Defence companies managing long-lifecycle products face growing challenges in spare parts management. Injection moulding (IM) becomes less suitable over time due to tool obsolescence, low part demand, and high storage costs. Additive manufacturing (AM) offers a flexible alternative for producing plastic spare parts through on-demand production and digital storage. However, AM adoption remains limited due to technical barriers, unclear internal roles, and the lack of structured tools for evaluation and sustainability.The purpose of this study is to evaluate the feasibility of replacing IM spare parts with AM in defence companies managing long product lifecycles, focusing on operational, design, and sustainabilityperspectives.MethodsThe study was conducted as a single-case study at a defence company. Data was collected through semi-structured interviews, internal documents, observations, and a validation workshop. The research design process followed three phases: (1) understanding the current situation, (2) developing a practical solution, and (3) validating the results. A combination of qualitative methods and triangulation was used to support the findings.ResultsFindings showed that AM is feasible for low-volume spare parts when IM tools are missing or demand is unpredictable. Motivations included shorter lead times, reduced storage needs, and easier access tolegacy parts, which are components no longer in production and for which the original tooling or drawings may be unavailable. The main barriers were limited AM knowledge, unclear responsibilities,and a lack of sustainability tools.Analysis and SynthesisBased on the findings, a structured framework called the Additive Manufacturing Spare Part Assessment and Realisation (AM-SPAR) was developed and applied. It guides users through spare part screening,redesign, validation, and integration into existing supply chains. The framework builds on existing knowledge about AM, spare part management, and sustainability, but has been adapted to address the specific challenges encountered in the defence industry.DiscussionThe study shows that AM works best as a complementary production method to IM. A dual sourcing strategy, utilising IM for initial production and AM for spare parts, was deemed adequate. The framework helped reduce uncertainty and showed that even in cautious organisations, AM can be implemented with clear routines and coordination. The study also showed the need for more accessible tools for assessing sustainability and managing digital spare parts.Conclusions AM is not a universal solution, but it offers clear advantages for specific spare part needs in long-lifecycledefence products. The AM-SPAR framework provides a practical and adaptable guide for evaluating andimplementing AM in real-world settings. It helps organisations move from initial testing to scalable and repeatable AM spare part applications.