Polymer Melt Flow Research Articles

Bottom-up self-organizing dissipative polymer nanostructures

Based on scaling concepts a methodology of research of non-equilibrium polymer systems has been elaborated. Polymers with flexible chains (melt-crystallized linear high density polyethylene is chosen as an example) are solutions in melt as well as in solid state, the ends of a chain serving as a solvent for it. At critical polymerization degree all phases (melt, solid isotropic or oriented state) are identical. The square of the neck draw ratio is equal to the product of the square of the draw ratio at break and the chain ends collision probability. This probability in its turn is proportional to the average thickness of amorphous layers in the isotropic material. Depending on the type of polymer statistics (Gauss or Lévy–Khinchin) and the number of components of an ordering field for the second case, the melt viscosity and the self-diffusion coefficient vs. the molecular weight of linear flexible-chain polymer follow the power laws with the 3.50, 3.41 or 3.33 and −2.50, −2.05 or −1.65 exponents, respectively, within the reptation model near the critical point. Vibrational–rotational Brownian motions of chain ends about the polymer melt flow direction were taken into account to find better agreement with the experiment. The recent experimental results of dynamic mechanic and dielectric spectroscopy show the value 3.5±0.1 for the viscosity exponent of long chains, while NMR data result in −2.3±0.1 for the self-diffusion coefficient exponent of short chains. Possible reasons are discussed.

Fatigue life estimation of injection mold core using simulation-based approach

The purpose of this study is to present a simulation-based approach capable of predicting the fatigue life of a mold core with the help of CAE technology based on injection molding simulation, core stress analysis and fatigue life prediction. Injection molding simulation is performed in order to obtain the non-uniform pressure acting on a mold core due to the polymer melt flow during injection molding process. Subsequently, the melt pressure profile is used as loading condition for core stress analysis via one-way coupling technique. The lifetime of a mold core is then predicted by fatigue analysis with stress-life approach. For verification of simulation-based approach to the fatigue life prediction, tests were also performed.

A correlation between the variable melt flow index and the molecular mass distribution of virgin and recycled polypropylene used in the manufacturing of battery cases

This study looked at establishing a correlation between the variable melt flow index (VMFI) values of molten polypropylene (PP) with different piston-load masses using a standard MFI analyser. The study was done using virgin PP and recycled PP obtained from recycling Pb-acid batteries. The study showed that the results would fit a suitable power function equation where the size of the exponent reflects the increase in flow characteristics of the polymer with increased piston-load mass. The established correlation was then compared to the average molecular weight distribution of virgin PP determined by gel permeation chromatography (GPC). Good agreement was obtained for the range of grades of virgin PP samples that correlated well with the Mark-Houwink power law where the inverse of the MFI (1/MFI) would be proportional to the average molecular weight to the power of 3.4 (Mw3.4). GPC analysis cannot be effectively used to study recycled PP, where a number of factors can influence the melt flow properties such as fillers, impurities and the presence of polyethylene in the polymer matrix. Instead, a comparative understanding of the flow behaviour of recycled PP to that of virgin PP was done by using the VMFI method to possibly show the dissimilar polymer melt flow behaviour of using virgin and recycled PP material in injection moulding of new battery cases and lids, or when attempting to seal the lid to the battery case during manufacturing.

Effect of Nano-Particles on Flow and Recovery of Polymer Nano-Composites in the Melt State

Abstract The effect of nano-particle geometry on flow and recovery of polymer melts based on ethylene vinyl acetate (EVA) was investigated. Two nano-particles, calcium carbonate (CaCO3) and montmorillonite clay, were used with concentrations between 2.5 and 15% by weight. First, by using small amplitude oscillatory shear and transient step shear tests, the linear and non-linear response of the nano-composites was studied. Then, to examine the structure recovery, the same behavior was studied on pre-sheared samples. The linear and non-linear data revealed that the effect of nano-particles in material flow is more important for anisometric particles which can be attributed to their ability to form a fractal structure controlling the rheological properties while spherical particles are not able to form such networks. Recovery experiments revealed that while nano-composites containing spherical particles have relatively similar response in subsequent tests, the rheological properties decreased significantly for platelet particles. The recovery results were also interpreted based on a filler-network mechanism and the reduction of the nano effect for anisometric particles was related to the breakdown of the filler network. These observations were validated by morphological investigations.

