Band 16, Heft 4

A new method for characterization of distributive mixing in processing equipment, based on Renyi entropies, was developed. This method was applied to a twin-flight single screw extruder, in which tracer positions were determined through computer simulations of the flow field. The various entropies were calculated using particle concentrations in equal area domains of the mixer. Renyi entropies, which are function of a parameter β, were calculated for extruders of different lengths. We discuss the merit of using Renyi entropies for different values of β by pointing to the different mixing characteristics they probe. The relative Renyi entropy varies between 0 and 1 and represents a measure of distributive mixing quality, with 1 corresponding to perfect mixing and 0 corresponding to poorest mixing. We compare this new method of distributive mixing characterization to traditional ones based on the concepts of Scale and Intensity of Segregation, and the calculations based on Pairwise Correlations and Correlation Sums. The results show good agreement between the relative Renyi entropy and the traditional methods. Other advantages of the Renyi entropy such as reduced calculation time and geometric independence are discussed. For the case of a twin-flight single screw extruder, it is shown that a longer extruder is not necessarily more beneficial to distributive mixing.

The focus of this investigation was to determine the effect of barrel rotation and screw rotation on the flow rate and viscous dissipation inside the screw pump-extruder. The screw pump-extruder was built to allow for barrel rotation and screw rotation independently. A Newtonian fluid, Polypropylene glycol, was chosen as the working fluid. The experimental results can be summarized by noting that the pumping capacity of the device was independent of which element was rotated at constant angular velocity and that barrel rotation generated higher temperature rise than screw rotation for the two screws evaluated. Also this analysis suggested that a large amount of viscous dissipation inside this extruder arises from the clearance flow. A new analytical model was proposed to evaluate the viscous dissipation for screw rotation. Calculations using this new mathematical model provided good agreement with the experimental data for both barrel and screw rotation.

A pair of related Residence Time Distribution (RTD) curves obtained experimentally during extrusion, are deconvoluted using a methodology based on the concept of equivalent residence times. Two points of two RTD curves are equivalent when the same percentage of tracer has exited the system. The time scale of the deconvoluted curve is obtained by subtracting the two equivalent time values of the available RTD curves. The method was tested using simulated pulse-shaped RTD curves and also carrying out measurements on a twin screw extruder. Despite the experimental errors involved, the two tests seem to demonstrate the usefulness of the approach.

Tightly intermeshing, co-rotating twin-screw extruders are commonly employed for tasks requiring good mixing. The modular structure of both barrel and screw makes it possible to optimize the extruder configuration for a given task. Increasing demands on polymers have resulted in the development of more sophisticated mixing elements. In the framework of this investigation we looked at the pressure-throughput behavior of mixing elements in a conveying, reconveying and conveying-reconveying configuration. Furthermore, the local residence time distribution within those elements was investigated. We developed physical-mathematical models to describe the pressure – throughput behavior observed in the experimental investigations.

The flow of molten polymers in corotating twin screw extruders has been largely studied, but little attention has been paid until now to the melting process. In the present work, we develop an experimental study of melting in twin screw extruders, including pressure and temperature measurements, dead-stop experiments and sampling, observation of cross sections and quantification of solid fraction. The influences of process parameters (screw speed and feed rate), screw profile, pellets size and extruder size are also investigated. An original model of the melting process is proposed, deduced from these observations. This model allows one to calculate the evolution of mean shear rate, pressure gradient, dissipated energy, pellet radius and both solid and liquid temperatures along the screws, for a sequence of right- and left-handed screw elements and blocks of kneading discs. The results of the model are in good agreement with the experiments.

The performance of a single-screw extruder for the processing of cross-linkable low-density polyethylene in cable coating applications is examined. In particular, the aim is to demonstrate that the melt quality is markedly dependent on the melting efficiency. An instrumented 120 mm, 24 : 1 L/D extruder is used to perform the study. So-called pull-out tests are carried out for investigating the melting behavior inside an extruder. At the beginning of the melting zone, a typical Maddock melting mechanism is observed. Thereafter, as melting proceeds, the solid polymer becomes totally surrounded by melt according to the Lindt-Decker melting mechanism, and finally a solid-bed break-up occurs in many circumstances. This undesirable phenomenon can be avoided by screw cooling which, however, has its own drawbacks. In addition to the experimental work, numerical simulations of the melting behavior are conducted. The results provided by the simulation are in accord with the experimental findings.

