JP2006341081A - Method for production of golf ball - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a golf ball having a cover with a small nominal thickness, exhibiting uneven thickness not that much, and being excellent in durability. <P>SOLUTION: A half shell 46 having a top part thickness of 65% or greater and 95% or less and a side part thickness of 100% or greater and 120% or less of the nominal thickness T of the cover is formed. Next, a core 4 composed of the center and the mid layer is fitted to two pieces of the half shell 46. Next, the core 4 and the half shell 46 are placed into a mold 32. Next, the mold 32 is clamped at a speed of 0.5 mm/sec or greater and 2.0 mm/sec or less. The thermoplastic resin composition of the half shell 46 is heated while being compressed in a spherical cavity to result in flow. This resin composition is hardened to give a cover having a thickness of 0.1 mm or greater and 0.8 mm or less. Dimples are formed on the cover by way of pimples 40. Recessed parts are formed on the mid layer by way of the pimples 40. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method for manufacturing a golf ball. In particular, the present invention relates to a compression molding method for golf balls.

  A general golf ball includes a core and a cover. The cover is made of a thermoplastic resin composition. The cover is formed by an injection molding method or a compression molding method. Dimples are formed on the surface of the cover.

  The injection molding method is excellent in mass productivity. In the injection molding method, the core is first held at the center of the spherical cavity by a holding pin. Next, the molten thermoplastic resin composition is injected into the gap between the cavity surface and the core. In the final stage of injection, the holding pin moves backward, so that the core may move from the center as the resin composition flows. The movement causes non-uniformity of the cover thickness (so-called uneven thickness). Air existing in the gap between the cavity surface and the core is discharged from the clearance of the vent hole or the holding pin according to the inflow of the resin composition. If this discharge is insufficient, an appearance defect due to residual air occurs. Production of high-quality golf balls by the injection molding method involves difficulties.

In the compression molding method, two half shells made of a thermoplastic resin composition and a core covered with these half shells are put into a mold. This mold consists of an upper mold and a lower mold. By tightening the mold, the resin composition is pressurized, and excess resin composition flows out of the parting line. Air existing between the core and the half shell is discharged from the parting line as the resin composition flows out. Such a compression molding method is disclosed in Japanese Patent Application Laid-Open No. 2004-344386.
JP 2004-344386 A

  In the compression molding method, the half shell undergoes thermal shrinkage during the clamping process. This heat shrinkage increases the volume of the resin composition in the vicinity of the top. In the obtained golf ball, the cover thickness of the pole (corresponding to the top part of the half shell) is larger than the cover thickness of the seam (corresponding to the side part of the half shell). In other words, uneven thickness of the cover occurs due to heat shrinkage. In particular, heat shrinkage tends to occur when the mold clamping speed is low. In the compression molding method, uneven thickness also occurs when the resin composition flows out in a specific direction. In particular, when the amount of outflow is large, uneven outflow tends to occur. The uneven thickness adversely affects the durability and other performances of the golf ball. In particular, in a golf ball having a thin cover with a nominal thickness, adverse effects on durability due to uneven thickness are large.

  The cover is thin just below the dimples. When a golf ball is repeatedly hit, the crack may be the starting point of the dimple and the cover may be damaged. Covers with a small nominal thickness are particularly susceptible to breakage.

  In recent years, golf balls having a thin cover have been proposed and marketed from the viewpoint of achieving both control performance and flight performance. In this golf ball, improvement in durability is urgent. An object of the present invention is to provide a golf ball having a cover with a small nominal thickness, having a small thickness deviation and excellent durability.

The manufacturing method according to the present invention includes
(1) A core molding step in which a center is obtained by covering the center with an intermediate layer made of a thermoplastic resin composition,
(2) a half shell molding step in which a half shell that is made of a thermoplastic resin composition and has a bowl shape is molded;
(3) The mold and the two half shells fitted in the core are formed into a mold having an upper mold and a lower mold having a hemispherical cavity surface and a large number of pimples on the surface. A charging process that is performed in an open state,
(4) When the mold is clamped, the half-shell thermoplastic resin composition is heated while being pressed in the spherical cavity, so that the excess thermoplastic resin composition flows out of the spherical cavity, and the intermediate layer is pimple. And (5) a solidification step in which the thermoplastic resin composition remaining in the spherical cavity is solidified to form a cover having a nominal thickness T of 0.1 mm to 0.8 mm. . The half shell obtained in this half shell molding step has a top thickness Tt of 65% to 95% and a side thickness Ts of 100% to 120% with respect to the nominal thickness T of the cover. In the present invention, the nominal thickness is a value obtained by subtracting the radius of the core from the radius of the golf ball before painting.

