## 4 Discussion
### Solution Extrusion-Printed Parts  
The experimental results in Figure 10B (factor level D3) show that we should adopt an oven temperature of 22 \({}^{\circ}\)C to print parts of the greatest mechanical strength. This is due to the fact that to produce parts of greater strength, it is necessary to enhance chain entanglements [17] at the interfaces of various extruded strips. During solution printing, evaporation of solvents and chain diffusion of polymers take place during the solidification process of polymeric materials, and chain entanglement is liable for the final mechanical properties of printed parts [18]. The previous study reported that for solution processing of polymeric materials, the best polymeric solution is a semi-dilute, moderately entangled regime, which takes place at Ce (critical entanglement concentration) [19]. This stage is the crossover point from the semi-dilute un-entangled regime to the semi-dilute moderately entangled regime. Within this range, enhanced entanglement at the strip interfaces can be expected. When the oven temperature is too high (high than 22 \({}^{\circ}\)C), the solvent can evaporate too fast, and there is insufficient time for the polymeric solution to undergo molecular entanglement at the interface. The tensile strengths of printed parts thus decreased. On the other hand, if the oven temperature is too low, it would take longer for the solvent to be evaporated. The excessive solvent might diffuse and dissolve the surrounding strips (as shown in Figure 13B) and diminish the molecular chain entanglement. Printed part strengths decreased accordingly. The data in Figure 10B suggest the optimal fill density to be 65% (factor level B2). The fill density defines the number of polymeric materials used in printing a part. A higher fill density indicates that there is more plastic on the inside of the print, leading to a stronger object. Nevertheless, as the fill density is too high, due to the excessive solvent, it might take longer to solidify the semi-dilute materials. The unevaporated solvent might diffuse and penetrate into the surrounding strips, and reduce the molecular chain entanglement and the relevant part strengths. Adopting the highest concentration of DCM (i.e., PLA/DCM = 5/5.8, factor level C2 in Figure 10B) increased the mechanical strength of the solution-printed parts. Increasing the solvent keeps the polymer in the semi-dilute state for a longer time. It is then easier for the polymeric strips to heal at the interfaces. Printed PLA parts thus exhibited higher strength. The empirical data suggested that a print speed of 35 mm/s (factor level A2 in Figure 10B) should be used to print parts with the highest strengths. As soon as the solution is extruded out of the nozzle, the solvent begins to evaporate. If the print speed is too high, the solvent evaporates too fast, and there is less time for the polymeric solution to undergo molecular entanglement at the interface. By contrast, if the print speed is too low, it would take a longer time for the solvent to evaporate. The excessive solvent might diffuse and diminish the molecular chain entanglement. The strength of the printed parts is thus decreased. Finally, the 45\({}^{\circ}\) orientation printed parts exhibited higher strength than the 0\({}^{\circ}\) printed parts. Due to the irregular fracture characteristic at the ductile interfaces (Figure 11B), the effect of print orientation on the solution extrusion-printed parts is relatively limited. This study has satisfactorily utilized the Taguchi technique to maximize the mechanical properties of both solutions and melt extrusion 3D-printed products. The same method can also be adapted to optimize the other properties of printed parts or the printing of other materials. This proves the eventuality of the empirical design in 3D printing products of different applications.