Multi-piece solid golf ball

Abstract

The invention provides a multi-piece solid golf ball having a solid core, an inner cover layer and an outer cover layer, which outer cover layer has numerous dimples on a surface thereof. The ball is characterized by combining an inner cover layer and an outer cover layer that are each formed to specific material hardnesses and thicknesses with dimples which satisfy specific conditions. This multi-piece solid golf ball is able to substantially reduce the distance traveled by the ball when struck at a high head speed, while at the same time holding down the decrease in distance when struck at a low head speed.

1 . A multi-piece solid golf ball comprising a solid core, an inner cover layer and an outer cover layer, which outer cover layer has numerous dimples on a surface thereof, wherein the inner cover layer has a thickness of from 0.8 to 3.0 mm and a material hardness, in terms of Shore D hardness, of from 10 to 60, the outer cover layer has a thickness of from 0.7 to 3.0 mm and a material hardness, in terms of Shore D hardness, of from 45 to 62, and the material hardness of the outer cover layer is higher than the material hardness of the inner cover layer; the dimples number at least 250 but not more than 50%, have a surface coverage (SR) of at least 70% and a volume ratio (VR) of at least 1.06%, are of at least three types of mutually differing dimple diameter (DM) and/or dimple depth (DP), and have an average depth of at least about 0.18 mm and an average diameter-to-depth ratio (DM/DP) of not more than about 23; and the ball has a coefficient of lift CL at a Reynolds number of 70,000 and a spin rate of 2,000 rpm which is maintained at 60% or more of a coefficient of lift CL at a Reynolds number of 80,000 and a spin rate of 2,000 rpm. 
     
     
         2 . The multi-piece solid golf ball of  claim 1  wherein, letting Da represent dimples having a diameter of at least 3.7 mm and Db represent dimples having a diameter of less than 3.7 mm, the ratio (total number of Db dimples)/(total number of Da dimples) is at least about 0.005 but not more than about 1. 
     
     
         3 . The multi-piece solid golf ball of  claim 2 , wherein the dimples Da having a diameter of at least 3.7 mm account for at least about 75% of the total dimple volume. 
     
     
         4 . The multi-piece solid golf ball of  claim 1 , wherein the value obtained by subtracting the material hardness of the inner cover layer from the material hardness of the outer cover layer (outer cover layer material hardness−inner cover layer material hardness) is, in terms of Shore D hardness, at least 5 but not more than 50. 
     
     
         5 . The multi-piece solid golf ball of  claim 1 , wherein the dimples have an average edge angle of from 11 to 17 degrees. 
     
     
         6 . The multi-piece solid golf ball of  claim 1 , wherein the proportion of dimples having an edge angle of from 12 to 16 degrees is more than 70% of the total number of dimples formed on the surface of the ball. 
     
     
         7 . The multi-piece solid golf ball of  claim 1 , wherein the value obtained by subtracting the inner cover layer material hardness from a surface hardness (Hs) of the core (Hs−inner cover layer material hardness) is, in terms of Shore D hardness, greater than −10 and less than +10. 
     
     
         8 . The multi-piece solid golf ball of  claim 1 , wherein the value obtained by subtracting the outer cover layer material hardness from a surface hardness (Hs) of the core (Hs−outer cover layer material hardness) is, in terms of Shore D hardness, at least −15 and not higher than +5. 
     
     
         9 . The multi-piece solid golf ball of  claim 1 , wherein the ratio of deflection by a sphere composed of the solid core encased by the inner cover layer (inner cover layer-encased sphere) when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf) to deflection by the solid core when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf), which ratio is represented as (inner cover layer-encased sphere deflection)/(solid core deflection), is from 0.82 to 0.92. 
     
     
         10 . The multi-piece solid golf ball of  claim 1 , wherein the ratio of deflection by the ball when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf) to deflection by the solid core when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf), which ratio is represented as (ball deflection)/(solid core deflection), is from 0.72 to 0.79. 
     
     
         11 . The multi-piece solid golf ball of  claim 1 , wherein the ratio of deflection by the ball when compressed under a final load of 5,880 N (600 kgf) from an initial load state of 98 N (10 kgf) to deflection by the ball when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf), which ratio is represented as (600 kgf deflection/130 kgf deflection), is from 3.2 to 3.7. 
     
     
         12 . The multi-piece solid golf ball of  claim 1 , wherein the core has a center hardness (Hc), a surface hardness (Hs) and a cross-sectional hardness (Hm) at an intermediate position between the core center and the core surface which, in terms of Shore D hardnesses, satisfy the following conditions:
     Hm−Hc= 0 to 7,       Hs−Hm= 11 to 25, and       Hs−Hc≧ 16.   
     
     
         13 . The multi-piece solid golf ball of  claim 1 , wherein the core has a center hardness (Hc), a surface hardness (Hs) and a cross-sectional hardness (Hm) at an intermediate position between the core center and the core surface which, in terms of Shore D hardnesses, satisfy the following condition:
     Hs−Hm >( Hm−Hc )×3.
   
     
     
         14 . The multi-piece solid golf ball of  claim 1  wherein dimples Da with a diameter of at least 3.7 mm have an average diameter of at least 3.7 mm but not more than 6 mm, and dimples Db with a diameter of less than 3.7 mm have an average diameter of at least 1 mm but less than 3.7 mm. 
     
     
         15 . The multi-piece solid golf ball of  claim 1 , wherein dimples Da with a diameter of at least 3.7 mm have an average depth of from 0.05 to 0.5 mm, and dimples Db with a diameter of less than 3.7 mm have an average depth of from 0.05 to 0.3 mm. 
     
     
         16 . The multi-piece solid golf ball of  claim 1 , wherein dimples Da with a diameter of at least 3.7 mm have an average volume of from 0.8 to 3.0 mm 3 , and dimples Db with a diameter of less than 3.7 mm have an average volume of from 0.2 to 1.5 mm 3 . 
     
     
         17 . The multi-piece solid golf ball of  claim 1 , wherein dimples Da with a diameter of at least 3.7 mm have an average diameter (Dm) to average depth (Dp) ratio Dm/Dp of from 7 to 25, and dimples Db with a diameter of less than 3.7 mm have an average diameter (Dm) to average depth (Dp) ratio Dm/Dp of from 10 to 30. 
     
     
         18 . The multi-piece solid golf ball of  claim 1 , wherein the cover is formed of a material comprising:
 (A) a thermoplastic polyurethane material, and   (B) an isocyanate mixture obtained by dispersing (B-1) an isocyanate compound having as functional groups at least two isocyanate groups per molecule in (B-2) a thermoplastic resin that is substantially non-reactive with isocyanate.   
     
     
         19 . The multi-piece solid golf ball of  claim 1 , wherein the cover is formed of a material comprising:
 (D) a thermoplastic polyurethane, and   (E) a polyisocyanate compound.