Evaluating the exit pressure method for measurements of normal stress difference at high shear rates

A challenge for polymer rheology is the reliable determination of shear dependent first normal stress difference (N-1 values) at high shear rates (>10 s(-1)). Here, we evaluate the correctness of the commonly applied exit pressure method focusing on polypropylene and high and low density polyethylene melts at 200 degrees C. It is demonstrated that the linear extrapolation of pressure values toward the die exit, which is a key step in the application of the exit pressure method, is affordable to determine N-1 values despite that these extrapolated exit pressure values are characterized by a relative deviation of 25%-40%. The validity of the exit pressure method is further supported by an excellent match with rheological data from the Laun rule (exponent close to 0.7) and a representative simulation of extrudate swelling data in the width and height direction, considering tuned parameters for the Phan-Thien-Tanner constitutive model. Also, the absence of a significant viscous heating effect near the die exit is highlighted based on numerical analysis. (c) 2020 The Society of Rheology

Shape evolution of 3D periodic structure fabricated by direct-write assembly of concentrated colloidal gels

Scope and Method of Study: 3D periodic structures were fabricated by direct-write assembly of concentrated colloidal gels with self-supporting features. The rheological behavior of the gel was characterized in linear viscoelastic regions. The flow behavior of the gel was modeled by using structural kinetics theory. Based on this model, the dynamic extrusion process of the gel was simulated by incorporating slip wall boundary conditions. A viscoelastic catenary model was developed to describe span shape and compare the results to previous results that used a simple elastic beam theory. The shape evolution (i.e., spanning behavior) of spanning filaments observed was related to shear stress conditions and a limited set of rheological parameters.Findings and Conclusions: The rate and magnitude of microstructure change within a colloidal gel ink are crucial factors for shape evolution of 3D structures assembled by direct write techniques. The events that set the equilibrium shape of 3D structure occur within the initial few seconds after deposition and gels microstructure recovery within this period is critical to geometric fidelity. The shape evolution of 3D structures may be predicted by knowledge of the rheological behavior of the colloidal gel in shear loading. Rheological behavior can be related to the structural recovery time of the colloidal gel and this may be measured with a series of equilibrium flow measurements. Successful completion of this research advances science-based ink design methods and optimization of deposition variables. Better control of shape evolution will lead to improvements in advanced applications such as photonic band gap structures, artificial bone structures, and metal-ceramic composites. The improved connections between time-dependent shear behavior and shape evolution in an extrusion process will also impact other industries (e.g., clay extrusion for catalytic converter substrates) and improve industrial productivity through better paste design. Although the current work is limited to colloidal gels, the knowledge gained here may be easily extended to other complex ink systems such as partially melted thermoplastic polymers and metals

