Golf ball manufacturing method

Abstract

SOLUTION: To provide a golf ball with a core and a cover that includes: a plurality of grooves or holes at a surface of the core made of a polybutadiene rubber; and a polybutadiene rubber having a composition different from the polybutadiene rubber forming a core body and filled in those grooves or holes, where those groove parts or hole parts into which the polybutadiene rubber is filled have a surface hardness that is higher than a surface hardness of the core body.EFFECT: A golf ball suppresses torsion caused by external force while maintaining a sufficient amount of flexure at the time of ball hitting and reduces a rate of spin without deteriorating resilience. The golf ball effectively increases a carry, especially, upon being hit by a driver.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a golf ball having a core and a cover, and particularly to a golf ball that can appropriately reduce the amount of spin on driver shots to increase the flight distance, and to a method for manufacturing the same. [Background technology]

[0002] Various proposals have been made on core structures to improve the performance of golf balls. For example, many proposals have been made on cores with two-layer structures, and the following Patent Documents 1 to 3 propose structures in which a number of recesses and protrusions are provided on the core surface.

[0003] On the other hand, there is a strong demand for increased distance, especially on driver shots, and an effective way to achieve this is to reduce the amount of spin on driver shots. Increasing the surface hardness of the core and cover effectively reduces the twist of the ball caused by external forces, but if the core or cover is made harder, the amount of deflection of the ball will also decrease, reducing its resilience and initial velocity upon impact, resulting in an insufficient increase in distance.

[0004] Therefore, there is a need for a method to increase the flight distance by suppressing twisting caused by external forces while maintaining sufficient deflection upon impact, thereby reducing the amount of spin without reducing resilience. However, with the conventional methods mentioned above, such as using a two-layer core or providing recesses on the core surface, it is not easy to achieve both deflection and suppression of twisting and effectively increase the flight distance. Therefore, there is a need for the development of technology that can achieve both the deflection of the ball and suppression of twisting to more effectively increase the flight distance. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] US2015-0018127 [Patent Document 2] US2015-0018124 [Patent Document 3] US2015-0018126 Summary of the Invention [Problem to be solved by the invention]

[0006] As mentioned above, the relationship between the hardness and deflection of a golf ball core is a trade-off in conventional technology, and it has been difficult to achieve both by reducing the spin rate without reducing the resilience. Therefore, there remains a major challenge in improving the core to increase the flight distance.

[0007] The present invention has been made in consideration of the above circumstances, and aims to provide a golf ball that can suppress twisting due to external forces while maintaining sufficient deflection upon impact, thereby reducing the amount of spin without reducing resilience and effectively increasing the flight distance. [Means for solving the problem]

[0008] As a result of extensive research into achieving the above-mentioned object, the inventors have discovered that, in a golf ball having a core and a cover, by forming a plurality of grooves or holes in the surface of a core made of polybutadiene rubber and filling these grooves or holes with polybutadiene rubber that is harder than the core itself, it is possible to obtain a golf ball that maintains sufficient deflection when hitting the ball while suppressing twisting due to external forces, thereby reducing the amount of spin without reducing resilience and effectively increasing the flight distance, and have completed the present invention.

