# An Approach to Improve the Resolution of DLP 3D Printing by Parallel Mechanism
## 4 Experiments and Discussion
### Parameters Optimization Based on the Orthogonal Method  
For the top-down projection molding method, the processing parameters affecting the fabrication resolution include the exposure time and the waiting leveling time, and the thickness of the bottom layer and every single layer. The bottom layer curing is mainly used to cover the drainage holes on the molding platform and support the printing model. In the experiment, it is found that the lack of exposure of the bottom layer will lead to the faults, the holes and other phenomena. However, the overexposure curing of the bottom layer will increase the difficulty of the separation between the bottom layer and the molding platform. Combined with the designed molding platform, under the large format before zoom and small format after zoom, the appropriate exposure time of the bottom layer is respectively 60 s and 50 s with 0.1 mm bottom layer. On this basis, it is found that the monolayer thickness, the monolayer exposure time and the waiting leveling time have the greatest influence on the model forming quality. So it is necessary to optimize its parameters. The orthogonal method can obtain the accurate test results by scientifically selecting some representative points and using fewer test times. It is widely used to study multi-factor levels for its high efficiency, the fast speed and so on. In this experiment, there are three factors, which are the monolayer thickness (A), the \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Supplier** & **Viscosity** & **Wavelength** & **Density** & **Color** \\ \hline CREALITY & 150–250 mPa.s 1 & For 405 nm & 1.05–1.15 g/cm3 & White \\ \hline \hline \end{tabular}  
* A low viscosity resin was chosen to reduce its influence on the leveling process of the free surface.  
\end{table} 
Table 1: Parameters of liquid resin. Figure 8: **(a)** Physical prototype; **(b)** Photograph to illustrate the large format printing stage; **(c)** Photograph to illustrate the small format printing stage after reducing the projection distance. monolayer exposure time (B) and the waiting leveling time (C). The values of each factor level under the small scale selected by the preliminary experiment are shown in Table 2.  
Nine group tests were completed on the prototype using the L9 (3\({}^{4}\)) orthogonal test table. The cross-section size of the test sample is 14 mm \(\times\) 13.7 mm (rounded rectangle), and the printing height is 2.4 mm. The printed sample is solidified with the bottom layer to eliminate errors caused by the thickness of the bottom layer. Using a vernier caliper (GENERTEC HARBIN MEASUURING & CUTTING TOOL CO., LTD, Harbin, China) to measure the test sample X, Y and Z directions. Each sample is measured 6 times to obtain the average value, and the Relative Size Error (RSE) is used to represent the dimensional accuracy of the printed sample. RSE = \(\mid\) Measured value \(-\) Theoretical value \(\mid\) / Theoretical value \(\times\) 100%. The RSE of each direction is represented by \(\Delta X\), \(\Delta Y\) and \(\Delta Z\) respectively. The printed samples are shown in Figure 9.  
The \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Level**} & \multicolumn{3}{c}{**Factors**} \\ \cline{2-4}  & \(A\) **(mm)** & \(B\) **(s)** & \(C\) **(s)** \\ \hline
1 & 0.06 & 18 & 4 \\
2 & 0.08 & 20 & 5 \\
3 & 0.1 & 22 & 6 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Factors level information. \begin{  
The greater the range \(R\), the greater the effect on optimization. From the range results, it can be concluded that the primary and secondary order of the influencing factors is \(A>C>B\). That is, the influence factors is the monolayer thickness, the waiting leveling time and the monolayer exposure time in turn. From the perspective of the optimization effect, the RSE should be as small as possible. So the factor level combination with the best dimensional accuracy is \(A_{2}B_{2}C_{3}\).