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.     

The numerical simulation of boger fluids: A viscometric approximation approach

A general-purpose finite element program has been used to simulate the flow of nonshear-thinning, highly elastic polymer solutions (Boger fluids). In particular, the creeping flow through an abrupt 4:1 circular and planar contraction is studied, as well as the flow at the exit of a capillary die for the determination of extrudate swell. Experimentally measured normal stress and viscosity data are included in a simple rheological model, based on the viscometric simplification of the CEF constitutive equation. Vortex size and intensity in the die entry and extrudate swell at the die exit increase rapidly, with elasticity level, in general agreement with experimental findings. It is shown that despite the limitations of the model, the viscometric approximation can be used to study the effect of normal stresses in cases where a main flow direction can unambiguously be defined. In die exit Flows, it can also provide an upper limit for the determination of extrudate swell, while Tanner's theory of elastic recovery provides the lower limit.     

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.     

Effect of molecular weight distribution on the rheological and mechanical properties of polypropylene

The effect of molecular weight distribution on the Theological and mechanical properties of a series of polypropylenes is evaluated. The polypropylenes tested were produced by controlled chemical degradation in a single-screw plasticating extruder. Measured properties include shear, extensional and intrinsic viscosity, melt flow index, extrudate swell, melting and crystallization temperatures, impact strength, flexural modulus, and tensile stress.     

Plastification or melting: A critical process for free radical grafting in screw extruders

This paper shows for the first time that when a monomer is to be grafted onto a polymer backbone by a free radical mechanism in a twin screw extruder, the grafting process occurs mainly, if not exclusively, in the plastification (melting) zone. For this purpose, the free radical grafting of glycidyl methacrylate (GMA) onto polypropylene (PP) and polyethylene (PE) was chosen as model systems. A co-rotating self-wiping twin screw extruder of type Werner Pfleiderer ZSK-30 (L = 42D) was used to process the grafting. Owing to its modular character in terms of barrel arrangement, screw element combination and barrel temperature, the position and length of the plastification zone can be adjusted virtually at will. This allowed us to follow up the grafting not only at the die exit, but also in the plastification zone under different grafting conditions. Our results clearly show that it is in the plastification (melting) zone that the entire grafting process occurs. This length is usually very short in a co-rotating twin screw extruder like ours. Under the grafting conditions, it varied from 1D to 5D. Thus, any relevant analysis or model of a free radical grafting process carried out in a screw extruder must be based on detailed information generated not only at the die exit, but also and most importantly in the plastification zone. Otherwise, it may lead to incomplete and/or wrong conclusions.     

Effects of plasticizer on shear modification of polystyrene

The elastic and viscous properties of polymer melts may be affected by the shear history of the polymer. The extrudate swell of a polymer melt is primarily a manifestation of the elasticity of the polymer melt. In this study, a single screw extruder was used to impose different shear histories on a polystyrene polymer which was processed with and without added plasticizer. The extrudate swell and apparent viscosity of these melts were measured with a capillary rheometer. These characteristics of unplasticized polystyrene are almost not affected by the various preshearing processes. However, the extrudate swell and viscosity of polystyrene containing plasticizer are affected by plasticizer level, shear history and thermal history. After most of the plasticizer in the presheared plasticized polystyrene was extracted, the extrudate swell was still lower than that of the parent sheared polystyrene with the same shear history and the same plasticizer content. These results were obtained without significant changes in molecular weight. Shear modification by conventional process equipment may become impractical if the shear field intensity or dwell time of the material in the apparatus is limited. In such cases, shear refinability by standard process equipment may be observed if the coupling density in the polymer is reduced by some additional means, such as blending with a plasticizer.     

Numerical simulations of annular extrudate swell of polymer melts

Numerical viscoelastic simulations were carried out using a K-BKZ type of separable integral constitutive equation. Both reversible and irreversible models were tried for several types of damping functions to calculate the annular extrudate behavior of high-density polyethylene (HDPE). There are two aims in this study; first, to clarify the properties of these dumping functions, and second, to investigate the influence of rheological characteristics on annular extrudate swell. In these numerical simulations, relaxation spectrum and shear viscosity were fixed, and the other characteristics were varied. The reversional response of the damping function mainly has an effect on the magnitude of the area swell even if the die is straight. The irreversible model expresses the experimental results of annular extrudate swell better than the reversible model. The accurate fitting of N1 by the damping model is important for predicting it. The magnitude of N1 predicted from the Wagner exponential model is lower than that of the PSM model, and the area swell shows the same tendency as N1. A modified PSM model that allows the N1 curve to shift can fit the magnitude of area swell. The relationship between the diameter and thickness of the extrudate depends on N2/N1, and it was estimated by simple linear elasticity of solids. The time dependent viscosity varies with the type of damping function, and it influences the time-dependent swell.     