[0009] Accordingly, the present invention provides the following golf balls and methods for producing the same. 1. a core manufacturing step of forming a sphere by pressure vulcanization molding polybutadiene rubber, and then drilling a plurality of grooves or holes in the surface of the sphere to manufacture a core body; a rubber filling step of filling the grooves or holes formed on the surface of the core body with polybutadiene rubber having a hardness higher than the surface hardness of the core body to produce a core having a smooth spherical surface; a cover-forming step of forming a cover around the core, either directly or via an intermediate layer; having a core and a cover,The core is made of polybutadiene rubber and has a plurality of grooves or holes on the surface thereof, and these grooves or holes are filled with polybutadiene rubber of a different composition from the polybutadiene rubber forming the core body, and the surface hardness of the groove or hole portions filled with this polybutadiene rubber is higher than the surface hardness of the core body. Get a golf ball A golf ball characterized by Manufacturing method. 2. In the rubber filling step, a pair of polybutadiene rubber materials formed in a half-cup shape are placed on the core body to cover it, and after pressure vulcanization molding, the surface of the core body is polished until the surface of the core body is exposed, thereby producing a core in which the grooves or holes formed on the surface of the core body are filled with polybutadiene rubber having a hardness higher than the surface hardness of the core body. 1 description A method for manufacturing a golf ball. 3. The difference in surface hardness between the groove or hole portion and the core body is 1 to 20 in JIS-C hardness. 3. A method for producing the golf ball according to 1 or 2. 4. The depth of the groove or hole formed on the core surface is 0.2 to 2 mm. 4. The method for producing the golf ball according to any one of 1 to 3 . 5. The total area of ​​the grooves or holes formed on the core surface accounts for 10 to 50% of the core surface. 5. A method for producing the golf ball according to any one of 1 to 4. 6. The hole formed on the core surface is a cylindrical hole. 6. A method for producing the golf ball according to any one of 1 to 5. 7. The grooves formed on the core surface are in a lattice pattern. 7. A method for producing the golf ball according to any one of 1 to 6. 8. The distance between adjacent grooves or holes is 2 to 10 mm. 8. A method for producing the golf ball according to any one of 1 to 7. 9. The hardness of the cover is higher than the surface hardness of the groove or hole portion of the core. 9. A method for producing the golf ball according to any one of 1 to 8. 10. The difference in hardness between the cover and the surface hardness of the groove or hole portion of the core is 1 to 25 in JIS-C hardness. 10. A method for producing the golf ball according to any one of 1 to 9. [Effects of the Invention]

[0010] The golf ball of the present invention maintains a sufficient amount of deflection when hitting the ball while suppressing twisting due to external forces, thereby reducing the amount of spin without reducing resilience and effectively increasing the flight distance, particularly on driver shots. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 2 is a front view showing an example of a plurality of grooves formed on the surface of a core in the present invention. [Figure 2] FIG. 2 is a front view showing an example of a plurality of holes formed on the surface of a core in the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0012] The present invention will be described in more detail below. The golf ball of the present invention has a core and a cover, and as described above, a core body made of polybutadiene rubber has a plurality of grooves or holes formed on the surface thereof, and these grooves or holes are filled with polybutadiene rubber having a harder surface than the core body.

[0013] As described above, the core used in the golf ball of the present invention is composed of a core body made of polybutadiene rubber with a plurality of grooves or holes formed on the surface, and a rubber filler made of polybutadiene rubber that fills these grooves or holes.

[0014] The core body and the rubber filler are made of polybutadiene rubber as described above, and can be produced from a rubber composition containing polybutadiene rubber as the base rubber as the main component, to which a co-crosslinking agent, an organic peroxide, an inert filler, an organic sulfur compound, etc. are compounded.

[0015] Examples of the co-crosslinking agent include unsaturated carboxylic acids and metal salts of unsaturated carboxylic acids. Specific examples of unsaturated carboxylic acids include acrylic acid, methacrylic acid, maleic acid, and fumaric acid, with acrylic acid and methacrylic acid being particularly preferred. Metal salts of unsaturated carboxylic acids are not particularly limited, but include, for example, those obtained by neutralizing the above-mentioned unsaturated carboxylic acids with desired metal ions. Specific examples include zinc salts and magnesium salts of methacrylic acid, acrylic acid, and the like, with zinc acrylate being particularly preferred.

[0016] The amount of the unsaturated carboxylic acid and / or metal salt thereof, per 100 parts by weight of the base rubber, is preferably at least 5 parts by weight, more preferably at least 9 parts by weight, and even more preferably at least 13 parts by weight, with the upper limit being preferably no more than 60 parts by weight, more preferably no more than 50 parts by weight, and even more preferably no more than 40 parts by weight. If the amount is too high, the ball may become too hard, resulting in an unbearable feel at impact, while if the amount is too low, the resilience may decrease.

