Free Form Extrusion : extrusion of 3D components using complex polymeric systems

Tese de doutoramento - Programa Doutoral em Ciência e Engenharia de Polímeros e CompósitosFree Form Extrusion (FFE) is a 3D fabrication process that involves depositing an extruded filament onto successive horizontal planes, in order to build a physic part with a specific geometry. The method is an adaptation of Fused Deposition Modeling (FDM), whereby a commercial pre-extruded ABS filament is replaced by a miniextruder capable of processing a wide range of materials, including homopolymers, polymer blends and nanocomposites. The mechanical performance of FFE parts is controlled by the bonding quality between adjacent filaments both in the horizontal and vertical plans. Adjacent filaments must be sufficiently hot to ensure adhesion, but should then cool down fast enough to avoid excessive deformation due to gravity and weight of the filaments on top. Therefore, it is important to know the evolution in time of the filaments temperature and how it is affected by the major process variables. Although several heat transfer models have been proposed in literature, most assume simplifications without verifying their effective importance. In this work, a detailed analysis of the contributions to the global heat transfer was made using the ABAQUS® software. Heat exchanges with ambient by convection and between adjacent filaments by conduction were found to be the most important. Consequently, these were taken in the energy balance, in order to obtain a rigorous FFE heat transfer analytical model. This analytical solution was then embedded into a routine capable of considering three different deposition patterns, for parts with specific geometries. Moreover, an adhesion criterion was implemented, in order to predict whether the operating conditions and deposition patterns selected for the manufacture of a given part are adequate. The deformation of the filaments was investigated by performing computational experiments with the ABAQUS® software, assuming a temperature dependent viscoelastic response. Since the maximum differences were shown to be quite small (< 0.2%), this phenomenon was considered negligible The consequence of this work is then a computer code that rigorously considers and inter-relates the three phenomena, with the potential of assisting the FFE user in selecting the ideal process parameters in order to obtain a part with good performance. This also allows testing the influence of many process variables and concluding about the most important.Free Form Extrusion (FFE) é um processo 3D de fabricação que envolve a deposição de um filamento extrudido em planos horizontais sucessivos, por forma a obter uma peça com uma geometria específica. O método é uma adaptação do Fused Deposition Modelling (FDM), através do qual um filamento de ABS comercial pré-extrudido é substituído por uma mini-extrusora com a capacidade de processar uma grande variedade de materiais, incluindo homopolímeros, misturas de polímeros e nanocompósitos. O desempenho mecânico das peças obtidas através da técnica FFE é influenciado pela qualidade de adesão entre filamentos adjacentes, nos planos horizontais bem como verticais. Os filamentos adjacentes devem ser suficientemente quentes para assegurar a adesão, mas ter um arrefecimento suficientemente rápido para evitar a deformação excessiva devido à gravidade e ao peso dos filamentos posicionados nas camadas superiores. Consequentemente, é importante o conhecimento da evolução da temperatura dos filamentos em função do tempo, e como este é afetado pelas principais variáveis do processo. Apesar de vários modelos de transferência de calor terem sido propostos na literatura, na maior parte, simplificações são assumidas sem verificar a sua efetiva importância. Neste trabalho, uma análise detalhada dos intervenientes na transferência de calor foi elaborada usando o software ABAQUS®. Trocas de calor com o ambiente por convecção e entre filamentos adjacentes por condução mostraram ser os mais importantes. Consequentemente, estes foram incluídos no balanço de energia, a fim de obter um modelo analítico rigoroso de transferência de calor para o processo FFE. Esta solução analítica foi então incorporada numa rotina com a capacidade de considerar três tipos diferentes de deposição, para peças com geometrias específicas. Além disso, um critério de adesão foi implementado, a fim de prever se as condições operatórias e os tipos de deposição selecionados para a construção de uma dada peça são adequados A deformação dos filamentos foi investigada através da realização de experiências com o software computacional ABAQUS®, assumindo uma resposta viscoelástica dependente da temperatura. Uma vez que as diferenças máximas mostraram ser muito baixas (< 0.2%), este fenômeno foi considerado negligenciável. A consequência deste trabalho é portanto um código computacional que considera rigorosamente e inter-relaciona os três fenómenos, com a potencialidade de auxiliar o utilizador do processo FFE na seleção ideal dos parâmetros de processo, a fim de obter uma peça com bom desempenho. Também permite testar a influência de diversas variáveis de processo e concluir quais são as mais importantes

