An object of the present invention is to provide a golf ball traveling a great distance on driver shots. The present invention provides a golf ball comprising a spherical core including an inner layer and an outer layer, wherein a difference (H X+1 −H X−1 ) between a hardness (H X+1 ) at a point outwardly away in a radial direction from a boundary between the inner layer and the outer layer of the spherical core by 1 mm and a hardness (H X−1 ) at a point inwardly away in the radial direction from the boundary between the inner layer and the outer layer of the spherical core by 1 mm is 0 or more in Shore C hardness, a surface hardness (H X+Y ) of the spherical core is more than 70 in Shore C hardness, an angle α of a hardness gradient of the inner layer is 0° or more, and a difference (α−β) between the angle α and an angle β of a hardness gradient of the outer layer is 0° or more.
1. A golf ball comprising a spherical core and a cover positioned outside the spherical core, wherein
the spherical core includes an inner layer and an outer layer,
a difference (H X+1 −H X−1 ) between a hardness (H X+1 ) at a point outwardly away in a radial direction from a boundary between the inner layer and the outer layer of the spherical core by 1 mm and a hardness (H X−1 ) at a point inwardly away in the radial direction from the boundary between the inner layer and the outer layer of the spherical core by 1 mm is 8 or more and 16 or less in Shore C hardness,
a surface hardness (H X+Y ) of the spherical core is more than 70 in Shore C hardness,
a hardness difference (H X+Y −Ho) between a center hardness (Ho) of the spherical core and the surface hardness (H X+Y ) is 18 or more and 35 or less in Shore C hardness,
a hardness difference (H X−1 −Ho) between the center hardness (Ho) and the hardness (H X−1 ) ranges from 4 to 14 in Shore C hardness,
an angle α of a hardness gradient of the inner layer calculated by a formula (1) is 15° or more and 75° or less,
an angle β of a hardness gradient of the outer layer calculated by a formula (2) is −20° or more and 20° or less, and
a difference (α−β) between the angle α and the angle β is 0° or more:
α=(180/π)× a tan [{ H X−1 −Ho }/( X− 1)] (1)
β=(180/π)× a tan [{ H X+Y −H X+1 }/( Y− 1)] (2)
wherein
X represents a radius (mm) of the inner layer,
Y represents a thickness (mm) of the outer layer,
Ho represents a center hardness (Shore C) of the spherical core,
H X−1 represents the hardness (Shore C) at the point inwardly away in the radial direction from the boundary between the inner layer and the outer layer of the spherical core by 1 mm,
H X+1 represents the hardness (Shore C) at the point outwardly away in the radial direction from the boundary between the inner layer and the outer layer of the spherical core by 1 mm, and
H X+Y represents the surface hardness (Shore C) of the spherical core.
2. The golf ball according to claim 1 , wherein the center hardness (Ho) of the spherical core is less than 70 in Shore C hardness.
3. The golf ball according to claim 1 , wherein a ratio (Y/X) of the thickness Y (mm) of the outer layer to the radius X (mm) of the inner layer ranges from 0.2 to 2.0.
4. The golf ball according to claim 1 , wherein a ratio (S 2 /S 1 ) of a cross-sectional area S 2 (mm 2 ) of the outer layer to a cross-sectional area S 1 (mm 2 ) of the inner layer on a cut plane of the spherical core obtained by cutting the spherical core into two semispheres ranges from 0.5 to 6.0.
5. The golf ball according to claim 1 , wherein a ratio (V 2 /V 1 ) of a volume V 2 (mm 3 ) of the outer layer to a volume V 1 (mm 3 ) of the inner layer ranges from 1.0 to 20.0.
6. The golf ball according to claim 1 , wherein the golf ball further comprises an intermediate layer between the spherical core and the cover.
7. The golf ball according to claim 6 , wherein the intermediate layer includes an inner intermediate layer and an outer intermediate layer positioned outside the inner intermediate layer.
8. The golf ball according to claim 1 , wherein the hardness (H X+1 ) ranges from 70 to 90 in Shore C hardness.
9. The golf ball according to claim 1 , wherein the surface hardness (H X+Y ) is 90 or less in Shore C hardness.
10. The golf ball according to claim 1 , wherein the center hardness (Ho) is 48 or more in Shore C hardness.
11. The golf ball according to claim 1 , wherein the difference (H X+Y −H X+1 ) ranges from −7 to 10 in Shore C hardness.
