Practice golf ball

Abstract

A practice golf ball with a core and a cover has, upon initial measurement, a deflection BH1 (mm) when compressed under a final load of 1,275 N from an initial load of 98 N and an initial velocity BV1 (m/s), and also has, when measured again after being left to stand for 350 days following initial measurement, a deflection BH2 (mm) when compressed under a final load of 1,275 N from an initial load of 98 N and an initial velocity BV2 (m/s), such that the difference BH2−BH1 is not more than 0.2 mm and the difference BV2−BV1 is not more than 0.3 m/s. The ball has excellent durability to cracking and durability of appearance, and also maintains a stable feel and flight performance over a long period of time.

1 . A practice golf ball comprising a core and a cover, the ball having, upon initial measurement, a deflection BH1 (mm) when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) and an initial velocity BV1 (m/s), and also having, when measured again after being left to stand for 350 days following initial measurement, a deflection BH2 (mm) when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) and an initial velocity BV2 (m/s), such that the difference BH2−BH1 is not more than 0.2 mm and the difference BV2−BV1 is not more than 0.3 m/s. 
     
     
         2 . The practice golf ball of  claim 1 , wherein the core is formed of a rubber composition comprising a base rubber and, as compounding ingredients: a co-crosslinking agent, a crosslinking initiator and a metal oxide, said co-crosslinking agent being methacrylic acid. 
     
     
         3 . The practice golf ball of  claim 1 , wherein the cover comprises a resin component composed primarily of polyurethane, has a thickness of from 0.3 mm to 1.9 mm, and has a material hardness, expressed as the Shore D hardness, of from 30 to 57. 
     
     
         4 . The practice golf ball of  claim 2 , wherein the compounding ingredients are included in respective amounts of from 10 to 40 parts by weight of methacrylic acid as the co-crosslinking agent, from 15 to 30 parts by weight of zinc oxide as the metal oxide, and from 0.3 to 5.0 parts by weight of the crosslinking initiator per 100 parts by weight of the base rubber. 
     
     
         5 . The practice golf ball of  claim 1 , wherein the ball initial velocity BV1 at the time of initial measurement is not more than 76 m/s. 
     
     
         6 . The practice golf ball of  claim 1 , wherein the ball has formed on a surface thereof a plurality of dimples, each dimple having a spatial volume below a flat plane circumscribed by an edge of the dimple, and the sum of the dimple spatial volumes, expressed as a percentage (VR) of the volume of a hypothetical sphere representing the ball were the ball to have no dimples on the surface thereof, being from 0.8% to 1.7%. 
     
     
         7 . The practice golf ball of  claim 1 , wherein the ball has formed on a surface thereof a plurality of dimples which satisfy conditions (1) and (2) below:
 (1) the dimples have a peripheral edge provided with a roundness represented by a radius of curvature R of from 0.5 mm to 2.5 mm; and   (2) the ratio ER of a collective number of dimples RA having a radius of curvature R to diameter D ratio (R/D) of at least 20%, divided by a total number of dimples N on the surface of the ball, is from 15% to 95%.   
     
     
         8 . The practice golf ball of  claim 7  which further satisfies condition (3) below:
 (3) the ball has thereon a plurality of dimple types of differing diameter, and the ratio DER of a combined number of dimples DE obtained by adding together dimples having an own diameter and an own radius of curvature larger than or equal to a radius of curvature of dimples of larger diameter than said own diameter plus dimples of a type having a largest diameter, divided by the total number of dimples N on the surface of the ball, is at least 80%. 
 
     
     
         9 . The practice golf ball of  claim 8  which further satisfies conditions (4) to (6) below:
 (4) the number of dimple types of differing diameter is 3 or more; 
 (5) the total number of dimples N is not more than 380; and 
 (6) the surface coverage SR of the dimples, which is the sum of individual dimple surface areas, each defined by a flat plane circumscribed by an edge of the dimple, expressed as a percentage of the surface area of a hypothetical sphere representing the ball were the ball to have no dimples on the surface thereof, is from 60% to 74%.