Chaotic mixing in a free-helix extruder using a new solution to the biharmonic equation

A recently published approach for modeling the cross flow in an extruder channel using a new solution to the biharmonic equation is utilized in a study of chaotic mixing in a free-helix single-screw extruder. This novel extruder was designed and constructed with the screw flight, also referred to as the helix, detached from the screw core. The flight-helix had straight sides that more closely emulated rectangular channel theory than the nominal sloped sides of a conventional single screw channel. Each of the screw elements could be rotated independently to obtain chaotic motion in the screw channel. Using the new extruder, experimental evidence for the increased mixing of a dye, for both a Dirac and droplet input, with a chaotic flow field relative to the traditional residence time distribution is presented. These experimental results are compared using the new biharmonic equation-based model. Comparing the experimental chaotic mixing with theoretical calculations was facilitated by a recently published technique for accurately placing the dye in the extruder channel. Because of the ability to periodically rotate only the flight/helix, the chaotic mixing results are minimally confounded by the existence of Moffatt eddies.     

Residence time distribution in a real single screw extruder

The residence time distribution in an industrial single screw extruder was investigated experimentally in the case of melt and plasticating extrusion. The investigations performed proved that the extrusion parameters influence strongly the residence time distribution in the extruder. It was found that the resistance to flow through the die-head of the extruder is very important from this point of view, as well as other parameters like rotational speed of the screw and the screw channel depth. Variation of these parameters can change the residence time distribution over a broad range between the extreme idealized cases of plug flow and flow with perfect mixing. In order to obtain quantitative dependences three moduli were used and a correlation equation was obtained. This equation enables an estimation of residence time distribution on the basis of experimental characteristics of the extruder and the actual extrusion parameters.     

Characteristic analysis on a reactive extrusion process for the imidization of poly(styrene-co-maleic anhydride) with aniline

The imidization of poly(styrene-co-maleic anhydride) (SMA) with aniline by reactive extrusion is investigated in a co-rotating twin-screw extruder. During this reactive extrusion, the process temperature is much higher than the boiling point of the aniline. Accordingly, most of the aniline should be vaporized immediately after being fed into the extruder, occupies the unfilled part of the extruder, and is transferred to the melt phase where it is consumed through the reaction. Based on the mechanism of this vapor-melt heterogeneous process and experimental data for residence time distribution (RTD) in the extruder, a continuous process model is developed. The effects of operating conditions including temperature, throughput, and screw rotation speed on the reaction kinetics are discussed by both experimental data and model simulation. The results indicate that the residence time and the mass transport of aniline from vapor to melt phase should play significant roles in this heterogeneous reactive extrusion process.     

Theoretical model for intermeshing twin screw extruders: Axial velocity profile for shallow channels

Positive displacement intermeshing twin screw extruders have been analyzed by a simple model for flow in the channel formed by the screw root and screw flights. The model considers the down channel flow to be a combination of drage flow resulting from the relative motion of the barrel and screw and pressure flow resulting from the positive displacement action of the device. The pressure flow in this situation is distinguished from pressure flow in a single screw extruder in that the pressure forces induce flow toward the die for the twin screw model rather than away from the die as in a single screw extruder. Comparison of the down channel shear rate profile of apositive displacement twin screw extruder with that of a single screw extruder with no net flow reveals that they are identical but inverted with respect to channel depth. The model presented does not consider leakage between the twin screws or the rotational motion of the second screw.     

Measurements of residence time distribution for the peroxide degradation of polypropylene in a single-screw plasticating extruder

Measurements of the residence time distribution (RTD) in a single-screw plasticating extruder were carried out during experimental studies of the peroxide-initiated controlled chemical degradation of polypropylene (PP). A radioactive tracer method was employed, and the effect of screw speed, temperature, and reaction on the RTD was examined. An increase of the peroxide concentration resulted in a broader distribution whereas an increase of the extrusion temperature was found to result in a narrower distribution. Use of low screw speeds simply increased the time delay through the extruder without affecting considerably the breadth of distribution. Results obtained from the present experiments were compared with several theoretical models.     

