Relationship between local residence time and distributive mixing in sections of a twin‐screw extruder

Local residence time and distributive mixing were measured in conveying sections and kneading blocks of a twin screw‐extruder. The residence time measurements were completed using carbon black as the tracer and an infrared temperature probe to detect the temperature decrease caused by the changing surface emissivity. The validity of this experimental technique was extensively evaluated. A mixing limited interfacial reaction between polymer tracers was used to directly measure the distributive mixing in the twin‐screw extruder. Possible relationships between mixing and residence time in the sections of the twin‐screw extruder were investigated by combining these two measurements. Distributive mixing in conveying sections was related to the local average residence time and the fill. In contrast, distributive mixing in kneading blocks was related to the local average number of screw revolutions experienced by the polymer. Forward stagger kneading discs achieved the greatest amount of distributive mixing, which was attributed to a combination of local stagnant flow regions and more frequent interfacial reorientation.     

Analysis of mixing during melt-melt blending in twin screw extruders using reactive polymer tracers

Mixing during melt-melt blending of segregated polypropylene melt streams in a co-rotating twin screw extruder was experimentally investigated. The mixing limited reaction between two polymer reactive tracers, which are terminally functionalized polyolefin oligomers, was used to determine the mixing performance of a kneading block section. The selected functional groups were succinic anhydride and a primary amine, and Fourier-Transform Infrared Spectrometry (FT-IR) was used to determine the anhydride conversion. In the absence of interfacial tension, the reaction conversion was directly related to the amount of interfacial area generated. Experiments were completed to study the effects of operating conditions, kneading block design, and polymer material properties. The screw speed effect was observed to be non-linear because of competing contributions from shear rate, residence time, channel fill, and viscous heating. The mixing performance of kneading blocks backed by a reverse conveying element was observed to follow the trend of: forward > reverse > neutral. For each kneading block design, the mixing performance decreased with an increase in polymer viscosity.     

Mixing mechanism of three‐tip kneading block in twin screw extruders

In recent years, twin screw extruders have been applied to various kinds of polymer processing. It has been important to find their optimum geometrical configurations and operational processing conditions for the best performance of extrusions and products. Many engineers have been evolving numerical and the experimental methods to characterize the mixing performance for twin screw extruders. We have carried out three‐dimensional flow simulations of kneading blocks in intermeshing co‐rotating twin screw extruders by using the finite element method to quantify their ability in distributive and dispersive mixing. We discuss their performance in distributive mixing for three different type of kneading blocks in terms of the residence time distribution and the nearest distance between markers at various periods of time, by using the marker tracking method. Those numerical techniques and applications of mixing indices have enabled us to quantify and evaluate their abilities in distributive mixing of kneading blocks in twin screw extruders.     

Numerical simulation and experimental validation of mixing performance of kneading discs in a twin screw extruder

This work aims at simulation by particle tracking the local residence time distributions (RTDs) of a co‐rotating twin‐screw extruder using computational fluid dynamics. Simulated results follow reasonably well the trend of experimental results obtained by an in‐line measuring instrument for different screw configurations and feed rates. To analyze the distributive mixing performance and overall efficiency of different types of kneading discs (KDs), mixing parameters such as area stretch ratio, instantaneous efficiency, and time‐average efficiency are calculated. Among KDs with stagger angles 45°, 60°, and 90°, the 90/10/64 with disc gaps is most efficient in terms of distributive mixing. The effects of the disc width and disc gap on the local RTD and distributive mixing are also discussed. This provides a numerical tool for assessing point‐by‐point information on the local RTD, flow, and mixing along the screw extruder. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

同向双螺杆挤出机的停留时间分布及填充度

引 言 双螺杆挤出机在高分子材料加工中已被广泛地应用于聚合物共混改性、反应挤出及高分子可控降解等各个方面[1].但先前对挤出过程研究较少,一般仅停留在"黑箱"型经验操作的层面,主要以定性的机械设计为主.     