  Preferably, the mold clamping speed in the mold clamping process is 0.5 mm / s or more and 2.0 mm / s or less.

  Preferably, the manufacturing method further includes a high pressure step in which the resin composition of the half shell is pressurized in the spherical cavity at a pressure higher than the pressure in the mold clamping step after the mold clamping step. Preferably, the compressive force in the high pressure step is not less than 17652N and not more than 40207N. Preferably, the time of the high pressure process is not less than 30 seconds and not more than 300 seconds.

  By this manufacturing method, a golf ball having a small difference between the pole cover thickness and the seam cover thickness is obtained. By this manufacturing method, uneven thickness is suppressed. According to this manufacturing method, a golf ball having a cover having a large number of dimples and an intermediate layer having a large number of recessed portions at a position corresponding to pimples is obtained. In the golf ball obtained by this manufacturing method, the cover thickness directly under the dimple is large. This golf ball is excellent in durability.

  Hereinafter, the present invention will be described in detail based on preferred embodiments with appropriate reference to the drawings.

  FIG. 1 is a partially cutaway sectional view showing a golf ball 2 obtained by a manufacturing method according to an embodiment of the present invention. The golf ball 2 includes a spherical core 4 and a cover 6 that covers the core 4. The core 4 includes a spherical center 8 and an intermediate layer 10 that covers the center 8. A large number of dimples 12 are formed on the surface of the cover 6. A portion of the surface of the cover 6 other than the dimples 12 is a land 14. The golf ball 2 includes a paint layer and a mark layer outside the cover 6, but these layers are not shown.

  The golf ball 2 has a diameter of 40 mm to 45 mm. The diameter is preferably 42.67 mm or more from the viewpoint that the American Golf Association (USGA) standard is satisfied. In light of suppression of air resistance, the diameter is preferably equal to or less than 44 mm, and more preferably equal to or less than 42.80 mm. The golf ball 2 has a mass of 40 g or more and 50 g or less. From the viewpoint of obtaining a large inertia, the mass is preferably 44 g or more, and more preferably 45.00 g or more. From the viewpoint that the USGA standard is satisfied, the mass is preferably equal to or less than 45.93 g.

  The center 8 is obtained by crosslinking the rubber composition. Examples of the base rubber of the rubber composition include polybutadiene, polyisoprene, styrene-butadiene copolymer, ethylene-propylene-diene copolymer, and natural rubber. Two or more kinds of rubbers may be used in combination. From the viewpoint of resilience performance, polybutadiene is preferred, and high cis polybutadiene is particularly preferred.

  For crosslinking of the center 8, a co-crosslinking agent is usually used. From the viewpoint of resilience performance, preferred co-crosslinking agents are zinc acrylate, magnesium acrylate, zinc methacrylate and magnesium methacrylate. It is preferable that an organic peroxide is blended with the co-crosslinking agent in the rubber composition. Suitable organic peroxides include dicumyl peroxide, 1,1-bis (t-butylperoxy) -3,3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-di (t- Butyl peroxy) hexane and di-t-butyl peroxide.

  In the rubber composition, various additives such as a filler, a sulfur compound, an anti-aging agent, a colorant, a plasticizer, and a dispersant are blended in appropriate amounts as necessary. Crosslinked rubber powder or synthetic resin powder may be blended with the rubber composition.

  The center 8 has a diameter of 30.0 mm or more, particularly 35.0 mm or more. The center 8 has a diameter of 41.5 mm or less, particularly 41.0 mm or less. The center 8 may be subjected to surface treatment such as polishing, brushing, framing, and plasma treatment. The center 8 may be composed of two or more layers. Another layer made of a thermoplastic resin composition may be provided between the center 8 and the intermediate layer 10.

  The mid layer 10 is made of a thermoplastic resin composition. Examples of the base polymer of the resin composition include ionomer resins, thermoplastic polyester elastomers, thermoplastic polyamide elastomers, thermoplastic polyurethane elastomers, thermoplastic polyolefin elastomers, and thermoplastic polystyrene elastomers. In particular, an ionomer resin is preferable. The ionomer resin is highly elastic. As will be described later, the cover 6 of the golf ball 2 is extremely thin. When the golf ball 2 is hit with a driver, the mid layer 10 is greatly deformed. The intermediate layer 10 using the ionomer resin contributes to the flight performance in a shot with a driver. When the ionomer resin and another resin are used in combination, the ionomer resin is the main component of the base polymer from the viewpoint of flight performance. The proportion of the ionomer resin in the total base polymer is preferably 50% by mass or more, more preferably 70% by mass or more, and particularly preferably 85% or more.

