Rheological Behavior Of Polymer Melts Research Articles

Rheological characterization of complex fluids through a table-top 3D printer

3D printing is changing the way we conceive, design, and build 3D objects in mechanical, biomedical, aerospace, construction, automotive and maritime industries. In the current work, the nonlinear rheological behaviour of polymer melts is measured through a table-top 3D printer (3D RheoPrinter) that, smartly modified, allows inline investigation of viscosity, extrudate swell and melt fracture. By using a piezoresistive mini-transducer, the innovative system is designed to be applicable to all Fused Deposition Modelling (FDM) 3D printers by a simple and cost-effective modification of a state-of-art nozzle. The measurements of the nonlinear rheological behaviour are compared with traditional, rotational rheology. Two biodegradable polymers, i.e. polylactic acid and polycaprolactone, are investigated as model systems to test the 3D RheoPrinter. The results of the shear viscosity and the first normal stress difference coefficient, as function of shear rate, show a good agreement between the 3D RheoPrinter and rotational rheometer with an error of about 6% for a confidence interval of 96%. Moreover, the 3D RheoPrinter can still be used as 3D printer. In the last part of this work, it is presented a printing test for building 3D structures in which the results show controllable resolution by means of the measured rheological information such as the extrudate swell. The vision of this work is that an inline rheological characterization, possible with the developed 3D RheoPrinter, can enable automatic process optimization and quality assurance to the 3D printing community. The social and scientific impacts of this work are maximized by the cost-efficiency and simplicity of the design that makes it within reach of the general public. The 3D RheoPrinter opens for a rheological experimentation to a broad audience and it offers important insights to bring FDM to the next level of resolution.

Melts of Linear Polymers in Fast Flows

We report on recent progress in our understanding of the rheological behavior of polymer melts, resulting from the work of several investigators, mostly on fast extensional flows. They contributed ...

Determination of the effect of pressure on viscosity at high shear rates by using an injection molding machine

ABSTRACTThe effect of pressure on rheological behavior of polymer melts is surely a significant phenomenon for polymer processing, and its determination has been the object of several studies which highlighted the difficulties in performing accurate measurements. Even more important is the determination of this effect at very high shear rates, which cannot easily be obtained by conventional rheometers. In this work, a slit rheometer was located at the nozzle of the injection molding machine to obtain rheological measurements by means of two pressure transducers. Pressure values were analyzed by an original method to evaluate the pressure coefficient and the viscosity at zero pressure. The zero pressure viscosity aligned with independent rheological measurements obtained in previous works. The effect of pressure on viscosity was described by means of a parameter whose values resulted to be in line with those obtained in previous works, although the highest shear rates explored in this work were at least one order of magnitude larger. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 45277.

Flow properties of polymer melt in longitudinal ultrasonic-assisted microinjection molding

Microplastic parts are usually fabricated by micro-injection molding (µIM) which is an effective and low cost method. But the defects, such as short shot, often appear during fabricating plastic parts with high aspect ratio structures or complex shapes. a longitudinal ultrasonic-assisted micro-injection molding (LUµIM) method effectively improve the molding quality. In the paper, the mechanism that the ultrasonic vibration impacts on the polymer melt is investigated. Considering from the point view of energy effect, mechanical energy transmission, and mechanical energy conversion, which are divided from the energy of ultrasonic vibration, are analyzed. The model of energy transmission and a new rheological equation including the parameters of ultrasonic vibration are established to describe the rheological behavior of polymer melt in micro cavity. The simulation results show that the ultrasonic vibration improves the viscosity field and the velocity field in complex shaped micro cavity, and leads to a better filling capability and uniformity of the polymer melt. This research achievement can be used to guide the process flow and parameter selection of LUµIM. POLYM. ENG. SCI., 2016. © 2016 Society of Plastics Engineers

New approach to develop a 3D non-isothermal computational framework for injection molding process based on level set method

