Yes / The microstructure and molecular orientation distribution over both the length and the thickness of microinjection‐molded linear low‐density polyethylene with a weld line were characterized as a function of processing parameters using small‐angle X‐ray scattering and wide‐angle X‐ray diffraction techniques. The weld line was introduced via recombination of two separated melt streams with an angle of 180° to each other in injection molding. The lamellar structure was found to be related to the mold temperature strongly but the injection velocity and the melt temperature slightly. Furthermore, the distributions of molecular orientation at different molding conditions and different positions in the cross section of molded samples were derived from Hermans equation. The degree of orientation of polymeric chains and the thickness of oriented layers decrease considerably with an increase of both mold temperature and melt temperature, which could be explained by the stress relaxation of sheared chains and the reduced melt viscosity, respectively. The level of molecular orientation was found to be lowest in the weld line when varying injection velocity, mold temperature, and melt temperature, thus providing an effective means to identify the position of weld line induced by flow obstacles during injection‐molding process. / Jilin Scientific and Technological Development Program. Grant Number: 20180519001JH; National Key R&D Program of China. Grant Number: 2018YFB0704200; National Natural Science Foundation of China. Grant Numbers: 21674119, 21790342; Newton Advanced Fellowship of Royal Society. Grant Number: NA150222

No description available.

Made available in DSpace on 2016-12-08T17:19:29Z (GMT). No. of bitstreams: 1 Elementos pre-textuais.pdf: 356258 bytes, checksum: 9fa8daf60f02856f1b21e1882468c798 (MD5) Previous issue date: 2006-12-13 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / The weld line is a common phenomenon in molded plastic articles and its consequences can be evaluated by several manners. This study treat weld line as a sharp crack, which length can be estimated based on the self-diffusion of molecular chains across the polymer-polymer interface. The temperature gradient from the contact surface mold/polymer up to the centre of the thickness was determined by simple heat transfer analyses equations. By combining the temperature gradient behavior through the part thickness with the polymer properties it is possible to determine the diffusion value across the interface and the interpenetration as a result of that. Above a critical interpenetration value, the weld line crack length will fall below a critical crack length, whereby the weld line will no longer initiate brittle fracture. The critical interpenetration is a function of the melt temperature and it is determined by strength tests for samples molded in several temperatures in the polymer manufacture recommended range. The results from strength tests, by which were possible analyze the weak area generated by weld line, were compared with theoretical results. It was evident, by comparing experimental and theoretical results, that weld lines are determined to be near-surface flaws, which embrittle notch sensitive materials such as most amorphous polymers. A software was developed to become possible to determine very quickly the crack length calculations as a function of the melt temperature and other properties related to diffusion. The software can be used to the calculations of any amorphous polymer, process condition and part thickness, obviously by the use of proper adjust equations. / A linha de solda é um fenômeno muito comum em produtos plásticos injetados e as conseqüências da mesma podem ser analisadas de várias maneiras. Este trabalho considerou a linha de solda como sendo uma trinca, cujo comprimento foi estimado a partir da difusão molecular na interface entre as duas frentes convergentes de fluxo. Um gradiente de temperatura desde a superfície de contato molde/polímero até o centro da espessura da peça foi determinado com equações para análise de troca térmica. E o comportamento deste gradiente de temperatura ao longo da espessura do produto somado as propriedades do polímero determinaram valores para a difusão molecular na interface e a conseqüente interpenetração. Acima de um determinado valor crítico de interpenetração, o comprimento da trinca da linha de solda cai para valores abaixo de um determinado valor crítico, pelo qual a linha de solda não mais será responsável por uma fratura frágil. A interpenetração crítica é função da temperatura de injeção e foi determinada via ensaios de tração para amostras injetadas em várias temperaturas dentro da faixa recomendada pelo fabricante da matéria-prima. Os resultados dos ensaios de tração, que possibilitaram a análise da perda de resistência à tração devido à trinca gerada pela linha de solda, foram comparados com os resultados obtidos teoricamente. Ficou evidente, devido a convergência dos resultados teóricos e experimentais, que a linha de solda é um fenômeno que produz um defeito superficial que fragiliza de maneira mais acentuada os materiais poliméricos amorfos, devido a sensibilidade dos mesmos ao entalhe. Um aplicativo foi desenvolvido para proporcionar o cálculo do comprimento da trinca em função da temperatura de injeção e demais propriedades. O aplicativo pode ser utilizado para o cálculo de qualquer polímero de estrutura amorfa, em qualquer condição de processo e espessura, mediante a utilização das adequadas equações de ajuste.

