## 1 Introduction
It was found that maximum strength and Young's modulus can be obtained when choosing 210 \({}^{\circ}\)C printing temperature, 0.30 mm layer thickness, triangular filling pattern, and 100% filling. To reduce dimensional errors, 190 \({}^{\circ}\)C printing temperature, 0.20 mm layer thickness, hexagonal fill pattern, and 20% fill percentage can be selected as printing parameters. [PERSON] et al. [19] used the DOE method to evaluate the effects of layer thickness, raster angle, and infill pattern on the tensile properties of PA6 parts reinforced with short CF processed using FFF. They found that the layer thickness and print pattern have a profound impact on the maximum tensile strength and tensile modulus. For discontinuous fiber-reinforced composites, it is still the matrix material that dominates the mechanical properties, and adding natural fibers even reduces the tensile and flexural strength of the composite. Compared with short fiber-reinforced composites, the tensile and flexural strengths of continuous fiber-reinforced composites can be seven and five times higher, respectively. A large number of studies have been published to demonstrate that adding continuous fibers to a pure matrix material can substantially improve the mechanical properties of the material, but it is still not as good as the mechanical properties of parts produced using traditional processing methods. To further improve the mechanical properties of continuous fiber-reinforced composite parts, some scholars have optimized the process parameters; some scholars have changed the printing method; and some scholars have applied pressure to the printed parts during printing. All these methods can improve the mechanical properties of the printed parts. For the study of improving mechanical properties by optimizing printing parameters, [PERSON] et al. [20] developed their own co-extrusion printer to find out the appropriate process parameters for printing continuous carbon fiber-reinforced PLA. The printing temperature was 200-230 \({}^{\circ}\)C, the recommended layer thickness was 0.4 to 0.6 mm, etc. The flexural strength of the printed composite specimens was 335 MPa, and the modulus was 30 GPa when the fiber content reached 27%. [PERSON] et al. [21] used continuous Kevlar fiber-reinforced nylon material for FFF printing and investigated different process parameters such as fiber orientation, number of fiber layers, fiber position, and nylon raster angle. The results showed that the modulus of elasticity of continuous fiber-reinforced nylon could reach 27 GPa, and the strength was 333 MPa, and the mechanical properties of continuous fibers were improved much more than those of short fibers. [PERSON] et al. [22] used a Markforged Mark One 3D printer to print continuous carbon fiber, continuousamid fiber, and continuous glass fiber-reinforced nylon composites, respectively, to investigate the effects of fiber orientation, fiber type, and volume fraction on mechanical properties. Tensile and bending experiments were also performed. The results showed that, among the studied fibers, the specimens made using carbon fibers showed the largest increase in mechanical strength per unit fiber volume. Their tensile strength values were 6.3 times higher than those using non-reinforced nylon polymers. As the volume content of carbon and glass fibers increases, the amount of air entrapment in the composite matrix also increases, which affects the mechanical properties. The maximum tensile strength was observed in glass specimens when the fiber content was close to 22.5%. [PERSON] et al. [23] used Kevlar fibers as reinforcing fibers for ABS, PLA, PLA- c, and PLA- cu thermoplastic materials with Taguchi orthogonal tests followed by tensile and flexural tests, which increased the tensile strength up to 3 times and flexural strength up to 1.6 times compared to the pure matrix material. [PERSON] et al.