The simulation of three-dimensional (3D) non-isothermal, non-Newtonian fluid filling process is an extremely difficult task and remains a challenging problem, which includes polymer melt flow with free surface coupled with transient heat transfer. This paper presents a full 3D non-isothermal two-phase flow model to predict the complex flow in melt filling process, where the Cross-WLF model is applied to characterize the rheological behavior of polymer melt. The governing equations are solved using finite volume method with SIMPLEC algorithm on collocated grids and the melt front is accurately captured by a high resolution level set method. A domain extension technique is adopted to deal with the complex cavities, which greatly reduces the computational burden. To verify the validity of the developed 3D approach, the melts filling processes in two thin rectangular cavities (one of them with a cylindrical insert) are simulated. The predicted melt front interfaces are in good agreement with the experiment and commercial software prediction. For a case with a rather complex cavity, the dynamic filling process in a hemispherical shell is successfully simulated. All of the numerical results show that the developed numerical procedure can provide a reasonable prediction for injection molding process.

Time-dependent rheological behavior of branched polymer melts in extensional flows

The viscoelastic flow of low-density polyethylene melts in a cross-slot channel is studied numerically for the double convected Pom–Pom (DCPP) and single modified DCPP (S–MDCPP) models. The equal low-order finite elements for velocity–pressure–stress variables are implemented to solve the flow fields using the pressure-stabilized iterative fractional step algorithm. The distributions of axial velocity along the channel centerline and the principal stress difference achieved by numerical predictions are in quantitative good agreement with the reported experimental results. Furthermore, the evolution of backbone stretch of macromolecule with flow time is demonstrated, and then an exponential decay function is proposed linking the microscale stretch relaxation time with the macroscale flow time, in order to reveal the stretch relaxation response of the branched polymer. The effects of the Weissenberg number and several constitutive parameters of the S–MDCPP model on the rheological behavior of polymer melts are discussed.

Evaluation of rheological properties of non-Newtonian fluids in micro rheology compounder: Experimental procedures for a reliable polymer melt viscosity measurement

The aim of this report is to develop an experimental method which provides relevant data about the rheological behaviour of polymer melts using a Haake minilab microcompounder. A slit capillary die integrated in the recirculation channel of the microcompounder allows determination of polymer flow characteristics using two pressure transducers specially designed for this purpose. According to a specific methodology detailed in the present paper, the effective flow rate and the corresponding apparent shear rate were carefully investigated as regards the repeatability of the data. The viscosity of different commercial polymeric materials (polyolefins and polyamide 6) exhibiting different melt flow rate properties was then calculated based on the Hagen-Poiseuille theory. The results show that the melt flow data obtained from the micro-rheology compounder overlap very well with the shear data obtained using oscillatory or rod capillary rheometers according to the Cox-Merz rule. Finally, a possible application of viscosity measurements of the developed method is given. Using the morphological analysis of a PP/PA6 (80/20 w/w) sample as a polymeric blend model, a deduction of an interfacial tension factor was highlighted and a pertinent value corresponding to that found in the literature was assessed.

Experimental and numerical investigation of pressure-driven microscale flows of polymer melts: New approach

The aim of this study is to develop an experimental method which provides relevant data about the rheological behavior of polymer melts in pressure-driven flows in microscale geometries. In order to get better description of the physics involved in such flows, numerical simulation with commercial and in-house softwares was implemented, especially with molecular dynamics constitutive models. Thus, a modular rheometrical slit die equipped with pressure and temperature transducers was designed and adapted to a capillary rheometer. Different gap dimensions between 50 μm and 200 μm are available. The measurements were performed with high density polyethylene and low density polyethylene. The device was assessed by comparing to usual rheometers. The wall slip was investigated according to Mooney's method. Simulations were conducted with MATLAB® by implementing the Doi–Edwards' tube model and the molecular stress function of Wagner to take into account the enhanced orientation of the molecules due to the very close vicinity of the die walls. The flow was also simulated with POLYFLOW®. The pressure effect on the geometry and on the polymer material was investigated. Experimental results were compared to calculations in terms of pressure values. The conclusions somewhat modify the prospects for future studies of such flows.

