# Optimisation of Shrinkage and Strength on Thick Plate Part Using Recycled LDPE Materials
## 3 Results and Discussion
### Experimental Works Results  
Through the FFD, the contributions of each input parameters are tabulated in Table 7. The table indicates that packing pressure is the most significant processing parameter that affects the shrinkage in the \(x\)-direction. The results are in line with those of [PERSON] et al. [44], where packing pressure is the most critical processing parameter that affects the shrinkage of part. The packing pressure pushes molten plastic into the mould as the moulded parts are cooled and shrank inside the mould cavities. The change in packing pressure regulates the compression of the melt along the flow path. Excessive shrinkage can happen if the packing pressure applied is insufficient. Meanwhile, mould temperature is seen to be the most significant processing parameter affecting shrinkage in \(y\)-direction for thick plate part moulded using recycled material (R30, R40 and R50). A high mould temperature allows more time for crystallisation to occur. An increase in crystallisation produces a high-density part, causing less shrinkage formation. This condition shows that changes in mould temperature and packing pressure result in an increase or decrease of shrinkage percentages in the \(x\) and \(y\) directions. Besides, it can be seen that the packing pressure was the most influential parameter affecting the tensile strength for parts moulded using R0 and R30 specimens. Increasing the packing pressure will increase the crystallinity of the materials as the polymer's crystallinity increases in strength, the intermolecular bonding becomes more significant in the crystalline phase [45; 46; 47]. Meanwhile, mould temperature is the most significant parameter affecting the tensile strength for parts moulded using R40 and R50. It was seen from the past works that an increase in mould temperature increases the material crystallinity thus producing parts with good tensile strength [32]. This is because the rise in mould temperature increases the plastic density and tensile strength of the moulded parts produced. Through the CCD, a mathematical model denoting the interaction between the response and input parameters was determined. The shrinkage of the thick plate part in the \(x\) and \(y\) directions and the tensile strength were optimised to the values as shown in Table 8 using the RSM optimisation method, and the results are plotted in Figure 9. In addition, Table 8 shows the optimal shrinkage of thick plate part in the \(x\) and \(y\) directions and tensile strength for thick plate part optimised using the PSO method. Subsequently, these results are plotted in Figure 10. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multicolumn{2}{c}{**Specimens**} & **R0** & **R30** & **R40** & **R50** \\ \hline \multirow{2}{*}{**Most significant parameter**} & \(x\) direction & Packing pressure & Packing pressure & Packing pressure & Packing pressure \\  & \(y\) direction & Packing pressure & Mould temperature & Mould Temperature & Mould temperature \\ \hline \hline \end{tabular}
\end{table} 
Table 7: The most contributing processing parameters. Figure 9: RSM optimisation method on the thick plate part for shrinkage and tensile strength. Compared with the results of R0 (RSM) shown in Table 8, it is apparent that the result of using RSM on R30, R40 and R50 in the \(x\) direction increased by 24.49%, 30.19% and 33.93% respectively, while shrinkage in the \(x\) direction, optimised using the PSO method, increased by 33.20%, 37.82% and 35.95% compared to R0 (PSO). Meanwhile, shrinkage improvement via the use of RSM for R30, R40 and R50 specimens improved by 4.48%, 6.34% and 7.69% respectively in the \(y\) direction, as compared to R0 (RSM). Shrinkage in the \(y\) direction optimised using PSO method for R40 and R50 increased by 4.48% and 35.95% respectively, while R30 decreased by 2.67% compared to R0 (PSO). Lastly, tensile strength optimised by RSM for R30, R40 and R50 specimens decreased by 0.51%, 2.07% and 35.95%, respectively.