Screw Channel Research Articles

Effect of the axial vibration of screw on residence time distribution in single‐screw extruders

AbstractAn analytic model has been developed to analyze the residence time distribution of melt in the screw channel of the melt conveying section in a novel extruder of which the screw can vibrate axially. A comparison of the residence time distribution of melt in screw channel with and without vibration shows that the residence time of melt increases with the apply of vibration and the larger the vibration frequency and amplitude are, the longer time it will take the melt to travel through the melt conveying section, which is in favor of the improvement of the effect of melt mixing. POLYM. ENG. SCI. 46:198–204, 2006. © 2005 Society of Plastics Engineers

Two Dimensional Description of Pressure-Throughput Behaviour of Newtonian Materials Considering Wall Slippage Effects

Abstract The description of pressure-throughput behaviour of wall adhering melts in plastifying machines has been issued in a number of publications, e. g. [1 to 4] in details. Nevertheless there are a variety of polymer melts as well as elastomers, polymer suspensions, ceramic materials and food stuff, which do not behave wall adhering in the course of manufacturing. The theoretical discussion of the pressure-throughput behaviour has not sufficiently been described up to now. Worth's [5] case studies include the isothermal one-dimensional flow of a Newtonian medium in an unwinded screw channel. He assumed that after exceeding a critical shear rate the slipping process starts on the barrel wall only – but not on channel wall and screw root surface. Mennig, [6 to 9] took up this statement again and assuming a shear rate on the barrel wall as well as on the screw root surface he derived simple relations for the radial velocity profile from it. Lawal and Kalyon [10 to 12] have developed an analytical model that describes the behaviour of visco-plastic liquids in flat channels for different quotients of wall slippage, which are derived from the wall slippage quotients on screw root surface and on barrel wall. A mathematical model describing the flow behaviour for the two-dimensional Newtonian, isothermal case has been developed for wall slipping materials in order to describe the pressure-throughput behaviour multi-dimensionally. In addition to the development of the analytical calculating model the flow behaviour of wall slipping polymer melts has been analysed by using the finite-element-calculation (FEM). Comparison of the pressure-throughput behaviour results show a high degree of conformity in the calculation results.

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.

In-line monitoring and analysis of polymer melting behavior in an intermeshing counter-rotating twin-screw extruder by ultrasound waves

An ultrasound in-line monitoring system that includes a homemade probe and a fast data acquisition system was used to investigate the melting behavior of linear low density polyethylene (LLDPE) in pellet form and four polyvinyl chloride (PVC) compounds in powder form in an intermeshing counter-rotating twin-screw extruder. Ultrasound signal patterns obtained from experiments revealed various melting phenomena in C-chambers, depending on materials, processing conditions, and screw configurations. The experimental results suggest that PVC particles were suspended in the polymer melt in the melting process of most PVC compounds, while melted film was still observed in the melting process of the PVC/dimethyl phthalate (DMP) system and the PVC/polybutylene adipate (PBA) system. Based on the analysis of wave attenuation, the normalized amplitude ratio K was used to characterize the melting level and uniformity across screw channels at screw speeds of 40 and 50 rpm, respectively. POLYM. ENG. SCI., 45:998–1010, 2005. © 2005 Society of Plastics Engineers

G211 射出成形における加熱シリンダ内現象の計測と実験解析(基調講演,OS-13 材料加工における熱工学関連技術I)

Resin pellets are plasticated inside the heating cylinder not only by heat transfer from outer heaters but also by shear heating of a rotating screw. This paper introduces, from the viewpoint of heat transfer, the following important phenomena dominating the plasticating process of resins; (1) temperature distribution inside flowing melts at the nozzle and screw channels, (2) heat flux changes at the inner surface of the cylinder, (3) torque axial distributions along the inner surface of the cylinder. The break-up phenomenon of the solid-bed (solid unmelted resin pellets) is also described based on visualization results and correlation with the above-mentioned measurement results.