  Preferably, an ionomer resin in which a part of a carboxylic acid in a copolymer of an α-olefin and an α, β-unsaturated carboxylic acid having 3 to 8 carbon atoms is neutralized with a metal ion is used. Preferred α-olefins are ethylene and propylene. Preferred α, β-unsaturated carboxylic acids are acrylic acid and methacrylic acid. Examples of the metal ions for neutralization include sodium ions, potassium ions, lithium ions, zinc ions, calcium ions, magnesium ions, aluminum ions, and neodymium ions. Neutralization may be performed with two or more metal ions. Particularly suitable metal ions from the viewpoint of resilience performance and durability of the golf ball 2 are sodium ion, zinc ion, lithium ion and magnesium ion.

  As specific examples of the ionomer resin, trade names “HIMILAN 1555”, “HIMILAN 1557”, “HIMILAN 1605”, “HIMILAN 1706”, “HIMILAN 1707”, “HIMILAN AM7311”, “HIMILAN AM7315” of Mitsui Dupont Polychemical Co., Ltd. "High Milan AM 7317", "High Milan AM 7318", "Hi Milan AM 7329" and "Hi Milan MK7320"; DuPont's trade names "Surlin 7930", "Surlin 7940", "Surlin 8140", "Surlin 8940", "Surlin 8945" "Surlin 9120", "Surlin 9910" and "Surlin 9945"; and Exxon's trade names "IOTEK7010", "IOTEK7030", "IOTEK8000" and "IOTEK8" 30 ", and the like.

  From the viewpoint of flight performance, the thickness of the intermediate layer 10 is preferably 0.3 mm or more, more preferably 0.5 mm or more, and particularly preferably 0.7 mm or more. In light of feel at impact, the thickness is preferably equal to or less than 2.5 mm, and more preferably equal to or less than 2.0 mm.

  From the viewpoint of the close contact between the intermediate layer 10 and the cover 6, it is preferable that the intermediate layer 10 is subjected to a surface treatment to increase its roughness. Specific examples of the surface treatment include brushing and polishing.

  The cover 6 is made of a thermoplastic resin composition. Examples of the base polymer of this resin composition include thermoplastic polyurethane elastomers, thermoplastic polyester elastomers, thermoplastic polyamide elastomers, thermoplastic polyolefin elastomers, thermoplastic polystyrene elastomers, and ionomer resins. A thermoplastic polyurethane elastomer is preferably used as the base polymer. The thermoplastic polyurethane elastomer is soft. When the golf ball 2 having the cover 6 made of a thermoplastic polyurethane elastomer is hit with a short iron, the spin rate is high. The cover 6 made of a thermoplastic polyurethane elastomer contributes to control performance in a shot with a short iron. The thermoplastic polyurethane elastomer also contributes to the scratch resistance performance of the cover 6.

  The thermoplastic polyurethane elastomer includes a polyurethane component as a hard segment and a polyester component or a polyether component as a soft segment. Examples of the curing agent for the polyurethane component include alicyclic diisocyanate, aromatic diisocyanate and aliphatic diisocyanate. In particular, alicyclic diisocyanates are preferred. Since the alicyclic diisocyanate does not have a double bond in the main chain, yellowing of the cover 6 is suppressed. In addition, since the alicyclic diisocyanate is excellent in strength, damage to the cover 6 is suppressed. Two or more diisocyanates may be used in combination.

Examples of alicyclic diisocyanates include 4,4′-dicyclohexylmethane diisocyanate (H ₁₂ MDI), 1,3-bis (isocyanatomethyl) cyclohexane (H ₆ XDI), isophorone diisocyanate (IPDI), and trans-1,4- Examples are cyclohexane diisocyanate (CHDI). From the viewpoint of versatility and workability, H ₁₂ MDI is preferable.

  Aromatic diisocyanates include 4,4'-diphenylmethane diisocyanate (MDI) and toluene diisocyanate (TDI). As the aliphatic diisocyanate, hexamethylene diisocyanate (HDI) is exemplified.

  Specific examples of thermoplastic polyurethane elastomers include BASF Japan's trade name “Elastolan XNY90A”, trade name “Elastolan XNY97A”, trade name “Elastolan XNY585”, and trade name “Elastolan XKP016N”; The trade name “Rezamin P4585LS” and the trade name “Rezamin PS62490” of the industrial company are listed.

  In the cover 6, when the thermoplastic polyurethane elastomer and another resin are used in combination, the thermoplastic polyurethane elastomer is a main component of the base polymer from the viewpoint of control performance. The ratio of the thermoplastic polyurethane elastomer in the total base polymer is preferably 50% by mass or more, more preferably 70% by mass or more, and particularly preferably 85% by mass or more.

