A mathematical model of the melting of polymeric materials in extruders. Circulation currents and temperature fields in the polymer. Pressure distribution along the length of the extruder screw

Conclusions Circulation currents of the polymer melt in the transverse section of the extruder screw channel, temperature fields in the polymer melt and in the solid plug, and the pressure distribution (pressure gradient) along the length of the melting zone have been investigated.It has been show that under definite conditions of extruder operation, hot regions are form at the bottom of the left channel wall, in which the polymer can be considerably overheated or thermally decomposed.Depending on the extruder output, the pressure along the length of the polymer melting zone can either increase or decrease.Translated from Khimicheskie Volokna, No. 5, pp. 42–44, September–October, 1984.     

A mathematical model of the fusion of polymeric materials in extruders. Effect of polymer physical properties and processing regimes on rate of fusion

Conclusions The effect of polymer physical properties, rate of extruder screw rotation, and preliminary heating of the polymer on its rate of fusion have been studied.It has been shown that in individual sections of the interface a freezing of previously melted polymer takes place.The rate of polymer fusion along the screw channel is not constant, and as the flake moves from the inlet pipe of the extruder to the exit pipe, it initially decreases, then rises, and then drops again.Translated from Khimicheskie Volokna, No. 6, pp. 46–48, November–December, 1984.     

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.     

Investigation of the melting behavior and morphology development of polymer blends in the melting zone of twin‐screw extruders

The melting behavior and the morphology development that runs parallel to it play central roles in the processing of polymer blends. We studied the impact of speed, melt throughput, continuous‐phase viscosity, screw configuration, and disperse‐phase content on the melting behavior and morphology development in the melting zone of a twin‐screw extruder. The polymer blend used incorporated polyamide‐6 (PA6) as its disperse phase and a high‐viscosity or low‐viscosity polypropylene as the matrix phase. The melting behavior of the polymer blend was investigated with press plates. A qualitative assessment was made of the processes, on basis of the optical impression gained from the transilluminated press plates. One key result was that the PA6 granules melted very rapidly in the polypropylene melt. We took samples over the length of the melting section to permit a quantitative assessment of the morphology. The results show a finely dispersed morphology already at the start of the melting section. This did not undergo any essential change as the blend passed through the extruder, and only a limited correlation was evident with the process parameters. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 1986–2002, 2001     

A mathematical model of the melting of polymeric materials in extruders. Effect of various heat sources on the form and size of the solid plug

Conclusions The form and size of the solid polymer plug in melting polymeric materials in plasticizing extruders depend on the source of heat: external, dissipation of mechanical energy, or preliminary warming of the flake.The length of the polymer melting zone is determined by the rate of rotation of the extruder screw.Translated from Khimicheskie Volokna, No. 4, pp. 49–50, July–August, 1984.     

Effect of fluctuations in polymer melt temperature of linear density of yarn

Conclusions The effect of polymer melt temperature fluctuation at the inlet to the metering pump on nonuniformity in yarn linear density has been studied.It has been shown that considerable fluctuation in yarn linear density, caused by change in melt temperature, takes place also in individual working sites.A considerable difference in the temperature of the melt going to individual working sites of machines for spinning technical yarns has been discovered.Fluctuation in the linear density of yarn caused by variation in melt temperature at the inlet to the metering pump may reach 0.3–0.8% in spinning technical yarns, depending on the construction (group or individual) of the spinning machine melting devices.Translated from Khimicheskie Volokna, No. 5, pp. 45–47, September–October, 1984.     

A Computer Model for Single-Screw Plasticating Extrusion

A fully predictive computer model has been developed for a single-screw plasticating extrusion (with conventional screws). The model takes into account five zones of the extruder (hopper, solids conveying, delay zone, melting zone, melt conveying) and the die, and describes an operation of the extruder-die system, making it possible to predict a mass flow rate of the polymer, pressure and temperature profiles along the screw channel and in the die, solid bed profile, and power consumption. Moreover, mixing degree, temperature fluctuation and viscoelastic properties of the polymer are estimated. The simulation parameters are the material and rheological properties of the polymer, the screw, hopper and die geometry, and the operating conditions (screw speed and barrel temperature profile). Such a comprehensive approach to the modeling of extrusion creates the possibility of optimizing the process, for example, from the point of view of the quality of extrusion. The model has been verified experimentally for a low-density polyethylene on a 45 mm diameter single-screw extruder.     