Design of complex profile extrusion dies through numerical modeling

The achievement of a balanced flow is one of the major tasks encompassed in the design of profile extrusion dies [1]. For this purpose numerical modeling codes may be a very useful aid. The research team involved in this work has been working during the last decade on the development of numerical tools to aid the conception of extrusion dies [1,2]. The design code developed so far carries out the automatic search of a final geometry via an optimization routine coupled with geometry and mesh generators and a 3D computational fluid dynamics (CFD) code based on the finite volume method (FVM). This CFD code is able to model the flow of polymer melts in confined channels, but is inadequate to deal with complex geometries, since it is limited to structured meshes. This work describes the recent efforts made to enlarge the scope of the design procedures, that are currently focused on the development of a modeling code able to deal with unstructured meshes. This code solves the continuity and linear momentum conservation equations, with generalized Newtonian fluids, using a SIMPLE based approach. This paper describes the developed numerical modelling code and its employment in a case study that involves the design of a medical catheter extrusion die, focused on the search of a balanced flow distribution. The results obtained show that the developed numerical code is able to deal with complex geometrical problems, being thus a valuable tool to aid the design of extrusion dies to produce complex profiles.

A numerical study of solidification and viscous dissipation effects on polymer melt flow in plane channels

Abstract The combined effects of solidification and viscous dissipation on the hydrodynamic and thermal behavior of polymer melt flow during the injection process in a straight plane channel of constant cross section are numerically studied by considering the shear-rate and temperature-dependent viscosity and transient-phase change behavior. A numerical finite volume method, in conjunction with a modified form of the Cross constitutive equation to account for shear rate, temperature-dependent viscosity changes and a slightly modified form of the method proposed by Voller and Prakash to account for solidification of the liquid phase, is used and a validation with an analytical solution is presented for viscous heating effects. The hydrodynamic and solidified layers growth under the influence of a transient phase-change process and viscous dissipation, are analyzed for a commercial polymer melt flow, polypropylene (PP) for different parametric conditions namely, inflow velocity, polymer injection (inflow) temperature, the channel wall temperature, and the channel height. The results demonstrate that the proposed numerical formulations, including conjugate effects of viscous heating and transient-solidification on the present thermal transport process, can provide an accurate and realistic representation of polymer melt flow behavior during the injection molding process in plane channels with less simplifying assumptions.

Assessing the practical utility of the hole-pressure method for the in-line rheological characterization of polymer melts

The most promising method capable of providing accurate measurements of the first and second normal-stress differences in shear flows at shear rates typical of polymer processing is the so-called hole-pressure method, but its use has not been as widespread as would be expected, namely due to the experimental difficulties associated with performing such experiments accurately. In this work, we use a small-scale modular slit die to assess the practical utility of the method for in-line monitoring of polymer melt flow. We provide a quantitative analysis of intrinsic error sources and use state-of-the-art data acquisition tools to minimize errors associated with pressure transducers. Our results demonstrate that the method can be used to accurately measure the viscosity and first normal-stress difference in melts but probably not the second normal-stress difference because the intrinsic errors are too high, even when the influence of all the potential error sources is minimized or eliminated.

Effects of agglomeration and pressing process on the properties of flat pressed WPC panels

This study discusses the influence of agglomeration process, press type and press parameters, on the physical and mechanical properties (water absorption (WA), thickness swelling (TS), internal bond strength, module of elasticity, and module of rupture) of flat pressed wood–plastic composite panels. Additionally, the effects of coupling agent and polymer's melt flow rate on panel properties are investigated. The compounding of the raw materials is done using a heating cooling mixer and a die ring agglomerator (DRA). Test panels are made using a laboratory scale single-daylight press, an industrial single-daylight press, and an industrial scale continuous double belt press (DBP). The best results were obtained using the DRA and the continuous DBP, with 24 h WA and TS values of below 1%, and a modulus of elasticity of more than 4500 N mm−2. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Computational Modeling of the Effects of Viscous Dissipation on Polymer Melt Flow Behavior During Injection Molding Process in Plane Channels

