Melt Flow Behavior Research Articles

Investigations of Non-Linear Viscoelastic Properties for Polypropylene/Clay-Nanocomposites through Melt Flow Birefringence and Damping Function

Rheological investigations are reported for pure polypropylene and its clay-nanocomposites to establish viscoelastic properties and filler concentration relationship. Flow birefringence is performed through a slit-die to obtain centerline principal stress difference during extensional flow. The centerline stress profile of clay-nanocomposite revealed additional viscoelastic nature even at low silicate concentrations whereas no exceptional strain hardening was reported. Effects of higher filler concentrations are further examined during the simple shearing flow to consider non-linear viscoelasticity in terms of damping function. The increase in damping coefficient with increasing clay concentration shows polymer-nanocomposites are more strain sensitive. The Wagner's exponential damping function could adequately describe the time-strain separability at all clay concentrations studied. The results of both investigations reveal that the polymers are time-strain separable at all clay concentrations studied during elongational and simple shearing flows, whereas filler orientations are found to be different for different melt flow behavior.

Measuring the degree of “nanotilization” of volcanic glasses: Understanding syn-eruptive processes recorded in melt inclusions

Iron and water content substantially affect the physical properties of natural silicate melts and may, therefore, influence magmatic and volcanic processes such as crystallization, degassing, flow behaviour and eruptive style. Here we present Raman spectroscopic data for a set of synthetic and natural multicomponent glasses with varying iron oxidation state and water content. We systematically study the effect of different Raman excitation sources on the spectral response of both crystal free and magnetite nanolite bearing glasses spanning basaltic to calc- and per-alkaline rhyolitic compositions. Based on these data we document changes in Raman spectra resulting from the formation of nano-scale crystals. We show that the peak located at ~970 cm−1 is directly related to the presence of Fe2O3 dissolved in the quenched melt structure and that this feature is present regardless of the chemical composition of the sample and the Raman excitation source. We further show that a peak between 670 and 690 cm−1, which is not related to any other spectral feature of the glass structure, reveals the presence of nanolites. Based on systematic spectral investigations of this feature, we present a new index that allows to identify if iron is present in the nanocrystalline state and/or bound in the glass structure. Since the melt structural and physical effects of the formation of nanolites can heavily affect the flow behaviour of melts and the eruptive style of volcanoes, the results presented in this study significantly broaden the application of Raman spectroscopy for investigations of nano-heterogeneity in synthetic and natural glasses.We apply this method to study both the degree of nanolitization as well as the H2O content and iron oxidation state of groundmass glasses as well as melt inclusions and glass embayments in explosive products from Pantelleria island (Italy). We observe that the process of nanotilization is not purely restricted to magnetite nanolites but that Raman spectroscopy may also identify the incipient crystalization of pyroxene and feldspar at sub-micron scale. The data document that nanolite formation correlates well with the observed intensity of the respective eruptions suggesting that structural changes in the melt, caused by incipient crystallization play an important role in defining the eruptive style of relatively low viscosity magmas.

Effect of binder melt flow on the leading edge flames of solid propellant sandwiches

This study reports the effect of binder melt flow on the burning behaviour, specifically the burning rate controlling sites referred to as the leading edge flames (LEFs), of different types of sandwich propellants, namely, pure, micro-aluminized and nano-aluminized binder. The distance between the LEFs anchored over the lamina interface edges of sandwiches is measured from the combustion images captured under high spatio-temporal resolution. Similarly, the extent of binder melt flow is also measured from the quenched surfaces of sandwiches. The burning rate experiments are performed as well on sandwiches with different middle lamina contents and thicknesses at pressures of 2, 4 and 7 MPa. Two different curing agents are considered to examine the melt flow behaviour of the binder. The curing agent significantly influences the inter-LEF distance mainly in the case of pure binder sandwiches, however, its effect is negligible in aluminized binder sandwiches because of the presence of Al particles that impedes the flow to appreciable extent. Substantial protrusion of the middle lamina relative to the lamina interfaces is observed in micro-aluminized binder sandwiches due to significant accumulation of Al particles on the burning surface. In the case of nano-aluminized binder sandwiches, such protrusion is relatively marginal since nano-Al particles burn quickly, which enables the gas phase flame to locate close to the burning surface, although the extent of Al accumulation is considerably more than in the former case. This causes the nano-aluminized binder sandwiches as a whole to burn significantly faster than the other two cases in the pressure range (<7 MPa) where the LEFs predominantly control the sandwich burning rates.

