## 2 Results of Literature Analysis
### Content Analysis
#### 2.2.3 Constraints  
As mentioned above, the AM process, decision-making, and strategies are governed by the constraints imposed by the business, government, and standards. The literature discussing these limitations and constraints to drive or adopt AM is discussed here. Sourcing  
In order to run the AM, material availability is one of the most important aspects. Companies can adopt single sourcing, dual sourcing, or competitive sourcing by controlling the quantities and negotiating prices. The focus in AM is on raw materials, for which the number of suppliers may be higher. The AM industry can treat these items as leverage or non-critical items rather than strategic or bottleneck items, and sourcing strategies can change. The company must adopt a long-term collaboration with the raw material supplier (even the technology supplier) if there are only a few suppliers in the market. However, due to the uncertainty of demand and better perception of conventionally manufactured products, industries would have to judge their sourcing strategy. In most cases, industries seem to focus on dual sourcing, one for AM and the other for CM-based spare parts development [11]. [PERSON] et al. [11] mentioned that sourcing with AM is better when the AM piece purchase price is high, and the demand rate and backorder costs are high. The reduced holding cost can be an advantage for the AM in this case. The authors also mentioned that in other cases, sourcing using the conventional method remains profitable. [PERSON] et al. [57] studied the value of sourcing spare parts using a mix of AM and CM methods. Their study focused on the aerospace industry. The authors analyzed sourcing through the optimization model, optimal inventory policy, optimal sourcing, and maintenance strategy. Their analysis shows that dual sourcing is the best in the aerospace \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Author**} & \multirow{2}{*}{**Stage**} & \multicolumn{2}{c}{**Parameters**} \\ \cline{3-4}  & & **Cost** & **Other** \\ \hline \multirow{3}{*}{[PERSON] [58]} & \multirow{3}{*}{Multi-stage} & Preparation, building the job assembly, setup, building & \multirow{3}{*}{-} \\  & & of the part, removing the part from the machine, & & \\ \cline{3-4}  & & separating the part from the substrate plate & & \\ \cline{3-4}  & & and postprocessing. & & \\ \hline \multirow{3}{*}{[PERSON] [52]} & \multirow{3}{*}{Single stage} & 3D printer’s maintenance, raw material, labor, & \multirow{3}{*}{-} \\  & & overhead, and sterilization. & & \\ \hline \multirow{3}{*}{[PERSON] [62]} & \multirow{3}{*}{Single stage} & Production, inventory holding, downtime and & \multirow{3}{*}{Performance benefit, \multirow{3}{*}{probability loss}} \\  & & repair, investment & & \\ \hline \multirow{3}{*}{[PERSON] [3]} & \multirow{3}{*}{Single stage} & Labor, software, 3D scanner, machine, building, & \multirow{3}{*}{Machine setup time, Machine} \\  & & energy, raw material, consumable, warehouse unitary & & clean-up time, Inspection time \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Economic models. industry. [PERSON] et al. [55], who studied AM-based spare parts for the army, also mentioned that a dual sourcing strategy, printing urgent spare parts instead of waiting for conventional parts to arrive at scheduled replenishments, is a better option. Although they assume that printed parts are less reliable, AM parts can fill the short-term gap until the CM-based parts are replenished. This type of policy provides a strategic advantage in decreasing the army's reliance on vulnerable supply lines and enables more efficient and effective operations on foreign missions. Currently, most firms and organizations rely on the dual sourcing option to experience the adoption of AM, as they are still hesitant about this technology. This review shows that sourcing is important in making the AM system work.