António Gaspar-cunha | Universidade do Minho (original) (raw)

Papers by António Gaspar-cunha

Mathematical and Computational Applications

Solving real-world multi-objective optimization problems using Multi-Objective Optimization Algor... more Solving real-world multi-objective optimization problems using Multi-Objective Optimization Algorithms becomes difficult when the number of objectives is high since the types of algorithms generally used to solve these problems are based on the concept of non-dominance, which ceases to work as the number of objectives grows. This problem is known as the curse of dimensionality. Simultaneously, the existence of many objectives, a characteristic of practical optimization problems, makes choosing a solution to the problem very difficult. Different approaches are being used in the literature to reduce the number of objectives required for optimization. This work aims to propose a machine learning methodology, designated by FS-OPA, to tackle this problem. The proposed methodology was assessed using DTLZ benchmarks problems suggested in the literature and compared with similar algorithms, showing a good performance. In the end, the methodology was applied to a difficult real problem in po...

Mathematical and computational applications, Jan 30, 2023

Engineering Applications of Artificial Intelligence, Nov 1, 2022

Journal of Polymer Science Part B: Polymer Physics, 1999

This article reports on a new phenomenon: The presence of a compatibilizer accelerates the meltin... more This article reports on a new phenomenon: The presence of a compatibilizer accelerates the melting/plastification of an immiscible polymer blend during melt blending. The increase in the rate of melting as a result of the addition of a compatibilizer is believed to be one of the important factors responsible for the fact that the morphology of compatibilized blends develops much faster than that of their uncompatibilized counterparts. To substantiate the above statement, blends based on polypropylene (PP) and polyamide 6 (PA6) were used as model systems. The compatibilizer was a graft copolymer (PP-g-PA6) with PP as the backbone and PA6 as grafts. Its presence in a PP/PA6 blend accelerated the rate of melting of the PA6. This effect was observed only when the compatibilizer itself was molten and migrated to the interfacial layer between the PA6 and PP phases. It is likely that the presence of the compatibilizer increased the chain entanglements at the PP and PA6 interface and consequently reduced the thermal resistance of the interfacial layer. Detailed mechanisms are discussed.

AIP Conference Proceedings, 2007

This work reports a computational and experimental effort to quantify (distributive and dispersiv... more This work reports a computational and experimental effort to quantify (distributive and dispersive) mixing in a single screw extruder. This is done by considering the presence of solid-liquid or liquid-liquid systems and following changes in the average size of the disperse phase along the channel. The experiments are performed in a prototype modular extruder that allows changes in screw geometry

Proceedings of the Fifth International Conference on Engineering Computational Technology

ABSTRACT

Apresenta-se um software para o processo de extrusao monofuso que congrega um tutorial, a monitor... more Apresenta-se um software para o processo de extrusao monofuso que congrega um tutorial, a monitorizacao, a modelacao e a optimizacao do processo. O programa foi desenvolvido por um consorcio nacion ...

International Journal of Material Forming, 2008

The present work presents numerical simulations of dispersive and distributive mixing of a two ph... more The present work presents numerical simulations of dispersive and distributive mixing of a two phase system being processed in a single screw extruder under various conditions. Models were developed to quantify the degree of mixing of liquid-liquid and solids-liquid systems, for a given set of material properties, operating conditions and extruder geometry.

Polymer Engineering & Science, 2013

ABSTRACT

Powder Technology, 2014

ABSTRACT

Devido essencialmente à sua estrutura e à dimensão nanométrica do reforço, os compósitos de NanoF... more Devido essencialmente à sua estrutura e à dimensão nanométrica do reforço, os compósitos de NanoFibras de Carbono (CNF) e matriz termoplástica apresentam um ”limiar” (threshold) de condutividade eléctrica a relativamente baixas concentrações de fibras. Este limiar, no entanto, é muito dependente da história de processamento do material. O presente trabalho descreve uma metodologia de optimização utilizada na produção de compósitos de CNF e nylon-6 numa extrusora monofuso. Esta metodologia é baseada no equilíbrio entre a deformação aplicada e a degradação das fibras durante o processamento. O procedimento de optimização combina uma rotina de modelação do processo de extrusão com Algoritmos Evolutivos Multi-Objectivo, capazes de ter em conta as características complexas deste problema. Os resultados obtidos mostram que existe uma relação directa entre os dois objectivos da optimização utilizados, a deformação aplicada e a degradação induzida no processo, e que é possível relacioná-los com a condutividade eléctrica dos compósitos. De uma forma mais geral, este trabalho permite também concluir que a metodologia desenvolvida tem um grande potencial de aplicação na produção optimizada de compósitos de CNF e matriz termoplástica