同向双螺杆熔融段螺杆组合对共混物相态变化的影响

研究了双螺杆熔融段不同螺杆组合对模型聚合物共混体系HDPE/PS相态变化的影响。发现错列角为正向30°及正向60°的捏合块具有较好的输送能力,但不利于物料的熔融和混合;90°错列角捏合块对共混物具有很好的熔融和混合效果,其混合性能甚至优于反向30°捏合块,接近反向60°捏合块;及向螺纹元件对共混物相态变化影响显著。     

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     

单螺杆挤出过程中固体粒子的变形、取向与熔融Ⅲ固体粒子熔融过程的理论与计算

采用带有在线显微观察和摄影系统的全程透明视窗单螺杆挤出机为实验装置 ,在对LDPE物料固体粒子的熔融过程进行实验观察和分析的基础上 ,提出了固体粒子熔融的物理模型和数学模型 ,并对有关模型进行了数学计算。研究结果表明 ,螺槽表层粒子熔融过程不发生熔体迁移的假设与实际情况差异较大 ,而存在熔体迁移的模型与实验现象较为接近 ;而螺槽心层粒子的封闭球壳结构模型与实际情况也存在一定的差异。研究结果进一步指出 ,螺槽心层粒子的熔融前期可以认为不发生熔体迁移 ,但在熔融后期将存在较明显的熔体迁移现象 ;对于固体粒子熔融过程的精确计算既要考虑不同模型的差别 ,还要考虑到粒子形状、粒子运动与变形等因素的影响。     

Melting of polymers in narrow annular shear devices

The melting process of a compacted solid bed of polymer forced through a narrow annular shear cell has been investigated. This melting device is used in a continuous ram extruder¹, where melting rates are independent of throughput rates and are governed mainly by the tangential shear rates and heat conducted from surrounding walls. Theoretical results show that increasing shear rates promotes faster melting, but the rate at which melting accelerates slows down significantly at excessive shear rates. This is due to the reduction in polymer melt viscosity as shear rates and temperature increase in the melt film where most of the energy for melting is generated by viscous dissipation. A comparison with melting in a single screw extruder of similar size and capacity indicates that the present device may have superior performance.     

Morphology of polymer blends in the melting section of co‐rotating twin screw extruders

The properties of polymer blends are largely determined by the morphological structure of the polymer combinations that are involved. In terms of extruder design, this means it is necessary to have models available for estimating the development of the morphology over the length of the screws. Since significant morphological changes are observed in the melting section, in particular, is it necessary to analyze not only the plasticizing process for binary material combinations but also the initial formation and further development of the morphology in this section of the extruder. In the framework of this study, experimental investigations were conducted into polypropylene/polyamide 6 (PP/PA6) blends with small components (by weight) of the disperse PA phase. Apart from varying the process conditions of screw speed and throughput, the viscosity ratio was also varied through the use of two different PP grades. The degree of melting and the development of the morphology over the length of the screws were determined for the individual tests. The study of blend morphology in the melting section reveals key findings that must be taken into account for modeling the initial formation and further development of the morphology. It is very clear that, on the second component, which melts at higher temperatures, a kind of melt film removal occurs at the surface of the granules as they melt. The drops of second component in the melting section, which are directly adjacent to components that have not yet fully melted in some cases, have already assumed dimensions (in the μm range) similar to those that are seen at the end of the extrusion process. This means that, in the melting section of the twin‐screw extruder, no volumes become detached from or are worn off the already‐molten granule surfaces. An evaluation of scanning electron micrographs also shows that, in the melting section of co‐rotating twin‐screw extruders, virtually all the degradation mechanisms that can essentially be distinguished, such as quasi‐steady drop breakup, folding, end pinching and decomposition through capillary instabilities, take place in parallel.     

Mathematical model of fusion of polymeric materials in extruders

Conclusions The melting of polycaproamide flake has been studied experimentally on a 32 mm laboratory extruder under various processing regimes. The lengths of the extruder charging and melting zones have been measured, plus the cross-sectional area of the polymer plug along the length of the melting zone.The results of the experimental studies agree well with the calculated data, which indicates adequacy of the theoretical model for a real process.Translated from Khimicheskie Volokna, No. 6, pp. 38–40, November–December, 1985.     

Shear modification of low density polyethylene

A single screw extruder was used to impose different shear histories on a low density polyethylene with broad molecular weight distribution and high molecular weight tail that had very little long chain branching. This polymer exhibits relatively high melt elasticity and the viscoelastic properties of its melt are strongly affected by preshearing. Such changes are accomplished without significant changes in molecular weight distribution or chemical structure. Measured viscous and elastic properties of the melt are different from piston-driven and screw extruder capillary rheometers. Shear modification effects in single screw extruders are enhanced by decreasing melt temperature, increased screw rotation speeds, and higher screw compression ratios. Melt elasticity can be cycled between high and relatively low values, for the particular polymer, by annealing or shearing the polymer melt.     

Application of ultrasound in the determination of fundamental extrusion performance: Residence time distribution measurement

By means of new probe design and rapid data acquisition, we have succeeded in in‐line ultrasonic monitoring of residence time distribution (RTD) at the melting, mixing, and pumping zones as well as at the die exit of a Werner & Pfleiderer 30‐mm twin‐screw extruder by mounting the ultrasonic probes on the extruder barrel over the screw elements and at the die. The experimental systems were LDPE, CaCO₃‐filled LDPE, and a Kraton/LDPE blend. The ultrasonic data at each of the extruder functional zones are presented. The ultrasonic results have been used to evaluate an opical RTD measurement method by using an optical sensor side by side with one ultrasonic probe at the mixing zone of the extruder. The comparison of the ultrasonic and optical results has shown that the presented ultrasonic technique could be a good complement to the optical technique in the monitoring and understanding of RTD during polymer extrusion processes.     

