## 2 Materials and Methods
### Empirical Factors and Orthogonal Design  
Five processing variables were chosen for analysis, including print orientation, fill density, oven temperature, nozzle temperature, and print speed. Before setting the values, a few test trials were first completed to identify the range of processing parameters that can successfully print the parts. For melt extrusion 3D printing, the print orientation was then set at either 0\({}^{\circ}\) or 45\({}^{\circ}\) (as shown in Figure 7). The fill density was 40%, 50%, or 60%. The oven temperature was maintained at 25 \({}^{\circ}\)C, 30 \({}^{\circ}\)C, or 40 \({}^{\circ}\)C, while the nozzle temperature was set at 210 \({}^{\circ}\)C, 220 \({}^{\circ}\)C, or 230 \({}^{\circ}\)C. The print speed (moving speed of the nozzle) was 50, 55, or 60 mm/s.  
For solution extrusion 3D printing, five parameters were selected for analysis, including print speed, fill density, PLA/DCM ratio, oven temperature, and print orientation. The print speed (moving speed of the nozzle) was 30, 35, or 40 mm/s. The fill density was 45%, 65%, or 75% (as shown in Figure 8). The PLA/DCM ratio (_w/v_) was set to be 5 g/5.5 mL, 5 g/5.8 mL, or 5 g/5.2 mL. The oven temperature (air temperature of the oven) was kept at 25 \({}^{\circ}\)C, 20 \({}^{\circ}\)C, or 22 \({}^{\circ}\)C. The print orientation selected was either 0\({}^{\circ}\) or 45\({}^{\circ}\). Tables 1 and 2 list the adopted factors and factor levels for the melt and solution printing experiments, respectively.  
Figure 6: Photos of square-shaped parts right after printing (**left**), and 7 days post printing (**right**).  
Figure 7: Print orientation and the optical image of printed parts (**A**) 0\({}^{\circ}\), (**B**) 45\({}^{\circ}\).  
A Taguchi L18 orthogonal array design [14] (Tables 3 and 4, respectively, for melt extrusion FDM and solution extrusion printings) was adopted for analysis. The array provides the method of completing the minimised number of experimental runs while providing the full data of all the parameters affecting the performance [15]. To optimise the printed products' mechanical properties, the optimised processing condition can be obtained by combining all the processing conditions that possess the highest tensile strengths.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Factor**} & **A: Print** & **B: Fill** & **C: Oven** & **D: Nozzle** & **E: Print Speed** \\  & **Orientation** & **Density (\%)** & **Temperature (\({}^{\circ}\)C)** & **Temperature (\({}^{\circ}\)C)** & **(mm/s)** \\ \hline Level 1 & 0\({}^{\circ}\) & 40 & 25 & 210 & 50 \\ Level 2 & 45\({}^{\circ}\) & 50 & 30 & 220 & 55 \\ Level 3 & 60 & 35 & 230 & 60 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Parameters used for melt-extruded 3D printing.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Factor**} & **A: Print Speed** & **B: Fill** & **C: PLA/DCM \({}^{*}\)** & **D: Oven** & **E: Print** \\  & **(mm/s)** & **Density (\%)** & **(gram/mL)** & **Temperature (\({}^{\circ}\)C)** & **Orientation** \\ \hline Level 1 & 30 & 45 & 5/5.5 & 25 & 0\({}^{\circ}\) \\ Level 2 & 35 & 65 & 5/5.8 & 20 & 45\({}^{\circ}\) \\ Level 3 & 40 & 75 & 5/5.2 & 22 & \\ \hline \hline \end{tabular} \({}^{*}\) PLA: polylactide; DCM: dichloromethane.  
\end{table}
Table 2: Parameters used for solution extruded 3D printing.  
Figure 8: Appearance and optical images of printed parts with different fill densities, (**A**) 45%, (**B**) 65%, (**C**) 75%.