Melting mechanism of a starved‐fed single‐screw extruder for calcium carbonate filled polyethylene

A study of starved‐fed single screw extrusion was initiated to understand the relation between its distinctive melting mechanism and the improved mixing capabilities attained during compounding of a calcium carbonate filler into HDPE. Experiments were carried out in a 63.5 mm single screw extruder, examining the effect of degree of starvation on a conventional and barrier feed screw. Interest was focused on the mixing/melting mechanism of starved‐fed solids‐conveying as it affects the size and number of filler agglomerates observed in the extrudate. The melting performance of both feed screws was examined using pressure and temperature measurements down the screw length as well as direct inspection of the polymer in the screw channel via rapid screw cooling. Both screws showed improved mixing quality with increased starvation.     

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。     

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.     

Modeling contraction flows and swell of polymer melts extruded from dies of various lengths

This article describes isothermal contraction flows and extrudate swell, using our recent model for fast (high Deborah number) contraction flows of polymers melts. The model analyzes the polymer flow in several regions as one continuous process. This approach makes possible to evaluate the extrudate swell as a result of complex contraction polymer flow in dies of various length. Using the asymptotic matching conditions for the change in flow type at the die exit allowed us to calculate the swelling profile for extrudate in the flow direction. The present calculations performed using a multi-mode viscoelastic constitutive equation of differential type, are compared with the experimental/direct numerical data including basic rheological tests. The presented complex model for contraction flow and swelling consists of no fitting parameter and is applicable for calculations using any viscoelastic constitutive equation. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Production of controlled-rheology polypropylene resins by peroxide promoted degradation during extrusion

Results of experimental and modeling studies of the peroxide promoted degradation of polypropylene (PP) are presented. Experiments were carried out, in glass ampoules and in a plasticating extruder. The initiator, 2.5-dimethyl-2,5-bis(tert-butylperoxy)hexane was used as a radical generator. The extruder used had a 38 mm diameter and 24:1 L/D single-screw. In these experiments, the effect of peroxide concentration and screw speed on the molecular weight distribution (MWD) of the polypropylene resin was studied. Samples collected from the experimental runs were analyzed for melt flow index (MFI), flow curve, extrudate swell, and MWD. The measured data are presented and correlations among various parameters are considered. Generally, it can be concluded that controlled-rheology (CR) resins with lower molecular weight, narrower MWD, and reduced viscosity and elasticity, can be produced, A kinetic model for the peroxide Initiated degradation of PP is proposed. Simulations based on this model are compared with experimental data for the production of CR resins. The experimental data were obtained from three sources: (i) industry, (ii) literature, and (iii) present experimental work. The comparison was done in terms of average molecular weights of the resin. Agreement between model predictions and experimental results is quite satisfactory suggesting that this model should find use in commercial practice.     

Effects of die temperature on extrudate swell in screw extrusion

Extrusion of a hot polymer melt through a cooler die zone substantially increases the extrudate swell of some thermoplastics. This effect was examined for commercial samples of low-density polyethylene, polypropylene, and polystyrene. Two conflicting effects come into play during extrusion of a thermoplastic. Colder melt temperatures promote increased extrudate swell, but the same conditions also facilitate molecular disentanglement and reduced melt elasticity and die swell. Since the extrusion process itself may affect the relation between die swell and melt temperature, laboratory-scale measurements for the design of processes like blow molding are better carried out with small-scale screw extruders than with capillary rheometers. For some applications it may be advantageous to use a polymer whose die swell is particularly responsive or unresponsive to die temperature variations. The procedure described in this article can be used effectively to monitor this characteristic.     

Predicting the effect of screw wear on the performance of plasticating extruders

The effect of screw wear on the performance of a 2.5 in. diameter extruder is studied with the aid of computer simulations. The effect of progressively increasing flight clearance on the extrusion of low density polyethylene, polypropylene and nylon 6/6 is presented. The remedial effect of increased screw speed and its side effects on melting behavior, solids content, extrudate temperature and power consumption are also described.     

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.     