Study on keyhole and melt flow behaviors of laser welding of aluminum under reduced ambient pressures

Laser welding of aluminum alloy under reduced ambient pressure can produce less porosity defects. In this paper, the effect of reduced ambient pressures on porosity was investigated by a combination of experimental and numerical method. A novel numerical model was proposed to describe the keyhole and melt flow behaviors under reduced ambient pressures. Numerical results showed that, compared with atmospheric pressure, reduced ambient pressure would produce a more stable keyhole. The vortex of melt flow in molten pool became unapparent and even disappeared under reduced ambient pressures. The flow velocity of the melt on the keyhole wall became faster under reduced ambient pressures. The difference between boiling point and melting point decreased under reduced ambient pressure, which made a contribution to the formation of a thinner keyhole wall and hence improved the stability of keyhole. Higher recoil pressure would be produced under reduced ambient pressures, which was responsible for the weakened vortex and enhanced melt flow velocity. The formation of porosity during laser welding process could be effectively inhibited based on the above combined effects.

Effects of ultrasonic injection molding conditions on the plate processing characteristics of PMMA

Abstract Polymer processing is a crucial and diverse field in the manufacturing industry. We investigated the process characteristics and effects of injection molding using ultrasonic vibration. An ultrasonic device was installed in an injection mold; polymer was directly vibrated during injection. An ultrasonic oscillation device 45 mm in diameter was placed in the cavity and used to vibrate a poly(methyl methacrylate) melt at 19 kHz. The amplitude of the acoustic unit was set at 15 μm for the measurements. Moreover, cavity pressure sensors were positioned at the front and rear sides of the vibration region to determine the melt flow behavior under ultrasonic-assisted injection molding conditions. Because of the absorption of ultrasonic energy, local heat was generated inside the resin, thus improving the flow characteristics of the melt. Moreover, the melt flow behavior around the skin layer was changed; the molecular orientation and high shear effect were reduced. Furthermore, the freezing rate of the melt was reduced; thus, the amount of melt pressure lost through the cavity was decreased and the residual stress inside the injection-molded component generated during the photoelastic stress analysis was lower.

Using a Visualization Mold to Discuss the Influence of Gas Counter Pressure and Mold Temperature on the Fountain Flow Effect

Abstract The fountain flow effect in a mold cavity results in molecular orientation that is likely to create flow-induced residual stresses, warpage of finished products, and excessive shrinkage, thus making it difficult to guarantee high precision control. This study uses a gas counter pressure technique to inhibit fountain flow and employs a visualization mold design to observe the influence of counter pressure on melt flow behavior, in order to discuss the impact of the counter pressure mechanism on the fountain flow. The visualization mold designed herein and the clip cavity help to test the counter pressure mechanism in injection molding, while the observed particles and high-speed camera assist in observing the influence of fluid flow behavior and counter pressure on the fountain flow effect. The study observes and tracks the flow trajectory of particles in the melt, with findings showing that the closer the flow line of the melt is to the mold wall, the shorter the offset distance will be to the outward flip. Moreover, the closer to the center, the longer the offset distance of the outward flip meaning that it flips outwards in the melt-front nearby the center line and stays on the mold wall surface to form a new frozen layer. The melt-front length changes under different counter pressures and different mold temperatures. The front length changes present the inhibitory effect of counter pressure on the fountain flow, which is more apparent at the far gate than at the near gate. The melt-front lengths of the counter pressure of 0 bar at mold temperatures of 40 °C and 20 °C increase 1.5% and 4.7%, respectively, meaning that the thicker the frozen layer, the more apparent the fountain effect.

