This invention relates to a dimpled multi-piece solid golf ball comprising an elastic solid core and a resin cover of a plurality of layers enclosing the solid core, and more particularly, to a dimpled multi-piece solid golf ball which ensures consistent flight and has improved controllability in that it suppresses substantial reduction of spin when hit in the wet state with a short iron. As used herein, the term xe2x80x9cwet statexe2x80x9d refers to the state of a golf course in rain weather, and the term xe2x80x9cdry statexe2x80x9d refers to the state of a golf course in fine weather.
One of known solid golf balls has the structure in which a rubbery elastic solid core is enclosed with a cover of relatively hard ionomer resin characterized by good external damage prevention such as cut resistance and abrasion resistance. A multiplicity of dimples are arranged on the outer surface of the cover so as to achieve a uniform distribution.
Molds used to mold the cover about the core are generally of the two-division type, that is, constructed from a pair of mold halves which are mated along a parting plane to define a spherical cavity. The parting plane is located in register with the equator of the spherical cavity. Therefore, for the convenience of mold manufacture, a mold in which the placement of dimples (exactly stated, dimple-forming protrusions) at positions that lie in the parting plane is avoided is generally used. This destroys the uniformity of dimple arrangement about the overall ball including the proximity of the equator, even if the uniform arrangement of dimples is accomplished on each of the hemispheres divided along the equator.
On account of such a non-uniform arrangement or non-uniform dispersion of dimples, the aerodynamic effect of dimples exerted when the ball is hit differs with the spin direction, that is, between the case where the ball spins about an axis passing the center of the ball and parallel to the equator plane and the case where the ball spins about an axis passing the opposite poles of the ball and perpendicular to the equator plane. This causes variations in the flight direction and distance of the ball when hit with a club which is designed to gain distance, as typified by a driver. The ball lacks flight consistency.
In connection with the material aspect and physical properties of the cover, the golf ball has the following problem. The ball performs well in the dry state or fine weather in that it travels a satisfactory distance when hit with a driver and receives a requisite spin when hit with an iron which demands controllability to the ball. In the wet state or rain weather, however, the ball becomes less susceptible to spin and therefore, becomes less controllable when hit with an iron club. In particular, the spin susceptibility of the ball when hit with a short iron having a loft of an 8-iron or greater is degraded. As a result, the ball will travel a longer distance than intended or will not stop immediately on the green.
In these regards, the prior art golf balls are not fully satisfactory to professional and low-handicap golfers. It is desired to overcome the above problems.
Therefore, an object of the invention is to provide a dimpled multi-piece solid golf ball comprising an elastic solid core and a resin cover of a plurality of layers, which ball is improved for short iron shots in that the percent retention of the spin the ball receives when hit with the short iron in the wet state from the spin the ball receives when hit with the short iron in the dry state is high and which ball exhibits flight consistency when hit with a club designed to gain distance, as typified by a driver.
According to the invention, there is provided a multi-piece solid golf ball comprising an elastic solid core and a resin cover enclosing the solid core, the cover being composed of a plurality of layers including an inner layer disposed adjacent to the solid core and an outer layer disposed radially outside the inner layer. The solid core has a hardness corresponding to a deflection of at least 1.1 mm under an applied load of 294 N (30 kgf), the cover inner layer has a Shore D hardness of 45 to 70, and the cover outer layer has a Shore D hardness of 35 to 55. The ball is provided on its surface with a multiplicity of dimples which are substantially uniformly arranged on the ball surface such that a great circle which does not intersect with the dimples is absent.
In a preferred embodiment, provided that the ball receives a spin rate S1 (rpm) in the dry state and a spin rate S2 (rpm) in the wet state when hit with a short iron having a loft of an 8-iron or greater, the percent spin retention given by (S2/S1)xc3x97100 is at least 47%.
Preferably the cover outer layer is softer than the cover inner layer. Also preferably, the cover inner layer has a Shore D hardness A and the cover outer layer has a Shore D hardness B, and A and B satisfy the relationship: Axc3x97Bxe2x89xa72300. Preferably, the cover outer layer is formed mainly of a thermoplastic polyurethane elastomer, thermosetting polyurethane elastomer, polyester elastomer or a mixture thereof. More preferably, the cover outer layer is formed mainly of a thermoplastic polyurethane elastomer obtained using an aromatic or aliphatic diisocyanate, or the reaction product of a thermoplastic polyurethane elastomer with an isocyanate compound. Also preferably, the cover inner layer is formed mainly of an ionomer resin. An adhesive layer may intervene between the cover inner and outer layers.
Preferably, the core has a specific gravity of 1.0 to 1.3, the cover inner layer has a specific gravity of 0.8 to 1.4, and the cover outer layer has a specific gravity of 0.9 to 1.3.
The invention ensures the manufacture of a seamless golf ball on the surface of which dimples are substantially uniformly arranged so that a great circle which does not intersect with the dimples is absent. The flight distance is consistent whether the ball is hit with a driver or an iron. The spin susceptibility of the ball when hit with a short iron having a loft of an 8-iron or greater is not reduced in the dry state or fine weather, nor is noticeably reduced even in the wet state or rain weather. Then the ball travels a distance as intended, immediately stops on the green, and is easy to control. Since the ball offers stable ready-to-strike conditions, flight performance and spin performance under any conditions covering from the fairway (driver) to the putting green (iron), it is a high-performance solid golf ball suited for professional and amateur low-handicap golfers to play with.