## 3 Results and Discussion
### Experimental
#### 3.1.2 The Effect of Layer Thickness on Anisotropy  
Three different layer thicknesses of 0.15 mm, 0.25 mm, and 0.35 mm were selected to investigate the influence of layer thickness on the mechanical properties of ABS parts printed in two longitudinal and transverse build orientations. Figure 7 shows the micro-CT images of the samples before any mechanical force is applied. The lower thickness can cause more air voids and this can potentially increase the chances of crack initiation and propagation in parts. However, the smaller diameter of the extruded filaments can also increase the layer adhesion between the deposited filaments because of the increased surface area. On the other hand, the higher layer thicknesses in Figure 7c show lower layer adhesion and therefore there might be a chance of the sample breaking sooner than the other two-layer thicknesses due to the delamination of the layers. Figure 8 visualises the SEM images for the samples of ABS printed in the longitudinal build orientation with different layer thicknesses. No mechanical force was applied to these samples and the samples were cut using a microtome. From those images, larger air voids for the higher thickness can be observed. The mechanical properties of the printed samples (both longitudinal and transverse) were obtained using the Instron machine and the cross-sections of the samples were visualised by SEM. Figure 9 represents the obtained ultimate tensile strengths, elongation at breaks, and Young's modulus values for ABS samples printed in two build orientations at three layer thicknesses of 0.15 mm, 0.25 mm, and 0.35 mm, respectively. The results show that at a layer thickness of 0.25 mm, the difference between the measured tensile strength and elongation at break for the two build orientations is the maximum. According to the SEM images shown in Figure 8, the 0.25 mm layer thickness created much denser structures than the other two cases, and that is why this layer thickness provided the strongest material. The 0.25 mm thickness results show higher tensile strength properties for longitudinal build orientation, while lower values are recorded for the transverse build orientation. [PERSON]'s modulus, on the other hand, showed a drop due to the higher elastic behaviour that this layer thickness can show compared to other layer thickness values. In  
Figure 8: SEM images of ABS samples printed with (a) 0.15 mm layer thickness, (b) 0.25 mm layer thickness, and (c) 0.35 mm layer thickness. Figure 7: Micro-CT images show different air voids and layer adhesion qualities at different layer thicknesses of 0.15 mm (a), 0.25 mm (b), and 0.35 mm (c) for ABS samples. another study, layer thickness was deemed as the most significant factor and the maximum tensile strength was also found at 0.25 mm [29]. Figure 10 shows the SEM images of samples at the cross-section of the break because of mechanical force. For longitudinal samples printed with a 0.15 mm layer thickness, a smaller and higher number of air voids are observed compared to the 0.25 mm and 0.35 mm layer thickness. This in addition to a smaller interface between the layers results in a lower tensile strength, lower elongation at break, and forming a scattered brittle fracture surfaces (highlighted in yellow when force is applied (Figure 10a)). For longitudinal 0.25 mm and 0.35 mm layer thicknesses (Figure 10b,c), larger diameter filaments result in fewer voids and stronger adhesion between the layers due to a larger interface. From Figure 10b,c, concentric cone-shaped fracture surfaces can be observed which justifies the results of higher tensile and elongation values compared to those from the 0.15 mm layer thickness. The slightly better values for 0.25 mm compared to 0.35 mm could be related to the smaller sizes of filaments and consequently, a higher surface area to adhere with the other adjacent rasters. For the transverse build orientation (TBO), the mechanical properties behaviour is completely different when changing layer thickness compared to the longitudinal build orientation.