Effects of screw configurations on the grafting of maleic anhydride grafted low-density polyethylene in reactive extrusion

The effects of screw configurations, that is, the staggering angles and disc widths of the kneading blocks, on grafting reactive extrusion for maleic anhydride grafted low-density polyethylene were investigated in a corotating twin-screw extruder. Samples were collected from three positions along the screw and the die exit. The grafting degree (GD) of the specimens was evaluated by titration. It was found that the kneading block configurations had a significant influence on the grafting reactive extrusion. In addition, another three groups of extrusion experiments were performed to explore the intrinsic relationship between the GD, the degree of fill in the screw channel, the residence time distribution (RTD), and the mixing intensity in various screw configurations. The experimental results indicated that the location of the melting endpoint significantly affected the position at which the reaction began; the degree of fill, RTD, and mixing performance of the screw played important roles in the grafting reaction. The reverse kneading blocks with a narrow disc width, which had a high degree of fill and good mixing capacity, enhanced the increase in GD along the screw during the reactive extrusion. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Evaluation of the mixing performance for one novel twin screw kneader with particle tracking

In this article, a novel continuous twin‐screw kneader was proposed. The end‐cross section of the screw rotor consists of convex arcs and cycloidal curves and the rotors profiles were presented. The mixing performance of the novel twin screw kneader was simulated using finite element method (FEM) combined with mesh superimposition technique (MST). Statistical analysis was carried out for flow field using particle tracking technique to research the effect of geometry parameters and working parameters on the mixing performance. To study the dispersive mixing performance, specifically the maximum shear rate, maximum shear stress, maximum mixing index, residence time distribution (RTD) and RTD density function of tracer particles, and dispersive mixing is evaluated using the mixing index in combination with the shear stress. The results show that the changes of centre distance between female and male rotor have little influence on dispersive mixing performance, the lead of rotor has little effect on maximum shear stress and maximum shear rate, while it has an obvious effect on mixing index, cumulative RTD, and RTD density function. The rotor speed has obvious influence on mixing performance, and average residence time of material decreases greatly and the mixing ability is weakened, while the self‐cleaning performance of rotor improved obviously with the increasing of rotor speed. POLYM. ENG. SCI., 54:2407–2419, 2014. © 2013 Society of Plastics Engineers     

Flow patterns and mixing mechanisms in the screw mixing element of a co‐rotating twin‐screw extruder

Non‐Newtonian and non‐isothermal flow simulations based on 3‐D FEM were applied to a special and conventional elements of a twin‐screw extruder. The screw mixing element (SME), a kind of special element, was a distributive mixing promoter consisting of a standard screw profile with slots cut across the flight tip to increase leakage flow. The full flight screw (FF) and the kneading block (KB) were examined as conventional elements in order to contrast the mixing behavior with the SME. The accuracy of numerical results was verified by experimentally measuring pressure and temperature. Additionally, marker particle tracking analysis was carried out to evaluate the distributive and dispersive mixing. Using the above analyses, the following results were obtained: The pumping capability of the SME was smaller than that of the FF and was the same as for the KB. The SME suppressed heat generation and showed the lowest temperature distribution of the three elements. For distributive mixing, the SME showed the best performance judging from the mixing coefficient G, residence time distribution, and area stretch distribution based on a laminar mixing mechanism. A higher rotational speed achieved better distributive mixing performance. For dispersive mixing defined by stress distribution, the SME showed the second best performance next to the KB. It also showed better dispersive mixing performance with increasing rotational speed. The SME had the advantages of low heat generation and good distributive mixing.     

单螺杆挤出机中非牛顿流体层流混合停留时间表征

基于有限差分数值模拟技术,提出了计算非牛顿流体在单螺杆挤出机内停留时间分布的半解析方法,得到了不同操作参数下的停留时间分布,该分布可用来表征聚合物熔体在加工过程中的混合情况.结果表明：半解析方法能够反映由流体的非牛顿性所导致的耦合流场及压力反流对混合的影响,能更真实地反映聚合物熔体在单螺杆挤出机内的混合程度.     

Local residence time,residence revolution,and residence volume distributions in twin‐screw extruders

This work was aimed at studying the overall, partial, and local residence time distributions (RTD); overall, partial and local residence revolution distributions (RRD) and overall, partial and local residence volume distributions (RVD) in a co‐rotating twin screw extruder, on the one hand; and establishing the relationships among them, on the other hand. Emphasis was placed on the effects of the type and geometry of mixing elements (a gear block and various types of kneading elements differing in staggering angle) and process parameters on the RTD, RRD and RVD. The overall and partial RTD were directly measured in‐line during the extrusion process and the local ones were calculated by deconvolution based on a statistical theory. The local RTD allowed comparing the mixing performance of mixing elements. Also it was confirmed both experimentally and theoretically that specific throughput, defined as a ratio of throughput (Q) over screw speed (N), controlled all the above three types of residence distributions, be they local, partial or overall. The RRD and RVD do not provide more information on an extrusion process than the corresponding RTD. Rather they are different ways of representing the same phenomena. POLYM. ENG. SCI., 48:19–28, 2008. © 2007 Society of Plastics Engineers     

