Thread-wound golf ball

Abstract

A thread-wound golf ball includes a solid center, a rubber thread layer wound around the surface of the solid center, and a cover enveloping the surface of the rubber thread layer, the cover having dimples formed thereon. An amount of deformation of the solid center under a load of 9.8 N-294 N is 1.0-1.5 mm. A difference between the diameter A1 of the solid center and the thickness A2 of the rubber thread layer represented by (A1−A2) is 34.5-36.5 mm. The cover has a Shore D hardness of 44-58. Dimple volume ratio Rv and the difference between the diameter of the solid center and the thickness of the rubber thread layer represented by (A1−A2) satisfy a relation: 24.5≦(A1−A2)×Rv≦29.0.

Description

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to thread-wound golf balls. In particular, the present invention relates to a thread-wound golf ball providing an improved carry while maintaining a desired shot feel that are achieved by defining a difference between the diameter of a solid center and the thickness of a rubber thread layer as well as an optimum dimple volume ratio.

[0003] 2. Description of the Background Art

[0004] It has been proposed to enhance the carry of a thread-wound golf ball by defining certain properties of the solid center of the thread-wound golf ball, for example, the diameter, hardness, specific gravity and resilience of the solid center, or defining the hardness for example of the cover.

[0005] For example, Japanese Patent Laying-Open No. 60-72573 discloses a thread-wound golf ball having a solid center of high resilience with a diameter of 33-38 mm and a JIS-C hardness of 65-90 and a rubber thread wound around the solid center to a thickness of 1-2.5 mm.

[0006] Japanese Patent Laying-Open No. 60-168471 discloses a thread-wound golf ball of 1.62 inches in size having a solid center with an outer diameter of 27-30 mm, a JIS-A hardness of 75-85 and a weight of 20.5-23.5 g as well as a thread-wound golf ball of 1.68 inches in size having a solid center with a diameter of 28-32 mm, a JIS-A hardness of 70-80 and a weight of 17.5-21.0 g. These golf balls each have a cover of ionomer resin with a Shore D hardness of at least 65 and a softening point of at least 50° C. and the cover has a thickness of 1.5-2.0 mm.

[0007] According to some recent proposals, the carry is improved not only by defining such factors as the properties of the solid center or the hardness for example of the cover as discussed above but by considering various factors in combination, i.e., considering such factors as properties of the solid center or the hardness for example of the cover as well as some properties of dimples like the number of dimples or the dimple volume ratio.

[0008] U.S. Pat. No. 5,704,853 discloses a thread-wound golf ball constituted of a solid center with an outer diameter of 27-38 mm, a deformation under a load of 30 kg of 1.5-3.5 mm and a rebound, when dropped from a height of 120 cm, of at least 96 cm, 350-500 dimples with a dimple volume ratio of 0.76-0.9%, an outer cover with a Shore D hardness of 40-55 and an inner cover with a Shore D hardness of 55-68.

[0009] The proposed thread-wound golf ball with the carry improved in consideration of only the properties of the solid center has improved initial flight conditions to increase the carry. However, no consideration is taken regarding optimization of the trajectory which affects the improvement of the carry.

[0010] The proposal based not only on the properties of the solid center but also on the properties of dimples takes consideration individually for initial flight conditions as well as optimization of the trajectory. However, nothing is considered about a further optimization of the trajectory in relation to initial flight conditions.

SUMMARY OF THE INVENTION

[0011] One object of the present invention is to solve the above-described problems by improving initial flight conditions in consideration of a combination of the diameter of a solid center and the thickness of a rubber thread layer and defining an optimum dimple volume ratio in relation to the initial flight conditions to optimize the trajectory and, accordingly, to improve the carry while maintaining a desired shot feel.

[0012] A thread-wound golf ball according to the present invention includes a solid center, a rubber thread layer wound around the surface of the solid center, and a cover enveloping the surface of the rubber thread layer, the cover having dimples formed thereon. An amount of deformation of the solid center under a load of 9.8 N-294 N is 1.0-1.5 mm. A difference between the diameter A1 of the solid center and the thickness A2 of the rubber thread layer, (A1−A2), is 34.5-36.5 mm. The cover has a Shore D hardness of 44-58. Dimple volume ratio Rv and the difference between the diameter of the solid center and the thickness of the rubber thread layer represented by (A1−A2) satisfy a relation 24.5≦(A1−A2)×Rv≦29.0.

[0013] For the thread-wound golf ball according to the present invention, preferably the solid center has a surface hardness greater than its center hardness that are measured by a JIS-C hardness tester and a difference between the surface hardness and the center hardness is 15-35.

[0014] For the thread-wound golf ball according to the present invention, preferably the solid center has a surface hardness of 70-90 measured by a JIS-C hardness tester.

