Golf ball 2 has a spherical core 4 , a reinforcing layer 6 positioned outside of the core 4 and a cover 8 positioned outside of the reinforcing layer 6 . The core 4 has a inner sphere 10 , a first mid layer 12 positioned outside of the inner sphere 10 , and a second mid layer 14 positioned outside of the first mid layer 12 . The principal component of the first mid layer 12 and the second mid layer 14 is an ionomer resin. The principal component of the cover 8 is a thermoplastic polyurethane elastomer. The second mid layer 14 has a hardness Hs which is greater than a hardness Hf of the first mid layer 12 and a hardness Hc of the cover 8 . The hardness Hc of the cover 8 is equal to or less than 50. A product (Tc·Hc) of a thickness Tc (mm) and a hardness Hc of the cover 8 is equal to or less than 25.
1. A golf ball which comprises a spherical core and a cover positioned outside of the core and formed by a thermoplastic resin composition,
said core having an inner sphere, a first mid layer positioned outside of the inner sphere and formed by a thermoplastic resin composition, and a second mid layer positioned outside of the first mid layer and formed by a thermoplastic resin composition,
said second mid layer having a hardness Hs which is greater than a hardness Hf of the first mid layer and a hardness Hc of the cover,
said cover having the hardness Hc of equal to or less than 50, and a product (Tc·Hc) of a thickness Tc (mm) of the cover and a hardness Hc of the cover being equal to or less than 25, wherein a sum (Tf+Ts+Tc) of a thickness Tf of said first mid layer, a thickness Ts of said second mid layer and a thickness Tc of said cover is 0.5 mm or greater and 3.7 mm or less, and wherein the hardness Hc of said cover is smaller than a central hardness Hi of the inner sphere.
2. The golf ball according to claim 1 , wherein a ratio (Hs/Hc) of a hardness Hs of the second mid layer to a hardness Hc of said cover is 1.4 or greater and 3.7 or less.
3. The golf ball according to claim 1 , wherein a difference (Hs−Hf) of the hardness Hs of said second mid layer and the hardness Hf of the first mid layer is 3 or greater.
4. The golf ball according to claim 1 , wherein a principal component of a base polymer of said first mid layer is an ionomer resin,
a principal component of a base polymer of said second mid layer is an ionomer resin, and
a principal component of a base polymer of said cover is a thermoplastic polyurethane elastomer.
5. The golf ball according to claim 1 , which further has a reinforcing layer with a thickness of 3 μm or greater and 50 μm or less and positioned between said second mid layer and said cover.
6. The golf ball according to claim 1 , wherein the thickness Tc of said cover is equal to or less than 1.0 mm.
7. A golf ball which comprises a spherical core and a cover positioned outside of the core and formed by a thermoplastic resin composition, said core having an inner sphere, a first mid layer positioned outside of the inner sphere and formed by a thermoplastic resin composition, and a second mid layer positioned outside of the first mid layer and formed by a thermoplastic resin composition,
said second mid layer having a hardness Hs which is greater than a hardness Hf of the first mid layer and a hardness Hc of the cover,
said cover having the hardness Hc of equal to or less than 50, and
a product (Tc·Hc) of a thickness Tc (mm) of the cover and a hardness Hc of the cover being equal to or less than 25, and wherein the hardness Hc of said cover is smaller than a central hardness Hi of the inner sphere.