On-line Visualization of PS/PP Melting Mechanisms in a Co-rotating Twin Screw Extruder

Abstract The melting and deformation mechanisms of polystyrene (PS) and polypropylene (PP) blends were investigated through on-line visualization of the co-rotating twin-screw extrusion process. Two compositions, PP/PS (80:20) and PS/PP (80: 20) were chosen as the model systems for this study. A sliding barrel technique was used to realize the on-line visualization using a glass window in the barrel. The axial temperature and pressure profiles along the screw channel were measured using the same sliding technique. It was found that in the PP/PS (80:20) blend, in which PP was the major phase, there was a combined melting of PS and PP, whereas in the PS/PP (80:20) blend, in which PS was the major phase, there was initial melting of PS alone and then combined melting of PS and PP. In the partially filled region, heat conduction from the hot barrel was the major source for heating polymer pellets under the conditions studied here; while in the fully filled region, viscous energy dissipation (VED) generated most of the heat for melting of polymer pellets. The pressure profiles along the extrusion channel gave us insight into the melting process for the two blends. It was found that if there was some molten polymer in the fully filled region, the overall melting process was accelerated due to heating from viscous dissipation.

Temperature gradients in the channels of a single‐screw extruder

AbstractWe used a temperature‐sensitive fluorescent dye, perylene, to monitor the true resin temperature during extrusion of polycarbonate. The measurement involved doping polycarbonate with perylene and detecting fluorescence with an optical sensor that accesses a standard instrumentation port on a barrel of a single‐screw extruder. The sensor's confocal optics design permits fluorescence intensity measurements as a function of position. Using a previously established calibration function, temperature and temperature gradients were obtained from the measured fluorescence. Because the origin of the measurement is the fluorescent dye molecule that is soluble in the resin, this method allows temperature measurement of the polymer without interference from the surrounding metal parts. With the sensor looking over the screw, temperature profiles from the barrel wall to the core of the screw were obtained as a function of screw speed, screw design and resin melt flow rate. Polym. Eng. Sci. 44:2148–2157, 2004. © 2004 Society of Plastics Engineers.This paper is a contribution from the National Institute of Standards and Technology, and, thus, is not subject to copyright in the United States.

Experiments on Characteristic Deformation and Breakup Behaviors of an Immiscible Drop in a Screw Channel Flow

Abstract Deformation and breakup mechanisms of an immiscible drop were investigated experimentally in a screw channel flow, which mimics the flow in the metering section of the single screw extruder. It was observed that the deformation/breakup mechanisms are far distinct from the well-known routes for the linear flow, mainly because of the complex flow history that can be characterized by the repetition of strong/weak shear, the continuously varying shear direction and the acceleration/deceleration. The experimental observations include: comet-like thread formation with the large bulbous head and the long narrow tail; the initiation of the breakup at the entrance of the low-shear zone; breakup by asymmetric end pinching; breakup by the growth of the capillary wave, propagating from the tail toward the head; monotonically decreasing droplet size distribution from the head to tail. The mechanism for the comet-like thread formation has been investigated by using the numerically obtained flow field.

Mixing of Polymers with Twin Screws

Blending of polymer melts in a twin-screw mixer with longitudinal circulation of liquid along one of the channels in the unit was investigated. Equations were obtained for determining the average shear rate and shear strain accumulated by the polymer melt, responsible for obtaining a mixture of the required quality. It was shown that the presence of a small radial gap separating the deep and shallow screw channels almost does not alter the pressure-flow characteristics of the unit but makes an important contribution to the value of the shear strain accumulated by the polymer melt.

