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.
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Some New Observations Relative to Melting in Single Screw Extruders
The focus of this investigation was to evaluate literature and run experiments to help understand the mechanism of melting in single screw extruders. Literature data was re-analyzed using video capture and analysis programs to determine the rate of loss of material in the cross channel and solid bed thickness directions. The analysis demonstrates that the polymer solid bed goes to zero dimension in the thickness direction well before the solid bed width is consumed. This observation was confirmed in our laboratory using a specially built glass barrel extruder. These results suggest that the melting in single screw extruders is dominated by the loss of bed thickness and not bed width as predicted by current literature.
The Influence of Morphology on the Failure of Polyethylene Pipes in Hydrostatic Pressure Tests
Failure times of plastic and metal pipes subjected to hydrostatic pressure at various levels assist pipe manufacturers to not only design pipes for certain applications, but also to give them an indication of the useful service life-times of these pipes. In order to understand the influence exerted by semicrystalline morphology on the failure of polyethylene pipes under hydrostatic pressure, a medium-density polyethylene resin was converted into various pipes by altering the extrusion processing conditions. These pipes were subsequently subjected to hydrostatic pressure at a constant hoop stress and the failure times were recorded. The failure times were observed to depend strongly on the morphology of the pipes.
Computer Aided Troubleshooting of Extrusion Problems
When a problem occurs in an extrusion operation it is important to diagnose the problem, determine possible solutions, and implement the best solution in the shortest possible time. In-house personnel are often not well trained in efficient troubleshooting techniques and problem solving. Outside technical assistance may not be immediately available and may not lead to an expedient solution of the problem. As a result, it is important for extrusion companies to have tools available in-house that can help in the troubleshooting and problem solving process.A new tool available to the plastics industry is an expert system that allows computer aided troubleshooting of extrusion problems. This program allows the user to systematically analyze a large number of extrusion problems. In the end the program presents the user with a number of possible solutions to the problem. This paper will describe the capabilities of the program.
Study of the Micropelletization Process
A study has been performed to examine the rheological impact of micropelletization on several polyethylene grades with melt index values between 1-5 g/10 min. The experiments were done on a 50 mm 30:1 L/D extruder with an underwater micropelletizer attached. A 2-D finite element simulation was used to assist in the analysis by comparing the observed results to the predicted shear stresses in the die. The average micropellet size collected was 0.525 mm diameter. Minor sharkskin was observed on the surface of micropellets due to the high stresses experienced in the pelletizer die. However, the rheological properties of the micropellets did not change in comparison to the virgin resins.
Detecting Defects and the Onset of Failure of Adhesive Bonds Using Electrochemical Impedance Spectroscopy
Tape sensors were mounted on the adherends and the health of the adhesive bonds was studied by electrochemical impedance spectroscopy. Results show that EIS measurements can discriminate bonded and unbonded areas in CFRP-Al systems. The EIS measurements are also sensitive to the weakening of adhesive bonds caused by exposure to humidity at high temperatures.
Microscopy and X-Ray Elemental Spectroscopy in Failure Analysis: Case Studies
Optical and Scanning electron microscopy (SEM) techniques, coupled with energy dispersive X-ray analysis (EDS) are effective tools in investigating the cause of failures in articles made from plastics. Practical examples and findings are reviewed that have been used to solve customer problems in real world applications.
Thermal Properties of ? Nucleated Polypropylenes
Commercially available isotactic polypropylene (iPP) was modified by various amounts of a specific ?- nucleating agent. Isothermal crystallization and subsequent melting behavior were analyzed using differential scanning calorimetry (DSC). The findings resulting from this work (such as crystallization halftimes, other kinetics parameters, glass transition temperatures, etc.) can significantly assist to explain not a few open questions concerning ?-nucleated polypropylenes and their processing.
High Thermal Conductivity Shape Memory Polymers
The ability for shape memory polymers (SMPs) to quickly actuate upon exposure to a heat transfer fluid or radiative heating depends significantly on thermal conductivity and heat capacity. Recognizing this, we have pursued the enhancement of thermal conductivity of several SMPs - developed previously in our laboratory - through the addition of thermally conductive inorganic filler. Our presentation will focus on both thermal and thermomechanical analyses for filled and crosslinked polycyclooctene, revealing the dependence of thermal conductivity and shape memory response on filler loading.