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。     

Mixing characteristics of different tracers in extrusion of polystyrene and polypropylene

To characterize the actual flow behavior of polymer, polystyrene (PS) and polypropylene (PP) macromolecular tracers were synthesized by copolymerization and graft routes. The two tracers were then used to obtain the residence time distribution (RTD) functions of PS and PP in a twin‐screw extruder. The effect of temperature on RTD is discussed using different tracers. The mixing characteristics of different tracers in the extrusion of PS and PP were compared by measuring the overall and partial RTDs. The dependence of the overall and partial RTDs on the tracer types was investigated. The effects of mixing intensity, miscibility, and melting difference on the measured distribution were also discussed. POLYM. ENG. SCI., 54:310–316, 2014. © 2013 Society of Plastics Engineers     

Assessing local residence time distributions in screw extruders through a new in‐line measurement instrument

This work aimed at developing a new instrument to measure in real time the residence time distribution (RTD) in screw extruders. The instrument followed the same principle as the one reported in the literature but possessed several important advantages. For example, the detection system had two probes that allowed to simultaneously measure RTDs at any two different locations of an extruder, thus providing the possibility of calculating the local RTD between them by a deconvolution method based on a statistical theory for the RTD. Its performance was evaluated on a corotating twin‐screw extruder using anthracene as tracer and polystyrene as flowing material. The effects of various process parameters such as feed rate and screw speed on the RTDs were investigated. The emphasis was placed, however, on the effect of the staggering angle of kneading discs on local RTDs both in the kneading zone itself and its neighboring upstream and downstream screw zones. This work is in support of an ongoing project on the simulation of flow in corotating twin‐screw extruders. POLYM. ENG. SCI., 46:510–519, 2006. © 2006 Society of Plastics Engineers.     

Residence time distribution in a twin screw extruder

The residence time distribution (RTD) in a fully intermeshing, corotating twin screw extruder was determined with a stimulus-response technique. In addition to varying three process parameters (i.e. throughput, screw rotational speed, and barrel temperature), two screw configurations were also studied: one containing four kneading block mixing sections, and the other consisting only of regular screw bushings. Although screw configuration was an important variable, it was found that for both configurations the throughput had the largest effect on RTD. The screw rotational speed was second in importance, and the barrel temperature change produced no effect. A fluid mechanical model based on the fluid flow in a partially-filled rectangular channel was used to explain the experimentally observed dependence of RTD on the process parameters. Reaction engineering approaches were adopted to compare the RTD results of two screw configurations with two idealized flows.     

Influence of the melt viscosity and operating conditions on the degree of filling,pressure, temperature,and residence time in a co‐kneader

The effect of the melt viscosity and operating conditions on processing parameters in a co‐kneader with a discharge die was experimentally investigated. Filling ratio, pressure, temperature, and residence time distribution were measured. Experiments were performed with polypropylene resins. The viscosity of the melt was varied either by changing the regulation temperature of the kneader or the molecular weight of the polymer. The filling pattern in the co‐kneader shows the conveying capability of the various elements without any effect of the melt viscosity. Experimental residence time distributions remain the same at a given feed rate and screw speed, regardless of the viscosity of the material. The global degree of filling in a zone combining conveying elements, kneading elements, and restriction ring was found to be nearly constant in the conditions of this study and therefore a simple relation exists between the mean residence time and the feed rate. Beside expected variations of the die pressure and melt temperature when the viscosity, screw speed or feed rates change, a model based on heat equation and experimental data demonstrate the high capability of the co‐kneader for heat exchange and for the control of self‐heating during mixing. POLYM. ENG. SCI., 58:133–141, 2018. © 2017 Society of Plastics Engineers     

Residence time distributions in a co‐kneader: A chemical engineering approach

In this article, we have studied the residence time distributions (RTD) in a modular co‐kneader. Several papers have already addressed the co‐kneader modeling and operating mode but there is still a lack of experimental data on RTD. To investigate the RTD, we have used a colored tracer dispersed in polypropylene (PP) that was injected in the flow during the compounding of neat PP. The effect of operating parameters such as temperature, feed rate, and screw configuration was investigated, focusing on the influence of mixing and conveying elements in a zone where the polymer is molten. As can be expected, results on various screw configurations show that increasing the number of kneading elements makes the RTD longer. More interestingly, for a defined set of elements, their position does not change the experimental RTD. A chemical engineering approach was used to model the RTD, with an equation derived from a cascade of continuous stirred tank reactors. The model allows to retrieve an elementary RTD for each section of a defined type of elements and to propose a law for their combination in good agreement with experiments. POLYM. ENG. SCI., 55:1237–1245, 2015. © 2015 Society of Plastics Engineers     

