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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Weldline Strength in Products Manufactured through Vibration-Assisted Injection Molding
Weldlines, which are a common feature in many injection molded parts, are known to be inherently weak. The present experimental study focused on improving weldline strength through the addition of oscillatory vibrational energy during molding processes. Both filled and unfilled polymers were studied, and comparisons between the tensile strength of ASTM specimens made with and without vibration were made. It was found that minimal product strength increases were realized when optimized vibration-assisted molding conditions were applied. Details related to the critical vibration-assistance parameters studied and optimization processes utilized will be presented and discussed.
Preparation and Properties of Polypropylene Nano-Composites
Polypropylene (PP) nano-composites are prepared by melt intercalation in this study. Nano-clay is mixed with PP by twin screw extrusion. Maleic anhydride modified polypropylene (PP-MA) is added to enhance the dispersion of the clay in PP. Nylon 6 is also added to this PP/PP-MA/clay composite to partially replace the content of PP-MA in order to decrease the material cost. The basal spacing of the clay in the composites is measured by X-ray Diffraction (XRD). The morphology of the composites is observed by SEM. The mechanical properties of the composites are also measured. It was found that the molecular weight and MA content strongly affect the nano-structure and the properties of PP nano-composites. PP-MA with a lower molecular weight (LMW) and a high MA content can lead to good clay dispersion in PP-MA. However, it causes a decrease in the mechanical properties of PP/PP-MA/clay composites. It was also found that the addition of nylon 6 further expands the basal spacing of the clay in PP composites. However, the mechanical strength decreases by adding nylon 6 to the nano-composites.
Design and Fabrication of Polymer Microfludic Platforms for Biomedical Applications
In this paper, design and fabrication of a CD platform, where different fluidic structures are micromachined into a plastic CD, are presented. Various ways to fabricate mold inserts (traditional CNC-machining of tool steel vs. photolithography/electroplating) and microfluidic platforms (micro-embossing, thin wall injection molding, and reactive molding) based on optically clear thermoplastic and thermoset polymers (e.g. optical quality polycarbonate, PMMA, PDMS) are introduced. The effects of mold inserts, molding methods, and feature size on replication accuracy and 'molded-in' stresses are also discussed.
Effect of Co-Promoter and Secondary Monomer on Shrinkage Control of Unsaturated Polyester Resins Cured at Low Temperatures
The presence of low profile additives (LPAs) can reduce the shrinkage of unsaturated polyester (UP)/styrene (St) resins cured at low temperatures, but the final shrinkage is still quite high. In this study, secondary monomers (e.g. divinylbenzene (DVB) and trimethylopropane trimethacrylate (TMPTMA)) and co-promoter 2,4-Pentandione (2,4-P) were added into the UP/St/LPA system to investigate their effect on shrinkage control of resins cured at low temperatures. Dilatometery results showed that the addition of both TMPTMA and 2,4-P resulted in earlier volume expansion during curing and in turn; better shrinkage control. The phase separation, reaction kinetics and viscosity changes in the LPA-rich and UP-rich phase during curing were investigated. Results revealed that the increased reaction rate in the LPA-rich phase led to an earlier formation of microvoids and, consequently, less volume shrinkage of the cured resin.
Effect of Resin Chemistry on Curing of Unsaturated Polyester and Vinylester Resins
Unsaturated polyester and vinylester resins are the two major thermoset resins used in low temperature composite manufacturing processes, such as the Seemann Composite Resin Infusion Molding Process (SCRIMP). Understanding the reaction kinetics and network formation can be critical to SCRIMP. A series of well-defined polyester resins are used to study the effect of resin chemistry on the reaction kinetics. The effects of styrene concentration and temperature on the cure kinetics of styrene/vinylester systems are also studied. The glass transition temperature of polyester/vinylester resins cured at different temperatures is measured and used to monitor the final conversion change. A mechanistic kinetic model is proposed to simulate both the reaction kinetics and the final conversions of various resins in low temperature copolymerization. The results may shed light on resin modification and process innovation.