Melt Flow and Flexural Properties of Polypropylene Composites Reinforced with Graphene Nano-Platelets

Abstract The effects of graphene nano-platelets (GNPs) content on melt flow behavior and flexural properties of reinforced polypropylene (PP) composites were investigated. The results showed that both the flexural modulus and strength at room temperature increased when GNPs weight fraction was lower than 0.4 wt.%, and then decreased with increasing GNPs weight fraction. During the melt flow of the PP composites in capillary extrusion within temperatures ranging from 180 to 230 °C and in apparent shear rates varying from 100 to 4 000 s−1, the melt shear flow followed the power law relationship, and the dependence of the melt shear viscosity on temperature obeyed the Arrhenius equation. The correlation between the melt shear viscosity and GNPs weight fraction was approximately linear under the given test conditions.

Simulation of Micropelletization Mechanisms in Polymer Melt – Air Systems

Abstract This paper presents a series of simulation approaches to model the mechanisms associated with the flow and breakup of a polymer melt in a new micropelletization technique. This has proven to be an alternative way of producing micropellets and powders with physical properties demanded in polymer processes such as sintering, rotomolding, injection molding, and extrusion. The new technique involves extruding a polymer melt strand through a capillary and stretching it with a stream of hot air, causing the formation of Rayleigh disturbances that lead to breakup of the strand into small particles. Experimental work has demonstrated that the viscoelastic response of the extruded thread influences the breakup process, affecting the final shape and size distribution of the particles. This work presents simulation results for the melt flow behavior that reproduces what has been observed experimentally during the breakup stage. A free surface viscoelastic solver, implemented in the OpenFOAM computational fluid dynamics software, allows visualization of viscoelastic effects on the melt as it leaves the extrusion die and meets the stream of air. Modeling of the viscoelastic fluid flow in this process is intended to serve as a tool for process control and understanding of influential factors in the development of micropellets, for a range of materials and process conditions.

STUDY OF THE BEHAVIOR OF CURRENT-CARRYING LIQUID IN THE BATH OF DC ARC FURNACE BY AN EXTERNAL VERTICAL MAGNETIC FIELD ON A PHYSICAL MODEL

When an external vertical magnetic field is applied to a current-carrying melt in the bath of a DC arc furnace, volumetric electromagnetic forces arise that drive it. Flows of metal and slag occur in the bath of the DC arc furnace. This can lead to their efficient mixing, but can also carry negative aspects,for example, associated with an increase in wear oflining in the region of the bottom electrode. The processes of conductive mixing in the bath of the DC arc furnace during smelting of metals and alloys remain poorly understood, there arise both theoretical questions related to the nature of the flows in the bath under the action of external and intrinsic magnetic fields of a given intensity, and practical with the absence of simple and reliable sources of external magnetic fields. The purpose of this paper isto qualitatively test the capabilities of a physical model, to study the nature of currents of a current-carrying liquid under the action of an external vertical magnetic field, and to analyze the possibility of transferring simulation results to processes occurring in a five-ton bath of an industrial DC arc furnace. The principal possibility of studying the current flow behavior of a current-carrying melt under the influence of external magnetic fields on models using non-metallic electrically conducting transparent liquids is shown. In this work the authors have used a water solution of table salt, it allowed to estimate the velocity of the liquid on itsfree surface and near the bottom electrode using video. The physical modeling of the effect of an external vertical magnetic field on the current flowing current in a bath was carried out for different combinations of connecting the bottom electrode and different currents flowing through the bath. The current flowing current in the bath under the influence of an external vertical magnetic field was established when the central electrode or the bottom electrode is displaced from the axis of the bath. It was revealed that when the axis of the bottom electrode is displaced from the bath axis, the average rotation speed of the liquid in the horizontal plane increases. An estimate of the value of the vertical magnetic field strength (about 5 kA/m) is obtained, which should be accompanied by conductive mixing the metal bath of the five-ton steel-smelting arc furnace.

Fluid-structure interaction model to predict deformation of mold cores in injection molding filling stage

In the present study, a numerical model of the injection molding filling stage was developed by combining non-Newtonian behavior, heat transfer, and thermo-elastic behavior in order to precisely predict mold deformation. In general, local deformation of an injection mold can be caused by two critical factors – elastic compression induced by the plastic melt and thermal expansion due to rapid heat transfer from the plastic melt. As severe mold deformation lowers the dimensional accuracy of the molded product or results in failure of the injection mold, the accurate prediction of mold deformation is critical to the design and manufacture of precision injection mold. In this regard, a numerical model considering the relevant physical behavior was developed and applied to a center-gated disc model. Both the melt flow behavior and effect of heat transfer inside the mold cavity were investigated, which subsequently revealed that the dominant influence is that of thermal expansion due to heat transfer.