  If necessary, the cover 6 includes a colorant such as titanium dioxide, a filler such as barium sulfate, a dispersant, an antioxidant, an ultraviolet absorber, a light stabilizer, a fluorescent agent, a fluorescent whitening agent, and the like. Appropriate amount is blended. For the purpose of adjusting the specific gravity, the cover 6 may be mixed with powder of a high specific gravity metal such as tungsten or molybdenum.

  The nominal thickness T of the cover 6 is 0.8 mm or less. As described above, the cover 6 has a low hardness. The low hardness cover 6 is disadvantageous in terms of the coefficient of restitution of the golf ball 2. In the shot with the driver, the mid layer 10 and the center 8 of the golf ball 2 are also greatly deformed. By setting the nominal thickness T of the cover 6 to 0.8 mm or less, even if the cover 6 has a low hardness, the cover 6 does not have a significant adverse effect on the coefficient of restitution upon a shot with a driver. From the viewpoint of flight performance and ease of molding, the nominal thickness T of the cover 6 is more preferably 0.6 mm or less, and particularly preferably 0.5 mm or less. From the viewpoint of easy forming of the cover 6, the nominal thickness T is preferably 0.1 mm or more, and more preferably 0.2 mm or more.

  FIG. 2 is a sectional view showing a part of the first mold 18 used for manufacturing the golf ball 2 of FIG. The first mold 18 includes an upper mold 20 and a lower mold 22. The upper mold 20 includes a flat part 24 and a convex part 26. The surface of the convex portion 26 is substantially hemispherical. The lower mold 22 includes a flat portion 28 and a concave portion 30. The surface of the recess 30 is substantially hemispherical. The radius of the convex portion 26 is smaller than the radius of the concave portion 30. When the upper mold 20 and the lower mold 22 are combined, a space is formed between the convex portion 26 and the concave portion 30. When the upper mold 20 and the lower mold 22 are combined, a space is also formed between the flat portion 24 of the upper mold 20 and the flat portion 28 of the lower mold 22. Preferably, the first mold 18 is preferably subjected to a fluororesin coating rough surface electroless plating treatment. In this process, first, electroless plating is applied to the main body of the first mold 18. Typically, electroless nickel plating is applied. Next, the surface of the plating layer is roughened by chemical treatment. Next, a fluororesin is applied to the plating layer and baked. By the baking, the fluororesin enters the minute recesses of the plating layer.

  FIG. 3 is a cross-sectional view showing a part of the second mold 32 used for manufacturing the golf ball 2 of FIG. The second mold 32 includes an upper mold 34 and a lower mold 36. Each of the upper mold 34 and the lower mold 36 includes a plurality of cavity surfaces 38, and a hemispherical cavity is formed by the cavity surfaces 38. A spherical cavity is formed by combining the upper mold 34 and the lower mold 36. A large number of pimples 40 are formed on the cavity surface 38.

  In the production of the golf ball 2, the base rubber, the crosslinking agent and various additives are first kneaded to obtain a rubber composition. Next, this rubber composition is put into a molding die (not shown) which is composed of an upper die and a lower die and has a spherical cavity. Next, the mold is tightened. Next, the rubber composition is heated through a mold. The rubber causes a crosslinking reaction by heating. The rubber composition is cured by the crosslinking. The mold is opened and the spherical center 8 is taken out.

  The center 8 is put into a molding die (not shown) that includes an upper die and a lower die and has a spherical cavity. A molten resin composition is injected around the center 8 by an injection molding method. This resin composition is solidified to form the intermediate layer 10. In this way, the core 4 composed of the center 8 and the intermediate layer 10 is obtained. The intermediate layer 10 may be formed by a compression molding method.

  Next, the thermoplastic resin and the additive are blended and extruded from an extruder to obtain a resin composition. Next, this resin composition is cut into a predetermined size. By cutting, pellets 42 (see FIG. 2) are obtained. Next, the pellet 42 is put into the first mold 18. As shown in FIG. 2, the pellet 42 is placed in the recess 30 of the lower mold 22. Next, the lower mold 22 rises relative to the upper mold 20 and the mold is clamped. Clamping is usually done by a press. The pellets 42 are pressurized and heated by clamping. The resin composition flows by pressurization and heating, and the space between the upper mold 20 and the lower mold 22 is filled with the resin composition. Next, the first mold 18 is cooled. By cooling, the temperature of the resin composition also decreases. When the temperature is sufficiently lowered, the first mold 18 is opened, and the preform is taken out. As described above, since the first molding die 18 is subjected to the fluororesin coating rough surface electroless plating treatment, the preform can be easily removed from the mold. As shown in FIG. 3, the preform 44 includes a number of half shells 46. The half shell 46 has a bowl shape. The half shell 46 may be formed by an injection molding method.

