Abstract:
Mold and method of compression molding shell half covers around a core to form golf balls. Opposed dies are pressed together such that some of the material trapped between the approaching dies is forced back into the mold to compensate for shrinkage and the remainder is forced outside of the die cavities where it can be drained off.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to the manufacture of two-piece golf balls. 
     The manufacture of two-piece golf balls, i.e., golf balls with only a solid core ball and a dimpled outer shell, has normally been done in an injection molding process. In this process, the solid core is first placed inside of a mold cavity, a set of pins then pushes upward through the bottom of the cavity to raise the core to a central position, and a molten material is then forced into the cavity and around the core to form the outer shell. 
     Although this process makes well-formed balls, it is a difficult one to control and requires skilled operators. Also, it requires expensive tooling and expensive maintenance to the tooling. Ideally, it would be preferred if a less expensive process could be found. 
     While manufacturers have had to use injection molding to form the dimpled shell on two-piece balls, they have been able to use a much less expensive process of compression molding for forming a similar shell on three-piece balls, i.e., the traditional golf ball with a small rubber core ball surrounded by elastic windings and then the outer shell. Obviously, the manufacturers would prefer to use compression molding in forming twopiece balls because the equipment and tooling are already available in their plants. Further, the general process of producing the outer shell would be less expensive because it would be easier to control and would therefore not require expensive, highly skilled operators. 
     Unfortunately, attempts to compression mold perfectly shaped, two-piece balls have failed because, unlike the wound centers of three-piece balls, the solid cores of two-piece balls are relatively incompressible. In three-piece balls the wound core has small voids between the latticework of elastic windings. During compression molding of the cover for a three-piece ball, the wound core is able to be compressed into a smaller volume because the elastic windings fill these voids. Since the windings seek to return to their original positions, this creates an internal &#34;back pressure&#34; within the molding cavity. This internal &#34;back pressure&#34; presses the cover stock for the shell firmly against the inside walls of the mold and results in the final product being an exact replica of the dimpled mold cavity. Conversely, since the solid core ball is relatively incompressible, it creates no appreciable internal &#34;back pressure&#34; during the compression molding of two-piece balls. On the contrary, the solid core material expands upon heating, and displaces cover stock from inside the mold. During the cooling cycle, the contents of the mold contract and pull away from the mold&#39;s sidewalls. Since there is no internal &#34;back pressure&#34;, this pulling away causes distortion of the ball&#39;s surface and makes the product unacceptable. 
     In 1902, Eleazer Kempshall attempted to solve this problem by using telescopically mating dies to compression mold a dimpled cover for two-piece balls. As described in his U.S. Pat. No. 695,867, Kempshall&#39;s process of compression molding includes a pair of male and female dies. As shown in FIGS. 5-7 of the Kempshall patent, the female die has a rim or step C on which a small amount of the cover material extrudes when the dies are initially pushed together to form the cover. The overflow material is trapped between the approaching edges B and C of the two dies. Continued axial pressure on the dies causes the edges B and C to move toward one another and force the trapped material to flow back inside the mold cavity to create an integral dimpled cover for the ball. 
     While the Kempshall attempt was noteworthy, it has its drawbacks. As explained in the patent, the produced cover is not uniform because it has a welt at its equator. This welt prevents the ball from being consistently hit the same distance with any single golf club by an accomplished golfer. Since predictability in the length of a &#34;shot&#34; is important for a low score in golf, this lack of consistency and the welt which causes it are highly undesirable. 
     The inconsistency occurs when the &#34;strike&#34; of the golfer&#39;s club head hits against the welt. Because this portion of the cover is fatter than the rest, the strike creates less compression of the ball than it would if any other portion of the cover were struck. This lesser compression causes the ball to go a shorter distance than it would normally travel. 
     In addition to creating a welt, the compression molding process of Kempshall allows the size of the welt to vary from ball to ball. Since the volume of the solid core for two-piece balls can vary substantially from one core to the next, it is difficult to precisely control the amount of cover stock extruded from the mold cavity during mating of the dies. With the Kempshall mold, if a large core were encountered, an excessive amount of cover stock would be extruded and the same large amount would be forced back into the mold cavity with the result being an even larger deformation or welt at the center of the finished ball cover. Also, wear would occur on the approaching edges B and C of the dies because of the extra pressure placed on the edges by the increased bulk between them. 
