MULTI-PIECE SOLID GOLF BALL

Abstract

The present invention provides a multi-piece solid golf ball including a core, a surrounding layer, an intermediate layer, and a cover, in which a large number of dimples are formed on an outside surface of the cover, the core is formed of a single layer or a plurality of layers of a rubber composition, the surrounding layer is formed of a single layer or a plurality of layers of a resin composition, the intermediate layer and the cover are both formed of a single layer of a resin composition, and a relationship between an initial velocity of an intermediate layer-encased sphere and an initial velocity of the ball satisfies the following condition: (initial velocity of ball)<(initial velocity of intermediate layer-encased sphere). Further, a deflection of the ball under a predetermined load is optimized, and a lift coefficient and a drag coefficient at a predetermined Reynolds number and spin rate of the dimples are set within a predetermined range.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This non-provisional application claims priority under 35 U.S.C. § 119 (a) on Patent Application No. 2023-171962 filed in Japan on Oct. 3, 2023, the entire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to a multi-piece solid golf ball including a core, a surrounding layer, an intermediate layer, and a cover, wherein a large number of dimples are formed on an outside surface of the cover.

BACKGROUND ART

In March 2022, manufacturers of golf balls were notified by the Royal and Ancient Golf Club of St Andrews (hereinafter, R&A) and the United States Golf Association (hereinafter, USGA) that they would start research to suppress distance by long hitters by changing test conditions for the Overall Distance Standard (hereinafter, ODS) of golf balls in the future. For this reason, it is preferable to provide a golf ball that does not simply reduce distance, but while making a distance for reducing a distance on shots with a driver by long hitters longer, by making a distance for reducing a distance on shots with an iron shorter, reduces an influence on play other than reducing the distance on shots with a driver by long hitters. In addition, due to the above changes, it is desirable to set spin characteristics in the short game to a high level so that a sense of discomfort does not occur even if professionals or advanced players use the golf ball with the reduced distance.

In the past, a golf ball has been proposed in the following Patent Documents 1 to 4 in which a design of a low-trajectory dimple is combined with the ball surface to greatly reduce the distance of the ball at a high head speed, and at a low head speed, to suppress the decrease in the distance as much as possible in spite of the reduction at the high head speed. In addition, Patent Documents 5 to 8 propose a golf ball in which a ball layer structure is a four-layer structure of a core, a surrounding layer, an intermediate layer, and a cover, a relationship between an initial velocity of the ball and an intermediate layer-encased sphere is optimized, and a core hardness gradient and a deflection are specified, as a golf ball having flight that can satisfy professionals or advanced players.

However, the golf ball of Patent Documents 1 to 4 has a distance that is excessively reduced compared to shots with a driver at a high head speed. In addition, the golf ball of Patent Documents 5 to 8 is intended to increase the distance on shots with a driver (W #1), and is not intended to suppress the distance of long hitters.

CITATION LIST

• Patent Document 1: JP-A 2011-218160
• Patent Document 2: JP-A 2011-218161
• Patent Document 3: JP-A 2011-218162
• Patent Document 4: JP-A 2011-240125
• Patent Document 5: JP-A 2007-319667
• Patent Document 6: JP-A 2008-068077
• Patent Document 7: JP-A 2009-095364
• Patent Document 8: JP-A 2012-071163

SUMMARY OF THE INVENTION

The present invention has been made in view of the above circumstances, and in order to address the possibility of there being a change to the rules in the future to suppress a distance by long hitters by changing test conditions for the ODS of golf balls, an object of the present invention is to provide a golf ball that does not simply reduce distance, and reduces a distance for reducing a distance on shots with a driver by long hitters, but does not suppress so much a distance for reducing a distance on shots with a driver by average hitters.

As a result of intensive studies to achieve the above object, the present inventor has found that a ball structure has a core, a surrounding layer, an intermediate layer, and a cover (outermost layer), and has a spin-type multilayer encasing layer structure in which an initial velocity of an intermediate layer-encased sphere is faster than an initial velocity of the ball, a deflection of an entire ball when a predetermined load is applied is set to be relatively small, and dimples on a ball surface are formed by a special dimple design, so that a distance on shots with a driver (W #1) by long hitters is suppressed, but a distance on shots with a driver (W #1) by average hitters is not suppressed so much, and has completed the present invention. In addition, the golf ball of the present invention has high controllability in the short game, and further has excellent durability to cracking on repeated impact.

The above “long hitters” mean users whose head speed on shots with a driver (W #1) is at least about 50 m/s, and the above “average hitters” mean users whose head speed on shots with a driver (W #1) is not more than about 45 m/s.

Accordingly, the present invention provides a multi-piece solid golf ball including a core, a surrounding layer, an intermediate layer, and a cover, wherein a large number of dimples are formed on an outside surface of the cover, the core is formed of a single layer or a plurality of layers of a rubber composition, the surrounding layer is formed of a single layer or a plurality of layers of a resin composition, the intermediate layer and the cover are both formed of a single layer of a resin composition, and a relationship between an initial velocity of an intermediate layer-encased sphere and an initial velocity of the ball satisfies the following condition:

(initial velocity of ball)<(initial velocity of intermediate layer-encased sphere), and

• • where a deflection when the ball is compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) is not more than 3.0 mm, a ratio CL1/CD1 of a lift coefficient CL1 at a Reynolds number of 218,000 and a spin rate of 2,800 rpm to a drag coefficient CD1 is denoted by A1, a ratio CL2/CD2 of a lift coefficient CL2 at a Reynolds number of 184,000 and a spin rate of 2,900 rpm to a drag coefficient CD2 is denoted by A2, and a ratio CL3/CD3 of a lift coefficient CL3 at a Reynolds number of 158,000 and a spin rate of 3,100 rpm to a drag coefficient CD3 is denoted by A3, the following two conditions are satisfied:

0.590≤A1≤0.640, and

$\left(A2+A3\right)/2\ge 0.67.$

In a preferred embodiment of the multi-piece solid golf ball according to the invention, the value of A1 is from 0.590 to 0.613, the value of A2 is from 0.635 to 0.668, and the value of A3 is from 0.695 to 0.734.

In another preferred embodiment, the value of A1 is from 0.614 to 0.640, the value of A2 is from 0.669 to 0.750, and the value of A3 is from 0.735 to 0.815.

In yet another preferred embodiment, the value of (A2+A3)/2 is from 0.670 to 0.783.

In still another preferred embodiment, the following three conditions are satisfied:

(initial velocity of intermediate layer-encased sphere)−(initial velocity of surrounding layer-encased sphere)≤0.70 (m/s)

(initial velocity of surrounding layer-encased sphere)−(initial velocity of core)≥0.20 (m/s)

1.00≤(initial velocity of intermediate layer-encased sphere)−(initial velocity of core)≤1.60 (m/s).

In a further preferred embodiment, the following condition is satisfied:

0.65≤(initial velocity of surrounding layer-encased sphere)−(initial velocity of core)≤1.00 (m/s).

In a yet further preferred embodiment, the following condition is satisfied:

• • surface hardness of ball<surface hardness of intermediate layer-encased sphere>surface hardness of surrounding layer-encased sphere>surface hardness of core
  • where the surface hardness of each sphere means Shore C hardness.

In a still further preferred embodiment, the intermediate layer contains an inorganic particulate filler, and the resin material of the intermediate layer has a specific gravity of at least 1.05.

In another preferred embodiment, a difference between a specific gravity of the cover and a specific gravity of the intermediate layer is not more than 0.15.

In yet another preferred embodiment, the resin composition of the intermediate layer contains a high-acid ionomer resin having an acid content of at least 16 wt %.

In still another preferred embodiment, the following condition is satisfied:

cover thickness<intermediate layer thickness≤surrounding layer thickness.

In a further preferred embodiment, the core has a hardness profile in which, letting a Shore C hardness at a core center be Cc, a Shore C hardness at a midpoint M between the core center and the core surface be Cm, Shore C hardnesses at positions 2 mm, 4 mm, and 6 mm inward from the midpoint M be Cm−2, Cm−4, and Cm−6 respectively, Shore C hardnesses at positions 2 mm, 4 mm, and 6 mm outward from the midpoint M be Cm+2, Cm+4, and Cm+6 respectively, and a Shore C hardness at the core surface be Cs, and defining surface areas A to F as follows:

• • surface area A: ½×2×(Cm−4−Cm−6)
  • surface area B: ½×2×(Cm−2−Cm−4)
  • surface area C: ½×2×(Cm−Cm−2)
  • surface area D: ½×2×(Cm+2−Cm)
  • surface area E: ½×2×(Cm+4−Cm+2)
  • surface area F: ½×2×(Cm+6−Cm+4)
  • the following condition is satisfied:

(surface area E+surface area F)−(surface area A+surface area B)≥1.0.

In a yet further preferred embodiment, the following condition is satisfied:

35≤(surface hardness of intermediate layer-encased sphere)−(core center hardness) where the hardnesses mean Shore C hardnesses.

Advantageous Effects of the Invention

To address the possibility of there being a change to the rules in the future by the R&A and the USGA to suppress the distance by long hitters by changing test conditions for the ODS of golf balls, with the golf ball of the present invention, instead of simply reducing distance, a distance on shots with a driver (W #1) by long hitters is suppressed, but a distance on shots with a driver (W #1) by average hitters is not suppressed so much, which may reduce an influence on play other than reducing the distance on shots with a driver by long hitters. In addition, when used by professionals or advanced players, while the golf ball of the present invention has a shorter distance on shots with a driver (W #1) than a conventional tour ball, the golf ball of the present invention has a high playability in the short game by making a level of spin rate in the short game equivalent. Furthermore, the golf ball of the present invention also has excellent durability to cracking on repeated impact.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a golf ball according to one embodiment of the present invention.

FIG. 2 is a graph that uses core hardness profile data in Example 1 to describe surface areas A to F in a core hardness profile.

FIG. 3 is a graph showing core hardness profiles in Examples 1 to 4 and Comparative Examples 1 to 5.

FIG. 4 is a graph showing core hardness profiles in Comparative Examples 5 to 9.

FIGS. 5A and 5B show an arrangement mode (pattern) of dimples (1) to (4) used in Examples 1 to 4 and Comparative Examples 1 to 9, where FIG. 5A shows a plan view of the dimples, and FIG. 5B shows a side view thereof.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, the present invention is described in more detail.

As shown in FIG. 1, the multi-piece solid golf ball of the present invention is a golf ball G having at least four multilayers including a core 1, a surrounding layer 2 encasing the core, an intermediate layer 3 encasing the surrounding layer, and a cover 4 encasing the intermediate layer. A large number of dimples D are formed on a surface of the cover 4 in order to obtain aerodynamic properties as intended properties of the present invention. In addition, although not particularly illustrated, a coating film layer is typically formed on the surface of the cover 4 by coating. The cover 4 is positioned at an outermost layer in a layer construction of the golf ball except for the coating film layer. The core 1 and the surrounding layer 2 are not limited to a single layer, and may be formed in a plurality of layers of at least two layers, but the intermediate layer 3 or the cover 4 is formed in a single layer.

The core is obtained by vulcanizing a rubber composition containing a rubber material as a chief material. If the core material is not the rubber composition, a rebound of the core may become low, and a desired distance may not be attainable on shots with a driver (W #1) by average hitters and on shots with an iron. The rubber composition typically contains a base rubber as the chief material, and is obtained with the inclusion of a co-crosslinking agent, a co-crosslinking initiator, an inert filler, an antioxidant, an organosulfur compound, or the like.

The base rubber may include a diene rubber. Examples of the diene rubber include polybutadiene, natural rubber, isoprene rubber, and ethylene propylene diene rubber. These base rubbers may be used singly or in a combination of at least two kinds thereof.

The co-crosslinking agent is an α,β-unsaturated carboxylic acid and/or a metal salt thereof. Specific examples of the unsaturated carboxylic acid include acrylic acid, methacrylic acid, maleic acid, fumaric acid, or the like, and in particular, acrylic acid and methacrylic acid are suitably used. The metal salt of the unsaturated carboxylic acid is not particularly limited, and examples thereof include those obtained by neutralizing the unsaturated carboxylic acid with a desired metal ion. Specific examples thereof include zinc salts and magnesium salts such as methacrylic acid and acrylic acid, and in particular, zinc acrylate is suitably used.

The unsaturated carboxylic acid and/or the metal salt thereof is typically included in an amount of at least 5 parts by weight, preferably at least 15 parts by weight, and even more preferably at least 25 parts by weight, and an upper limit thereof is typically not more than 60 parts by weight, preferably not more than 50 parts by weight, and even more preferably not more than 40 parts by weight per 100 parts by weight of the base rubber. If the compounding amount is too large, the core may become too hard, giving the ball an unpleasant feel at impact, and if the compounding amount is too small, rebound may become low.

The co-crosslinking initiator is typically an organic peroxide. As the organic peroxide, an organic peroxide having a relatively high thermal decomposition temperature is suitably used. Specifically, a high-temperature organic peroxide having a one-minute half-life temperature of about 165 to 185° C. is used, and examples thereof include dialkyl peroxides. Examples of the dialkyl peroxides include a dicumyl peroxide (“Percumyl D” manufactured by NOF Corporation), a 2,5-dimethyl-2,5-di (t-butylperoxy) hexane (“Perhexa 25B” manufactured by NOF Corporation), and a di (2-t-butylperoxyisopropyl) benzene (“Perbutyl P” manufactured by NOF Corporation), and a dicumyl peroxide may be suitably used. These may be used singly, or two or more may be used in combination. The half-life is one of the indices representing a degree of a decomposition rate of the organic peroxide, and is indicated by a time required for the original organic peroxide to be decomposed and its active oxygen amount to reach ½. A vulcanization temperature in the core-forming rubber composition is typically within a range of from 120 to 190° C., and in that range, an organic peroxide having a one-minute half-life temperature of a high temperature, which is about 165° C. to 185° C., is thermally decomposed relatively slowly. With the rubber composition used in the present invention, by adjusting an amount of free radicals produced, which increases with the lapse of a vulcanization time, it is possible to obtain a core that is a rubber cross-linked product having a specific internal hardness shape described later.