[0017] Commercially available organic peroxides can be used, such as Percumyl D (manufactured by Nippon Oil & Fats Co., Ltd.), Perhexa C-40, Perhexa 3M (manufactured by Nippon Oil & Fats Co., Ltd.), and Luperco 231XL (manufactured by Atochem). These may be used alone or in combination. The amount of organic peroxide added is preferably at least 0.1 parts by weight, more preferably at least 0.3 parts by weight, even more preferably at least 0.5 parts by weight, and most preferably at least 0.6 parts by weight, per 100 parts by weight of the base rubber. The upper limit is preferably at most 5 parts by weight, more preferably at most 4 parts by weight, even more preferably at most 3 parts by weight, and most preferably at most 2.5 parts by weight. If the amount added is too high or too low, it may be difficult to achieve a satisfactory feel, durability, and resilience.

[0018] Other examples of compounding agents that can be compounded into the base rubber include inert fillers, such as zinc oxide, barium sulfate, and calcium carbonate. These may be used alone or in combination. The amount of inert filler compounded into the inner core layer is preferably at least 40 parts by weight, more preferably at least 50 parts by weight, per 100 parts by weight of the base rubber, with the upper limit being preferably no more than 100 parts by weight, more preferably no more than 90 parts by weight, and even more preferably no more than 80 parts by weight. If the amount is too high or too low, it may be difficult to achieve the appropriate weight and appropriate resilience.

[0019] The organic sulfur compound is intended to improve the resilience of golf balls, and examples thereof include thiophenols, thionaphthols, halogenated thiophenols, and their metal salts. More specific examples include pentachlorothiophenol, pentafluorothiophenol, pentabromothiophenol, parachlorothiophenol, zinc salts of pentachlorothiophenol, zinc salts of pentafluorothiophenol, zinc salts of pentabromothiophenol, zinc salts of parachlorothiophenol, and diphenyl polysulfides containing 2 to 4 sulfur atoms, such as dibenzyl polysulfide, dibenzoyl polysulfide, dibenzothiazoyl polysulfide, and dithiobenzoyl polysulfide. The amount of the organic sulfur compound is preferably at least 0 parts by weight, more preferably at least 0.05 parts by weight, and even more preferably at least 0.1 parts by weight, per 100 parts by weight of the base rubber. The upper limit is preferably 5 parts by weight, more preferably 3 parts by weight, and even more preferably 2.5 parts by weight. If the blending amount is too high, the expected improvement in resilience (especially on shots with a W#1) will be limited, the entire core will become too soft, or the feel may be poor. Conversely, if the blending amount is too low, the expected improvement in resilience will be lost.

[0020] Furthermore, an antioxidant can be added as needed, and examples of commercially available products include Nocrac NS-6 and Nocrac NS-30 (manufactured by Ouchi Shinko Chemical Industry Co., Ltd.), Yoshinox 425 (manufactured by Yoshitomi Pharmaceutical Co., Ltd.), etc. These may be used alone or in combination of two or more.

[0021] The amount of antioxidant blended per 100 parts by weight of the base rubber is preferably at least 0 part by weight, more preferably at least 0.05 part by weight, and particularly preferably at least 0.1 part by weight, with the upper limit being preferably at most 3 parts by weight, more preferably at most 2 parts by weight, particularly preferably at most 1 part by weight, and most preferably at most 0.5 parts by weight. If the blended amount is too high or too low, it may be difficult to obtain suitable rebound properties and durability.

[0022] Here, in the present invention, as described above, the hardness of the polybutadiene rubber of the rubber filler (surface hardness of the groove or hole portion) is set higher than the hardness of the polybutadiene rubber of the core body (surface hardness of the core body). The hardness of the polybutadiene rubber can be adjusted by adjusting the amount of the co-crosslinking agent, although there are no particular limitations on this adjustment.