Processing Nanocomposites on a Kneader Reciprocating Single Screw Compounding System
Interest in nanocomposites is based on the premise that with a relatively small loading of properly dispersed treated clay, polymers can exhibit substantial improvement in properties. These include thermal properties such as HDT, mechanical properties such as flexural strength and modulus (without significant loss of impact), barrier properties, flame resistance, and abrasion resistance. While the concept behind nanocomposites is sound, implementation has been flawed. Dispersion of 8 micron clay particles into a million or so high aspect ratio platelets is a difficult task. Both chemical modification of the clay for improved compatibility, and development of specific compounding technology have shown promise but not been fully successful. In previous work (1) unit operations that have significant impact on clay dispersion in the co-rotation twin-screw extruder were identified. This presentation will review the design flexibility associated with the reciprocating single screw, or kneader, compounding system and discuss key unit operations necessary to obtain well dispersed clay. It will also compare results from the reciprocating single screw system with those obtained from previous work on a twin-screw extruder.
Wave Interfacial Instabilities at Coextrusion in Flat Dies: Simulation and Experimental Research
The work presents both, simulation and experimental findings of the research of two LDPE melts using a flat coextrusion flow visualization cell. The simulation is performed by FEM analysis with full u-v-p-? numerical scheme employing viscoelastic constitutive equations. The predicted stresses, velocities and interface location have been found to be in a good agreement with the measurements. The experimental analysis shows that pronounced wave instabilities are caused by the minor layer break at the merge point of the layers, and extensional viscosity is a driving parameter here. Finally, recently proposed ‘TNSD sign criterion‘ has been successfully tested for the prediction of the onset of wave interfacial instabilities in this type of the die geometry.
Experimental Observations and Analysis of LDPE Melt Flow in Coextrusion Geometries
The study of two low density polyethylene melt flows in slit coextrusion flow cells with 30° and 90° confluent geometries is presented. The stream flows and polymerpolymer interface in the confluent region and die land are observed through side windows of the cell. Birefringence and image processing techniques are used to quantify stress and velocity fields in the upstream and down stream melt flows. Interfacial instabilities were observed in the LDPE melt flows in both geometries. Instabilities occurred at certain stream mass flow ratios. Digital image processing shows the wave type extrudate instabilities have the same periodicity as perturbation in the flow field in the die land. A recently proposed “TNSD sign criterion” reasonably predicted the onset of the interfacial instabilities in these die geometries.
A Non-Isothermal 3D FEM Study of Spiral Mandrel Dies with Non-Symmetrical Input
The design of an optimal spiral mandrel die suited to given requirements is quite a complicated task for the die designer. Even for a properly designed spiral section, there is always a question how variations in the flow prior to the spiral channels can influence the melt distribution at the exit of the spiral distribution section. This study uses 3D FEM analysis to investigate an effect of having uneven inputs. Specifically, the influence of uneven mass and temperature input distributions are studied. The influence of the die distributive system is investigated by analyzing the temperature and velocity fields at the exit.
Sensitivity of Flow Distribution and Flow Patterns in Profile Extrusion Dies
Fluctuations of the operating conditions or slight variations of the polymer rheology may occur during longterm productions, affecting the performance of the die in an extent dependent on its flow distribution sensitivity. In this work, four extrusion dies are optimised (balanced) using different design methodologies. These are compared in terms of their performance and stability to some operating conditions and polymer rheological properties. A finite-volume based computational code is used to perform the required simulations of the non-isothermal three-dimensional flows, under conditions defined by a statistical Taguchi technique. Correlation between the flow patterns developed and flow distribution sensitivity is also investigated.
Method for Measuring the Kinetics of Isometric Crystallization of Thermoplastic Sheets and Patient Fixation Devices
A new method for crystallization kinetics of stretched polymer samples and real size polymer articles under isometric conditions is developed. The internal shrinkage force caused by the volume contraction of the sample during crystallization is measured incessantly. The kinetics parameters of isometric crystallization are calculated by the Avrami equation and discussed in terms of controlled molecular structure and design of large polymer sheet samples and of patient fixation devices for medical application in radiation therapy.