[0015] Preferably, the thread-wound golf ball according to the present invention has dimple volume ratio Rv of 0.600-0.950%.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] The present invention is now described in detail. According to the present invention, a thread-wound golf ball includes a solid center, a rubber thread layer wound around the solid center and a cover enveloping the rubber thread layer.

[0017] According to the present invention, an amount of deformation of the solid center under a load of 9.8 N-294 N is 1.0-1.5 mm, preferably 1.1-1.4 mm and more preferably 1.2-1.4 mm. The amount of deformation less than 1.0 mm results in a smaller launch angle and a higher spin rate to decrease the carry, while the amount of deformation exceeding 1.5 mm means that the solid center is too soft and accordingly the golf ball has an inappropriate hardness resulting in a lower resilience.

[0018] The center hardness, measured by a JIS-C hardness tester, of the solid center according to the present invention is 50-70, preferably 52-68 and more preferably 54-66. The center hardness less than 50 means that the golf ball has an inappropriate hardness resulting in a lower resilience which decreases the carry, while the center hardness exceeding 70 means that the golf ball is too hard which impairs shot feel.

[0019] The surface hardness, measured by a JIS-C hardness tester, of the solid center according to the present invention is 70-90, preferably 73-87 and more preferably 76-85. The surface hardness of the solid center less than 70 means that the golf ball has an inappropriate hardness resulting in a lower resilience which decreases the carry, while the surface hardness exceeding 90 means that the golf ball is too hard which impairs shot feel.

[0020] Moreover, according to the present invention, preferably the solid center has a surface hardness (Hs) greater than a center hardness (Hc) that are measured by a JIS-C hardness tester and, a difference between the surface hardness and the center hardness of the solid center that is represented by (Hs−Hc) is 15-35, preferably 16-33 and more preferably 18-32. This hardness difference less than 15 results in a smaller launch angle and a higher spin rate to decrease the carry, while the difference exceeding 35 results in a lower resilience which decreases the carry.

[0021] A difference between the diameter A1 of the solid center and the thickness A2 of the rubber thread layer that is represented by (A1−A2) is 34.5-36.5 mm, preferably 34.8-36.3 mm and more preferably 35.0-36.0 mm. The difference (A1−A2) less than 34.5 mm means that the diameter of the solid center is relatively small or the rubber thread layer is relatively thick, resulting in a higher spin rate at impact to decrease the carry, while the difference (A1−A2) exceeding 36.5 mm means that the rubber thread layer is thin which hinders the rubber thread from sufficiently exhibiting its resilience and thus the carry decreases.

[0022] According to the present invention, the cover has a Shore D hardness of 44-58, preferably 45-57 and more preferably 46-55. The Shore D hardness of the cover that is less than 44 increases the spin rate at impact to decrease the carry, while the Shore D hardness exceeding 58 means that the cover is too hard which impairs the shot feel of the resultant golf ball.

[0023] A large number of depressions called dimples are formed as required over the cover when the cover is molded or after the cover has been molded.

[0024] Moreover, dimple volume ratio Rv and the difference between the diameter of the solid center and the thickness of the rubber thread layer which is represented by (A1−A2) satisfy a relation 24.5≦(A1−A2)×Rv≦29.0, preferably 25.0≦(A1−A2)×Rv≦28.7 and more preferably 25.3≦(A1−A2)×Rv≦28.7.

[0025] Here, dimple volume ratio Rv (%) is determined by a mathematical expression: Rv (%)=100×VT/VG, where VT (mm3) is the total volume of dimples, the volume of an individual dimple corresponding to a depression of the dimple terminating at the flat surface which is defined by the straight line between the dimple edges, and VG (mm3) is the volume of the golf ball which is supposed to be a sphere without dimples on the surface.

[0026] As discussed above, the difference (A1−A2) ranging from 34.5 to 36.5 mm provides a proper spin rate and a proper resilience to improve the carry. However, a relatively small difference (A1−A2) within the range 34.5-36.5 mm tends to provide a higher spin rate while a relatively large difference (A1−A2) within the range 34.5-36.5 mm tends to provide a lower spin rate. In order to further improve the carry, the dimples may be designed with respect to the difference (A1−A2), in consideration of a relation between the difference (A1−A2) and dimple properties as described in detail below.

[0027] In general, a higher dimple volume ratio Rv decreases the lift which should be created by dimples, while a lower dimple volume ratio Rv increases the lift which should be created by dimples. Therefore, if any difference (A1−A2) within the range 34.5-36.5 mm is close to 34.5 mm to provide a higher spin rate, dimple volume ratio Rv may be increased to decrease the lift and thus enhance the carry. On the other hand, if any difference (A1−A2) within the range 34.5-36.5 mm is close to 36.5 mm to provide a lower spin rate, dimple volume ratio Rv may be decreased to increase the lift and thus enhance the carry.

