Sustainability in Manufacturing Processes

Abstract

This thesis explores the application of life cycle assessment (LCA) methodology and the life cycle energy (LCE) framework in manufacturing processes through four case studies developed during the PhD programme. The main objective is to evaluate significant technologies, systems and innovations for the manufacturing sector, contributing to the scientific discussion on environmental sustainability. In a general perspective, the environmental impact of case studies applied to the automotive field was evaluated. This thesis begins with an introduction to sustainability in major manufacturing processes, outlining the LCA methodology, its history, relevant regulations and the steps required to conduct a life cycle analysis. Factors influencing environmental impact are examined, looking at three of the main subtractive, additive and casting manufacturing techniques and analysing their potential to improve environmental performance throughout the life cycle. The first case study analyses a component from the automotive field, comparing three of the main production techniques by applying the process of topological optimisation; thus, optimising the geometry of components to assess how lightweighting can contribute to sustainability. The results show that the benefits of topological optimisation applied to the additive manufacturing (AM) process are evident with a reduction in cumulative energy demanded (CED) of approximately 65% compared to approximately 41% and 25% for standard machining (SM) and casting process (CP), respectively. The latter considerations are valid without considering the impact of the moulds for the CED of CP in order to make a comparison with AM and SM without taking into account the number of parts to be produced. However, from an environmental sustainability point of view, CP is influenced by the energy contribution of the moulds, which weigh on the production of a few parts. The study was therefore completed by also considering the contribution of the moulds and weighting it against the batch size. Nevertheless, the results show that, to emphasise this influence and weigh the contribution of each phase, it is necessary to carry out more detailed LCA studies with different geometries, changing the percentage of volume to be removed from the initial billet before obtaining the desired product. In fact, in general, LCA is competitive with AM if the shape to be produced is simple and, therefore, far from being topologically optimised. Starting from these results, additive manufacturing as an alternative to conventional processes is analysed, examining different sustainability parameters and also considering different recycling scenarios and energy mixes. These were chosen to assess the impact, both in terms of energy (CED) and environmental sustainability, of countries using energy from renewable and non-renewable sources. The advantage of SM as a less environmentally impactful process compared to AM was observed at least up to a 90% reduction in billet mass due to the energy consumed in the manufacture of the product. This result is only consistent if the chip generated in the SM process is recycled properly, otherwise the material energy impact for the SM process is markedly impactful, with the break-even point between the two processes being between 40% and 80% of the billet material reduction. Furthermore, as the production phase increases, the environmental impact of the choice of production site begins to become increasingly relevant due to the peculiarities of the countries' energy sources. In fact, looking at the CED, the environmental impact of oil sources is more relevant when compared to nuclear and/or hydroelectric energy sources. Nuclear energy, on the other hand, loses its environmental competitiveness, even compared to oil, when specific midpoint and endpoint indicators are taken into account. On the other hand, by analysing components in the automotive sector, it was possible to assess how the use phase has a considerable impact on the environmental impact of the component, so in order to lighten the masses being transported as much as possible, the focus of the thesis shifted to the analysis of another case study that is lightened by joining two different materials, namely steel and composite materials. Specifically, a conventional solid steel gear, a lightweight gear and a hybrid gear were compared from the point of view of sustainability, using life cycle energy quantification. In addition, two end-of-life (EoL) scenarios were considered: a conventional open-loop scenario with partial recycling and a closed-loop scenario with full recycling, including thermal recycling for carbon-fibre reinforced plastics. The overall CED rating for the ‘greener’ approach results in the values of 898.92 MJ, 649.10 MJ, 697.87 MJ considering full, lightweight and hybrid gear, respectively. In this scenario, the lightweight and hybrid solutions are comparable, with a CED difference of approximately 7.50 per cent. On the other hand, the hybrid gear achieves a CED saving of 28.82% compared to the full gear. Finally, the focus shifted to the end-of-life processes of polymer matrix composites, evaluating recycling and remanufacturing strategies from an energy (CED) perspective in order to develop sustainable approaches according to material type and field of application. In detail, polypropylenecarbon fibres (PP-CF), polypropylene-glass fibre (PP-GF), polyether-ether-ketone-carbon fibre (PEEK-CF) and polyether-ether-ketone-glass fibre (PEEK-GF) at different reinforcement volume fraction percentages and three end-of-life processes were compared, namely, energy recovery by combustion, reuse by thermoforming and finally recycling of the reinforcement by pyrolysis. When analysing the different end-of-life scenarios and filling percentages, the main results show that if the composite has a low matrix value (PP) and a high reinforcement value (CF), both end-of-life processes, i.e. recycling and reforming, minimise the energy impact. On the other hand, if the composite consists of a high-value matrix (PEEK) and a low-value fibre (GF), reforming is more energy-efficient, although its advantages are more evident for low-value fibres; recycling is not always more advisable than combustion. In fact, if the composite consists of low-value (GF) fibres, combustion is preferable. To present a comprehensive overview of the work, the graphical abstract in Figure 0.1 was provided. The conclusions offer a discussion of the main issues that emerged, highlighting how the LCA approach can be used not only to evaluate, but also to guide the development of more environmentally friendly technologies, providing a baseline for the future of the manufacturing sector.