FIELD OF THE INVENTION

Molds and methods for making golf balls, especially those having a thermoplastic polyurethane cover.

BACKGROUND OF THE INVENTION

The present invention relates to molds and methods for making golf balls, especially those including a thermoplastic polyurethane (TPU) cover since golf balls incorporating at least one thin TPU cover layer possess and produce many advantageous physical and playing performance properties and characteristics.

Both professional and amateur golfers alike generally use multi-piece, solid golf balls today. Typically, an outermost cover layer surrounds a golf ball subassembly which may be comprised of any number and combination of layers such as an inner core, outer core layer(s), intermediate layer(s), and/or inner cover layer(s).

An example of a common two-piece golf ball construction is a solid inner core, protected and surrounded by a cover. Three-piece, four-piece, and even five-piece balls have become more popular over the years, due to new manufacturing technologies, lower material costs, and desirable ball playing performance properties. In this regard the core itself can be multi-layered—for example, in a “dual-core” construction, the inner core can be made of a relatively soft and resilient material, while the surrounding outer core layer is made of a harder and more rigid material.

Meanwhile, specific materials are selected for a given golf ball layer and/or construction based on the desired resulting overall golf ball properties and playing features. For example, the inner core is often made of a natural or synthetic rubber such as polybutadiene, styrene butadiene, or polyisoprene, while outer layers incorporate ethylene acid copolymer ionomers, polyamides, polyesters, polyurethanes, and polyureas.

Ionomer compositions comprising an ethylene acid copolymer containing acid groups that are at least partially neutralized are commonly used to make golf ball covers. Suitable ethylene acid copolymers that may be used to form the cover layers are generally referred to as copolymers of ethylene; C3to C8α, β-ethylenically unsaturated mono- or dicarboxylic acid; and optional softening monomer. Commercially available ionomer compositions that can be used to make such covers include Surlyn® (DuPont) and Escor® and Iotek® (Exxon) ionomers. In recent years, there has also been high interest in using polyurethane compositions to make golf ball covers. Basically, polyurethane compositions contain urethane linkages formed by reacting an isocyanate group (—N═C═O) with a hydroxyl group (OH). Polyurethanes are produced by the reaction of a multi-functional isocyanate with a polyol in the presence of a catalyst and other additives. The chain length of the polyurethane prepolymer is extended by reacting it with hydroxyl-terminated and amine curing agents.

Different molding operations can be used to form the cover over the core or sub-assembly of the ball. For example, compression-molding, casting, and injection-molding processes can be use. These molding processes normally use molds having an upper mold cavity and lower mold cavity. Each mold cavity is hemispherical-shaped and one-half of the size of a finished ball. The mold cavities have interior walls with details defining the dimple pattern of the cover that will be produced. The upper and lower mold cavities are joined together under sufficient heat and pressure. The polyurethane material in the cavities encapsulates the ball subassembly and forms the cover of the ball.

Compression-molding typically involves using multiple pairs of mold cavities, each pair comprising first and second mold cavities that mate to form a spherical recess. In one exemplary compression molding process, a cover material is pre-formed into half-shells, which are placed, respectively, into each of a pair of compression mold cavities. The core is placed between the cover material half-shells and the mold is closed. The core and cover combination is then exposed to heat and pressure, which cause the cover half-shells to combine and form a full cover.

Casting processes also typically use pairs of mold cavities. In a casting process, a cover material is introduced into a first mold cavity of each pair. A core is then either placed directly into the cover material or is held in position (e.g., by an overhanging vacuum or suction apparatus) to contact the cover material in what will be the spherical center of the mold cavity pair. Once the cover material is at least partially cured (e.g., to a point where the core will not substantially move), the cover material is introduced into a second mold cavity of each pair, and the mold is closed. The closed mold is then subjected to heat and pressure to cure the cover material thereby forming a cover on the core. Casting is a common method used for producing a thermoset polyurethane cover layer on a golf ball. However, the thermoset polyurethane materials typically used in casting require a relatively long gel time. Long gel times have the disadvantage of requiring long cure times for the material to set so that the ball can be demolded, or removed from the mold. Additionally, once demolded, cast golf balls usually require subsequent buffing and other finishing process steps.

Injection molding is a conventional method for forming thermoplastic polyurethane covers. Injection molding generally utilizes a mold and an injection unit. The lower mold cavity fits into a lower mold plate (frame) and defines a hemispherical molding cavity for receiving the core or ball sub-assembly. The plate defines a runner system for transporting the molten, polyurethane cover material to one or more gates that allow the material to enter the cavity from the runner system.

In one example of an injection-molding process, each mold cavity may also include retractable positioning pins to hold the core in the spherical center of the mold. Once the core is positioned in the first mold cavity, the respective second mold cavity is mated to the first to close the mold. A cover material is then injected into the closed mold. The positioning pins are retracted while the cover material is still flowable in order to allow the material to fill in any holes caused by the pins once the retractable pins are withdrawn. Thus, once the material being molded is at least partially cured, the covered core is removed from the mold. Different molds and molding systems have been used in the past to form golf ball covers, and these systems have been general effective. For example, Puniello et al., U.S. Pat. Nos. 7,223,085; 7,135,138; 6,877,974; and 6,235,230 describe different molding systems.