[0023] The surface hardness of the core body is preferably 65 or more, more preferably 70 or more, and even more preferably 75 or more, in JIS-C hardness, with an upper limit of preferably 95 or less, more preferably 90 or less, and even more preferably 85 or less. Expressed in Shore D, this surface hardness is preferably 41 or more, more preferably 45 or more, and even more preferably 49 or more, with an upper limit of preferably 64 or less, more preferably 60 or less, and even more preferably 57. If this value is too high, crack resistance may be poor when repeatedly hit. Conversely, if this value is too low, spin may increase on full shots, preventing the desired distance from being achieved.

[0024] The surface hardness of the groove or hole portions is set higher than the surface hardness of the core body, and is preferably 70 or higher, more preferably 75 or higher, and even more preferably 79 or higher in JIS-C hardness, with an upper limit of preferably 98 or lower, more preferably 94 or lower, and even more preferably 91 or lower. When expressed in Shore D, this surface hardness is preferably 45 or higher, more preferably 49 or higher, and even more preferably 52 or higher, with an upper limit of preferably 66 or lower, more preferably 63 or lower, and even more preferably 61 or lower.

[0025] The difference in surface hardness between the groove or hole portion and the core body is not particularly limited, but is preferably 1 or more, more preferably 3 or more, and even more preferably 7 or more in JIS-C hardness, with the upper limit being preferably 20 or less, more preferably 17 or less, and even more preferably 15 or less.

[0026] The shape and form of the grooves or holes formed in the core are not particularly limited as long as they are uniformly arranged over the entire core surface, but when grooves are formed, for example, it is preferable to arrange multiple grooves in a lattice pattern so that the grooves are uniformly distributed over the entire core surface without significant bias. Specifically, for example, the lattice pattern shown in Figure 1 can be exemplified. In the example shown in Figure 1, three mutually perpendicular axes, i.e., x-axis, y-axis, and z-axis, are set in the core, and latitude lines are set evenly for each of these three axes, and grooves are formed along those latitude lines.

[0027] The holes formed in the core may have any shape, as long as they are columnar and extend from the surface toward the center of the core, and their cross-sectional shape may be circular, square, polygonal, etc., or a combination of holes of multiple shapes may be used, although cylindrical holes with a circular cross-sectional shape are usually preferred. There are no limitations on the arrangement of these holes, as long as they are uniformly arranged without significant bias over the entire surface of the core, and an example of the arrangement shown in Figure 2 is an example.

[0028] The depth of the grooves and holes is set appropriately depending on the size of the core, the hardness of the core body and filled rubber, the difference in hardness between them, and the like, and is not particularly limited, but is preferably 0.2 mm or more, more preferably 0.3 mm or more, and even more preferably 0.4 mm or more, with the upper limit being preferably 2 mm or less, more preferably 1.6 mm or less, and even more preferably 1.2 mm or less.

[0029] The number of the grooves and holes is appropriately set depending on the size of the grooves and holes, the size of the core, etc., and is not particularly limited, but the ratio of the total area of ​​the grooves and holes to the core surface is preferably 10% or more, more preferably 14% or more, and even more preferably 17% or more. The upper limit of this ratio is preferably 50% or less, more preferably 46% or less, and even more preferably 43 % or less. Although not particularly limited, the interval between adjacent grooves or holes is preferably 2 mm or more, more preferably 3 mm or more, and even more preferably 4 mm or more. The upper limit of this interval is 10 mm or less, more preferably 8 mm or less, and even more preferably 6 mm or less.

[0030] The core is first prepared using the above-mentioned rubber composition mainly containing polybutadiene rubber. The core body is prepared. A suitable method for this purpose is, for example, a conventional method of molding the core body by heating and compressing the rubber composition at 140°C to 180°C for 10 to 60 minutes to form a spherical shape. The method for forming the grooves or holes in the core body is not limited. For example, a method of molding a sphere having a smooth spherical surface and then drilling multiple grooves or holes in the surface of the sphere, or a method of transferring multiple protrusions or projections formed in a mold cavity to the surface of the core body during molding by heating and compressing the core body, thereby forming the grooves or holes, can be used. For industrial production, a method of transferring the protrusions or projections of the mold cavity during core molding is preferably used, in terms of production efficiency.