As the inert filler, for example, zinc oxide, barium sulfate, calcium carbonate, or the like may be suitably used. These may be used singly, or two or more may be used in combination. A compounding amount of the filler may be preferably at least 4 parts by weight, more preferably at least 8 parts by weight, and even more preferably at least 12 parts by weight per 100 parts by weight of the base rubber. In addition, an upper limit of the compounding amount is preferably not more than 50 parts by weight, more preferably not more than 40 parts by weight, and even more preferably not more than 30 parts by weight per 100 parts by weight of the base rubber. If the compounding amount is too large or too small, it may not be possible to obtain an appropriate weight and a suitable rebound.

As an antioxidant, for example, commercially available products such as Nocrac NS-6, Nocrac NS-30, Nocrac NS-200, and Nocrac MB (all manufactured by Ouchi Shinko Chemical Industrial Co., Ltd.) may be employed. These may be used singly, or two or more may be used in combination.

The compounding amount of the antioxidant is not particularly limited, but is preferably 0.05 parts by weight or more, and more preferably 0.1 parts by weight or more, and the upper limit is preferably 1.0 part by weight or less, more preferably 0.7 parts by weight or less, and even more preferably 0.5 parts by weight or less per 100 parts by weight of the base rubber. If the compounding amount is too large or too small, a suitable core hardness gradient cannot be obtained, and it may not be possible to obtain suitable rebound, durability, and a spin rate-lowering effect on full shots.

The organosulfur compound may be included in order to control the rebound of the core so that it is increased. As the organosulfur compound, specifically, it is recommended to include thiophenol, thionaphthol, halogenated thiophenol, or a metal salt thereof. More specifically, examples of the organosulfur compound include zinc salts such as pentachlorothiophenol, pentafluorothiophenol, pentabromothiophenol, p-chlorothiophenol, and pentachlorothiophenol, and any of the following having 2 to 4 sulfur atoms: diphenylpolysulfide, dibenzylpolysulfide, dibenzoylpolysulfide, dibenzothiazoylpolysulfide, and dithiobenzoylpolysulfide. In particular, diphenyldisulfide and the zinc salt of pentachlorothiophenol are suitably used.

An upper limit of a compounding amount of the organosulfur compound is preferably not more than 5 parts by weight, more preferably not more than 4 parts by weight, even more preferably not more than 3 parts by weight, and most preferably not more than 2 parts by weight per 100 parts by weight of the base rubber. If the compounding amount is too large, the core hardness becomes too soft or the rebound of the core becomes too high, and the distance on shots with a driver by long hitters may be too long. On the other hand, a lower limit of the compounding amount is preferably at least 0.1 parts by weight, more preferably at least 0.2 parts by weight, and even more preferably at least 0.3 parts by weight per 100 parts by weight of the base rubber. If the compounding amount is too small, the rebound of the core may be too low, and the distance on shots with a driver by average hitters and with an iron by both long hitters and average hitters may be shortened too much.

A sulfur may also be included in the rubber composition. Specific examples of the sulfur include trade names “SANMIX S-80N” (manufactured by Sanshin Chemical Industry Co., Ltd.) and “SULFAX-5” (manufactured by Tsurumi Chemical Industry Co., Ltd.). The addition of the sulfur may increase a difference in hardness of the core. If the compounding amount of the sulfur is too large, rebound may be greatly reduced, or a durability on repeated impact may worsen.

The core can be manufactured by vulcanizing and curing the rubber composition containing the above components. For example, a molded body can be manufactured by intensively mixing the rubber composition using a mixing apparatus such as a Banbury mixer or a roll mill, subsequently compression molding or injection molding the mixture using a core mold, and curing the resulting molded body by appropriately heating it at a temperature sufficient for the organic peroxide or the co-crosslinking agent to act, such as at a temperature of from 100 to 200° C., and preferably at a temperature of from 140 to 180° C., for 10 to 40 minutes.

In the present invention, the core is formed as a single layer or a plurality of layers, although it is preferably formed as a single layer. If a rubber core is produced as a plurality of layers of rubber, layer separation at an interface may arise when the ball is repeatedly struck, possibly leading to cracking at an earlier stage.

A core diameter is preferably at least 35.1 mm, more preferably at least 35.3 mm, and even more preferably at least 35.4 mm. An upper limit of the core diameter is preferably not more than 41.3 mm, more preferably not more than 39.2 mm, and even more preferably not more than 38.3 mm. If the core diameter is too small, an initial velocity of the ball may become too low, or a deflection of the entire ball may become small, so that a spin rate of the ball on full shots may increase, and a desired distance may not be attainable on shots with a driver (W #1) by average hitters. On the other hand, if the diameter of the core is too large, the spin rate on full shots may rise, and the desired distance of average hitters may not be attainable, or a durability to cracking on repeated impact may worsen.

The deflection (mm) when the core is compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) is not particularly limited, although the deflection is preferably at least 2.9 mm, more preferably at least 3.2 mm, and even more preferably at least 3.5 mm, and an upper limit thereof is preferably not more than 5.0 mm, more preferably not more than 4.4 mm, and even more preferably not more than 4.2 mm. If the deflection of the core is too small, that is, the core is too hard, a desired distance may not be attainable on shots with a driver (W #1) by average hitters, or the feel at impact may be too hard. On the other hand, if the deflection of the core is too large, that is, the core is too soft, a ball rebound may become too low and a desired distance may not be attainable on shots with a driver (W #1) by average hitters, or the feel at impact may be too soft, or the durability to cracking on repeated impact may worsen.

Next, the core hardness profile is described. The hardness of the core described below means Shore C hardness. The Shore C hardness is a hardness value measured with a Shore C durometer conforming to the ASTM D2240 standard.

A core center hardness (Cc) is preferably at least 50, more preferably at least 52, and even more preferably at least 54, and an upper limit thereof is preferably not more than 61, more preferably not more than 59, and even more preferably not more than 57. If this value is too large, a desired distance on shots with a driver (W #1) by average hitters may not be attainable, and the feel at impact may be too hard. On the other hand, if the above value is too small, the rebound becomes low and a good distance may not be achieved, a desired distance may not be attainable on shots with a driver (W #1) by average hitters, or the durability to cracking on repeated impact may worsen.

A hardness (Cm-6) at a position 6 mm inward from a position M (hereinafter also referred to as “midpoint M”) between the core center and the core surface is not particularly limited, although the hardness may be preferably at least 51, more preferably at least 53, and even more preferably at least 55, and an upper limit thereof is also not particularly limited, and may be preferably not more than 61, more preferably not more than 59, and even more preferably not more than 57. Hardnesses that deviate from these values may lead to undesirable results similar to those described above for the core center hardness (Cc).

A hardness (Cm-4) at a position 4 mm inward from the midpoint M between the core center and the core surface is not particularly limited, although the hardness may be preferably at least 52, more preferably at least 54, and even more preferably at least 56, and an upper limit thereof is also not particularly limited, and may be preferably not more than 63, more preferably not more than 61, and even more preferably not more than 59. Hardnesses that deviate from these values may lead to undesirable results similar to those described above for the core center hardness (Cc).

A hardness (Cm−2) at a position 2 mm inward from the midpoint M of the core is not particularly limited, although the hardness may be preferably at least 53, more preferably at least 55, and even more preferably at least 57, and an upper limit thereof is also not particularly limited, and may be preferably not more than 63, more preferably not more than 61, and even more preferably not more than 59. Hardnesses that deviate from these values may lead to undesirable results similar to those described above for the core center hardness (Cc). Hardnesses that deviate from these values may lead to undesirable results similar to those described above for the core center hardness (Cc).

A cross-sectional hardness (Cm) at the midpoint M of the core is not particularly limited, although the hardness may be preferably at least 55, more preferably at least 57, and even more preferably at least 59. In addition, an upper limit thereof is not particularly limited, although the upper limit may be preferably not more than 66, more preferably not more than 64, and even more preferably not more than 62. Hardnesses that deviate from these values may lead to undesirable results similar to those described above for the core center hardness (Cc).

A core surface hardness (Cs) is preferably at least 78, more preferably at least 80, and even more preferably at least 82. An upper limit thereof is preferably not more than 90, more preferably not more than 88, and even more preferably not more than 86. If this value is too large, the durability to cracking on repeated impact may worsen, or the feel at impact may be too hard. On the other hand, if the above value is too small, the rebound becomes low and a good distance may not be achieved, or the spin rate on full shots may rise and a desired distance may not be attainable on shots with a driver (W #1) by average hitters.

A hardness (Cm+2) at a position 2 mm outward from the midpoint M of the core toward the core surface is not particularly limited, although the hardness may be preferably at least 60, more preferably at least 62, and even more preferably at least 64, and an upper limit thereof is not particularly limited, and may be preferably not more than 71, more preferably not more than 69, and even more preferably not more than 67. Hardnesses that deviate from these values may lead to undesirable results similar to those described above for the core surface hardness (Cs).

A hardness (Cm+4) at a position 4 mm outward from the midpoint M of the core is not particularly limited, although the hardness may be preferably at least 65, more preferably at least 67, and even more preferably at least 69, and an upper limit thereof is also not particularly limited, and may be preferably not more than 78, more preferably not more than 76, and even more preferably not more than 74. Hardnesses that deviate from these values may lead to undesirable results similar to those described above for the core surface hardness (Cs).

A hardness (Cm+6) at a position 6 mm outward from the midpoint M of the core is not particularly limited, although the hardness may be preferably at least 70, more preferably at least 72, and even more preferably at least 74, and an upper limit thereof is also not particularly limited, and may be preferably not more than 83, more preferably not more than 81, and even more preferably not more than 79. Hardnesses that deviate from these values may lead to undesirable results similar to those described above for the core surface hardness (Cs).

In a core hardness profile, surface areas A to F are defined as follows:

• • surface area A: ½×2×(Cm−4−Cm−6)
  • surface area B: ½×2×(Cm−2−Cm−4)
  • surface area C: ½×2×(Cm−Cm−2)
  • surface area D: ½×2×(Cm+2−Cm)
  • surface area E: ½×2×(Cm+4−Cm+2)
  • surface area F: ½×2×(Cm+6−Cm+4)

and are characterized in that a value of (surface area E+surface area F)−(surface area A+surface area B) is preferably at least 1.0, more preferably at least 2.0, and even more preferably at least 4.0, and an upper limit thereof is preferably not more than 20.0, more preferably not more than 16.0, and even more preferably not more than 12.0. If this value is too large, the durability to cracking on repeated impact may worsen. On the other hand, if this value is too small, the spin rate on full shots may rise and a desired distance may not be attainable on shots with a driver (W #1) by average hitters.

In addition, a value of (surface area D+surface area E)−(surface area B+surface area C) is preferably at least 1.0, more preferably at least 2.0, and even more preferably at least 3.0, and an upper limit thereof is preferably not more than 20.0, more preferably not more than 16.0, and even more preferably not more than 12.0. If this value is too large, the durability to cracking on repeated impact may worsen. On the other hand, if this value is too small, the spin rate on full shots may rise and a desired distance may not be attainable on shots with a driver (W #1) by average hitters.

The surface areas A to F preferably satisfy the following conditions:

surface area A<surface area C<(surface area E+surface area F), and

surface area B<surface area C<(surface area E+surface area F)

• • and even more preferably satisfy the following conditions:

surface area A<surface area C<surface area D<(surface area E+surface area F),

and

surface area B<surface area C<surface area D<(surface area E+surface area F).

If these relationships are not satisfied, the spin rate on full shots may rise and a desired distance on shots with a driver (W #1) by average hitters may not be attainable.

FIG. 2 shows a graph describing the surface areas A to F using the core hardness profile data of Example 1. In this way, the surface areas A to F are surface areas of each triangle whose base is a difference between each specific distance and whose height is a difference in hardness between each position at these specific distances.

An initial velocity of the core is preferably at least 75.5 m/s, more preferably at least 75.9 m/s, and even more preferably at least 76.2 m/s. An upper limit thereof is preferably not more than 77.2 m/s, more preferably not more than 76.9 m/s, and even more preferably not more than 76.6 m/s. If the initial velocity value is too high, the initial velocity of the ball becomes too fast and may be against the rules. On the other hand, if the initial velocity of the core is too low, the ball rebound on full shots may become low, or the spin rate may rise excessively and a desired distance on shots with a driver (W #1) by average hitters may be unattainable. The value of the initial velocity in this case is a numerical value measured by a device for measuring a coefficient of restitution (COR) (Golf Ball Testing Machine) of the same type as the R&A. Specifically, a Golf Ball Testing Machine manufactured by Hye Precision USA is used. As a condition, at the time of measurement, an air pressure is changed in four stages and measured, a relational expression between the incident velocity and the COR is constructed, and the initial velocity at an incident velocity of 43.83 m/s is determined from the relational expression. For a measurement environment of the Golf Ball Testing Machine, a ball temperature-controlled for three hours or more in a thermostatic bath adjusted to 23.9±1° C. is used, and measurement is performed at a room temperature of 23.9±2° C.

Next, the surrounding layer is described.

The surrounding layer has a material hardness on the Shore C hardness scale which, although not particularly limited, is preferably at least 72, more preferably at least 75, and even more preferably at least 78, and an upper limit thereof is preferably not more than 92, more preferably not more than 90, and even more preferably not more than 88. The material hardness on the Shore D hardness scale is preferably at least 47, more preferably at least 49, and even more preferably at least 51, and an upper limit thereof is preferably not more than 62, more preferably not more than 60, and even more preferably not more than 57.

