## 3 Orthogonal Material Properties of 3D Printing  
In this study, parts were manufactured through extrusion molding, using 3D printing technology based on FDM. Composite material mechanics suggest that the stacking characteristics of FDM are similar to those of stacking fibers, and finished products of stacked fiber materials have anisotropic properties. Therefore, in this study we assumed that orthotropic materials also exhibit anisotropic properties with the following stress-strain relationship:  
\[ \begin{bmatrix}\varepsilon_{1}\\ \varepsilon_{2}\\ \varepsilon_{3}\\ \gamma_{4}\\ \gamma_{5}\\ \gamma_{6}\end{bmatrix} = \begin{bmatrix}\frac{1}{\varepsilon_{x}}&-\frac{v_{yx }}{E_{y}}&-\frac{v_{yx}}{E_{z}}&0&0&0\\ -\frac{v_{yx}}{E_{x}}&\frac{v_{y}}{E_{y}}&-\frac{v_{yx}}{E_{z}}&0&0&0\\ -\frac{v_{yx}}{E_{x}}&-\frac{v_{yx}}{E_{y}}&\frac{1}{E_{z}}&0&0&0\\ 0&0&0&\frac{1}{\varepsilon_{yx}}&0&0\\ 0&0&0&0&\frac{1}{\varepsilon_{xx}}&0\\ 0&0&0&0&\frac{1}{\varepsilon_{xy}}&0\\ 0&0&0&0&\frac{1}{\varepsilon_{xy}}\end{bmatrix}\begin{bmatrix}\sigma_{1}\\ \sigma_{2}\\ \sigma_{3}\\ \tau_{4}\\ \tau_{5}\\ \tau_{6}\end{bmatrix}  \tag{14}\]  
This section mainly explores the influence of stacking direction on the material properties in FDM. A high polymer, namely, polylactic acid (PLA), was used to manufacture the test pieces. In addition, G-code files were generated by the slicing software Cura, through which the placement position and raster angle of the 3D printer were adjusted to achieve  
Figure 2: Setup of steel ball drop test. different stacking directions. In terms of settings for the test pieces, the PLA material was printed with a spacing of 0.4 mm in the \(y\)-direction, and a spacing of 0.2 mm in the \(z\)-direction; the printing schematic and actual product are shown in Figure 3[30]. Note that the finished product is symmetrical. Therefore, it is assumed to be orthotropic, and the nine material constants must be found. With different placement positions and printing directions, a total of six test pieces with different stacking methods were printed, as shown in Table 1. Hereinafter, the test pieces are named according to the plane formed by the length and width of the test piece, followed by the direction in which the length of the test piece lies. Taking the first image in Table 1 as an example, its length and width forms the \(xy\) plane, and its length is in the \(x\)-direction; consequently, this test piece is named as \(xy\) plane-\(x\) direction. All the other test pieces are named in the same manner. It is noted that the specimens were directly printed strips to measure by drop test, not sliced from the large cube. The 3D-printed folding area influences the stiffness. In the study, the specific elastic modulus concerned the whole structure, ignoring the microstructure effect. Based on the inversely calculated material constants, it is known that each test piece corresponds to an \(E\) and a \(G\), i.e., the six test pieces correspond to six \(E\)s and six \(G\)s. However, given that orthotropic materials have only \(E_{x}\), \(E_{y}\), \(E_{z}\), \(G_{xy}\), \(G_{xz}\), and \(G_{yz}\), it is inferred that the \(E\)s and \(G\)s obtained from the test are in pairs. Moreover, the subscript of \(E\) is the same as the second part of the name of the test piece, while the subscript of \(G\) is the same as the first part of the name of the test piece. For example, the material constants \(E_{x}\) and \(G_{xy}\) of the \(xy\) plane-\(x\) direction test piece were obtained from the steel ball drop test. In the follow-up, the material constants of the six test pieces were inversely calculated, and the accuracy of this inference was verified. The \(xy\) plane-\(x\) direction test piece was printed first. The print head was set to a temperature of 190 \({}^{\circ}\)C, and the platform was set to a temperature of 55 \({}^{\circ}\)C. The test piece was 166 \(\times\) 24 \(\times\) 2.4 mm\({}^{3}\) in size with a weight of 10.85 g. The boundary condition during the test was such that the cantilever beam was fixed on one end.