Chaos is a relatively new subject in mathematical science. It has rapidly, however, found applications in a diverse range of areas in pure science and engineering. This work considers the application of chaos in viscous fluid mixing, fluids that are, for example, found in the food or polymer industry. It has been shown by many authors that under steady state conditions laminar fluids flow in a streamline motion. It has also been shown that it is possible to increase the degree of disorder in such a system by introducing time dependant moving boundaries. It is well established however, that chaotic motion is not dependent on the variation in streamlines alone. The system is required to be operated within a specific range of boundary conditions for chaos to occur. This work presents further investigations of chaotic flow domains concentrating on a property that relates to the divergence and separation of particles originating from small generation zones within the flow domain. The property of divergence of particles is noted to yield conclusions consistent with previously published work in two dimensions. It is then applied to investigate the degree of disorder in three three-dimensional continuous chaotic systems, operating with a range of time and spatially periodic boundary conditions. In order to highlight the benefits of increased levels of mixing when operating the systems under specific conditions characteristic plots, for incremental variations of operating conditions, are presented. It is hoped that by the development of the characteristic plots and concepts presented here that engineers will be able to predict and characterise the degree of disorder in future industrial chaotic mixing equipment.

The properties of blown polyethylene (PE) films depend on various factors, including crystallinity, morphology, and orientation, in addition to chemical composition. It has been shown that the optical properties are strongly influenced by surface morphology. In this work, non-contact atomic force microscopy (AFM) and polarized light microscopy (PLM) were used to visualize surface and bulk morphology. Various techniques, such as surface and line roughness, surface and line fractal dimension, pair-correlation function and nearest neighbor distance distribution function, are employed to quantify the description of morphology and to compare the morphological characteristics of a number of polyethylene (PE) films of commercial interest. A comprehensive quantitative analysis of surface topography has been performed. The co-monomer of the PE resins was found to play a significant role in the formation and the orientation of spherulite-like domains. The film cross-section microstructure has been evaluated qualitatively by using both AFM and PLM. However, quantitative analysis of bulk morphology cannot be obtained due to knife effects.

The levels in biaxial orientations and mechanical properties of stretched extrusion cast polyamide 12 films were investigated with birefringence, wide angle X-ray diffraction (WAXD) pole figure and mechanical testing. Introduction of a pseudo-orthogonal crystalline symmetry enabled the calculation of White-Spruiell biaxial orientation factors using WAXD pole figure data. The orientation was uniaxial or biaxial depending on the stretch ratios along two principal stretching directions that are the machine direction (MD) and the transverse direction (TD). According to the WAXD pole figure patterns, planar alignments of hydrogen bonded layers were formed with respect to the film surface from most of the stretched films. Balanced stretching at higher stretch ratios enhanced the planarity. It was found that the mechanical properties (tensile strength and elongation at break) of the films had a close correlation with the out-of-plane birefringences.

The simulation of injection moulding is essential to meet quality requirements and to increase the efficiency of the design process. In this paper a 3D-finite element model for the simulation of injection moulding is presented. The model's capability comprises the calculation of velocity, pressure and temperature fields as well as the prediction of the flow front position. The numerical approach is mainly based on the Galerkin formulation. Special emphasis is placed on the handling of polymer compressibility in order to enable a realistic simulation of the packing phase. The numerical efficiency has been largely enhanced through the employment of an automatic time step control. In order to demonstrate the code's capabilities and outline advantages and disadvantages over other simulation systems, the calculation results are compared to experimental data and to results obtained from other numerical models.

Flow marks in injection moulding is quite often encountered on different type of polymers. This defects is known as successive and periodic bright and dull bands oriented perpendicular to the flow direction. Their appearances meanly depend on processing and observation conditions. This study was lead on two industrial polypropylene polymers with a simplified polished rectangular mould geometry. We show that this defect appears during the filling stage and comes from a local change of morphological and molecular orientation along the filling direction on the surface. An instability of the flow front due to a limited sliding at the contact combined to the strong changed of molecular orientation before and after the stagnation point are invoked to explain this instability.

This paper presents the development of a hybrid process combining forming of a superplastic zinc-aluminum sheet with injection molding. The product is a plastic part covered with a superplastic zinc-aluminum layer used mainly for electromagnetic interference (EMI) shielding. This paper investigates the moldability of Zn/Al sheet covered plates with various bosses. The effects on the moldability of various parameters, including: melt temperature, injection pressure, sheet thickness, various boss shapes and boss location in the mold, are investigated. It is found that bosses formed with thick Zn/Al sheet and located close to melt entrance exhibit good moldability. Among boss geometries, boss depth is the most critical parameter; a little increase in boss depth sharply lowers the mold-ability. Large angle improves moldability. To help interpret the effects of geometrical parameters, sheet deformation is observed by measuring the deformations in printed-grids and thickness. The base circle of the boss is found to be the most severely deformed region. The thickness reduction ratio in the center of base circle could help predicting moldability.