Ce travail de thèse, inscrit dans le cadre du projet FUI ConProMi, se propose d'étudier les mécanismes intervenant durant la phase de réplication par micro-injection de pièces en polymères amorphes. Dans ce but, un outil spécifique dénommé « La Rotonde » a été développé, pour réaliser des pièces d'épaisseurs variables, tout en suivant les conditions de pressions et températures locales. Les mesures de ces conditions ont montré que les écoulements de polymère à l'état fondu, dont d'un copolymère d'oléfines cycliques (COC), variaient en fonction de l'épaisseur de la cavité. L'analyse de ces résultats a permis de proposer une nouvelle loi de comportement reliant les pertes de charge dP à l'épaisseur de la cavité, selon une loi Puissance: dP~1/e^a. L'extrapolation de cette loi à des épaisseurs décroissantes nous a permis de prédire la limite physique de fabrication d'objets par micro-injection, connue de façon empirique dans l'industrie de la plasturgie ; la fabrication de pièces d'épaisseurs inférieures à 0.19 mm (ou de facteur de forme supérieur à 45) est en effet impossible du fait des pertes de charges trop importantes lors de l'écoulement du polymère dans la cavité. Il a ainsi été démontré que La Rotonde est un véritable rhéomètre interne, mesurant les viscosités du COC dans la cavité lors de son écoulement, en intégrant l'influence de la compressibilité de la matière. Une attention particulière a enfin été portée sur l'échantillon de plus faible épaisseur injectable (0.27 mm), afin de mieux appréhender les mécanismes s'opposant à l'écoulement du polymère fondu. Ces travaux ont révélé des viscosités élevées et des contraintes de cisaillement supérieures aux valeurs seuils rapportées dans la littérature pour expliquer le phénomène de glissement des chaînes macromoléculaires à l'interface paroi/polymère. Une analyse morphologique fine a été entreprise afin d'observer et identifier les mécanismes impliqués lors du remplissage des pièces. Cette étude spécifique a permis de révéler la présence de multiples écoulements secondaires, localisés à proximité du seuil et dont le nombre croit fortement avec la réduction d'épaisseur. Ces défauts ont tendance à disparaitre le long de l'écoulement et à former des lignes de recollement. Ce résultat original montre que le modèle d'écoulement Fontaine relatif au remplissage des cavités en injection a été mis en défaut dans le cas de la micro-injection. La dernière partie de cette étude a été consacrée à l'analyse des conséquences de ces écoulements secondaires sur les propriétés physiques des pièces en COC. Les mesures de biréfringence ont permis de mettre en évidence des niveaux de contraintes internes particulièrement élevés à proximité du seuil, qui sont favorisées par la phase de maintien. Ces contraintes ont été par ailleurs directement corrélées avec la réduction d'épaisseur. Les variations des propriétés physiques et en particulier mécaniques dynamiques des micro-pièces ont été reliées à l'histoire thermomécanique subie par les échantillons, avec notamment des augmentations des propriétés élastiques et de densité liées aux cinétiques de refroidissement croissantes avec la réduction d'épaisseur. Les variations de mobilité moléculaire à grande distance des chaînes macromoléculaires du polymère considéré ont été corrélées au phénomène de vieillissement physique, ou à la présence de défauts structurels générés lors de l'écoulement. D'un point de vue industriel, une telle étude a permis de proposer des préconisations sur l'optimisation des conditions de mise en œuvre en micro-injection d'objets de tailles réduites, en expliquant l'origine du verrou technologique relevé industriellement. A la lumière de ces résultats, des solutions technologiques pourront être proposées, pour reculer les limites de fabrication de micro-objets en favorisant le glissement des chaines macromoléculaires à l'interface paroi/polymère ou formuler de nouveaux polymères spécifiques pour ce secteur d'activité. / This work, done within the framework of a FUI project (ConProMi), endeavoured to study the mechanisms involved within amorphous polymers during the replication of micro-parts by microinjection moulding. A specific mould called “La Rotonde” has been developed to realise parts with variable thicknesses, and to follow the local pressure and temperature conditions during the moulding phase. The in-situ measurements show that the polymer flow is greatly affected by the cavity thickness, in the particular case of cyclic olefin copolymer (COC). Indeed, the pressure drops dP increases with the thickness e and respect a power law, through the expression dP~e^a. The extrapolation to lower thicknesses gives a physical limit for the manufacturing of micro-parts by injection moulding, known empirically in the plastic industry. The moulding of a 0,19mm thick part (or aspect ratio upper than 45) is impossible due to the pressure drops