Development of Temperature Control System of Multi-Function and All-Electric Rheometer

In order to study the nonlinear viscoelasticity of polymer melts at high shear rate, multi-function and all-electric rheometer (MAR) was designed by the authors. Development of temperature control system of MAR is important because rheological behavior of polymer melts is affected by temperature significantly. The design principle of the temperature control system of MAR was studied. According to the temperature upper limit the barrel need to reach, and the power the heating system can offer to maintain the temperature, the suitable type of heating system was determined. Considering the requirement of heater and the measurement error, the suitable type of temperature transducer was determined. The measured values of temperature were compared with the set values of them to evaluate the temperature control error of MAR. The results show that the temperature fluctuation is smaller than ±0.4°C in 3.5 minute. Therefore, the temperature control error of MAR is small enough for rheological test. And the temperature control stability of MAR is very well.

Rheological behavior of PS polymer melt under ultra high speed injection molding

Determination of the rheological behavior of a polymer melt under high shear rate is generally considered to be crucial for accurate simulation modeling of ultra high speed injection molding, a process that has been attracting more attention from researchers. In this study, a melt viscosity measurement system under high shear rate was established using an instrumented injection molding machine combined with a slit die. Polystyrene melt was studied. From measured pressure drop and volumetric flow rate, the slit flow model was used for the calculation of viscosity at wall shear strain rates up to 105 s−1. Good agreement in shear viscosity was found between results from conventional capillary rheometry and in-line measurement made using an injection molding machine for strain rates of 7 × 103 to 104 s−1. Measured shear viscosity in the test range was found to follow shear thinning power law behavior at shear rates below 7 × 104 s−1. However, the measured viscosity values are significantly lower (28%–46%) than the database obtained from the Moldflow package software (Cross-WLF model) at melt temperatures of 200, 215, and 230 °C. Further, the reduction in the viscosity value increases as the injection speed increases. It seems that the wall-slip effect plays a dominant role, becoming more significant as the melt temperature increases. Alternatively, as shear strain rates approached or exceeded 7 × 104 s−1, shear viscosity deviations from shear thinning behavior were observed and reached a rate-independent plateau. The behavior is thought to be influenced by pressure effects on viscosity. In the present study we emphasize that the measured results are significant for the design and simulation of ultra high speed injection processes such as micromolding and thin-walled injection molding.

A study of novel viscous dissipation measurement for polymer melts in microchannels

Studies on the rheological behaviour of polymer melts, flowing through microchannels, are complicated because a large number of factors affect the melt viscosity. One such factor, viscous dissipation, is investigated in the current work through a novel experimental technique that is used in determining the viscous dissipation of a polymer melt flowing through microchannels. Relative tests are conducted using melts of high-density polyethylene (HDPE) extruded through several capillary dies at different temperatures. Experimental results indicate that the temperature rises due to viscous dissipation increase with increasing shear rate. In addition, simulations considering viscous dissipation are carried out. The comparison of the experimental results with those predicted from the simulations at different melt temperatures indicates that the maximum temperature rise deviation is about 15 per cent. Therefore, the measurement method of viscous dissipation is available, which is helpful to better understand the flow characteristics of microchannels.

Continuous lubricated squeezing flow: A novel technique for equibiaxial elongational viscosity measurements on polymer melts

A novel experimental technique to study the rheological behavior of polymer melts and other viscous fluids in equibiaxial elongational deformations is described. The new technique is a modification of lubricated squeezing flow (LSF) wherein deformation is induced by compression of the sample between parallel plates having a thin film of a low-viscosity fluid or lubricant. In previous work, we have demonstrated that despite its simplicity, LSF is limited to rather small strains because of uncontrolled lubricant film thinning. The new technique, which we call continuous lubricated squeezing flow (CLSF), resolves the lubricant thinning problem by replenishing the lubricant films with a continuous and uniform flow of lubricant through porous plates. The CLSF technique is validated in constant strain-rate deformations by direct comparison with published data obtained using a considerably more complicated device developed by Meissner and co-workers [Hachmann, P., and J. Meissner, “Rheometer for equibiaxial and ...

