Short distance golf ball and production thereof.

This invention is concerned with a short distance golf ball, that is a golf ball which has performance characteristics comparable to that of a conventional golf ball except that it plays from 30%-70% shorter in distance than a conventional golf ball.

A conventional 18-hole golf course occupies approximately 180 acres (73 hectares) of land. Because of the availability and cost of land in metropolitan areas and in resort areas where golf courses are frequently built, it is desirable to be able to design a golf course which uses substantially less area but at the same time presents all of the challenges of a conventional golf course. By providing a short distance golf ball which will play approximately 50% of the distance of a conventional golf ball, the land requirements for a golf course can be reduced 67% to 50%.

In order for a golfer to realize the ordinary training and practice benefits as well as the enjoyment associated with playing a conventional ball on a conventional course, the short distance golf ball must perform in a manner substantially similar to a conventional ball except that the distance it flies must be approximately 50% shorter. In order to play comparably to a conventional ball, the short distance golf ball must be manoeuverable in play, which means that the golfer must be able to draw or fade, to hook or slice, or to hit high or low shots with the short distance ball much the same as with a conventional golf ball. In addition, the short distance ball when struck by a putter on a green must perform essentially the same as a conventional golf ball. The golfer must also be able to impart sufficient backspin to the short distance ball when properly struck by a medium or short iron to make the short distance golf ball "bite" or "hold" the green to the same degree possible with a conventional golf ball when it is struck in the same manner. The short distance golf ball must perform the same as a conventional ball in terms of flight and green holding when it is hit from sand bunkers.

If the short distance golf ball has the above listed attributes, the golfer can play the short distance golf ball on a short golf course, use all of his clubs, and achieve the same practice and training benefits as well as the enjoyment associated with playing on a conventional golf course, in about half the time on a golf course that occupies about half the acreage of a conventional golf course.

We have discovered that a short distance golf ball having the characteristics described above cannot be provided by simply reducing the resilience of a conventional golf ball so as to reduce the initial velocity of the ball off of the face of the club. Such a "dead ball", even with a special aerodynamic dimple design to maximize lift, will not produce the lift necessary to give the trajectory of a conventional ball which weighs approximately 45 grams.

We have further found that to provide a short distance golf ball which has essentially the same performance characteristics as a conventional golf ball except that its playing distance is from 30% to 70% shorter, it is necessary that the golf ball be lighter than a conventional ball so that it is easier to spin off of the face of the club and has less gravitational force to overcome. These requirements are met by a one-piece golf ball which is moulded from a thermoplastic material which is sufficiently light (that is, has a sufficiently low specific gravity) and at the same time has the resilience to fly properly and the rigidity to withstand the impact of standard golf clubs without permanent deformation.

According to the present invention, there is provided a one-piece short distance golf ball which is a moulded mixture of a foamed thermoplastic polymer and a filler material, the ball having an increasing density gradient from the centre to the surface.

The present invention also comprises a process for making a one-piece short distance golf ball, which comprises: <ul id="ul0001" list-style="none"><li>a. forming a mixture of thermoplastic resin, a chemical blowing agent, and a filler material;</li><li>b. injecting a predetermined amount of the mixture into a golf ball mould cavity;</li><li>c. holding the mixture in the mould cavity for a predetermined time;</li><li>d. opening the mould cavity and removing the golf ball therefrom; and</li><li>e. quenching the golf ball in cooled water.</li></ul>

The golf ball preferably has a cellular core structure and a dense outer skin, the latter preferably having a thickness of from 0.125 inch (3.175 mm) to 0.625 inch (15.88 mm). The thickness of the dense outer skin determines the moment of inertia of the ball which, in turn, determines how much spin can be imparted to the ball when struck by a golf club. The range of thicknesses mentioned corresponds to the preferred range of moments of inertia for the ball to have desirable playing characteristics.

The preferred filler material is microscopic glass bubbles and it is preferred to use from 0.5 to 15% of filler material, based on the weight of the thermoplastic polymer.

For the better understanding of the invention, a preferred embodiment of golf ball will now be described, by way of example, with reference to the accompanying drawings, in which: <ul id="ul0002" list-style="none"><li>Figure 1 is a plan view of a golf ball of the present invention; and</li><li>Figure 2 is a cross section of the golf ball of Figure 1 as taken along line 2-2.</li></ul>

Referring to Figure 1, the golf ball 10 is approximately 1.68 inch (42.67 mm) in diameter, the same size as a conventional golf ball, and weighs from 15 to 30 grams as compared to approximately 45 grams for a conventional golf ball. The golf ball 10 is a one-piece golf ball made from a mixture 12 of a thermoplastic material and a filler material. The golf ball 10 is formed by injection moulding the mixture together with a chemical blowing agent, as described in greater detail below.

With reference to Figure 2, the cross section of the golf ball 10 shows that the density of the ball material increases along the radius 16 of the ball from the centre 20 to the surface 14. The ball 10 has an outer dense skin 22 of from 0.125 inch to 0.625 inch (3.175 to 15.88 mm) thickness. The skin thickness, which is inversely related to the amount of blowing agent used in making the golf ball, is most preferably approximately 0.250 inch (6.35 mm). The core 24 of the ball 10 has a blown cellular structure. The boundary 26 between the cellular core 24 and the skin 22, while not sharply delineated, is observable from an inspection of a cross sectioned sample of a golf ball made in accordance with the present invention.