Viscoelastic fluids in profile extrusion: relevance and characterization

Tese de doutoramento em Engineering and Science of Polymers and CompositesA extrusão de perfis termoplásticos é uma técnica de fabrico contínua essencialmente empregue na produção de produtos de seção transversal constante. O projeto de cabeças de extrusão requer uma modelação realista do escoamento que ocorre no seu canal de fluxo e, portanto, necessita de uma caracterização reológica rigorosa do polímero fundido. A presente tese de doutoramento foca-se em dois assuntos, nomeadamente: (i) os efeitos da viscoelasticidade no escoamento confinado que ocorre no canal de fluxo da cabeça de extrusão, e (ii) os efeitos de erros (dimensões das amostras e temperatura) que ocorrem comummente em de testes de reometria extensional uniaxial realizados com a plataforma Sentmanat (SER), com fluidos inelásticos e fluidos viscoelásticos. Os estudos computacionais foram realizados recorrendo à biblioteca computacional OpenFOAM. Relativamente ao primeiro assunto, inicialmente é implementado um sistema de cálculo computacional, e os resultados obtidos com modelos inelástico e viscoelástico são comparados. Para permitir uma comparação adequada dos modelos, primeiramente os comportamentos linear e não linear do material são caracterizados experimentalmente, e os dados obtidos ajustados com um modelo viscoelástico de Giesekus. Em seguida, este modelo é usado para gerar a curva de fluxo (viscosidade de corte versus taxa de corte), e os dados gerados são ajustados, com um código desenvolvido para o efeito, a um modelo inelástico de Bird Carreau. Subsequentemente, os modelos constitutivos viscoelástico e inelástico equivalente são empregues na modelação do escoamento numa cabeça de extrusão de perfil, para aferir o efeito do modelo utilizado na queda de pressão e na distribuição do escoamento previstas. Os resultados obtidos demonstram que a viscoelasticidade desempenha um papel significativo tanto na distribuição do fluxo quanto na queda de pressão, pelo que, idealmente, deveria ser considerada no projeto de cabeças de extrusão de perfil. Relativamente ao segundo assunto estudado, é desenvolvido um modelo computacional que replica os testes de reometria extensional uniaxial efetuados com a plataforma Sentmanat (SER). Numa primeira fase, o trabalho realizado com um modelo inelástico permitiu definir os requisitos computacionais adequados. Esta fase permitiu também concluir que os resultados obtidos com a estratégia de captura de superfície baseada num método Volume-de-Fluido (VOF) geométrico, são melhores que aqueles obtidos com a alternativa algébrica. Tendo em consideração os resultados obtidos com o modelo inelástico, foi desenvolvido um novo código para modelar o escoamento viscoelástico multifásico, usando o método VOF geométrico, para permitir capturar a interface ar-polímero de modo mais preciso. Para além disso, o utilitário do OpenFOAM usado para calcular forças e binários foi adaptado para fluidos viscoelásticos. O sistema computacional desenvolvido é então usado para avaliar o efeito dos erros comuns acima mencionados. Para quantificar o efeito dos erros induzidos, são comparadas as viscosidades extensionais resultantes da modelação numérica com os valores teóricos. Os resultados obtidos mostram que o efeito de erros relativos à temperatura de ensaio é mais significativo do que o correspondente às dimensões da amostra, principalmente quando se utilizam modelos constitutivos viscoelásticos.Thermoplastics profile extrusion is a continuous manufacturing technique that is mostly employed to produce constant cross-section polymeric products. Proper extrusion die design involves realistic modeling of the flow occurring inside the die flow channel and, therefore, accurate polymer melt characterization. The present thesis focuses on two subjects, namely: (i) the effects of viscoelasticity in the confined flow that takes place inside the extrusion die flow channel, and (ii) the effects of common experimental error sources (sample dimensions and test temperature) on the accuracy of the uniaxial extensional rheometry tests performed with the Sentmanat Extensional Rheometer (SER), both for inelastic and viscoelastic fluid models. The computational studies were carried out with the OpenFOAM computational library. Concerning the first subject, a computational framework is developed, and the results obtained from the inelastic and viscoelastic fluids model are compared. To allow a proper model comparison, first, the material linear and nonlinear behavior are characterized experimentally, and the collected data is fitted with the Giseskus viscoelastic model. Afterwards, the fitted Giesekus model is used to generate the material flow curve (shear viscosity versus shear rate), and the data is fitted, with an in-house code, to a Bird Carreau (inelastic) model. Subsequently, the viscoelastic and corresponding inelastic models are employed in a computational study, aiming at comparing the effect of viscoelasticity on the calculated pressure drop and flow distribution. The results obtained demonstrate that viscoelasticity plays a relevant role in both the flow distribution and pressure drop obtained, and, consequently, it should be taken into account when designing profile extrusion dies. Regarding the second subject, a computational setup is devised to model uniaxial extensional rheometry tests performed with the SER device. The work carried out initially with an inelastic model allowed defining of the appropriate computational setup requirements (the computational domain geometry, mesh refinement level, and initial and boundary conditions). This part of the study allowed concluding that the results obtained from a surface-capturing approach based on the geometric Volume-of-Fluid (VOF) method were better than the ones provided with the algebraic counterpart. Having in mind the results obtained with the inelastic model, a new multiphase viscoelastic flow solver was implemented using the geometric VOF, to allow capturing a sharper polymer-air interface. Also, the OpenFOAM utility devised to calculate forces and torques, was adapted to viscoelastic fluids models. The computational framework was then used to assess the effect of the common errors mentioned above. To quantify the effect of the induced errors, the extensional viscosities resulting from the numerical computational studies and the ones achieved with the theoretical counterpart were compared. The results obtained show that the effect of the test temperature errors is more significant than the one corresponding to the sample dimensions, especially when viscoelastic constitutive models are employed.I gratefully acknowlagde funding FEDER funds through the COMPETE 2020 Program and National Funds through FCT - Portuguese Foundation for Science and Technology under the projects UIDB/05256/2020/, UIDP/05256/2020, CPCA/A2/6202/2020, CPCA_A2_6231_2020, NORTE-08- 5369-FSE-000034, under program IMPULSE – Polímeros e Compósitos: Drivers da Inovação Tecnológica e da Competitividade Industrial. I also acknowledge the support of the computational clusters Search-ON2 (NORTE-07-0162-FEDER- 000086) and Minho Advanced Computing Center (MACC)

A Personal Perspective on the Use of Modelling Simulation for Polymer Melt Processing

Copyright 2015 Carl Hanser Verlag. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the Carl Hanser Verlag. The authors are grateful to the publisher, Carl Hanser Verlag, for letting the manuscript being archived in this Open Access repository. The final publication is available at = http://dx.doi.org/10.3139/217.3020International audienceThis paper gives a personal view on the state of art in relation to the modelling of polymer melt processing. The paper briefly reviews both industrial, laboratory and modelling developments over the last forty years and highlights the key aspects now required for realistic modelling of polymer melt processing. The paper summarizes elements relating to the numerical simulation of specific and general polymer processes and also provides topical examples of the application of numerical modelling to certain commercial processes. The paper concludes with identifying areas of polymer processing that still remain a challenge in relation to accurate prediction

A Lagrangian-Eulerian simulation method for viscoelastic flows applied to adhesive joining

