Games using tangible projectile – Golf – Ball
Reexamination Certificate
1999-03-05
2001-05-29
Graham, Mark S. (Department: 3711)
Games using tangible projectile
Golf
Ball
Reexamination Certificate
active
06238306
ABSTRACT:
This invention relates to thread-wound golf ball and a method for preparing the same.
BACKGROUND OF THE INVENTION
In the prior art, thread-wound golf balls are prepared by forming a cover around a wound core by a compression molding technique. The compression molding technique uses a molding press having for example 40 to 60 cavities. Owing to variances of temperature and pressure at the molding press surface and variances of working precision of cavities, wound golf balls as finished by the compression molding technique have large variations of the outer diameter and roundness as compared with injection molded solid golf balls. As compared with the injection mold, the molding press used in the compression molding technique must be provided with a greater number of cavities and thus requires a greater investment, which becomes a bar against cost-effective manufacture.
For wound golf balls, the injection molding of the cover as in the case of solid golf balls is desired in order to improve the productivity and stabilize the quality of wound golf balls. Various attempts were made to this end.
(1) For example, JP-A 47873/1980 and JP-A 115270/1982 describe golf balls prepared by impregnating a wound core with a latex containing 30 to 60% by weight of ionomer resin solids for causing the latex solids to penetrate into the network structure of the thread rubber layer for thereby integrally joining the cover to the thread rubber layer.
(2) JP-A 112618/1986 discloses the preparation of a golf ball by wrapping a pair of half-caps preformed from thermoplastic film over a wound core, compression molding the caps to form a protective layer, and injection molding a cover over the protective layer. AS to the compression molding of thermoplastic film, it is described to heat at a temperature at least 10° C. higher than the softening point of the caps and lower than 105° C.and to clamp the mold under a force of 100 to 1,000 kg for ½ to 3 minutes, preferably within one minute.
(3) JP-A 112619/1986 discloses a method for preparing a golf ball involving the consecutive steps of wrapping a wound core with a resilient thermoplastic film, causing heat shrinkage of the film to form a protective layer of substantially uniform gage closely bonded to the wound core, and injecting molding a cover over the surface of the protective layer.
(4) JP-A 109879/1988 discloses a golf ball which is prepared by wrapping a wound core with a thermoplastic resin film of 50 to 300 &mgr;m thick showing an area shrinkage factor of less than 10% when heated at a temperature of 100 to 180° C., and injection molding the same material as the film as a cover stock to enclose the wound core with the cover stock. The film is fused by the thermal inertia of the cover stock to thereby integrate the film with the cover stock.
Of these attempts of injection molding the cover of wound golf balls, the method of impregnating the thread rubber layer with the ionomer latex and the method of wrapping the wound core with the thermoplastic film eliminate the possibility that the heat applied during injection molding can degrade the thread rubber or cause the thread rubber to be ruptured and exposed at the cover surface, but have the drawback that the durability comparable to compression molding is not achievable because it is difficult to infiltrate the cover material deeply into interstices of the thread rubber layer for enhancing the bond of the cover to the thread rubber layer.
The injection molding method (2) of JP-A 112618/1986 fails to cause the half-cap material to deeply penetrate the thread rubber layer because the temperature and time of compression molding of the half-caps onto the wound core is low and short, respectively. No firm bond is established between the cover and the wound core.
SUMMARY OF THE INVENTION
Therefore, an object of the present invention is to provide a wound golf ball of quality wherein the bond of the cover to the wound core is improved so that the ball is fully durable against repetitive shots. Another object of the present invention is to provide a highly productive method for preparing such wound golf balls to consistent quality.
The inventors have found that crucial points in improving the durability of wound golf balls against repetitive shots are to protect thread rubber from the heat applied during molding of the cover, and to fully infiltrate the cover material into interstices of the thread rubber layer to enhance the joint of the cover to the thread rubber layer. For improving the cut resistance against iron topping, it is necessary to make the cover harder.
In a first aspect, the present invention provides a thread-wound golf ball comprising a wound core composed of a center and a layer of thread rubber wound onto the center, and a cover enclosing the core. The cover has a two layer structure consisting essentially of an inner layer and an outer layer having a lower hardness than the cover inner layer. The cover inner layer is formed by compression molding, and the cover outer layer is formed by injection molding. The cover inner layer and the cover outer layer have a total thickness of 1.0 to 2.5 mm. The cover inner layer penetrates to the thread rubber layer to a depth of 1.0 to 3.0 mm from the surface of the thread rubber layer. In preferred embodiments, the thread rubber layer has a density of 0.65 to 0.9 g/cm
3
; the cover inner layer has a thickness of 0.2 to 1.0 mm and a Shore D hardness of 50 to 66,and the cover outer layer has a thickness of 0.8 to 2.0 mm and a Shore D hardness of 40 to 60; the center is a solid center having a diameter of 30 to 36 mm or a liquid center having a diameter of 26 to 32 mm.
The choice of these parameters enables the cover inner layer to be formed by compression molding without the risk of degrading the thread rubber. This also permits the cover inner layer material to evenly and fully penetrate into interstices of the thread rubber layer so that the bond of the cover inner layer to the thread rubber layer is significantly improved, resulting in an outstanding improvement in durability against repetitive shots. Since the cover inner layer plays the role of protecting the thread rubber, the cover outer layer can be formed by an injection molding technique featuring greater molding efficiency and productivity than by compression molding. As a consequence, there are obtained wound golf balls of quality having improved cut resistance, flight performance and ease of control.
In a second aspect, the invention provides a method for preparing a thread-wound golf ball comprising a wound core composed of a center and a layer of thread rubber wound onto the center, and a cover enclosing the core and consisting essentially of an inner layer and an outer layer, the method comprising the steps of encasing the wound core in a pair of half-cups preformed from a cover inner layer material, placing the assembly in one cavity of a molding press, and effecting compression molding to form the cover inner layer around the wound core; and placing the wound core with the cover inner layer in a cavity of an injection mold, and injecting a cover outer layer material into the cavity to form the cover outer layer on the cover inner layer. Preferably, each of the cover inner layer material and the cover outer layer material is based on an ionomer resin, and the compression molding of the cover inner layer is effected at a temperature of 125 to 160° C. for 2 to 5 minutes.
According to the method of the invention, when the cover inner layer is molded over the wound core, a conventional molding press may be used without a need for precision adjustment. The wound core enclosed with the cover inner layer at this point has greater variations of diameter and roundness than injection molded solid golf balls. Since the cover inner layer protects the thread rubber from heat and pressure, the cover outer layer can be molded over the cover inner layer by a conventional injection molding technique. Then wound golf balls of consistent quality having a minimal variation of diameter and a high
Bridgestone Sports Co. Ltd.
Gorden Raeann
Graham Mark S.
Sughrue Mion Zinn Macpeak & Seas, PLLC
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