[0028] Thus, in order to further improve the carry, the relation between the difference (A1−A2) and dimple volume ratio Rv is optimized by defining the relation 24.5≦(A1−A2)×Rv≦29.0. Dimples with dimple volume ratio Rv adapted to the diameter A1 of the solid center and the thickness A2 of the rubber thread layer may accordingly be formed over the cover to further improve the carry.

[0029] According to the present invention, dimple volume ratio Rv is 0.600-0.950%, preferably 0.620-0.930%, more preferably 0.640-0.910%, and most preferably 0.700-0.810%. A dimple volume ratio Rv lower than 0.600% helps to uplift the trajectory of the golf ball and thus decreases the carry, while a dimple volume ratio Rv higher than 0.950% helps to drop the trajectory of the golf ball and thus decreases the carry as well.

[0030] The number of dimples according to the present invention is 250-550, preferably 300-500 and more preferably 320-480. The number of dimples smaller than 250 increases the diameter of the individual dimples formed on the cover, and accordingly the sphericity of the golf ball is deteriorated. The number of dimples greater than 550 decreases the diameter of the individual dimples formed on the cover, which means that the effect of the dimples causing disturbance in the air flow is not fully exhibited.

[0031] The solid center of the thread-wound golf ball according to the present invention is formed of a rubber composition containing a base rubber, a co-crosslinking agent, an organic peroxide and a filler for example.

[0032] The base rubber is any natural or synthetic rubber which has conventionally been employed for the solid golf ball. The base rubber is preferably polybutadiene of a high resilience, and particularly high cis polybutadiene is preferred. The polybutadiene rubber may be mixed with a natural rubber, polyisoprene rubber, styrene-polybutadiene rubber, or ethylene-propylene-diene rubber (EPDM) for example as desired.

[0033] The co-crosslinking agent is a metal salt of &agr;,&bgr;-unsaturated carboxylic acid, particularly including &agr;,&bgr;-unsaturated carboxylic acids with 3-8 carbons such as acrylic acid and methacrylic acid neutralized with monovalent or divalent metal ions like zinc or magnesium salt. Zinc acrylate is particularly preferable that provides a high resilience. Per 100 parts by weight of the base rubber, 10-40 parts by weight, preferably 15-35 parts by weight of the co-crosslinking agent is blended with the base rubber. If more than 40 parts by weight of the co-crosslinking agent is blended, the resultant golf ball is too hard resulting in poor shot feel, while less than 10 parts by weight of the co-crosslinking agent deteriorates the resilience to decrease the carry.

[0034] The organic peroxide serves as a crosslinking agent or curing agent and is preferably dicumyl peroxide, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, or di-t-butyl peroxide. Per 100 parts by weight of the base rubber, 0.5-2.0 parts by weight, preferably 0.8-1.5 parts by weight of the organic peroxide is blended with the base rubber. If the amount of the blended organic peroxide is less than 0.5 parts by weight, the resultant golf ball is too soft which deteriorates the resilience and decreases the carry, while the amount of the blended organic peroxide exceeding 2.0 parts by weight makes the golf ball too hard which impairs the shot feel.

[0035] The filler may be any which is usually included in the core of the golf ball. For example, the filler is an inorganic salt (e.g. zinc oxide, barium sulfate or calcium carbonate), a metal powder of high specific gravity (e.g. tungsten powder or molybdenum powder), or a mixture thereof. Per 100 parts by weight of the base rubber, 20-70 parts by weight, preferably 25-60 parts by weight of the filler is blended with the base rubber. The amount of the filler less than 20 parts by weight results in a lighter core and accordingly a lighter golf ball, while the amount of the filler exceeding 70 parts by weight results in a heavier core and accordingly a heavier golf ball.

[0036] Moreover, the solid center of the present invention may include any component which may usually be employed for fabricating the core of the solid golf ball, such as antioxidant, peptizing agent or the like. The amount of the blended antioxidant is preferably 0.2-0.5 parts by weight per 100 parts by weight of the base rubber.

[0037] The solid center may be produced by kneading the rubber composition as described above and then vulcanizing and molding the rubber composition in a mold. The solid center of the thread-wound golf ball according to the present invention has a diameter larger than the general one to some extent that is namely 35.0-38.0 mm, preferably 35.5-37.7 mm and more preferably 36.0-37.5 m. The diameter of the solid center smaller than 35.0 mm increases the spin rate at impact to decrease the carry while the diameter of the solid center larger than 38.0 mm means that the rubber thread layer is thin so that the resilience of the rubber thread is not sufficiently exhibited and accordingly the carry decreases, or means that the cover is thin and thus has a poor durability.