Numerical simulation of fluid flow and heat transfer in a single-screw extruder with different dies

In a plasticating screw extruder, a polymer melt forms in the melting zone of the extruder. Pressurization of the molten polymer takes place in the melting and the metering sections so that the melt can flow through the restricted passage of the die and assume a desired shape. In a melt fed extruder, the throughput is governed by the pressure rise over the entire length of the extruder. The pressure developed in the screw channel may also be employed in rapid filling of molds, such as those in injection molding. When the geometry of the screw, the barrel temperature, and the die are selected, a unique set of operating parameters arise for a particular flow rate or screw speed. In the present study, numerical and analytical methods are used to calculate the transport in the extruder and the pressure drop in the die. An iterative numerical method based on solving the equations of motion and energy in the screw channel and a correction scheme to couple the die with the screw channel is discussed. The numerical algorithm is capable of handling an arbitrary variation of the viscosity of the polymeric fluid with the shear rate and temperature. The results obtained by simulating the fluid flow in the screw channel are compared with available numerical and experimental results in the literature, indicating good agreement. The performance characteristics of the extruder, for chosen thermal boundary conditions and screw geometry, are presented for different die geometries and different fluids. The important considerations that arise in the numerical simulation of the extrusion process are also discussed.     

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.     

新型偏心三螺杆挤出机流体混合特性分析

在传统的三螺杆挤出机的基础上,设计了一种新型偏心三螺杆挤出机。该三螺杆挤出机中具有螺杆几何结构偏心、螺槽构型呈梯度变化特殊以及较高的面积利用系数等特征。利用有限元法对聚丙烯（PP）熔体在新型偏心三螺杆挤出机中流动和混合规律进行三维数值模拟,给出偏心三螺杆挤出机中压力和速度分布规律,计算了3种偏心螺杆挤出流场的停留时间分布、分布指数、分离尺度、最大剪切应力等混合表征参数。结果表明,螺杆偏心距不仅决定了螺杆端面形状,也改变了螺槽梯度的变化程度。随着螺槽梯度的逐渐减小,挤出机内粒子团聚效应逐渐降低、物料剪切作用逐渐增强。在3种新型偏心三螺杆挤出机中,偏心距e=3 mm的新型偏心螺杆挤出机的混合性能相对较好。     

Non-Newtonian and isothermal analysis of flow of poly(methyl methacrylate) in a model twin screw extruder. Part I

The flow analysis network (FAN) method was modified to analyze the flow of poly(methyl methacrylate) (PMMA) in a model counter-rotating nonintermeshing screw extruder. The numerical prediction of the pressure profiles was compared with the experimental results. Flow patterns in the screw elements of the model counter-rotating nonintermeshing twin screw extruder were also predicted. A new flow path method was developed to calculate the residence time distribution. This result will be applied to analyze the flow during the reaction in the model twin screw extruder.     

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.     

A new approach to analyzing residence time and mixing in a co-rotating twin screw extruder

A series of experiments was conducted to determine what correlations exist between an experimental parameter, percent drag flow, and other parameters such as head, tail and mean residence time. Experimentation was carried out on two polymer systems, a model system of near-Newtonian fluid and a viscoelastic system of polyisoprene with several additives. To aid in the residence time analysis, data from three literature sources were cited and replotted. A family of residence time curves for a partially filled system can be combined into one curve by plotting the number of screw revolutions carrying the tracer to the extruder exit versus the percent drag flow. This method of plotting the data for each screw configuration estimates the mean residence time for any throughput and screw speed once a few data points are taken. In all four sets of experiments, the number of screw revolutions carrying the tracer to the exit decreases with increasing percent drag flow. The filled volume of the extruder was calculated from residence time data to show that percent drag flow is linearly related to extruder filled volume. When percent drag flow increased in the viscoelastic system the following results were recorded: fraction of polymer residence time spent in conveying elements increased, fraction of residence time spent in mixing elements decreased, polymer Mooney viscosity increased, number and weight average molecular weights increased and polydispersivity increased.     