Extrudate roughness of high-density polyethylene and linear low-density polyethylene: Determination of key parameters using screening design

A screening design is used to establish the contribution of various parameters to the roughness of cylindrical extrudates. A dimensionless response variable is proposed to characterize the extrudate roughness, and the effects of ten parameters on this roughness response were examined. Two polyethylenes (one high density and one linear low density) were investigated using a 45 mm single screw extruder. The results show that the main parameters affecting the extrudate roughness are, in order of importance, apparent shear stress at the die wall, die diameter, ratio of die length to diameter, and type of polymer. The other six parameters (the use of an additive, recycling, type of entrance adapter, die material, die temperature, melt temperature) were found to have a non-significant contribution to roughness.     

Study of poly(ethylene terephthalate)/polypropylene microfibrillar composites. I. Morphological development in melt extrusion

Poly(ethylene terephthalate)/polypropylene (PET/PP) blends of different compositions were extruded through a 2‐mm capillary die using a corotating twin‐screw extruder. The extrudates were cryogenically fractured and examined using scanning electron microscopy. The viscosity ratio of the constituent polymers alone was found to be unsuitable for explaining the polymer blend morphology. At a PET concentration of 20%, the extrudate consists of three regions. The skin layer, which is about 10 μm thick, has a lower concentration of the dispersed PET phase than the overall concentration. The intermediate region, which is about 400 μm thick, has profuse PET fibers and some small PET particles. The central region, which is approximately 800 μm in diameter, mainly contains PET particles that are generally bigger. A low barrel temperature, low die temperature, and fast cooling rate helped to retain the fibers near the extrudate skin. Meanwhile, the variation of the barrel temperature, die temperature, and cooling media did not produce a significant affect on the PET particle size distribution in the central region of the extrudate. A high screw speed and a high postextrusion drawing speed were very effective in producing fibers in the extrudates through elongation of particles. At a PET concentration of 30%, coalescence of the PET phase was prevalent, leading to the formation of PET platelets near the extrudate skin and irregular PET networks in the central region of the extrudate. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 1743–1752, 2003     

Study on PET/PP microfibrillar composites. I. Morphological development in melt extrusion

Poly(ethylene terephthalate)/polypropylene (PET/PP) blends of different compositions were extruded through a 2‐mm capillary die using a corotating twin‐screw extruder. The extrudates were cryogenically fractured and examined using scanning electron microscopy. The viscosity ratio of the constituent polymers alone was found not suitable for explaining the polymer blend morphology. At a PET concentration of 20%, the extrudate consists of three regions: The skin layer, about 10 μm thick, has a lower concentration of the dispersed PET phase than that of the overall concentration. The intermediate region, about 400 μm thick, has profuse PET fibers and some small PET particles. The central region, approximately 800 μm in diameter, contains mainly PET particles that are generally bigger. A low barrel temperature, low die temperature, and fast cooling rate helped to retain the fibers near the extrudate skin. Meanwhile, variation of the barrel temperature, die temperature, and cooling media did not affect the PET particle‐size distribution significantly in the central region of the extrudate. A high screw speed and a high postextrusion drawing speed were very effective in producing fibers in the extrudates through elongation of the particles. At a PET concentration of 30%, coalescence of the PET phase was prevalent, leading to the formation of PET platelets near the extrudate skin and irregular PET networks in the central region of the extrudate. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 3100–3109, 2003     

管壁厚度对微挤出成型的影响分析

基于Bird-Carreau黏度模型,运用有限元方法对三维等温微管挤出成型流动模型进行了数值分析,主要研究了管壁厚度对微管挤出成型过程中挤出胀大、速度分布、剪切速率和口模压降等重要指标的影响。结果表明,当熔体入口体积流率相等时,随着管壁厚度的增大,挤出物挤出胀大率和横截面尺寸变化量增大;口模出口端面上熔体的二次流动增强,但挤出速度和剪切速率减小;熔体在口模内的压力降明显下降;适当增加管壁厚度,有利于提高微管挤出质量。     

Experimental study on extrudate swell and die geometry of profile extrusion

The objective of this study was to determine the die swell behavior of a polymer melt and to design a die for forming a polymeric extrudate with a desired shape using profile extrusion. Polystyrene pellets were chosen to perform the profile extrusion experiments. First, the polystyrene pellets were melted and pushed through a quarter ring profile. The profile of the swelled extrudate agreed with the numerical predictions. A modified die was designed to produce a quarter ring profile extrudate based on the direct extrusion problem (DEP) prediction. Polystyrene pellets were also melted and pushed through the modified die. The experimental results were close to the computational results. The melting temperature, die length, and melting residence time affect die swell behavior. The die swell ratio becomes smaller as the melting temperature and melting residence time are increased. As the die length is increased, the die swell ratio is lowered. According to the die geometry predictions, an extrudate with the desired profile can be made precisely.