  Next, as shown in FIG. 3, the core 4 is sandwiched between the two preforms 44. Two half shells 46 are fitted to the core 4. Next, the preformed body 44 and the core 4 are put into the opened second mold 32. The half shell 46 and the core 4 are usually placed on the cavity surface 38 of the lower mold 36.

  Next, the lower mold 36 rises relative to the upper mold 34, and the lower mold 36 approaches the upper mold 34. This operation is usually performed by a press machine. The approach speed is 3.0 mm / sec or more and 200.0 mm / sec or less. This speed is fast. Fast cycle times achieve short cycle times. This process is called an approach process.

  When the distance between the upper mold 34 and the lower mold 36 reaches a predetermined value, the approaching process ends. Thereafter, the lower die 36 approaches the upper die 34 at a speed of 0.5 mm / sec or more and 2.0 mm / sec or less. The thermoplastic resin composition of the half shell 46 is heated while being pressurized in the spherical cavity. The resin composition flows by pressurization and heating, and covers around the core 4. Excess resin composition flows out of the spherical cavity. This process is referred to as a mold clamping process. In the mold clamping process, the pimple 40 presses the half shell 46 and the intermediate layer 10. By this pressing, the half shell 46 and the intermediate layer 10 are recessed. The dimple 12 is formed by the depression of the half shell 46.

  Next, the pressure of the press is increased. The resin composition of the half shell 46 is pressurized at a pressure higher than the pressure in the mold clamping process in the spherical cavity. This process is referred to as a high pressure process. By the high pressure process, the upper half shell 46 and the lower half shell 46 are firmly bonded. The dimple 12 in which the shape of the pimple 40 is accurately reflected is formed by the high pressure process.

  Next, the second mold 32 is cooled while the second mold 32 is tightened. By cooling, the resin composition of the half shell 46 is solidified. This resin composition constitutes the cover 6. This process is called a solidification process. After solidification, the second mold 32 is opened and the golf ball 2 is taken out.

  As described above, in the mold clamping process, the lower mold 36 approaches the upper mold 34 at a speed of 0.5 mm / sec or more and 2.0 mm / sec or less. This speed is extremely slow. Since the second mold 32 moves slowly, the air existing between the half shell 46 and the core 4 and the air existing between the half shell 46 and the cavity surface 38 are surely moved out of the spherical cavity. Discharged. In this manufacturing method, the remaining of air on the golf ball 2 is suppressed. Since the second molding die 32 moves slowly, uneven outflow of the resin composition is unlikely to occur. In this manufacturing method, uneven thickness due to uneven outflow hardly occurs. The golf ball 2 in which the remaining air and uneven thickness are suppressed is excellent in durability. From the viewpoint of durability, the speed at which the lower mold 36 approaches the upper mold 34 in the mold clamping step is more preferably 1.5 mm / sec or less, and particularly preferably 1.0 mm / sec or less.

  The compression force in the high pressure process is preferably 17652 N or more. By setting the compressive force to 17652 N or more, excessive outflow of the resin composition of the half shell 46 is suppressed, and uneven thickness is suppressed. By setting the compressive force to 17652 N or more, the outflow of the resin composition of the mid layer 10 is also suppressed. In this respect, the compressive force is more preferably 20594N or more, and particularly preferably 26478N or more. If the compression force is excessive, expensive equipment is required, so the compression force is preferably 40207 N or less. The compression force is a value obtained by dividing the maximum force applied to the second mold 32 by the press in the high pressure process by the number of spherical cavities that the second mold 32 has. The force applied to the second mold 32 by the press machine is a value obtained by multiplying the ram pressure of the press machine by the cross-sectional area of the ram.

  The time for the high pressure step is preferably 30 seconds or more and 300 seconds or less. By setting the time to 30 seconds or more, the upper and lower half shells 46 are firmly joined. In this respect, the time is more preferably equal to or greater than 70 seconds, and particularly preferably equal to or greater than 100 seconds. By setting the time to 300 seconds or less, the outflow of the resin composition of the intermediate layer 10 is suppressed. In this respect, the time is more preferably equal to or less than 260 seconds, and particularly preferably equal to or less than 230 seconds. From the viewpoint of suppressing uneven thickness, it is preferable that the pressure of the press is increased immediately after the mold clamping process.

