## 3 Results and Discussion
### Analysis of Complex Insert and Produced Samples  
According to the results obtained from the experiments performed on the flat thin plates, the V00 configuration was selected for the production test of a typical LOC device with a resin insert for microinjection moulding.  
Figure 9 shows a general overview of the microstructured insert cavity, with inverted channels having a trapezoidal cross-section and a smaller base size of 200 (T200) (Figure 9a), as well as details of the lower circular reservoir (Figure 9b) and of the cross of the microchannels close to the bottom square reservoir (Figure 9c). The residuals of wax, discussed in the previous section, are clearly visible as a white layer all around the microstructures.  
In Figure 10, examples of resin inserts with inverted trapezoidal (T300) and semi-circular (S200) microchannels after twenty moulding cycles are shown. All the inserts present the same defects: the generation of a crack along the cross-section (as observed in Figure 4a), a crack on the top surface located in the middle (Figure 10a,c), and the loss of resin along the microstructures and reservoirs of microfluidic features (Figure 10a-d). These defects should be attributed to the high forces involved in the \(\upmu\)IM process and the brittle behaviour of the resin, as discussed in Section 3.1.  
Figure 10: Overview of the main defects on resin inserts after moulding cycles.  
Figure 9: General and detailed views of bulk insert T200.  
The experimental test showed that the hemispherical channel cross-section was less resistant to the injection moulding process compared to the trapezoid shape. Due to the absence of draft angles, the hemispherical microchannels were under greater stress and had a higher failure rate during the production of the PMMA parts. Additionally, the significance of the position of the microstructures along the insert cavity in relation to the failure mechanism was noted. During \(\upmu\)IM, the injection rate increases near the gate and decreases toward the end of the insert cavity due to the pressure loss. As a result, microstructures located closer to the gate are more susceptible to greater damage.  
The subsequent analysis of the PMMA samples was useful to identify the number of moulding cycles and, consequently, the part numbers produced before the defects became evident. All samples, except for T200, present evident defects starting from the tenth, whereas T200 presents evident defects from the first one. In Figure 11, it can be seen that the moulded parts have faithfully replicated the insert cavity, exhibiting cracks and irregularities in the channels and pockets due to the damage of the insert microstructures (Figure 11a,b). Figure 11c, in particular, shows the channel shape in Zone 4 of the 1 st, 10 th, and 20 th moulded parts. The change of channel shape is gradual up to the 20 th sample, in contrast to the channel depth, which remains constant, around 200 \(\upmu\)m as designed. Brittle failure was the dominant failure mode for most inserts. The results also highlighted that the features with the highest aspect ratio (e.g., microchannels) were damaged, as expected, more than those with a lower aspect ratio, such as the bottom square pocket (Zone 4), which showed no defects up to the 20 th sample. The experimentation showed that the T300 insert exhibited less damage during moulding cycles than the other configurations tested. The T300 geometry was the most effective in faithfully reproducing the desired microfluidic features on the PMMA moulded parts. Based on these results, the sample parts produced with the T300 insert were considered the most suitable for the preliminary microfluidic tests.  
Figure 11: General and detailed views of fifth sample produced with T300 insert and channel shape during \(\upmu\)IM production.