The SPE Library contains thousands of papers, presentations, journal briefs and recorded webinars from the best minds in the Plastics Industry. Spanning almost two decades, this collection of published research and development work in polymer science and plastics technology is a wealth of knowledge and information for anyone involved in plastics.
Because of the effects of die swell, the final shape of an extrudate is often substantially different from that of the exit opening of the die. As a result, the design of profile dies producing complex shapes often involves more than just “balancing” the die but also compensating for the effects of die swell. Typically, a successful design of such dies is only achieved through much “cut and try”. However, with the use of a fully three-dimensional finite element flow algorithm along with quick mesh generating capabilities, the usual cut and try involved in the design of many profile dies can be greatly reduced if not eliminated.This paper demonstrates how the effects of die swell can be compensated for in the design of profile dies. For profiles with one plane of symmetry, this includes compensating for the sideways translation of the extrudate as well as the change in shape that the extrudate experiences. Completely asymmetric profiles undergo a “twisting” downstream of the die. This twisting, which appears not to have been reported in the literature (at least for isothermal extrusion) is also accounted for here along with the change in shape that the extrudate undergoes.
Feedblock Coextrusion is considered to have been commercialized with the Dow Chemical patents issued in 1971. As Cloeren and Nissel introduced their own Coextrusion technology, a common thread throughout all technologies was design of Coextrusion dies. For decades extrusion die manifold designs remained unchanged. The ‘90s ushered in new commercial manifold designs aimed at solving age old problems with flow uniformity, die deflection, and coextrusion performance. This paper will examine the progression of die manifold designs, their impact on extruded products, and their implications on future coextruded structures.
The development of crystallinity during film blowing of a linear low-density polyethylene at different processing conditions has been investigated using online Raman spectroscopy. The obtained trends for crystallization rates were explained using the concept of flow-induced crystallization. Further, direction dependent (machine and transverse) tensile properties were studied as a function of take-up ratio (TUR), inflation air pressure and cooling conditions. Based on the results for development of crystallinity at different conditions, the reciprocal crystallization half time was suggested as a likely essential parameter to formulate processing-structure-property relationships.
Transient behavior and stability of the film blowing process have been studied using the governing equations consisting of two well-known force balances and a Phan- Thien and Tanner constitutive equation. A newly-devised numerical scheme, which incorporates an orthogonal collocation on finite elements, was employed to yield transient solutions of the film blowing operation which are hitherto difficult to obtain due to severe numerical stability problems when the system is in an instability called draw resonance. Thus obtained simulation results make possible more systematic analysis of draw resonance, e.g., draw resonance criterion based on the traveling times of kinematic waves on the free surfaces, and the effects of fluid viscoelasticity on the stability.
Stagnation of polymer materials at the screw tip is a common problem for many extrusion processes with thermoset or thermal sensitive materials. Cured or burnt materials are often found at the screw tip and cause quality problems. Another example is a frequent color change. It may take a long time to purge the old color completely and creates a large amount of scrap.A mathematic model has been developed to simulate the polymer flow at the screw tip. Streamlines and residence time distributions have been obtained to understand the stagnating flow. The screw tip designs have been evaluated with the model. Also evaluated are the gaps between the screw tip and screen pack. The potential solutions to the stagnating flow are discussed.
In this paper we will discuss the differing solids conveying rates of several polymers using a “general purpose square pitch screw.” It is often desired to change polymer in an extruder as the products that are sold in a company change a the time or capital needed to pull the screw an replace it are not available. Often the results are confounding because the rates one obtains are dramatically different for the new polymer. In this paper we will present data for six polymers which demonstrate that the output of a typical solids conveying screw changes substantially as back pressure is increased on the solid bed being conveyed.
Various temperature measurement techniques have been used to investigate thermal profiles in the die of a 60mm single screw extruder, including invasive and wall mounted thermocouples, thermocouple meshes, ultrasound, and infrared. For a general-purpose polyolefin screw, large temperature differences of the order of 20°C were observed over short time periods of less than 1 second, in the exit melt temperature. These instabilities were evident during measurement of bulk melt temperature and are believed to be a result of melting instabilities. Predictive temperature plots obtained from CFD simulations using a commercial software package show qualitative agreement with the empirical results.
Thermocouple meshes have been used to measure the temperature profile of polyethylene melt at the exit of a 60mm single screw extruder. Measurements have been made across a 38mm cross-section using a mesh with seven junctions. The meshes are located across the melt and provide temperatures at discrete positions across the flow, from which a thermal map can be constructed. The effect of screw geometry is investigated, with comparisons made between a general purpose polyolefin screw with gradual melting zone, a rapid transition screw, and a barrier flighted screw with a Maddock mixer. Significant differences in melting and temperature profiles were observed between the three extruder screws.
