Golf balls are generally composed of a core and a cover covering the core, and the cover is generally formed from ionomer resin and has a thickness of 2.4 mm or less. The cover thickness of less than 2.4 mm has been required in view of the producing process.
However, it has been recently proposed that the cover be made thicker than the conventional thickness to add another function. For example, the cover is made having a thickness of 3.0 to 5.0 mm and the core is made softer, which attains a higher hit angle and lower spin amount and results in enhancement of flight performance. As mentioned above, however, a thick covering on the core gives rise to some problems in the producing process. The covering process is generally conducted by injection molding or press molding, but the following three problems would occur:
(1) The cover thickness is non-uniform and the sphericity of the resulting golf ball is deteriorated in comparison with the conventional golf ball having a cover thickness of up to 2.4 mm.
(2) When injection molding, the cores are held by holding pins in a mold and the traces of the holding pin remain in the produced golf balls. The holding pin traces produce bad effects in the durability of the golf balls.
(3) When injection molding, the cover resin contracts a little. The thicker the cover, the larger the contraction, and the resulting golf balls often have a smaller diameter than required by he PGA rules.