[0031] Next, the grooves or holes are filled with polybutadiene rubber having a hardness higher than that of the core body to prepare a core. The method for this is not particularly limited, but a method that can be suitably employed is, for example, to form a pair of half cups using the polybutadiene rubber composition in a sheet form, cover the core body with the pair of polybutadiene rubber materials formed in the half cup shape, and then pressure vulcanize-molded, and then polish the surface until the core body surface is exposed, thereby preparing a core in which the grooves or holes formed in the core body surface are filled with polybutadiene rubber having a hardness higher than that of the core body surface.

[0032] In this case, in the heat vulcanization molding of the above-mentioned half-cup shaped polybutadiene rubber material, an unvulcanized sheet-like rubber material may be semi-vulcanized (primary vulcanization) to produce a semi-vulcanized half cup, which may then be placed on the core body for full vulcanization (secondary vulcanization), or an unvulcanized sheet-like rubber material may be pressed into a hemispherical shape to produce an unvulcanized half cup, which may then be placed on the core body for full vulcanization in one go.

[0033] Next, the cover that encases the core will be described. There are no particular restrictions on the cover material, but various known materials used in golf balls, such as ionomer resins and urethane elastomers, can be used, and these materials can be used to form a cover having a one-layer structure or, in some cases, two or more layers.

[0034] To further reduce the spin rate of the ball, it is particularly preferable to use a highly neutralized ionomer material in the layer adjacent to the core. Specifically, it is preferable to use a material containing the following components (i) to (iv): (i-1) an olefin-unsaturated carboxylic acid binary random copolymer and / or a metal ion-neutralized product of an olefin-unsaturated carboxylic acid binary random copolymer; (i-2) an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester ternary random copolymer and / or a metal ion neutralized product of an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester ternary random copolymer in a mass ratio of 100:0 to 0:100 (i) a base resin, and (ii) a non-ionomer thermoplastic elastomer in a mass ratio of 100:0 to 50:50 (ii) a resin component, (iii) 5 to 80 parts by mass of a fatty acid and / or a derivative thereof having a molecular weight of 228 to 1500; (iv) 0.1 to 17 parts by mass of a basic inorganic metal compound capable of neutralizing unneutralized acid groups in the above components (i) and (iii). In particular, when using a mixed material of the above components (i) to (iv), it is preferable to use one in which 70% or more of the acid groups have been neutralized.

[0035] When the cover has two or more layers, it is preferable that the material of the outermost layer be primarily made of ionomer resin or urethane material, especially ionomer resin.

[0036] To obtain the cover, for example, the core is placed in a mold, the mixture is heated, mixed, and melted, and then injection-molded to form a desired cover around the core. In this case, the cover can be manufactured under conditions that ensure excellent thermal stability, fluidity, and moldability, and the final golf ball has high resilience, a good feel on impact, and excellent abrasion resistance. In addition to the above, the cover can also be formed by, for example, molding a pair of hemispherical half cups from the cover material in advance, encasing the core in these half cups, and pressure-molding the resulting half cups at 120 to 170°C for 1 to 5 minutes.

[0037] When the cover has one layer, its thickness can be 0.3 to 3 mm. When the cover has two layers, the outer cover layer can be 0.3 to 2.0 mm thick, and the inner cover layer can be 0.3 to 2.0 mm thick. There are no particular restrictions on the Shore D hardness of each layer (cover layer) constituting the cover, but it is preferably 40 or greater, more preferably 45 or greater, and the upper limit is preferably 70 or less, more preferably 65 or less.

