Numerical simulation of fluid flow and heat transfer in twin-screw extruders for non-Newtonian materials

A new simplified approach has been proposed for the numerical simulation of the thermal transport in corotating, tangential, and self-wiping twin-screw extruders. It is assumed that the flow domain in a twin-screw extruder can be divided into (i) the translation region (T-region), which represents a flow similar to that in a single-screw channel and (ii) the intermeshing region (I-region), which is located between the two screws. The two regions are simulated separately and then coupled for each screw section to model the overall transport in tangential and self-wiping twin-screw extruders. A finite difference method is employed for the developing flow and temperature fields in the T-region, in order to minimize the computing effort, while a finite element method is employed for determining the interchannel flow mixing and the thermal transport in the I-region. Results are obtained in terms of temperature, velocity, and pressure variations along the screw channels and mixing between the two screws.     

A 2-d numerical study of intermeshing self-wiping screws in biopolymer extrusion

A knowledge of flow behavior is important in the study of laminar flow in twin-screw extrusion processes to predict the velocity distribution and to understand the mixing process. The flow of a power law fluid in self-wiping twin-screw extruders is examined using a two-dimensional finite element analysis of a mid-channel section of intermeshing screws. Theory is compared with experiment using food biopolymer and plastic materials. Comparisons showing overprediction of throughputs, but similarities in behavior, suggest that this model could provide an upper limit for melt conveying. For most of the throughput range examined, pumping of intermeshing self-wiping screws appears to be almost independent of the power law flow index of the melt extruded, but the value of the flow index determines the degree of influence intermeshing has on the overall pressure gradient generated in the extruder.     

Corotating Twin-Screw Extrusion Reactive Extrusion-Devolatilization Model and Software

The modular corotating twin-screw extruder is an increasingly widely used polymer processing machine. This paper describes analyses of reactive extrusion and devolatilization in a modular corotating twin-screw extruder. The results of the analyses are intergrated into a comprehensive model and computer software for the modular corotating twin-screw extrusion process.     

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.     

Resin distribution along axial and circumferential directions of self-wiping co-rotating parallel twin-screw extruder

A self-wiping co-rotating twin-screw extruder (TSE) is operated in a starved state in which the screws are partially filled with resin. Understanding the resin distribution on the screw surface of a TSE in this state is essential for the design, operation, and maintenance of the twin-screw extrusion process. Accordingly, in this study, the circumferential and axial distribution of resin in a TSE were simulated using a novel method combining the mathematical formulation of Hele–Shaw flow, the finite element method, and a newly developed down-wind pressure updating scheme. The results of the simulation were found to be in good agreement with experimental measurements. The proposed simulation method enables the detailed visualization of resin distribution in the entire axial and circumferential directions over the length of a TSE, improving the ability to determine both the devolatilization and fiber attrition during the extrusion process.     

A global computer software for polymer flows in corotating twin screw extruders

A global computation model for self-wiping corotating twin screw extruders is proposed. Based on a ID approximated approach, it has been validated by comparison with experimentation and more sophisticated numerical models. It allows one to obtain, for any screw profile including left-and right-handed screw elements and kneading discs, the profile along the screws of the main flow variables, such as pressure, mean temperature, residence time, and filling ratio. Owing to the approximations made, this model can be easily and rapidly run on a personal computer or a workstation. Important applications may be found in screw profile design, scaleup, compounding or reactive extrusion.     

The modeling of continuous mixers. Part I: The corotating twin-screw extruder

In many operations in polymer processing, such as polymer blending, devolatilization, or incorporation of fillers in a polymeric matrix, continuous mixers are used; e.g., corotating twin-screw extruders (ZSK), Buss Cokneaders and Farrel Continuous Mixers. Theoretical analysis of these machines tends to emphasize the flow in complex geometries rather than generate results that can be directly used (1–5). In this paper, a simple model is developed for the hot melt closely intermeshing corotating twin-screw extruder, analogous to the analysis of the single-screw extruder carried out in 1922 and 1928 (6, 7). With this model, and more specifically with its extension to the complete nonisothermal, non-Newtonian situation, it is possible to understand the extrusion process and to calculate the energy, specific energy, and temperature rise during the process with respect not only to the viscosity of the melt, but also to the screw geometry (location and number of transport elements, kneading sections and blisters, pitch, positive or negative, screw clearance, and flight width) and screw speed. To support the theoretical analysis, model experiments with a Plexiglas-walled twin-screw extruder were performed, in addition to practical experiments with melts on small- and large-scale extruders, with very reasonable results, In Part 2, the Buss Cokneader will be analyzed analogously.     