[0038] The rubber thread layer is then formed over the solid center produced as described above. The rubber thread wound around the solid center may be any similar to those that have conventionally been employed for the rubber thread layer of the thread-wound golf ball. For example, the rubber thread may be a vulcanized rubber composition containing a natural rubber or a blend of a natural rubber, and a synthetic polyisoprene rubber as a rubber component that is blended with sulfur, vulcanization aid, vulcanization accelerator and antioxidant for example.

[0039] The rubber thread layer of the present invention may be wound around the solid center by any method of fabricating a thread-wound core of the conventional thread-wound golf ball. The rubber thread layer has a thickness of 1.0-2.0 mm, preferably 1.1-1.8 mm and more preferably 1.2-1.6 mm. The thickness of the rubber-thread layer smaller than 1.0 mm means that the resilience of the rubber thread is not sufficiently exhibited to decrease the carry. The thickness of the rubber thread layer larger than 2.0 mm increases the spin rate at impact to decrease the carry.

[0040] The cover is then formed over the rubber-thread layer.

[0041] The cover of the present invention contains a base resin component which may be ionomer resin only, a heat mixture of ionomer resin and epoxy group-modified thermoplastic elastomer, a heat mixture of ionomer resin, maleic anhydride-modified thermoplastic elastomer and glycidyl group-modified thermoplastic elastomer, or a heat mixture of ionomer resin and a terpolymer of ethylene-unsaturated carboxylic acid ester-unsaturated carboxylic acid. Alternatively, the base resin component of the cover may be a heat mixture of ionomer resin, maleic anhydride-modified thermoplastic elastomer or thermoplastic elastomer having terminal OH group, and epoxy group-modified thermoplastic elastomer.

[0042] The ionomer resin is a copolymer of ethylene and (meth)acrylic acid, with at least a portion of carboxyl groups neutralized with metal ion, or a terpolymer of ethylene, (meth)acrylic acid and &agr;,&bgr;-unsaturated carboxylic acid ester, with at least a portion of carboxyl groups neutralized with metal ion. Preferably, 5-80% of carboxyl groups of the ionomer resin is neutralized with metal ion.

[0043] Examples of the ionomer resin which is the copolymer of ethylene and (meth)acrylic acid with at least a portion of carboxyl groups neutralized with metal ion are: “Hi-milan 1555 (Na),” “Hi-milan 1557 (Zn),” “Hi-milan 1601 (Na),” “Hi-milan 1605 (Na)” and “Hi-milan 1706 (Zn)” that are commercially available from Dupont-Mitsui Polychemicals Co., Ltd., and “Surlyn AD8511 (Zn)” and “Surlyn AD8512 (Na)” commercially available from DuPont USA.

[0044] Examples of the ionomer resin which is the terpolymer of ethylene, (meth)acrylic acid and &agr;,&bgr;-unsaturated carboxylic acid ester with at least a portion of carboxyl group neutralized with metal ion are “Hi-milan 1856 (Na),” “Hi-milan 1855 (Zn)” and “Hi-milan AM7316 (Zn)” that are commercially available from Dupont-Mitsui Polychemicals Co., Ltd., and “Surlyn AD8265 (Na)” and “Surlyn AD8269 (Na)” that are terpolymer-based ionomer resins commercially available from DuPont USA. It is noted that elements Na and Zn for example in parentheses following the trade names of the ionomer resins represent any neutralizing metal ion.

[0045] Examples of the epoxy group-modified thermoplastic elastomer which includes epoxy group in an elastomer molecule are styrene-butadiene-styrene (SBS) block copolymers having polybutadiene blocks with epoxy group, commercially available from Daicel Chemical Industries Co., Ltd. under trade names “Epofriend A1010,” “Epofriend A1005” and “Epofriend A1020.”

[0046] Examples of the maleic anhydride-modified thermoplastic elastomer are maleic anhydride adducts of hydrogenated styrene-butadiene-styrene block copolymers commercially available from Asahi Kasei Corporation under trade name “Taftek M series”, ethylene-ethyl acrylate-maleic anhydride terpolymers commercially available from Sumitomo Chemical Co., Ltd. under trade name “Bondine,” and products obtained by graft-modifying ethylene-ethyl acrylate copolymers with maleic anhydride commercially available from Dupont Mitsui Polychemicals Co., Ltd. under trade name “AR series,” that are suitably employed according to the present invention.

[0047] Examples of the glycidyl group-modified thermoplastic elastomer are ethylene-glycidyl methacrylate copolymer, ethylene-glycidyl methacrylate-methyl acrylate terpolymer, ethylene-glycidyl methacrylate-vinyl acetate terpolymer commercially available from Sumitomo Chemical Co., under trade name “Bondfast,” glycidyl methacrylate adducts of hydrogenated styrene-butadiene-styrene (SBS) block copolymers commercially available from Asahi Kasei Corporation under trade names “Taftek Z514” and “Taftek Z513” for example, and ethylene-acrylic ester-glycidyl methacrylate terpolymer commercially available from DuPont USA under trade name “Elvaloy-AS.” Although the glycidyl group is broadly included in the epoxy group, the term “glycidyl group” is herein used instead of the term “epoxy group” for specifying the glycidyl group.

