Single-screw Plasticating Extruder Research Articles

Experimental validation of a quasi three-dimensional conjugate heat transfer model for the metering section of a single-screw plasticating extruder

This paper presents a quasi three-dimensional computer model and its validation with experimental results of the thermal transport processes in the metering section of a single-screw plasticating extruder. A finite volume based approach (SIMPLEC) has been used to obtain flow, thermal and pressure fields of a power-law fluid in the screw channel. The accuracy of the present model has been confirmed by the satisfactory tally of the numerical results with the experimental data for the extruder with fully open exit valve, and also by the excellent matching of the dimensionless exit pressure gradient with the experimental data.

SSEM: a computer model for a polymer single-screw extrusion

A fully predictive computer model SSEM (single-screw extrusion model) has been developed for the single-screw plasticating extruders (with conventional and non-conventional screws of different geometry, and different dies as well). 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 mass flow rate of the polymer, pressure and temperature profiles along the extruder screw channel and in the die, solid bed profile, and power consumption. The simulation parameters are the material and rheological properties of the polymer; the screw, hopper and die geometry; and the extruder operating conditions (screw speed and barrel temperature profile).

Chemomechanical Systems: Study of Contraction and Mechanical Work of Poly(Acrylonitrile) Gel Fibers

A fully predictive computer model SSEM (Single-Screw Extrusion Model) has been developed for the single-screw plasticating extruders (with conventional and mixing screws). The model takes into account five zones of the extruder (hopper, solids conveying, delay zone, melting zone, and melt conveying) and the die, and it describes an operation of the extruder-die system, making it possible to predict the mass flow rate of the polymer, the pressure and temperature profiles along the screw channel and in the die, the solid-bed profile, and power consumption. Moreover, the mixing degree, the temperature fluctuation, and the viscoelastic characteristic of the polymer are evaluated. The model also makes it possible to predict the morphological changes of the polymer blend during the compounding process in the single-screw extruder. 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 a possibility of optimizing the process, for example, from the point of view of the quality of extrusion. The model has been verified experimentally on a 45-mm-diameter single-screw extruder.

A Mathematical Model for Predicting the Dynamic Temperature Behavior of a Single Screw Plasticating Extruder

Article A Mathematical Model for Predicting the Dynamic Temperature Behavior of a Single Screw Plasticating Extruder was published on July 1, 1993 in the journal Journal of Polymer Engineering (volume 12, issue 3).

Mixing on Melting in Single Screw Extrusion

Abstract A new approach to assessing mixing in single screw plasticating extruders is proposed. It is based on image analysis of microtomed sections obtained from the polymer helix formed in a suddenly cooled split barrel extruder fed with a mixture of black and white polymer granules. The observations suggest that most of the mixing occurs on melting with little improvement thereafter. A simple laminar mixing model is drawn in the melting zone the results of which compare favourably with experimental data. In particular, it is shown that improved mixing can be achieved with increasing screw speed in complete contradiction with the classical strain theories which consider mixing in the melt conveying zone only.

Existing Scale-up Rules for Single-screw Plasticating Extruders

Abstract This paper gives a review of existing scale-up rules for single screw plasticating extruders. It is shown that there were and still are rather varied trends. The main three are: – scale-up rules under the boundary conditions of an invariant shear rate, – scale-up rules including the power law, – scale-up rules including the zones of operation. In Europe, at least, the scale-up rules, which take the power law-flow characteristics into account, seem to have been established in practical terms. For economical reasons conventional extruders are designed in such a way, that the output increases with the square of the diameter. This however, results in the problem that the melting rate does not follow the same scale-up rules as the pumping rate and the solid conveying rate. Thus the discussion continues, to calculate the operating section lengths according to varying scale-up rules. Finally the scale-up problem is discussed once more with respect to the dominant dimensionless numbers, and despite several similarity theoretical difficulties a standardized solution will be attempted to be found.

A Computer Model for the Simulation of Conventional and Barrier Screws

Abstract Extrusion is one of the most important operations in the polymer-processing industry. In order to improve their productivity, polymer processors must rely on computer-aided design, engineering, and manufacturing (CAD/CAE/CAM). The performance of an extruder is governed by its capacity to convey, melt, and pump the polymeric material. In the past, a great deal of effort, theoretical and experimental, has been put into the analysis of plasticating single-screw extruders. Despite these developments, the perfect model that could help the practicing polymer engineer to design extruder screws does not exist. A comprehensive model has been developed for steady-state computer simulation of a single-screw plasticating extruder (barrier and conventional screws) in order to predict the performance of the extruder, the optimum screw design, and the operating conditions. The model takes into account the three zones of the extruder: the solids-conveying section, the plasticating section, and the metering section.

A Model for Single-screw Plasticating Extruders

Abstract A mathematical model has been developed to describe the steady-state behavior of single-screw plasticating extruders. The model is capable of predicting the solid bed profile, pressure, and polymer temperature profiles along the screw channel. The model is based on concepts developed by Tadmor and Klein [1], but a modified model for plastication rate is proposed and the pressure profile is calculated using a unidirectional power-law flow model, in which correction factors for flight walls, curvature, and varying channel height have been incorporated. A finite difference technique (Crank-Nichols on method) is used to compute the polymer temperature profile. The overall extruder performance (pressure, temperature and solid bed profiles) is obtained with short computer times on a personal computer (typically one minute on IBM AT). A series of menus and graphics allow for selective simulation and rapid visualization of effects of key parameters, making the model attractive for screw design and optimization of operating conditions. The predictions are in good agreement with experimental data obtained for a polyethylene on a 45 mm diameter screw extruder.

