3‐D non‐isothermal flow field analysis and mixing performance evaluation of kneading blocks in a co‐rotating twin srew extruder

We have developed a three‐dimensional non‐Newtonian and non‐isothermal flow analysis of the twin screw extruder (TSE) using the finite element method. This code can simulate the fully filled parts of several kinds of screw elements, such as full flight screws, kneading blocks, rotors and their combinations. A marker particle tracking analysis has also been developed to evaluate the mixing performance of the screw elements. In this paper, simulations for the kneading blocks in a co‐rotating TSE were carried out. The screw configurations are combinations of 2‐lobe kneading blocks with several stagger angles and disk widths. The effects of screw configurations on pressure and temperature distributions are examined. We also evaluate the dispersive and distributive mixing via stress magnitude and area stretch obtained by marker particle tracking analysis. Additionally, we discuss the desirable stagger, disk width and their combinations that promote the mixing performance.     

啮合异向双螺杆挤出机中捏合块元件对PE-HD/PS不相容体系混合效果的研究

在常规啮合异向双螺杆上引入不同错列角和不同捏合盘厚度的捏合块元件,将其用于高密度聚乙烯/聚苯乙烯（PE-HD/PS）不相容体系的混合,观察混合体系的相态并分析分散相的粒径。结果表明,增加捏合块元件后啮合异向双螺杆挤出机的分散混合能力得到了提高;捏合块的错列角和捏合盘的厚度对PE-HD/PS不相容体系分散相PS的重均粒径和粒径分布指数有较大影响。     

Study of mixing efficiency in kneading discs of co-rotating twin-screw extruders

Co-rotating twin-screw extruders are widely used for compounding and blending in polymer processing. In a modular machine, the dominant elements determining dispersive mixing efficiency are the kneading discs. A fluid dynamics analysis package - FIDAP, using the finite element method was implemented to simulate the 3-D isothermal flow patterns in the kneading disc region of a Werner & Pfleiderer ZSK-53 co-rotating twin-screw extruder. The kneading discs simulated are three-lobe discs. The flow field characteristics relevant for dispersive mixing are shear stresses generated and elongational flow components. We compare these flow characteristics for the three-lobe kneading discs of the ZSK-53 with the two-lobe discs of a ZSK-30 machine. We also discuss the influence of processing parameters on the flow field mixing efficiency.     

双螺杆挤出中聚合物熔体混合状态的统计学分析

利用POLYFLOW对啮合同向和异向双螺杆挤出机中的三维等温流场进行了数值模拟，在获得大量粒子轨迹的基础上，运用统计学指标-分配尺度和分散指数对聚合物熔体在两种挤出机中的混合状态进行了定量描述。     

高分散混合元件设计及混合性能的研究

根据啮合同向双螺杆挤出机的啮合原理,设计了一种高分散混合双螺杆元件。运用Polyflow有限元分析软件对该双螺杆元件的3种螺杆构型的流场进行了模拟分析,并且对这3种螺杆构型进行了实验研究。研究表明,错列角为150°的元件的分散混合性能最好,其次是错列角为30°的元件,错列角为90°的元件的分散混合性能最差。     

Melt-mixing by novel pitched-tip kneading disks in a co-rotating twin-screw extruder

Melt-mixing in twin-screw extruders is a key process in the development of polymer composites. Quantifying the mixing performance of kneading elements based on their internal physical processes is a challenging problem. We discuss melt-mixing by novel kneading elements called “pitched-tip kneading disk (ptKD)”. The disk-stagger angle and tip angle are the main geometric parameters of the ptKDs. We investigated four typical arrangements of the ptKDs, which are forward and backward disk-staggers combined with forward and backward tips. Numerical simulations under a certain feed rate and screw revolution speed were performed, and the mixing process was investigated using Lagrangian statistics. It was found that the four types had different mixing characteristics, and their mixing processes were explained by the coupling effect of drag flow with the disk staggering and pitched-tip and pressure flows, which are controlled by operational conditions. The use of a pitched-tip effectively controls the balance of the pressurization and mixing ability.     