Effects of micro particle reinforcement on mechanical properties of 3D printed parts

PurposeThe purpose of this paper is to characterize the effects of different micro particle reinforcement with same weight ratio in acrylonitrile-butadiene-styrene (ABS) feed-stocks for 3D printing process.Design/methodology/approachIn this study, composite filaments were produced by using a co-rotational twin screw extruder and used as building material to print samples in a commercial fused deposition modeling (FDM) 3D printer. The reinforcement particles, ZrB2 and Al, have different properties, including density, surface area, purity and particle morphology, and were expected to improve mechanical properties of 3D printed samples. Differential calorimetry scanning and melt flow index studies were applied on the materials to observe the change in glass transition temperatures and melt flow behaviors, respectively. Also, to evaluate the mechanical properties, tensile and three-point bending test were carried out. Fractured surfaces were characterized via energy-dispersive X-ray spectroscopy for validation of the reinforcements in the ABS matrix. Moreover, scanning electron microscope micrograph examination was conducted on the fractured surfaces to characterize fracture modes.FindingsFor 3D printed samples, a strain increase of at least 82.5 per cent was achieved by using micro particle reinforcement with a weight ratio of 1.5 per cent.Research limitations/implicationsHigher filler ratios of the reinforcement particles cause loss on the printability of the feed-stocks.Practical implicationsReinforced ABS stands out as a possible solution to overcome robustness problems in FDM printing.Originality/valueEven though the effects of printing parameters on the mechanical properties of 3D printed parts have been vastly studied in the literature, studies conducted on improvement of the building materials are limited. This paper proposes to create novel feed-stock materials for achieving printed parts with superior properties using polymer composites.

Phase transition and anomalous rheological behaviour of polylactide/graphene nanocomposites

Polylactide (PLA) nanocomposites with graphene nanoplatelet (GNP) contents of 0–15 wt% were prepared and characterised in terms of rheology and electrical conductivity. As expected for rigid nanofillers, GNP incorporation significantly enhanced the viscoelastic properties of the nanocomposites. Furthermore, above a certain GNP concentration the liquid-like melt flow behaviour changed into a solid-like behaviour. However, this rheological percolation threshold was found to decrease from near 9 wt% at 180 °C to below 6 wt% at 220 °C, detected and measured by Van Gurp-Palmen plots and Winter-Chambon method, respectively. It was also found that the viscoelastic properties of some of the nanocomposites increased with increasing temperature, which is in contrast to the rheology of ideal polymeric melts. The Electrical percolation threshold of the PLA/GNP system, which is determined by a sudden increase in DC conductivity and is indicative of establishment of interconnected conductive structures of GNPs within the matrix, was observed to be between 6 and 9 wt% GNPs. Morphological studies also showed physical contact between the platelets at 9 wt% while at 6 wt% they were still separated by the PLA matrix. These observations challenge the attribution of rheological percolation threshold to the formation of a percolating network of only filler particles within the matrix. A percolating network consisting of both polymer chains and filler particles might be the reason for the temperature dependency of rheological percolation threshold in the PLA/GNP system.

Non-linear viscoelastic behavior and damping factor of polypropylene/clay nanocomposites

The non-linear viscoelastic properties of pure polypropylene and its clay nanocomposites are studied to establish structure–property relationship in conjunction with clay concentrations. First, flow birefringence is performed through a slit-die to obtain centerline principal stress difference during elongational flow for clay nanocomposites. The centerline stress profile of clay nanocomposite reveals additional viscoelastic nature even at low silicate concentrations, while similar short-time chain relaxation is observed. The effects of higher clay concentrations are further examined during the simple shearing flow to consider damping properties of the clay nanocomposites. The step strain, dynamic shear and steady shear are performed. All the samples show time-strain separable melt flow behavior adequately demonstrated through Wagner’s exponential damping function. The damping coefficient is found to be strongly dependent on clay percentage revealing viscoelastic differences therein. We have also used a time-strain separable Kaye-Bernstein Kearsley Zapas (K-BKZ) type constitutive equation to predict steady shear stress. The suggested constitutive model satisfies simple shear at lower fractions of clay while the damping function behaves similar to pure polymer thought to result from the absence of filler–filler interactions and chain length degradation. The unusual rheological behavior for maximum clay concentration studied is explained on experimental as well as theoretical basis. Thus, the results of this investigation would improve the theoretical understanding of possible molecular orientations at different clay concentrations during elongational and shearing flows.