[0048] Examples of the terpolymer of ethylene-unsaturated carboxylic acid ester-unsaturated carboxylic acid are ethylene-isobutyl acrylate-methacrylic acid terpolymers commercially available from Dupont-Mitsui Polychemicals Co., Ltd. under trade names “Nucrel AN4212C” and “Nucrel N0805J.”

[0049] The thermoplastic elastomer with terminal OH group has OH group in an elastomer molecule, and an example thereof is hydrogenated styrene-isoprene-styrene (SIS) block copolymer commercially available from Kuraray Co., Ltd.

[0050] For the cover composition including ionomer resin and other components to achieve a desired hardness, the ratio between the ionomer resin and another resin or elastomer (epoxy group-modified thermoplastic elastomer, maleic anhydride-modified thermoplastic elastomer or terpolymer of ethylene-unsaturated carboxylic acid ester-unsaturated carboxylic acid) is preferably 95:5-20:80.

[0051] To the cover of the present invention containing the base resin component as described above, such additives as pigment, dispersant, antioxidant, UV absorber and photostabilizer may be added as required.

[0052] If the ionomer resin and another resin or elastomer are employed for the cover composition, both are required to undergo heat mixing usually for 1-20 minutes. In actual, an extruder is used for the heat mixing with the temperature adjusted.

[0053] According to the present invention, the cover composition may optionally contain, in addition to the above-described base resin as a main component, a filler such as barium sulfate, a pigment such as titanium dioxide and other additives such as dispersant, antioxidant, UV absorber, photostabilizer and a fluorescent agent or a fluorescent brightener, as long as the additives do not deteriorate desired characteristics achieved by the cover of the golf ball. The amount of the pigment is preferably 1.0-6.0 parts by weight.

[0054] The cover layer of the present invention may be formed by any known method employed generally for producing the cover of the golf ball and the method is thus not limited to a specific one. The cover composition may be molded into a hemispherical half-shell, and two half-shells are used to enclose the thread-wound core to be pressure-molded at 130-170° C. for 1-5 minutes. Alternatively, the cover composition may directly be injection-molded around the thread-wound core to enclose the solid core.

[0055] The cover of the present invention has a thickness of 1.0-2.0 mm, preferably 1.1-1.9 mm and more preferably 1.2-1.6 mm. The thickness of the cover that is smaller than 1.0 mm results in deterioration in the durability of the cover, while the cover thickness exceeding 1.9 mm increases the spin rate at impact to decrease the carry.

[0056] The geometrical arrangement of dimples may be any arbitrary one such as octahedral and icosahedral arrangements, and any dimple pattern such as square, hexagonal and triangular patterns may be used.

[0057] From aesthetic and commercial considerations, the thread-wound golf ball of the present invention is usually finished with paint and marked with a stamp for example to be brought to the market.

[0058] The present invention thus provides a golf ball with an improved carry without impairing the excellent shot feel specific to the conventional thread-wound golf ball.

EXAMPLES

[0059] The present invention is now described in more detail in connection with examples, while the present invention is not limited to such specific examples.

[0060] (1) Production of Solid Center

[0061] Any of rubber compositions for the center shown in Table 1 and Table 2 was kneaded and hot-pressed in a mold at 165° C. for 23 minutes to produce a spherical solid center. Respective amounts of the components in Tables 1 and 2 are expressed in parts by weight. The tables also show the diameter, weight, center hardness, surface hardness, hardness difference and amount of deformation under a load of 9.8 N-294 N of solid centers thus produced. The center hardness of the solid center was measured, by a C-type spring hardness tester defined by JIS-K6301, at a cross section of the solid center where the center was cut into equal two hemispheres. The surface hardness of the solid center was also measured, by a C-type spring hardness tester defined by JIS-K6301, at a cross section of the solid center where the surface of the solid center was divided into equal two halves. The amount of deformation of the solid center under a load from an initial load of 9.8 N to a final load of 294 N was measured. 1 TABLE 1 center composition A B C D BR18 (*1) 100 100 100 100 zinc acrylate 28 28 28 35 dicumyl peroxide 0.9 0.9 0.9 0.9 zinc oxide 10.2 14.7 8.4 8.2 barium sulfate 20.8 20.8 20.8 20.8 diphenyl diSulfide 0.5 0.5 0.5 0.5 vulcanization temperature (° C.) 165 165 165 165 conditions time (min) 23 23 23 23 diameter of center (mm) 37.0 35.8 38.0 37.0 weight of center (g) 32.4 29.9 34.4 32.4 center hardness 60 61 59 62 (JIS-C hardness) Hc surface hardness 80 80 81 82 (JIS-C hardness) Hs hardness difference (Hs − Hc) 20 19 22 20 deformation under load (mm) 1.34 1.32 1.35 1.02 (*1) high cis polybutadiene rubber manufactured by JSR Corporation

