# 3D Printing of Objects with Continuous Spatial Paths by a Multi-Axis Robotic FFF Platform
## 2 Related Work  
### Multi-DOF 3D Printing  
Researchers have two main approaches to achieving multi-DOF printing: extending the DOF of a traditional 3D printer or using a multi-axis robotic arm as a platform. The hardware platforms of the above two methods are different. However, their essence is to introduce more DOF to extend the printing process from parallel planes to spatial trajectories.  
Some research efforts are aimed at adding more freedom to traditional FDM printers. [PERSON] et al. [5] developed a hybrid 3D printing system by adding two axes to the gantry milling machine platform. Their hybrid system can mill the part's surface after printing to achieve higher finish quality. However, the system can only print the simple shapes on parallel planes in multiple directions, and the effect of the process on mechanical properties is not discussed in detail. [PERSON] et al. [6] proposed a multi-DOF 3D printing system assisted by visual surveillance. Moreover, the system added two more axes of rotation to the platform to increase the printing freedom. The platform can print more complex models without any support, but it is still based on multi-directional flat plane printing. Traditional FDM printing equipment is more suitable for printing on flat surfaces but has difficulty in multi-axis cooperating. Multi-DOF printing needs the precise coordination of multiple axes, which is difficult to control and guarantee accuracy. At the same time, the complicated mechanical structure further reduces the printing space.  
More research works are devoted to building a robotic arm-based 3D printing platform. [PERSON] et al. [7] conducted a preliminary case study on the 3D printing of the robotic arm. Their platform is based on a six-axis robotic arm fitted with different end effectors for additive manufacturing, milling, and sculpture. [PERSON] et al. [8] took inspiration from the spider web structure and customized a four-extruder printing nozzle on the robot arm platform. The nozzle can print a self-supporting frame structure in the space. The structure is more decorative than practical. [PERSON] et al. [9] developed a metal 3D printing system that coupled a six-axis robotic arm with a two-axis tilt and rotation positioning system. The 8-DOF system can print metal structures such as propellers without support by collaboration. [PERSON] et al. [10] proposed a method to deposit material on curved layers using a multi-axisprinting system based on a robot arm, significantly reducing the need for support structures and avoid collisions. In order to enhance the strength of the printed parts, [PERSON] et al. (2018) used a robotic printing platform to print continuous carbon fiber on multiple axes. However, the platform had a sophisticated end-effector to achieve multi-material extrusion. Their experiment result shows that the strength of the carbon fiber reinforced parts is about six times higher than that of the non-reinforced parts, but the specific test model and method are not given. Most of the relevant work focuses on support-free printing methods, but there is little research on evaluating the strength of multi-DOF printing.