# An Approach to Improve the Resolution of DLP 3D Printing by Parallel Mechanism
## 1 Introduction
This method dynamically switches the projected patterns while moving the DLP projection device, which improves the printing size and improves the problems existing in the previous step splicing. [PERSON] et al. [34] proposed a DMD-based Double Mask Projection Stereolithography (DMPSL) system. The system includes two types of projection modules, large format and high resolution, which provide a printing format of 55 mm \(\times\) 41 mm and a pixel size of 6.6 um, respectively. Then combined with the stitching process, large-sized parts with small-sized features can be printed. However, a problem with all the above methods is that due to motion errors and other reasons, it is difficult to achieve the desired effect in the forming at the splicing point. In addition, the time required for splicing is even increased by \(n\) times (\(n\) is the number of single-layer splicing). The second type is the method of photocuring molding by hybrid exposure. That is, on the basis of fixed projection resolution, combined with laser scanning. Specifically, [PERSON] et al. [35] proposed a method combining laser scanning and mask projection. A high-resolution laser is used to scan the boundaries of the cured layer, while a dynamic mask is used to rapidly cure the inner area of the layer. Therefore, while realizing size printing, the edge printing resolution is improved. But this study only provides a theoretical hybrid exposure method. [PERSON] et al. [36] developed a hybrid exposure printing system with an area projection range of 144 mm \(\times\) 90 mm and a laser scanning spot size of 20 \(\upmu\)m. The system increases the projected area by increasing the size of the pixel points, and uses laser scanning to shape the contours, which can provide a good surface quality similar to the area projection light curing system. But the match between the edge of the DLP mask and the laser profile affects the final forming accuracy. In general, the above researches have promoted the development of DLP 3D printing technology. But they all solve the contradiction between size and resolution in DLP printing on the basis of fixed projection resolution. However, in reality, the feature resolution requirements in parts are not exactly the same, and not all require high-resolution printing. Furthermore, it is also difficult to apply to scenarios that the feature resolution is improved on demand. To this end, we developed a molding approach with adjustable resolution in the printing process. It adjusts the projection distance during the printing process and prints with different resolution formats. This approach allows for improved resolution of features over a certain print size. The proposed approach is described in the following sections. Section 2 introduces the design principle and the overall scheme. According to the principle that the change of the projection distance will cause the change of the optimal projection format and projection resolution, the tripetron parallel mechanism is introduced into the structure of DLP 3D printing. The projector is driven to move in space to expose and print at different positions with the adjusted projection resolution. In Section 3, the experimental prototype is designed and built, including the structural characteristics of tripetron parallel mechanism with orthogonal 3-DoF decoupling, the mechanical structure composition of the printing system, and the printing control process. In Section 4, to evaluate the manufacturing scale of the prototype, the orthogonal experiments are designed to optimize the printing process parameters, and the minimum features size that the system can print under different formats are experimentally tested. In addition, the idea of improving the resolution of features forming is verified by the features forming comparison experiment under the same size object, which proves that the proposed approach is feasible. The conclusions are provided in Section 5.