Viscoelastic flows are important for many industrial processes, such as adhesive joining, polymer extrusion and additive manufacturing. Numerical simulations enable virtual evaluation and product realization, which can support the design phase and reduce the amount of costly physical testing. However, such applications are challenging to simulate. Thus, efficient, robust and user-friendly simulation methods are needed. In this thesis, a Lagrangian--Eulerian simulation framework for viscoelastic flow is presented. The constitutive equation is solved at Lagrangian nodes, convected by the flow, while the momentum and continuity equations are discretized with the finite volume method. The volume of fluid method is used to model free-surface flow, with an injection model for extrusion along arbitrary nozzle paths. The solver combines an automatic and adaptive octree background grid with implicit immersed boundary conditions. In contrast to boundary-conformed mesh techniques, the framework handles arbitrary geometry and moving objects efficiently. Furthermore, novel coupling methods between the Lagrangian and Eulerian solutions as well as unique treatment of the Lagrangian stresses at the fluid-fluid interface are developed. Consequently, the resulting method can simulate the complex flows associated with the intended applications, without the need for advanced stabilization techniques. The framework is validated for a variety of flows, including relevant benchmarks as well as industrial adhesive joining applications. The latter includes robot-carried adhesive extrusion onto a car fender as well as a hemming application. The results agree with the available experimental data. As such, the research presented in this thesis can contribute to enable virtual process development for joining applications

Three-dimensional flow simulations for polymer extrudate swell out of slit dies from low to high aspect ratios

The impact of the slit die geometry and the polymer melt flow characteristics on the extrudate swell behavior, which is a key extrusion operating parameter, is highlighted. Three-dimensional (3D) numerical simulations based on the finite element method are compared with their conventional two-dimensional (2D) counterparts at the same apparent shear rates using ANSYS Polyflow software. The rheological behavior is described by the differential multimode Phan-Thien-Tanner constitutive model, with polypropylene as a reference. It is shown that increasing the aspect ratio of the die geometry (width/height ratio variation from 1 to 20) contributes to a significant change in the 3D extrudate deformation (relative changes of 10% in several directions; absolute changes up to 30%) and delays the equilibrium axial position (up to a factor 10). High aspect ratios induce a switch to contract flow (swell ratio <1) for the edge height swell. The 3D extrudate swell strongly deviates from the 2D simplified case due to the die effect near the wall, even for higher aspect ratios. Also a different relation with the material parameters is recorded. The initially large swell behavior is followed by a small shrinkage flow in the middle height direction which cannot be captured by the 2D counterpart. The findings are supported by a comprehensive analysis of the velocity and stress fields in and out of the slit dies

Experimental and Numerical Investigation of the Extrusion and Deposition Process of a Poly(lactic Acid) Strand with Fused Deposition Modeling

[Abstract] In the last decade, Fused Deposition Modeling (FDM) has gained popularity for allowing the fabrication of pieces with complex shapes. The final quality of the pieces is strongly linked to the shape, size and surface finish of the strands deposited successively, which themselves depend on the printing parameters and extruded material properties. In this work, we present an experimental characterization of an extruded and deposited single strand of Poly-Lactic Acid (PLA), by means of high-speed visualization of the bead region between the substrate and the nozzle, where the molten polymer is still in liquid phase. A Computational Fluid Dynamics (CFD) model proposed in literature, and, based on isothermal and viscous flow assumptions, is validated with this data in terms of strand height and meniscus shape. The characteristics of the printed layer are also confronted to the measurements of the solidified strands by microscopy, with a good agreement. The focus on high printing speeds allows extending the conclusions of previous studies. Regarding the surface finish, the roughness patterns detected on the printed strands are correlated to the velocity fluctuations of the printing head. The CFD model does not capture those thickness variations, however, due to not accounting for solidificationXunta de Galicia; ED431C 2019/1

Modelling volume change and deformation in food products/processes: An overview

Volume change and large deformation occur in different solid and semi-solid foods during processing, e.g., shrinkage of fruits and vegetables during drying and of meat during cooking, swelling of grains during hydration, and expansion of dough during baking and of snacks during extrusion and puffing. In addition, food is broken down during oral processing. Such phenomena are the result of complex and dynamic relationships between composition and structure of foods, and driving forces established by processes and operating conditions. In particular, water plays a key role as plasticizer, strongly influencing the state of amorphous materials via the glass transition and, thus, their mechanical properties. Therefore, it is important to improve the understanding about these complex phenomena and to develop useful prediction tools. For this aim, different modelling approaches have been applied in the food engineering field. The objective of this article is to provide a general (non-systematic) review of recent (2005–2021) and relevant works regarding the modelling and simulation of volume change and large deformation in various food products/processes. Empirical-and physics-based models are considered, as well as different driving forces for deformation, in order to identify common bottlenecks and challenges in food engineering applications.Fil: Purlis, Emmanuel. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Centro de Investigación y Desarrollo en Criotecnología de Alimentos. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigación y Desarrollo en Criotecnología de Alimentos. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Centro de Investigación y Desarrollo en Criotecnología de Alimentos; ArgentinaFil: Cevoli, Chiara. Università di Bologna; ItaliaFil: Fabbri, Angelo. Università di Bologna; Itali