[0062] 2 TABLE 2 center composition E F G BR18 (*1) 100 100 100 zinc acrylate 38 28 28 dicumyl peroxide 0.9 0.9 0.9 zinc oxide 8.0 12.0 9.4 barium sulfate 20.8 20.8 20.8 diphenyl disulfide 0.5 0.5 0.5 vulcanization temperature (° C.) 165 165 165 conditions time (min) 23 23 23 diameter of center (mm) 37.0 36.4 37.4 weight of center (g) 32.4 31.5 33.5 center hardness (JIS-C hardness) Hc 62 60 60 surface hardness (JIS-C hardness) 83 80 81 Hs hardness difference (Hs − Hc) 21 20 21 deformation under load (mm) 0.84 1.33 1.34 (*1) high cis polybutadiene rubber manufactured by JSR Corporation

[0063] (2) Formation of Thread-Wound Layer

[0064] Around the solid center as described above, rubber thread containing a base rubber which is a blend of natural rubber and a low cis isoprene rubber (“Shell IR-309” commercially available from Shell Chemicals Ltd.) with a weight ratio of 50/50 was wound to produce a thread-wound core.

[0065] (3) Preparation of Cover Composition

[0066] Materials in the cover composition shown in Table 3 were mixed by a kneading-type twin screw extruder to produce a pelletized cover composition. Respective amounts of components in Table 3 are expressed in parts by weight. The extrusion was performed under the conditions of:

[0067] a screw diameter of 45 mm;

[0068] a screw rate of 200 rpm; and

[0069] a screw L/D of 35.

[0070] The composition was heated to 200-260° C. at the position of the die of the extruder.

[0071] The resultant cover composition had a Shore D hardness as shown in Table 3. The Shore D hardness was measured according to ASTMD-2240. Specifically, a thermal press molded sheet of approximately 2 mm in thickness formed of the cover composition was preserved at 23° C. for two weeks and then, a D-type spring hardness tester was used to measure the hardness of at least three such sheets laid on each other. 3 TABLE 3 cover composition a b c d Surlyn 8945 (*2) 30 35 25 50 Surlyn 9945 (*3) 30 35 25 50 Epofriend A1010 (*4) 10 7 15 — Cepton HG-252 (*5) 30 23 35 — titanium oxide 4 4 4 4 hardness of cover 51 54 46 63 (Shore D hardness) (*2) sodium ion-neutralized ethylene-methacrylic acid copolymer ionomer resin, manufactured by Dupont Co. (*3) zinc ion-neutralized ethylene-methacrylic acid copolymer ionomer resin, manufactured by Dupont Co. (*4) styrene-butadiene-styrene block copolymer having polybutadiene block with epoxy group, manufactured by Daicel Chemical Industries Co., Ltd. (*5) hydrogenated styrene-isoprene/butadiene-styrene block copolymer with terminal OH groups, manufactured by Kuraray Co., Ltd.

[0072] (4) Production of Golf Galls for Examples and Comparative Examples

[0073] The cover composition was molded into two hemispherical half-shells with which the thread-wound core was covered, the resultant product was press-molded through thermal compression in a mold, and the surface thereof was painted to produce a golf ball with a diameter of 42.8 mm for Examples and Comparative Examples. The golf gall had 410 dimples composed of 50 dimples A of 4.3 mm in diameter, 210 dimples B of 3.9 mm in diameter, 110 dimples C of 3.6 mm in diameter, and 40 dimples D of 3.3 mm in diameter. The wall shape in the cross section of each dimple was single curvature. The dimples had depths varied to have dimple volume ratio Rv as shown in the tables.

[0074] Flight performance of golf balls thus produced was evaluated in terms of launch angle, spin rate, elevation angle of trajectory, elevation angle of trajectory-launch angle, carry, and shot feel at impact as shown in Table 4 (Examples 1-4), Table 5 (Examples 5-8), Table 6 (Comparative Examples 1-4) and Table 7 (Comparative Examples 5-7). Here, “elevation angle of trajectory” refers to the angle formed between the ground surface and a straight line connecting the point of shot and the peak of the trajectory seen from the point of shot. “Elevation angle of trajectory-launch angle” is determined by subtracting the launch angle from the elevation angle of trajectory. If this angle is small, the trajectory is a dropping trajectory. If this angle is large, the trajectory is an uplifting trajectory.

[0075] (5) Test Method

[0076] (i) Flight Performance (Launch Angle, Backspin Rate, Carry)

[0077] A No. 1 wood club (driver) with a metal head was mounted to a swing robot manufactured by True Temper Co., Ltd. to hit a golf ball at a head speed of 50 m/sec. Then, the launch angle immediately after the hit, the backspin rate immediately after the hit, and the carry ending at the point where the ball lands were measured.