[0038] The surface hardness of the cover that constitutes the surface of the ball is not particularly limited, but is preferably at least 75, more preferably at least 80, and even more preferably at least 85, on the JIS-C hardness scale. The upper limit of this surface hardness is preferably at least 99mm, more preferably at least 97mm, and even more preferably at least 95mm. Expressed in Shore D, this surface hardness is preferably at least 49, more preferably at least 53, and even more preferably at least 57, with the upper limit being preferably at most 67, more preferably at most 66, and even more preferably at most 64.

[0039] Although not particularly limited, the hardness of the cover is preferably set higher than the hardness of the groove or hole portions of the core, thereby effectively reducing spin rate on driver shots. In this case, the difference in hardness between the cover and the surface hardness of the groove or hole portions of the core is not particularly limited, but is preferably 1 or greater, more preferably 4 or greater, and even more preferably 7 or greater, on the JIS-C hardness scale. The upper limit of this hardness difference is preferably 25 or less, more preferably 20 or less, and even more preferably 16 or less.

[0040] Dimples can be formed on the surface of the cover in the usual manner, and the surface can also be painted as appropriate. Furthermore, the golf ball of the present invention can be a two-piece golf ball consisting of the core and cover, but in some cases, it can also be a multi-piece golf ball by forming one or more intermediate layers between the core and cover. [Example]

[0041] EXAMPLES The present invention will be specifically explained below with reference to examples and comparative examples, but the present invention is not limited to the following examples.

[0042] [Examples 1 to 3, Comparative Example 1] Core formation A rubber composition having composition A shown in Table 1 below was prepared and then vulcanized at 155°C for 13 minutes to produce a sphere having an outer diameter of 40.2 mm and a smooth spherical surface. Next, cylindrical holes with the dimensions shown in Table 3 were formed by milling in the arrangement shown in Figure 2 to obtain the core body for Examples 1 and 2. Grooves with the dimensions shown in Table 3 were formed in the surface of the sphere using a lathe in the lattice pattern shown in Table 1 to obtain the core body for Example 3. A similar rubber composition was vulcanized at 155°C for 15 minutes to produce a sphere having an outer diameter of 40.2 mm. This core for Comparative Example 1 was left with a smooth spherical surface without forming any grooves or holes. The compressive deformation (deflection) of the resulting core bodies for Examples 1 to 3 and the core for Comparative Example 1 was measured using the following method. The results are shown in Table 3.

[0043] Next, a rubber composition having composition B shown in Table 1 below was prepared. This rubber composition was placed in a mold having a hemispherical cavity and sandwiched between a convex mold having the same radius as the core. It was then heated at 155°C for 1 minute, removed from the mold, and a half-cup-shaped rubber material was prepared. Further, a half-cup-shaped rubber material was prepared in the same manner, to prepare a pair of half-cup-shaped rubber materials. These were placed over the core body having the hole or groove formed therein, and vulcanization-molded at 155°C for 13 minutes to obtain a sphere whose surface was coated with a filler rubber material. The surface of the obtained sphere was polished until the surface of the core body was exposed, producing each of the cores for Examples 1 to 3. The surface hardness of the exposed core body surface and the groove or hole portion filled with rubber was measured. The surface hardness of the core for Comparative Example 1 was also measured in the same manner. The results are shown in Table 3.

[0044] Cover formation The resulting cores for Examples 1 to 3 and Comparative Example 1 were each injection-molded with a cover material having composition a shown in Table 2 to form a 1.25 mm thick cover, thereby producing golf balls. The same dimples were formed on the cover surface of each golf ball. The surface hardness of the cover was measured for each of the resulting golf balls. The results are shown in Table 3.

[0045] Performance evaluation The amount of compressive deformation (deflection) of each golf ball and the amount of spin when hit with a driver (W#1) were measured using the following methods. The results are shown in Table 3. Amount of compression deformation (deflection) of the core and ball The core or ball is placed on a hard board and the deflection is measured from the initial load of 98N (10kgf) to the final load of 1275N (130kgf). The measurement is carried out at a temperature of 23.9°C. Spin amount when hitting with a driver (W#1) The amount of backspin is measured when a Bridgestone Sports PHYZ driver (loft angle 10.5°) is attached to a golf hitting robot and hit at a head speed (HS) of 45 m / s.