ABS复合材料耐溶剂的研究

以苯乙烯-马来酸酐共聚物(SMA)为增容剂,研究了不同浸泡时间与ABS复合材料力学性能的关系。结果表明:随浸泡时间的延长,ABS复合材料力学性能有明显变化;与ABS/PA6复合材料相比,ABS/PBT复合材料具有更优异的耐溶剂性能。     

Pumping characteristics of self-wiping twin-screw extruders—A theoretical and experimental study on biopolymer extrusion

The flow of Newtonian and non-Newtonian fluids which obeys a power law relationship between shear stress and shear rate has been modeled in the melt conveying section of a self-wiping co-rotating twin-screw extruder using a finite element analysis of an unwound channel section. Predictions of throughput against pressure gradient are compared with experimentally obtained results for maize grits which is represented as a power law material. Rheological data applicable to extrusion simulation were obtained from capillary rheometry. Comparisons are reasonable with predicted characteristic showing similar behavior.     

啮合同向双螺杆挤出机普通螺纹元件中物料停留时间计算

采用有限元方法对啮合同向双螺杆挤出机的普通螺纹元件流动进行了三维等温非牛顿模拟计算，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螺纹元件中的三维流动路径，利用物料的三维流动路径计算结果分析了普通螺纹元件中物料的停留时间分布和平均停留时间随螺杆转速和挤出机产量的变化规律。     

生物型降解母料的挤出特点分析

本文针对生物型降解母料的特点，结合在同向双螺杆挤出机挤出造粒过程中所出现的现象、从双螺杆挤出机的主机结构、配置、操作等方面进行了分析探讨，并介绍了我们在生物降解母料双螺杆挤出造粒方面的最新进展。     

Computation of the morphological changes of a polymer blend along a twin-screw extruder

The control of the morphology of an immiscible polymer melt is of vital importance for the mastering of the final properties of the product. As polymer blends are produced using corotating twin-screw extruders, understanding and modeling the changes experienced by the blend during this process is of great interest. In the present study, starting from Ludovic software, developed for computing flow parameters in the twin-screw extrusion process, we present a computation of the droplet morphology development, based on the basic mechanisms of break-up and coalescence. Depending on the value of a local capillary number and on local flow conditions, different changes may occur: affine deformation, drop splitting, break-up by capillary instability, and coalescence. It is thus possible to follow, all along the screws, the changes in morphology, either for a single particle or for a particle distribution. Examples of these different computations are presented and compared with experimental results. Generally speaking, orders of magnitude of droplet size and tendencies when modifying processing conditions are correctly described, but the model still suffers from the absence of descrption of the melting process.     

Transport in a twin-screw extruder for the processing of polymers

The fluid flow and heat transfer in polymer extrusion in a twin-screw extruder was studied numerically by using the finite volume method. In the mathematical model, the coordinate system is fixed to the screw so that it is held stationary and the barrel is moved to simplify the complicated geometry. The screw channel of a twin-screw extruder is approximated as two regions: translation and intermeshing. The flow in the translation region is similar to that in a shallow single screw extruder and is treated by the numerical methods given in the literature. In the nip or intermeshing region, strong mixing effects are expected, along with the diffusion of energy and momentum. The full governing equations are solved in this region to determine the velocity components in all the three coordinate directions. The energy equation is coupled with the equations of motion through viscosity, since the viscosity of the polymeric, non-Newtonian, fluids considered here is dependent upon the shear rate and temperature. There is no clear physical demarcation between the nip region and the translation region. Therefore, a domain matching was employed at an arbitrary location that was varied numerically to ensure that the results were independent of this location. The variation of pressure and bulk temperature along the helical channel of the twin-screw extruder is obtained, along with the shear rate. An experimental investigation of the velocity profiles in the translation region of a self-wiping twin-screw extruder, which is often used in practical applications, was carried out using a Laser Doppler Anemometer. The numerically predicted velocity profiles are compared with those from the experiments, yielding fairly close agreement.     

