Golf ball and method for manufacturing the same

1. A method for manufacturing a golf ball that comprises a spherical core and at least one cover layer covering the spherical core, comprising the steps of:

preparing a rubber composition containing a base rubber, an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or a metal salt thereof as a co-crosslinking agent, crosslinking initiator and a carboxylic acid and/or a salt thereof not including the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or the metal salt thereof, provided that the rubber composition further contains a metal compound in the case of containing only the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms as the co-crosslinking agent;

heat pressing the rubber composition at a press temperature ranging from t-40° C. to t-15° C. to mold the spherical core having a hardness difference (Hs−Ho) between a surface hardness Hs and a center hardness Ho thereof of 16 or more in JIS-C hardness, wherein t ° C. is a one-minute half-life temperature of the crosslinking initiator, provided that the heat press temperature is adjusted to satisfy said range relative to one-minute half-life temperatures of all of the crosslinking initiators in the case of blending two or more kinds of the crosslinking initiators in the rubber composition; and

forming at least one cover layer covering the spherical core.

2. The method for manufacturing the golf ball according to claim 1 , wherein the rubber composition contains the carboxylic acid and/or the salt thereof in a content ranging from 0.5 part to 30 parts by mass with respect to 100 parts by mass of the base rubber.

3. The method for manufacturing the golf ball according to claim 1 , wherein the crosslinking initiator is a peroxy ketal and/or a dialkyl peroxide.

4. The method for manufacturing the golf ball according to claim 1 , wherein the rubber composition further contains an organic sulfur compound.

5. The method for manufacturing the golf ball according to claim 4 , wherein the organic sulfur compound includes at least one compound selected from the group consisting of thiophenols and/or metal salts thereof, thionaphthols and/or metal salts thereof, diphenylpolysulfides and thiuramdisulfides.

6. The method for manufacturing the golf ball according to claim 4 , wherein a content of the organic sulfur compound is 0.05 part to 5 parts by mass with respect to 100 parts by mass of the base rubber.

7. The method for manufacturing the golf ball according to claim 1 , wherein a content of the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or the metal salt thereof is 15 parts to 50 parts by mass with respect to 100 parts by mass of the base rubber.

8. The method for manufacturing the golf ball according to claim 1 , wherein the rubber composition contains the metal salt of the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms as the co-crosslinking agent.

9. The method for manufacturing the golf ball according to claim 1 , wherein a content of the crosslinking initiator is 0.2 part to 5 parts by mass with respect to 100 parts by mass of the base rubber.

10. The method for manufacturing the golf ball according to claim 1 , a difference between a maximum value and a minimum value among one-minute half-life temperatures of the crosslinking initiators is 25° C. or less in the case of using at least two of the crosslinking initiators in combination.

11. The method for manufacturing the golf ball according to claim 1 , the carboxylic acid and/or the salt thereof has 1 to 17 carbon atoms.