Supercritical Carbon Dioxide Assisted Polymer Blending in Twin-Screw Extrusion: Phase Inversion, Morphology, and Mechanical Properties
Supercritical carbon dioxide (scCO2) was added during compounding of polystyrene (PS)/poly(ethylene-methyl acrylate) copolymer (SP 2207) and poly(methyl methacrylate) (PMMA)/SP 2207 blends and the resulting phase inversion, morphology, and mechanical properties were studied. SP 2207 is a 80 wt.% ethylene-20% methyl acrylate copolymer used to increase the toughness of polystyrene and PMMA. Carbon dioxide was added at 2.0 wt.% based on polymer melt flow rates. Carbon dioxide injected into the extruder sped up the phase inversion of the two components by reducing the glass transition temperature of PMMA and PS. This allowed for an enhanced dispersion of the rubber phase after phase inversion. An earlier extruder location at which phase inversion occurred along with a reduced viscosity ratio greatly improved the dispersion of the rubber copolymer in the polystyrene and PMMA matrix. An increase in impact strength and a reduction in the flexural modulus of blends prepared with CO2 were measured, indicating that CO2 aided in dispersing the rubber into the brittle matrices of polystyrene and PMMA. Reprocessing did not result in a loss of morphology for PMMA/SP 2207 blends.
A Novel Resin Design for Styrenic Resin-Based Room Temperature Vacuum Infusion Resin Transfer Molding-Scrimp
For low temperature composite manufacturing processes, a major concern for the fabricators is how to design and control the mold filling and curing time. Curing agents like inhibitors or retarders are needed to prevent premature gel and provide a sufficiently long time to complete mold filling. However, the addition of inhibitors or retarders tends to result in a slow mold curing rate and low resin conversion. In this study, a chelating agent, 2,4-pentanedione (2,4-P), is used to manipulate the resin gelation and curing. 2,4-P can interact with promoters (i.e., metal compounds such as cobalt carboxylates) and affect the catalytic activity of the promoter to decompose the initiators, by either retarding or promoting the resin system depending on the initiator and resin system used. It was found that the function of 2,4-P as a retarder or co-promoter for the co-polymerization of styrenic resins depends greatly on the acidity of the resin system used. Based on this observation, a series of SCRIMP molding experiments using a novel resin design was carried out. This design allows 2,4-P to serve as a retarder during mold filling to achieve a long gel time; it then becomes a co-promoter during curing to increase the curing rate and conversion when the acidity of the resin changes.
Instabilities in Startup Flows of Polyolefin Melts in Axisymmetric Contraction Geometries
A flow visualization cell with an abrupt entry pseudo-axisymmetric geometry has been used in the study of a low density polyethylene melt flow. The flow cell was mounted on a Davis Standard Betol BK38 single screw extruder. Birefringence and imaging velocimetry techniques have been used to observe changes in the stress and velocity fields in the melt at various processing conditions. Flow development of the melt was observed using pigment tracers. Start-up and steady state flow regimes are investigated. A transient instability occurred on start-up flows. The time stabilization of the flow increased with dwell time between extrusion trials. The instability was observed as oscillatory motion of the stress birefringence pattern and fluctuations in measured pressure.
Flow Visualisation of Polymer Melt Contraction Flows for Validation of Numerical Simulations
Full field velocity measurements in planar contraction geometries are reported - these having been evaluated using the techniques of particle image and particle streak velocimetry for HPDE and LDPE melt flows. Measurements were made in a specially designed flow cell that enables laser sheet lighting to be used to illuminate planes of the polymer at precise locations across the die, mounted on a Betol BK38 extruder. Stress measurements obtained by flow birefringence complement the velocity field information. Steady state and start up flow regimes were investigated with results presented in a form suitable for direct comparison with ongoing numerical simulations.
Combined Process Variables and Process Energy Monitoring for Injection Moulding
Good injection moulding machine control is a necessary requirement for control of the process, however there is an acknowledged lack of process understanding, related in turn to a lack of understanding of the polymer under process conditions which inhibits the development of standardised route to process control. In our laboratory, specific pressure indices in an identified low noise region of the primary injection stage of injection moulding have been found to provide a sensitive indicator of changes in a polymer, including batch to batch changes and process-induced changes, which in turn allows meaningful Statistical Process Control to be undertaken. Growing concern for environmental issues, including international standards agreements such as ISO14001, demonstrate a clear requirement to conserve energy for both environmental and cost issues. Detailed energy measurements on injection moulding machines both in the laboratory and in industry demonstrate the potential of process energy measurements as an aid to the development of a systematic management approach to the environmental concerns of an organization. Laboratory DOE studies allow a further insight into the influence of a variety of machine variable settings on the total energy consumption. We are currently in the process of combining both process variable and process energy measurements, to provide processors with the richest level of process information.