啮合同向双螺杆挤出过程新型混合元件—六棱柱元件实验研究

前文^[1]对啮合同向双螺杆挤出过程中的新型混合元件－六棱柱元件的流场了三维等温牛顿模拟，并将其结果与捏合块元件作了对比，得到了在相同压差下六棱柱元件比捏合块元件产率高，剪切速率场和剪切应力场分布更均匀，且两种元件分散混合能力相近的结论。本文对六棱柱元件的流场模拟结果进行了实验验证，证明流场模拟计算的方法可行，得到的结果可靠。进而利用多种有效方法，对六棱柱元件的挤出，混合特性进行了实验研究、实验结果表明，六棱柱元件比捏合块元件比能耗小，出料温度低，分布混合能力强，且六棱柱元件适于玻璃纤维增强改性加工过程。     

Modeling of a cokneader for the manufacturing of composite materials having absorbent properties at ultra‐high‐frequency waves. Part 1: Modeling of flow from residence time distribution investigations

The purpose of this study is to gain better understanding of flow patterns and mixing conditions in a particular single‐screw extruder: the Buss Cokneader. To this end, the residence time distribution (RTD) of the polymer was investigated experimentally for different combinations of the operating variables (i.e. feed rate, screw rotation speed). The measurement of RTD used a standard stimulus‐response technique. Two kinds of tracer were used: free anthracene and anthracene grafted on the polymer. It was shown that only the second could characterize the actual flow of the polymer in the extruder. It does not perturb the flow and has the same rheological behavior as the studied fluid. Thanks to the RTD data, a model of the extruder based on the combination of ideal reactors, such as continuous stirred tank reactors or plug flow reactors, was finally set up. The establishment of relationships between model parameters and extrusion conditions allowed the prediction of RTD with good agreement.     

Flow behavior in a corotating twin‐screw extruder of pure polymers and blends: Characterization by fluorescence monitoring technique

The objective of this work was to investigate the flow behavior of pure polymers and blends, especially miscible polymer/polymer systems, in a corotating twin‐screw extruder (TSE) using an online fluorescence monitoring device. An immiscible blend was also studied for the sake of comparison. The fluorescence signal was obtained by using synthesized fluorescence tracers added to the melt at very low concentrations. These tracers consisted of two styrene‐maleic anhydride copolymers (SMA) labeled with anthracene. The investigated blends were SMA8 (8 wt % of MA in SMA)/polystyrene (PS), SMA14 (14 wt % of MA in SMA)/styrene acrylonitrile copolymer (SAN), and poly(methyl methacrylate) (PMMA)/ethyl acrylate‐methyl methacrylate copolymer (PMMAEA). The residence time distribution (RTD), the mean residence time (t ), the dimensionless variance (σ_θ²), the Peclet number (Pe) and the fluorescence peak intensity distribution of pure polymers and binary polymer systems were investigated and interpreted in terms of polymers rheological properties. It was observed that polymers presenting higher viscosity or higher pressure showed longer residence time. A difference in behavior was also observed for the RTD of miscible and immiscible blends. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Numerical analysis of mixing in block‐head mixing screws

The research reported here used 3D non‐Newtonian flow simulations to investigate the pumping and mixing capability of block‐head mixers. Block‐head mixers are distributive mixing screws that are widely used to homogenize the polymer melt and eliminate thermal gradients. The polymer‐processing industry employs a variety of block‐head mixers, with little consensus on design and distribution of screw flights and mixing blocks. This analysis addresses this issue based on a computational design study in which the influence of three geometrical parameters was examined: (1) the number of flights at a mixing block, (2) the number of blocks along the screw, and (3) the stagger angle between the blocks. To examine the flow behavior of the mixing screws, the pressure consumption and energy dissipation is evaluated. Distributive mixing is analyzed using residence time distribution functions, kinematic stretching parameters, and the scale of segregation. Dispersive mixing is assessed by means of the mixing index and the shear stress. The results of this design study increase the understanding of block‐head mixers and contribute to the design and optimization of such geometries. The findings can further be applied to mixing screws of similar geometry, including pin‐type and knob mixers. POLYM. ENG. SCI., 59:E88–E104, 2019. © 2018 The Authors. Polymer Engineering & Science published by Wiley Periodicals, Inc. on behalf of Society of Plastics Engineers.