[0078] (ii) Shot Feel at Impact

[0079] The intensity of shot was evaluated by 10 (ten) professional golfers using a No. 1 wood club with a metal head to actually hit a ball. The intensity of shot at impact was evaluated in accordance with the following criteria to select one of the ratings. The evaluation of a certain golf ball was determined as any of the ratings selected by the majority of the golfers.

[0080] Evaluation Criteria

[0081] ◯: soft and good shot feel

[0082] &Dgr;: normal

[0083] X: hard and poor shot feel 4 TABLE 4 Example 1 2 3 4 center composition A B C D diameter of center A1 (mm) 37.0 37.0 37.0 37.0 deformation of center (mm) 1.34 1.34 1.34 1.02 thickness of rubber thread layer 1.3 1.3 1.3 1.3 A2 (mm) A1 − A2 (mm) 35.7 35.7 35.7 35.7 cover composition a b c a thickness of cover (mm) 1.6 1.6 1.6 1.6 Shore D hardness of cover 51 54 46 51 dimple volume ratio Rv (%) 0.758 0.758 0.758 0.758 (A1 − A2) × Rv 27.1 27.1 27.1 27.1 weight of ball (g) 45.4 45.4 45.4 45.4 diameter of ball (mm) 42.8 42.8 42.8 42.8 flight performance launch angle (degree) 9.9 10.0 9.7 9.6 spin rate (rpm) 2300 2200 2500 2500 elevation angle of trajectory 11.3 11.2 11.3 11.1 (degree) elevation angle of trajectory- 1.4 1.2 1.6 1.5 launch angle (degree) carry (m) 234 235 230 231 shot feel at impact ◯ ◯ ◯ ◯

[0084] 5 TABLE 5 Example 5 6 7 8 center composition A A F G diameter of center A1 (mm) 37.0 37.0 36.4 37.4 Deformation of center (mm) 1.34 1.34 1.33 1.34 thickness of rubber thread layer 1.3 1.3 1.6 1.1 A2 (mm) A1 − A2 (mm) 35.7 35.7 34.8 36.3 cover composition a a a a thickness of cover (mm) 1.6 1.6 1.6 1.6 Shore D hardness of cover 51 51 51 51 dimple volume ratio Rv (%) 0.709 0.805 0.758 0.725 (A1 − A2) × Reference voltage 25.3 28.7 26.4 26.3 weight of ball (g) 45.4 45.4 45.4 45.4 diameter of ball (mm) 42.8 42.8 42.8 42.8 flight performance launch angle (degree) 9.9 9.9 9.8 10.0 spin rate (rpm) 2300 2300 2500 2100 elevation angle of trajectory 11.5 11.0 11.2 11.4 (degree) elevation angle of trajectory- 1.6 1.1 1.4 1.4 launch angle (degree) carry (m) 231 231 230 230 shot feel at impact ◯ ◯ ◯ ◯

[0085] 6 TABLE 6 Comparative Example 1 2 3 4 center composition B C E A diameter of center A1 (mm) 35.8 38.0 37.0 37.0 deformation of center (mm) 1.32 1.35 0.84 1.34 thickness of rubber thread layer 1.9 0.8 1.3 1.3 A2 (mm) A1 − A2 (mm) 33.9 37.2 35.7 35.7 cover composition a a a d thickness of cover (mm) 1.6 1.6 1.6 1.6 Shore D hardness of cover 51 51 51 63 dimple volume ratio Rv (%) 0.758 0.758 0.758 0.758 (A1 − A2) × Reference voltage 25.7 28.2 27.1 27.1 weight of ball (g) 45.4 45.4 45.4 45.4 diameter of ball (mm) 42.8 42.8 42.8 42.8 flight performance launch angle (degree) 9.7 10.1 9.4 10.1 spin rate (rpm) 2700 2200 2600 2100 elevation angle of trajectory 11.3 11.4 11.0 11.2 (degree) elevation angle of trajectory- 1.6 1.3 1.5 1.1 launch angle (degree) carry (m) 224 225 225 236 shot feel at impact ◯ ◯ &Dgr; X

[0086] 7 TABLE 7 Comparative Example 5 6 7 center composition A A A diameter of center A1 (mm) 37.0 37.0 37.0 deformation of center (mm) 1.34 1.34 1.34 thickness of rubber thread layer 1.3 1.3 1.3 A2 (mm) A1 − A2 (mm) 35.7 35.7 35.7 cover composition a a d thickness of cover (mm) 1.6 1.6 1.6 Shore D hardness of cover 51 51 63 dimple volume ratio Rv (%) 0.660 0.856 0.709 (A1 − A2) × Rv 23.6 30.6 25.3 weight of ball (g) 45.4 45.4 45.4 diameter of ball (mm) 42.8 42.8 42.8 flight performance launch angle (degree) 9.9 9.9 10.1 spin rate (rpm) 2300 2300 2100 elevation angle of trajectory 11.8 10.7 11.5 (degree) elevation angle of trajectory- 1.9 0.8 1.4 launch angle (degree) carry (m) 225 227 234 shot feel at impact ◯ ◯ X

