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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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 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.
Statistical Process Control (SPC) is a powerful tool that can be extremely useful in assessing the performance of the extrusion process, provided it is applied properly. Histograms, normal probability plots and control charts have been used in this work to assess the operation of the extrusion process, as part of a fuller statistical and dynamic study of the extrusion process. It was observed that the extrusion process changed its behaviour with time in a way that affected statistical measures (mean and standard deviation) of key extrusion variables (melt pressure, screw speed, motor current, extrudate diameter). Such observed behaviour of the extrusion variables is categorised as statistically unstable" in classical SPC and control charts cannot be applied to the extrusion process variables directly. A treatment (a transform) is suggested for use with the extrusion variables which might show a stable behaviour allowing control charts to be used."
Recent work has shown that hyperbranched polymers have promise for use as processing aids for polyolefins and as toughening agents for thermosetting resins. This promise stems from the high reactivity and unique rheological properties of hyperbranched polymers, which are attributable for the unusual molecular structure. However, there are difficulties associated with the blending of these small, branched molecules into high molecular weight polymers, and processing can be challenging. In this study, we investigate the blending of hyperbranched polyester polyols (HBP) and high molecular weight polystyrene using batch and continuous processing techniques. The overall size, or generation, of the individual hyperbranched polyols is varied, as is the reactivity of the thermoplastic matrix toward to the polyol. Large reductions in system viscosity resulted from the addition of the hyperbranched polymers to the polystyrene. High energy processing and reactive compatibilization were effective in producing finely dispersed morphology in the blends. The processing characteristics, compatibility, and morphology of the blends are reported as a function of HBP generation, reactivity, type of processing, and shear rate.
Studies estimate there are two billion scrap tires in U.S. landfills with over 270 million tires added yearly. An overview of a simple technique for recycling thermosets will be discussed. In short, it is possible to recycle rubber powders made from scrap tires with the application of only heat and pressure and achieve good mechanical properties. An investigation of the mechanical properties of typical consolidated rubber powders as a function of the molding variables is be shown. To date every type of cross-linked elastomer investigated could be sintered, including silicone rubbers, natural rubbers, ethylene-propylene-diene rubbers, styrene-butadiene rubbers and fluoroleastomers.
Developing blown film resins for the production of films that exhibit the toughness properties of LLDPE with the stiffness and tensile characteristics of HDPE has historically been difficult to achieve for both resin producers and film converters. Resin producers have attempted to push the density higher for LLDPE film resins and lower for HDPE film resins. Separately, film producers are continuously searching for the same hybrid film through blending low, linear low, medium and high-density polyethylenes. Producing successful film blends for heavy-duty applications with densities over 0.918 g/cc usually results in a high performance LLDPE film resin blended with low levels of a MDPE or HDPE. As the level of MDPE or HDPE increases, loss of critical toughness properties such as machine direction tear (MD tear) and dart impact strength is observed. There now exists a novel method of producing films that possess a full density range between 0.918 g/cc and 0.938 g/cc. This approach is based on the utilization of a new family of lower density, high molecular weight medium density (HMW-MDPE) film resins. Blending a HMW-HDPE with a resin density of 0.938 g/cc with high performance LLDPE film resins, films can be fabricated over a wide range of densities without the loss of tear and impact properties. These blends can be produced at thin or thick gauge on either high stalk or conventional blown film equipment.
Resorcinol-Formaldehyde-Latex (RFL) adhesive and its modification were used to bond rubber to polyethylene terephthalate (PET) cord. The modification of RFL was done by adding the activating materials such as a chlorophenol condensate (DK) and chlororesorcinol condensate (CRA). The effect of heat treatment time, test temperature, aging, and cure conditions on adhesion were studied. Overcure conditions were used to simulate long service time. It was shown that adhesive strength decreases with increasing test temperature and deteriorates with overcure. Bond strength increases with increasing heat treatment time. In addition, adhesion increases with increasing ratio of DK or CR to RFL.
This paper presents the results of a study on the effect of cavity pressure transducer location on process consistency, when the transducer is used for control of the velocity to pressure transfer during the injection phase of the injection molding process. Specifically, a three-stage molding process, where a velocity phase is used to pack the part out slowly, was studied. This study looks at the weight and dimensional stability, when the process is subjected to material variation and check ring wear, of parts molded with the transducer near the gate versus near the last place to fill. It is found that, in some instances, the consistency of the parts can be improved by placing the controlling pressure transducer at the last place to fill in the mold cavity.
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ANTEC 2016 - Indianapolis, Indiana, USA May 23-25, 2016. [On-line].
Society of Plastics Engineers
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