Energy Efficiency and Decarbonisation Pathways in Injection Moulding: A Life Cycle Assessment of End-of-Life Allocation Methods

Life Cycle Assessment (LCA) is extensively employed to support sustainability evaluation in waste management and manufacturing systems; however, outcomes are highly sensitive to methodological decisions, particularly end-of-life (EoL) allocation approaches. This study examines how different allocation methods—primarily cut-off and substitution approaches—affect the interpretation of energy performance and decarbonisation potential in plastic waste management and injection moulding systems. The analysis applies cut-off logic to open-loop scenarios to establish a baseline impact, while substitution-based modelling is utilised for semi-closed and fully closed-loop configurations to quantify environmental credits and avoided burdens. A dual framework is adopted: first, a literature review examines methodological sensitivities in EoL modelling, focusing on allocation logic and system boundaries; second, a quantitative case study assesses open-loop, semi-closed, and fully closed-loop injection moulding scenarios for polyethylene (PE) products using LCA and hot-spot analysis. The results demonstrate that allocation choices can significantly influence calculated energy savings and greenhouse gas reduction potentials, sometimes reversing the relative ranking of configurations. Substitution-based approaches tend to report higher decarbonisation benefits by crediting avoided primary production, whereas cut-off approaches provide more conservative estimates. In the case study, increased internal material and water looping lead to measurable reductions in energy demand, although trade-offs across impact categories persist. These findings highlight that circular economy (CE) evaluations are strongly shaped by methodological assumptions, with direct implications for energy policy and decarbonisation pathways. The study emphasises the need for transparent allocation decisions and robust frameworks to ensure reliable decision-making in the transition toward low-carbon and energy-efficient systems.