## 3 Results and Discussions
### Findings from DOE1, 2 and 3  
Each of the six experimental set designs for DOE 4, 5, 6, 7, 8 and 9 has nine cases. The thermal analysis results in terms of the cooling times of these cases are shown in Table 9. The first six columns are for cylindrical cases, and the later ones are for conical cases. From Table 9, we can see that when all the other design variables are kept unchanged, the cooling time does vary if the cross-section geometry changes. This means that the effect of the cross-section is important when designing conformal cooling channels for injection molds. From the results, it is also be seen that no single cross-section geometry provides the best performance amongst all of the scenarios. For DOE-4, the minimum cooling time obtained is 13.51 s, which occurs in case 4.1. This is a circular-shaped conformal channel with D = four mm, P = eight mm, and L = six mm. The design cases 4.2 and 4.3 have the exact same P and L values, but rectangular and square sections provide slightly higher cooling times of 14.32 s and 13.87 s, respectively. The other design cases in DOE-4 provide cooling times in the range of 14 s to 20 s. It is reasonable to mention here that this DOE-4 is only for cylindrical shapes that have one-mm thick plastic parts only, whereas DOE-5 and DOE-6 have 3.5-mm and six-mm plastic parts, respectively. As mentioned earlier, a cylindrical-shaped plastic part with one-mm thickness is cooled down to its ejection temperature at around 28 s. Hence, all of the design cases in DOE-4 (4.1-4.9) are acceptable design solutions, while a mold designer needs to do design one. It is possible for a mold designer to choose and even mix and match from any of these cases if there are some design constraints that do not permit him to choose the most effective one (case 4.1 in this scenario). For DOE-5, the best result, 23.73 s, is achieved in case 5.3, which is a square section channel (6.3 mm \(\times\) 6.3 mm), with P = 16 mm and L = 12 mm. The similar design cases (same P and L as case 5.3) provide higher cooling times. Also, the other design cases in DOE-5 provide cooling times between 25-30 s. Again, for DOE-6 (six-mm thickness plastic part), the best solution comes in case 6.7, which is 26.89 s. The channel dimensions in this case are D = 14 mm, P = 24 mm, and L = 18 mm. The overall cooling time range in DOE-6 is 27 s-30 s. Hence, it is clear that a longer time is required to cool the thick plastic parts compared to the thinner ones. All of these design cases provide significant improvement from the traditional straight-drilled designs of injection molds. These also provide insight into a range of variables in which the mold designers can work. It is notable here that, starting from DOE-4, the variation of cooling time decreases in DOE-5, and then again in DOE-6. This indicates that when designing a conformal cooling channel configuration for a thick plastic part, a very small change can cause an improvement in cooling time. While designing such channels, mold designers need to conduct detailed design of experiments with a wide range of design variables and a small gap between levels to find out the most effective design for their purpose. \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline  & \multicolumn{4}{c}{**Cylindrical Part**} & \multicolumn{4}{c}{**Conical Part**} \\ \hline
**Case** & **Cooling** & **Case** & **Cooling** & **Case** & **Cooling** & **Case** & **Cooling** & **Case** & **Cooling** & **Case** & **Cooling** \\
**Number** & **Time (s)** & **Number** & **Time (s)** & **Number** & **Time (s)** & **Number** & **Time (s)** & **Number** & **Time (s)** & **Number** & **Time (s)** \\ \hline
**4.1** & 13.51 & **5.1** & 28.82 & **6.1** & 27.65 & **7.1** & 12.7 & **8.1** & 22.63 & **9.1**  
On the other hand, when designing for comparatively thin plastic parts, it is easier to find the most effective configuration, as a small change causes a noteworthy difference in cooling time, and designers can work on a specific range of design variables.