Development of Constitutive Equations for Solid Phase Deformation of Polymers with Time-Varying Temperature
The large deformation, nonlinear viscoelastic behaviour of polymers has been explored at elevated temperatures. Experiments consist of uniaxial tensile stress relaxation experiments. For isothermal experiments, the results can be represented using models consisting of Eyring processes and elastic networks. Experiments have also been carried out on specimens subject to controlled cooling, to simulate the development of the room- temperature mechanical properties of processed products. Progress in extending the Eyring-based theories to these conditions is reported.
Injection Moulding Machine Performance Inter-Comparisons
An extensive study has been carried out to compare the performance of four highly instrumented injection moulding machines, using the same polymer (HDPE) and tensile test specimen mould. A 50 tonne servo-electric machine, a 30 tonne servo-electric machine, a 75 tonne servo-hydraulic machine and a 60 tonne proportional- hydraulic machine were studied, to compare accuracy, repeatability and process control. In each case, nozzle melt pressure and temperature, tool temperature and Viscosity Index (a primary injection pressure integral) were monitored at 50Hz during each cycle. A Design of Experiments methodology together with long production runs were employed to compare machine performance at identical moulding conditions across the full process window. Comparable product quality was observed between the four machines but significant differences in process stability and control were highlighted by the in- process measurements.
Extrusion Temperature Field Measurement: A 'Soft Sensor' Approach
A range of temperature measurement techniques have been employed in single and twin screw extrusion in conjunction with CFD modelling in an attempt to build up an accurate picture of the temperature field across a flowing melt. A motorised traversing thermocouple provided a temperature profile at a range of extrusion conditions, with corrections made for conduction losses along its length. A non-intrusive infra red sensor was used to measure melt temperature several millimetres into the melt flow and non-intrusive high temperature ultrasonic sensors gave an indication of bulk temperature across the flow. A commercial 3D computational fluid dynamics package was used to model the melt flow and provide predicted thermal plots. A 'soft sensor approach', combining measurement routes and modelling, enables a detailed temperature field to be obtained in extrusion. This improving understanding is being used in conjunction with commercial 'DualTherm' barrel temperature control, with a view to enhanced control of polymer melt temperatures during extrusion.
Small Scale Flow Visualisation of Polymer Melts in a Recirculation Extruder
A novel small scale (< 30g of polymer) recirculation flow cell enables the study of the in-process behaviour of newly synthesised polymer melts. The system incorporates a single screw extruder, gear pump and variable geometry inserts. This paper extends our previous work by presenting results obtained using the techniques of stress birefringence and particle image velocimetry for a variety of polymeric materials including melts of LDPE, HDPE and PS, in a simple contraction flow. Process-induced change in a polymer is also observed. The new facility complements the larger extrusion studies performed in our laboratory, and comparisons between the two flow scales are also given.
Combined Infrared Spectroscopy and Ultrasound Studies of Changing Melt Composition during Single Screw Extrusion
In-Line ultrasound velocity measurements have been made simultaneously with on-line mid- and near- infrared spectroscopy during single screw extrusion of a range of blends of an HDPE and a PP, to assess the sensitivity of these techniques to blend composition. Ultrasound velocity was sampled at IHz, using in-house process monitoring technology; mid-infrared data was obtained at I 'sample' per minute (16 scans at 2cm-1) resolution). Under these conditions, it was found that the ultrasound technique could successfully resolve a change of l wt% PP in HDPE, whereas the infrared spectroscopy (using specific peak height change) could not resolve better than ~ 2wt%. The techniques showed very close agreement in monitoring the dynamics of change from one blend composition to another. The original blends were made by mixing pellets; twin-screw compounded blends of the PE/PP are being investigated, to assess any effects of blending process history.
