## 3 Results and Discussion  
### Influence of Different Process Parameters on MLA  
#### 3.1.1 Molding Temperature  
Molding temperature is one of the most critical parameters of the molding process. The glass transition point temperature Tg of PMMA is 104 \({}^{\circ}\)C. When it is lower than the Tg point, PMMA is glassy with extensive elastic recovery and weak deformability. The temperature must be kept constant during the hot embossing because any temperature change will affect the polymer substrate's fluid resistance, resulting in pattern defects [27].  
In the preliminary experiment, we fabricated the microlens when the hot embossing temperature was lower than Tg. At the molding temperature of 100 \({}^{\circ}\)C, molding pressure of 400 N, and holding temperature of 70 \({}^{\circ}\)C (See Figure 4a), PMMA could not be filled into the mold, resulting in an incomplete structure of the copied microlens array. The contour of the microlens array measured by the confocal laser microscope was used to analyze contour replication.  
Figure 4b shows a comparison between the microlens contour and the mold contour. The contours of the lens and the mold are very different. When the temperature is lower than the glass transition temperature, the deformation of PMMA is limited and the elastic recovery is extensive, so it is difficult to obtain a designed aspheric lens below the transition temperature. The minimum hot-embossing temperature needs to be higher than the glass transition point temperature of PMMA (104 \({}^{\circ}\)C). When the molding temperature is higher than the glass transition temperature Tg, PMMA is in a viscoelastic state with large deformation and small elastic recovery.  
Figure 4c shows the molded aspheric lens profile with the molding temperature of 110 \({}^{\circ}\)C, molding pressure of 400 N, and holding temperature of 70 \({}^{\circ}\)C. The contour of the molded aspheric lens coincides with the contour of the mold. With the increased temperature, the viscosity and flow deformation ability of PMMA gradually increase. Therefore, under the same conditions, as the molding temperature increases, better fluidity and higher profile replication will be achieved. When the molding temperature is 130 \({}^{\circ}\)C, the maximum deviation between the lens profile and the molding profile is about 4 \(\upmu\)m.  
However, when the temperature is higher than a certain range threshold (see Figure 4e), bubbles will be generated inside the PMMA. They will react quickly with the mold surface, thereby affecting the surface quality of the microlens. When the temperature is too high, a massive change in the cooling phase's temperature will result in a low surface finish of the replicated lens. At high molding temperatures, the PMMA surface adheres and falls off as a result of the damaging effect, which affects the lens's surface roughness. Therefore, according to the above analysis, it is advantageous to have a temperature slightly higher than Tg, because lower viscosity and higher fluidity facilitate the molding process. However, a  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Levels**} & \multicolumn{3}{c}{**Factors**} \\ \cline{2-4}  & **Embossing Temperature (\({}^{\circ}\)C)** & **Embossing Force (N)** & **Holding Time (s)** \\ \hline
1 & 110 & 300 & 50 \\
2 & 130 & 400 & 70 \\
3 & 150 & 500 & 90 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Molding test process parameters.  
too high temperature causes PMMA to stick to the mold, and massive cooling temperature changes cause low surface finish and bubbles.