# Spare Parts Made by Additive Manufacturing to Improve Preventive Maintenance
## 1 Introduction
In this context, the characteristics of additive manufacturing improve manufacturing systems, enabling them to produce more complex parts in a shorter time and in a more cost-efficient way. There are studies that claim that the use of additive manufacturing for spare parts has a potential that has not yet been fully explored. AM enables the production of old spare parts, facilitates the management of stock levels of those products with a high level of customisation, and decentralises production. Integration of AM in the design of supply chains can be considered a strategic decision that is part of the fourth industrial revolution [14]. The concept of digital spare parts has not yet gained much ground in the manufacturing industry. Researchers [PERSON] et al. identified some reasons for its selective rejection. This study has been conducted from a corporate perspective, outlining a holistic structure of the supply chain network, to visualise different digital spare parts distribution scenarios, and assessing the technical and economic capabilities of additive manufacturing [15]. Additionally, other threats regarding the use of digital spare parts have to be explored, such as the cyber security issues inherent to the Industry 4.0 paradigm during the transfer, execution and storage of the digital models [16]. Despite its interest, particularly in developing specific cyber security infrastructures and solutions for the AM scenario, this topic is out of the scope of this paper. [PERSON] et al. have analysed, through discrete event simulation, the use of AM as a replacement for traditional warehouse inventory for an on-demand spare parts supply system. A generic AM system based on a powder bed fusion was used in the model, and the same modelling approach could be applied to other types of AM processes. Using this model, this paper studied the impact of the characteristics of spare parts demand (e.g., part size attributes, demand rates) and the way the AM operation characteristics (e.g., machine size and deferral strategy) affect the performance of using AM to supply spare parts [17]. [PERSON] et al. analysed the manufacturing of the spare part at the place where the equipment needs it, which provides an interesting opportunity to implement additive manufacturing techniques in the field. They have conducted a case study of a spare part, particularly a valve cover used in electric diesel generators, needed by the German Federal Armed Forces, with a methodology for handling the data in the integrated design and manufacturing process [18]. [PERSON] et al. have compared the reliability and cost of a jet engine with components manufactured using additive manufacturing or welded parts. In the process, key reliability characteristics of AM are found, and overhaul schedules are defined for a fleet of aircraft manufactured with AM aircraft components. The simplicity and performance demonstrated in the comparison make the proposed method a powerful engineering tool for the evaluation of additive manufacturing in aeronautics [19]. [PERSON] et al. have identified potential benefits in the automotive aftermarket for transforming this business by providing individualised spare parts on demand and on-site, without the need for costly tooling. However, their results also revealed that neither of the two concepts is technically feasible from today's point of view. Nevertheless, additive technologies have advantages, as outlined in this study, which will affect the automotive aftermarket in the future [20]. [PERSON] et al. have proposed a decision support system (DSS) to automatically determine the suitability of a spare part or assembly for AM applications, based on machine learning (ML) and carefully selected choice criteria. In order to support efficient candidate selection in the early conceptual design phase, these criteria are individually decoded into decisive parameters that can be extracted from digital models or resource planning databases.