Using matlab to compute heat transfer In free form extrusion

Capítulo 2

Computer aided definition of the printing conditions of parts made by FFF

The growing worldwide use of 3D printing techniques requires the support of scientific research to improve the process and the quality of 3D printed parts. In a previous work, the authors developed a computer code that predicts the temperature evolution and the adhesion at any location of a 3D part produced by Fused Filament Fabrication (FFF). Here, a 3D printed scalpel handle is used as a case study to evaluate the usefulness of the simulation tool in the definition of the printing conditions. Considering a printer with a convection oven, the best built orientation is selected. The results demonstrate the complexity of the heat transfer mechanisms that develop during the deposition stage. For this particular case study, the importance of using a 3D printer fitted with a convection oven is demonstrated, as its positive effect on adhesion cannot be compensated by tuning other process parametersThis work has been partially supported by national funds through FCT – Fundação para a Ciência e Tecnologia through project UIDB/04728/2020. Partial support for this research has been provided by the Search-ON2: Revitalization of HPC infrastructure of Uminho, (NORTE-07-0162-FEDER-000086), co-funded by the North Portugal Regional Operational Programme (ON.2-O Novo Norte), under the National Strategic Reference Framework (NSRF), through the European Regional Development Fund (ERDF), at the University of Minho, Portugal

Studying the cooling stage in fused filament fabrication

Fused Filament Fabrication (FFF) is one of the available techniques that is capable of producing parts by additive manufacturing, i.e., by depositing thin filaments of thermoplastic polymers or composites onto a support as a vertical series of horizontal 2D slices of a 3D part. This chapter approaches FFF from a phenomenological point of view, and then focus on the deposition and cooling stage. A code capable of predicting the evolution of temperature during deposition and until cooling is completed, as well as of the final bonding between filaments is presented. The tool is then used to enlighten the effect of major processing parameters on the quality of parts

The effect of a phase change on the temperature evolution during the deposition stage in fused filament fabrication

Additive Manufacturing Techniques such as Fused Filament Fabrication (FFF) produce 3D parts with complex geometries directly from a computer model without the need of using molds and tools, by gradually depositing material(s), usually in layers. Due to the rapid growth of these techniques, researchers have been increasingly interested in the availability of strategies, models or data that may assist process optimization. In fact, 3D printed parts often exhibit limited mechanical performance, which is usually the result of poor bonding between adjacent filaments. In turn, the latter is influenced by the temperature field history during deposition. This study aims at evaluating the influence of the phase change from the melt to the solid state undergone by semi-crystalline polymers such as Polylactic Acid (PLA), on the heat transfer during the deposition stage. The energy equation considering solidification is solved analytically and then inserted into a MatLab® code to model cooling in FFF. The deposition and cooling of simple geometries is studied first, in order to assess the differences in cooling of amorphous and semi-crystalline polymers. Acrylonitrile Butadiene Styrene (ABS) was taken as representing an amorphous material. Then, the deposition and cooling of a realistic 3D part is investigated, and the influence of the build orientation is discussed.This work has been partially supported by national funds through FCT- Fundação para a Ciência e Tecnologia through project UIDB/04728/2020. Partial support for this research has been provided by the Search-ON2: Revitalization of HPC infrastructure of Uminho, (NORTE-07-0162- FEDER-000086), co-funded by the North Portugal Regional Operational Programme (ON.2-O Novo Norte), under the National Strategic Reference Framework (NSRF), through the European Regional Development Fund (ERDF)

Predicting the effect of build orientation and process temperatures on the performance of parts made by fused filament fabrication

Purpose: The performance of the parts obtained by fused filament fabrication (FFF) is strongly dependent on the extent of bonding between adjacent filaments developing during the deposition stage. Bonding depends on the properties of the polymer material and is controlled by the temperature of the filaments when they come into contact, as well as by the time required for molecular diffusion. In turn, the temperature of the filaments is influenced by the set of operating conditions being used for printing. This paper aims at predicting the degree of bonding of realistic 3D printed parts, taking into consideration the various contacts arising during its fabrication, and the printing conditions selected. Design/methodology/approach: A computational thermal model of filament cooling and bonding that was previously developed by the authors is extended here, to be able to predict the influence of the build orientation of 3D printed parts on bonding. The quality of a part taken as a case study is then assessed in terms of the degree of bonding, i.e. the percentage of volume exhibiting satisfactory bonding between contiguous filaments. Findings: The complexity of the heat transfer arising from the changes in the thermal boundary conditions during deposition and cooling is well demonstrated for a case study involving a realistic 3D part. Both extrusion and build chamber temperature are major process parameters. Originality/value: The results obtained can be used as practical guidance towards defining printing strategies for 3D printing using FFF. Also, the model developed could be directly applied for the selection of adequate printing conditions.This work is funded by National Funds through FCT – Portuguese Foundation for Science and Technology, References UIDB/ 05256/2020, UIDP/05256/2020 and UIDB/04728/2020. Partial support for this research has been provided by the Search-ON2: Revitalization of HPC infrastructure of UMinho, (NORTE-07–0162- FEDER-000086), co-funded by the North Portugal Regional Operational Programme (ON.2-O Novo Norte), under the National Strategic Reference Framework (NSRF), through the European Regional Development Fund (ERDF), at the University of Minho, Portugal

