# An Approach to Improve the Resolution of DLP 3D Printing by Parallel Mechanism
## 1 Introduction  
Additive Manufacturing (AM), commonly known as 3D printing, represents a set of processes that typically build parts through accumulating material layer by layer, which is different from the traditional material removal methods such as turning, milling, planning and grinding. It delivers unique advantages in manufacturing highly geometrical complex parts [1; 2; 3]. As a representative subversive technology in manufacturing, it has been widely studied in printing speed [4; 5], printing quality [6], mechanical properties of printing parts [7; 8] and other aspects since its emergence in the 1980s. At present, Stereolithography (SL) [9; 10], Fused Deposition Modeling (FDM) [11; 12], Selective Laser Sintering/melting (SLS/SLM) [13], and other 3D printing technologies have been created, and are deeply integrated with biomedicine [14; 15; 16], aerospace [17], automobile manufacturing [18], building [19] and other fields [20; 21]. Among these techniques, the DLP 3D printing method belonging to SL has garnered wide attention for its advantages such as low temperature, energy saving, large materials scope, superior fabrication accuracy, fast printing speed, and good surface quality [22; 23; 24; 10; 25]. The DLP 3D printing uses the dynamic mask reflected by the Digital Micromirror Device (DMD) to expose the surface of liquid resin, and then solidifies layer by layer to stack objects. DMD is a Micro Electro-Mechanical System (MEMS) composed of hundreds of thousands of independent mirrors, which can control the on-off of the optical path of a single pixel in the mask pattern. When the DMD is combined with imaging lenses of different magnification, the molding resolution ranges from several microns to severalhundred microns [26]. Due to its excellent performance, the projection stereolithography employing a dynamic mask is extensively applied in fabrication areas. Despite the rapid development of this technology, the number of pixels of DMD is fixed. Specially, when printing at a higher resolution, the optics need to scale to project smaller pixels, resulting in a smaller print area. Conversely, when printing with a larger build area, it causes less molding resolution. For example, [PERSON] et al. [26] developed a DMD-based micro-stereolithography system, which they used to construct complex structures with 0.6 \(\upmu\)m features, but with a print size was only 2 mm \(\times\) 1 mm. In fact, the inherent contradiction between size and resolution is a dilemma in the practical applications of this technology [27; 28]. For this problem, the currently proposed solutions can be divided into two categories. The first category is fixed projection distance, that is, fixed projection resolution stitching method. Specifically, [PERSON] et al. [29] proposed a Multi-Projector DLP with Energy Homogenization (EHMP-DLP) scheme, which improves the printing area on the basis of ensuring the printing resolution by placing multiple DLP projectors. But it increases the development cost. Based on the fixed projection distance, [PERSON] et al. [30] developed a molding system for size and resolution printing by placing the projection device on the X-Y mobile platform and using the mobile exposure stitching method. This is different from the traditional printing system which mostly uses the single degree of freedom mechanical structure to control the platform lifting. But the splicing method improves the requirement of motion control accuracy. In the 3D printing system for exposure of mobile splicing surface, [PERSON] et al. [31; 32] first applied the Delta 3-DoF parallel mechanism to DLP photocuring printing. The forming platform using parallel mechanism can be moved in space, thereby enabling large scale stitching printing. However, there are singular points in the mechanical structure, and the motion control algorithm is complex. [PERSON] et al. [33] improved the splicing method and proposed a Scanning-Projection Stereolithography (SPSL) method.