啮合盘元件中粉体聚集体分散混合的数值模拟

采用有限元方法对啮合同向双螺杆挤出机的啮合盘元件流场进行了三维等温模拟计算，根据流场计算所得到的速度场通过编程计算得到了物料在啮合盘元件中的三维流动路径,结合粉体聚集体的分散模型和三维流动路径，计算得到了粉体聚集体在啮合盘元件出口的平均直径及其分布。     

三螺杆挤出机端面二维流场混沌混合拉格朗日拟序结构分析

三螺杆挤出机是一种新型的聚合物流体加工设备,其独有的中心区呈现出几何结构和受力状态的周期性变化,混合机理非常复杂。区别于传统的线性混合分析,从拉格朗日体系的新视角对三螺杆挤出机混沌混合进行拉格朗日拟序结构分析,利用有限时间Lyapunov指数（FTLE）、拉格朗日拟序结构（LCS）,结合Poincaré截面和粒子可视化技术研究三螺杆挤出机二维流场的流体输运和混沌混合机理,讨论了中心区动态结构特性对FTLE和LCS分布的影响,并与单螺杆挤出机和双螺杆挤出机进行了对比分析。结果表明,LCS将三螺杆挤出机流域划分为近螺杆区、远螺杆区和中心区3个具有不同运动特性的区域,扭结是连接近螺杆区、远螺杆区和中心区物质交换的桥梁。随着混合时间的增加,扭结的弯曲和折叠程度逐渐增大,增强了3个区域的物质交换,强化了三螺杆流场的混沌混合。三螺杆挤出机啮合区附近存在3个双曲固定点,混合能力较好。Poincaré截面中椭圆周期点的出现说明在流域中心有非混沌区存在,因此三螺杆挤出机的中心区混合能力相对较弱。     

三螺杆挤出机中剪切-拉伸交变流场对PP/PE-HD体系增韧效果的影响

利用倒三角形排列三螺杆挤出机与传统双螺杆挤出机对不同黏度比的聚丙烯/高密度聚乙烯(PP/PE-HD)体系进行混炼加工,研究了三螺杆挤出机中剪切拉伸交变流场对于高黏度比聚合物两相体系的分散混合及分布混合效果的影响;并通过高黏度比体系的混合效果证明三螺杆挤出机中剪切拉伸交变流场的作用效果及机理。结果表明,在高黏度比体系的加工中,倒三角形排列三螺杆挤出机分散与分布混合效果明显优于双螺杆挤出机,且三螺杆挤出机的增韧效果随着体系黏度比的增大而提升。     

Three-dimensional flow modeling of a self-wiping corotating twin-screw extruder. Part II: The kneading section

Three-dimensional flow simulations of kneading elements in an intermeshing corotating twin-screw extruder are performed by solving the Navier Stokes equations with a finite element package, Sepran. Instead of using the whole geometry of the 8-shaped barrel a simplified geometry is used, representing a large part of the geometry during the rotating action of the kneading paddle. The goal of these calculations is to study the dependence of several factors that influence mixing, such as shear rate, elongation rate, pressure, and the flow profile in the extruder on various extruder parameters, such as fluid viscosity, rotation speed, and throughput. The shear and elongation rate and the pressure drop are calculated for varying viscosities. The various stagger angles possible for disc configurations in the corotating twin-screw extruder are modeled. The axial backflow volume is calculated for varying values of rotation speed and throughput.     

波状双螺杆元件混合性能的研究

对同向波状双螺杆元件和常规螺纹元件的分散混合性能进行了数值模拟,分析了2种元件流场的平均剪切应力、最大剪切应力分布、最大拉伸应力分布等混合性能参数,并对其进行了实验研究。模拟结果与实验结果基本吻合。结果表明,波状双螺杆元件有较强的分散混合能力。     

双螺杆挤出共混物分散相粒子的分散和分布——捏合段数量的影响

采用啮合同向双螺杆挤出机不同的螺杆组合形式，研究了熔融塑化区中捏合段数量对双螺杆挤出聚合物共混物表层和内层分散相粒子粒径及其分布的影响。结果表明，增大塑化区中螺杆前段的剪切作用和物料停留时间可明显减小分散相粒子的尺寸；在塑化区域增加剪切捏合段段数对表层分散相粒子尺寸没有明显作用，但可有效减小内层分散相粒子尺寸；对于在塑化区内采用前后两段捏合段的螺杆组合，增大后段剪切作用和物料停留时间比在前段增加更为有效，可同时显著降低共混物分散相表层和内层粒子尺寸，改善其分布特性。     

啮合同向双螺杆挤出机中齿形盘元件不同螺杆构型的性能分析

在相同的模拟条件下，通过对啮合同向双螺杆挤出机中齿形盘元件不同螺杆构型的流量、回流系数、剪切速率和剪切应力的分析，得到混合或(和)输送较佳的螺杆构型，并分析了其原因。     