Introduction Experimental studies of melting of homopolymers in single screw extruders were initi... more Introduction Experimental studies of melting of homopolymers in single screw extruders were initiated by Maddock (1959) and allowed a qualitative phenomenological understanding of the underlying mechanisms, while Tadmor et al. (1966) developed the corresponding mathematical description. The solid pellets are compacted and form a continuous bed separated from the inner barrel wall by a thin melt film, while the melt deposits in a pool near to the screw pushing flight. In contrast with the Maddock/Tadmor mechanism, melting of individual solid pellets suspended in the melt was observed by Jakopin and Nichols (1984), when using specific screw geometries. Rauwendaal (1989) put forward the first mathematical approach of this dispersed melting sequence. Since single-screw extruders are nowadays often used to manufacture extrudates from complex polymer systems such as polymer blends, it is important to use Maddock-type experiments to understand whether the conventional melting behavior is generally valid or otherwise. Materials PA6 (DSM Akulon F 130), PP (REPSOL ISPLEN 030G1E) and PP-g-MA (ARKEMA OREVAC CA 100) were used to prepare PA6/PP blends, both in their physical and in situ compatibilized forms (in this case, PA6/PP/PP-g-MA). Experimental Procedure The blends were prepared in a prototype modular single screw extruder, with a screw diameter of 30 mm and L/D = 30, fitted with material sampling devices and pressure transducers along the barrel and with a mechanical system for fast screw extraction. Maddock-type experiments were performed in order to investigate the effect on melting of changes in blend composition, screw speed, barrel temperature profile and screw geometry. Cross-sections were cut from polymer helices at regular down-channel intervals and immersed in epoxy for 24h before polishing. Later, the samples were submitted to a dyeing bath in order to tint PA6 and thus distinguish between PA6 and PP. The samples were photographed and the images were subsequently analyzed with the Image-Pro Plus 4.5 software. Results For each experiment, observation of sequences of cross-sections revealed that the melting mechanism of the PA6/PP system is much more complex than the traditional one observed in homopolymers. Usually, the evolution of melting along the screw channel is depicted in terms of a reduced solids width, X/W, as the solid bed is assumed as approximately rectangular. Figure 1 shows also the evolution of melting in terms of the relative area of solids (As/At where As is the total cross-sectional area occupied by solids and At is the total area of the cross-section) and number of solid particles suspended in the melt.

Mathematical and Computational Applications

Solving real-world multi-objective optimization problems using Multi-Objective Optimization Algor... more Solving real-world multi-objective optimization problems using Multi-Objective Optimization Algorithms becomes difficult when the number of objectives is high since the types of algorithms generally used to solve these problems are based on the concept of non-dominance, which ceases to work as the number of objectives grows. This problem is known as the curse of dimensionality. Simultaneously, the existence of many objectives, a characteristic of practical optimization problems, makes choosing a solution to the problem very difficult. Different approaches are being used in the literature to reduce the number of objectives required for optimization. This work aims to propose a machine learning methodology, designated by FS-OPA, to tackle this problem. The proposed methodology was assessed using DTLZ benchmarks problems suggested in the literature and compared with similar algorithms, showing a good performance. In the end, the methodology was applied to a difficult real problem in po...

Engineering Applications of Artificial Intelligence

Feature selection plays a central role in predictive analysis where datasets have hundreds or tho... more Feature selection plays a central role in predictive analysis where datasets have hundreds or thousands of variables available. It can also reduce the overall training time and the computational costs of the classifiers used. However, feature selection methods can be computationally intensive or dependent of human expertise to analyze data. This study proposes a neuroevolutionary approach which uses multiobjective evolutionary algorithms to optimize neural network parameters in order to find the best network able to identify the most important variables of analyzed data. Classification is done through a Support Vector Machine (SVM) classifier where specific parameters are also optimized. The method is applied to datasets with different number of features and classes.