The difference (Tf−Fc) between the molding temperature Tf and the flow start temperature Fc of the thermoplastic resin composition of the half shell 46 (that is, the cover 6) is preferably 20 ° C. or higher. By setting this difference (Tf−Fc) to 20 ° C. or more, the cover 6 is firmly attached to the core 4. In this respect, the difference (Tf−Fc) is more preferably 25 ° C. or higher. The molding temperature Tf means the highest temperature reached by the second mold 32 between the charging process and the solidifying process. The molding temperature Tf is measured at the pole of the cavity surface 38. The flow start temperature Fc is measured by “FLOWSTER CFT-500” manufactured by Shimadzu Corporation. The measurement conditions are as follows.
Plunger area: 1 cm ²
DIE LENGTH: 1mm
DIE DIA: 1mm
Load: 588.399N
Starting temperature: 30 ° C
Temperature increase rate: 3 ° C / min
Prior to measurement, the sample is kept in an environment of 70 ° C. for 8 hours and dried.

  FIG. 4 is an enlarged cross-sectional view showing the half shell 46 of FIG. The half shell 46 includes a top portion 48 and a side portion 50. In FIG. 4, what is indicated by a symbol B is a boundary between the top portion 48 and the side portion 50. As is clear from FIG. 4, the top 48 is thinnest at the center, and gradually increases from the center toward the boundary B. In FIG. 4, what is indicated by a double arrow Tt is the thickness of the top 48. The thinnest portion of the top 48 is determined, and the thickness Tt is measured at this portion. The top 48 may have a uniform thickness. The side portion 50 has a uniform thickness. In FIG. 4, what is indicated by a double arrow Ts is the thickness of the side portion 50. Such a half shell 46 can be obtained by devising the shape and dimensions of the first mold 18.

  The ratio of the thickness Tt of the top 48 to the nominal thickness T of the cover 6 is 95% or less. In other words, the top 48 is thin. In the manufacturing method according to the present invention, since the speed of the mold clamping process is slow, thermal shrinkage of the half shell 46 occurs in the mold clamping process. Due to this heat shrinkage, the volume of the resin composition in the vicinity of the top 48 increases. Since the top 48 before heating is thin, it is possible to prevent the pole thickness of the cover 6 from becoming excessive even if heat shrinkage occurs. In this respect, the ratio is more preferably equal to or less than 90%, and particularly preferably equal to or less than 85%. If this ratio is too small, bears are likely to occur. Therefore, the ratio is preferably 65% or more, more preferably 70% or more, and particularly preferably 75% or more.

  From the viewpoint that the upper and lower half shells 46 are firmly joined, the ratio of the thickness Ts of the side portion 50 to the nominal thickness T of the cover 6 is preferably 100% or more, and more preferably 105% or more. From the viewpoint of preventing uneven thickness due to excessive outflow of the resin composition, this ratio is preferably 120% or less, particularly preferably 115% or less.

  From the viewpoint of preventing uneven thickness, the central angle θ of the top portion 48 is preferably 10 ° or more, and more preferably 15 ° or more. From the viewpoint of firm joining of the half shell 46, the central angle θ is preferably 60 ° or less, and more preferably 45 ° or less.

  The ratio of the total volume of the upper and lower half shells 46 to the volume of the cover 6 is preferably 110% or more and 130% or less. By setting this ratio to 110% or more, it is difficult for air to remain. In this respect, the ratio is more preferably equal to or greater than 120%. By setting this ratio to 130% or less, uneven thickness due to excessive outflow of the resin composition is prevented. In this respect, the ratio is more preferably equal to or less than 125%.

  FIG. 5 is an enlarged cross-sectional view showing a part of the golf ball 2 of FIG. In this figure, the cover 6, the intermediate layer 10 and the center 8 are shown. The cover 6 includes dimples 12. The intermediate layer 10 includes a recessed portion 52. The dimple 12 and the recessed portion 52 are formed by the pimple 40 in the mold clamping process and the high pressure process. The presence of the recessed portion 52 ensures the thickness of the cover 6 immediately below the dimple 12. In the golf ball 2, the dimple 12 is prevented from starting a crack when repeatedly hit. This golf ball 2 is excellent in durability. From the viewpoint of durability, the depth De of the recessed portion 52 is preferably 0.05 mm or more, and particularly preferably 0.10 mm or more. The depth De of the recessed portion 52 is preferably 0.5 mm or less.

  In order to form the recessed portion 52, the intermediate layer 10 needs to be softened in the mold clamping process or the high pressure process. In this respect, the difference (Tf−Fm) between the molding temperature Tf and the flow start temperature Fm of the thermoplastic resin composition of the intermediate layer 10 is preferably 10 ° C. or higher, and more preferably 20 ° C. or higher. From the viewpoint of suppressing the outflow of the intermediate layer 10, the difference (Tf−Fm) is preferably 60 ° C. or less.