Optical Rheology of a Polydimethylsiloxane Fluid in Jeffrey-Hamel Type Flow

The rheo-optical behavior of a viscoelastic polydimethylsiloxane (PDMS) fluid was examined at room temperature for various pressure drops (flow rates) across a Jeffrey Hamel type converging wedge flow cell. The strain-rates were computed from local velocity measurements using laser Doppler anemometry (LDA), and the planar extensional flow behavior of the polymer melt was studied via birefringence measurements along the centerline of the flow cell. The linear relation between the stress and polarizability tensors were confirmed over a range of strain rates that extended well into the non-Newtonian region. The first normal stress difference (FNSD) was calculated from the local velocity measurements using a two-term Goddard-Miller model with a single Maxwell-type relaxation time constant of 0.0174 s computed from Rouse model and a zero-shear viscosity of 300 Pa.s. A linear stress-optical coefficient of 1.41 x 10-10 Pa-1 was obtained for PDMS in planar extensional flow at room temperature from the flow birefringence measurements and the first normal stress difference (FNSD) computed using the Goddard-Miller model. This compares well with values for PDMS in the range of 0.909 – 1.84 x 10-10 Pa-1 at room temperature as reported by various researchers.

Smoothed particle hydrodynamics simulation for injection molding flow of short fiber-reinforced polymer composites

The injection molding process of short fiber-reinforced polymer composites was investigated using smoothed particle hydrodynamics method. The polymer melt was modeled as a power law fluid, and the fibers were considered as rigid bodies. The flow behavior of short fiber-reinforced polymer composite melt and the motion and orientation of fibers were studied. The results showed that U-shaped fountain flow was generated at the flow front, and the center of the flow front gradually sank during the injection molding process; fibers were aligned to the flow direction in the cavity and near the wall in the sprue, and fibers accumulated at some points in the cavity. Additionally, the initial fiber configuration in the model influenced the final fiber orientation slightly. It was also found that the fiber orientation factor increased with the increase of fiber aspect ratio, and decreased with the increase of fiber content.

Improving additive manufacturing processability of hard-to-process overhanging structure by selective laser melting

In this paper, a selective laser melting (SLM) physical model describing the melt pool dynamics and the response of downward-facing surface morphology evolution of overhanging structure under different laser processing conditions was proposed, in which an enormous difference in thermal conductivity and laser absorption capacity between the as-fabricated part and powder material was taken into consideration. The underlying thermal physical mechanism of the dross formation phenomenon during SLM preparing overhanging surface was revealed by numerical simulation analysis and experimental studies. It was found that both high and low laser volume energy density (ω) resulted in an inferior downward-facing surface quality. As an optimal processing parameter (60–80J/mm3) was settled, the overhanging structure obtained a relatively smooth downward-facing surface due to the sound melt pool dimension and steady melt flow behavior. The experimental studies were compared with the simulated results, showing a good agreement with the predictions obtained in the simulations. It was interesting to find that the variation rules of surface quality and densification level of overhanging structure with different ω were exactly converse. As the ω decreased from 80J/mm3 to 60J/mm3, the surface roughness could be reduced from 59μm to 33μm while, contrarily, the porosity was elevated from 3.2% to 8.4%. In order to fabricate complicated metal parts with lower risk, four solutions for improving the processability of hard-to-process overhanging structure were provided.