2016 IEEE Congress on Evolutionary Computation (CEC), 2016

Multi-Objective Optimization (MOO) problems might be subject to many modeling or manufacturing un... more Multi-Objective Optimization (MOO) problems might be subject to many modeling or manufacturing uncertainties that affect the performance of the solutions obtained by a multi-objective optimizer. The decision maker must perform an extra step of sensitivity analysis in which each solution should be verified for its robustness, but this post optimization procedure makes the optimization process expensive and inefficient. In order to avoid this situation, many researchers are developing Robust MOO, where uncertainties are incorporated in the optimization process, which seeks optimal robust solutions. We introduce a coevolutionary approach for robust MOO, without incorporating robustness measures neither in the objective function nor in the constraints. Two populations compete in the environment, one representing solutions and minimizing the objectives, another representing uncertainties and maximizing the objectives in a worst case scenario. The proposed coevolutionary method is a coevolutionary version of MOEA/D. The results clearly suggest that these competing co-evolving populations are able to identify robust solutions to multi-objective optimization problems.

Apresenta-se um modelo matemático que quantifica a eficácia da mistura em extrusoras monofuso. Co... more Apresenta-se um modelo matemático que quantifica a eficácia da mistura em extrusoras monofuso. Consideram-se as componentes distributiva e dispersiva da mistura, bem como a eventual ocorrência de coalescência, calculando-se para o efeito a variação de espessura de gotas virtuais, assumidas como dispersas na matriz polimérica, tendo em conta os efeitos da taxa de corte, do tempo de rotura e da probabilidade de coalescência. As rotinas desenvolvidas são então inseridas num programa global de modelação do processo de extrusão monofuso. Torna-se assim possível averiguar qual a influência de alguns parâmetros do modelo nas características da mistura (por exemplo, tensão superficial, viscosidades dos polímeros, dimensão inicial da gota, fracção volumétrica da fase dispersa) e do processo (nomeadamente, velocidade de rotação do parafuso e temperatura do cilindro)

Mathematical and Computational Applications

Solving real-world multi-objective optimization problems using Multi-Objective Optimization Algor... more Solving real-world multi-objective optimization problems using Multi-Objective Optimization Algorithms becomes difficult when the number of objectives is high since the types of algorithms generally used to solve these problems are based on the concept of non-dominance, which ceases to work as the number of objectives grows. This problem is known as the curse of dimensionality. Simultaneously, the existence of many objectives, a characteristic of practical optimization problems, makes choosing a solution to the problem very difficult. Different approaches are being used in the literature to reduce the number of objectives required for optimization. This work aims to propose a machine learning methodology, designated by FS-OPA, to tackle this problem. The proposed methodology was assessed using DTLZ benchmarks problems suggested in the literature and compared with similar algorithms, showing a good performance. In the end, the methodology was applied to a difficult real problem in po...

Mathematical and computational applications, Jan 30, 2023

Engineering Applications of Artificial Intelligence, Nov 1, 2022

Journal of Polymer Science Part B: Polymer Physics, 1999

This article reports on a new phenomenon: The presence of a compatibilizer accelerates the meltin... more This article reports on a new phenomenon: The presence of a compatibilizer accelerates the melting/plastification of an immiscible polymer blend during melt blending. The increase in the rate of melting as a result of the addition of a compatibilizer is believed to be one of the important factors responsible for the fact that the morphology of compatibilized blends develops much faster than that of their uncompatibilized counterparts. To substantiate the above statement, blends based on polypropylene (PP) and polyamide 6 (PA6) were used as model systems. The compatibilizer was a graft copolymer (PP-g-PA6) with PP as the backbone and PA6 as grafts. Its presence in a PP/PA6 blend accelerated the rate of melting of the PA6. This effect was observed only when the compatibilizer itself was molten and migrated to the interfacial layer between the PA6 and PP phases. It is likely that the presence of the compatibilizer increased the chain entanglements at the PP and PA6 interface and consequently reduced the thermal resistance of the interfacial layer. Detailed mechanisms are discussed.