The present finite volume method based fluid flow solutions investigate the boundary-layer flow and heat transfer characteristics of polymer melt flow in a rectangular plane channel in the presence of the effect of viscous dissipation and heat transfer by considering the viscosity and density variations in the flow. For different inflow velocity boundary conditions and the injection polymer melt temperatures, the viscous dissipation effects on the velocity and temperature distributions are studied extensively to analyze the degree of interactions of thermal flow field dominated by the viscous heating and momentum diffusion mechanism with varying boundary conditions. The modified forms of Cross constitutive equation and Tait equation of state are adopted for the representation of viscosity variations and density change, respectively, in the polymer melt flow. These models together with the viscous dissipation terms are successfully incorporated into the finite volume method based fluid flow solutions to realistically represent the heat effects in the plane channel. The numerical results presented for two different commercial polymer melt flows, namely, polymer Polyacetal POM-M90-44 and polypropylene (PP), demonstrate that proposed mathematical formulations for viscosity and density variations including viscous heating terms into the energy equations, which are fully coupled with momentum equations, lead to more accurate representation of the fluid flow and heat transfer phenomena for the polymer melt flows in plane channels.

Thermal stabilities of drops of burning thermoplastics under the UL 94 vertical test conditions

The properties of polymer melts will strongly affect the fire hazard of the pool induced by polymer melt flow. In this study the thermal stabilities of eight thermoplastic polymers as well as their melting drops generated under the UL 94 vertical burning test conditions were investigated by thermogravimetric experiments. It was found that the kinetic compensation effect existed for the decomposition reactions of the polymers and their drops. For polymethylmethacrylate (PMMA), high impact polystyrene (HIPS), poly(acrylonitrile-butadiene-styrene) (ABS), polyamide 6 (PA6), polypropylene (PP) and low density polyethylene (LDPE), the onset decomposition temperature and the two decomposition kinetic parameters (the pre-exponential factor and the activation energy) of the drop were less than those of the polymer. However, the onset decomposition temperature and the two kinetic parameters of PC's drop were greater than those of polycarbonate (PC). Interestingly, for polyethylenevinylacetate (EVA18) the drop hardly contained the vinyl acetate chain segments. Similarly, for the PMMA/LDPE blends and the PMMA/PP blends, when the volume fraction of PMMA was less than 50% the drop hardly contained PMMA, implying that the blend would not drip until PMMA burned away and its surface temperature approached the decomposition temperature of the continuous phase composed of LDPE or PP.

Flow-induced nucleation in polymer melts: a study of the GO model for pure and bimodal blends, under shear and extensional flow

We present a rapid rescaling algorithm that enables a systematic comparison between the Graham and Olmsted (GO) model for flow-induced nucleation of polymer melts (Graham and Olmsted, Phys Rev Lett 103(11): 115702, 2009) and direct nucleation rate measurements from a flowing polymer melt. We consider polymer melts consisting of pure long chains and bimodal blends of long and short chains. We simulate the nucleation rate for a wide range of free energy barriers under a wide range of applied shear and extensional flows by using an accelerated nucleation algorithm. We then develop a semi-analytical technique to compute efficiently the nucleation rate under flow for monodisperse melts. We extend our approach to bimodal blends using a method to rescale reference data. This allows us to compare the GO model to experimentally measured nucleation rates at several different temperatures. The GO model is able to consistently account for the effect of temperature on flow-induced nucleation. Our modelling will also contribute to the derivation of computationally inexpensive molecular models of flow-induced nucleation in polymers.

Analysis of steady non-isothermal flow of polymer melts in a single-screw extruder

It is established that in addition to forced flow and counter flow of the melt during reworking of polymer materials, a flow generated by temperature variation of the melt over the height of the screw channel exists in a single-screw extruder.