A sphere obtained by encasing the core with the surrounding layer has a surface hardness on the Shore C hardness scale which is preferably at least 80, more preferably at least 83, and even more preferably at least 86. The upper limit is preferably not more than 97, more preferably not more than 95, and even more preferably not more than 93. The surface hardness on the Shore D hardness scale is preferably at least 53, more preferably at least 55, and even more preferably at least 57, and an upper limit thereof is preferably not more than 68, more preferably not more than 66, and even more preferably not more than 63.

If the surrounding layer includes a plurality of layers, a numerical value of the hardness refers to a hardness of an outer surrounding layer, and the surface hardness refers to a surface hardness of a layer-encased sphere encasing the outer surrounding layer.

If the material hardness and the surface hardness of the surrounding layer are too soft in comparison with the above ranges, the ball may be too receptive to spin on full shots, the initial velocity may become low, and a desired distance may not be attainable on shots with a driver (W #1) by average hitters. On the other hand, if the material hardness and the surface hardness of the surrounding layer are too hard in comparison with the above ranges, the feel at impact may become hard, and the durability to cracking on repeated impact may worsen.

The surrounding layer has a thickness that is preferably at least 0.8 mm, more preferably at least 1.0 mm, and even more preferably at least 1.2 mm. On the other hand, the surrounding layer thickness has an upper limit that is preferably not more than 2.0 mm, more preferably not more than 1.6 mm, and even more preferably not more than 1.4 mm. If the thickness of the surrounding layer falls outside of the above ranges, the spin rate-lowering effect on full shots is insufficient and an intended distance on shots with a driver (W #1) by average hitters may be unattainable. If the surrounding layer is too thin, the durability to cracking on repeated impact may worsen.

In relation to the thickness of the intermediate layer to be described later, the thickness of the surrounding layer preferably satisfies the condition of (thickness of intermediate layer)< (thickness of surrounding layer).

The surrounding layer is made of a single layer or a plurality of layers of a resin material, and is an essential member, particularly for securing excellent durability on repeated impact. The material of the surrounding layer is not particularly limited, although a known resin may be used, and particularly preferable examples of the material may include the following components (a) to (c):

• • a base resin of (a) an olefin-unsaturated carboxylic acid random copolymer and/or a metal ion neutralization product of an olefin-unsaturated carboxylic acid random copolymer blended with (b) an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer and/or a metal ion neutralization product of an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer in a weight ratio between 100:0 and 0:100, and
  • (c) a non-ionomeric thermoplastic elastomer
  • in a weight ratio between 100:0 and 0:100.

As the components (a) to (c), for example, a resin material of an intermediate layer described in JP-A 2010-253268 may be suitably adopted, and the resin material is a material that is flexible, has high rebound, and is suitable for achieving the characteristic of a low spin rate on full shots.

Any additive may be appropriately included in the resin material according to the application. For example, various additives such as a pigment, a dispersant, an antioxidant, an ultraviolet absorber, and a light stabilizer can be included. If these additives are included, a compounding amount thereof is preferably at least 0.1 parts by weight, and more preferably at least 0.5 parts by weight, and an upper limit thereof is preferably not more than 10 parts by weight, and more preferably not more than 4 parts by weight per 100 parts by weight of a total of the base resin.

The resin material may contain an inorganic particulate filler. This inorganic particulate filler is not particularly limited, although zinc oxide, barium sulfate, titanium dioxide, and the like may be appropriately used. Barium sulfate may be preferably used, and particularly preferably precipitated barium sulfate may be suitably used from the viewpoint of excellent durability to cracking on repeated impact.

A mean particle size of the inorganic particulate filler is not particularly limited, although the mean particle size may be preferably from 0.01 to 100 μm, and more preferably from 0.1 to 10 μm. If the mean particle size of the inorganic particulate filler is too small or too large, dispersibility during material preparation may be deteriorated. The above-mentioned mean particle size means a particle size measured by dispersing the particles in an aqueous solution together with an appropriate dispersant and measuring the particles with a particle size distribution measuring apparatus.

A compounding amount of the inorganic particulate filler is not particularly limited, although the compounding amount is preferably included within a range of not more than 30 parts by weight per 100 parts by weight of the base resin of the surrounding layer material. A specific gravity of the surrounding layer is preferably within a range of from 0.95 to 1.25.

The sphere (surrounding layer-encased sphere) in which the core is encased with the surrounding layer has an initial velocity that is preferably at least 76.0 m/s, more preferably at least 76.5 m/s, and even more preferably at least 77.0 m/s. An upper limit thereof is preferably not more than 78.0 m/s, more preferably not more than 77.7 m/s, and even more preferably not more than 77.5 m/s. If the initial velocity value is too high, the initial velocity of the ball becomes too fast and may be against the rules. On the other hand, if the initial velocity is too low, the ball rebound on full shots may become low, or the spin rate may rise, and an intended distance on shots with a driver (W #1) by average hitters may not be attainable. The initial velocity in this case is measured with the same device and under the same conditions as described above in the measurement of the initial velocity of the core.

Next, the intermediate layer is described.

The intermediate layer has a material hardness on the Shore C hardness scale which, although not particularly limited, is preferably at least 90, more preferably at least 92 and even more preferably at least 93, but is preferably not more than 100, more preferably not more than 98, and even more preferably not more than 96. The material hardness on the Shore D hardness scale is preferably at least 64, more preferably at least 66, and even more preferably at least 68, and an upper limit thereof is preferably not more than 75, more preferably not more than 72, and even more preferably not more than 70.

A sphere obtained by encasing the surrounding layer-encased sphere with the intermediate layer (intermediate layer-encased sphere) has a surface hardness which, on the Shore C hardness scale, is preferably at least 95, more preferably at least 96, and even more preferably at least 97. The upper limit is preferably not more than 100, more preferably not more than 99, and even more preferably not more than 98. The surface hardness on the Shore D hardness scale is preferably at least 68, more preferably at least 69, and even more preferably at least 70. The upper limit is preferably not more than 78, more preferably not more than 75, and even more preferably not more than 72.

If the material hardness and the surface hardness of the intermediate layer are too soft in comparison with the above ranges, the spin rate may increase excessively on full shots, the initial velocity may become low, and a desired distance on shots with a driver (W #1) by average hitters may not be attainable. On the other hand, if the material hardness and the surface hardness of the intermediate layer are too hard in comparison with the above ranges, the durability to cracking on repeated impact may worsen, or the feel at impact on shots with a putter or on short approaches may become too hard.

The intermediate layer has a thickness that is preferably at least 0.7 mm, more preferably at least 0.8 mm, and even more preferably at least 1.0 mm. On the other hand, the intermediate layer thickness has an upper limit that is preferably not more than 1.8 mm, more preferably not more than 1.4 mm, and even more preferably not more than 1.2 mm. It is preferable for the intermediate layer to be thicker than the subsequently described cover. If the intermediate layer thickness falls outside of the above ranges or is thinner than that of the cover, the spin rate-lowering effect on the ball on shots with a driver (W #1) may be inadequate, and a desired distance by average hitters may not be increased. Also, if the intermediate layer is too thin, the durability to cracking on repeated impact may worsen.

As a material of the intermediate layer, it is suitable to employ an ionomer resin as a chief material.

The ionomer resin material suitably contains a high-acid ionomer resin having an unsaturated carboxylic acid content (also referred to as “acid content”) of at least 16 wt %.

An amount of the high-acid ionomer resin included per 100 wt % of the resin material is preferably at least 20 wt %, more preferably at least 50 wt %, and even more preferably at least 60 wt %. An upper limit thereof is preferably not more than 100 wt %, more preferably not more than 90 wt %, and even more preferably not more than 85 wt %. If the content of this high-acid ionomer resin is too low, the spin rate of the ball on full shots may rise and a good distance may not be attained. On the other hand, when the content of this high-acid ionomer resin is too high, the durability to repeated impact may worsen.

If the ionomer resin is employed as the chief material, an aspect that uses in admixture a zinc-neutralized ionomer resin and a sodium-neutralized ionomer resin as the chief material is desirable. The blending ratio in terms of zinc-neutralized ionomer resin/sodium-neutralized ionomer resin (weight ratio) is from 5/95 to 95/5, preferably from 10/90 to 90/10, and more preferably from 15/85 to 85/15. If the zinc-neutralized ionomer and the sodium-neutralized ionomer are not included in these ratios, the rebound may become too low, a desired flight may not be attainable, and the durability to cracking on repeated impact at room temperature may worsen. Further, the durability to cracking at a low temperature (below zero) may worsen.

In the intermediate layer material, an optional additive may be appropriately included depending on the intended use. For example, various additives such as a pigment, a dispersant, an antioxidant, an ultraviolet absorber, and a light stabilizer can be included. If these additives are included, the compounding amount thereof is preferably 0.1 parts by weight or more, and more preferably 0.5 parts by weight or more, and an upper limit thereof is preferably 10 parts by weight or less, and more preferably 4 parts by weight or less per 100 parts by weight of the base resin.

For the intermediate layer material, it is suitable to abrade the surface of the intermediate layer in order to increase the degree of adhesion to a polyurethane suitably used in a cover material described later. Further, it is preferable that a primer (adhesive agent) is applied to the surface of the intermediate layer after the abrasion treatment, or an adhesion reinforcing agent is added to the intermediate layer material.

The material of the intermediate layer may contain an inorganic particulate filler. This inorganic particulate filler is not particularly limited, although zinc oxide, barium sulfate, titanium dioxide, and the like may be appropriately used. Barium sulfate may be preferably used, and particularly preferably precipitated barium sulfate may be suitably used from the viewpoint of excellent durability to cracking on repeated impact.

A mean particle size of the inorganic particulate filler is not particularly limited, although the mean particle size may be preferably from 0.01 to 100 μm, and more preferably from 0.1 to 10 μm. If the mean particle size of the inorganic particulate filler is too small or too large, dispersibility during material preparation may be deteriorated. The above-mentioned mean particle size means a particle size measured by dispersing the particles in an aqueous solution together with an appropriate dispersant and measuring the particles with a particle size distribution measuring apparatus.

The compounding amount of the inorganic particulate filler is not particularly limited, although the compounding amount may be preferably at least 0 parts by weight, more preferably at least 10 parts by weight, and even more preferably at least 15 parts by weight per 100 parts by weight of the base resin of the intermediate layer material. In addition, an upper limit thereof is not particularly limited, although the upper limit is not more than 50 parts by weight, preferably not more than 40 parts by weight, and even more preferably not more than 30 parts by weight. If the compounding amount of the inorganic particulate filler is too small, the durability to cracking on repeated impact may worsen. On the other hand, if the compounding amount of the inorganic particulate filler is too large, the ball rebound may become low, or the spin rate of the ball on full shots may rise, and an intended distance may not be increased on shots with a driver (W #1) by average hitters.

A specific gravity of the intermediate layer is preferably at least 1.05, more preferably at least 1.07, and even more preferably at least 1.09, and an upper limit thereof is preferably not more than 1.25, more preferably not more than 1.20, and even more preferably not more than 1.15. If the specific gravity of the intermediate layer is too small, the durability to cracking on repeated impact may worsen. On the other hand, if the specific gravity of the intermediate layer is too large, the ball rebound may become low, or the spin rate of the ball on full shots may rise, and an intended distance may not be increased on shots with a driver (W #1) by average hitters.

The sphere (intermediate layer-encased sphere) in which the surrounding layer-encased sphere is encased with the intermediate layer has an initial velocity that is preferably at least 77.0 m/s, more preferably at least 77.4 m/s, and even more preferably at least 77.7 m/s. An upper limit thereof is preferably not more than 78.5 m/s, more preferably not more than 78.2 m/s, and even more preferably not more than 77.9 m/s. If the initial velocity value is too high, the initial velocity of the ball becomes too fast and may be against the rules. On the other hand, if the initial velocity is too low, the ball rebound on full shots may become low, or the spin rate may rise, and an intended distance may not be increased on shots with a driver (W #1) by average hitters. The initial velocity in this case is measured with the same device and under the same conditions as described above in the measurement of the initial velocities of the core and the surrounding layer-encased sphere.

Next, the cover is described.

The cover has a material hardness on the Shore C hardness scale which, although not particularly limited, is preferably at least 50, more preferably at least 57, and even more preferably at least 63, and an upper limit thereof is preferably not more than 80, more preferably not more than 74, and even more preferably not more than 71. The material hardness on the Shore D hardness scale is preferably at least 30, more preferably at least 35, and even more preferably at least 40, and an upper limit thereof is preferably not more than 53, more preferably not more than 50, and even more preferably not more than 47.

A sphere (ball) obtained by encasing the intermediate layer-encased sphere with the cover has a surface hardness on the Shore C hardness scale which is preferably at least 73, more preferably at least 78, and even more preferably at least 83, and an upper limit thereof is preferably not more than 95, more preferably not more than 92, and even more preferably not more than 90. The surface hardness on the Shore D hardness scale is preferably at least 50, more preferably at least 53, and even more preferably at least 56, and an upper limit thereof is preferably not more than 70, more preferably not more than 65, and even more preferably not more than 60.

If the material hardness and the surface hardness of the cover are too soft in comparison with the above ranges, the spin rate may rise on full shots, and the distance on shots with a driver (W #1) by average hitters may not be increased. On the other hand, if the material hardness and the surface hardness of the cover are too hard in comparison with the above ranges, the ball may not be fully receptive to spin on approach shots, or a scuff resistance may worsen. In addition, there is a possibility that the distance on shots with a driver (W #1) by long hitters increases too much and does not conform to new ODS rules that may be changed in the future.

