## 3 Results and Discussion
### Experimental
#### 3.1.3 The Effects of Raster Angle on Anisotropy
The Effects of Infill Percentage on Anisotropy  
For this study, infill percentages of 40%, 60%, 80%, and 100% were considered for ABS polymers and both build orientations: longitudinal and transverse. Another influential machine parameter on mechanical anisotropy is infill percentage. Although a lower infill percentage can lead to a faster print and therefore a more profitable production method, the lower infill can tremendously influence the mechanical integrity of the printed part. Figure 16 shows the micro-CT images from uninterrupted samples at different infill percentages. The number and the volume of the air voids reduce enormously when the infill percentage is increased from 40% to 100%. The tensile strength results for samples printed with ABS polymer in the longitudinal build orientation showed a gradual increase of 1 Mpa when the infill percentage was increased from 40% to 80%. The strength showed a jump of 38% when the infill increased from 80% to 100%. Like the UTS values, Young's modulus values also increased by 17%, 28%, and 54% when the infills increased from 40% to 100%, respectively. On the other hand, the elongation at break showed a steady decline by increasing the infill percentage. The increased tensile strength and Young's modulus could be related to the better failure resistance and enhanced material bonding of the part due to a denser internal structure when the infill percentage increases, and the decrease in the elongation at break could be related to a decrease in the number of air voids, as well as the stiffer structure with less flexibility. A similar upward trend for the tensile strength and peak point at 100% infill when the infill percentage changed from 20% to 100% was recorded by [PERSON] et al. [32]. The same trend was also recorded when the transverse build orientation was selected. A steady increase was recorded when the infill percentage was increased from 40% to 100%. The same trend was recorded for the elongation at break. Interestingly, the values  
Figure 16: Micro-CT images of ABS samples showing different air void numbers and sizes when the infill percentage changes; (a) 40%, (b) 60%, (c) 80%, (d) 100%. Figure 15: Mechanical anisotropy (\(\Delta\) (UTSL) and \(\Delta\) (UTSI)) for ABS parts printed at different raster angles. of the elongation at break for 100% infill for both the longitudinal and transverse build orientations were very close. Figure 17 represents the obtained ultimate tensile strengths, elongation at break, and [PERSON]'s modulus values for ABS-made samples in the two build orientations--transverse and longitudinal--at two different infill percentages of 40%, 60%, 80%, and 100%, respectively. Like the mechanical testing results for ABS samples, from SEM images for the longitudinal build orientation, no significant structural change can be traced for infill percentages between 40% and 80% (Figure 18a-c). However, at 100% infill, a more unified structure with smaller air voids can be seen (Figure 18d). SEM images for ABS samples printed in the transverse build orientation showed that by increasing the infill percentage from 40% infill to 100%, the air voids shrank, and more uneven fracture surfaces were observed (Figure 18e-h). The uneven broken surface structures suggested a better layer adhesion. However, at lower infills in comparison with the 100% infill, the air voids in the structure and lower layer adhesions between the rasters were dominant and that could be the reason for the lower mechanical properties of the lower infill percentages. The effects of infill percentage on the mechanical properties of the printed ABS samples were investigated and the results can be seen in Table 5.  
Figure 18: SEM images of 3D-printed ABS samples. (**a**–**d**) represent LBO and (**e**–**h**) represent the TBO at 40%, 60%, 80%, and 100% infills, respectively. Yellow marks show the unevenly fractured surfaces, whereas the red marks represent the unwanted air voids.