Multi-piece solid golf ball

Abstract

A golf ball for amateur golfers is endowed with both an excellent flight when hit by golfers whose head speed is not very high and a soft yet good feel at impact. The ball has a core, an envelope layer, an intermediate layer and a cover. The core is formed primarily of a base rubber and the intermediate layer is formed so as to be thicker than the envelope layer and the cover. The respective surface hardnesses of the core, envelope layer-encased sphere, intermediate layer-encased sphere and ball satisfy a specific relationship. The core hardness profile is designed such that the surface areas A to F calculated from hardness differences between positions located at specific distances in the core and differences between the specific distances satisfy a specific formula.

1 . A multi-piece solid golf ball comprising a core, an envelope layer, an intermediate layer and a cover, wherein the core is formed primarily of a base rubber and the intermediate layer is formed so as to be thicker than the envelope layer and the cover; the core has a surface hardness, the sphere obtained by encasing the core with the envelope layer (envelope layer-encased sphere) has a surface hardness, the sphere obtained by encasing the envelope layer-encased sphere with the intermediate layer (intermediate layer-encased sphere) has a surface hardness and the ball has a surface hardness which together satisfy to the following relationship:
   surface hardness of ball >surface hardness of intermediate layer-encased sphere >surface hardness of envelope layer-encased sphere <surface hardness of core;   
       and the core has a hardness profile in which, letting Cc be the Shore C hardness at a center of the core, Cs be the Shore C hardness at the core surface, C M  be the Shore C hardness at a midpoint M between the core center and surface, C M+2.5 , C M+5.0  and C M+7.5  be the Shore C hardnesses at, respectively, positions 2.5 mm, 5.0 mm and 7.5 mm from the midpoint M toward the core surface side, and C M−2.5 , C M−5.0  and C M−7.5  be the Shore C hardnesses at, respectively, positions 2.5 mm, 5.0 mm and 7.5 mm from the midpoint M toward the core center side, the surface areas A to F defined as follows
 surface area A: ½×2.5×(C M−5.0 −C M−7.5 ), 
 surface area B: ½×2.5×(C M−2.5 −C M−5.0 ), 
 surface area C: ½×2.5×(C M −C M−2.5 ), 
 surface area D: ½×2.5×(C M+2.5 −C M ), 
 surface area E: ½×2.5×(C M+5.0 −C M+2.5 ), 
 surface area F: ½×2.5×(C M+7.5 −C M+5.0 ), 
 
       satisfy the condition
   (surface area  D +surface area  E +surface area  F )−(surface area  A +surface area  B +surface area  C )>0.
 
 
     
     
         2 . The golf ball of  claim 1 , wherein the surface areas A to F in the core hardness profile satisfy the condition
   (surface area  D +surface area  E )−(surface area  A +surface area  B +surface area  C )≥0.
   
     
     
         3 . The golf ball of  claim 1 , wherein the hardness difference between the surface and center of the core (Cs−Cc), expressed in terms of Shore C hardness, is at least 20. 
     
     
         4 . The golf ball of  claim 1 , wherein the surface areas A to F in the core hardness profile satisfy the condition
   0.15≤[(surface area  D −surface area  E +surface area  F )−(surface area  A +surface area  B +surface area  C )]/( Cs−Cc )≤0.6.
   
     
     
         5 . The golf ball of  claim 1 , wherein the cover has a thickness of 1.2 mm or less. 
     
     
         6 . The golf ball of  claim 1 , wherein the cover has a plurality of dimples formed on a surface thereof, the ball has arranged thereon at least one dimple with a cross-sectional shape that is described by a curved line or a combination of straight and curved lines and specified by steps (i) to (iv) below, and the total number of dimples is from 250 to 380:
 (i) letting the foot of a perpendicular drawn from a deepest point of the dimple to an imaginary plane defined by a peripheral edge of the dimple be the dimple center and a straight line that passes through the dimple center and any one point on the edge of the dimple be the reference line:   (ii) dividing a segment of the reference line from the dimple edge to the dimple center into at least 100 points and computing the distance ratio for each point when the distance from the dimple edge to the dimple center is set to 100%;   (iii) computing the dimple depth ratio at every 20% from 0 to 100% of the distance from the dimple edge to the dimple center; and   (iv) at the depth ratios in dimple regions 20 to 100% of the distance from the dimple edge to the dimple center, determining the change in depth ΔH every 20% of said distance and designing a dimple cross-sectional shape such that the change ΔH is at least 6% and not more than 24% in all regions corresponding to from 20 to 100% of said distance.   
     
     
         7 . The golf ball of  claim 1 , wherein the cover has a paint film layer formed on a surface thereof, which paint film layer has a hardness on the Shore C scale of from 40 to 80.