Analysis and experimental evaluation of twin screw extruders

Twin screw extruders can he classified according to their geometrical configuration. The main distinction is made between intermeshing and nonintermeshing extruders. Another distinguishing characteristic is the sense of rotation. The most important characteristics of the various twin screw extruders are examined, with particular emphasis on the effect of screw geometry on the conveying characteristics. A brief review is given of the state of the art in theoretical analysis of twin screw extruders. Experiments with two lab scale, intermeshing twin screw extruders are described, one co- and one counterrotating. Results are presented on power consumption, residence time distribution, and mixing characteristics of the two extruders. The counterrotating extruder exhibits a narrower residence time distribution and better dispersive mixing capability. The corotating extruder showed a better distributive mixing capability. These results can be explained in terms of the conveying and mixing mechanisms in both extruders. The overall extruder performance seems to be dominated by the effect of the intenneshing region. Any realistic, theoretical analysis of twin screw extruders should be centered around the flow behavior and mixing characteristics of the intermeshing region. The corotating extruder appears to be best suited for melt blending operations, while the counterrotating extruder seems to be preferred in operations where solid fillers have to be dispersed in a polymer matrix.     

反应挤出聚酰胺6／蒙脱土纳米复合材料的双螺杆螺杆元件组合设计

研究了已内酰胺单体通过反应型双螺杆反应挤出的方法合成聚酰胺6／蒙脱土纳米复合材料。讨论了反应挤出双螺杆螺杆元件组合设计，螺杆组合与停留时间的关系，以及挤出段螺槽充满程度与排气、冒料的关系。     

双螺杆挤出丁苯透明抗冲树脂溶液脱挥工艺

将质量分数为20．00％的丁苯透明抗冲树脂溶液经浓缩器提浓至质量分数不小于30．00％后,送入同向啮合式双螺杆挤出机,在多段真空状态下脱除溶剂,研究了双螺杆挤出脱挥工艺中的温度、真空度、停留时间和螺杆转速工艺务件,并与湿法凝聚工艺进行了产品分子参数和能耗对比。结果表明,双螺杆挤出脱除溶剂工艺适用于丁苯透明抗冲树脂溶液脱除溶剂,其能耗远低于湿法凝聚,所得产品的性能与国外同类产品基本相当。     

Experimental investigation of the drag flow assumption in extruder analysis

A new screw pump or extruder has been constructed with the goal of testing the drag flow analysis currently used in the evaluation of polymeric extruders. The new device has been constructed in a manner that allows the barrel, screw helix, and screw core to be rotated independently or in pairs. The experimental results of the flow produced for rotation of the barrel agree well with the theory. However, the flow produced by rotation of the screw elements does not agree with the predictions of the currently accepted theory. The experimental results indicate that the primary element that produced fluid propulsion when the screw is rotated is the screw helix. A new qualitative theory is proposed that is in agreement with the data.     

Infrared melt temperature measurement of single screw extrusion

An infrared temperature sensor has been used to provide real time quantification of the thermal homogeneity of polymer extrusion. The non‐intrusive sensor was located in the barrel of a single screw extruder, positioned such that it provided a measurement of melt temperature in the channel of the metering section of the extruder screw. The rapid response of the technique enabled melt temperature within the extruder screw channel to be monitored in real time, allowing quantification of the thermal stability of the extrusion process. Two polyethylenes were used in experiments with three extruder screw geometries at a range of screw speeds. Data generated by the infrared sensor was found to be highly sensitive to thermal fluctuations relating to the melting performance of the extruder screw. Comparisons made with an intrusive thermocouple grid sensor located in the extruder die suggested that the infrared technique was able to provide a similar level of information without disturbing the process flow. This application on infrared thermometry could prove highly useful for industrial extrusion process monitoring and optimization. POLYM. ENG. SCI., 55:1059–1066, 2015. © 2014 The Authors. Polymer Engineering & Science published by Wiley Periodicals, Inc. on behalf of Society of Plastics Engineers     

