## 4 Conclusions  
This study aimed at the determination of the effect of eight process parameters (flow rate, melt temperature, mold temperature, holding pressure, runner diameter, gate diameter, gate angle, and included angle) on the blush defect in PVC bushings produced by injection molding. Some prediction models have been created to estimate the area of the blush defect using ANOVA and ANNs. Among the prediction models used, the basic ANN method with a training accuracy of 99.99% has shown the best performance compared to other prediction methods for predicting the FEA results. Results showed that flow rate, melt temperature, and runner diameter have a particularly strong effect on the blush defect and can be related to the viscosity of the molten material. The viscosity can increase with rapid cooling of the material, making the material's shear stress exceed the allowable limit and, in turn, promoting blush. Figure 13: The initial defected bushing area is in simulation (**a**) and experimental test (**b**); and the bushing is after optimization in simulation (**c**) and experimental test (**d**). \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Title**} & **Melt** & **Flow** & **Holding** & **Runner** & **Defec** \\  & **Temperature** (\({}^{\circ}\)**C**) & **Rate** & **Pressure** & **Diameter** & **tArea** \\  & & **(cm\({}^{3}\)/s)** & **(mm)** & **(mm)** & **(mm)** \\ \hline
**Initial bushing** & 197.0 & 12.0 & 35.0 & 10.0 & 1978 \\
**Optimal bushing suggested by CCD optimization** & 197.8 & 26.0 & 56.4 & 6.7 & 517 \\
**Optimal bushing suggested by GA** & 197.1 & 25.6 & 84.9 & 6.4 & 362 \\
**Experimental test on the dataset suggested by GA** & 197.1 & 25.6 & 84.9 & 6.4 & 366 \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Comparison of the defect area before and after optimization. Holding pressure has also affected the blush defect. With the increase in holding pressure, the blush defect decreases. Lower values of holding pressure can cause some semi-cooled material (which has a high viscosity) to enter the mold cavity. Reflowing the material in these high viscosity conditions can cause high amounts of shear rate, which is the underlying reason of blush defect occurrence. The parameters of mold temperature, gate diameter, gate angle, and included angle have a negligible effect on the result. The key results of the research can be summarized as follows:  
* After optimization using the GA through the trained ANN as the cost function, values of 197.1 \({}^{\circ}\)C, 25.6 cm\({}^{3}\)/s, 84.9 MPa, and 6.4 mm were introduced as the optimal levels for the parameters of melt temperature, injection flow rate, holding pressure, and runner diameter, respectively. * After applying the optimized parameter levels by GA in a new FEA, the defect area was reduced by 81.7% compared to the pre-optimization defect area. Methodology, A.M.A. and G.T. ; Software, A.M.A. ; Validation, A.M.A., R.S. and G.T. ; Formal analysis, R.S. ; Investigation, A.M.A. ; Resources, G.A. ; Writing--original draft, A.M.A. ; Writing--review & editing, G.A., Y.S., M.C., J.H.H., M.K. and G.T. ; Visualization, A.M.A. ; Supervision, G.A. and G.T. ; Project administration, G.A. All authors have read and agreed to the published version of the manuscript. This research work was partially supported by the DIGIMAN4.0 project (\"DIGItal MANu-facturing Technologies for Zero-defect Industry 4.0 Production\", [[https://www.digiman4-0.meek.dtu.dk/](https://www.digiman4-0.meek.dtu.dk/)]([https://www.digiman4-0.meek.dtu.dk/](https://www.digiman4-0.meek.dtu.dk/)), accessed on 13 February 2023). DIGIMAN4.0 is a European Training Network supported by Horizon 2020, the EU Framework Programme for Research and Innovation (Project ID: 814225). Not applicable.