Effect of process variables on melt velocity profiles in extrusion process using single screw plastics extruder

Abstract

Single screw extruders are used to generate a continuous flow of molten polymer in many industrial polymer processes. The melt velocity profile as extruded is important in determining the properties of the final product and influences process related phenomena such as die swell and the onset of sharkskin. The factors that influence the velocity profile would be expected to be the melt temperature (this affecting the viscosity of the melt), the screw and die geometry, and the output rate from the extruder. In the present work a thermocouple mesh sensor coupled with a cooled stainless tube has been used to determine velocity profiles in melts exiting from the screw of a single screw extruder. The results show that the technique can be used successfully to determine velocity profiles in the extrusion process.

It was found that the main influence on the magnitude of the melt velocity was the extruder screw speed. Melt temperature, and hence melt viscosity, were found to have little effect on the velocity profiles measured. The flow in the centre of the duct was retarded slightly owing to the flow across the screw tip and no rotational component of flow was observed. The velocity profiles measured seemed to be reasonably stable, only small changes being observed in the velocity profiles as the melt flowed along a duct of uniform cross-section, although these changes were limited in nature. Die diameter and length had a limited effect on the velocity profiles generated, although the die entry angle did have a significant effect on the shape of the velocity profile at higher screw speeds.     

Effect of screw axial vibration on polymer melting process in single‐screw extruders

A model for investigating the melting process of polymer in a vibration‐induced single‐screw (VISS) extruder is presented. The key feature of this model is as follows: vibration force field is introduced into the overall course of extrusion by the axial vibration of the screw, and the velocity distribution in the polymer melt behaves strongly nonlinear and time‐dependent. To analyze this model, half‐open barrel visible experimental method and low‐density polyethylene material are adopted to investigate the effect of the vibration parameters on the melting process, which goes into further details of study and research on the melting mechanism, and thus, a novel physical melting model is derived. Combining the conservation equations of mass, movement, energy, and constitutive, analytical expressions of the melting rate, the energy consumption, the length of melting section, and the distribution of solid bed are obtained. This model enables the prediction of the processing and design parameters in the VISS extruders from which the optimum conditions for designing VISS extruder and polymer processing are obtained. The theory is supplemented by a calculation sample and experiment, which shows that the introduction of vibration force field can improve the melting capacity and decrease the power consumption of extruder greatly. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 3860–3876, 2006     

Nonisothermal flow of polymer melts and solutions in spinneret channels

Conclusions The effect of dissipation of mechanical energy on the temperature fields of polymer melts and solutions in the channels of spinnerets for fibre spinning has been investigated.It has been shown that the flow of polymer melts in spinneret channels takes place with only slight heat evolution, that is, practically under isothermal conditions. The flow of polymer solutions takes place significantly under nonisothermal conditions, with a large drop in temperature over the channel radius.Translated from Khimicheskie Volokna, No. 4, pp. 42–44, July–August, 1986.     

Simulation of nonisothermal flow of melt during melting process of vibration‐induced polymer extruder

A simplified 2D melt film model was established to simulate the nonisothermal melt flow during the melting process of the vibration‐induced polymer extruder of which the screw can vibrate axially. Since polymer has time‐dependent nonlinear viscoelastic characteristic with vibration force filed (VFF), a self‐amended nonisothermal Maxwell constitutive equation that can reflect the relaxation time spectrum of polymer was adopted. Using the 2D melt film model, melt films of two kinds of thickness representing different melting stages were simulated to investigate the influence tendency of the same VFF on the different melting stage. Special flow patterns and temperature distribution of melt in the melt film between the driving wall and the solid/melt interface with various vibration force fields were systematically simulated. It is found out that within a certain range of vibration strength, the application of vibration can optimize the time‐averaged shear‐rate distribution, improve the utilization efficiency of energy, and promote melting process; and the thinner the melt film is, the more intense the nonlinear viscoelastic response becomes with the same VFF; moreover, there exists optimum vibration strength to make the melting process fastest, which is in accord with the visualization experimental results. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 5825–5840, 2006     

Effect of moisture in polycaproamide flake and of types of melting devices on polymer properties after repeated melting

Conclusions A calculation of the equilibrium moisture content of polycaproamide flake has been given, with allowance for specific conditions of synthesis and processing on spinning machines.In processing flake with an equilibrium moisture content on machines of the extruder type, a lack of change in polymer molecular weight after repeated melting and a high chemical uniformity are assured.Translated from Khimicheskie Volokna, No. 3, pp. 44–46, May–June, 1986.