The cover has a thickness of preferably at least 0.3 mm, more preferably at least 0.5 mm, and even more preferably at least 0.6 mm. On the other hand, an upper limit of the cover thickness is preferably not more than 1.2 mm, more preferably not more than 0.9 mm, and even more preferably not more than 0.8 mm. If the cover is too thick, the ball rebound on full shots may be inadequate or the spin rate may rise, and accordingly, the distance on shots with a driver (W #1) by average hitters may not be increased. On the other hand, when the cover is too thin, the scuff resistance may worsen or the ball may not be receptive to spin on approach shots and may thus lack sufficient controllability.

As the cover material, various urethane resins used as a cover material in golf balls may be used from the viewpoints of spin controllability and scuff resistance in the short game. Furthermore, from the viewpoint of mass productivity, it is suitable to use a resin material mainly composed of a thermoplastic polyurethane. Further, the cover is suitably formed of a resin blend containing (I) a thermoplastic polyurethane and (II) a polyisocyanate compound as principal components.

The total weight of the components (I) and (II) is recommended to be 60% or more, and more preferably 70% or more with respect to the total amount of the resin composition of the cover. The components (I) and (II) are described in detail below.

Describing the thermoplastic polyurethane (I), the construction of the thermoplastic polyurethane includes a soft segment composed of a polymeric polyol (polymeric glycol), which is a long-chain polyol, and a hard segment composed of a chain extender and a polyisocyanate compound. Here, as the long-chain polyol serving as a starting material, any of those hitherto used in the art related to thermoplastic polyurethane can be used, and are not particularly limited, and examples thereof can include polyester polyol, polyether polyol, polycarbonate polyol, polyester polycarbonate polyol, polyolefin polyol, conjugated diene polymer-based polyol, castor oil-based polyol, silicone-based polyol, and vinyl polymer-based polyol. These long-chain polyols may be used singly, or two or more may be used in combination. Among them, a polyether polyol is preferable from the viewpoint that a thermoplastic polyurethane having a high rebound resilience and excellent low-temperature properties can be synthesized.

As the chain extender, those hitherto used in the art related to thermoplastic polyurethanes can be suitably used, and for example, a low-molecular-weight compound having on the molecule two or more active hydrogen atoms capable of reacting with an isocyanate group and having a molecular weight of 400 or less is preferable. Examples of the chain extender include, but are not limited to, 1,4-butylene glycol, 1,2-ethylene glycol, 1,3-butanediol, 1,6-hexanediol, 2,2-dimethyl-1,3-propanediol, or the like. Among them, the chain extender is preferably an aliphatic diol having from 2 to 12 carbon atoms, and is more preferably 1,4-butylene glycol.

As the polyisocyanate compound, those hitherto used in the art related to thermoplastic polyurethane can be suitably used, and are not particularly limited. Specifically, one or more selected from a group consisting of 4,4′-diphenylmethane diisocyanate, 2,4-toluene diisocyanate (or) 2,6-toluene diisocyanate, p-phenylene diisocyanate, xylylene diisocyanate, 1,5-naphthylene diisocyanate, tetramethylxylene diisocyanate, hydrogenated xylylene diisocyanate, dicyclohexylmethane diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, norbornene diisocyanate, trimethylhexamethylene diisocyanate, and dimer acid diisocyanate may be used. However, it may be difficult to control a crosslinking reaction during injection molding depending on the type of isocyanate. In the present invention, 4,4′-diphenylmethane diisocyanate, which is an aromatic diisocyanate, is most preferable from the viewpoint of providing a balance between stability during production and physical properties to be manifested.

As specific examples of the thermoplastic polyurethane serving as the component (I), commercially available products can be used such as Pandex T-8295, Pandex T-8290, and Pandex T-8260 (all manufactured by DIC Covestro Polymer, Ltd.).

Although not an essential component, a thermoplastic elastomer other than the thermoplastic polyurethane can be included as a separate component (III) with the components (I) and (II). By including the component (III) in the resin blend, a flowability of the resin blend can be further improved, and various physical properties required of the golf ball cover material can be increased, such as rebound and scuff resistance.

A compositional ratio of the components (I), (II), and (III) is not particularly limited, but in order to sufficiently and effectively exhibit the advantageous effects of the present invention, the compositional ratio (I): (II): (III) is preferably in the weight ratio range of from 100:2:50 to 100:50:0, and more preferably from 100:2:50 to 100:30:8.

Furthermore, various additives other than the components constituting the thermoplastic polyurethane can be included in the resin blend as necessary, and for example, a pigment, a dispersant, an antioxidant, a light stabilizer, an ultraviolet absorber, an internal mold lubricant, or the like can be appropriately included.

A specific gravity of the cover is preferably at least 1.00, more preferably at least 1.03, and even more preferably at least 1.06, and an upper limit thereof is preferably not more than 1.20, more preferably not more than 1.17, and even more preferably not more than 1.14. If the specific gravity of the cover is too small, a blend ratio of ionomer or the like increases, so that the scuff resistance may worsen or the durability to cracking on repeated impact may worsen. On the other hand, if the specific gravity of the cover is too large, an amount of a filler added increases, the rebound becomes too low, and an intended distance on shots with a driver (W #1) by average hitters may not be increased.

The manufacture of a multi-piece solid golf ball in which the above-described core, surrounding layer, intermediate layer, and cover (outermost layer) are formed as successive layers may be performed by a customary method such as a known injection molding process. For example, each material of the surrounding layer and the intermediate layer is sequentially injected around the core with each mold for injection molding to obtain each layer-encased sphere, and finally, the cover material, which is the outermost layer, is injection molded to obtain the multi-piece golf ball. In addition, as each encasing layer, it is also possible to produce a golf ball by preparing two half-cups pre-molded into hemispherical shapes, enclosing the layer-encased sphere within the two half-cups, and molding the encased spheres under applied heat and pressure.

The golf ball has a deflection when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) that is preferably at least 2.0 mm, more preferably at least 2.3 mm, and even more preferably at least 2.5 mm. On the other hand, an upper limit of the deflection is preferably not more than 3.0 mm, and more preferably not more than 2.9 mm. If the deflection of the golf ball is too small, that is, if the sphere is too hard, the spin rate of the ball increases excessively and the distance on shots with a driver (W #1) by average hitters may not be increased, or the feel at impact may be too hard. On the other hand, if the deflection is too large, that is, if the sphere is too soft, the feel at impact may be too soft, or the durability to cracking on repeated impact may worsen.

An initial velocity of the sphere (ball) in which the intermediate layer-encased sphere is encased with the cover is preferably at least 76.8 m/s, more preferably at least 77.0 m/s, and even more preferably at least 77.2 m/s. An upper limit thereof is not more than 77.724 m/s. If this initial velocity value is too high, the official rules of the R&A and the USGA are not satisfied. On the other hand, if the initial velocity is too low, the distance on shots with a driver (W #1) by average hitters may not be increased. The initial velocity value in this case is measured with the same device and under the same conditions as described above in the measurement of the initial velocities of the core and each layer-encased sphere.

[Relationships Between Initial Velocities of Each Sphere]

In the present invention, from the viewpoint that a relationship between the initial velocity of the intermediate layer-encased sphere and the initial velocity of the ball is compatible with a superior distance on shots with a driver (W #1) by average hitters and controllability in the short game, it is required that the following condition is satisfied: (initial velocity of ball)< (initial velocity of intermediate layer-encased sphere).

A value obtained by subtracting the initial velocity of the ball from the initial velocity of the intermediate layer-encased sphere is more than 0.0 m/s, preferably at least 0.10 m/s, and more preferably at least 0.30 m/s, and an upper limit thereof is preferably not more than 1.00 m/s, more preferably not more than 0.70 m/s, and even more preferably not more than 0.50 m/s. If this value is too large, the spin rate on full shots may increase, an actual initial velocity may become low, and an intended distance may not be attainable on shots with a driver (W #1) by average hitters. On the other hand, in a case where this value is too small, if the value is caused by the cover, the cover becomes hard, the ball is not receptive to spin in the short game, and the durability on repeated impact is inferior. In addition, in a case where this value is too small, if the value is caused by the intermediate layer, the spin rate increases on full shots, and an intended distance may not be attainable on shots with a driver (W #1) by average hitters.

A value obtained by subtracting the initial velocity of the surrounding layer-encased sphere from the initial velocity of the intermediate layer-encased sphere is preferably at least 0.10 m/s, more preferably at least 0.20 m/s, and even more preferably at least 0.30 m/s. An upper limit thereof is preferably not more than 0.70 m/s, more preferably not more than 0.60 m/s, and even more preferably not more than 0.57 m/s. If this value is too large, in the case for the value being caused by the intermediate layer, the durability to cracking on repeated impact may worsen. In addition, in a case where this value is large, if the value is caused by the surrounding layer, the spin rate on full shots increases, and an intended distance may not be attainable on shots with a driver (W #1) by average hitters.

A value obtained by subtracting the initial velocity of the core from the initial velocity of the surrounding layer-encased sphere is preferably at least 0.20 m/s, more preferably at least 0.65 m/s, and even more preferably at least 0.80 m/s. An upper limit thereof is preferably not more than 1.05 m/s, more preferably not more than 1.00 m/s, and even more preferably not more than 0.95 m/s. If this value is too large, the durability to cracking on repeated impact may worsen. If this value is too small, the spin rate on full shots increases and an intended distance may not be attainable on shots with a driver (W #1) by average hitters.

A value obtained by subtracting the initial velocity of the core from the initial velocity of the intermediate layer-encased sphere is preferably at least 1.00 m/s, more preferably at least 1.15 m/s, and even more preferably at least 1.30 m/s. An upper limit thereof is preferably not more than 1.60 m/s, more preferably not more than 1.55 m/s, and even more preferably not more than 1.50 m/s. If this value is too large, the durability to cracking on repeated impact may worsen. If this value is too small, the spin rate on full shots increases and an intended distance may not be attainable on shots with a driver (W #1) by average hitters.

[Core Diameter and Ball Diameter]

A relationship between the core diameter and a ball diameter, that is, a value of (core diameter)/(ball diameter), is preferably at least 0.820, more preferably at least 0.824, and even more preferably at least 0.828. On the other hand, an upper limit thereof is preferably not more than 0.970, more preferably not more than 0.920, and even more preferably not more than 0.900. If this value is too small, the initial velocity of the ball may become low, or the deflection of the entire ball becomes small and the ball may become too hard, the spin rate of the ball on full shots may increase, and the distance on shots with a driver (W #1) by average hitters may be shorter than the intended distance. On the other hand, if the above value is too large, the spin rate of the ball on full shots may increase, the distance on shots with a driver (W #1) by average hitters may be shorter than the intended distance, or the durability to cracking on repeated impact may worsen.

[Relationships Between Surface Hardnesses of Each Sphere]

Expressed on the Shore C hardness scale, a value obtained by subtracting the core center hardness from the surface hardness of the surrounding layer-encased sphere is preferably at least 24, more preferably at least 27, and even more preferably at least 30, and an upper limit thereof is preferably not more than 42, more preferably not more than 39, and even more preferably not more than 36. If the above value is too large, the durability to cracking on repeated impact may worsen, or the actual initial velocity may become lower and a desired distance may not be attainable on shots with a driver (W #1) by average hitters. If the above value is too small, the spin rate of the ball on full shots may increase, and a desired distance may not be attainable on shots with a driver (W #1) by average hitters.

Expressed on the Shore C hardness scale, a value obtained by subtracting the core surface hardness from the surface hardness of the surrounding layer-encased sphere is preferably larger than 0, more preferably at least 2, and even more preferably at least 4. An upper limit thereof is preferably not more than 25, more preferably not more than 15, and even more preferably not more than 10. If the above value falls outside of the above ranges, the spin rate of the ball on full shots may increase, and an intended distance may not be attainable on shots with a driver (W #1) by average hitters.

Expressed on the Shore C hardness scale, a value obtained by subtracting the core center hardness from the surface hardness of the intermediate layer-encased sphere is preferably at least 35, more preferably at least 38, and even more preferably at least 40, and an upper limit thereof is preferably not more than 52, more preferably not more than 48, and even more preferably not more than 45. If the above value is too large, the durability to cracking on repeated impact may worsen, or the actual initial velocity may become lower and a desired distance may not be attainable on shots with a driver (W #1) by average hitters. If the above value is too small, the spin rate of the ball on full shots may increase, and a desired distance may not be attainable on shots with a driver (W #1) by average hitters.

Expressed on the Shore C hardness scale, a value obtained by subtracting the surface hardness of the surrounding layer-encased sphere from the surface hardness of the intermediate layer-encased sphere is preferably at least 1, more preferably at least 3, and even more preferably at least 5, and an upper limit thereof is preferably not more than 25, more preferably not more than 17, and even more preferably not more than 14. If the above value falls outside of the above ranges, the spin rate of the ball on full shots may increase, and an intended distance may not be attainable on shots with a driver (W #1) by average hitters.

Expressed on the Shore C hardness scale, a value obtained by subtracting the ball surface hardness from the surface hardness of the intermediate layer-encased sphere is preferably at least 2, more preferably at least 4, and even more preferably at least 6, and an upper limit thereof is preferably not more than 25, more preferably not more than 17, and even more preferably not more than 14. If the above value is too small, controllability in the short game may worsen. If the above value is too large, the spin rate on full shots may rise, and an intended distance may not be attainable on shots with a driver (W #1) by average hitters.

[Regarding Specific Gravity Relationship Between Intermediate Layer and Cover]

A difference between the specific gravity of the cover and the specific gravity of the intermediate layer is typically recommended to be within +0.15, preferably within +0.10, and more preferably within +0.05. That is, if the difference in specific gravity is too large, in a case where the intermediate layer material and/or the cover material cannot be molded completely concentrically with the layers located inside these layers and is eccentric, when the ball is struck with a putter, the ball may greatly wobble to the left or right.