Peroxide crosslinking of LLDPE during reactive extrusion

AbstractAn experimental study of the free‐radical‐induced crosslinking of linear‐low‐density polyethylene (LLDPE) in the melt phase has been carried out. Experiments have been performed both in a single‐screw plasticating extruder and in an extrusion plastometer using a peroxide as the free‐radical source. Concentration of the peroxide was in the range of 0.005–0.08 wt % for rheological investigation and was extended to 5 wt % for gel content examination. Results in the form of melt flow index (MFI) values, viscosity curves, extrudate swell, and gel content of the produced resins are presented here.

Controlled Degradation of Polypropylene: A Comprehensive Experimental and Theoretical Investigation

Abstract Experimental and modeling studies of the free-radical-induced degradation of polypropylene (PP) in the melt phase have been carried out. Experiments have been performed in a single-screw plasticating extruder using a peroxide as the free-radical source. Concentration of the peroxide was in the range 0.01–0.6 wt%. Results in the form of melt flow index (MFI) values, viscosity curves, and molecular weight distribution (MWD) of the produced resins are presented here. Based on these results, a constitutive equation describing the shear viscosity of the melt as a function of shear rate, temperature, and molecular weight has been derived. The extensional viscosity of these resins has been determined as a function of strain rate using Cogswell's analysis of converging flows. A previously developed kinetic model (plug flow) has been used to simulate the changes of the average molecular weights of the MWD, and a sensitivity analysis of this model has been carried out.

Process control of polymer extrusion. Part I: Feedback control

AbstractOn‐line computer control of extrudate thickness was carried out using a 2–1/2 Inch single screw plasticating extruder. Predried poly(methyl methacrylate) (PMMA) was extruded through a slit die. Two feedback control methods, a conventional PI controller and a Smith predictor dead time compensation, were tried for both set point changes (i. e., extrudate thickness changes) and load changes (i. e., screw speed changes). Results showed that both the PI feedback control and the Smith predictor were satisfactory for long term set point changes but not for load changes. Since the Smith predictor may compensate the process dead time, it would be useful for regulating short term set point changes such as barrel temperature settings.

Dynamic behavior of a single screw plasticating extruder part I: Experimental study

AbstractThe dynamic responses of a 2–1/2 inch single screw plasticating extruder and extrusion line were investigated. Step changes in screw speed, take‐up speed, back pressure, and processing materials were used to determine the transient responses of barrel pressures, die pressure, melt temperature, and extrudate thickness. Dynamic responses of the entire extrusion line can be explained by the flow mechanism of the extruder and the logical properties of the polymer used. A capillary rheometer was also used to determine if it could simulate pressure responses in the extruder for screw speed changes. Results showed that capillary rheometer was helpful in estimating the short term pressure responses in the die.

Dynamic behavior of a single screw plasticating extruder part II: Dynamic modeling

AbstractThe dynamic response of a 2.5 inch plasticating extruder and the extrusion line are modeled using high density polyethylene and acrylics us extrudate. Screw speed, back pressure valve position, and material changes are used as forcing functions. Three fundamental transfer functions in the Laplace domain: a first order, a second order, and a lead‐lag, are developed to simulate the short term and long term responses of temperatures, pressures, and extrudate thickness. A kinetic‐elastic model which can predict rheological properties of non‐Newtonian, viscoelastic materials is also applied to the pressure responses of the extrusion process. This model can fit the experimental data well but due to the complexity involved in its parameter setting, more modifications are required before it can be applied for the control of extrusion process.

Mathematical model of a single-screw plasticating extruder

A five zone mathematical model of a plasticating extruder is presented. Its application in the design of new and improvement of existing extruders is briefly described. The model is based on theories proposed by Darnell and Mol, Tadmor, Broyer, McKelvey, Klein, Schneider, Fenner, Poon and Jankov. A comparison between experiments and theoretical calculations is included.

Surging in screw extruders

Output uniformity is one of the main factors limiting the maximum output obtainable from a single screw plasticating extruder, and is adversely affected by surging. Several causes of surging have been identified, perhaps the most important being instabilities in the melting process. These are caused by periodic break-up of the bed of compacted solid polymer formed in the screw channel. Solid bed break-up is shown, both experimentally and theoretically, to be associated with rapid acceleration of the bed in the downstream direction parallel to the screw flight. A novel method of measuring solid bed velocity and hence acceleration is described. The theoretical model of the melting process is shown to be capable of predicting this acceleration reliably, and therefore the tendency for a particular combination of screw design, material and operating conditions to cause surging.

Theoretical analysis of residence time distribution functions and strain distribution functions in plasticating screw extruders

AbstractThe residence time distribution (RTD) function in a single screw plasticating extruder was theoretically calculated. The calculation is based on the solids conveying, melting, and melt conveying models in extruders. The screw channel is divided into small axial increments and the path of each exiting fluid particle is followed from hopper to die. In addition to the residence times the total shear deformation or strain imposed on the fluid particles was also calculated. This together with the RTD function has led to the definition and calculation of the strain distribution function (SDF). This function is proposed for quantitative characterization of the mixing performance of screw extruders as well as other laminar mixers. Some simple idealized batch and continuous laminar mixers are analyzed in terms of the SDF. Finally, the effect of extruder operating conditions and screw design on the RTD and SDF were investigated by computer simulations.