Le PTFE ne peut pas être mis en forme par les procédés classiquement employés pour les polymères. La production de pièce peut se faire par compaction de poudre et frittage. Des charges peuvent être ajoutées à la poudre vierge. Afin de prédire les propriétés de la pièce à vert et d'avoir un outil fiable pour optimiser les paramètres du procédé de compaction, une modélisation du comportement mécanique de poudres de PTFE chargées, ou non, à température ambiante lors du pressage industriel est proposée. La caractérisation expérimentale des matériaux est réalisée grâce à un outil de compaction 3D installé dans la machine triaxiale ASTREE. A partir de ces essais originaux, un modèle Drucker-Prager/cap est identifié. Les variations importantes de densités et de propriétés sont décrites. Le changement de phase cristalline, prenant place à température ambiante et sous pression est également caractérisé, modélisé et implémenté dans le code EF. Ensuite, l'interaction entre le PTFE et l'outil métallique est vue comme un frottement interne entre le film de transfert de PTFE et le reste de la pièce. Elle est modélisée à partir des paramètres déjà identifiés, à savoir le coefficient de frottement interne du Drucker-Prager et la cohésion. Deux tests sont mis en place pour valider le modèle: un outil ¿dométrique instrumenté et un outil original " en V " offrant la possibilité de mesurer le champ de déplacement par Corrélation d'Image Numérique. Des simulations EF de l'ensemble du procédé sont finalement effectuées et comparées aux données industrielles. Les premiers liens entre les étapes de compaction et de frittage sont établis par caractérisation de la texture cristalline par DRX. / PTFE is not melt-processible. One of the production methods of PTFE parts consists in the powder compaction at room temperature followed by a thermal treatment, the sintering. Fillers can be added to the virgin powder. In order to be able to predict the properties of the obtained green parts and to have a reliable tool to optimize the parameters of the process, modeling of the mechanical behavior of PTFE compounds during their industrial pressing in big billets is proposed. Experimental characterization is made thanks to a 3D compaction tool, installed in the triaxial machine ASTREE. From original and complex loadings, a Drucker-Prager/cap model is identified, where the variations of density and properties are described. In addition to the elastoplastic model, a phase transformation in the crystalline structure at ambient temperature under pressure is experimentally characterized, modeled and implemented in the FE code. Then the interaction of the PTFE with a metallic counterpart is described as a friction between the PTFE transfer film and the bulk PTFE, characterized by the internal friction coefficient of the Drucker-Prager line once the cohesion of the material is reached. No additional parameter needs to be identified. Two laboratory tests allow the validation of the model, an instrumented œdometric tool and an original ‘V’ tool where the displacement field of the material during the compaction is measured by Digital Image Correlation. FE simulation of the whole compaction process is finally made and compared to industrial data. First links between the compaction and the sintering are established by a characterization of the crystalline texture thanks to XRD.

Submitted by Alison Vanceto (alison-vanceto@hotmail.com) on 2016-12-19T10:10:32Z No. of bitstreams: 1 DissFSM.pdf: 13052828 bytes, checksum: d7bc48b1bd94b3c0b5c9de86b2b4f85d (MD5) / Approved for entry into archive by Marina Freitas (marinapf@ufscar.br) on 2017-01-16T17:50:01Z (GMT) No. of bitstreams: 1 DissFSM.pdf: 13052828 bytes, checksum: d7bc48b1bd94b3c0b5c9de86b2b4f85d (MD5) / Approved for entry into archive by Marina Freitas (marinapf@ufscar.br) on 2017-01-16T17:50:20Z (GMT) No. of bitstreams: 1 DissFSM.pdf: 13052828 bytes, checksum: d7bc48b1bd94b3c0b5c9de86b2b4f85d (MD5) / Made available in DSpace on 2017-01-16T17:50:29Z (GMT). No. of bitstreams: 1 DissFSM.pdf: 13052828 bytes, checksum: d7bc48b1bd94b3c0b5c9de86b2b4f85d (MD5) Previous issue date: 2016-08-05 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / One of the methods for the processing of refractory material is cold pressing of the powder material, followed by sintering. Heterogeneous distribution of density can occur in the green compact during pressing because of the friction between the particles of the material and the pressing tools, that may hinder the sintering step. In this context, the simulation of the pressing process constitutes an important tool for the study and improvement of this step, to obtain green compacts with good microstructural homogeneity and also analyze the pressing tools to optimize its design in industrial applications. The identification of a constitutive model to represent the powder material is required to perform the simulation and it is the most complex step. The Drucker-Prager/Cap model implemented in the commercial software of finite elements, AbaqusTM, is suitable primarily for soil compaction study and is able to simulate the mechanical compaction of particulate materials. The parameters of this model have been obtained for a silico-aluminous refractory by a combination of simple, diametrical and hydrostatic compression tests. In studies available in the literature, the material models are partially identified, and in many cases, inadequate simplifications are applied. This study aimed to identify the parameters of the material model more precisely using the digital images correlation technique in the mechanical tests, which enabled a greater understanding of the yielding mechanisms and the achievement of data not obtained by conventional techniques. This methodology allowed the identification of a constitutive model and it was valitaded by the good agrrement between experimental results and those obtained in computer simulations, applied to a uniaxial case followed by isostatic pressing. / Um dos métodos para o processamento de materiais refratários é a prensagem a frio do material particulado, seguida da etapa de sinterização. Durante a prensagem pode ocorrer uma distribuição heterogênea de densidades no compacto verde, causada pelo atrito entre as partículas do material e as paredes da cavidade do ferramental de prensagem, que pode prejudicar a etapa de sinterização. Neste contexto, a simulação computacional do processo de prensagem constitui-se em uma ferramenta importante para o estudo e aperfeiçoamento desta etapa, visando obter compactos verdes com boa homogeneidade microestrutural e também analisar as solicitações no ferramental a fim de otimizar seu projeto em aplicações industriais. A identificação de um modelo constitutivo que represente o material é necessária para realização das simulações e trata-se da etapa de maior complexidade. O modelo de Drucker-Prager/Cap implementado no software comercial de elementos finitos AbaqusTM é adequado principalmente para o estudo de compactação de solos e é capaz de simular o adensamento mecânico de materiais particulados. Os parâmetros deste modelo para um material refratário sílico-aluminoso foram identificados pela combinação de ensaios de compressão simples, diametral e hidrostática. Em trabalhos disponíveis na literatura, os modelos de material são parcialmente identificados e, em muitos casos, são aplicadas simplificações inadequadas. O presente estudo visou uma identificação mais precisa dos parâmetros do modelo de material, uma vez que os ensaios mecânicos foram auxiliados pela técnica de correlação de imagens digitais, que possibilitou a avaliação mais profunda dos mecanismos de escoamento e a obtenção de dados não alcançados por técnicas convencionais. Essa metodologia permitiu a identificação do modelo constitutivo e a validação consistiu na comparação entre resultados experimentais e os obtidos em simulações computacionais, aplicados a um caso de prensagem uniaxial seguida de prensagem isostática, apresentando boa concordância.