  FIG. 6 is a cross-sectional view showing a half shell 54 used in a manufacturing method according to another embodiment of the present invention. In the half shell 54, the thickness gradually increases from the apex P toward the end E. The thickness Tt of the top 56 is measured at the apex P. The thickness Ts of the side portion 58 is measured at the end portion E. Also by using this half shell 54, a golf ball in which uneven thickness is suppressed can be obtained.

  Also in this half shell 54, the ratio of the thickness Tt of the top portion 56 to the nominal thickness T of the cover is preferably 95% or less, more preferably 90% or less, and particularly preferably 85% or less. The ratio is preferably 65% or more, more preferably 70% or more, and particularly preferably 75% or more.

  Also in this half shell 54, the ratio of the thickness Ts of the side portion 58 to the nominal thickness T of the cover is preferably 100% or more, and more preferably 105% or more. This ratio is preferably 120% or less, and particularly preferably 115% or less.

  The ratio of the total volume of the two half shells 54 to the volume of the cover 6 is preferably 110% or more, and more preferably 120% or more. This ratio is preferably 130% or less, and more preferably 125% or less.

  Hereinafter, the effects of the present invention will be clarified by examples. However, the present invention should not be construed in a limited manner based on the description of the examples.

[Example 1]
100 parts by weight of polybutadiene (trade name “BR18” from JSR), 35 parts by weight of zinc acrylate, 5.0 parts by weight of zinc oxide, 13.4 parts by weight of barium sulfate, 0.8 parts by weight of diphenyl disulfide (Sumitomo Seika Chemical Co., Ltd.) and 0.5 parts by mass of dicumyl peroxide were kneaded with a closed kneader to obtain a rubber composition. This rubber composition was put into a mold composed of an upper mold and a lower mold each having a hemispherical cavity and heated at 170 ° C. for 15 minutes to obtain a center having a diameter of 38.5 mm.

  50 parts by weight of sodium neutralized binary ionomer resin (previously referred to as “HIMILAN 1605”) and 50 parts by weight of zinc neutralized ionomer resin (previously referred to as “HIMILAN AM7329”) were blended and extruded with a twin screw extruder. A resin composition was obtained. The flow start temperature Fm of this resin composition is 92.2 ° C. This resin composition was coated around the center by an injection molding method to obtain an intermediate layer. The thickness of this intermediate layer is 1.62 mm.

100 parts by mass of thermoplastic polyurethane elastomer (“Elastollan XNY97A” described above) and 4 parts by mass of titanium dioxide were blended and extruded with a twin screw extruder to obtain a resin composition. Extrusion conditions are as follows.
Temperature: 230 ° C
Screw diameter: 45mm
Screw rotation speed: 200rpm
Screw L / D: 35
The flow initiation temperature Fc of this resin composition is 115 ° C. This resin composition was cut to obtain cylindrical pellets. The pellet has a diameter of 20 mm and a mass of about 2 g. One pellet was put into each of the concave portions of the first mold, and a preform with many half shells was obtained by compression molding. This half shell has the shape shown in FIG. The conditions of the compression molding method are as follows.
Molding temperature: 170 ° C
Compression force of primary pressurization process: 7845N
Time of primary pressurization process: 2 minutes Compression force of secondary pressurization process: 26478N
Secondary pressurization process time: 3 minutes Cooling process compression force: 26478 N
Cooling process time: 5 minutes

A core composed of a center and an intermediate layer was sandwiched between two preforms, put into a second mold, and a cover was molded by a compression molding method. The conditions of the compression molding method are as follows.
Molding temperature: 140 ° C
Clamping process speed: 1.0 mm / s
Pressure of high pressure process: 26478N
High pressure process time: 150 seconds The surface of this cover was painted to obtain a golf ball. The golf ball has a diameter of 42.75 mm.

[Examples 2 to 4 and Comparative Examples 1 to 3]
A golf ball was obtained in the same manner as in Example 1 except that the shape of the half shell was as shown in Table 1 below.

[Examples 5 to 6 and Comparative Example 4]
A golf ball was obtained in the same manner as in Example 1 except that the thickness of the intermediate layer and the thickness of the half shell were as shown in Table 2 below.

[Examples 7 to 9]
A golf ball was obtained in the same manner as in Example 1 except that the conditions of the compression molding method were as shown in Table 2 below.

[Comparative Example 5]
A golf ball was obtained in the same manner as in Example 1 except that a thermoplastic polyamide elastomer (trade name “PEBAX 7233” manufactured by Toray Industries, Inc.) was used as the base polymer of the resin composition of the intermediate layer. The flow start temperature Fm of this resin composition is 165 ° C.