The Effects of a Submerged Entry Nozzle on Flow and Initial Solidification in a Continuous Casting Bloom Mold with Electromagnetic Stirring

The melt flow, level fluctuation, temperature field, and solidification behavior coupled with electromagnetic stirring (EMS) effects in the continuous casting mold region of U71Mn steel bloom were numerically analyzed by commercial computational fluid dynamics (CFD) software named ANSYS FLUENT. The effects of submerged entry nozzle (SEN) structures and the installation methods for optimized four-port SEN on the flow pattern, level fluctuation, heat transfer and initial solidification behavior in a bloom mold loaded with EMS were investigated. The aim is to propose a better SEN condition for the big bloom casting of high railway steel. The water simulation experiments were conducted to show the flow characteristics under different SEN conditions and verify the numerical model of flow pattern. The experimental and numerical simulation results showed that the optimized four-port SEN with diagonal installation cannot only improve the flow pattern of the molten steel by alleviating the level fluctuation and reducing the impact pressure to the wall. It is also beneficial for temperature variation at both bloom surface and corner, as well as the local solidified shell thinning phenomena due to the elimination of impingement effect.

Numerical visualization and optimization on the core penetration in multi-cavity co-injection molding with a bifurcation runner structure

Co-injection molding and multi-cavity molding are common processes for plastic products manufacturing. These two systems are sometimes combined and applied in the manufacture of bifurcation-structure products. However, how the influential factors truly affect the core penetration behavior and the detailed mechanism of core penetration behavior has not yet been fully understood. In this study, it has focused on studying the multi-cavity co-injection system with a bifurcation runner structure. The results showed that when the skin-to-core ratio is fixed (say 72/28), the melt flow behavior of a co-injection system, utilizing the same material for both skin and core, is very similar to that of a single shot injection molding. Specifically, the non-symmetrical bifurcation runner structure will influence the flow behavior greatly and cause the core distribution imbalance between different cavities. However, it is observed that when the flow rate is increased, the core material will occupy more volume space in the upstream portion of the runner and the core penetration distance will be reduced in the flow direction downstream. This feature is very useful to further manipulate the skin/core interface in a multi-cavity system. Moreover, regarding how to improve a poor inter-cavity balance of core material distribution, using a suitable adjustment of the skin-to-core ratio will be greatly helpful. However, the core break-through defect can be a common problem in co-injection molding when an unsuitable skin-to-core ratio is used. To prevent the core break-through defect, increasing the flow rate properly can be one of the good options that we can use. Hence, it is concluded that a suitable adjustment of the skin-to-core ratio and a proper flow rate control can be used to optimize the core material distribution in multi-cavity co-injection molding with a bifurcation runner structure. Lastly, in order to validate the inference and the effectiveness of this proposal to improve the inter-cavity imbalance and core break-through problem, a series of experimental studies were performed. And, all experimental results are in good agreement with those of our numerical predictions to further validate the feasibility of our proposed method to gain a better control of the core material distribution with a bifurcation runner structure in multi-cavity co-injection molding.

Effect of gas rate and position of powder injection on RH refining process using numerical simulation

In this study, the gas–liquid two phase flow behavior in the vacuum vessel during the Rheinstahl-Heraeus (RH) refining process was investigated to improve the refining effect. Special attention is paid to investigate double powder injection lances system which is used to improve the refining efficiency during RH refining process. The melt flow behavior, circulation flow rate and mean residence time of the tracers with different powder injection gas flow rates and different configurations of the double powder injection lances are simulated using three-dimension numerical model based on Eulerian-Eulerian approach. The results show that, with the increasing gas flow rate of the power injection, the circulating flow rate and the mean residence time of tracer increases. It is also noted that the mean residence time of tracer varies with the different positions of the powder injection lances. It is found that there is an optimum location of the positions of the powder injection lances corresponding to the increasing of the residence time of powder and can improve the refining effect.

Melt Filling Behaviors and Primary Si Particle Distribution on Horizontal Centrifugal Casting in B390 Aluminum Alloy

Both computational and experimental methods were employed to study the behavior of melt flow and distribution of primary Si particles of B390 aluminum. The rotational frequency of the mold has a dominant effect on the melt flow and distribution of primary Si particles. As the rotational frequency of the mold increased, the speed of melt flow increased, leading to a turbulent flow. The numerical estimation of 1200 rpm rotational frequency for the homogeneous distribution of primary Si particles across the wall of the cylinder showed good consistency with the experiment results. As the rotational frequency of the mold increased, the functionally graded materials (FGM) microstructure of Si particles was more evident toward the inner side of the cylinder.