Dans l’industrie pharmaceutique, la granulation sèche par compactage à rouleaux est un procédé d’agglomération de poudres en granulés pour améliorer les propriétés d’écoulement nécessaire pour le procédé de compression en matrice. Comprendre le procédé de compactage à rouleaux et optimiser l’efficacité de production est limitée par l’utilisation de l’approche expérimentale à cause du coût élevé des poudres, le temps des essais et la complexité du procédé. Dans ce travail, une méthode d’éléments finis en 3D, est développée dans le but d’identifier les paramètres critiques du matériau et du procédé pour le contrôle de la qualité de la production. Le modèle de comportement de Drucker-Prager Cap est utilisé pour décrire le comportement en compression de poudres et sa calibration est déterminée à partir des essais standard. Pour surmonter la complexité liée à l’existence de deux mécanismes différents, l’alimentation en poudre par une vis sans fin et le compactage entre les rouleaux, une nouvelle méthode d’interfaçage entre la méthode des éléments discrets (DEM) employée pour décrire l’écoulement dans l’alimentation et la méthode des éléments finis (FEM) utilisée pour le compactage entre les rouleaux est développée. Enfin, pour une modélisation de compactage de rouleaux plus réaliste, prenant en compte la variation de l’entrefer entre les rouleaux, une nouvelle approche de couplage Euler-Lagrange est proposée. Les résultats de simulations par éléments finis montrent clairement l’effet des différents paramètres du procédé sur les distributions de pression et de densité dans la zone de compactage. En outre, les résultats montrent que l'utilisation de plaques de confinement de la poudre entre les rouleaux, développe une distribution de pression et de densité non homogène dans le compact, avec une densité plus élevée au centre et plus faible aux bords. D'autre part, l’utilisation de rouleaux dont l’un est surmonté d’une jante de confinement, a montré une distribution de propriétés globalement plus uniforme sur la largeur du compact avec des valeurs légèrement plus élevées aux bords qu’au centre. La méthodologie combinant les méthodes DEM & FEM montre clairement une corrélation directe entre la vitesse des particules entraînées par la vis dans la zone d’alimentation et la pression du rouleau. Tous les deux oscillent avec la même période. Cela se traduit par un compact anisotrope avec un profile de densité variant de manière sinusoïdale le long de sa largeur. Afin d'étudier la capacité du modèle à prédire les propriétés des compacts produits par compactage à rouleaux, les prédictions par simulations numériques sont comparées aux données de la littérature et validées par des mesures spécifiques. / In the pharmaceutical industry, dry granulation by roll compaction is a process of size enlargement of powder into granules with good flowability for subsequent die compaction process. Understanding the roll compaction process and optimizing manufacturing efficiency is limited using the experimental approach due to the high cost of powder, time-consuming and the complexity of the process. In this work, a 3D Finite Element Method (FEM) model was developed to identify the critical material properties, roll press designs and process parameters controlling the quality of the product. The Drucker-Prager Cap (DPC) model was used to describe the powder compaction behavior and was determined based on standard calibration method. To overcome the complexity involving two different mechanisms of powder feeding by the screw and powder compaction between rolls, a novel combined approach of Discrete Element Method (DEM), used to predict the granular material flow in the feed zone and the Finite Elements Method (FEM) employed for roll compaction, was developed. Lastly, for a more realistic roll compaction modelling, allowing the fluctuation of the gap between rolls, a Coupled-Eulerian Lagrangian (CEL) approach was developed. FEM simulation results clearly show the effect of different process parameters on roll pressure and density distribution in the compaction zone of powder between the rolls. Moreover, results show that using a cheek-plates sealing system causes a nonuniform roll pressure and density distribution with the highest values in the middle and the lowest at the edges. On the other hand, the resultant pressure and density distributions with the rimmed-roll obtained higher values in the edges than in the middle and overall a more uniform distribution. The combined DEM-FEM methodology clearly shows a direct correlation between the particle velocity driven by the screw conveyor to the feed zone and the roll pressure, both oscillating in the same period. This translates into an anisotropic ribbon with a density profile varying sinusoidally along its length. To validate the results, the simulations are compared with literature and experimentally measured values in order to assess the ability of the model to predict the properties of the produced ribbons.