Physico-Mathematical Model for the Description of the Temperature Development and the Power Consumption in Co-Rotating Twin Screw Extruders

Abstract Tightly intermeshing, co-rotating twin-screw extruders are commonly employed for tasks requiring good mixing. The modular constitution of both barrel and screw makes it possible to optimise the extruder configuration for a given task. Even today this optimisation is frequently done by applying the “Trial and Error”-method. Physico-mathematical models enable the process engineer to predict the process behaviour of a chosen extruder configuration and to optimise existing extrusion processes. Increasing demands in mixing quality and efficiency of the processing unit result in efforts to optimise existing extruders with respect to those factors. For this reason knowledge concerning the temperature profile along the screw and the power consumption is essential. We present a new physico-mathematical model for the description of the temperature development. At stages where the analytical solution itself would result in a not satisfying degree of accuracy we used descriptions based on finite element simulation results to achieve the desired exactness. As a result we got a model to describe the temperature development in the screw channel and a model to describe the power consumption in different screw sections. Applying these models it is possible to optimise the process parameters and the screw configuration with only a minimum of preceding experimental investigations.

Screw Machines with Longitudinal Circulation of Fluid. Communication 3. Intensification of Mixing Processes

Flow of melts and solutions of polymers with Newton's law of flow was investigated in a double screw mixer with screw channels of different depth and width in the presence of a radial gap on one screw ridge. With the radial gap, the pressure gradients along both channels are the same, the pressure along the entire length of the shallow and deep channels differ by a constant value and the higher pressure is always established in the second channel along the path of movement of the upper plate. By intensive exchange between channels, flow which gives zero flow of fluid in each section in the direction across the channels is organized in the deep and shallow channels of the mixer. The overall pressure differential and output of the mixer do not decrease in the presence of a radial gap on one of the ridges of the screw.

Numerical Simulation of Twin-Screw Extrusion with Wall Slip

Wall slip boundary condition is first introduced into twin-screw extrusion with the Navier slip law. Three-dimensional isothermal flow in the twin-screw extruder is simulated by using the finite element package POLYFLOW. Profiles of velocity contours in the screw channel and shear rate distributions in the intermeshing region are presented for different slip coefficients. Curves of axial pressure difference, average shear rate and dispersive mixing index vs. the slip coefficient are plotted and discussed. Comparisons are also made between the wall slip conditions and the non-slip condition. The simulation results indicate that, as the level of wall slip decreases, the axial pressure difference rises, the shear effect is intensified and the axial mixing is also enhanced. All these flow characteristics seem to level off with the increase of the slip coefficient. However, because of the inherent limitation of the Navier slip law, use of an overestimated slip coefficient would predict an over-sticky state between the screw surface and the polymer melt.

Steady and transient flow of a non‐Newtonian chemically reactive fluid in a twin‐screw extruder

AbstractThe flow of chemically reactive non‐Newtonian materials, such as bio‐polymers and acrylates, in a fully intermeshing, co‐rotating twin‐screw extruder is numerically investigated. A detailed study of the system transient behavior is carried out. The main transient aspects, including response time, variation of system variables, and instability of operation, are studied for both single‐ and twin‐screw extruders, since single‐screw extruder modeling closely approximates the region away from the intermeshing zone in a twin‐screw extruder. The effect of a time‐dependent variation in the boundary conditions is studied. The coupling due to conduction heat transfer in the screw barrel is found to be very important and is taken into account for single‐screw extruders. In the absence of this conjugate coupling, the response time is much shorter. Several other interesting trends are obtained with respect to the dependence of the transient response on the materials and operating conditions. Steady state results are obtained at large time. The calculated velocity distributions in the screw channel are compared with experimental results in the literature for steady state flow and good agreement has been obtained. The calculated results for transient transport agree with the few experimental observations available on this system. Chemical reaction, leading to chemical conversion of the material, is also considered and the resulting effects on the flow and transport determined. These results will be useful in the design, control and optimization of polymer extrusion processes.

