## 3 Results and Discussion
### Experimental
#### 3.1.5 The Effect of Nozzle Temperature Variation on Anisotropy  
The effect of the printing temperature or nozzle temperature on material anisotropy is less studied in the literature. In this study, temperatures of 220 \({}^{\circ}\)C, 240 \({}^{\circ}\)C, 260 \({}^{\circ}\)C, and 280 \({}^{\circ}\)C were selected as the nozzle temperatures for printing ABS samples in the longitudinal and transverse build orientations. A good understanding of the nature of the polymer and its behaviour when the temperature changes is the key to generating parts with fewer air voids and, consequently, better structure with maximum resistance to deformation and break. Figure 20a-d show the micro-CT images of the ABS samples at different nozzle temperatures. The effects of print temperature on ABS samples printed in the longitudinal build orientation suggested an optimum temperature of 260 \({}^{\circ}\)C. Tensile strength results show an increase of 43% when nozzle temperature was increased from 220 \({}^{\circ}\)C to 240 \({}^{\circ}\)C followed by another 19% improvement when the temperature was changed from 240 \({}^{\circ}\)C to 260 \({}^{\circ}\)C. A drop of 9% was recorded when the temperature increased to 280 \({}^{\circ}\)C which suggests that the optimum tensile strength can be achieved at 260 \({}^{\circ}\)C. The elongation at break showed an increase by increasing the temperature from 220 \({}^{\circ}\)C to 280 \({}^{\circ}\)C. Almost the same trend for tensile strength was observed for samples printed in the transverse build orientation. A minimal increase from 220 \({}^{\circ}\)C to 240 \({}^{\circ}\)C followed by a peak of 19 MPa at 260 \({}^{\circ}\)C and a drop of 59% at 280 \({}^{\circ}\)C print temperature proves the optimum printing temperature condition can be achieved at 260 \({}^{\circ}\)C. Figure 21 represents the measured ultimate tensile strengths, elongation at breaks and Young's modulus values for ABS samples in two build orientations--transverse and longitudinal--at different nozzle temperatures. Figure 19: Mechanical anisotropy \(\Delta\) (ABS) for parts printed at infill percentages and build orientations for printed samples. Figure 20: Micro-CT images of the ABS polymer when printing in the LBO and at different nozzle temperatures of (**a**) 220 \({}^{\circ}\)C, (**b**) 240 \({}^{\circ}\)C, (**c**) 260 \({}^{\circ}\)C, and (**d**) 280 \({}^{\circ}\)C.  
The mechanical behaviours of the printed sample based on the mechanical tests can also be confirmed by SEM images. For the longitudinal build orientation, at 220 \({}^{\circ}\)C and 240 \({}^{\circ}\)C (Figure 22a and b, respectively), the poor layer adhesion can be confirmed by the missing perimeter layers and massive formed air voids in the structure of the samples. By increasing the temperature to 260 \({}^{\circ}\)C, layer adhesion improves enormously and more uneven fractured surfaces with fewer air voids compared to lower nozzle temperatures appear. At 280 \({}^{\circ}\)C, structural integrity partially persisted but compared to the 260 \({}^{\circ}\)C nozzle temperature, the relatively bigger air voids were the main contributing factors in lowering mechanical properties. For samples printed in the transverse build orientation, the 220 \({}^{\circ}\)C nozzle temperature, although there are no obvious air voids in the structure, a flat and smooth fracture surface suggests minimal resistance from the layers towards the axial force. This fracture behaviour was changed in the sample printed at 240 \({}^{\circ}\)C with a more uneven fractured surface. However, more air voids between rasters are proof of the lack of proper layer adhesion between rasters. At 260 \({}^{\circ}\)C, the fracture surface shows almost no obvious air voids, and the uneven break surface justifies the better tensile strength and elongation at break results. For 280 \({}^{\circ}\)C like 240 \({}^{\circ}\)C, the air voids in the sample became more prominent and can be the reason for the decrease in mechanical properties compared to the 260 \({}^{\circ}\)C nozzle temperature.