One drawback with using conventional molds and molding systems is that it can be difficult to ventilate fast enough the large volume of air/gases that can be produced/present/trapped within the mold. For example, molding thin cover TPU (thermoplastic polyurethane) golf balls and other thin multi-layer golf ball constructions require ultra-fast cavity filling (˜200 to 400 ms) during injection molding in order to prevent flow front freezing before the entire cover geometry is formed and packed. This faster fill, combined with the very steep viscosity vs. shear rate characteristics of TPU in a thin dimpled cover molding, makes cavity/mold balancing and concentric flow front very difficult.

In fact, cavity/mold balancing and flow front variation can be significant even with slight changes in melt temperature and/or regrind and/or fill profile. Additionally, the location of cavities and runner systems introduces variation in shear—which, in turn, causes flow imbalances. These conditions can result in the production of tiny unfills, flow marks, and/or other defects because gases are not released fast enough or the vent path is not located at the point of gas trap.

Accordingly, there remains a need for new, cost-effective, efficient molding methods and molds wherein air and other gasses collecting within the mold at points of gas trap during molding can be exhausted/eliminated/removed quickly, without meanwhile negatively impacting desired golf ball durability physical properties, and performance characteristics. The molds and molding methods of the present invention address and solve these needs and are particularly suited for ventilating air and other gasses while molding thermoplastic polyurethane covers about golf ball subassemblies.

SUMMARY OF THE INVENTION

In one embodiment, the method of the invention for molding a golf ball comprises providing a mold assembly comprising a mold having an upper mold cavity and a lower mold cavity; wherein each mold cavity has an arcuate inner surface defining an inverted dimple and fret pattern and collectively create a mold having an interior spherical cavity for holding a golf ball subassembly when mated together. A portion of the inverted dimple and fret pattern that is located within a pole region of each mold cavity comprises at least four mold cavity vents, each vent extending through an entire thickness of the mold and is adapted to eliminate gases that collect within the interior spherical cavity during molding.

The method may further comprise loading the golf ball subassembly into the interior spherical cavity of the mold; securing the subassembly within the mold using a plurality of retractable pins; injecting a polymeric material into the interior spherical cavity to mold a cover layer about the golf ball sub-assembly within the mold and form a molded golf ball; withdrawing the retractable pins before molding is finished; then separating the upper mold cavity and lower mold cavity and removing the molded golf ball from the mold.

In one embodiment, at least some of the mold cavity vents are disposed within the inverted dimples of the mated upper and lower mold cavities. In another embodiment, at least some of the mold cavity vents are disposed within frets of the mated upper and lower mold cavities.

In a specific embodiment, at least four stationary flow-through pins are located in each pole region. Each stationary flow-through pin comprises an elongated body housing a primary vent section, a hollow channel, and an exhaust aperture. The primary vent section is in fluid communication with each of the interior spherical cavity, the hollow channel and the exhaust aperture during molding; wherein fluid communication between the interior spherical cavity and the primary vent section occurs through a mold cavity vent; such that air/gas produced in the interior spherical cavity during molding is drawn from the interior spherical cavity through each mold cavity vent, into each primary vent section, through each hollow channel, and out each exhaust aperture when sufficient suction is provided thereto from outside the mold. In this stationary flow-through pin construction, gas(es) may be drawn from the primary vent section into the hollow channel through an inlet groove located within the primary vent section.

The primary vent section may be a channel that extends circumferentially about the perimeter of the stationary flow-through pin adjacent to a top head of the stationary flow-through pin. In some embodiments, the stationary flow-through pin may have a secondary vent section that comprises a channel that extends circumferentially about the perimeter of the stationary flow-through pin, is parallel to the primary vent section, and is located in a region of the stationary flow-through pin that is closer to the exhaust aperture than to the top head of the stationary flow-through pin and is not in fluid communication with any of the primary vent section, the hollow channel, and/or the exhaust aperture.

The elongated body of each stationary flow-through pin may have a smaller outer diameter at locations of the primary vent section and the secondary vent than its diameter there between. The primary vent section may have a channel depth of from about 0.0001 inches to about 0.002 inches.

In a particular embodiment, each primary vent section and any secondary vent section is an elliptical-shaped channel.

The retractable pin may have a free-end planar surface and may be movable between an extended position, wherein the free end surface contacts the ball subassembly and a retracted position, wherein the planar surface forms a portion of the inner wall of the inner surface of the mold cavity. The free-end planar surface of the retractable pin has a diameter that is less than the diameter of a bore in the mold cavity for inserting the pin.

