| 摘要 |
PROBLEM TO BE SOLVED: To reduce a significant decrease in the amount of eccentricity of the internal layer in relation to the external layer of a core, realize mass production and provide outstanding durability and flying performance. SOLUTION: When forming a core, after an unvulcanized material 12 is semi-vulcanized or vulcanized to form a hemispherical shell-shaped external layer half 13, an unvulcanized internal layer forming material is pinched by a pair of the external layer halves 13 and 13 and vulcanized and press-molded. The internal layer is penetrated into at least a part of the seam portion of the external layer of the core. The volume of the part of the internal layer that exudes to the external layer is 1/300 to 1/5 of the volume of the external layer when it is presumed to be a perfectly spherical shell.
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