Viscous dissipation influencing viscosity of polymer melt in micro channels

Determination of melt rheological behavior within micro-structured geometry is very important for the accurate simulation modeling of micro-molding. Yet studies on the rheological behavior of polymer melts, flowing through micro channels, are complicated due to a large number of factors affecting the melt viscosity. One factor, viscous dissipation, is investigated in the current work through a novel experimental technique to determine the viscous dissipation of a polymer melt flowing through several micro channels with identical aspect ratio. Relative tests are conducted with the melt of high density polyethylene (HDPE) at different temperatures being extruded through the capillary dies with diameters 1000μm, 500μm and 350μm, respectively. It was found that the temperature rise due to viscous dissipation decreases significantly with the reduction of the characteristic size of micro channel at the same shear rate. In addition, based on the suggested model of radial temperature distribution, the influence of viscous heating on the melt viscosity is investigated. The results indicate that viscous dissipation does not play a significant role.

An investigation on the correlation between rheology and morphology of nanocomposite foams based on low‐density polyethylene and ethylene vinyl acetate blends

AbstractThis article presents the correlation between rheology and morphology of nanocomposite foams of low‐density polyethylene (LDPE), ethylene vinyl acetate (EVA), and their blends. LDPE/EVA nanocomposites were prepared via melt mixing and then foamed using batch foaming method. To assess the rheological behavior of polymer melts, frequency sweep and creep recovery tests were done. Morphology of the samples was also studied by scanning electron microscopy and X‐ray diffraction. The results showed that with increase in clay content, storage modulus, complex and zero shear viscosities will be increased, which affect the foam morphology. In addition, elasticity plays an important role in foaming process, in a way that samples with more elasticity percentage have the highest cell density and the lowest cell size. POLYM. COMPOS., 31:1808–1816, 2010. © 2010 Society of Plastics Engineers.

Rheological behavior of POM polymer melt flowing through micro-channels

Determination of the rheological behavior of the polymer melt within micro-structured geometry is vital for accurate simulation modeling of micro-molding. The lack of commercial equipment is one of main hurdles in the investigation of micro-melt rheology. In this study, a melt viscosity measurement system for POM melt flowing through micro-channels was established. For measured pressure drop and volumetric flow rate, both capillary and slit flow models were used for the calculation of viscosity. The calculated results were also compared with those of PS resin to discuss the effect of morphology structure on the viscosity characteristics of polymer within micro-channels. It was found that the measured POM viscosity values in the test ranges are significantly lower (about 29–35% for a channel size of 150 μm) than those obtained with a traditional capillary rheometer. Meanwhile, the percentage reduction in the viscosity value and the ratio of slip velocity relative to mean velocity all increase with decreasing micro-channel size, but less significantly when compared with PS resin. In the present study we emphasize that the rheological behavior of the POM resin in microscopic scale is also different from that of macroscopic scale as PS resin but displays a less significant lower. It also revealed that the wall slip occurs more easily for the PS resin within micro-channels than POM resin due to enlarge the effect of molecular weight.

Rheological behavior of polymer melt in the electric field

The electric field is applied to polymer processing, a theoretical framework is established for understanding the physical characters of the procedure. The theoretical analysis shows that electric field can result in decreasing the elasticity, the entanglement density and the viscosity of polymer melts.

Analyses of dynamic rheological behavior of polymer melt based on novel experimental equipment and procedures

By introducing a vibrating, force field into the extrusion process of polymer melt, an experimental equipment of constant velocity type dynamic rheometer of capillary (CVDRC) was designed. A set of experimental procedures was established, by which the dynamic rheological parameters of polymer can be acquired, and a set of data management methods to undergo time-domain or frequency-domain analysis was set up for dynamic rheological data of polymer melt. Meantime, the characterization formula of polymer melt’s rheological behavior in a vibrating force field was set up. The instantaneous value of capillary entry pressure, capillary volume flow rate and their phase difference were measured and analyzed, and the melt apparent viscosity, which describes the rheological behavior of polymer melt in a vibrating force field, was obtained.