Non-Newtonian behavior of polydisperse polymer melts as a consequence of the evolution of their relaxation spectra

The non-Newtonian flow of polydisperse polymer melts is shown to be described by a model according to which an increase in the shear rate leads to the suppression of the dissipative losses of the relaxation modes of each fraction. The higher the shear rate, the greater the suppression. The relaxation spectrum of each monodisperse fraction is represented by the Rouse distribution, and only this form of spectrum leads to a “spurt” effect at the critical shear stress. Hence, the physical content of the model that relates the non-Newtonian behavior of polymer melts to their molecular-mass distributions consists in the fact that the relaxation modes responsible for energy dissipation are gradually truncated from the side of high relaxation times. The higher the M of a given fraction, the greater the contribution of this part of the spectrum to the total viscous losses. In this case, the truncation of the spectrum from the side of high relaxation times is equivalent to the gradual “elimination” of high-molecular-mass fractions of the polydisperse polymer from the contribution to dissipation. The shear-rate-dependent evolution of the relaxation spectrum of the medium is the structural mechanism that causes the non-Newtonian flow of polymer melts. The efficiency of the proposed model is shown through calculation of the flow curves for polymers with known molecular-mass distributions. The calculation results are in agreement with the experimental data. The theoretical ideas developed with the use of the γ function to describe molecular-mass distributions have made it possible to solve the inverse problem, i.e., to establish a quantitative relationship between the shape of the flow curve and the molecular-mass distribution and, thus, to calculate the molecular-mass distributions according to the shearrate dependence of the apparent viscosity.

Constant force elongational flow of polymer melts: Experiment and modelling

Characterizing elongational behavior of polymer melts in constant elongation rate or constant tensile stress experiments is hampered by the onset of “necking” instabilities according to the Considère criterion: For an elastic filament, homogeneous uniaxial extension is not guaranteed for strains larger than the strain at which a maximum occurs in the force versus extension curve. Although simulations and experiments seem to indicate that in viscoelastic fluids viscosity effects delay the onset of necking to higher strains, integral measurements of elongational viscosities in Meissner- or Münstedt-type elongational rheometers are often affected by inhomogeneous deformations. A simple means of avoiding the consequences of the Considère criterion consists in operating at constant tensile force, but little attention has been paid so far to constant force elongational rheometry since the pioneering work of Raible et al. [J. Non-Newtonian Fluid Mech. 11, 239 (1982)]. Constant force elongation is also of great industrial relevance, since it is the correct analog of steady fiber spinning, as, e.g., exemplified by the so-called Rheotens test. We present experimental data for two long-chain branched polyethylene melts obtained in constant elongation rate and constant tensile force modes by use of a Sentmanat extensional rheometer in combination with an Anton Paar MCR301 rotational rheometer. The accessible experimental window and experimental limitations are discussed. Constant force elongation can be subdivided into three distinct deformation regimes: At small deformations (regime 1), constant force elongation is equivalent to creep at constant tensile stress. This is followed by regime 2, which is characterized by a steep increase in tensile stress at roughly constant strain rate, while regime 3 corresponds to elongation of a viscous power-law fluid. All three regimes can be modelled quantitatively by a single integral constitutive equation, and a constitutive analysis reveals substantial underestimation of the effective strain-hardening effect observed in constant strain-rate elongation in comparison to constant tensile force extension, which may be indicative of the effect of necking under constant elongation-rate conditions.

Study on viscosity of polymer melt flowing through microchannels considering the wall‐slip effect

AbstractFor polymers with long, complicated, branched chains, it is difficult to measure the real shear viscosity and slip velocity, using the capillary rheometer based on the adsorption–desorption mechanism. In this study, a double‐barrel capillary rheometer was used to investigate the viscosities of four polymers including polypropylene, high‐density polyethylene, polystyrene, and polymethylmethacrylate in a microchannel. A general model of polymer viscosity based on the entanglement–disentanglement was presented. The proposed model is important in understanding the mechanism of wall slip. This general model can be transferred to the other different models when changing the parameters. Actually, the entanglement–disentanglement model can also be transformed to the adsorption–desorption model. Using the model, it was found that the viscosities of polystyrene and polymethylmethacrylate were reduced with decreasing die diameter, and the slip velocities were increased with the increase of shear stress which agrees well with polymer microrheology based on the microscale effect. For polymers with long, complicated, branched chains, the proposed model improves the accuracy of the calculated viscosity and gains the real slip velocity when polymer melt flows through a microchannel. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers

Characterization of thermo-rheological behavior of polymer melts during the micro injection moulding process

In-line process monitoring and rheological characterization can help to understand the behaviour of polymer melt flows during manufacture and to make injection moulding a measurable process for manufacturing high quality parts. This work developed an in-line rheology measurement system using a slit die attached to a micro injection moulding machine. A series of dumbbell mould inserts was used to form the slit die with thickness ranging from 600 μm to 200 μm. Two combined pressure and temperature sensors were embedded into the slit die to measure the pressure drop. Based on the slit flow model, it was found that the viscosity of Pebax melts depends on the slit thickness in the actual injection moulding process. The competing effects of wall slip and non-isothermal conditions will determine the melt viscosity. The plastication induced thermo-mechanical history can also influence polymer viscosity, although it is neglected in conventional rheology measurements.

