# Embedded Based Real-Time Monitoring in the High-Pressure Resin Transfer Molding Process for CFRP
## 4 Results and Discussion  
### Process Analysis  
To obtain a uniform CFRP quality, the temperature and pressure of the resin and hardener should be maintained at a constant so that the viscosity of the resin can be kept at a constant. Figure 9 shows the injection pressure and temperature of the mixing head and the cavity pressure of the mold in the injection process. As shown in Figure 9a, the output pressure of the mixing head at the set pressure of 125 bar had changed by about 5% in both the resin and hardener. As soon as the resin of the mixing \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Conditions** \\ \hline Press force (kN) & 3000 \\ \hline Mixing head pressure (bar) & 125 \\ \hline Resin temperature (\({}^{\circ}\)C) & 80 \\ \hline Hardener temperature (\({}^{\circ}\)C) & 45 \\ \hline Mixing ratio & 100:20 \\ \hline Mold temperature (\({}^{\circ}\)C) & 110 \\ \hline Injection rate (g/s) & 64.7 \\ \hline Injection time (s) & 9.5 \\ \hline End-gap (mm) & 0.5 \\ \hline Curing time (s) & 500, 600 \\ \hline Carbon sheet number & 7, 8 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Experimental conditions. Figure 8: Experimental setup for **(a)** Mixing head; **(b)** Mold; **(c)** The HP-RTM equipment and monitoring system. head was injected into the mold, the cavity pressure CP1 increased. The distances between the cavity pressure CP1, CP2, and CP3-4 were each 190 mm, and the resin reached CP2 after about two seconds and CP3-4 about three seconds after CP1 in the injection step. As seen in Figure 9b, the temperature of the resin was maintained at 79 \({}^{\circ}\)C relative to the set temperature of 80 \({}^{\circ}\)C and the hardener was retained at 45.5 \({}^{\circ}\)C against the set temperature of 45 \({}^{\circ}\)C.  
Figure 10 shows the cavity pressure and temperature inside the mold. Figure 10a shows the cavity pressure according to the vacuum (1), injection (2) and curing (3) steps. In the curing step, the pressing force increased from 0 kN to 3000 kN for 20 s. The cavity pressure rose up to 64 bar upon the completion of compression and was maintained at a constant pressure while curing the resin as 100 s passed. In general, the cavity pressure CP1 should be kept constant as in CP2. However, in the HP-RTM equipment used in this experiment, when the resin was cured due to the structure of the mixing head, the specimen adhered to the injection nozzle of the mixing head and reached 0 bar as no pressure was applied to the CP1 pressure sensor in the lower mold opposite the injection port. Figure 10b shows the internal temperature of the mold. The initial temperature in the vacuum process was about 108-113 \({}^{\circ}\)C. The reason for this temperature deviation was that the temperature of the mold was influenced by the convection of the outside air due to the end-gap. Since the mold temperature sensor T1 was located on the injection port opposite the mold, it was reduced by about 2 \({}^{\circ}\)C due to the supply temperature of the resin (80 \({}^{\circ}\)C), the maximum reaction temperature (TR, max) was 4 \({}^{\circ}\)C and then gradually decreased. The resin flow reached the position of the temperature sensors T2-4 with the heat reaction and slowly decreased after increasing by 4 \({}^{\circ}\)C. The temperature inside the mold increased due to the exothermic reaction of the resin in the curing step rather than in the vacuum step because the temperature change was the same when the resin amount was equal. Figure 11 shows the end-gaps measured with the LVDT and the thickness of the finished CFRP specimen in the HP-RTM process. In Figure 11a, the end-gap of the vacuum step was 0.5 mm and the deviation of the end-gap was 0.1-0.2 mm in the curing step, which maintained a steady pressure after compression. Figure 11b shows the CFRP thickness that was obtained with the average value measured three times at nine positions. The thickness deviation of the CFRP specimen was \(\pm\)0.05 mm on average. Figure 12 shows a photo of a CFRP specimen in which the resin flow appeared well.