## 4 Characteristics of Direct-Pressure Vulcanization Technology  
### Energy-Saving Performance  
#### 4.1.1 Vulcanization Efficiency  
The thermal resistance had a significant influence on curing efficiency. In the conventional process, the thermal conductivity of the bladder was about 0.09 w/mk and the thickness of heat transmission was generally 5 mm. The thermal conductivity of the internal mold made up by steel was about 46 w/mk. The skin effect of electromagnetic induction could cause a rapid increase in the surface temperature of the internal mold, so thickness of heat transmission tended to be zero, therefore, the \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Location**} & \multicolumn{2}{c}{**Maximum Difference**} & \multicolumn{2}{c}{**Maximum Error**} \\ \cline{2-4}  & **Temperature** & **t\({}_{50}\)** & **Temperature** & **t\({}_{50}\)** \\ \hline Node 2 & 7 \({}^{\circ}\)C & 23 s & 3.78\% & 3.58\% \\ Node 8 & 5 \({}^{\circ}\)C & 17 s & 2.70\% & 2.64\% \\ Node 10 & 6 \({}^{\circ}\)C & 36 s & 3.24\% & 5.55\% \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Differences between the simulation and experimental results. Figure 8: Relation of (**a**) temperature versus time and (**b**) state of cure (SOC) versus time at Node 10. Figure 7: Locations of the selected points for results collection. Nodes 1 and 6: chafer end on the internal profile, Nodes 3 and 4: chafer end on the external profile, Nodes 2 and 5: center of the apex, Nodes 7 and 9: center of shoulder, Nodes 10 and 11: the second belt end, and Node 8: center of tread. internal mold had essentially no thermal resistance. Accordingly the thermal resistance of the bladder was much higher than one of internal mold. Furthermore, the process steps differed between the new and traditional technologies. As shown in Figure 9a, in the traditional process, the bladder was first injected with 0.03-0.08 MPa of nitrogen for shaping tire before closing the mold until raw tire was expanded to the diameter of D\({}^{\prime}\) (step 1), which satisfies  
\[\text{D-D${}^{\prime}$}=\text{h}, \tag{8}\]  
where h is the pattern depth, and D is the overall diameter of the cured tire. Then the mold was closed (step 2). Next, the bladder was filled with saturated steam (step 3) and the diameter of the raw tire continued to expand to D. In another word, once the in-mold vulcanization started, the raw tire had not immediately contacted the external mold. In addition, the temperature of bladder needs to rise slowly repeatedly from lower than 80 \({}^{\circ}\)C at the start of curing cycle since it is vacuumed at the end of the previous cycle to unload tire. Therefore the temperature rise of the raw tire would lag behind the start of the curing cycle to some extent. With direct-pressure vulcanization technology, as shown in Figure 9b, when the tire was shaped by the internal mold before closing mold by step 1, the diameter of the raw tire directly expanded to the maximum diameter D. Then the raw tire immediately contacted the external mold when the mold was closed in step 2. Moreover, the internal and external molds were kept at a constant temperature throughout the entire curing cycle. Therefore, when the curing cycle was started, the inner and outer surface of the raw tire was heated without delay. The changes in the internal heat-conduction mode and process steps directly affect the temperature field of the tire. The apex, shoulder, and tread of the tire are generally difficult to be cured in traditional curing process, as shown in Figure 10a-c, however, these positions are heated up more rapidly with direct-pressure vulcanization technology. According to Table 3, in the condition of the same curing temperature, the press time per curing cycle estimated by FEA was 723 s (Node 10) for the traditional process, the press time of DPCT was 643 s and the vulcanization efficiency was increased by 11.07% with the new technology.