## 1 Introduction  
Three-dimensional printing is also known as additive manufacturing, because objects are manufactured by stacking materials layer by layer. In addition to manufacturing hollow parts with complicated shapes, it also greatly reduces the manufacturing time, and simplifies the manufacturing process when compared to conventional manufacturing methods. Moreover, it allows for fast customization and production in small quantities, which is much in line with the development trend of Industry 4.0. In the past decade, various processes and manufacturing methods were invented as 3D printing technology continued to develop [1, 2, 3, 4, 5, 6, 7, 8]. At present, the most widely used manufacturing processes include fused deposition modeling (FDM), stereolithography, and selective laser sintering [9, 10, 11, 12, 13]; their manufacturing cost was greatly reduced, making it possible to apply them in various fields, including aerospace, construction, mold making, dentistry, and healthcare. Simultaneously,various orthotropic materials were also studied and applied in additive manufacturing, including metals, plastics, ceramics, and concrete [14; 15; 16; 17; 18; 19; 20]. In FDM, the print head of the 3D printer is heated first, such that the wire material passing through the print head turns into a molten state and is stacked on the platform or the previously printed layer; the molten materials then fuse with each other during the printing process. Therefore, the layer thickness, width, and direction of the wire material are the main printing parameters affecting the properties of the part [21], and the deformation between layers is considered the cause of inferior mechanical properties [22]. Three-dimensional printed parts have the advantages of low cost and easy customization, but also the disadvantages of weak mechanical strength and uneven surfaces [23]. In particular, the fiber direction and void size are the main challenges for 3D-printed composites [24; 25]. In 2002, [PERSON] and [PERSON] [26] explored the influence of printing parameters on FDM. A total of four test pieces were printed with different fiber densities. The material constants were obtained through a tensile test and then compared. Subsequently, the test pieces were printed with different fiber directions, and the tensile test was also carried out. The results show that the test piece at [45\({}^{\circ}\)/ \(-\)45\({}^{\circ}\)] led to the smallest [PERSON]'s modulus. Also in 2002, [PERSON] [27] used acrylonitrile butadiene styrene as the material for FDM, exploring the printing parameters including raster angle, line width, and air gap, and measuring the tensile and compressive strengths of the samples. In 2013, [PERSON] [28] performed tensile tests on test pieces with different printing directions to obtain the nine material constants, and simulated the force balance of the 3D-printed test pieces when stretched in all directions using a composite laminate in ABAQUS. In 2015, [PERSON] [29] determined the nine material constants, tensile stress at yield, and ultimate strength of orthotropic material by performing tensile tests on test pieces. A simple part was designed and made using polycarbonate, which was subjected to different bending moments and torques. The results indicate that the part should be printed in the same direction as the tensile stress for higher strength. In 2021, [PERSON] [30] defined three material directions to describe the orthogonality of 3D-printed materials, based on the results of electron microscope observations. A total of 27 test pieces were printed for static tests, which reveal that the Young's modulus notably varies in different material directions. A finite element model with different printing directions and different thickness layers was developed to simulate its natural resonance frequency. The error with respect to the test results is very small in the follow-up studies. Three-dimensional printing technology is bound to greatly affect human life in the future.