The Estrangement Challenge

Board game developed by Prof. Yvette Taylor, Dr. Cristina Costa, Claire Goodfellow and Sidonie Ecochard

Experimental study of PLA thermal behavior during fused filament fabrication

Fused filament fabrication (FFF) is an additive manufacturing technique that is used to produce prototypes and a gradually more important processing route to obtain final products. Due to the layer-by-layer deposition mechanism involved, bonding between adjacent layers is controlled by the thermal energy of the material being printed, which strongly depends on the temperature development of the filaments during the deposition sequence. This study reports experimental measurements of filament temperature during deposition. These temperature profiles were compared to the predictions made by a previously developed model. The two sets of data showed good agreement, particularly concerning the occurrence of reheating peaks when new filaments are deposited onto previously deposited ones. The developed experimental technique is shown to demonstrate its sensitivity to changing operating conditions, namely platform temperature and deposition velocity. The data generated can be valuable to predict more accurately the bond quality achieved in FFF parts

The evolving SARS-CoV-2 epidemic in Africa: Insights from rapidly expanding genomic surveillance

INTRODUCTION Investment in Africa over the past year with regard to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) sequencing has led to a massive increase in the number of sequences, which, to date, exceeds 100,000 sequences generated to track the pandemic on the continent. These sequences have profoundly affected how public health officials in Africa have navigated the COVID-19 pandemic. RATIONALE We demonstrate how the first 100,000 SARS-CoV-2 sequences from Africa have helped monitor the epidemic on the continent, how genomic surveillance expanded over the course of the pandemic, and how we adapted our sequencing methods to deal with an evolving virus. Finally, we also examine how viral lineages have spread across the continent in a phylogeographic framework to gain insights into the underlying temporal and spatial transmission dynamics for several variants of concern (VOCs). RESULTS Our results indicate that the number of countries in Africa that can sequence the virus within their own borders is growing and that this is coupled with a shorter turnaround time from the time of sampling to sequence submission. Ongoing evolution necessitated the continual updating of primer sets, and, as a result, eight primer sets were designed in tandem with viral evolution and used to ensure effective sequencing of the virus. The pandemic unfolded through multiple waves of infection that were each driven by distinct genetic lineages, with B.1-like ancestral strains associated with the first pandemic wave of infections in 2020. Successive waves on the continent were fueled by different VOCs, with Alpha and Beta cocirculating in distinct spatial patterns during the second wave and Delta and Omicron affecting the whole continent during the third and fourth waves, respectively. Phylogeographic reconstruction points toward distinct differences in viral importation and exportation patterns associated with the Alpha, Beta, Delta, and Omicron variants and subvariants, when considering both Africa versus the rest of the world and viral dissemination within the continent. Our epidemiological and phylogenetic inferences therefore underscore the heterogeneous nature of the pandemic on the continent and highlight key insights and challenges, for instance, recognizing the limitations of low testing proportions. We also highlight the early warning capacity that genomic surveillance in Africa has had for the rest of the world with the detection of new lineages and variants, the most recent being the characterization of various Omicron subvariants. CONCLUSION Sustained investment for diagnostics and genomic surveillance in Africa is needed as the virus continues to evolve. This is important not only to help combat SARS-CoV-2 on the continent but also because it can be used as a platform to help address the many emerging and reemerging infectious disease threats in Africa. In particular, capacity building for local sequencing within countries or within the continent should be prioritized because this is generally associated with shorter turnaround times, providing the most benefit to local public health authorities tasked with pandemic response and mitigation and allowing for the fastest reaction to localized outbreaks. These investments are crucial for pandemic preparedness and response and will serve the health of the continent well into the 21st century

The Effect of a Phase Change on the Temperature Evolution during the Deposition Stage in Fused Filament Fabrication

Additive Manufacturing Techniques such as Fused Filament Fabrication (FFF) produce 3D parts with complex geometries directly from a computer model without the need of using molds and tools, by gradually depositing material(s), usually in layers. Due to the rapid growth of these techniques, researchers have been increasingly interested in the availability of strategies, models or data that may assist process optimization. In fact, 3D printed parts often exhibit limited mechanical performance, which is usually the result of poor bonding between adjacent filaments. In turn, the latter is influenced by the temperature field history during deposition. This study aims at evaluating the influence of the phase change from the melt to the solid state undergone by semi-crystalline polymers such as Polylactic Acid (PLA), on the heat transfer during the deposition stage. The energy equation considering solidification is solved analytically and then inserted into a MatLab® code to model cooling in FFF. The deposition and cooling of simple geometries is studied first, in order to assess the differences in cooling of amorphous and semi-crystalline polymers. Acrylonitrile Butadiene Styrene (ABS) was taken as representing an amorphous material. Then, the deposition and cooling of a realistic 3D part is investigated, and the influence of the build orientation is discussed