Throughout the present study, guaranteed soundness of heavy section spheroidal graphite cast iron has been pursued by means of a reliable C and Si ranges design. For this purpose, the three main parameters of superheating temperature, mushy zone and total volume change during solidification are chosen to calculate the optimized range for casting soundness using the conventional thermodynamic software. As a result, a triangle region of C and Si allowable range, where C content is over 3.7 wt%, Si content is over 1.3 wt% and carbon equivalent value is below 4.5 wt%, was obtained by superimposing the individual C and Si range of each thermodynamic parameter for the prevention of casting defects. In order to verify the calculation results, a comparative study was performed with a previous study, and test castings were performed both at laboratory scale and mass production sites. Regarding the laboratory casting, various casting defects were observed off the calculated allowable ranges. Within the calculated allowable ranges for the guarantee of the sound casting, both laboratory and mass production castings revealed an absence of defect. Graphical Abstract