Viscosity of Blowing Agent Laden Polymer
The viscosity of a blowing agent laden polymer was measured using a Helical Barrel Rheometer (HBR). Temperature, pressure and shear rate dependence of viscosity at a fixed blowing agent concentration was computed from the experimental data. Technique was developed to identify the onset of bubble nucleation by measuring the shear viscosity as function of hydrostatic pressure.
Structure-Property Relationships in Coextruded Foam/Film Microlayers
It has been demonstrated for the first time that microcellular foam structures can be produced using the microlayer coextrusion technology. Cell size can be reduced considerably by increasing the number of layers without adversely affecting the density. Unique mechanical properties can be achieved through material choice and layer structure. The cell morphology and the compressive response of these multilayered foam/film structures are similar to that of cork.
Simulation of Polymeric Flow in a Twin Screw Extruder: An Analysis of Elongational Viscosity Effects
Flow of a polymer in an intermeshing co-rotating twin-screw extruder is simulated. Effect of elongational viscosity on the flow is analyzed using independent power-law models for the shear and elongational viscosities. Axial component of the velocity is found to be maximum in the intermeshing region of the extruder. Axial component of velocity, which determines the throughput of the extruder, decreased as the elongational viscosity of the polymer used for the flow simulation was increased. The pressure in the extruder decreased from a very high positive value on the leading edge to a very large negative value on the trailing edge of the screw. For the same rotational speed the pressure build-up in the twin-screw extruder increased as the elongational viscosity of the polymer was increased.
Plastic Energy Dissipation (PED) of Uncompatibilized and Compatibilized Polymer Blend Systems
It has been demonstrated experimentally, that Plastic Energy Dissipation (PED) is the most important heating/melting mechanism, especially in the initial stage of heating of compacted solid polymer particulates in polymer compounding equipment, where forced particulate deformation takes place. Our previous work has also found that the melting of multi-component polymer blends is totally different from that of the individual blend component. The difference has been correlated to the different PED response of individual blend components in the deformation of polymer blends. In this work, we study the PED behavior of model polymer blends (PE/PS) at different compositions as well as the effect of a compatibilizer on the blend PED. The Specific Mechanical Energy (SME) input associated with PED of both uncompatibilized polymer blends and compatibilized polymer blends during uniaxial compressive deformation is calculated and compared.
Simulation of the Fluid Flow of Deeper Screw Flights for Co-Rotating Twin Screw Extruders
Co-rotating twin-screw extruders are frequently used in the field of polymer compounding processes. The extruders are build in a modular design with different conveying and mixing elements to be adapted of specific processes. The user operating the machines often has an optimization problem: On the one hand a gentle mechanical treatment during the flow through the extruder are required. Otherwise there is the desire for a flow rate as much as possible. Therefore Coperion Werner & Pfleiderer has developed a new model series with deeper screw flights, because the ratio of outer versus root diameter is one of the characteristic dimensions of a screw extruder.To estimate and evaluate the success of the new model series, numerical simulations have been carried out of the three dimensional fluid flow in such twin-screw extruders. The approximation based on a finite volume method with contour-adapted meshes. The calculations are made in a relative system, where the flow appears in a steady state.In this paper we present results of numerical investigations of single- and double-flighted screw elements and compare these with calculated values of an old model range with smaller diameter ratio. We discuss not only the conveying- and power-characteristics for Newtonian and nonnewtonian highly viscous materials but also the distribution of shear rate and shear stress. It is shown that the deeper screw flights are a good choice to take care of the polymer material and reduce the energy consumption of many processes.
Development and Verification of a Method to Optimize Individual Screw Elements for Co-Rotating Twin Screw Extruders
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 optimize the extruder configuration for a given task. This research focuses on the optimization of individual screw elements, thus the development of an optimized geometry for different materials and operating conditions.We calculated the pressure profile, the temperature development and the power consumption using one-dimensional models. A non-linear optimization algorithm was used to vary the geometrical data of the screw elements of interest. The goal was to find a geometry that shows an optimum between the contradicting goals of a minimum temperature increase, a minimum power consumption and a capability to build up a certain pressure with in a minimum distance (i.e. maximum pressure gradient). The quality of the geometry was characterized and evaluated by quality functions. We verified the optimization method by systematic experimental investigations.
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