# Research and Implementation of Axial 3D Printing Method for PLA Pipes
## 1 Introduction
[14] proposed a curved layer-based process planning algorithm for multi-axis printing of an arbitrary freeform dense part, which is effective and feasible in eliminating the need for supporting structures. [PERSON] et al. [15] proposed a five degrees of freedom (DOF) 3D printing algorithm, which can quickly and efficiently calculate the possible material deposition path, so that the fabrication can be performed in a support-free way. [PERSON] et al. [16] proposed an improved Chopper model block unsupported printing method based on the algorithm of binary space partitioning (BSP) tree generation, which can divide large complex models into advanced small models for block 3D printing, and finally, glued the individual components to realize the model without support printing. [PERSON] et al. [17] proposed an approach of printing overhanging structures using inclined layer printing technology, which can easily print complex models with ordinary MEX printers, without generating supporting structures. Typical studies in improving the mechanical properties of MEX parts are as follows. [PERSON] et al. [18] solved the problem of non-interaction between layers of fiber-reinforced composite materials, by printing a 0/90 woven structure in one layer and carried out open-hole tensile tests. The tensile strength of the printed sample of this woven structure was 44% higher than that of the punched specimen. [PERSON] et al. [19] used a continuous carbon fiber 3D printer to manufacture sandwich structures with honeycomb, rhombus, rectangle, and circle core shapes as a single piece. Three-point bending tests showed maximum load, and flexural modulus increased as the effective density increased for all core shapes, but the rhombus core shape was the strongest. [PERSON] et al. [20] used a 3D printer on a robot arm to realize multi-plane layered printing. Compared with the tensile specimen produced by a traditional 3D printer, the elastic modulus, yield strength and ultimate tensile strength of the tensile specimen in the upright printing orientation have been improved. [PERSON] et al. [21] proposed a curved layer adaptive slicing algorithm, combining adaptive flat layer and curved layer slicing together. The three-point bending test for different thickness of the curved layer proves that the thicker the curved layer is, the better the mechanical properties are. [PERSON] et al. [22] applied the surface layer printing method to the 3D printing process of concrete, and compared with the existing flat printing path; the surface strength was higher. [PERSON] et al. [23] used a six-axis industrial robot arm to deposit the filaments along the path derived from the principal stress line and applied 2.5D surface geometry to improve the strength of the print. According to stress calculations from a finite element analysis of the components, [PERSON] et al. [24] constructed the tool path parallel to the direction of maximum principal stress, which efficiently improves the mechanical performance of the printing specimen, but this method was only applied to the plane layer. It can be seen that, initially, path planning algorithms had the aim to improve the surface quality of AM printed parts, and reduce or avoid support structures. However, path planning algorithms have started to be used to improve the mechanical properties of 3D printing parts, mainly by changing the printing path of flat or curved layers [18; 19; 20; 21; 22; 23; 24]. To our knowledge, no print space path method has been used to increase the mechanical performance of printed parts. Therefore, this paper aims to enhance the mechanical properties of equal-section pipes and studies the method of planning the printing path according to the actual stress of the pipes. Finally, a multi-axis MEX platform was built to achieve enhanced printing along the force direction, with standard polylactic acid (PLA) filaments. Moreover, through mechanical testing, we aim to verify the feasibility of the method and the correctness of the planned path.