## 4 Influence of Printing Parameters on Material Properties
### Raster Angle  
The raster angle is defined as the angle between the printing direction and the \(x\)-axis (\(\theta\)), as shown in Figure 7. If \(\theta\) = 0\({}^{\circ}\), the print head prints along the \(x\)-axis, i.e., the aforementioned \(x\) direction; if \(\theta\) = 90\({}^{\circ}\), the print head prints along the \(y\)-axis, i.e., the aforementioned \(y\) direction. The 3D printer was used to print five test pieces with \(\theta\) = 0\({}^{\circ}\), 30\({}^{\circ}\), 45\({}^{\circ}\), 60\({}^{\circ}\), and 90\({}^{\circ}\), as depicted in Figure 8. These test pieces were subjected to the steel ball drop test to measure the changes in \(E\) and \(G\). Figure 8: Photos of test pieces with different raster angles. Figure 7: Schematic of raster angle. In terms of printing parameters, the spacing between each printed line of material is set to be 0.4 mm, and the spacing between each layer in the height direction is set to be 0.2 mm. The \(x\)-axis and \(y\)-axis correspond to the length and width of the test piece, respectively, which is166 \(\times\) 24 \(\times\) 2.4 mm\({}^{3}\) in size. Therefore, the test piece with \(\theta\) = 0\({}^{\circ}\) in this section is the same as the \(xy\) plane-\(x\) direction test piece in Section 3, while the test piece with \(\theta\) = 90\({}^{\circ}\) is the same as the \(xy\) plane-\(y\) direction test piece in Section 3. The values of \(E\) and \(G\) corresponding to each test piece can be obtained by the inverse calculation of material constants, as shown in Table 5. Using the corresponding cross-ply angle, the elastic constants are calculated from the off-axis transformation by the nine orthotropic material constants listed in Table 4. The longitudinally elastic constant, \(E_{x^{\prime}\chi^{\prime}}\), is listed in Table 5 as the \(E_{\textit{off-axis}}\). In addition, Figure 9 displays the comparison between \(E_{x^{\prime}\chi^{\prime}}\), \(E\), and \(G\) among the five test pieces. It can be seen from Figure 9 that \(E\) decreases as the raster angle (\(\theta\)) increases. Given that the material constants indicate \(E_{x}\) = 3.19 GPa and \(E_{y}\) = 2.56 GPa, the value of \(E\) measured with \(\theta\) = 0\({}^{\circ}\)-90\({}^{\circ}\) falls between \(E_{x}\) and \(E_{y}\), and it is reasonable that the value decreases as the raster angle increases. Moreover, from a microscopic point of view, the test piece with \(\theta\) = 0\({}^{\circ}\) under steel ball impact draws its stiffness from the wire material strength, while the test piece with \(\theta\) = 90\({}^{\circ}\) under steel ball impact draws its stiffness from the binding force between the wire materials, which is much lower in comparison. As such, the measured value of \(E\) is indeed lower in the latter case. Similarly, the stiffness of the test piece, initially drawn from the wire material strength, becomes gradually more dependent on the binding force between the wire materials as \(\theta\) increases, causing the stiffness and the measured value of \(E\) to decrease. It is also seen in Figure 9 that \(G\) is maximum when \(\theta\) = 45\({}^{\circ}\), and shows a symmetrical distribution, i.e., the value of \(G\) with \(\theta\) = 30\({}^{\circ}\) is very close to that with \(\theta\) = 60\({}^{\circ}\), and the value of \(G\) with \(\theta\) = 0\({}^{\circ}\) is very close to that with \(\theta\) = 90\({}^{\circ}\). It is, thus, speculated that the material is generally subject to the maximum shear stress in the direction of 45\({}^{\circ}\) under torsion. Therefore, the test piece printed at a raster angle of 45\({}^{\circ}\) can withstand the maximum torsional force, i.e., the stronger the material under torsion, the greater the value