# Solution Extrusion Additive Manufacturing of Biodegradable Polycaprolactone
## 4 Discussion  
Two factors might affect the mechanical properties of an additively manufactured product and determine the success of the product in a given application. The first factor is the healing or fusion of layer interfaces, and the second is the size of the pores. To produce parts of greater strength in solution extrusion additive manufacturing, the enhancement of chain entanglements [15] at the strip interface is essential and is responsible for the final mechanical properties of additively manufactured products [16]. In solution extrusion, solvent exhalation and diffusion of molecular chains occur at the solidification stage of polymeric materials. It has been reported that in the solution processing of polymers, the optimum solution is a semi-dilute, mildly entangled pattern, which occurs at the critical entanglement concentration Ce [17] (10-12 w/v% for PCL when DCM is used as the solvent [18]). Figure 14: DSC curves of PCL: as purchased, after solution extrusion additive manufacturing, and after solution extrusion manufacturing and annealing. Figure 13: Various PCL parts additively manufactured using the optimum processing conditions. This period is a critical point between a semi-dilute unentangled pattern and a semi-dilute mildly entangled pattern. In this range, enhanced healing at the strip interfaces can be anticipated. Despite the PCL concentration used (approximately 7%) in this work being less than optimal, fusion and healing at the interface was still expected. As for the second factor, a solution extrusion additively manufactured part may have tiny incompletely fused pores and cracks, which may lead to stress concentrations when subjected to external forces. The strength of fabricated parts is thus decreased. The experimental results in Figure 5 suggest that the strength of the manufactured parts increased with the volume of DCM. The parts made with PCL/DCM = 5 g/7.2 mL had the highest mechanical strength. When less solvent was used, the solvent can volatilize too rapidly, leading to insufficient time for the polymeric solution to go through molecular entanglement at the strip interfaces. A higher concentration of DCM maintains the polymeric materials in a semi-dilute state for a sustained period of time and helps the polymeric strips to fuse and close up at the interfaces. This occurs not only in the same layer but also across various layers, as shown in Figure 9. Thus, the parts exhibited higher strength. The data in Figure 6 suggest that the mechanical properties of the parts increased with the fill density. The fill density is defined as the amount of polymer used in fabricating a specimen. A higher fill density implies more plastic material within the manufactured samples, thus resulting in a stronger product. Furthermore, filling the parts with more material also leads to smaller pore sizes (Figure 10), which increases the manufactured part's quality. Meanwhile, as pointed out earlier, the strength of the manufactured parts also increased with the volume of DCM. Therefore, there exists a balance between volume of DCM and amount of polymer that leads to the maximum strength. The empirical data in Figure 7 suggest that the strength decreases with the print speed. Once the solution is expelled from the dispensing tip, the solvent starts to volatilize. When the tip is too fast, not as much time is allowed for the solution to go through molecular entanglement at the strip's interface (Figure 11). In addition, the solvent may not have enough time to evaporate. The excessive solvent may spread and dissolve the neighboring strips and decrease the chain entanglement. The strength decreased accordingly. The parts fabricated with a 45\({}^{\circ}\) print orientation of infill exhibited the lowest strength, while 90\({}^{\circ}\) resulted in the greatest part strength. One possible explanation is that the 90\({}^{\circ}\) orientation of extruded strips limits the amount of solvent that quickly evaporates.