Numerous dimples may be formed on the outside surface of the cover. Although not particularly limited, the number of dimples arranged on the surface of the cover may be preferably at least 280, more preferably at least 300, and even more preferably at least 310, and an upper limit thereof may be preferably not more than 450, more preferably not more than 400, and even more preferably not more than 350. If the number of dimples deviates from the above ranges, the distance on shots with a driver (W #1) by average hitters may be shortened.

As for the shape of the dimples, one type or a combination of two or more types such as a circular shape, various polygonal shapes, a dewdrop shape, and other oval shapes can be appropriately used. For example, if circular dimples are used, the diameter can be about 2.5 mm or more and 6.5 mm or less, and the depth can be 0.08 mm or more and 0.30 mm or less.

A dimple coverage ratio of the dimples on a spherical surface of the golf ball, specifically, a ratio (hereinafter, SR value) of a sum of individual dimple surface areas, each defined by a flat plane circumscribed by an edge of a dimple, to a ball spherical surface area on the assumption that the ball has no dimples, is preferably at least 75%, more preferably at least 80%, and even more preferably at least 84%. An upper limit thereof is not more than 90%, more preferably not more than 88%, and even more preferably not more than 86%. If the SR value deviates from the above ranges, the distance on shots with a driver (W #1) by average hitters may be shortened.

A VR value of a sum of volumes of the individual dimples formed below the flat plane circumscribed by the edge of a dimple to a ball spherical volume on the assumption that the ball has no dimples is at least 0.79%, preferably at least 0.80%, and more preferably at least 0.81%. An upper limit thereof is not more than 0.89%, more preferably not more than 0.88%, and even more preferably not more than 0.87%. If this VR value is larger than the above ranges, the distance on shots with a driver (W #1) by long hitters may be too short, or the intended distance on shots with a driver (W #1) by average hitters may not be attainable. In addition, in this case, a ball trajectory may become lower, it may become difficult for the ball to carry, and it may become difficult for the ball to go over a valley or a pond. On the other hand, if the above value is too small, the extent to which the distance is reduced on shots with a driver (W #1) by long hitters is inadequate, and there is a possibility that the distance is too long compared with the standard distance of the new distance rules assumed by the R&A and the USGA.

A value V₀ obtained by dividing a spatial volume of the dimples below the flat plane circumscribed by the edge of each dimple by a volume of a cylinder whose base is the flat plane and whose height is a maximum depth of the dimple from the base is preferably at least 0.35, more preferably at least 0.38, and even more preferably at least 0.40. An upper limit thereof is not more than 0.80, more preferably not more than 0.70, and even more preferably not more than 0.60. If the V₀ value deviates from the above ranges, the distance on shots with a driver (W #1) by long hitters and average hitters may be shorter than the intended distance.

In the golf ball of the present invention, when a ratio (CL1/CD1) of a lift coefficient CL1 at a Reynolds number of 218,000 and a spin rate of 2,800 rpm to a drag coefficient CD1 is denoted by A1, a ratio (CL2/CD2) of a lift coefficient CL2 at a Reynolds number of 184,000 and a spin rate of 2,900 rpm to a drag coefficient CD2 is denoted by A2, and a ratio (CL3/CD3) of a lift coefficient CL3 at a Reynolds number of 158,000 and a spin rate of 3,100 rpm to a drag coefficient CD3 is denoted by A3, the dimples are appropriately designed to satisfy the following two conditions:

0.590≤A1≤0.640, and

$\left(A2+A3\right)/2\ge 0.67.$

In the present specification, the lift coefficients (CL1, CL2, CL3) and drag coefficients (CD1, CD2, CD3) are measured in accordance with an Indoor Test Range (ITR) defined by the USGA. The lift coefficients and the drag coefficients may be adjusted by adjusting a configuration of the dimples of the golf ball (arrangement, diameter, depth, volume, number, shape, and the like). The lift coefficients and the drag coefficients are independent of an internal configuration of the golf ball. The Reynolds number (Re) is a dimensionless number used in the field of hydrodynamics. The Reynolds number (Re) is calculated by the following condition (1).

$\begin{array}{cc}\mathrm{Re}=\rho \mathrm{vL}/\mu & \left(1\right)\end{array}$

In the above condition (1), ρ represents a density of a fluid, v represents an average velocity of an object relative to a flow of the fluid, L represents a characteristic length, and μ represents a viscosity coefficient of the fluid.

In the present invention, the ratio CL1/CD1 of the lift coefficient CL1 at the Reynolds number of 218,000 and the spin rate of 2,800 rpm to the drag coefficient CD1 is defined as A1, the ratio CL2/CD2 of the lift coefficient CL2 at the Reynolds number of 184,000 and the spin rate of 2,900 rpm to the drag coefficient CD2 is defined as A2, and the ratio CL3/CD3 of the lift coefficient CL3 at the Reynolds number of 158,000 and the spin rate of 3,100 rpm to the drag coefficient CD3 is defined as A3.

If a condition of the Reynolds number 218,000 and the spin rate 2,800 rpm under which the lift coefficient CL1 and the drag coefficient CD1 are measured is described, this high-speed condition corresponds to a condition provided by a long hitter with a driver (W #1), this Reynolds number corresponds to a ball speed when the golf ball is driven out at a head speed (HS) of 54 m/s, and the spin rate 2,800 rpm is an average spin condition of a player with a head speed (HS) of 54 m/s.

If a condition of the Reynolds number 184,000 and the spin rate 2,900 rpm under which the lift coefficient CL2 and the drag coefficient CD2 are measured is described, this middle-speed condition corresponds to a condition provided by an average hitter with a driver (W #1) at a head speed (HS) of 45 m/s, this Reynolds number corresponds to the ball speed when the golf ball is driven out at a head speed (HS) of 45 m/s, and the spin rate 2,900 rpm is the average spin condition of a player with a head speed (HS) of 45 m/s.

If a condition of the Reynolds number 158,000 and the spin rate 3,100 rpm under which the lift coefficient CL3 and the drag coefficient CD3 are measured is described, this low-speed condition corresponds to a condition provided by an average hitter with a driver (W #1) at a head speed (HS) of 40 m/s, this Reynolds number corresponds to the ball speed when the golf ball is driven out at a head speed (HS) of 40 m/s, and the spin rate 3,100 rpm is the average spin condition of a player with a head speed (HS) of 40 m/s.

The ratio between the lift coefficient CL1 and the drag coefficient CD1, that is, the value of CL1/CD1=A1, is at least 0.590, preferably at least 0.595, and more preferably at least 0.600, and an upper limit thereof is not more than 0.640, preferably not more than 0.634, and more preferably not more than 0.627. If this value is too large, an effect of suppressing the distance on shots with a driver (W #1) by long hitters is insufficient and the distance may be too long. On the other hand, if the above value is too small, an actual distance may be shortened too much compared with the intended distance.

When the value of A1 is from 0.590 to 0.613, the ratio between the lift coefficient CL2 and the drag coefficient CD2, that is, the value of CL2/CD2=A2, is preferably at least 0.635, more preferably at least 0.645, and even more preferably at least 0.655, and an upper limit thereof is preferably not more than 0.668, more preferably not more than 0.666, and even more preferably not more than 0.664. Furthermore, when the value of A1 is from 0.614 to 0.640, the value of A2 is preferably at least 0.669, more preferably at least 0.671, and even more preferably at least 0.673, and an upper limit thereof is preferably not more than 0.750, more preferably not more than 0.725, and even more preferably not more than 0.700. If the above value deviates from the above ranges, the ball may blow up or a trajectory may occur in which the ball does not carry on shots with a driver (W #1) by average hitters, and an intended total distance may not be attainable.

When the value of A1 is from 0.590 to 0.613, the ratio between the lift coefficient CL3 and the drag coefficient CD3, that is, the value of CL3/CD3=A3, is preferably at least 0.695, more preferably at least 0.705, and even more preferably at least 0.715, and an upper limit thereof is preferably not more than 0.734, more preferably not more than 0.731, and even more preferably not more than 0.728. Furthermore, when the value of A1 is from 0.614 to 0.640, the value of A3 is preferably at least 0.735, more preferably at least 0.738, and even more preferably at least 0.741, and an upper limit thereof is preferably not more than 0.815, more preferably not more than 0.780, and even more preferably not more than 0.760. If the above value deviates from the above ranges, the ball may blow up or a trajectory may occur in which the ball does not carry on shots with a driver (W #1) by average hitters, and an intended total distance may not be attainable.

An average value of the above A2 and A3, that is, the value of (A2+A3)/2, is at least 0.670, preferably at least 0.680, and more preferably at least 0.690, and an upper limit thereof is preferably not more than 0.783, more preferably not more than 0.775, and even more preferably not more than 0.765. If this value is too low, it becomes difficult for the ball to carry on shots with a driver (W #1) by average hitters, and the intended total distance may not be attainable. On the other hand, if the above value is too high, the ball trajectory is blown up on shots with a driver (W #1) by average hitters, and the intended distance may not be attainable.

The multi-piece solid golf ball of the invention can be made to conform to the Rules of Golf for play. The inventive ball may be formed to a diameter which is such that the ball does not pass through a ring having an inner diameter of 42.672 mm and to a weight which is preferably between 45.0 and 45.93 g.

EXAMPLES

Hereinafter, the present invention is specifically described with reference to Examples and Comparative Examples, but the present invention is not limited to the following Examples. That is, the present invention can implement many aspects, and should not be construed as being limited to content described in the embodiments. Numerical values described in the following Examples are merely examples, and the interpretation of the present invention is not limited by these numerical values.

Examples 1 to 4 and Comparative Examples 1 to 9

[Formation of Core]

In Examples 1 to 4 and Comparative Examples 1 to 9, a rubber composition of each Example shown in Table 1 is prepared, and then vulcanization molding is performed under vulcanization conditions according to each Example shown in Table 1 to produce a solid core.

TABLE 1
Core formulation	Example	Comparative Example
(pbw)	1	2	3	4	1	2	3	4	5	6	7	8	9
Polybutadiene A												100	100
Polybutadiene B	100	100	100	100	100	100	100	100	100	100	100
Zinc acrylate	37.8	36.5	35.2	35.2	37.8	37.8	33.7	32.7	31.7	35.0	38.5	37.0	34.0
Zinc methacrylate												1.0
Organic peroxide	0.6	0.6	0.6	0.6	0.6	0.6	0.6	0.6	0.6	1.0	0.6	1.0	1.0
Water	0.8	0.8	0.8	0.8	0.8	0.8	0.8	0.8	0.8	0.8	0.8	0.4	0.4
Antioxidant	0.1	0.1	0.1	0.1	0.1	0.1	0.1	0.1	0.1	0.1	0.1	0.1	0.1
Zinc oxide	17.5	18.1	18.6	18.6	17.5	17.5	19.2	19.6	20.0	25.6	9.9	14.8	16.5
Zinc salt of	0.7	0.7	0.7	0.7	0.7	0.7	0.7	0.7	0.7	1.0	1.0	1.0	1.0
pentachlorothiophenol
Vulcanization	Temp.	152	152	152	152	152	152	152	152	152	158	158	150	150
conditions	(° C.)
	Time	19	19	19	19	19	19	19	19	19	13	13	19	19
	(min)

Details of the above formulations are as follows.

• • Polybutadiene A: Trade name “BR 01”, (manufactured by ENEOS Materials Corporation)
  • Polybutadiene B: Trade name “BR 730”, (manufactured by ENEOS Materials Corporation)
  • Zinc acrylate: Trade name “ZN-DA85S” (manufactured by Nippon Shokubai Co., Ltd.)
  • Zinc methacrylate: Trade name “ZDA-90” (manufactured by Asada Chemical Industry Co., Ltd.)
  • Organic peroxide: Dicumyl peroxide, trade name “Percumyl D” (manufactured by NOF Corporation)
  • Water: Pure water (manufactured by Seiki Co., Ltd.)
  • Antioxidant: 2,2-methylenebis(4-methyl-6-butylphenol), trade name “Nocrac NS-6” (manufactured by Ouchi Shinko Chemical Industrial Co., Ltd.)
  • Zinc oxide: Trade name “Grade 3 Zinc Oxide” (manufactured by Sakai Chemical Industry Co., Ltd.)
  • Zinc salt of pentachlorothiophenol: Manufactured by FUJIFILM Wako Pure Chemical Corporation

[Formation of Surrounding Layer]

Next, in each of the Examples and Comparative Examples, the surrounding layer is formed around the core by an injection molding method using the surrounding layer material having a formulation No. 1 shown in Table 2. In Comparative Examples 7 to 9, the surrounding layer is not formed.

[Formation of Intermediate Layer]

Next, the intermediate layer is formed around the surrounding layer-encased sphere obtained above by an injection molding method using the intermediate layer materials having the formulations No. 3 to No. 6 shown in Table 2.

[Formation of Cover (Outermost Layer)]

Next, the cover (outermost layer) is formed around the intermediate layer-encased sphere in each of the above Examples by an injection molding method using the cover materials having the formulations No. 7 to No. 9 shown in Table 2.

TABLE 2
Formulation	High-acid	Metal
(pbw)	ionomer	type	No. 1	No. 2	No. 3	No. 4	No. 5	No. 6	No. 7	No. 8	No. 9
HPF 1000	Not applicable	Mg	100					56
Himilan 1605	Not applicable	Na						44
Himilan 1557	Not applicable	Zn			15		15
Himilan 1706	Not applicable	Zn				15
AM7318	Applicable	Na			85	85	85		75
AM7327	Not applicable	Zn							25
Titanium oxide									4	3	3
Barium sulfate					20
Trimethylolpropane					1.1	1.1	1.1
Hytrel 4001				100
TPU (1)										100
TPU (2)											100

Details of the blending components in Table 2 are as follows.

