## 3 Results and Discussion
### Experimental
#### 3.1.2 The Effect of Layer Thickness on Anisotropy
In this build orientation, the lowest layer thickness shows the best performance followed by the highest layer thickness value, and then 0.25 mm. In the 0.15 mm layer thickness (Figure 10d), the uneven fractured surface shows a better layer adhesion with fewer air voids compared to 0.25 mm (Figure 10e). In the 0.35 mm layer thickness (Figure 10f), there are no evident air voids, and the layers are well-defused, but the layers are thicker than 0.15 mm and the number of rasters in each layer, as well as their effective surface area, is less and consequently, the tensile stress and strain are slightly lower than 0.15 mm. The effects of layer thickness on the mechanical properties of the printed ABS samples were investigated and the results of the ultimate tensile strength (UTS) strain at break (\(\Delta\)), elastic modulus (E), and their associated root square error percentages (SQRT error mean) of the ABS polymer in the two build orientations--longitudinal (LBO) and transverse (TBO)--are mentioned in Table 3.  
Figure 9: (**a**) Tensile strength, (**b**) elongation, and (**c**) Young’s modulus values for ABS samples at different layer thicknesses. The mechanical degree of anisotropy for the longitudinal (\(\Delta\) (UTSL)) and transverse (\(\Delta\) (UTST)) build orientations at different layer thicknesses for the ABS polymer are shown in Figure 11. Both graphs exhibit comparable trends in terms of anisotropy behaviour. The lowest degree of anisotropy at the 0.15 mm layer thickness for the longitudinal and transverse build orientation was found with values of 65.5% and 39.6%, respectively. At this layer thickness, the longitudinal and transverse build orientations show the lowest difference in tensile strength value which shows an ideal condition for 3D printing a part when isotropic mechanical properties are the priorities. Although the printing time can noticeably increase at this layer thickness, the characteristics of the final printed product can show less profound anisotropy features. At the same time, the elasticity of the part can be compromised because of the build orientation influence which shows less ductile behaviour when printing in the transverse build orientation. \begin{table}
\begin{tabular}{l l l l l l l} \hline \hline \multicolumn{6}{l}{**Longitudinal Build Orientation**} \\ \hline Layer & UTS\({}_{\text{(mean)}}\) & UTS\({}_{\text{SQRT Error Mean}}\) & \(\Delta_{\text{(mean)}}\) & \(\Delta_{\text{SQRT Error Mean}}\) & E\({}_{\text{(mean)}}\) & E\({}_{\text{SQRT Error Mean}}\) \\ thickness & (MPa) & (MPa) & (\%) & (\%) & (MPa) & (MPa) \\
0.15 mm & 17.12 & 0.49 & 0.16 & 0.02 & 1883 & 58.34 \\
0.25 mm & 12.21 & 0.19 & 0.12 & 0.006 & 1645 & 77.61 \\
0.35 mm & 14.28 & 0.36 & 0.15 & 0.016 & 1649 & 102.19 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: UTS, \(\Delta\), and E values for ABS samples printed in the LBO and TBO at different layer thicknesses. Figure 10: SEM images of 3D-printed ABS samples: (**a**) (0.15 mm), (**b**) (0.25 mm), and (**c**) (0.35 mm) represent LBO, and (**d**) (0.15 mm), (**e**) (0.25 mm), and (**f**) (0.35 mm) represent TBO. Yellow marks show the unevenly fractured surfaces and red marks represent the air voids.