AIP Conference Proceedings, 2007

This work reports a computational and experimental effort to quantify (distributive and dispersiv... more This work reports a computational and experimental effort to quantify (distributive and dispersive) mixing in a single screw extruder. This is done by considering the presence of solid-liquid or liquid-liquid systems and following changes in the average size of the disperse phase along the channel. The experiments are performed in a prototype modular extruder that allows changes in screw geometry

Proceedings of the Fifth International Conference on Engineering Computational Technology

ABSTRACT

Apresenta-se um software para o processo de extrusao monofuso que congrega um tutorial, a monitor... more Apresenta-se um software para o processo de extrusao monofuso que congrega um tutorial, a monitorizacao, a modelacao e a optimizacao do processo. O programa foi desenvolvido por um consorcio nacion ...

International Journal of Material Forming, 2008

The present work presents numerical simulations of dispersive and distributive mixing of a two ph... more The present work presents numerical simulations of dispersive and distributive mixing of a two phase system being processed in a single screw extruder under various conditions. Models were developed to quantify the degree of mixing of liquid-liquid and solids-liquid systems, for a given set of material properties, operating conditions and extruder geometry.

Polymer Engineering & Science, 2013

ABSTRACT

Powder Technology, 2014

ABSTRACT

Devido essencialmente à sua estrutura e à dimensão nanométrica do reforço, os compósitos de NanoF... more Devido essencialmente à sua estrutura e à dimensão nanométrica do reforço, os compósitos de NanoFibras de Carbono (CNF) e matriz termoplástica apresentam um ”limiar” (threshold) de condutividade eléctrica a relativamente baixas concentrações de fibras. Este limiar, no entanto, é muito dependente da história de processamento do material. O presente trabalho descreve uma metodologia de optimização utilizada na produção de compósitos de CNF e nylon-6 numa extrusora monofuso. Esta metodologia é baseada no equilíbrio entre a deformação aplicada e a degradação das fibras durante o processamento. O procedimento de optimização combina uma rotina de modelação do processo de extrusão com Algoritmos Evolutivos Multi-Objectivo, capazes de ter em conta as características complexas deste problema. Os resultados obtidos mostram que existe uma relação directa entre os dois objectivos da optimização utilizados, a deformação aplicada e a degradação induzida no processo, e que é possível relacioná-los com a condutividade eléctrica dos compósitos. De uma forma mais geral, este trabalho permite também concluir que a metodologia desenvolvida tem um grande potencial de aplicação na produção optimizada de compósitos de CNF e matriz termoplástica

Introduction Experimental studies of melting of homopolymers in single screw extruders were initi... more Introduction Experimental studies of melting of homopolymers in single screw extruders were initiated by Maddock (1959) and allowed a qualitative phenomenological understanding of the underlying mechanisms, while Tadmor et al. (1966) developed the corresponding mathematical description. The solid pellets are compacted and form a continuous bed separated from the inner barrel wall by a thin melt film, while the melt deposits in a pool near to the screw pushing flight. In contrast with the Maddock/Tadmor mechanism, melting of individual solid pellets suspended in the melt was observed by Jakopin and Nichols (1984), when using specific screw geometries. Rauwendaal (1989) put forward the first mathematical approach of this dispersed melting sequence. Since single-screw extruders are nowadays often used to manufacture extrudates from complex polymer systems such as polymer blends, it is important to use Maddock-type experiments to understand whether the conventional melting behavior is generally valid or otherwise. Materials PA6 (DSM Akulon F 130), PP (REPSOL ISPLEN 030G1E) and PP-g-MA (ARKEMA OREVAC CA 100) were used to prepare PA6/PP blends, both in their physical and in situ compatibilized forms (in this case, PA6/PP/PP-g-MA). Experimental Procedure The blends were prepared in a prototype modular single screw extruder, with a screw diameter of 30 mm and L/D = 30, fitted with material sampling devices and pressure transducers along the barrel and with a mechanical system for fast screw extraction. Maddock-type experiments were performed in order to investigate the effect on melting of changes in blend composition, screw speed, barrel temperature profile and screw geometry. Cross-sections were cut from polymer helices at regular down-channel intervals and immersed in epoxy for 24h before polishing. Later, the samples were submitted to a dyeing bath in order to tint PA6 and thus distinguish between PA6 and PP. The samples were photographed and the images were subsequently analyzed with the Image-Pro Plus 4.5 software. Results For each experiment, observation of sequences of cross-sections revealed that the melting mechanism of the PA6/PP system is much more complex than the traditional one observed in homopolymers. Usually, the evolution of melting along the screw channel is depicted in terms of a reduced solids width, X/W, as the solid bed is assumed as approximately rectangular. Figure 1 shows also the evolution of melting in terms of the relative area of solids (As/At where As is the total cross-sectional area occupied by solids and At is the total area of the cross-section) and number of solid particles suspended in the melt.