• • “HPF 1000” manufactured by The Dow Chemical Company
  • “Himilan 1605”, “Himilan 1557”, “Himilan 1706”, “AM7318”, and “AM7327” ionomer resins manufactured by Dow-Mitsui Polychemicals Co., Ltd.
  • “Precipitated Barium Sulfate 300” barium sulfate manufactured by Sakai Chemical Industry Co., Ltd.
  • “Trimethylolpropane” (TMP) manufactured by Tokyo Chemical Industry Co., Ltd.
  • “Hytrel 4001” polyester elastomer manufactured by Toray Celanese Co., Ltd.
  • Trade name “Pandex” ether-type thermoplastic polyurethane (TPU (1)), material hardness (Shore D) 43, manufactured by DIC Covestro Polymer Ltd.
  • Trade name “Pandex” ether-type thermoplastic polyurethane (TPU (2)), material hardness (Shore D) 50, manufactured by DIC Covestro Polymer Ltd.

For the dimples of Examples and Comparative Examples, the following dimples (1) to (4) were used. Each dimple mode includes eight types of circular dimples of No. 1 to No. 8 having different diameters and depths. Details thereof are listed in Table 3 below. In addition, an arrangement mode (pattern) of the dimples (1) to (4) is shown in FIGS. 5A and 5B. FIG. 5A is a plan view of the dimples, and FIG. 5B is a side view thereof.

TABLE 3
						Cylinder
			Diameter	Depth	Volume	volume	SR	VR
	Type	Quantity	(mm)	(mm)	(mm3)	ratio Vo	(%)	(%)
Dimple	No. 1	12	4.63	0.122	1.009	0.491	84	0.68
(1)	No. 2	198	4.50	0.119	0.919	0.486
	No. 3	36	3.92	0.115	0.655	0.472
	No. 4	12	2.87	0.086	0.245	0.442
	No. 5	36	4.49	0.126	0.965	0.483
	No. 6	24	3.92	0.124	0.711	0.473
	No. 7	6	3.31	0.133	0.550	0.479
	No. 8	6	3.21	0.132	0.457	0.430
	Total	330
Dimple	No. 1	12	4.69	0.144	1.220	0.493	86	0.81
(2)	No. 2	198	4.54	0.140	1.100	0.486
	No. 3	36	3.96	0.134	0.766	0.467
	No. 4	12	2.92	0.109	0.322	0.441
	No. 5	36	4.54	0.146	1.141	0.485
	No. 6	24	3.96	0.144	0.827	0.468
	No. 7	6	3.36	0.136	0.590	0.491
	No. 8	6	3.22	0.127	0.433	0.421
	Total	330
Dimple	No. 1	12	4.70	0.149	1.252	0.487	86	0.85
(3)	No. 2	198	4.55	0.146	1.144	0.483
	No. 3	36	3.97	0.140	0.815	0.472
	No. 4	12	2.91	0.103	0.298	0.435
	No. 5	36	4.55	0.152	1.183	0.480
	No. 6	24	3.97	0.150	0.864	0.467
	No. 7	6	3.40	0.140	0.627	0.495
	No. 8	6	3.33	0.138	0.510	0.426
	Total	330
Dimple	No. 1	12	4.68	0.164	1.391	0.493	85	0.93
(4)	No. 2	198	4.53	0.161	1.258	0.485
	No. 3	36	3.95	0.154	0.883	0.468
	No. 4	12	2.90	0.114	0.331	0.440
	No. 5	36	4.53	0.168	1.294	0.480
	No. 6	24	3.95	0.165	0.949	0.470
	No. 7	6	3.36	0.154	0.663	0.487
	No. 8	6	3.26	0.152	0.538	0.426
	Total	330

[Definition of Dimple]

• • Edge: Highest point in a cross-section passing through a center of the dimple
  • Diameter: Diameter of the flat plane circumscribed by the edge of the dimple
  • Depth: Maximum depth of the dimple from the flat plane circumscribed by the edge of the dimple
  • SR: Ratio of the sum of individual dimple surface areas, each defined by the flat plane circumscribed by the edge of the dimple, to the ball spherical surface area on the assumption that the ball has no dimples
  • Dimple volume: Volume of the dimple under the flat plane circumscribed by the edge of the dimple
  • Cylinder volume ratio: Ratio of the dimple volume to the cylinder volume having the same depth and diameter as the dimple
  • VR: Ratio of the sum of volumes of the individual dimples formed below the flat plane circumscribed by the edge of the dimple to the ball spherical volume on the assumption that the ball has no dimples

The ratio CL1/CD1=A1 of the lift coefficient CL1 at the Reynolds number of 218,000 and the spin rate of 2,800 rpm to the drag coefficient CD1, the ratio CL2/CD2=A2 of the lift coefficient CL2 at the Reynolds number of 184,000 and the spin rate of 2,900 rpm to the drag coefficient CD2, and the ratio CL3/CD3=A3 of the lift coefficient CL3 at the Reynolds number of 158,000 and the spin rate of 3,100 rpm to the drag coefficient CD3 of the balls with the above dimples (1) to (4) formed on their cover surfaces are listed in the table below. These lift coefficients and drag coefficients are measured in accordance with the Indoor Test Range (ITR) defined by the USGA.

	TABLE 4
		Dimple	Dimple	Dimple	Dimple
		(1)	(2)	(3)	(4)
	CL1	0.151	0.147	0.145	0.143
	CD1	0.230	0.237	0.239	0.244
	CL1/CD1 = A1	0.657	0.620	0.607	0.586
	CL2	0.168	0.162	0.160	0.156
	CD2	0.233	0.240	0.242	0.246
	CL2/CD2 = A2	0.721	0.675	0.661	0.634
	CL3	0.190	0.182	0.179	0.173
	CD3	0.242	0.245	0.247	0.250
	CL3/CD3 = A3	0.785	0.743	0.725	0.692

For each resulting golf ball, various physical properties such as internal hardnesses at various positions of the core, outer diameters of the core and each layer-encased sphere, thicknesses and material hardnesses of each layer, surface hardnesses of each layer-encased sphere, and ball initial velocities are evaluated by the following methods, and are shown in Tables 5 to 8.

[Core Hardness Profile]

The core surface is spherical, but an indenter of a durometer is set substantially perpendicular to the spherical core surface, and a core surface hardness expressed on the Shore C scale is measured in accordance with ASTM D2240. With respect to the core center and a predetermined position of the core, the core is cut into hemispheres to obtain a flat cross-section, the hardness is measured by perpendicularly pressing the indenter of the durometer against a center portion and the predetermined positions shown in Tables 5 and 6, and the hardnesses at the center and each position are shown as Shore C hardness values. For the measurement of the hardness, a P2 Automatic Rubber Hardness Tester manufactured by Kobunshi Keiki Co., Ltd. equipped with a Shore C durometer is used. For the hardness value, a maximum value is read. All measurements are carried out in an environment of 23±2° C. The numerical values in the table are Shore C hardness values.

In addition, in the core hardness profile, letting Cc be the Shore C hardness at the core center, Cm be the Shore C hardness at the midpoint M between the core center and the core surface, Cm−2, Cm−4, and Cm−6 be the respective Shore C hardnesses at positions 2 mm, 4 mm, and 6 mm inward from the midpoint M, Cm+2, Cm+4, and Cm+6 be the respective Shore C hardnesses at positions 2 mm, 4 mm, and 6 mm outward from the midpoint M, and Cs be the Shore C hardness at the core surface, the surface areas A to F are calculated as follows:

• • surface area A: ½×2×(Cm−4−Cm−6)
  • surface area B: ½×2×(Cm−2−Cm−4)
  • surface area C: ½×2×(Cm−Cm−2)
  • surface area D: ½×2×(Cm+2−Cm)
  • surface area E: ½×2×(Cm+4−Cm+2)
  • surface area F: ½×2×(Cm+6−Cm+4)
  • and values of the following three conditions are determined.

surface area E+surface area F  (1)

(surface area E+surface area F)−(surface area A+surface area B)  (2)

(surface area D+surface area E)−(surface area B+surface area C)  (3)

The surface areas A to F in the core hardness profile are described in FIG. 2, which shows a graph that represents the surface areas A to F using the core hardness profile data in Example 1.

In addition, FIG. 3 and FIG. 4 show graphs of core hardness profiles for Examples 1 to 4 and Comparative Examples 1 to 9.

[Outer Diameters of Each Sphere of Core, Surrounding Layer-Encased Sphere, and Intermediate Layer-Encased Sphere]

At a temperature adjusted to 23.9±1° C. for at least three hours or more in a thermostatic bath, five random places on the surface are measured in a room with a temperature of 23.9±2° C., and, using an average value of these measurements as a measured value of each sphere, an average value for the diameter of 10 such spheres is determined.

[Ball Diameter]

At a temperature adjusted to 23.9±1° C. for at least three hours or more in a thermostatic bath, a diameter at 15 random dimple-free places is measured in a room at a temperature of 23.9±2° C., and, using an average value of these measurements as a measured value of one ball, an average value for the diameter of 10 balls is determined.

[Deflections of Core, Surrounding Layer-Encased Sphere, Intermediate Layer-Encased Sphere, and Ball]

Each subject layer-encased sphere is placed on a hard plate, and a deflection when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) is measured. The deflection in each case is a measurement value measured in a room at a temperature of 23.9±2° C. after temperature adjustment to 23.9±1° C. for at least three hours or more in a thermostatic bath. As a measuring device, a high-load compression tester manufactured by MU Instruments Trading Corp. is used, and a down speed of a pressure head that compresses the core, the layer-encased sphere of each layer, or the ball is set to 10 mm/s.

[Material Hardnesses of Surrounding Layer, Intermediate Layer, and Cover (Shore C and Shore D Hardnesses)]

The resin material of each layer is molded into a sheet having a thickness of 2 mm and left at a temperature of 23±2° C. for two weeks. At the time of measurement, three such sheets are stacked together. The Shore C hardness and the Shore D hardness are each measured with a Shore C durometer and a Shore D durometer conforming to the ASTM D2240 standard. For the measurement of the hardness, the P2 Automatic Rubber Hardness Tester manufactured by Kobunshi Keiki Co., Ltd. to which a Shore C durometer or a Shore D durometer is mounted is used. For the hardness value, a maximum value is read. The measurement method is in accordance with the ASTM D2240 standard.

[Surface Hardnesses of Each Sphere of Surrounding Layer-Encased Sphere, Intermediate Layer-Encased Sphere, and Ball]

A measurement is performed by perpendicularly pressing the indenter against the surface of each sphere. A surface hardness of a ball (cover) is a measured value at a dimple-free area (land) on the surface of the ball. The Shore C hardness and the Shore D hardness are each measured with a Shore C durometer and a Shore D durometer conforming to the ASTM D2240 standard. For the measurement of the hardness, the P2 Automatic Rubber Hardness Tester manufactured by Kobunshi Keiki Co., Ltd. to which a Shore C durometer or a Shore D durometer is mounted is used. For the hardness value, a maximum value is read. The measurement method is in accordance with the ASTM D2240 standard.

[Initial Velocity of Each Sphere]

The initial velocity of each sphere is measured at a temperature of 23.9±2° C. using a device for measuring COR manufactured by Hye Precision Products of the same type as the R&A. A measurement principle is as follows.

An air pressure is changed to four stages of 35.5 psi, 36.5 psi, 39.5 psi, and 40.5 psi, and a ball is fired at four stages of incident velocity by respective air pressures, collided with a barrier, and its COR is measured. That is, a correlation equation between the incident velocity and the COR is created by changing the air pressure in four stages. Similarly, a correlation equation between the incident velocity and a contact time is created.

Then, from these correlation equations, the coefficient of restitution (COR) and the contact time (μs) at an incident velocity of 43.83 m/s are determined and substituted into the following initial velocity conversion equation to calculate an initial velocity of each sphere.

$\mathrm{IV}=136.8+136.3e+0.019\mathrm{tc}$

(Here, e is a coefficient of restitution, and tc is a contact time (μs) at a collision speed of 143.8 ft/s (43.83 m/s).)

In the initial velocity measurement of each sphere, the barrel diameter is selected such that the clearance on one side with respect to the outer diameter of the object being measured is from 0.2 to 2.0 mm. For the core, a barrel diameter of 38.23 mm in Examples 1 to 4 and Comparative Examples 1 to 5, 36.58 mm in Comparative Example 6, and 39.88 mm in Comparative Examples 7 to 9 is selected. A barrel of 39.88 mm is selected in all Examples for the surrounding layer-encased sphere, a barrel of 41.53 mm is selected in all Examples for the intermediate layer-encased sphere, and a barrel of 43.18 mm is selected in all Examples for the ball.

