A high-pressure differential scanning calorimeter chamber was used to investigate double crystal melting peak generation in polypropylene. The double crystal melting peak forms during the batch-based, expanded polypropylene (EPP) bead foaming process. This double peak structure is required for good sintering and the desired geometric shape of the final bead foam products. To form the double peak structure, isothermal treatment over a certain period of time is necessary at a high temperature and saturated pressure during the batch foaming process. This study investigated the influences of various saturation temperatures, time, and pressure on double crystal melting peak behavior. It was seen that the temperature was the most sensitive parameter on double peak generation. Also, the longer saturation time increased the amount of perfect crystals with a high melting temperature. Furthermore, as the saturation pressure increased, the required saturation temperature for generating the second peak with perfected crystals decreased through the plasticization effect of CO2.