This study investigated the influence of cooling rate and Sn addition on the microstructure formation of as-cast GCD700 spheroidal graphite cast irons. Changes in cooling rate manifested as step cast thickness differences. Optical microstructures of as-cast GCD700 alloys revealed α-ferrite and pearlite and dispersed graphite nodules. In all examined thicknesses without Sn, the α-ferrite, rather than the pearlite, surrounding graphite nodules appeared to dominate microstructures, and the graphite looked well rounded, whereas microstructure containing 0.09% Sn had a significantly expanded pearlite area. Image analysis showed numbers of graphite nodules increased only on decreasing cast thickness. However, the phase fractions of ferrite and pearlite were not dependent on thickness. For samples containing Sn, pearlite fractions significantly increased with Sn content. Thermodynamic calculations and scanning electron microscopy-based microstructural analysis confirmed that the Sn contents examined had no significant effect on phase formation, Sn segregation, or the relationships between ferrite and Fe3C orientations in pearlite.

The design of the injection mold cooling system is important. The cooling time consumes 70-80% of the injection molding cycle, so a well-designed cooling system can shorten the molding time and improve productivity significantly. Recently, many studies have been conducted for rapid cooling of a hot-spot area using CO₂ in injection molding. In this study, a cooling module based on CO₂ was designed and manufactured for uniform and rapid cooling of an injection mold with a large cavity, and cooling characteristics were investigated through experiments. As the CO₂ supply pressure increased, the cooling effect increased significantly, while the cooling uniformity decreased relatively. In the case of using the heat exchanger, the cooling effect increased by 10oC on average compared to the case without the heat exchanger, whereas the effect on the cooling uniformity was insignificant. When the CO₂ was injected from both sides, the cooling effect increased by approximately 8oC on average compared to the case of injection from one side, and the cooling uniformity was approximately 10% higher. By using a heat exchanger and applying CO₂ bidirectional supply, a cooling rate of up to 5.78℃/s and an average of 4.9℃/s could be achieved.