TABLE 5
	Example	Comparative Example
	1	2	3	4	1	2	3
Construction (piece)	4P	4P	4P	4P	4P	4P	4P
Core	Outer diameter (mm)	36.27	36.27	36.27	36.27	36.27	36.27	36.29
	Weight (g)	29.31	29.31	29.31	29.31	29.31	29.31	29.34
	Specific gravity	1.17	1.17	1.17	1.17	1.17	1.17	1.17
	Deflection (mm)	3.7	4.0	4.2	4.2	3.7	3.7	4.5
	Initial velocity (m/s)	76.48	76.43	76.38	76.38	76.48	76.48	76.33
	Cs [surface] (Shore C)	86.0	84.1	82.2	82.2	86.0	86.0	79.8
	Cm + 6 (Shore C)	78.4	76.4	74.5	74.5	78.4	78.4	72.1
	Cm + 4 (Shore C)	73.3	71.6	69.9	69.9	73.3	73.3	68.4
	Cm + 2 (Shore C)	66.2	65.2	64.1	64.1	66.2	66.2	62.9
	Cm (Shore C)	61.1	60.3	59.5	59.5	61.1	61.1	58.4
	Cm − 2 (Shore C)	59.0	58.1	57.2	57.2	59.0	59.0	56.1
	Cm − 4 (Shore C)	58.4	57.3	56.3	56.3	58.4	58.4	55.1
	Cm − 6 (Shore C)	56.8	55.9	55.0	55.0	56.8	56.8	53.9
	Cc [center] (Shore C)	56.6	55.6	54.7	54.7	56.6	56.6	53.5
	Cs − Cc (Shore C)	29.4	28.5	27.5	27.5	29.4	29.4	26.3
	(Cs − Cc)/(Cm − Cc)	6.5	6.1	5.7	5.7	6.5	6.5	5.4
	Surface area A	1.6	1.4	1.3	1.3	1.6	1.6	1.2
	Surface area B	0.7	0.8	0.9	0.9	0.7	0.7	1.0
	Surface area C	2.1	2.2	2.3	2.3	2.1	2.1	2.3
	Surface area D	5.1	4.9	4.6	4.6	5.1	5.1	4.5
	Surface area E	7.0	6.4	5.9	5.9	7.0	7.0	5.5
	Surface area F	5.1	4.8	4.6	4.6	5.1	5.1	3.7
	Surface area E + surface area F	12.1	11.3	10.4	10.4	12.1	12.1	9.2
	(Surface area E + surface area F) −	9.8	9.1	8.3	8.3	9.8	9.8	7.0
	(surface area A + surface area B)
	(Surface area D + surface area E) −	9.4	8.3	7.3	7.3	9.4	9.4	6.7
	(surface area B + surface area C)

TABLE 6
	Comparative Example
	4	5	6	7	8	9
Construction (piece)	4P	4P	4P	3P	3P	3P
Core	Outer diameter (mm)	36.28	36.27	35.20	38.06	38.65	38.67
	Weight (g)	29.32	29.30	27.82	32.65	35.09	35.12
	Specific gravity	1.17	1.17	1.22	1.13	1.16	1.16
	Deflection (mm)	4.7	4.8	4.6	4.0	2.9	3.4
	Initial velocity (m/s)	76.27	76.22	76.19	76.85	76.70	76.75
	Cs [surface] (Shore C)	78.6	77.4	78.8	81.8	87.4	83.8
	Cm + 6 (Shore C)	71.0	69.8	74.1	76.4	80.4	77.9
	Cm + 4 (Shore C)	66.7	64.9	71.1	71.3	75.8	75.9
	Cm + 2 (Shore C)	62.0	61.1	66.9	65.6	70.8	69.8
	Cm (Shore C)	58.1	57.7	63.0	60.7	66.3	65.7
	Cm − 2 (Shore C)	55.5	54.8	60.7	56.5	65.6	64.7
	Cm − 4 (Shore C)	54.4	53.7	59.6	55.1	64.9	63.6
	Cm − 6 (Shore C)	53.5	53.0	58.4	54.7	63.3	62.4
	Cc [center] (Shore C)	53.0	52.5	57.2	54.3	62.6	60.3
	Cs − Cc (Shore C)	25.6	24.9	21.6	27.5	24.8	24.8
	(Cs − Cc)/(Cm − Cc)	5.0	4.8	3.7	4.3	6.7	4.4
	Surface area A	0.9	0.7	1.2	0.4	1.6	1.2
	Surface area B	1.1	1.1	1.1	1.4	0.7	1.1
	Surface area C	2.6	2.9	2.3	4.2	0.7	1.0
	Surface area D	3.9	3.4	3.9	4.9	4.5	4.1
	Surface area E	4.7	3.8	4.2	5.7	5.0	6.1
	Surface area F	4.3	4.9	3.0	5.1	4.6	2.0
	Surface area E + surface area F	9.0	8.7	7.2	10.8	9.6	8.1
	(Surface area E + surface area F) −	7.0	6.9	4.9	9.0	7.3	5.8
	(surface area A + surface area B)
	(Surface area D + surface area E) −	4.9	3.2	4.7	5.0	8.1	8.1
	(surface area B + surface area C)

TABLE 7
	Example	Comparative Example
	1	2	3	4	1	2	3
Surrounding	Material	No. 1	No. 1	No. 1	No. 1	No. 1	No. 1	No. 1
layer	Thickness (mm)	1.32	1.32	1.32	1.32	1.32	1.32	1.32
	Specific gravity	0.96	0.96	0.96	0.96	0.96	0.96	0.96
	Material hardness (Shore C)	85	85	85	85	85	85	85
	Material hardness (Shore D)	55	55	55	55	55	55	55
Surrounding	Outer diameter (mm)	38.91	38.91	38.91	38.91	38.91	38.91	38.93
layer-	Weight (g)	34.87	34.87	34.87	34.87	34.87	34.87	34.90
encased	Deflection (mm)	3.4	3.6	3.8	3.8	3.4	3.4	4.0
sphere	Initial velocity (m/s)	77.35	77.33	77.30	77.30	77.35	77.35	77.26
	Surface hardness (Shore C)	90	90	90	90	90	90	89
	Surface hardness (Shore D)	58	58	58	58	58	58	57
Surface hardness of surrounding layer-encased sphere -	33	34	35	35	33	33	36
core center hardness
Surface hardness of surrounding layer-encased sphere -	4	6	8	8	4	4	9
core surface hardness
Intermediate	Material	No. 3	No. 3	No. 3	No. 3	No. 3	No. 3	No. 3
layer	Thickness (mm)	1.08	1.08	1.08	1.08	1.08	1.08	1.07
	Specific gravity	1.09	1.09	1.09	1.09	1.09	1.09	1.09
	Material hardness (Shore C)	95	95	95	95	95	95	95
	Material hardness (Shore D)	69	69	69	69	69	69	69
Intermediate	Outer diameter (mm)	41.06	41.06	41.06	41.06	41.06	41.06	41.06
layer-	Weight (g)	40.69	40.69	40.69	40.69	40.69	40.69	40.70
encased	Deflection (mm)	2.9	3.0	3.2	3.2	2.9	2.9	3.3
sphere	Initial velocity (m/s)	77.82	77.83	77.83	77.83	77.82	77.82	77.86
	Surface hardness (Shore C)	97	97	97	97	97	97	97
	Surface hardness (Shore D)	71	71	71	71	71	71	71
Surface hardness of intermediate layer-encased sphere -	40	41	42	42	40	40	44
core center hardness (Shore C)
Surface hardness of intermediate layer-encased sphere -	7	7	7	7	7	7	8
surface hardness of surrounding layer-encased sphere
(Shore C)
Surrounding layer thickness - intermediate layer thickness	0.24	0.24	0.24	0.24	0.24	0.24	0.25
(mm)
Cover	Material	No. 8	No. 8	No. 8	No. 8	No. 8	No. 8	No. 8
	Thickness (mm)	0.83	0.83	0.83	0.83	0.83	0.83	0.83
	Specific gravity	1.12	1.12	1.12	1.12	1.12	1.12	1.12
	Material hardness (Shore C)	66	66	66	66	66	66	66
	Material hardness (Shore D)	43	43	43	43	43	43	43
Dimples	Type	(3)	(3)	(3)	(2)	(1)	(4)	(3)
	Quantity	330	330	330	330	330	330	330
	Surface area coverage ratio: SR (%)	86	86	86	86	84	85	86
	Volume occupancy ratio: VR (%)	0.85	0.85	0.85	0.81	0.68	0.93	0.85
	A1: CL1/CD1	0.607	0.607	0.607	0.620	0.657	0.586	0.607
	A2: CL2/CD2	0.661	0.661	0.661	0.675	0.721	0.634	0.661
	A3: CL3/CD3	0.725	0.725	0.725	0.743	0.785	0.692	0.725
	Average value of A2 and A3	0.693	0.693	0.693	0.709	0.753	0.663	0.693
Ball	Outer diameter (mm)	42.72	42.72	42.72	42.72	42.72	42.72	42.72
	Weight (g)	45.5	45.5	45.5	45.5	45.5	45.5	45.5
	Deflection (mm)	2.7	2.8	2.9	2.9	2.7	2.7	3.1
	Initial velocity (m/s)	77.36	77.36	77.36	77.36	77.36	77.36	77.45
	Surface hardness (Shore C)	86	86	86	86	86	86	85
	Surface hardness (Shore D)	59	59	59	59	59	59	58
Surface hardness of intermediate layer-encased sphere -	11	11	11	11	11	11	12
ball surface hardness (Shore C)
Core diameter/ball diameter (g)	0.849	0.849	0.849	0.849	0.849	0.849	0.849
Intermediate layer thickness - cover thickness (mm)	0.25	0.25	0.25	0.25	0.25	0.25	0.24
Cover specific gravity -	0.03	0.03	0.03	0.03	0.03	0.03	0.03
intermediate layer specific gravity (Shore C)
Initial	Intermediate layer-encased sphere - ball (m/s)	0.46	0.47	0.47	0.47	0.46	0.46	0.41
velocity	Intermediate layer-encased sphere -	0.47	0.50	0.53	0.53	0.47	0.47	0.60
	surrounding layer-encased sphere (m/s)
difference	Surrounding layer-encased sphere - core (m/s)	0.87	0.90	0.92	0.92	0.87	0.87	0.93
	Intermediate layer-encased sphere - core (m/s)	1.34	1.40	1.45	1.45	1.34	1.34	1.53

TABLE 8
	Comparative Example
	4	5	6	7	8	9
Surrounding	Material	No. 1	No. 1	No. 2	—	—	—
layer	Thickness (mm)	1.32	1.32	1.24	—	—	—
	Specific gravity	0.96	0.96	1.12	—	—	—
	Material hardness (Shore C)	85	85	67	—	—	—
	Material hardness (Shore D)	55	55	40	—	—	—
Surrounding	Outer diameter (mm)	38.92	38.91	37.68	—	—	—
layer-	Weight (g)	34.90	34.90	33.50	—	—	—
encased	Deflection (mm)	4.2	4.3	4.3	—	—	—
sphere	Initial velocity (m/s)	77.25	77.28	76.23	—	—	—
	Surface hardness (Shore C)	89	89	72	—	—	—
	Surface hardness (Shore D)	57	57	47	—	—	—
Surface hardness of surrounding layer-encased sphere -	36	37	15	—	—	—
core center hardness
Surface hardness of surrounding layer-encased sphere -	10	12	−7	—	—	—
core surface hardness
Intermediate	Material	No. 3	No. 3	No. 6	No. 3	No. 4	No. 5
layer	Thickness (mm)	1.07	1.08	1.30	1.51	1.17	1.21
	Specific gravity	1.09	1.09	0.95	1.09	0.95	0.95
	Material hardness (Shore C)	95	95	86	95	94	94
	Material hardness (Shore D)	69	69	57	69	67	66
Intermediate	Outer diameter (mm)	41.06	41.06	40.28	41.07	40.99	41.09
layer-	Weight (g)	40.70	40.72	39.38	40.59	40.63	40.86
encased	Deflection (mm)	3.5	3.6	3.7	3.1	2.5	2.8
sphere	Initial velocity (m/s)	77.84	77.81	76.95	77.69	77.50	77.60
	Surface hardness (Shore C)	97	97	93	98	97	97
	Surface hardness (Shore D)	71	71	63	71	71	71
Surface hardness of intermediate layer-encased sphere -	44	45	36	44	34	37
core center hardness (Shore C)
Surface hardness of intermediate layer-encased sphere -	8	8	21	—	—	—
surface hardness of surrounding layer-encased sphere
(Shore C)
Surrounding layer thickness - intermediate layer thickness	0.25	0.24	−0.06	—	—	—
(mm)
Cover	Material	No. 8	No. 8	No. 7	No. 8	No. 9	No. 8
	Thickness (mm)	0.83	0.83	1.20	0.82	0.85	0.81
	Specific gravity	1.12	1.12	0.97	1.12	1.12	1.12
	Material hardness (Shore C)	66	66	96	66	71	66
	Material hardness (Shore D)	43	43	64	43	50	43
Dimples	Type	(3)	(3)	(3)	(3)	(1)	(1)
	Quantity	330	330	330	330	330	330
	Surface area coverage ratio: SR (%)	86	86	86	86	84	84
	Volume occupancy ratio: VR (%)	0.85	0.85	0.85	0.85	0.68	0.68
	A1: CL1/CD1	0.607	0.607	0.607	0.607	0.657	0.657
	A2: CL2/CD2	0.661	0.661	0.661	0.661	0.721	0.721
	A3: CL3/CD3	0.725	0.725	0.725	0.725	0.785	0.785
	Average value of A2 and A3	0.693	0.693	0.693	0.693	0.753	0.753
Ball	Outer diameter (mm)	42.72	42.72	42.68	42.71	42.69	42.70
	Weight (g)	45.5	45.5	45.3	45.3	45.5	45.5
	Deflection (mm)	3.2	3.3	3.2	2.8	2.3	2.5
	Initial velocity (m/s)	77.41	77.37	77.19	77.31	77.15	77.18
	Surface hardness (Shore C)	85	85	97	85	87	85
	Surface hardness (Shore D)	58	58	68	58	61	59
Surface hardness of intermediate layer-encased sphere -	12	12	−4	13	10	12
ball surface hardness (Shore C)
Core diameter/ball diameter (g)	0.849	0.849	0.825	0.891	0.905	0.906
Intermediate layer thickness - cover thickness (mm)	0.24	0.25	0.10	0.68	0.32	0.40
Cover specific gravity -	0.03	0.03	0.02	0.03	0.17	0.17
intermediate layer specific gravity (Shore C)
Initial	Intermediate layer-encased sphere - ball (m/s)	0.43	0.44	−0.24	0.38	0.35	0.42
velocity	Intermediate layer-encased sphere -	0.59	0.53	0.72	—	—	—
	surrounding layer-encased sphere (m/s)
difference	Surrounding layer-encased sphere - core (m/s)	0.98	1.06	0.04	—	—	—
	Intermediate layer-encased sphere - core (m/s)	1.57	1.59	0.76	0.84	0.80	0.85

The flight (W #1), the controllability on approach shots, and the durability on impact of each golf ball are evaluated by the following methods. The results are shown in Table 9.