Study on Extrusion Characteristics of the Tri-Screw Extruder

A novel kind of extruding machinery is proposed—a tri-screw extruder (TRISE), in which three intermeshed screws are arranged in the three corners of a triangle; three meshing regions and a center zone also comprise the special construction. The flow rate and the pressure-generating ability of the melt in the thread-zone flow field of the tri-screw extruder, especially the pumping formed by the action of taking in and sending out melt in the unique center zone, are also described. The results of simulation and experiments clearly show that the tri-screw extruder has stronger conveying capacity. The shear frequency of material in the tri-screw extruder is higher than that in the twin-screw extruder, which means that filler could be loaded to a much higher point in the TRISE than in the twin-screw. Results from the TRISE also show that energy consumption of the TRISE is lower than that of the twin-screw extruder.     

Polymerization of ε-caprolactam in an extruder: Process analysis and aspects of industrial application

The polymerization of caprolactam into polyamide 6 on corotating twin-screw extruders provides the processor with technically and economically interesting aspects of application. Using the extruder as a small reactor, it is possible, on the one hand, for small batches of polyamide to be optimized for specific applications, and, on the other hand, for large quantities of polymerized polyamide melts to be fed into different processes. Apart from the extrusion of filaments, films, and profiles, the continuous polymerization process also can be coupled up to discontinuous processes. This so-called rapid polymerization in the extruder not only brings price advantages as compared with standard polyamides, but also gives higher molecular weights and, hence, improved properties.     

挤出加工方法对制备PP/PET原位微纤共混物微观形态的影响

分别使用双螺杆挤出机、配备不同结构螺杆或强剪切机头的单螺杆挤出机对聚丙烯(PP)/聚对苯二甲酸乙二醇酯(PET)进行熔融共混挤出,并用扫描电子显微镜观察了产物的微观形态。结果表明,使用双螺杆挤出机或使用配备三段式螺杆的单螺杆挤出机挤出PP/PET,只能制备出PET以球状形态均匀分散在PP连续相中的共混物,不含有任何微纤;使用配备有头部直槽混炼件的单螺杆挤出机挤出PP/PET,部分PET会形成短而粗的微纤;采用熔融挤出热拉伸淬冷法挤出PP/PET,可生产出微纤直径约为5 μm、长径比超过20的原位微纤共混物;采用强剪切机头及头部配备有直槽混炼件螺杆的单螺杆挤出机挤出PP/PET,可生产出微纤直径约为5~7 μm、长径比超过20的原位微纤共混物,且该方法操作简单、辅助设备少、具有工业可行性。     

Characterization of polyamide 6 made by reactive extrusion. I. Synthesis and characterization of properties

The anionic polymerisation of ε-caprolactam in a corotating, intermeshing twin-screw extruder is discussed in terms of the materials formulation, extruder screw profile, and processing parameters such as temperature and screw speed. Using a variety of analytical characterization techniques, it is demonstrated that molecular mass, residual monomer content, and mechanical properties of the polyamide 6 are very sensitive to the reactive processing conditions employed. © 1994 John Wiley & Sons, Inc.     

Reactive processing of nonmiscible polymers: Shear rate effect

The effect of shearing in a corotating twin‐screw extruder on the apparent kinetic rate of model reactions was studied. Oxazoline‐terminated oligomers with different molar masses were prepared and used in condensation reactions with carboxylic acid terminated oligomers. A miscibility study of these oligomers allowed the choice of miscible and nonmiscible reactive systems. Reactions were first conducted in a glass reactor and then in the kneading‐disc area of a corotating twin‐screw extruder modified to neutralize the transport along the screws. For nonmiscible reactive systems, reactions were slower than equivalent reactions with miscible reactants. A shear increase for these nonmiscible reactants resulted in an increase in the apparent kinetic rate, which was in all cases lower than the rate obtained for miscible reactants. For very high shear rates, mechanical degradation was observed. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 2357–2362, 2004     

啮合同向双螺杆挤出过程停留时间分布实验研究

通过对啮合同向双螺杆挤出过程常规螺纹元件螺杆组合及引入轴向循环段的螺杆组合停留时间的实验研究，分析了轴向循环段的引入对啮合同向双螺杆挤出过程停留时间及其分布的影响。