## 2 Additively Manufactured Digital Spare Parts as a Concept  
Spare parts are replacements for failed or lifetime-expired components through repair (repairable) or reproduction (consumable). Currently, spare part businesses are affected by three main issues: Availability, cost, and time [19]. Ideally, companies store a sufficient amount of spare parts in an inventory to avoid downtime, while keeping the number of spare parts as low as possible to avoid high storage costs. Due to significant improvements in AM technologies during recent years and the expected further enhancements [20], a new concept called digital spare parts (DSP), is gaining importance within the manufacturing sector. Companies theoretically have the opportunity to make the storing facilities and traditional manufacturing processes related to spare parts redundant by sending digital models directly to AM service providers or by producing components additively in-house [21; 22; 23]. Potential benefits of DSP could arise from drastically simplified corporate supply chains and decreased inherent complexity, resulting in novel distribution networking models with enhanced operative flexibility [5]. The DSP concept shows major advantages in reduced lead times, part obsolescence in storages, as well as diminished administration and transportation costs. AM is economically feasible in numerous cases, especially when the components are of highly complex and are needed in low quantities [24]. Several AM decision support systems have been developed to support the part selection process [25; 26]. Thus, it is applied to a growing number of industrial companies, not only for prototyping purposes but also for the serial production of end-use components [27]. The DSP concept could be applied to industrial sectors such as aerospace, automotive, and consumer electronics. AM processes/process groups such as plastic and metal-based powder bed fusion (PBF), plastic and metal-based binder jetting, material jetting, vat photopolymerization, and filament-based extrusion have been verified successfully for end-use components, representing the manufacturing processes to be assessed in this study [28; 29; 30; 31]. However, there are few examples [32; 33], in which companies implement this concept in their spare part supply chains. To visualize the changes when applying AM in supply chains, the well-known networking model from [PERSON] et al. is taken as a basis [34]. In Figure 1, this example supply chain network structure is divided into seven hierarchy levels, interacting through convergent or divergent connections. Black lines between hierarchy levels show unidirectional material flow between them, the orange arrows show spare part demands. According to this example model, the demand for spare parts can originate from an employee in e.g., maintenance in the original equipment manufacturer (OEM) company (internal) and from end-use customers (external). External demand can be addressed either to the distributors, which order the part from the OEM, or directly to component suppliers if AM is offered as the manufacturing process. The model comprises the centralized distribution model (component passes all fields) and the decentralized distribution model (component passes through dark grey fields only). Inside the OEM, a digital warehouse management system is needed to store, transfer, and use digital parts for the spare part production and to make decisions regarding whether it is favorable (e.g., in cost and/or lead times) to produce spare parts in-house or forward the order to suppliers. To be able to store and transfer large numbers of digital object files and the necessary additional part related information, a strong enterprise resource planning (ERP)-system must be implemented to face the challenge of the increasing amount of data.