Golf ball

Abstract

A golf ball 2 has a spherical core 4 , a mid layer 6 situated on the external side of the core 4 , and a cover 8 situated on the external side of the mid layer 6 . The core 4 has a spherical center 10 and an envelope layer 12 situated on the external side of the center 10 . At all points P included in a zone away from the central point of the core 4 at a distance of 1 mm or greater and 15 mm or less, the following mathematical expression is satisfied. H 2− H 1≦5 In the above mathematical expression, H 1 represents the JIS-C hardness at a point P 1 that is located inside the point P along the radial direction and away from the point P at a distance of 1 mm, and H 2 represents the JIS-C hardness at a point P 2 that is located outside the point P along the radial direction and away from the point P at a distance of 1 mm.

1 . A golf ball comprising a core, a mid layer situated on the external side of the core, and a cover situated on the external side of the mid layer, wherein:
 the proportion of the volume of the core relative to the volume of the phantom sphere of the golf ball is no less than 76%;   the difference (He−Ho) between the JIS-C hardness He of the surface of the core and the JIS-C hardness Ho of the central point of the core is 20 or greater and 40 or less;   the JIS-C hardness Hc of the cover is less than the hardness Ho; and   at all points P included in a zone away from the central point of the core at a distance of 1 mm or greater and 15 mm or less, the following mathematical expression is satisfied:
     H 2− H 1≦5
 
   
       in the above mathematical expression, H 1  represents the JIS-C hardness at a point P 1  that is located inside the point P along the radial direction and away from the point P at a distance of 1 mm, and H 2  represents the JIS-C hardness at a point P 2  that is located outside the point P along the radial direction and away from the point P at a distance of 1 mm. 
     
     
         2 . The golf ball according to  claim 1 , wherein the JIS-C hardness Hc of the cover is no greater than 65. 
     
     
         3 . The golf ball according to  claim 1 , wherein the cover has a thickness of no greater than 0.8 mm. 
     
     
         4 . The golf ball according to  claim 1 , wherein:
 a principal component of the base material of the cover is a thermoplastic polyurethane; and   a polyol component of the thermoplastic polyurethane is a polytetramethylene ether glycol having a number average molecular weight of no greater than 1,500.   
     
     
         5 . The golf ball according to  claim 1 , wherein the JIS-C hardness Hm of the mid layer is no less than 90. 
     
     
         6 . The golf ball according to  claim 1 , wherein the mid layer has a thickness of no greater than 1.5 mm. 
     
     
         7 . The golf ball according to  claim 1 , wherein the JIS-C hardness Hm of the mid layer is greater than the hardness He. 
     
     
         8 . The golf ball according to  claim 1 , wherein the hardness He is 70 or greater and 95 or less. 
     
     
         9 . The golf ball according to  claim 1 , wherein the hardness Ho is 40 or greater and 80 or less. 
     
     
         10 . The golf ball according to  claim 1 , wherein the difference (Ho−Hc) between the hardness Ho and the hardness Hc is 3 or greater and 15 or less. 
     
     
         11 . The golf ball according to  claim 1 , wherein the core has a center, and an envelope layer situated on the external side of the center. 
     
     
         12 . The golf ball according to  claim 11 , wherein:
 the center has a diameter of 10 mm or greater and 20 mm or less; and   the envelope layer has a thickness of 8 mm or greater and 18 mm or less.   
     
     
         13 . The golf ball according to  claim 11 , wherein the hardness He is greater than the surface hardness of the center. 
     
     
         14 . The golf ball according to  claim 11 , wherein the difference (He−Hi) between the hardness He and the hardness Hi of the innermost point of the envelope layer is 10 or greater and 25 or less.