Core molding method and core molding device
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.
1. 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=kσπD 2 L/2≤T max , 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 T max denotes maximum torque produced during die removal when the core die is removable from the core.
2. The core molding method according to claim 1 , wherein the core die is removed from the core while stress σ per unit area produced in the core during die removal is maintained so as to satisfy the following relationship:
0<σ=2 hT max /πD 2 L≤σ min ,
where h denotes a coefficient, T max denotes maximum torque produced during die removal when the core die is removable from the core, 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, and σ min denotes minimum compression strength of the core during die removal.
3. The core molding method according to claim 1 , wherein the core die is removed from the core while stress σ per unit area produced in the core during die removal is maintained so as to satisfy the following relationship:
0<σ=2 hT max /πD 2 L≤σ min ,
where h denotes a coefficient, T max denotes maximum torque produced during die removal when the core die is removable from the core, 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, and σ min denotes minimum compression strength of the core during die removal.
4. The core molding method according to claim 1 , wherein the sand is new sand or reclaimed sand having polygonal or spherical grains, a size of which is 130 AFS or smaller.
5. The core molding method according to claim 1 , wherein the resin is an acid-setting furan resin containing furfuryl alcohol and a content of the resin with respect to the sand is 0.8%.
6. The core molding method according to claim 5 , wherein the hardener is a hardener acquired by mixing a xylene-sulfonic-acid-based hardener and a sulfuric-acid-based hardener and a content of the hardener with respect to the resin is 40%.