Numerical analysis of a reactive extrusion process. Part II: Simulations and verifications for the twin screw extrusion

A newly designed extruder reactor for grafting vinyl monomers onto polyolefins was studied experimentally and theoretically. The process was made up of a self‐wiping co‐rotating twin screw extruder with a separated reaction zone and two vent zones. The reactive extrusion was performed using a linear low density polyethylene, vinyltrimethoxysilane and di‐t‐butylperoxide under different operation conditions. For the purpose of process analysis, we built a computer simulation based on the reaction kinetics and rheological models studied in the preceding paper. The flow field in the extruder was calculated by the flow analysis network (FAN) method with non‐isothermal non‐Newtonian flow conditions. The iterative procedure was organized to predict local pressure, filling factor, cumulative residence time and temperature along the extruder. Furthermore, we succeeded in representing the profiles of reaction conversion and shear viscosity. Calculated results showed good agreements with the experimental data.     

Behavior of fully filled regions in a non‐intermeshing twin‐screw extruder

Twin‐screw extruders are operated with sequential filled and partially filled regions in order to perform the required unit processes. Channel fill length, defined as the length of fully filled regions in an extrusion screw, is gaining importance as a design parameter because of its implications on residence time distribution, distributive and dispersive mixing, and also process stability. A detailed study—experimental and theoretical—of the behavior of fill lengths in response to operating conditions (throughput, screw speed) and screw geometry is presented in this paper. Mean residence times were also measured for each geometry and operating condition. The apparatus consisted of a non‐intermeshing counter‐rotating twin‐screw extruder (NITSE) with a transparent (acrylic) barrel, fed with corn syrup (Newtonian at room temperature). Fill length exhibits a nonlinear relationship with specific throughput (Q/N), with the slope increasing monotonously as the throughput Q increases at a given screw speed N. The mean residence time exhibits a strong linear relationship with inverse specific throughput and inverse fill length. A theoretical model was developed to predict the filled length based on pressure‐throughput relationships taken from literature for this system, and the predictions were found to agree very well with experimental observations.     

橡胶在销钉机筒挤出机中的流动分析

建立了销钉机筒挤出机的非牛顿流体流动模型,并计算出压力场、流动场和螺杆的特性曲线,结果表明,随着流体非牛顿特性的增强,螺杆的生产能力降低;随着螺杆螺槽上切口的增多,逆流增多,挤出产量降低,机筒上的销钉对挤出特性影响很小,销钉式挤出机的挤出特性与螺杆上有切口的挤出机的挤出特性相似。     

Modelling the solids inflow and solids conveying of single-screw extruders using the discrete element method

A new solids-conveying model for the single-screw extruder based on the Discrete Element Method (DEM) is proposed in this work. The polymer solids are treated as spherical particles moving in a 3-D environment which includes the feed hopper, the solids-inflow zone, and the solids-conveying region of an extruder, without inclusion of the plug flow assumption common to continuum models. Normal and tangential forces resulting from inelastic collisions with neighboring particles and surfaces dictate how each polymer pellet is conveyed through the model extruder. The DEM technique was implemented in this work to allow fundamental study of the local transport phenomena within the screw channel. Reported in this paper are results examining the cross- and down-channel velocity profile of solids in the screw; the residence time distribution; the cross-channel temperature profile; and the coordination number distribution. Two exit conditions were evaluated by the model: i) the open-discharge case where no compaction of the solids occurred; and ii) the restricted case where the axial pressure increased as the solids flowed towards the barrel exit. The predictions of the DEM simulations allowed for detailed observations of the solids movement in the screw, providing insight into the inherent flow fluctuations of extrusion systems.