The ball 10 has dimples 28 which are formed during the injection moulding process by the mould pattern. The dimples 28 are formed at the surface 14 in any conventional dimple pattern that can be used on a conventional golf ball. The golf ball is finished in the conventional manner by painting the ball white, yellow or orange, or any other suitable colour.

The weight and density distribution of the material of the ball ensures that the golf ball will perform in most respects in the same way as a conventional golf ball except that its playing distance will be shorter. The short distance golf ball of the present invention has a rebound of from 50% to 80%, and preferably 67%, and it has a compression of from 0 to 100, preferably 10, as measured on the Atti compression tester.

The preferred material 12 from which the ball is made comprises a thermoplastic polymer and microscopic glass bubbles distributed uniformly throughout the polymer. The thermoplastic polymer is preferably the product of the reaction of an olefin and metallic salt of an unsaturated monocarboxylic acid. Suitable ionomer resins for producing such thermoplastic polymers are sold by the Dupont Company, Polymer Products Department, Ethylene Polymers Division, Wilmington, Delaware 19898, under the trademark SURLYN. The Surlyn resin is available both as a zinc ionic copolymer and as a sodium ionic copolymer. It has been found that each copolymer is useful in carrying out the present invention and that mixtures of the two copolymers are also useful in carrying out the present invention.

The thermoplastic material, as previously stated, includes microscopic glass bubbles which serve as filler or extender. In the finished product the glass bubbles are distributed uniformly throughout the thermoplastic polymer. Preferred glass bubbles for carrying out the present invention are manufactured by the 3M Company, St. Paul, Minnesota 55101 and range in density from 0.12 to 0.18 g/cc. Other inorganic fillers, such as titanium dioxide or calcium carbonate, can also be used. Glass bubbles are, however, preferred because they improve impact resistance by functioning as microscopic shock absorbers to dissipate energy and thus help to reduce the distance the ball travels when struck.

Suitable blowing agents for use in the process according to the invention are, for example, Freon, nitrogen gas, and carbon dioxide; these may, in particular, be used with Surlyn. The blowing agent used should preferably have a decomposition temperature of from 230° to 435⁰F (110° to 224°C). Two preferred chemical blowing agents are sold under the trade designation Celogen TSH and Celogen RA by Uniroyal Chemical, Naugatuck, Connecticut 06770. Norteck brand foam concentrate sold by Northern Petro Chemical Company, Clinton, Massachusetts 01510 also works well.

While the process can be operated over a wide range of blowing agent decomposition temperatures, higher decomposition temperatures are preferable because the risk of premature expansion can be minimized. Thus decomposition temperatures of from 350° to 450OF (177° to 232⁰C) are preferred.

The following formulations have been found to produce acceptable golf balls which have approximately 0.250 inch (6.35 mm) thick skin and which have the physical and performance characteristics desired for short distance golf balls.

Example 1

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Example 2

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Example 3

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Example 4

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Example 5

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Example 6

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Example 7

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Example 8

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Example 9

<tables id="tabl0009" num="0009"><img file="EP0154735A2_D0009.tif" /></tables>The thickness of the skin of the finished golf ball is inversely proportional to the amount of blowing agent. For example, reducing the amount of Celogen TSH to 0.50 parts will produce a skin thickness of approximately 0.5 inch (12.7 mm) in the finished golf ball. Therefore a range of 0.50 to 2.00 parts of blowing agents should produce skin thickness of from 0.5 to 0.125 inch (12.7 to 3.88 mm) respectively.

In order to form the short distance golf ball of the present invention, the ingredients specified are first mixed together prior to injection moulding.

A conventional screw injection machine as used to manufacture conventional two-piece moulded golf balls is suitably modified for foam moulding as follows. The injection nozzle is provided with a shut-off valve to ensure that only a predetermined amount of the mixture is injected into each mould cavity. In particular, it is preferred that only about 15 to 30 grams of the mixture per golf ball be injected. The injection machine must generate sufficient injection pressure to be able to inject the material into the mould cavity in one second or less to minimize premature gas expansion. Also flow channels must be kept short and provide equidistance flow to the extremities of the cavity to achieve uniform skin thickness for each ball moulded.

In order to ensure that the resulting short distance golf ball has the proper skin thickness, it is important that the process parameters be controlled. The initial temperature of the mixture is room temperature. The mould cavity is chilled by 40°F (4.4°C) water to approximately 40° to 70⁰F (4.4 to 21.1°C). The injection cylinder is provided with a temperature gradient along its length to the nozzle. The rear part of the cylinder is kept at a lower temperature (preferably about 325°F; 163⁰C) to reduce premature gas expansion, and the nozzle is maintained at a higher temperature (preferably about 400°F; 204°C) to make rapid injection easier by reducing viscosity of the mixture. The mould is then held closed (elapsed time) for from 60 to 240 seconds (depending on the skin thickness) while maintaining the mould temperature at approximately 40° to 70⁰F (4.4 to 21.1⁰C). The process requires about 60 seconds per 0.125 inch (3.18 mm) of skin thickness to ensure that the skin is fully moulded before the mould is opened. After the specified time has elapsed, the mould is opened, the ball is removed and immediately quenched in cold water to curtail any further blowing.

By mixing the requisite amount of blowing agent and regulating the process within the parameters specified above, the density of the thermoplastic material which forms the short distance golf ball will have the desired skin thickness to provide the performance characteristics required.

We have achieved best results with the following process parameters for golf balls having a skin thickness of approximately 0.250 inch (6.35 mm) and manufactured from the mixtures specified in Examples 1 to 9^:<tables id="tabl0010" num="0010"><img file="EP0154735A2_D0010.tif" /></tables>