[0087] (6) Results of the Test

[0088] According to Comparative Examples 1 and 2, the amount of deformation of the solid center is in the range from 1.0 to 1.5 mm, the cover has a Shore D hardness in the range from 44 to 58 and the relation 24.5≦(A1−A2)×Rv≦29.0 is satisfied. However, with respect to the difference (A1−A2) which should be in the range from 34.5 to 36.5 mm, the difference (A1−A2) of Comparative Example 1 is 33.9 mm which results in a higher spin rate, while the difference (A1−A2) of Comparative Example 2 is 37.2 mm which results in a lower resilience of the rubber thread. Consequently, Comparative Examples 1 and 2 provide the shorter carry compared with Examples.

[0089] According to Comparative Example 3, the difference (A1−A2) is in the range from 34.5 to 36.5 mm, the cover has a Shore D hardness in the range from 44 to 58 and the relation 24.5≦(A1−A2)×Rv≦29.0 is satisfied. However, the amount of deformation of the solid center is 0.84 mm which is out of the range from 1.0 to 1.5 mm and accordingly, the launch angle is smaller and the spin rate is higher, resulting in the shorter carry compared with Examples.

[0090] According to Comparative Example 4, the carry is substantially equivalent to that of Examples. However, the cover has a Shore D hardness of 63 which means the golf ball is too hard, resulting in an inferior shot feel at impact to that of Examples.

[0091] According to Comparative Examples 5 and 6, the amount of deformation of the solid center is in the range from 1.0 to 1.5 mm, the difference (A1−A2) is in the range from 34.5 to 36.5 mm and the cover has a Shore D hardness in the range from 44 to 58. However, (A1−A2)×Rv of Comparative Example 5 is 23.6 and that of Comparative Example 6 is 30.6 and thus the relation 24.5≦(A1−A2)×Rv≦29.0 is not satisfied. Consequently, the trajectory of the golf ball of Comparative Example 5 tends to uplift while that of Comparative Example 6 tends to drop, resulting in a shorter carry than that of Examples.

[0092] According to Comparative Example 7, the carry is substantially equivalent to that of Examples. However, the cover has a Shore D hardness of 63 which means that the golf ball is too hard. Then, the shot feel at impact is inferior to that of Examples.

[0093] On the other hand, Examples of the present invention meet all of the requirements: the amount of deformation of the solid center is in the range from 1.0 to 1.5 mm; the difference (A1−A2) is in the range from 34.5 to 36.5 mm; the Shore D hardness of the cover is in the range from 44 to 58; and the relation 24.5≦(A1−A2)×Rv≦29.0 is satisfied. Examples of the present invention are thus generally superior to Comparative Examples in that a longer carry is achieved without sacrificing excellent shot feel.

[0094] According to the present invention, the golf ball has the amount of deformation of the solid center under a load, the diameter of the solid center, the thickness of the rubber thread layer and the hardness of the cover that are defined within respective specific ranges. Accordingly, the initial flight conditions are optimized without sacrificing excellent shot feel specific to the thread-wound golf ball, and an optimum dimple volume ratio is defined in relation to the initial flight conditions to optimize the trajectory. A further increase in the carry is thus accomplished.

[0095] Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.

Claims

1. A thread-wound golf ball comprising a solid center, a rubber thread layer wound around said solid center and a cover enveloping said rubber thread layer, said cover having dimples formed thereon, wherein

an amount of deformation of said solid center under a load of 9.8 N-294N is 1.0-1.5 mm,

a difference between the diameter A1 of said solid center and the thickness A2 of said rubber thread layer represented by (A1−A2) is 34.5-36.5 mm,

said cover has a Shore D hardness of 44-58, and

dimple volume ratio Rv and the difference between the diameter of said solid center and the thickness of said rubber thread layer represented by (A1−A2) satisfy a relation:

24.5≦(A1−A2)×Rv≦29.0.

2. The thread-wound golf ball according to claim 1, wherein

said solid center has a surface hardness greater than its center hardness that are measured by a JIS-C hardness tester, and a difference between the surface hardness and the center hardness is 15-35.

3. The thread-wound golf ball according to claim 1, wherein

said solid center has a surface hardness of 70-90 measured by a JIS-C hardness tester.

4. The thread-wound golf ball according to claim 1, wherein

dimple volume ratio Rv is 0.600-0.950%.