[Measurement of cover thickness]
The golf ball was cut along a plane passing through the upper pole and the lower pole, and an image with an enlarged cross section was obtained. In this image, the thickness T1 of the upper pole, the thickness T2 of the lower pole, the thickness T3 of one seam, and the thickness T4 of the other seam were measured with a caliper. The thicknesses T1, T2, T3, and T4 were measured directly below the land while avoiding dimples. Then, the average of the thickness T1 and the thickness T2 is calculated as the pole thickness P, the average of the thickness T3 and the thickness T4 is calculated as the seam thickness S, and the difference between the pole thickness P and the seam thickness S (PS) ) Was calculated. Further, a value obtained by subtracting the minimum value from the maximum value among the thicknesses T1, T2, T3, and T4 was calculated and used as the thickness deviation value. Average values of data obtained by measuring 24 golf balls are shown in Tables 1 and 2 below.

[Visual observation]
The appearance of 24 golf balls was visually observed, and the presence or absence of a golf ball in which a bear was generated was determined. At the same time, the presence or absence of a golf ball with air remaining inside the cover was determined. The results are shown in Tables 1 and 2 below.

[Durability test]
A driver with a metal head was attached to a swing machine manufactured by Tsurutemper. The golf ball was hit 100 times under the condition that the head speed was 45 m / sec. Six golf balls were subjected to the test, and the presence or absence of cracks in them was visually determined. The results are shown in Tables 1 and 2 below.

  As is clear from Tables 1 and 2, the golf balls obtained by the production methods of the examples have a small (PS) and excellent durability. On the other hand, in the golf balls obtained by the manufacturing methods of Comparative Examples 1 to 4, (PS) is large. Further, the golf ball obtained by the manufacturing method of Comparative Example 5 is inferior in durability. From this evaluation result, the superiority of the present invention is clear.

  The golf ball according to the present invention is particularly suitable for use in golf competitions.

FIG. 1 is a partially cutaway sectional view showing a golf ball obtained by a manufacturing method according to an embodiment of the present invention. FIG. 2 is a cross-sectional view showing a part of the first mold used for manufacturing the golf ball of FIG. FIG. 3 is a cross-sectional view showing a part of a second mold used for manufacturing the golf ball of FIG. FIG. 4 is an enlarged cross-sectional view showing the half shell of FIG. FIG. 5 is an enlarged cross-sectional view showing a part of the golf ball of FIG. FIG. 6 is a cross-sectional view showing a half shell used in a manufacturing method according to another embodiment of the present invention.

Explanation of symbols

2 ... Golf ball 4 ... Core 6 ... Cover 8 ... Center 10 ... Intermediate layer 12 ... Dimple 14 ... Land 18 ... First mold 20, 34 Upper mold 22, 36 ... Lower mold 24, 28 ... Flat part 26 ... Convex part 30 ... Concave part 32 ... Second mold 38 ... Cavity surface 40 ... Pimple 42 ... Pellets 44 ... Preliminary compacts 46,54 ... Half shells 48,56 ... Top 50,58 ... Side 52 ... Recess

Claims (5)

A core molding step in which a center is obtained by covering the center with an intermediate layer made of a thermoplastic resin composition; and
A half shell molding step in which a half shell that is made of a thermoplastic resin composition and is shaped like a bowl is molded;
The mold and the two half shells fitted to the core are opened into a mold having an upper mold and a lower mold having a hemispherical cavity surface and a large number of pimples on the surface. An input process that is input in a
By tightening the mold, the half-shell thermoplastic resin composition is heated while being pressed in the spherical cavity, and the excess thermoplastic resin composition flows out of the spherical cavity, and the intermediate layer is pressed against the pimples. And the mold clamping process
A solidification step in which the thermoplastic resin composition remaining in the spherical cavity is solidified to form a cover having a nominal thickness T of 0.1 mm or more and 0.8 mm or less,
The half shell obtained in the half shell molding process has a top thickness Tt of 65% to 95% with respect to the nominal thickness T and a side thickness Ts of 100% to 120% with respect to the nominal thickness T. Golf ball manufacturing method.   The manufacturing method according to claim 1, wherein a clamping speed in the clamping process is 0.5 mm / s or more and 2.0 mm / s or less.   The manufacturing method of Claim 1 or 2 further equipped with the high pressure process by which the resin composition of a half shell is pressurized in a spherical cavity with the pressure higher than the pressure of a mold clamping process after the said mold clamping process.   The manufacturing method according to claim 3, wherein the compression force in the high-pressure step is 17652N or more and 40207N or less.   The manufacturing method according to claim 3 or 4, wherein a time of the high-pressure step is 30 seconds or more and 300 seconds or less.

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