The mold may further include one stationary center venting pin. Suction may occur between each retractable pin and the stationary center venting pin. Two or more stationary flow through pins may comprise inner venting pins, each which surrounds the stationary center pin. Two or more of the stationary flow-through pins may comprise outer venting pins, each which surrounds the inner venting pins.

In a different embodiment, each of the plurality of mold cavity vents further extends longitudinally between stationary pins from a proximal end thereof, located within a dimple and/or fret of the pole region, to a distal end thereof, located proximate to at least one channel portion of the mold. Each mold cavity vent is in fluid communication with the interior spherical cavity as well with as each of the least one channel portion and an entire length of an exterior surface of at least one retractable pin; such that the air/gas produced in the interior spherical cavity during molding is drawn from the interior spherical cavity, into and through each mold cavity vent, and then transfers within each channel portion and along the outer surfaces of retractable pins. The air/gas is then exhausted from the mold and out into the atmosphere when sufficient suction/vacuum is provided/applied at the distal end of the retractable pins.

In one particular embodiment, the polymeric material may be a thermoplastic polyurethane composition. In another particular embodiment, the polymeric material may be a thermoplastic ethylene acid copolymer ionomer composition.

In one embodiment, the golf ball subassembly may comprise a core formed from a polybutadiene rubber composition. In another embodiment, the golf ball subassembly may comprise a core formed from a polybutadiene rubber composition, and an intermediate layer formed from an ethylene acid copolymer ionomer composition.

The invention meanwhile also relates to the mold used in connection with the method of the invention for forming a golf ball cover about a subassembly as described herein and depicted in the accompanying figures.

In different embodiments, the invention provides a method for molding a golf ball, comprising the steps of: a) providing a mold having a lower mold cavity and upper mold cavity, each mold cavity having an arcuate inner surface defining an inverted dimple pattern; so that when the upper and lower mold cavities are mated together, they define a mold having an interior spherical cavity for holding a golf ball subassembly; b) loading the golf ball subassembly into the interior spherical cavity of the mold, wherein the mold further includes two or more retractable pins for holding the golf ball within the spherical cavity, each retractable pin having a primary vent section, secondary vent section, and tertiary vent section, the primary vent being in fluid connection with the secondary vent and the secondary vent being in fluid connection with the tertiary vent for removing gasses from the interior spherical cavity; c) injecting a polymeric material into the spherical cavity to form a spherical cover over the golf ball sub-assembly; and d) detaching the lower and upper mold cavities and removing the molded golf ball from the mold.

Each retractable pin preferably has a free-end planar surface and the pin is movable between an extended position, wherein the free end surface contacts the ball subassembly and a retracted position wherein the planar surface forms a portion of the inner wall of the inner surface of the mold cavity. The mold may contain other pins in addition to the retractable pins. For example, the mold may contain a set of high venting inner pins; high venting outer pins; and a stationary high venting center pin.

In one embodiment, the retractable pin has an upper region and the primary vent is a channel defined along a side of the upper region. In another embodiment, the retractable pin has an upper region and the primary vent is a non-channel and defined along the flat side of the upper region. The upper region of the pin has a diameter that is less than the diameter of a bore in the mold cavity for inserting the pin. This primary vent can be referred to as a primary ring vent. A small circular gap is created between the upper region of the pin and inner surface of the mold cavity for ventilating the trapped air and other gasses.

In a preferred embodiment, the secondary vent is an elliptical-shaped channel that is positioned below the primary vent and extends around the perimeter of the retractable pin. This secondary vents acts as an elliptical air reservoir for removing large volumes of gas quickly. The gasses enter the secondary vent and flow-through the elliptical-shaped channel and around the perimeter of the retractable pin. In one embodiment, the tertiary vent is a channel that is positioned below the secondary vent and is defined along a side of the retractable pin.

Preferably, the polymeric material used for making the cover is a thermoplastic polyurethane composition. In another example, the polymeric material is thermoplastic ethylene acid copolymer ionomer composition. In one example, the ball subassembly comprises a core formed from a polybutadiene rubber composition. In another example, the ball subassembly comprises a core formed from a polybutadiene rubber composition, and an intermediate layer formed from an ethylene acid copolymer ionomer composition.

The present invention also includes molds for forming a golf ball cover, the mold comprising: i) a lower hemispherical-shaped mold cavity; and ii) an upper hemispherical-shaped mold cavity; wherein each mold cavity has an arcuate inner surface defining an inverted dimple pattern; so that when the upper and lower mold cavities are mated together, they define a mold having an interior spherical cavity for holding a golf ball subassembly; and each mold cavity comprises at least one retractable pin for holding the golf ball within the spherical cavity, each retractable pin having a primary vent section, secondary vent section, and tertiary vent section, the primary vent being in fluid connection with the secondary vent and the secondary vent being in fluid connection with the tertiary vent for removing gasses from the interior spherical cavity. The primary, secondary, and tertiary vents can have the structures as described above.

