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Alvarez-Miranda, E., Pereira, J., Torrez-Meruvia H., & Vila, M. (2021). A Hybrid Genetic Algorithm for the Simple Assembly Line Balancing Problem with a Fixed Number of Workstations. Mathematics, 9(17), 2157.
Abstract: The assembly line balancing problem is a classical optimisation problem whose objective is to assign each production task to one of the stations on the assembly line so that the total efficiency of the line is maximized. This study proposes a novel hybrid method to solve the simple version of the problem in which the number of stations is fixed, a problem known as SALBP-2. The hybrid differs from previous approaches by encoding individuals of a genetic algorithm as instances of a modified problem that contains only a subset of the solutions to the original formulation. These individuals are decoded to feasible solutions of the original problem during fitness evaluation in which the resolution of the modified problem is conducted using a dynamic programming based approach that uses new bounds to reduce its state space. Computational experiments show the efficiency of the method as it is able to obtain several new best-known solutions for some of the benchmark instances used in the literature for comparison purposes.
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Alvarez-Miranda, E., Pereira, J., Vargas, C., & Vila, M. (2022). Variable-depth local search heuristic for assembly line balancing problems. Int. J. Prod. Res., Early Access.
Abstract: Assembly lines are production flow systems wherein activities are organised around a line consisting of various workstations through which the product flows. At each station, the product is assembled through a subset of operations. The assembly line balancing problem (ALBP) consists of allocating operations between stations to maximise the system efficiency. In this study, a variable-depth local search algorithm is proposed for solving simple assembly line balancing problems (SALBPs), which are the most widely studied versions of the ALBP. Although the state-of-the-art techniques for solving the SALBP consist of exact enumeration-based methods or heuristics, this paper proposes a local search-based heuristic using variable-length sequences that allow the solution space to be efficiently explored. The proposed algorithm improves the best solution known for multiple instances reported in the literature, indicating that its efficiency is comparable to those of the state-of-the-art method for solving the SALBP. Moreover, the characteristics of the instances for which the proposed procedure provides a better solution than previously reported construction procedures are investigated.
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Vallejos Baier, R., Contreras Raggio, J. I., Millán Giovanetti, C., Palza, H., Burda, I., Terrasi, G., et al. (2022). Shape fidelity, mechanical and biological performance of 3D printed polycaprolactone-bioactive glass composite scaffolds. Mater. Sci. Eng. C, 134, 112540.
Abstract: Direct ink writing (DIW) is a promising extrusion-based 3D printing technology, which employs an ink-deposition nozzle to fabricate 3D scaffold structures with customizable ink formulations for tissue engineering applications. However, determining the optimal DIW process parameters such as temperature, pressure, and speed for the specific ink is essential to achieve high reproducibility of the designed geometry and subsequent mechano-biological performance for different applications, particularly for porous scaffolds of finite sizes (total volume > 1000 mm3) and controlled pore size and porosity. The goal of this study was to evaluate the feasibility of fabricating Polycaprolactone (PCL) and bio-active glass (BG) composite-based 3D scaffolds of finite size using DIW. 3D-scaffolds were fabricated either as cylinders (10 mm diameter; 15 mm height) or cubes (5 × 5 × 5 mm3) with height/width aspect ratios of 1.5 and 1, respectively. A rheological characterization of the PCL-BG inks was performed before printing to determine the optimal printing parameters such as pressure and speed for printing at 110 °C. Microstructural properties of the scaffolds were analyzed in terms of overall scaffold porosity, and in situ pore size assessments in each layer (36 pores/layer; 1764 pores per specimen) during their fabrication. Measured porosity of the fabricated specimens�PCL: =46.94%, SD = 1.61; PCL-10 wt%BG: = 48.29%, SD = 5.95; and PCL-20 wt% BG: =50.87%, SD = 2.45�matched well with the designed porosity of 50%. Mean pore sizes�PCL [ = 0.37 mm (SD = 0.03)], PCL-10%BG [ = 0.38 mm (SD = 0.07)] and PCL-20% BG [ = 0.37 mm (SD = 0.04)]�were slightly fairly close to the designed pore size of 0.4 mm. Nevertheless there was a small but consistent, statistically significant (p < 0.0001) decrease in pore size from the first printed layer (PCL: 0.39 mm; PCL-10%BG: 0.4 mm; PCL-20%BG: 0.41 mm) to the last. SEM and micro-CT imaging revealed consistent BG particle distribution across the layers and throughout the specimens. Cell adhesion experiments revealed similar cell adhesion of PCL-20 wt% BG to pure PCL, but significantly better cell proliferation � as inferred from metabolic activity � after 7 days, although a decrease after 14 days was noted. Quasi-static compression tests showed a decrease in compressive yield strength and apparent elastic modulus with increasing BG fraction, which could be attributed to a lack of adequate mechanical bonding between the BG particles and the PCL matrix. The results show that the inks were successfully generated, and the scaffolds were fabricated with high resolution and fidelity despite their relatively large size (>1000 mm3). However, further work is required to understand the mechano-biological interaction between the BG particle additives and the PCL matrix to improve the mechanical and biological properties of the printed structures.
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Vukasovic, T., Vivanco, J. F., Celentano, D., & Garcia-Herrera, C. (2019). Characterization of the mechanical response of thermoplastic parts fabricated with 3D printing. Int. J. Adv. Manuf. Technol., 104(9-12), 4207–4218.
Abstract: 3D printing has gained great popularity due to its main feature of manufacturing complex geometries. The building process by adding successive layers generates mechanical properties that depend on the printing parameters, where build orientation is one of the most relevant factors. Due to this, the characterization of the mechanical response of these pieces is a challenging task of practical importance to estimate their lifespan. The aim of this study is to characterize the mechanical behavior and define a 3D constitutive model of polymer materials commonly used in 3D printing manufacturing. Hence, ABS and PLA were used with a low-cost desktop printer with which specimens were manufactured in two orthogonal orientations: flat and upright. Tensile and compression tests were performed to this end, where the Young's modulus, yield, and maximum stresses were determined. In the tensile tests, the samples with vertical (upright) orientation showed lower values in the evaluated mechanical properties than the corresponding to the horizontal (flat) orientation. However, no significant difference caused by the printing orientations was observed in the compression tests. Different values of Young's modulus and maximum strength were found between tensile and compression tests for the same material and orientation. Moreover, in order to describe the observed material response, a linear isotropic bimodular model is proposed. This constitutive model, which is fed with the previously obtained tensile and compression data, is used in the simulation of a four-point bending test where it is found to adequately represent the experimentally measured elastic behavior in the load-deflection curve. Thus, the combination of experiments and a bimodular constitutive model contributes to making better predictions of the mechanical response of structures made with 3D printing.
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