挤出口模内的流动进行了研究,得到了挤出物直径的挤出胀大比方程。模拟结果表明,熔体在滑移段的平均停留时间与材料松弛时间之比同挤出胀大比之间存在指数衰减关系。通过HDPE的气辅挤出成型实验,发现不同螺杆转速及挤出温度下挤出物直径的实验值与计算值达到很好的吻合,该计算方法可用于指导气辅口模设计。     

聚合物挤出过程中壁面滑移现象研究

对聚合物挤出加工中引起表面质量缺陷的壁面滑移现象及其产生机理进行了总结，并对滑移产生的机理作了以下几种解释：滑移发生在口模壁面的吸附一解吸，发生在口模与熔体间润滑层的缠结一解缠，滑移既可能是口模壁面的吸附一解吸，也可能是在滑移层的缠结一解缠或两种情况同时出现，与口模材料和聚合物熔体有关。阐述了壁面滑移区域、滑移速度的测定和计算方法，介绍了用在口模与聚合物熔体之间建立气垫膜层采用气体辅助挤出等完全滑移方式挤出以提高挤出速度的几种方法。     

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 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.     

Rheology of shaped fiber formation

A rheological interpretation is given for the formation of shaped fibers whose cross section is different from the spinnerette shape. For the study, a rectangular die having an aspect ratio of 6 was constructed, with dimensions 0.1 in. by 0.6 in. Measurements were then taken of the distributions of wall normal stresses at two adjacent walls of the rectangular duct. The measurements show the nonuniform distributions of wall normal stresses over the long and short sides of the rectangle, which is in accord with the nonuniform distributions of extrudate swell on both sides of the rectangle. Explanations are then given for the observed fact that extrudate swell is more pronounced at the long side than at the short side.     

Effect of die temperature on the flow of polymer melts. Part II: Extrudate swell

The effect of die wall temperature on the extrudate swell of polymer melts flowing through dies with single and dual circular channels was studied. Extrudate swell was measured at constant flow rates using an Instron capillary rheometer with a modified die section. It was found that under isothermal conditions, extrudate swell plotted against the average wall shear stress gave rise to a temperature independent correlation for polystyrene. Under non-isothermal conditions, such a correlation did not exist, which might be due to the change of wall shear stress in the axial direction. The extrudate swell in the non-isothermal cases can be better correlated with the wall shear stress at die exit. For the two-hole die, changes of die wall temperature varied both the flow rate ratio and the extru date swell ratio. The latter is, however, much less sensitive to the die wall temperature than the former.     

Numerical simulation of double‐layer flow from tube‐in‐orifice spinnerets

Numerical simulations of double‐layer flow from tube‐in‐orifice spinnerets have been carried out using a differential constitutive equation. The streamline‐upwinding finite element method with 4 X 4 sub‐elements on stress components and over‐relaxation method for equation of motion were used. In addition, the extrudate swell and interface shape have been experimentally observed for PVP(polyvinyl‐pyrrolidone) aq./water system to check the validity of numerical results and to relate the flow geometry to the extrudate swell and interface shape. It was found through both numerical predictions and experiments that extrudate swell shape is strongly dependent on the location of the center tube edge in a tube‐in‐orifice spinneret. In particular, the minimum of the outer diameter swell was found when the center‐tube edge was flush with the outer‐orifice edge. These results give useful information for choosing the optimal design of a tube‐in‐orifice spinneret.     

Experimental studies on extrudate swell behavior of PS and LLDPE melts in single and dual capillary dies

Extrudate swell behavior of polystyrene (PS) and linear low‐density polyethylene (LLDPE) melts was investigated using a constant shear rate capillary rheometer. Two capillary dies with different design configurations were used, one being a single flow channel and the other being a dual flow channel. A number of extrudate swell related parameters were examined, and used to explain the discrepancies in the extrudate swell results obtained from the single and dual flow channel dies, the parameters including output rate and output rate ratio, power law index, wall shear rate, wall shear stress, melt residence time, pressure drop induced temperature rise, flow channel position relative to the barrel centerline, and the flow patterns. It was found in this work that the power law index (n value) was the main parameter to determine the output rate ratio and the extrudate swell between the large and small holes for the dual flow channel die: the greater the n value the lower the output rate ratio and thus decreased extrudate swell ratio. The differences in the extrudate swell ratio and flow properties for PS and LLDPE melts resulted from the output rate ratio and the molecular chain structure, respectively. The extrudate swell was observed to increase with wall shear rate. The discrepancies in the extrudate swell results from single and dual dies for a given shear rate were caused by differences in the flow patterns in the barrel and die, and the change in the melt velocities flowing from the barrel and in the die to the die exit. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1713–1722, 2003     

工艺参数对聚合物气辅共挤成型的影响

利用流体力学有限元分析软件对二维气辅共挤出过程进行数值模拟,研究各工艺参数对聚合物气辅共挤成型的影响.结果表明,速度场的峰值和压力峰值会随着流量的增加而增加;气辅共挤时熔体流动稳定,呈柱塞状挤出;气辅共挤可以有效提高挤出速率,防止制品表面的"鲨鱼皮"现象.     

Extrudate swell from slit and capillary dies: An experimental and theoretical study

A comparative experimental study of extrudate swell from long slit and capillary dies is reported for rheologically characterized polystyrene and polypropylene melts. Generally extrudate swell from a slit is greater than that from a capillary die. At low die wall shear rates it goes to a value of about 1.2 as opposed to about 1.1 found for capillary dies. The onset and character of extrudate distortion have been studied. The experimental results are compared with theories of swell based on unconstrained recovery from Poiseuille flow in these geometries. A detailed analysis of such theories of extrudate swell based on the original work of Tanner has been carried out. The analysis is placed in a more general form which should be valid for a range of die cross-sections.     

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。     

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。