Core molding method and core molding device

A core molding method for molding a core having a helical shape using a core die, the method comprising:

• a hardening step in which the core die is disposed in a frame and then self-hardening sand acquired by mixing sand, a resin, and a hardener is filled into the frame and left to harden;
• and
• a die removal step in which the core die is removed from the core, resulting from hardening of the self-hardening sand, while being rotated around an axis of the core die,
• wherein, in the die removal step, a time for hardening the self-hardening sand, a frictional force exerted between the core and the core die during die removal, and strength of the core during die removal are optimized wherein the core die is removed from the core while a moment M corresponding to torque and resulting from friction between the core and the core die during die removal is maintained so as to satisfy the following relationship:
• 0<M=kσπD2L/2≤Tmax, where k denotes a friction coefficient, D denotes a diameter of a cylinder having a contact area equivalent to a contact area over which the core die and the core touch each other, L denotes a length of the cylinder, σ denotes stress per unit area produced in the core, and Tmax denotes maximum torque produced during die removal when the core die is removable from the core.