Study on rheological behavior of polymer melt flowing through micro-channels considering the wall-slip effect

Micro molding is attracting more attention nowadays and determination of the rheological behavior of the polymer melt within micro structured geometry is considered to be very important for the accurate simulation modeling of micro molding. The lack of commercial equipment is one of the main hurdles in the investigation of micro melt rheology. In this study, the melt viscosity measurement system for PS (polystyrene) melt flowing through a micro-channel was established using a micro-channel mold operated at a mold temperature as high as the melt temperature. From measured pressure drop and volumetric flow rate both the capillary flow model and the slit flow model were used for the calculation of viscosity utilizing Rabinowitsch and Walters corrections. It was found that the measured viscosity values in the test ranges are significantly lower (decreased by a factor of about 1.4–4.1) than those obtained from the traditional capillary rheometer at a melt temperature of 200 °C using both the capillary flow model and the slit flow model. As the micro-channel size decreases, the reduction in the viscosity value increases when compared with data obtained from the traditional capillary rheometer. The ratio of slip velocity relative to mean velocity was also found to increase with decreasing size of micro-channels. It seems that wall slip plays a dominant role when melt flows through micro-channels and would result in a greater percentage in apparent viscosity reduction when the size of the micro-channel decreases. In addition, the wall-slip effect becomes more significant as the melt temperature increases. In the present study we emphasize that the rheological behavior of the melt in the microscopic scale is different from that of the macroscopic scale and that current simulation packages are not suitable for micro molding simulation without considering this difference.

High‐pressure rheology and viscoelastic scaling predictions of polymer melts containing liquid and supercritical carbon dioxide

AbstractHigh‐pressure rheological behavior of polymer melts containing dissolved carbon dioxide (CO2) at concentrations up to 6 wt % were investigated using a high‐pressure extrusion slit die rheometer. In particular, the steady shear viscosity of poly(methyl methacrylate), polypropylene, low‐density polyethylene, and poly(vinylidene fluoride) with dissolved CO2 were measured for shear rates ranging from 1 to 500 s−1 and under pressure conditions up to 30 MPa. The viscosity of all samples revealed a reduction in the presence of CO2 with its extent dependent on CO2 concentration, pressure, and the polymer used. Two types of viscoelastic scaling models were developed to predict the effects of both CO2 concentration and pressure on the viscosity of the polymer melts. The first approach utilized a set of equations analogous to the Williams–Landel–Ferry equation for melts between the glass‐transition temperature (Tg) and Tg + 100 °C, whereas the second approach used equations of the Arrhenius form for melts more than 100 °C above Tg. The combination of these traditional viscoelastic scaling models with predictions for Tg depression by a diluent (Chow model) were used to estimate the observed effects of dissolved CO2 on polymer melt rheology. In this approach, the only parameters involved are physical properties of the pure polymer melt that are either available in the existing literature or can be measured under atmospheric conditions in the absence of CO2. The ability of the proposed scaling models to accurately predict the viscosity of polymer melts with dissolved high‐pressure CO2 were examined for each of the polymer systems. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 3055–3066, 2001

Rheological behavior of polymer melts in monodimensional flow through low‐angle convergence and spiralling dies

AbstractAn experimental study was carried out to examine the rheological behavior of polymer melts in laminar flows through channels with small‐angle convergences (2–3°) and spiralling walls (6°/mm along the flow direction), using an extrusion grade of low‐density polyethylene. The results show that convergence is the main factor responsible for the observed deviations from steady state laminar shear flow, particularly with respect to additional pressure requirements to maintain a constant mass output. The swell ratio, calculated from the dimensions of the channel at the exit, was found to be considerably higher for flow‐through converging channels. Only a small increase in swell ratio could be attributed, on the other hand, to the rotational elements of the die configuration. It is inferred that even for small angles of convergence the increased level of swelling is associated with elongational stress components, which were grouped together and denoted as the “additional stress.” This was obtained from the difference between the calculated average shear stress at the wall, using the recorded pressure at the die entry, and the value of the average shear stress at the wall calculated from the shear viscosity data for the melt. © 2001 John Wiley & Sons, Inc. Adv Polym Techn 20: 99–107, 2001