[Evaluation of Flight (W #1, HS 54 m/s)]

A driver is mounted on a golf swing robot, and a spin rate and a distance traveled (total) by a ball when struck at a head speed (HS) of 54 m/s are measured. The club used is a TOUR B XD-5 Driver/loft angle 8.5° (2016 model) manufactured by Bridgestone Sports Co., Ltd. and evaluation is performed according to the following rating criteria.

[Rating Criteria]

• • Good: Total compared with Comparative Example 8 is not more than-7.0 m, and at least-20.0 m.
  • Fair: Total compared with Comparative Example 8 is less than-20.0 m.
  • NG: Total compared with Comparative Example 8 is greater than-7.0 m.
    [Evaluation of Flight (W #1, HS 45 m/s)]

The driver is mounted on the golf swing robot, and the spin rate and the distance traveled (total) by a ball when struck at a head speed (HS) of 45 m/s are measured. The club used is a JGR Driver/loft angle 9.5° (2016 model) manufactured by Bridgestone Sports Co., Ltd. and is evaluated according to the following rating criteria.

[Rating Criteria]

• • Good: Total compared with Comparative Example 8 is at least-5.0 m.
  • Fair: Total compared with Comparative Example 8 is at least-10.0 m and less than-5.0 m.
  • NG: Total compared with Comparative Example 8 is less than-10.0 m.
    [Evaluation of Flight (W #1, HS 40 m/s)]

The driver is mounted on the golf swing robot, and the spin rate and the distance traveled (total) by a ball when struck at a head speed (HS) of 40 m/s are measured. The club used is a JGR Driver/loft angle 9.5° (2016 model) manufactured by Bridgestone Sports Co., Ltd. and is evaluated according to the following rating criteria.

[Rating Criteria]

• • Good: Total compared with Comparative Example 8 is at least +1.0 m.
  • Fair: Total compared with Comparative Example 8 is at least −4.0 m and less than +1.0 m.
  • NG: Total compared with Comparative Example 8 is less than −4.0 m.
    [Evaluation of Spin Rate on Approach Shots (SW, HS 15 m/s)]

A judgment is made based on a spin rate when the sand wedge is mounted on the golf swing robot and a ball is struck at an HS of 15 m/s. Similarly, the spin rate immediately after the ball is struck is measured by a device for measuring initial conditions. The sand wedge used is a TOURSTAGE TW-03 (loft angle) 57° 2002 model manufactured by Bridgestone Sports Co., Ltd.

[Rating Criteria]

• • Good: Spin rate is at least 4,500 rpm.
  • NG: Spin rate is less than 4,500 rpm.

[Durability on Repeated Impact]

The durability of the ball is evaluated with an ADC Ball COR Durability Tester manufactured by Automated Design Corporation (U.S). The tester has a function of firing a golf ball pneumatically and causing the golf ball to repeatedly strike two metal plates installed in parallel. The incident velocity on the metal plates is set to 43 m/s. The number of times of firing required until the golf ball cracks is measured, and an average value of the measured values of 10 golf balls is calculated. The average value is evaluated according to the following criteria.

[Rating Criteria]

• • Good: At least 181 times.
  • NG: Not more than 180 times.

TABLE 9
	Example	Comparative Example
	1	2	3	4	1	2	3
Flight	W#1	Spin rate (rpm)	2,581	2,545	2,510	2,510	2,581	2,581	2,467
	HS	Total (m)	275.4	275.2	275.1	278.1	287.7	263.5	274.9
	54 m/s	Total (m)	−10.2	−10.4	−10.5	−7.5	2.2	−22.1	−10.7
		compared to
		Comp. Ex. 8
		Rating	Good	Good	Good	Good	NG	Fair	Good
	W#1	Spin rate (rpm)	3,005	2,970	2,936	2,936	3,005	3,005	2,894
	HS	Total (m)	222.3	222.5	222.6	226.4	232.8	215.7	222.8
	45 m/s	Total (m)	−8.9	−8.7	−8.6	−4.8	1.6	−15.5	−8.4
		compared to
		Comp. Ex. 8
		Rating	Fair	Fair	Fair	Good	Good	NG	Fair
	W#1	Spin rate (rpm)	2,993	2,969	2,945	2,945	2,993	2,993	2,917
	HS	Total (m)	198.2	197.3	196.5	199.3	198.5	196.4	195.5
	40 m/s	Total (m)	3.4	2.5	1.7	4.5	3.7	1.6	0.7
		compared to
		Comp. Ex. 8
		Rating	Good	Good	Good	Good	Good	Good	Fair
Approach	SW	Spin rate (rpm)	4,761	4,732	4,702	4,702	4,761	4,761	4,667
	HS	Rating	Good	Good	Good	Good	Good	Good	Good
	15 m/s
Durability on	Number of	219	207	195	195	219	219	180
Repeated	cracks
Impact	Rating	Good	Good	Good	Good	Good	Good	NG
	Comparative Example
	4	5	6	7	8	9
Flight	W#1	Spin rate (rpm)	2,442	2,434	2,274	2,620	2,588	2,819
	HS	Total (m)	274.8	274.2	274.8	275.2	285.6	281.8
	54 m/s	Total (m)	−10.8	−11.3	−10.7	−10.4	0.0	−3.8
		compared to
		Comp. Ex. 8
		Rating	Good	Good	Good	Good	NG	NG
	W#1	Spin rate (rpm)	2,871	2,863	2,708	3,042	3,011	3,235
	HS	Total (m)	223.0	223.6	222.9	222.5	231.2	228.5
	45 m/s	Total (m)	−8.2	−7.6	−8.3	−8.7	0.0	−2.7
		compared to
		Comp. Ex. 8
		Rating	Fair	Fair	Fair	Fair	Good	Good
	W#1	Spin rate (rpm)	2,901	2,873	2,836	3,071	2,986	3,231
	HS	Total (m)	194.9	197.4	195.4	193.5	194.8	193.8
	40 m/s	Total (m)	0.1	2.6	0.6	−1.3	0.0	−1.0
		compared to
		Comp. Ex. 8
		Rating	Fair	Good	Fair	Fair	Fair	Fair
Approach	SW	Spin rate (rpm)	4,647	4,651	3,807	4,769	4,785	4,996
	HS	Rating	Good	Good	NG	Good	Good	Good
	15 m/s
Durability on	Number of	172	160	134	163	372	256
Repeated	cracks
Impact	Rating	NG	NG	NG	NG	Good	Good

As shown in the results in Table 9, the golf balls of Comparative Examples 1 to 9 are inferior to the golf balls according to the present invention (Examples) in the following respects.

In Comparative Example 1, the value of A1 is larger than 0.655, and there is an excessively long distance with the driver (W #1) at the head speed (HS) of 54 m/s.

In Comparative Example 2, the value of A1 is less than 0.590, the value of (A2+A3)/2 is less than 0.670, there is an excessively short distance with the driver (W #1) at the head speed (HS) of 54 m/s, and the distance is inferior with the driver (W #1) at the head speed (HS) of 45 m/s.

In Comparative Example 3, the deflection of the ball is greater than 3.0 mm, and the durability on repeated impact is inferior to that of each Example.

In Comparative Example 4, the deflection of the ball is greater than 3.0 mm, and the durability on repeated impact is inferior to that of each Example.

In Comparative Example 5, the deflection of the ball is greater than 3.0 mm, and the durability on repeated impact is inferior to that of each Example.

In Comparative Example 6, the deflection of the ball is greater than 3.0 mm, the initial velocity of the ball is faster than the initial velocity of the intermediate layer-encased sphere, the spin rate on approach shots decreases, and the durability on repeated impact is inferior.

Comparative Example 7 is a ball having a three-piece structure without a surrounding layer, and the durability on repeated impact is inferior to that of each Example.

Comparative Example 8 is a first embodiment of a tour ball used by professionals and advanced players. The ball has a three-piece structure without a surrounding layer, the value of A1 is larger than 0.655, and there is an excessively long distance with the driver (W #1) at the head speed (HS) of 54 m/s.

Comparative Example 9 is a second embodiment of the tour ball used by professionals and advanced players. The ball has a three-piece structure without a surrounding layer, the value of A1 is larger than 0.655, and there is an excessively long distance with the driver (W #1) at the head speed (HS) of 54 m/s.

Japanese Patent Application No. 2023-171962 is incorporated herein by reference. Although some preferred embodiments have been described, many modifications and variations may be made thereto in light of the above teachings. It is therefore to be understood that the invention may be practiced otherwise than as specifically described without departing from the scope of the appended claims.

Claims

1. A multi-piece solid golf ball comprising a core, a surrounding layer, an intermediate layer, and a cover, wherein a large number of dimples are formed on an outside surface of the cover, the core is formed of a single layer or a plurality of layers of a rubber composition, the surrounding layer is formed of a single layer or a plurality of layers of a resin composition, the intermediate layer and the cover are both formed of a single layer of a resin composition, and a relationship between an initial velocity of an intermediate layer-encased sphere and an initial velocity of the ball satisfies the following condition: ( A ⁢ 2 + A ⁢ 3 ) / 2 ≥ 0.67.

(initial velocity of ball)<(initial velocity of intermediate layer-encased sphere), and

where a deflection when the ball is compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) is not more than 3.0 mm, a ratio CL1/CD1 of a lift coefficient CL1 at a Reynolds number of 218,000 and a spin rate of 2,800 rpm to a drag coefficient CD1 is denoted by A1, a ratio CL2/CD2 of a lift coefficient CL2 at a Reynolds number of 184,000 and a spin rate of 2,900 rpm to a drag coefficient CD2 is denoted by A2, and a ratio CL3/CD3 of a lift coefficient CL3 at a Reynolds number of 158,000 and a spin rate of 3,100 rpm to a drag coefficient CD3 is denoted by A3, the following two conditions are satisfied: 0.590≤A1≤0.640

and

2. The multi-piece solid golf ball according to claim 1, wherein the value of A1 is 0.590 to 0.613, the value of A2 is 0.635 to 0.668, and the value of A3 is 0.695 to 0.734.

3. The multi-piece solid golf ball according to claim 2, wherein the value of A1 is 0.614 to 0.640, the value of A2 is 0.669 to 0.750, and the value of A3 is 0.735 to 0.815.

4. The multi-piece solid golf ball according to claim 1, wherein the value of (A2+A3)/2 is 0.670 to 0.783.

5. The multi-piece solid golf ball according to claim 1, wherein the following three conditions are satisfied:

(initial velocity of intermediate layer-encased sphere)−(initial velocity of surrounding layer-encased sphere)≤0.70 (m/s)

(initial velocity of surrounding layer-encased sphere)−(initial velocity of core)≥0.20 (m/s)

1.00≤(initial velocity of intermediate layer-encased sphere)−(initial velocity of core)≤1.60 (m/s).

6. The multi-piece solid golf ball according to claim 1, wherein the following condition is satisfied:

0.65≤(initial velocity of surrounding layer-encased sphere)−(initial velocity of core)≤1.00 (m/s).

7. The multi-piece solid golf ball according to claim 1, wherein the following condition is satisfied:

ball surface hardness<surface hardness of intermediate layer-encased sphere>surface hardness of surrounding layer-encased sphere>core surface hardness

where the surface hardness of each sphere means Shore C hardness.

8. The multi-piece solid golf ball according to claim 1, wherein the intermediate layer contains an inorganic particulate filler, and the resin material of the intermediate layer has a specific gravity of at least 1.05.

9. The multi-piece solid golf ball according to claim 1, wherein a difference between the specific gravity of the cover and the specific gravity of the intermediate layer is not more than 0.15.

10. The multi-piece solid golf ball according to claim 1, wherein the resin composition of the intermediate layer contains a high-acid ionomer resin having an acid content of at least 16 wt %.

11. The multi-piece solid golf ball according to claim 1, wherein the following condition is satisfied:

cover thickness<intermediate layer thickness≤surrounding layer thickness.

12. The multi-piece solid golf ball according to claim 1, wherein the core has a hardness profile in which, letting a Shore C hardness at a core center be Cc, a Shore C hardness at a midpoint M between the core center and the core surface be Cm, Shore C hardnesses at positions 2 mm, 4 mm, and 6 mm inward from the midpoint M be Cm−2, Cm−4, and Cm−6 respectively, Shore C hardnesses at positions 2 mm, 4 mm, and 6 mm outward from the midpoint M be Cm+2, Cm+4, and Cm+6 respectively, and a Shore C hardness at the core surface be Cs, and defining surface areas A to F as follows:

surface area A: ½×2×(Cm−4−Cm−6)

surface area B: ½×2×(Cm−2−Cm−4)

surface area C: ½{grave over (×)}2×(Cm−Cm−2)

surface area D: ½×2×(Cm+2−Cm)

surface area E: ½×2×(Cm+4−Cm+2)

surface area F: ½×2×(Cm+6−Cm+4)

the following condition is satisfied: (surface area E+surface area F)−(surface area A+surface area B)≥1.0.

13. The multi-piece solid golf ball according to claim 7, wherein the following condition is satisfied:

35≤(surface hardness of intermediate layer-encased sphere)−(core center hardness) where the hardnesses mean Shore C hardnesses.