[0046] [Examples 4 to 6, Comparative Examples 2 to 4] Core formation A rubber composition having composition A (Comparative Examples 2 to 4) or composition D (Examples 4 to 6) shown in Table 1 below was prepared, and then vulcanized and molded at 155°C for 13 minutes to produce a sphere having a smooth spherical surface and an outer diameter of 40.2 mm. Next, cylindrical holes having the dimensions shown in Table 3 were formed by milling in the arrangement shown in Figure 2 to obtain the core bodies for Example 6 and Comparative Examples 2 and 3. Grooves having the dimensions shown in Table 3 were formed in the surface of the sphere by lathe processing in the lattice pattern shown in Table 1 to obtain the core bodies for Examples 4 and 5 and Comparative Example 4. The compressive deformation (deflection) of the resulting core bodies for Examples 4 to 6 and Comparative Examples 2 to 4 was measured by the above method, and the results are shown in Table 3.

[0047] Next, rubber compositions B (Example 6), C (Comparative Examples 2-4), and E (Examples 4 and 5) were prepared as shown in Table 1 below. Each rubber composition was placed in a mold with a hemispherical cavity and sandwiched between a convex mold with the same radius as the core. The mixture was heated at 155°C for 1 minute and then removed from the mold to produce a half-cup-shaped rubber material. Further half-cup-shaped rubber materials were produced in the same manner, resulting in a pair of half-cup-shaped rubber materials. These were placed over the core body with holes or grooves formed therein and vulcanized at 155°C for 13 minutes to obtain a sphere in which the surface of the core body was coated with a filler rubber material. The surface of the resulting sphere was polished until the surface of the core body was exposed, producing the cores for Examples 4-6 and Comparative Examples 2-4. The surface hardness of the exposed core body surface and the groove or hole portion filled with rubber was measured for each core, and the results are shown in Table 3.

[0048] Cover formation The resulting cores for Examples 4 to 6 and Comparative Examples 2 to 4 were each injection-molded with a cover material having a composition a (Example 4 and Comparative Examples 2 to 4), b (Example 6), or c (Example 5) shown in Table 2 to form a 1.25 mm thick cover, producing golf balls. The same dimples as in Example 1 were formed on the cover surface of each golf ball. The surface hardness of the cover for each of the resulting golf balls was measured, and the results are shown in Table 3.

[0049] Performance evaluation For each golf ball, the amount of compressive deformation of the ball and the amount of spin when hit with a driver (W#1) were measured in the same manner as in Example 1 above. The results are shown in Table 3.

[0050] [Table 1] Polybutadiene: JSR Corporation, product name "BR01" Zinc acrylate: "ZN-DA85S" manufactured by Nippon Shokubai Organic peroxide (I): Dicumyl peroxide, NOF Corporation, trade name "Percumyl D" Organic peroxide (II): a mixture of 1,1-di(t-butylperoxy)cyclohexane and silica, manufactured by NOF Corporation, trade name "Peroxa C-40" Antioxidant: 2,2-methylenebis(4-methyl-6-butylphenol), manufactured by Ouchi Shinko Chemical Industry Co., Ltd., product name "Nocrac NS-6" Zinc oxide: Sakai Chemical Industry Co., Ltd., product name "Zinc Oxide Type 3" Pentachlorothiophenol: ZHEJIANG CHO&FU CHEMI CO., LTD.

[0051] [Table 2] Himilan 1605: Ionomer resin manufactured by Mitsui Dow Polychemicals Himilan 1855: Ionomer resin manufactured by Mitsui Dow Polychemicals AM7327: Ionomer resin manufactured by Mitsui Dow Polychemicals Titanium oxide: Sakai Chemical Industry Co., Ltd.

