When a core die is being removed from a core while being rotated around its axis, the hardening time of self-hardening sand, the frictional forces generated between the core and the core die during die removal, and the strength of the core during die removal are optimized.

When a core die is being removed from a core while being rotated around its axis, the hardening time of self-hardening sand, the frictional forces generated between the core and the core die during die removal, and the strength of the core during die removal are optimized.