Process Description for the Extrusion of Rubber Compounds – Development and Evaluation of a Screw Design Software

AbstractThe development of a mathematical model to be used in a simulation software for the description of rubber extrusion operating points is presented. A discrete model for the flow has been introduced, which considers the flow in the main direction and in the cross section of the screw channel. Moreover the flow model is connected with a discrete model for the computation of the temperature within the screw channel, an FDM approach has been used here. The combination permits the non‐isothermal flow simulation along extruder screws. Beside the isothermal and non‐isothermal option the simulation software, obtained on the basis of the models described, allows a multi‐variation of parameters as well. It is shown how such a variation mode can be used within a screw geometry optimization process and a short comparison of simulated and experimental data for the optimized concept is given. Further examples of comparison simulation/experiment are demonstrating that results can prone to error. The occuring lack of the model is most likely linked to an insufficient description of the rheology of rubber compounds. Further imperfection discussed are the processes within the feeding section. For example feed rolls and grooved barrels are often installed and effective for the process but not considered within theoretical models. magnified image

A study on the rheological properties and processability of polycarbonate

The transport theory for the solids conveying zone in a single-screw extruder was applied to calculate the pressure distributions along the screw channel for several bisphenol A polycarbonate resins based on the screw revolution speed and the flow rate. The pressure distributions and the flow rates of the resins were related to the structural and rheological properties. When polymers have the same chemical structure and number-average molecular weight and the same mechanical properties, the polymer having a broader molecular weight distribution showed a lower glass transition temperature. For the polymer with broader MWD a relatively low pressure was developed along the screw channel, and an increased flow rate was observed. A relatively short melting length was also observed for this polymer and, accordingly, it was concluded that the polymer with a broader MWD has a better processability. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 2921–2929, 2002

Incompressible Model of Solids Conveying in a Single-Screw Extruder

Experiments in corn meal extrusion [Zhang, S. and Sernas, V. (1998). Proc. 1998 ASME Int ME. Congress and Exposition, Anaheim, CA, HTD-Vol. 361-1. pp. 309-315] have shown that the flow of solids in the screw channel of a single-screw extruder has a helical pattern. This observation implies that there is a cross-channel velocity component in the solids conveying zone of the extruder. Existing solids conveying models [Broyer, E. and Tadmor, Z. (1972). Polym. Eng. Sci., 12(1): 12-24; Campbell, G.A. and Dontula, N. (1995). International Polymer Process X. Hanser Publishing. pp. 30-35; Darnell, W.H. and Mol, E.A. (1956). Soc. Plast. Eng. J., 12(4), 20-29; Hyun, K.S. and Spalding, M.A. (1997). ANTEC ’97 Proc. 163: 211-218] treat the solids moving in the channel as a plug flow without a cross-channel velocity component. The two-dimensional powder conveying model proposed in this paper contains both a down-channel component and a cross-channel component. The stress generating mechanism is much more complicated in a powder flow than in a fluid flow. The incompressible model incorporates a constitutive equation for the powder flow with slip boundary conditions. The energy equation with appropriate boundary conditions is also included in the model. The numerically solved model shows that the predicted down-channel pressure development, velocity and temperature distributions are all reasonable.

Experiments on chaotic mixing in a screw channel flow

AbstractMixing patterns of a passive dye were investigated experimentally in an unwound screw channel flow with periodic barriers that serve to induce a chaotic flow. Continuous cross‐sectional and longitudinal mixing patterns were observed as a function of the barrier fraction and were interpreted in terms of dynamical systems theory, along with 3‐D numerical simulations. Observations include: periodically invariant cross‐sectional mixing patterns due to the resonance bands; pile‐up of dye streaks caused by stretching and folding of manifolds near hyperbolic points; unmixed zones due to KAM tori; longitudinal deformation patterns with exit time distributions; and effects of the barrier fraction on the size of unmixed islands, the thickness of the band of dye streaks, and the distribution of exit times.

