# Design of a Combined Redrawing-Ironing Process to Manufacture a CNG Pressure Vessel Liner
## 3 Finite Element Analysis for Combined Redrawing-Ironing Process
### Simulation Results
#### 3.2.3 Discussion of Combined Redrawing-Ironing Process  
To verify the theoretical formula to calculate the combined redrawing-ironing load, Figure 14 compares the theoretical loads with those obtained from FEA. The loads were almost 923-929 tons at DIRs from 1.1 to 1.4, while they decreased over the DIR of 1.5. The errors between the FEA and theory decreased when the DIR was higher and were in good agreement in cases of DIRs over 1.6, as shown Table 5. At the lower DIR, which causes a sudden change in thickness, the theoretical load assuming constant thickness and \(\beta=0\) in Equation (16) shows relatively large error.  
\begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Case**} & \multicolumn{2}{c}{**Maximum Thickness**} & \multirow{2}{*}{**Formability of 3 rd Stage**} \\  & **Reduction Rate in 2 nd Stage** & \\ \hline DIR 1.1 & 55\% & Tearing \\ DIR 1.2 & 50\% & Tearing \\ DIR 1.3 & 45\% & Tearing \\ DIR 1.4 & 40\% & Tearing \\ DIR 1.5 & 35\% & No defect \\ DIR 1.6 & 20\% & No defect \\ DIR 1.7 & 10\% & No defect \\ \hline \hline \end{tabular}
\end{table}
Table 4: Formability of 3 rd stage according to the DIRs.  
Figure 13: Forming time and load of the combined redrawing-ironing process.  
The results of the new redrawing-ironing combined process were compared with those of the second conventional stage and are shown in Table 6. Figure 15 shows schemes of the new D.D.I. process with a combined redrawing-ironing die and conventional D.D.I. process. By substituting the three existing dies with one combined redrawing-ironing die, the total die height was reduced by 46.9% (800 mm (400 mm + 200 mm + 200 mm) \(\rightarrow\) 425 mm). The punch diameter was also reduced by 2.8% (394.1 mm \(\rightarrow\) 383.0 mm) because the DIR in the second stage was increased to 1.5 (higher than the existing draw ratio, 1.35). Compared to the conventional D.D.I. process, the ironing rate increased from 33.4% (12.8 mm \(\rightarrow\) 8.52 mm) to 35.0% (12.8 mm \(\rightarrow\) 8.32 mm) in the second stage, and the processing time calculated from the punch velocity (15 mm/s), die heights, and die distances decreased by 49.7% (39.8 s \(\rightarrow\) 20 s). This will contribute to a reduction in manufacturing cost and improvement in productivity.  
After carrying out the new and conventional process, based on the results of the punch and die diameters, the reduction rates of cross sectional areas (RA) of the final product are compared in Table 7. The reduction ratio (54%) of the cross sectional area by the D.D.I. process in the second stage, which exceeds the limit reduction ratio of cross that is carried out in the conventional second stage, exceeds the limit reduction ratio of the cross sectional area, 50%, by improving the phosphate coating and heat treatment condition at  
\begin{table}
\begin{tabular}{c c c} \hline \hline  & **2 nd Conventional Stage** &
\begin{tabular}{c} **New Combined Redrawing-** \\ **Ironing Process** \\ \end{tabular} \\ \hline Number of die & 3 & 1 \\ \hline Die height (mm) & 800 & 425 (46.9\% \(\downarrow\)) \\ \hline Punch diameter (mm) & 394.1 & 383 (2.8\% \(\downarrow\)) \\ \hline Ironing rate (\%) & 33.4\% & 35\% \\ \hline Process time (s) & 39.8 & 20.0 (49.7\% \(\downarrow\)) \\ \hline \hline \end{tabular}
\end{table}
Table 6: Comparison of the new combined redrawing-ironing process with conventional 2 nd stage.  
Figure 14: Comparison of the combined redrawing-ironing load obtained from FEA and that of theoretical formula by varying DIR.  
the third stage in the actual field, which is carried out carefully on that account. However, the thickness after using the new second stage was reduced from 8.52 mm to 8.32 mm by using the redrawing-ironing die, so the reduction rate of the cross sectional area was decreased from 54% to 51% in the third stage, which can be performed more reliably than the conventional D.D.I. process.