Thermal optimisation of polymer extrusion using in-process monitoring techniques

Polymer extrusion is an energy intensive process, which is often run at less than optimal conditions. The extrusion process consists of gradual melting of solid polymer by thermal conduction and viscous shearing between a rotating screw and a barrel; as such it is highly dependent upon the frictional, thermal and rheological properties of the polymer. Extruder screw geometry and extrusion variables should ideally be tailored to suit the properties of individual polymers, but in practice this is rarely achieved due to the lack of understanding of the process. Here, in-process monitoring techniques have been used to characterise the thermal dynamics of the extrusion process. Novel thermocouple grid sensors have been used to measure melt temperature fields within flowing polymer melts at the entrance to an extruder die in conjunction with infra-red thermometers and real-time quantification of energy consumption. A commercial grade of polyethylene has been examined using three extruder screw geometries at different extrusion operating conditions to understand the process efficiency. Extruder screw geometry, screw rotation speed and set temperature were found to have a significant effect on the thermal homogeneity of the melt and process energy consumed.

A single‐mode rheological model for steady flows of polymer melts

AbstractThe steady shear viscosity (ηs), the steady first normal stress coefficient (Ψ1), the steady second normal stress coefficient (Ψ2), and extensional viscosity (ηe) are four important parameters for polymer melts during polymer processing. In this article, we propose a stress and rate‐dependent function to describe creation and destruction of polymer junctions. Moreover, we also introduce a movement expression to describe nonaffine movement of network junctions. Based on network theory, a nonaffine single‐mode rheological model is presented for the steady flow of polymeric melts, and the equations of ηs, Ψ1, Ψ2, and ηe are derived from the model accordingly. Furthermore the dependences of ηs and ηe on model parameters are discussed for the model. Without a complex statistical simulation, the single‐mode model with four parameters yields good quantitative predictions of the steady shear and extensional flows for two low density polyethylene melts reported from previous literature in very wide range of deformation rates. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Numerical Simulation of Mixing Performance of Intermeshing Co-Rotating Tri-Screw and Twin-Screw Extruders

Parallel arranged tri-screw extruder (PATSE) is a new machine of polymer processing and first manufactured in recent years in China. Compared with the traditional twin-screw extruder, PATSE adds a screw, and added an intermeshing region. It is well known that material going though intermeshing region will acquire higher shear rate and stretching rate, which is beneficial to mixing processing. In order to know the mixing performance in cross-section for PATSE, polymer melt flow field simulation and mixing simulation were conducted on PATSE with 2D model and a Carreau flow model to evaluate velocity profiles, particle trajectories, max shear rate, max stretching rate, dispersive mixing, distributive mixing, segregation scale, length of stretch, mixing efficiency with the commercial CFD package Polyflow and compared with those of twin screw extruder (TSE). The results show that PATSE has better mixing performance than TSE.

Effects of raw materials and process parameters on the physical and mechanical properties of flat pressed WPC panels

For the production of WPC, the flat pressing technology can be considered as an alternative to common techniques, particularly when manufacturing large-dimensioned panels. The aim of the present study was to identify and analyze the main influencing parameters on sheets made of wood flour (WF) imbedded in a polypropylene matrix. Test panels were made to measure the effects of panel density, polymer melt flow rate (MFR), WF content, coupling agent, WF size and press temperature on water absorption (WA), thickness swelling (TS), internal bond strength, MOE and MOR. Density, WF content and MFR were identified as main influencing parameters. The increase of density typically leads to an improvement of properties. Lower WF contents cause reduced WA and TS, while the MOE seems to have a maximum at a WF level of 50–70%, depending on raw material used. As an explanation for improving properties when using a high-MFR polymer, a better distribution on the wood surface is hypothesized.