Attaining lightweight design of mechanical parts and components is an ever-lasting goal of designers during design process. In this paper, a systematic lightweight design methodology is proposed for mould and die design enabling us to avoid conventional ways of empirical approaches, leading to substantial cost reduction in part's material utilization and minimizing environmental impacts in product realization. At the initial stage, the characteristics of a lightweight mould or die are identified using the so-called lightweight coefficient defined as the ratio of the part's functional performance to its mass. The proposed design methodology was implemented to attain an optimum lightweight coefficient resulting in desirable mass reduction as well as overall functional performance improvements of the final products. In the case study, the design was started with numerical simulation to model the process-part interaction for a typical extrusion operation, which allowed the identification of the numerical design parameters that affect the required performances as well as the threshold values that must be met. Structural optimization scheme was then employed to achieve the targeted performances with structural components of minimum mass by applying the design criterion of reducing the thickness of components' walls. The study was done on the redesign of an actual polymer pipe extrusion die. The results showed that 21.67% (3325.26 kg) mass reduction of the underlying extrusion die, together with the evaluated increase of lightweight coefficient by 25.23%, could be obtained with the specified performances of productivity, static stiffness, compressive strength and assembly property satisfied. All in all, this work provides a foundation for dealing with lightweight mechanical parts design aimed at conventional heavy duty machine components with complex functionalities such as moulds and dies. The proposed lightweight coefficient design methodology proves to be effective in reducing parts' weight without sacrificing the part's specifications and desired functionalities. In future work other design criteria such as shortening the length of the flow channel or reducing the number of assembly parts are to be investigated to further improve the design outcomes. Moreover, life cycle assessment (LCA) will be performed to evaluate the positive impact of lightweight design on the environment due to material saving and machining wastes reduction in realizing the parts.