## 4 Results and Discussion
### Quantitative Analysis of the Impact of AM, Lean and Green Manufacturing
Products can be kept in the form of digital inventory and produced only when necessary [115]. As a result, the environmental burden associated with overproduction and keeping physical inventory is eliminated. Another benefit of on-demand manufacturing is that it is localized; hence, it eliminates the negative environmental impact of transportation. Additive manufacturing can reduce waste in the manufacturing process in several ways, thus contributing towards lean manufacturing. One major contribution is reducing the supply chain lead time through direct conversion of digital models to physical [129]. This helps to eliminate several production steps associated with conventional manufacturing [144]. As a result, the overall operational efficiency in manufacturing products is improved. The digitalized manufacturing system employed in AM eliminates human errors associated with traditional production methods, thereby improving quality [1]. Another contribution of the digitalized systems is that it allows products to be stored in the form of digital files as opposed to physical inventory [115]. Digital files can be printed locally and on demand. Accordingly, the costs associated with keeping unnecessary physical inventory are eliminated. Overproduction can be avoided since products can only be produced when required. The localized production opportunity offered by AM reduces the waiting time associated with outsourcing products from elsewhere [116]. AM can produce products with optimized designs that perform better than their conventional counterparts. The freedom of design offered by AM provides the opportunity to make products with improved quality and durability [108]. Some of the studies in the literature focused on the production of AM-based injection molding and forming tools [112; 113]. The usage of such tools has led to a reduction in the overall cycle time and improved quality. The AM technology can promote green, lean and sustainable manufacturing with the potential for reducing environmental degradation and material usage. Although existing studies differ in the amount of energy consumed during the AM vis-a-vis conventional manufacturing, the environmental friendliness and reduction in carbon footprints exceed those in the conventional manufacturing process. AM provides a feasible option for producing complex parts with improved geometry with the tendency for weight reduction and considerable energy savings. Thus, the energy saved using lightweight materials can compensate for the energy required for the AM main processes and post-processing. Evidence from exiting works indicated that the positive or negative impacts of the AM processes on the environment vary with AM processes employed. For instance, electron beam melting (EBM), selective laser melting (SLM) and selective laser sintering (SLS) can contribute positively to the environment through improvement in design to reduce the total energy consumption and recycling of materials. However, extra energy may be consumed during material processing and the raw material consumed, such as in the form of gases, compressed air, argon, and nitrogen, which may increase the production cost and contribute to marine, terrestrial and freshwater toxicity [66; 67; 145; 146; 147]. For AM processes such as stereolithography (SLA), the energy consumption due to post-processing is relatively low compared to others and the process residues are also negligible, making the post-processing process more environmentally friendly. However, the preprocessing stage is highly energy intensive. For fused deposition modelling (FDM), the raw material consumption is low and provides the option of material recycling. The generated process residues are usually negligible. However, the main processing stage is energy intensive and may contribute to freshwater and marine eutrophication [148]. Existing studies found that the evolution of AM technology has positive impact on the economic, social and environmental factors of sustainability and the industrial revolution [149; 150].