A golf ball 2 includes a center 12 , a mid layer 14 , a reinforcing layer 6 , a cover 8 and a paint layer 10 . The golf ball 2 has a large number of dimples 16 on the surface thereof. The cover 8 is formed of a resin composition including thermoplastic polyurethane (A) and a polyisocyanate mixture (B). The polyisocyanate mixture (B) includes a urethane prepolymer (B1) having two or more isocyanate groups or an isocyanate compound (B2) having three or more isocyanate groups. The polyisocyanate mixture (B) further includes a thermoplastic resin (B3) which substantially does not react with an isocyanate group. The golf ball 2 is obtained by a mold having protrusions which project from the equator of the mold. Each protrusion includes a part of a pimple.
1. A golf ball comprising a core, a cover positioned outside the core, and a large number of dimples formed on a surface of the cover, wherein,
the core includes a center and a mid layer positioned outside the center,
the cover is formed of a resin composition including thermoplastic polyurethane (A) and a polyisocyanate mixture (B),
the polyisocyanate mixture (B) includes a urethane prepolymer (B1) having two or more isocyanate groups and a thermoplastic resin (B3) which substantially does not react with an isocyanate group,
the golf ball is formed by a mold including a pair of mold halves and having a large number of pimples on a cavity face of the mold for forming the dimples,
each mold half has a plurality of protrusions which project from the equator of the mold,
each protrusion includes a part of one of the pimples, and
when one of the mold halves is mated with the other mold half, a central angle between any two adjacent protrusions is equal to or greater than 10° and equal to or less than 20°.
2. The golf ball according to claim 1 , wherein the cover has a thickness of 0.1 mm or greater and 0.7 mm or less.
3. The golf ball according to claim 1 , wherein the cover has a hardness of 50 or less which is measured with a Shore D type spring hardness scale.
4. The golf ball according to claim 1 , wherein an amount of isocyanate group (NCO %) in the polyisocyanate mixture (B) is equal to or greater than 0.1% by weight and equal to or less than 30% by weight.
5. The golf ball according to claim 1 , wherein an amount of the polyisocyanate mixture (B) in the resin composition of the cover is equal to or greater than 1 part by weight and equal to or less than 50 parts by weight, per 100 parts by weight of the thermoplastic polyurethane (A).
6. The golf ball according to claim 1 , wherein,
the mold includes a protrusion (a); a protrusion (b) adjacent to the protrusion (a); and a protrusion (c) adjacent to the protrusion (a), and
central angle θ 1 between the protrusion (a) and the protrusion (b) is different from central angle θ 2 between the protrusion (a) and the protrusion (c).
7. The golf ball according to claim 6 , wherein a proportion P 1 of a number of the protrusions (a), each of which has the central angle θ 1 with the protrusion (b) different from the central angle θ 2 with the protrusion (c), to the total number of the protrusions is equal to or greater than 50%.
8. The golf ball according to claim 1 , wherein the pimple of which part is included in the protrusion has a diameter of 4.0 mm or greater.
9. A method for manufacturing a golf ball comprising the steps of:
forming two half shells of a resin composition including thermoplastic polyurethane (A) and a polyisocyanate mixture (B) which includes a urethane prepolymer (B1) having two or more isocyanate groups and a thermoplastic resin (B3) which substantially does not react with an isocyanate group;
covering a core, including a center and a mid layer, with the two half shells;
placing the core and the two half shells into a mold which includes a pair of mold halves and has a large number of pimples on a cavity face of the mold for forming dimples, each mold half having a plurality of protrusions which project from the equator of the mold, each protrusion including a part of one of the pimples, a central angle between any two adjacent protrusions being equal to or greater than 10° and equal to or less than 20° when one of the mold halves is mated with the other mold half; and
allowing the resin composition to flow in the mold, thereby forming the dimples having a shape inverted from the shape of the pimples and crosslinking the thermoplastic polyurethane (A) with the urethane prepolymer (B1).