A golf ball, especially a spherical rubber core of a golf ball is generally produced by a method wherein a rubber composition for the core is inserted into a semispherical mold and covered with another semispherical mold, followed by heating under pressure. It is also suggested to make a core two-layered. The two-layered core is generally prepared by a method wherein a center is formed by vulcanizing a rubber composition and then a bowl-shaped half-shell is separately formed from a rubber composition by molding under pressure at a temperature which does not cause vulcanization, using a half-shell mold and a core mold, followed by inserting the vulcanized center into the bowl-shaped half-shell, covering another bowl-shaped half-shell, and vulcanizing by heating under pressure. A hollow thread wound golf ball has also been proposed which is prepared by winding thread rubber on a hollow iron sphere in a stretched condition and then covering with a cover resin (Japanese Patent Kokai Publication No. 18787/198).
The rebound characteristics of golf balls have been remarkably improved these years by attaining a high launch angle and reducing the spin amount. However, shot feel at the time of hitting is desired to be more improved, because golf balls which have been commercially available do not meet golfers' satisfaction in shot feel. In order to improve the shot feel, it has been suggested to make a golf ball with a hollow center. However, it is a difficult technique to mold vulcanized rubber into a hollow sphere. Particularly, when a core of the hollow golf ball is produced either by press-molding in a core mold shown in FIG. 5 or by injection-molding to form a bowl-shaped half-shell followed by vulcanizing two of the bowl-shaped half-shells in a mold shown in FIG. 6, the resulting hollow portion can be easily broken by the expansion of the rubber composition and does not form a perfect spherical shape and thus provides much distribution in the thickness of the core wall. It has also been suggested that two bowl-shaped half-shells can be obtained by vulcanizing the rubber composition and then adhering the shells to each other by an adhesive agent etc. to form a hollow core. However, the hollow core has a defect in its durability because the hollow core often breaks at the adhesive surface by the very strong impact force created at the time of hitting the golf ball with a golf club.