铝粉高填充材料混合流场数值模拟及实验研究

应用Polyflow软件模拟计算了4种总长度为440 mm的螺杆熔体混合段内铝粉高填充复合材料的三维混合流场。通过对各个螺杆组合流道的剪切速率、剪切应力、轴向速度分布等指标进行分析,研究了这4种螺杆组合的流场分布规律和混合性能;分别对每个螺杆组合上的不同元件的混合特性进行直观的对比。数值模拟与实验分析结果表明,带有反向螺纹元件和捏合块的螺杆对填充材料中的超细铝粉具有更优的综合混合能力。     

含能材料锥形双螺杆安全混合径向间隙的研究

以锥形双螺杆挤出机挤出段、过渡体和挤出机头为研究对象,基于Polyflow非等温稳态和瞬态的有限元模拟,对不同径向间隙下流场中各场量分布情况和变化曲线进行分析,研究含能材料在挤出段、过渡体和挤出机头流道中的安全和混合问题。从安全和混合效果两方面考虑,建立了综合评价方法,优选径向间隙。结果表明,最大剪切速率、最高剪切应力和最高黏性耗散热均在挤出段流道入口附近,最高压力在挤出段流道出口处,最高温度在机头处;锥形双螺杆的混合效果随着间隙的增加而增加;在径向间隙为0.5 mm时,流道的安全性能最差,径向间隙为1 mm时,流道的安全性能最好。     

数值模拟研究螺纹元件的混合性能

使用FEM方法数值模拟了同向双螺杆螺纹混合元件和普通螺纹元件流道中硬聚氯乙烯的流动过程,计算了2种螺纹元件流道内熔体的非等温流场,使用粒子示踪法统计分析了2种螺纹元件流道内熔体的混合性能.研究结果表明,螺纹混合元件具有较均匀的剪切速率分布、较平缓的温度分布和较强的分散与分布混合能力,但是其输送能力较小.     

立式捏合机捏合间隙影响CFD分析

本文利用计算流体力学(CFD)方法研究捏合间隙对立式捏合机混合性能的影响。首先，根据立式捏合机搅拌桨叶运动特点以及被混物料的流变特性，进行立式捏合机混合流场数值模拟。其次，确定立式捏合机混合性能宏观评价指标。最后，分析了捏合间隙的变化对混合性能评价指标的影响。分析结果表明：间隙越小，立式捏合机的混合性能越好；当空心桨叶两侧的捏合间隙大小相等时，立式捏合机的功耗最小。     

同向双螺杆捏合盘厚度对MAH接枝LDPE反应挤出过程的影响

研究了同向双螺杆挤出机中,捏合盘元件的不同厚度对低密度聚乙烯(LDPE)接枝马来酸酐(MAH)反应挤出过程的影响,并将其与常规螺纹元件进行对比。对挤出沿程中的3个位置和挤出机出口处进行取样,用滴定法测定这些样品的接枝率。结果表明,捏合盘元件所对应的样品接枝率高于常规螺纹元件,并且随着捏合盘厚度的增加,其所对应的样品接枝率下降。此外对于接枝反应而言,捏合盘元件的分布混合能力所起到的作用要高于其分散混合能力。     

Polymer blend mixing and dispersion in the kneading section of a twin-screw extruder

This paper examines the mechanisms by which a polymer is dispersed in a co-rotating twin-screw extruder. An experimental investigation of the morphological evolution has been carried out on a 45-mm co-rotating twin-screw extruder. Polyethylene/polystyrene (PE/PS) blends in the low concentration range (i.e., 5–15 wt% of PE) were used as a model system. The following general trends were observed. First, the minor phase right after melting is predominantly in a fibrillar form. Secondly, droplet and fiber diameter at this early stage of compounding are already in the micron or sub-micron range. Even though a wide variety of mixing section configurations were used, the fibers created in the early compounding stages were relatively stable throughout extrusion. Morphological evolution after melting must therefore be discussed in terms of variation in the fiber fraction (i.e., fiber to droplet transition) rather than in a change in particle diameter. A control volume model for the flow in kneading blocks is used to interpret the morphological results and to predict the deformation and breakup of dispersed phase fibers under shear and in absence of coalescence. Theoretical results indicate that fiber breakup under shear is not likely in the kneading block under the normal processing conditions, which is confirmed by morphological observations made at the mixing section exit. The influence of several geometrical parameters on mixing and pumping in kneading blocks is also discussed with the use of flow model results.     

啮合型同向旋转双螺杆挤出机的螺杆特性及其应用

介绍了啮合型同向旋转双螺杆挤出机的起源和发展,着重论述了该型挤出机挤压系统中的螺杆型式、螺纹元件和捏合元件的输送及混炼特性、螺杆组合型式和操作条件以及应用情况。