Mathematical and Computational Applications

Solving real-world multi-objective optimization problems using Multi-Objective Optimization Algor... more Solving real-world multi-objective optimization problems using Multi-Objective Optimization Algorithms becomes difficult when the number of objectives is high since the types of algorithms generally used to solve these problems are based on the concept of non-dominance, which ceases to work as the number of objectives grows. This problem is known as the curse of dimensionality. Simultaneously, the existence of many objectives, a characteristic of practical optimization problems, makes choosing a solution to the problem very difficult. Different approaches are being used in the literature to reduce the number of objectives required for optimization. This work aims to propose a machine learning methodology, designated by FS-OPA, to tackle this problem. The proposed methodology was assessed using DTLZ benchmarks problems suggested in the literature and compared with similar algorithms, showing a good performance. In the end, the methodology was applied to a difficult real problem in po...

Engineering Applications of Artificial Intelligence

Feature selection plays a central role in predictive analysis where datasets have hundreds or tho... more Feature selection plays a central role in predictive analysis where datasets have hundreds or thousands of variables available. It can also reduce the overall training time and the computational costs of the classifiers used. However, feature selection methods can be computationally intensive or dependent of human expertise to analyze data. This study proposes a neuroevolutionary approach which uses multiobjective evolutionary algorithms to optimize neural network parameters in order to find the best network able to identify the most important variables of analyzed data. Classification is done through a Support Vector Machine (SVM) classifier where specific parameters are also optimized. The method is applied to datasets with different number of features and classes.

2016 IEEE Congress on Evolutionary Computation (CEC), 2016

Multi-Objective Optimization (MOO) problems might be subject to many modeling or manufacturing un... more Multi-Objective Optimization (MOO) problems might be subject to many modeling or manufacturing uncertainties that affect the performance of the solutions obtained by a multi-objective optimizer. The decision maker must perform an extra step of sensitivity analysis in which each solution should be verified for its robustness, but this post optimization procedure makes the optimization process expensive and inefficient. In order to avoid this situation, many researchers are developing Robust MOO, where uncertainties are incorporated in the optimization process, which seeks optimal robust solutions. We introduce a coevolutionary approach for robust MOO, without incorporating robustness measures neither in the objective function nor in the constraints. Two populations compete in the environment, one representing solutions and minimizing the objectives, another representing uncertainties and maximizing the objectives in a worst case scenario. The proposed coevolutionary method is a coevolutionary version of MOEA/D. The results clearly suggest that these competing co-evolving populations are able to identify robust solutions to multi-objective optimization problems.

Apresenta-se um modelo matemático que quantifica a eficácia da mistura em extrusoras monofuso. Co... more Apresenta-se um modelo matemático que quantifica a eficácia da mistura em extrusoras monofuso. Consideram-se as componentes distributiva e dispersiva da mistura, bem como a eventual ocorrência de coalescência, calculando-se para o efeito a variação de espessura de gotas virtuais, assumidas como dispersas na matriz polimérica, tendo em conta os efeitos da taxa de corte, do tempo de rotura e da probabilidade de coalescência. As rotinas desenvolvidas são então inseridas num programa global de modelação do processo de extrusão monofuso. Torna-se assim possível averiguar qual a influência de alguns parâmetros do modelo nas características da mistura (por exemplo, tensão superficial, viscosidades dos polímeros, dimensão inicial da gota, fracção volumétrica da fase dispersa) e do processo (nomeadamente, velocidade de rotação do parafuso e temperatura do cilindro)