Processors need to know if an air cooled extruder is adequate to their requirements or if they need the higher capacity of water cooling. This decision is often made based on experience, tempered with a fear of guessing wrong. If water cooling was used in the past, many processors are reluctant to try air cooling on a new machine. This paper compares the experimental data of various temperature settings and other operating conditions on a 90mm extruder, precisely repeated with an air and water cooled barrel heater on the same machine with the same screw. The data is then compared and quantified to establish useful guidelines for the application of water vs. air cooling.
High plastication rates and high quality extrudates are often difficult to produce using single-screw extruders, especially at low discharge temperatures. A new screw called a variable barrier energy transfer (VBET) screw was recently developed to plasticate at high rates, low discharge temperatures, and with high melt qualities. The fundamental operation of the screw along with performance aspects will be presented. A comparison is made between the melting, pumping, and mixing characteristics of an Energy Transfer (ET) screw and VBET screw.
A novel method of using single screw extruder as measuring viscosity and analyzing morphology evolution of polymer blend was elucidated. Combination of a batch wise operation of single screw extruder (the closed discharge condition) and using the modernized high-speed data acquisition system was a key technology of the method. The viscosity measured by “Screw Rheometer” was well matched with that by conventional capillary rheometer. Also the viscosities of rubber like polymers and low viscous materials such as polyamide were also successfully measured. The device also used to observe evolution of morphology for the immiscible polymer blend in controlled shearing state.
In recent years, pressure from economic and environmental requirements has been experienced in the field of polymer production. This trend towards single stage operational units, in processes such as low shear devolatilization of elastomer solutions, radical polymerization and polycondensation reactions, phase changing processes and the conversion from batch to continuous operation continues and has lead to the development of large volume, twin shaft horizontal processors (LVPs).These processors have been designed for applications requiring medium to long residence times (20-120 minutes).As an example of this new family of processors, the multi-purpose Reasol®, a new counter-rotating twin shaft processor, is introduced. Trials with model polymers have been performed in a 60L unit at the developer's test center. Its performance is described here by power consumption, RTD (residence time distribution) and self-cleaning and devolatilization efficiency.Trials show that product transport through the new LVP is characterized by a narrow RTD with a high degree of self-cleaning. Typically, the RTD exhibits a Peclet number in the region of 25-35. It is also shown that, unlike typical twin-screw extruders, the shape of the RTD curve is largely unaffected by the rotor speed or mass rate. Furthermore, rotor speed has a relatively small effect on mean residence time thus allowing the freedom to optimize rotor speed with respect to other processing objectives such as heat transfer, surface renewal or shear rate.The indications are that the characteristics of the RTD, power consumption and devolatilization are analogous to more traditional equipment such as twin screw extruders in spite of the larger free volume and residence time and the lower shear.The new LVP is commercially available up to sizes of 12,500 litres net processing volumes.
It has been experimentally demonstrated that Plastic Energy Dissipation (PED) is a dominant melting mechanism in twin-rotor co- and counter-rotating processing equipment. Such devices force compacted polymer particulates to undergo repeated compressive, volume-wise, large compressive deformations, resulting in massive dissipative heating of the solid particulates and rapid melting. The large magnitude of PED has two important consequences: it results in very rapid end efficient volume-wise melting, enabling very fast processing rates and produces a very narrow “age distribution” melt, important for reactive processing. To date the experimental evaluation of PED, during large unconfined compressive cylindrical sample deformations, at appreciable strain rates, can be used to arrive at reasonable engineering estimates of the melting lengths for given polymers in specific processing equipment operating under given processing conditions. But this task involves a large number of PED-evaluation experiments, because of the lack of constitutive equations which are capable of describing the compressive stress-strain behavior of solid polymers at “engineering” strains and strain rates. The object of this presentation is to evaluate and modify existing constitutive relations for amorphous polymers using the large experimental date that have been generated in our laboratories. An acceptable constitutive relation will not only reduce the number of experiments necessary to evaluate PED and estimate melting lengths, but a needed element in the development of PED melting simulation models.
