Practice golf ball

Abstract

A practice golf ball has a core and a cover. The core is made of a rubber composition which includes a base rubber, a co-crosslinking agent which is methacrylic acid, a crosslinking initiator and a metal oxide. The respective ingredients in the rubber composition are formulated in specific amounts, and the core has an optimized hardness profile. The cover includes a resin component which is composed primarily of polyurethane. The cover has a thickness of 0.3 mm to 1.9 mm, and a material hardness, expressed as the Shore D hardness, of 30 to 48. The respective deflections of the core and the ball under a specific load, and the relationship therebetween, are optimized. The volume ratio VR of dimples on the surface of the ball is optimized.

1 . A practice golf ball comprising
 a core made of a rubber composition comprising a base rubber and, as compounding ingredients: a co-crosslinking agent, a crosslinking initiator and a metal oxide; and   a cover which encases the core and comprises a resin component,   
       wherein the co-crosslinking agent is methacrylic acid; the metal oxide is zinc oxide; the core has a hardness profile in which, letting A be the JIS-C hardness at a surface of the core, B be the JIS-C hardness at a position 2 mm inside the core surface, C be the JIS-C hardness at a position 5 mm inside the core surface, D be the JIS-C hardness at a position 10 mm inside the core surface, E be the JIS-C hardness at a position 15 mm inside the core surface, and F be the JIS-C hardness at a center of the core: A is from 65 to 83, B is from 59 to 78, C is from 61 to 80, D is from 59 to 75, E is from 56 to 70, and F is from 53 to 67; the relative hardness conditions A>B<C≧D>E>F are satisfied; the value A-F is not more than 19; the core is formed in such a way that A has the highest value among A to F; the value A-C is from 0 to 8; the core has a specific gravity of from 1.05 to 1.2; the resin component of the cover is composed primarily of polyurethane; the cover has a thickness of from 0.3 mm to 1.9 mm; the cover has a material hardness, expressed as the Shore D hardness, of from 30 to 48; when the core and the ball are each compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf), letting deflection by the core be CH and deflection by the ball be BH 1 , the core deflection CH is from 2.8 mm to 7.0 mm and the ratio CH/BH 1  is from 0.95 to 1.1; and 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 0.95% to 1.7%. 
     
     
         2 . The practice golf ball of  claim 1 , wherein the ball has, upon initial measurement, a deflection BH 1  (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 BV 1  (m/s), and also has, when measured again after being left to stand for 350 days following initial measurement, a deflection BH 2  (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 BV 2  (m/s), such that the difference BH 2 −BH 1  is not more than 0.2 mm and the difference BV 2 −BV 1  is not more than 0.3 m/s. 
     
     
         3 . 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%.   
     
     
         4 . The practice golf ball of  claim 3  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%. 
 
     
     
         5 . The practice golf ball of  claim 4  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%. 
 
     
     
         6 . The practice golf ball of  claim 1 , wherein the polyurethane in the resin component of the cover is a thermoplastic polyurethane elastomer. 
     
     
         7 . The practice golf ball of  claim 6 , wherein the thermoplastic polyurethane elastomer comprises soft segments formed from a polymeric polyether polyol and hard segments formed from an aromatic diisocyanate. 
     
     
         8 . The practice golf ball of  claim 1 , wherein the ball has, upon initial measurement, a deflection BH 1  when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) of from 2.8 mm to 7.0 mm. 
     
     
         9 . The practice golf ball of  claim 1 , wherein the ball has, upon initial measurement, an initial velocity BV 1  of not more than 76 m/s. 
     
     
         10 . The practice golf ball of  claim 1 , wherein the compounding ingredients in the rubber composition are included in respective amounts of from 10 to 40 parts by weight of methacrylic acid, from 15 to 30 parts by weight of metal oxide, from 0.3 to 0.88 part by weight of crosslinking initiator, and from 0.1 to 1.0 part by weight of an antioxidant, per 100 parts by weight of the base rubber.