Melt Flow Simulation and Measurement of Extensional Viscosity in Planar Hyperbolic Dies
An adaptation of the 'separable' KBKZ constitutive equation for polymer melts is presented. This permits prediction of both strain hardening and shear thinning (for a single parameter set), consistent with measurements made on branched polymers such as low density polyethylene (LDPE) melts. Simulations using a new finite-element based code are presented showing that the constitutive model can be used to predict both stress and vortex growth for planar as well as axisymmetric contraction flows. Flow visualisation has been used to study the flow behaviour of a LDPE melt as it is processed through a planar hyperbolic die on a commercial scale extruder. The die profile consists of a planar hyperbolic section that gradually merges with a parallel slit and is designed to promote constant extensional strain rates at the centre line of flow. Extensional strain rates were determined by performing particle velocimetry along the flow axis. Stress, strain and strain rate data for the low density polyethylene melt is presented, and compared to simulation; it is shown that good agreement and a constant extension rate is achieved along the centre-line.
Visualisation of Melt Interface in a Co-Extrusion Geometry
Past studies, principally confined to numerical simulations, have led to several theories being proposed to explain the source of interfacial instabilities that often occur during coextrusion of polymer melts. However the mechanisms, kinematics and rheological correlations leading to the onset of instability are not yet fully understood. Instability is thought to arise from one of two locations within a die. One location is that coincident with the point where an interface is created when the melt streams combine; the other region is a point just prior to the die exit region where the interface usually experiences a maximum shear stress. It is further suggested that interracial instability can develop when the layer thickness ratio of the melt streams exceeds a critical limit. This paper presents the results of flow visualization studies performed in order to gain a better understanding of the interracial instability phenomena of polyolefin melts within a flat film coextrusion die. Visualisation studies have been initially confined to melts of the same viscoelastic properties, using a new modular flow cell. The objective of the experiments is to determine the effects of geometry and processing on the source and nature of flow instabilities in coextrusion.
Gas Assisted Injection Moulding: 3D Finite Element Modelling Using a Pseudo Concentration Method and Experimental Studies
A 3D implementation of a pseudo-concentration method is used to simulate cavity filling in the Gas Assisted Injection Moulding process. The pseudo-concentration method represents two different fluids (i.e. gas and polymer) within the same domain. Linear quadrilateral finite elements form a fixed grid on which to generate the solution using a time stepping procedure. Simulation results are compared with specimens manufactured on an industrial Gas Assisted Injection Moulding machine. Reasonable agreement is observed between simulation and experiment. The pseudo-concentration implementation discussed here is capable of quantitative predictions of cavity filling. Simulation experiments are ongoing to determine the wall thickness relationship with temperature.
Sensitivity of Ultrasonic Velocity and Attenuation Measurements to Temperature Pressure and Filler Concentration for Magnesium Hydroxide Filled LDPE and HDPE
Samples of magnesium hydroxide filled Low density polyethylene (LDPE) melts were examined ultrasonically, over a range of temperatures and pressures, to determine the effect of melt temperature, pressure and filler concentration on ultrasonic velocity through the melt. Tests were carried out on static samples of melt under conditions of no flow. These data were used with extrusion processing data to predict filler concentration. It was found that for a unit change ultrasonic velocity was most sensitive to changes in temperature and least sensitive to changes in pressure. Signal attenuation was determined through samples of magnesium hydroxide filled HDPE over a range of temperatures and pressures.
A 'Bleed' on Line Rheometer for Polymer Melts
A process-driven on line rheometer has been designed and used to obtain at-process assessment of variation in polymer melt shear viscosity, in conjunction with statistical and dynamics studies of the extrusion process. No gear pumps are involved in this device: melt flow through a chosen die/dies is driven by the melt pressure in ,the extruder. The bleed rheometer mounts via a standard Dynisco pressure transducer fitting. Results for an LDPE and an LLDPE are presented, for differing die geometries and extrusion screw speeds. These compare well with on-line rheometry studies in our laboratory. The effects of extruder conditions on the results obtained from the bleed rheometer are discussed. It is clear that such a simple device could be used for process quality assessment.
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