A new 2D composite model with the hybrid FEM/FDM was developed for simulating the fully filled and starved regions with the associated pressure profiles of a modular intermeshing co-rotating twin screw extruder. 1D composite models combine the screw characteristic curve of individual element to analysis flow of an entire modular screw and the flow fields of the whole region are not calculated again. Based on the linear relationship of the drag flow rate and the screw rotating speed under the single screw extrusion theory, the new mesh with artificial screw rotation speed boundary conditions was used to simulate the entire flow analysis for the co-rotating twin screw extrusion process in our 2D composite models.To demonstrate applicability, the results provided by our 2D composite models were found to similar to those of 1D composite models, through Fenner’s single screw example. The pressure and filling factor profiles in a modular TEX44 extruder provided by 2D composite models show good agreements with Fukuoka's calculated results and experiment data.
In order to improve the mixing ability and elongate the mean residence time in twin-screw extruders, the axial circular flow was introduced in this study. A non-intermeshing section was set in the co-rotating twin-screw extruder, in which one screw was equipped with reverse conveying elements and the other with forward conveying elements. In the current work, a three-dimensional simulation of the flow in the axial circular flow section was developed by FEM package, ANSYS. Based on the calculated pressure and velocity profile, the pressure-throughput behavior and mixing ability of the axial circular flow section were discussed. Compared with the conventional conveying elements, the circular flow section has reduced ability to conveying but its distributive mixing capability was largely increased. Visualization experiments showed that the simulation results were in good agreement with the experimental outcomes.
The polymerization of ?-caprolactone in fully intermeshing conveying screw elements of co-rotating twin screw extruders was simulated with Fluent. The effects of the pitch on the element, temperature, flow rate, and screw rotating speed upon the reaction progression were investigated. It was found that the back-flow in the screw element affects the progress of polymerization. The simulation result from 3-D model differs significantly from 1-D model, especially in relation to slow reactions. This is because reaction is very sensitive to temperature change, and the value of heat transfer coefficient at the barrel and screw surface used in 1-D model may not represent the real conditions in extruders.
PSE (Polygon Screw Element) is a new screw element, designed to meet the development of co-rotating twin-screw extruders toward high rotating speed and excellent mixing capability. In the present study, a three-dimensional flow field simulation of PSE element was carried out by the ANSYS FEM package. Pressure profile and shear stress profile were obtained. The mixing ability of PSE element was also analyzed. The simulation results of the flow field in PSE element were verified by experiments. It was found that the energy consumed in PSE element is smaller, whereas its mixing ability is better than the kneading block. Furthermore, PSE element is suitable for the fiberglass reinforced processing.
Polyethylene (PE) or polycarbonate (PC) drop breakup process in PE melt under shear flow was investigated using volume-of-fluid method. Real properties of polymers, and temperature and shear rate dependent viscosity model were incorporated in the modeling. An erosion mechanism was found for both PE and PC drops. Local flow information, such as shear rate, viscosity and shear stress, was obtained from the simulation results. Highest shear stress was observed at the interface, which could explain the erosion breakup mechanism.
Foamed products based on renewable raw material have a high application potential e.g. for packaging because of their biodegradeability. This may permit renewable raw materials to substitute polymers like polystyrene in some applications.A common way to process renewable raw material like starch is to produce starch based resins with twin screw extruders. These resins can be used on conventional polymer processing machines, but the step of compounding the starch on twin screw extruders causes costs which make these resins economically unattractive.Due to a new extrusion technology these costs can be reduced by a direct processing of starchy material like maize. A characteristic of this extruder is a very short (2 L/D), conic, multiple flighted screw in a barrel with spiral grooves. The energy for the plasticizing process is yielded just by the transfer of mechanical energy of the rotating screw into friction in a shear gap between screw and barrel.In order to understand the process different geometries of screw and barrel have been used in the experiments, additionally the process parameters have been varied. The results lead to an optimised configuration of the extruder and to a better understanding of the influence of process parameters on the product properties.
The importance of slip for applications such as extrusion, cable coating, thermoforming, etc. has been widely discussed in the literature. Recent experimental works suggested that slipping along the rotors also impacts the quality of the mixing in batch mixers typically used for Carbon Black dispersion. In batch applications, the slip reduces the mixing efficiency, hence requiring a longer process to get a given mixing quality.Recent developments in CFD allow to include the slip in the numerical models. The advantage of numerical simulation is the ability to turn on or off this phenomenon. Also, the effect of particles behavior along the rotors on both the flow pattern and the mixing efficiency of internal mixer is studied. Two 3D transient simulations are performed with the POLYFLOW package: one simulation assumes sticking boundary conditions along the rotors whereas the other involves a full slip condition.The results obtained in both simulations are compared and validated against experimental results: the slip condition modifies dramatically both the velocity and the shear rate fields. Therefore, the distributive and dispersive mixings generated by the two models are significantly different. Eventually, we observe a better match between numerical and experimental results when the slip condition is taken into account.
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