12. The golf ball according to claim 1 , wherein the difference (α−β) is 85° or less.
13. The golf ball according to claim 1 , wherein the radius (X) ranges from 8 mm to 16 mm, and the thickness (Y) ranges from 3 mm to 12 mm.
14. The golf ball according to claim 7 , wherein a hardness difference (Hmin-Hmou) between a slab hardness (Hmin) of a composition constituting the inner intermediate layer and a slab hardness (Hmou) of a composition constituting the outer intermediate layer ranges from 5 to 30 in Shore D hardness.
15. The golf ball according to claim 7 , wherein a ratio (Tmin/Tmou) of a thickness (Tmin) of the inner intermediate layer to a thickness (Tmou) of the outer intermediate layer ranges from 0.3 to 2.5.
16. A golf ball comprising a spherical core and a cover positioned outside the spherical core, wherein
the spherical core includes an inner layer and an outer layer,
a difference (H X+1 −H X−1 ) between a hardness (H X+1 ) at a point outwardly away in a radial direction from a boundary between the inner layer and the outer layer of the spherical core by 1 mm and a hardness (H X−1 ) at a point inwardly away in the radial direction from the boundary between the inner layer and the outer layer of the spherical core by 1 mm is 8 or more and 16 or less in Shore C hardness,
a center hardness (Ho) of the spherical core is 48 or more and less than 70 in Shore C hardness,
the hardness (H X−1 ) ranges from 63 to 82 in Shore C hardness,
the hardness (H X+1 ) ranges from 84 to 90 in Shore C hardness,
a surface hardness (H X+Y ) of the spherical core is more than 70 and 90 or less in Shore C hardness,
a radius (X) of the inner layer ranges from 8 mm to 16 mm,
a thickness (Y) of the outer layer ranges from 3 mm to 12 mm,
a hardness difference (H X+Y −Ho) between the center hardness (Ho) and the surface hardness (H X+Y ) is 18 or more and 35 or less,
a hardness difference (H X−1 −Ho) between the center hardness (Ho) and the hardness (H X−1 ) ranges from 4 to 14 in Shore C hardness,
an angle α of a hardness gradient of the inner layer calculated by a formula (1) is 15° or more and 75° or less,
an angle β of a hardness gradient of the outer layer calculated by a formula (2) is −20° or more and 20° or less, and
a difference (α−β) between the angle α and the angle β is 0° or more:
a =(180/π)× a tan [{ H X−1 −Ho }/( X− 1)] (1)
β=(180/π)× a tan [{ H X+Y −H X+1 }/( Y− 1)] (2)
wherein
X represents the radius (mm) of the inner layer,
Y represents the thickness (mm) of the outer layer,
Ho represents the center hardness (Shore C) of the spherical core,
H X−1 represents the hardness (Shore C) at the point inwardly away in the radial direction from the boundary between the inner layer and the outer layer of the spherical core by 1 mm,
H X+1 represents the hardness (Shore C) at the point outwardly away in the radial direction from the boundary between the inner layer and the outer layer of the spherical core by 1 mm, and
H X+Y represents the surface hardness (Shore C) of the spherical core.
17. The golf ball according to claim 16 , wherein the center hardness (Ho) of the spherical core is 60 or more, and less than 70 in Shore C hardness,
the hardness (H X−1 ) ranges from 70 to 82 in Shore C hardness,
the hardness (H X+1 ) ranges from 84 to 90 in Shore C hardness, and
the surface hardness (H X+Y ) ranges from 84 to 90 in Shore C hardness.
18. The golf ball according to claim 1 , wherein the hardness difference (H X−1 −Ho) is 6 or more and 14 or less in Shore C hardness,
a hardness difference (H X+Y −H X+1 ) between the hardness (H X+1 ) and the surface hardness (H X+Y ) is −5 or more and 5 or less in Shore C hardness, and
the difference (α−β) is 10° or more and 75° or less.
19. The golf ball according to claim 16 , wherein the hardness difference (H X−1 −Ho) is 6 or more and 14 or less in Shore C hardness,
a hardness difference (H X+Y −H X+1 ) between the hardness (H X+i ) and the surface hardness (H X+Y ) is −5 or more and 5 or less in Shore C hardness, and
the difference (α−β) is 10° or more and 75° or less.
20. The golf ball according to claim 1 , wherein the center hardness (Ho) of the spherical core is 63 or more and less than 70 in Shore C hardness.