involved. Therefore, “La Rotonde”, as an internal rheometer, allows measuring the COC viscosity under real process conditions, integrating the material compressibility. A focus has been made for the smaller parts (0,27mm thick) to study the mechanisms involved within the polymer flow. Higher viscosities and shear stresses are found for this cavity, and close to the values corresponding to the appearance of wall-slip phenomenon at the interface between the polymer and the cavity. A morphological analysis of short-shots reveals the presence of multiple secondary flows close to the injection gate, creating weld-lines. This phenomenon is clearly broadened with decreasing thicknesses, but it seems to disappear away from the injection gate. Therefore, this original result show that the classical fountain flow, used to describe the polymer flow behaviour, is not sufficient in the case of polymer flows within micro-cavities. As a consequence, each sample's morphology has been studied and related to the physical properties of COC. A specific analysis of the local birefringence has been developed in order to quantify the residual internal stresses. The internal stresses profiles normal to the flow direction are parabolic for all the samples, and the levels of maximum stresses reached increases with reducing thickness. The dynamic mechanical properties are history-dependent according to the former thermomechanical conditions. The elastic properties tend to increase together with the density when the thickness decreases, partly explained by the enhancement of the cooling rates. The differences observed for the molecular mobility of the macromolecular chains have been correlated to physical ageing and/or at the presence of structural defects during the moulding phase. With regards to the results observed, some recommendations are drawn regarding the optimization of the process conditions for the manufacturing of parts by micro-injection moulding. At last, different solutions are given to overcome the physical limitation to produce micro-parts, like controlling the wall-slip phenomenon at the interface between polymer and cavity or compounding new polymers with specific rheological behaviours.

With increasing emphasis and regulations on the environmental footprint in industries, the integration of reduced carbon dioxide equivalent (CO2eq) plastic materials is desirable. Fiber-reinforced plastic materials mechanical properties differ with varying fiber orientations. Similarly, the welding line phenomenon, commonly present in more complex injection molded parts, decreases the mechanical performance. This thesis aims to experimentally investigate tensile behavior on reduced CO2eq reinforced plastics in different fiber orientations and weld line configurations.  Ten materials with reduced CO2eq are investigated, the types of materials are as follows: PA6 (Polyamide6), PP (Polypropylene), and PA6/PP blend materials. Both short fiber-reinforced polymers (SFRP) and long fiber-reinforced polymers (LFRP) are investigated. The screening resulted in three selected materials for further investigation: one recycled PA6, one bio-based PA6/PP, and one alternative PP. The further investigation involves tensile testing in the five directions and three weld line configurations with non-standardized geometry specimens punched out from an injection molded plate with controlled fiber orientation. Two types of uniform fiber orientation plates are manufactured for the testing conditions, one with holes for weld line testing and one without for testing of orientation. The evaluated fiber orientations are 0° (fibers parallel to load direction), 22.5°, 45°, 67.5°, and 90° (fibers transverse to load direction). The weld line configuration consists of three consecutive holes with 96.5, 146.5, and 196.5 mm distances from the gating system. Three weld line test specimens are generated from each plate, they are denoted W1, W2, and W3 from their respective distance from the gating system, with W1 being closest to the gate. Optical microscopy of fiber orientation and failure modes for the test specimens are performed to investigate and validate the testing conditions.  Varying fiber orientation was found to greatly affect the stress-strain behavior in all four materials investigated. The tensile strength was reduced from longitudinal to transverse fiber orientation, with the most significant reduction near flow direction. High variations were present for the brittle materials supposedly from their weakness to stress concentrations. Strain tended to increase from the lowest at 0° to the maximum at 45°, from which it again decreased to a mid-value at 90° for all materials. The weld line strength reduced significantly for the brittle materials, whereas the ductile materials experienced a much smaller reduction. The three weld line cases failed at similar stresses, while having different stiffness.