[0052] [Table 3]

[0053] As shown in the results in Table 3, the golf balls of Examples 1 to 6 according to the present invention maintain good resilience (amount of compression deformation (amount of deflection)) while keeping the spin rate low on driver shots, and can effectively increase the flight distance. On the other hand, the golf balls of Comparative Examples 1 to 4 are inferior to Examples 1 to 6 in terms of reducing the spin rate, as follows:

[0054] The golf ball of Comparative Example 1 has no grooves or holes in the core, and therefore it is presumed that it has a higher spin rate than the balls of Examples 1 to 6, resulting in a shorter flight distance. The golf ball of Comparative Example 2 has a core surface hardness greater than that of the core body but less hardness at the hole. This suggests that the golf ball of Comparative Example 2 has a higher spin rate and a shorter flight distance than the ball of Example 1, which has the same specifications except for the hole. The golf ball of Comparative Example 3 has a core surface hardness greater than that of the core body but less hardness at the hole. This suggests that the golf ball of Comparative Example 3 has a higher spin rate and a shorter flight distance than the ball of Example 2, which has the same specifications except for the hole. The golf ball of Comparative Example 4 has a core surface hardness greater than that of the core body but greater than that of the grooves. This suggests that the ball has a higher spin rate and a shorter flight distance than the ball of Example 3, which has the same specifications except for the grooves.

Claims

1. A core manufacturing process comprising: forming a sphere by pressure vulcanization molding polybutadiene rubber, and then drilling a plurality of grooves or holes in the surface of the sphere to manufacture a core body; a rubber filling step of filling the grooves or holes formed on the surface of the core body with polybutadiene rubber having a hardness higher than the surface hardness of the core body to produce a core having a smooth spherical surface; a cover-forming step of forming a cover around the core, either directly or via an intermediate layer; A method for manufacturing a golf ball, comprising: a core and a cover; a plurality of grooves or holes on the surface of the core made of polybutadiene rubber; and a polybutadiene rubber having a different composition from the polybutadiene rubber forming the core body, the grooves or holes filled with the polybutadiene rubber, the surface hardness of the grooves or holes filled with the polybutadiene rubber being higher than the surface hardness of the core body.

2. A method for manufacturing a golf ball as described in claim 1, wherein in the rubber filling process, a pair of polybutadiene rubber materials formed in a half-cup shape are placed over the core body to cover it, and after pressure vulcanization molding, the surface of the core body is polished until the surface of the core body is exposed, thereby creating a core in which the grooves or holes formed on the surface of the core body are filled with polybutadiene rubber having a hardness higher than the surface hardness of the core body.

3. The method for manufacturing a golf ball according to claim 1, wherein the difference in surface hardness between the groove or hole portions and the core body is 1 to 20 in JIS-C hardness.

4. A method for manufacturing a golf ball described in any one of claims 1 to 3, wherein the depth of the grooves or holes formed on the core surface is 0.2 to 2 mm.

5. A method for manufacturing a golf ball described in any one of claims 1 to 4, wherein the total area of ​​the grooves or holes formed on the core surface accounts for 10 to 50% of the core surface.

6. A method for manufacturing a golf ball described in any one of claims 1 to 5, wherein the hole formed in the core surface is a cylindrical hole.

7. A method for manufacturing a golf ball described in any one of claims 1 to 6, wherein the grooves formed on the core surface are lattice-shaped.

8. A method for manufacturing a golf ball described in any one of claims 1 to 7, wherein the spacing between adjacent grooves or holes is 2 to 10 mm.

9. A method for manufacturing a golf ball as described in any one of claims 1 to 8, wherein the hardness of the cover is higher than the surface hardness of the groove or hole portion of the core.

10. A method for manufacturing a golf ball as recited in any one of claims 1 to 9, wherein the difference between the hardness of the cover and the surface hardness of the groove or hole portion of the core is 1 to 25 in JIS-C hardness.

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