Morphology development during blending of immiscible polymers in screw extruders

AbstractThe present work reports evolution of morphology from initial (presence of striation) to final (droplet formation) stages in a single‐screw extruder. Morphology development during the blending process controls the final size of the dispersed phase, which in turn significantly affects the properties of the blends. The experiments were carried out using a 70/30 wt% polypropylene/ethylene vinyl acetate (PP/EVA) blend; samples were collected along the length of the screw, by screw pullout experiment, to analyze the size and size distribution of the dispersed phase present both as striated layers and subsequently as droplets. Average size of the dispersed phase and standard deviation were taken into account to monitor the morphology evolution along the length of the screw. Pre‐breakup morphology development was studied by analyzing the sample collected from the feed zone of the extruder in terms of upper and lower layers along the cross section of screw channel. Examination of micrographs revealed the existence of pattern of ordered striations along the length of the melting zone containing striations from average size of 1000 μm down to 50 μm decreasing rapidly along the length of the screw. The breakup process was captured at the start of compression zone where step‐up in the shear as well as elongational flow was applied due to decrease in the channel depth along the compression zone. The observed droplet size formed by the breakup of filaments is found to be in accordance with theory. The final droplet size is found to be governed by the emulsification process occurring as a result of stretching, breakup and coalescence in the metering section of the screw and is in the order of 2 μm.

A 3D Numerical Study of Fluid Flow and Heat Transfer in a Single Screw Extruder

Abstract A numerical study of three-dimensional fluid flow and heat transfer in the metering section of a single screw extruder has been performed. The screw channel was simplified by unwinding the channel and the coordinate system was fixed on the screw root in the present model. The pressure distribution over the inlet region was to be extrapolated from the solution of interior pressure. The prescribed mass flow rate was imposed on the outlet of the unwinding screw channel. The three-dimensional velocity fields, temperature, viscosity, and pressure profiles in the screw channel were calculated by using the commercial code, STAR-CD, based on the finite volume method. Some of the results of the present full 3D model were compared with the results from the marching-type 3D model. It is shown that the present work can capture the back flow by the pressure gradient near the screw root in the cases of small mass flow rates. There exist significant discrepancies in temperature between the marching-type 3D model and the present 3D model. The present work well describes the effects of heating-up through the circulation flow and viscous dissipation. The cross-section contours of velocities, temperature, viscosities and pressures along the down-channel direction are also shown in this study.

Validation of fluoroscopy-based navigation in the hip region: what you see is what you get?

Fluoroscopy-based navigation systems can be used for internal fixation of intracapsular femoral neck fractures, with the object of optimizing positioning of the implant and reducing radiation exposure. With this technique, the virtual position and direction of a reamer can be simultaneously superimposed on anteroposterior (AP) and axial (AX) fluoroscopic images. However, surgeons have to rely on the accuracy of these systems, because the only intraoperative feedback on the true position of the reamer is the projection of a virtual reamer superimposed on two fluoroscopic images. The objective of this study was to evaluate the accuracy of the displayed position of the virtual reamer in relation to the true position of the instrument when using a fluoroscopy-based navigation system (medivision, Oberdorf, Switzerland). Secondary to this, the accuracy of the drill-channel measuring tool of the system was analyzed. The study was performed on 20 sawbones. To evaluate the position of the virtual reamer, an 8-mm Perspex bar was inserted in predefined drill channels in each sawbone. AP and AX fluoroscopic images of the sawbones with the Perspex bar were loaded into the workstation. The Perspex bar was then removed and exchanged for a navigated dynamic hip screw (DHS) reamer. The position of the Perspex bar in the images represented the true position of the reamer. Subsequently, the difference between the position of the virtual reamer and the Perspex bar was measured with a dedicated computer program. Drill-channel lengths measured with the system were compared with measurements obtained with a digital ruler. The mean difference in position of the Perspex bar and reamer at a predefined point was 0.90 mm (range: 0.00-3.21 mm) in 360 images. The mean difference in length measurements between the medivision system and the digital ruler was 1.00 mm (p = 0.01, SD =1.33). Reaming and measuring the screw channel of a DHS with a medivision fluoroscopy-based navigation system can be performed with an acceptable error margin.