## 4 Discussion  
The optimization of the surface quality towards reflective optical elements made by thermoset injection molding showed promising results. Surface quality with values close to 50 nm were possible. Technical optics with the purpose of signal reflection may be possible, in particular in combination with subsequent sputter processes such as Cr/Au, Ag or other reflective coatings. While the achieved values may appear low, the conducted measurements provide only a first insight into the potential of using thermosets as optical elements.  
Conducting the measurements presented a challenge for itself. On one side, the high reflectivity caused strong difficulties applying optical measurement tools due to highly reduced angles of acceptance, on the other side tactile measurements may destroy the polymeric surfaces during measurement, thus presenting no valid method. Furthermore, the surfaces created using a radius mill bit are prone to effects such as waviness. More dedicated analysis methods are required, which allow filtering of those effects from the measurement of the surface roughness. Fast fourier transformations on surface area roughness (Sa) measurements according to ISO25178 need to be applied. Also, fly cutting the optical inserts may be implemented into the tool cavity to avoid milling patterns in future investigations. However, with the requirement of absolute tolerances of features within a mold cavity, this option would increase the tolerances significantly.  
The dimensional analysis showed shrinkage values in the range of 0.5% dependent on the parameter set chosen. On the course to achieve optical elements, injection molding with a precision of \(\pm 2\) um and better is desired. Iterations in tool making can be applied to achieve the best absolute  
Figure 9: Evaluation of warpage of spherical glass-filled thermoset. (**Top**) Pareto diagram with the threshold for significance. (**Bottom**) main effect analysis.  
values. Consequently, further analysis should focus on the variations of shrinkage dependent on environmental parameters and varying material batches. Nonetheless, the best warpage result with an absolute tolerance of down to +4.5 \(\upmu\)m could be achieved with the best parameter set.  
A preceding application development employing the technique for functional components may provide insights into the anticipated functionality: within this development, micromirrors were machined by ultra precision milling resulting in a mold roughness Ra 25 nm. An orthogonal direction of injection was chosen, leading to a molded component with a surface roughness of Ra with 66.9 nm with a standard deviation of 29.2 nm. The thermoset components were sputter-coated with 60 nm chromium and 300 nm gold after molding and the transmission loss was measured. An average transmission loss of 1.25 dB could be achieved. The high standard deviations could be attributed to highly differing values in the center of the mirror structure compared to the outer areas. With the improved processing parameters in addition to extended measures of mold flow control such as drip catchers and overflow structures, further improvements on homogeneity and quality of surface properties and, thus, on transmission can be expected, lowering the transmission loss towards 0.6 dB. However, further studies towards surface replication by thermoset molding with the focus on inhomogeneities are required.