     Accordingly, a need exists for an improved set of compression dies by which the deficiencies of the Kempshall dies can be overcome so that a consistently well-shaped dimpled cover for two-piece golf balls can be formed by compression molding. 
     It is therefore the primary object of the present invention to provide an improved set of dies by which well-formed dimpled covers can be compression molded for two-piece golf balls. 
     It is another primary object to provide a new mold for compression molding the cover onto two-piece golf balls wherein the mold has a unique valving system for preventing or minimizing the welt formed in golf balls manufactured by the Kempshall method. 
     It is a more specific object to provide an improved set of telescopic dies for compression molding two-piece golf balls in which the &#34;Kempshall&#34; welt is prevented by draining off excess extrusion during mating of the dies so that the amount of extruded material between them that is forced back into the mold cavity is only the amount precisely needed to produce a well-shaped dimpled cover. 
     It is yet another object to provide a set of improved dies that are economical in design, yet extremely durable and effective to use. 
     The above and other objects and advantages of this invention will become more readily apparent when the following description is read in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a view, partly in cross section, of a two-piece golf ball that is formed by compression molding with the improved mold and process of the present invention; 
     FIG. 2 is an elevational view of the &#34;prior art&#34; mold disclosed in U.S. Pat. No. 695,867 to Kempshall; 
     FIG. 3 is a cross-sectional, elevational view of an improved mold made in accordance with the present invention, wherein the mold has telescopic halves or dies that are shown mated together; 
     FIG. 4 is an enlarged, fragmentary view of the FIG. 3 dies prior to their being closed together; 
     FIG. 5 is a fragmentary, cross-sectional view of the telescopic dies during the molding process for the FIG. 1 ball, with the dies being shown in their respective positions after they have started to mate but before they have reached their fully closed positions shown in FIG. 3; 
     FIG. 6 is a view similar to FIG. 5, but with the dies fully closed together; and, 
     FIGS. 7 and 8 are views similar to FIGS. 5 and 6, showing the problems of ball-making as they occurred in the &#34;prior art&#34; mold disclosed in U.S. Pat. No. 695,867. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the drawings in detail, an improved mold for compression molding the outer shell or cover of two-piece golf balls is illustrated and generally designated by the reference numeral 10. The mold includes a pair of male and female dies or telescopic mold halves 12, 14 which are attached to a hydraulic press (not shown) for movement toward or away from one another. By operating the press, the dies can be mated or pushed together to compression mold the outer cover 16 onto the two-piece ball 18 shown in FIG. 1. 
     As best shown in FIGS. 3-6, the male and female dies 12, 14 have opposing, substantially hemispherical mold cavities 20, 22. When the dies mate and bottom out against one another (see FIG. 3), the hemispherical cavities form a single spherical mold cavity 23 with an inner surface 24 that is basically a mirror image of the cover 16. Each of the cavity halves 20, 22 has a plurality of rounded projections or mold surfaces 26 that are equidistantly spaced from one another and which form the well known dimples 28 on the cover. 
     As best shown in FIGS. 3 and 4, male die 12 has an annular projection or piston 30 that encircles the lip of mold cavity 22. Further, it has a stepped surface 32 outside the piston that acts as a bottoming shoulder when the dies 12, 14 are mated together in their fully closed positions shown in FIG. 3. 
     Female die 14 is basically a &#34;negative image&#34; of die 12. It includes a circular bottoming shoulder 34 and an adjacent recessed step 36 with a rim 38 located at the start of mold cavity 22. The step includes a vertical sidewall 40 with a canted or sloped lip 42 (here, angled approximately 15°) for guiding the piston 30 of the male die into the recessed portion 36 when the dies are initially pushed together into their telescopic positions shown in FIG. 5. 
     Like the Kempshall process described in U.S. Pat. No. 695,867, the disclosure of which is incorporated herein by reference, an integral cover 16 for a two-piece golf ball is formed by first placing a hemispherical shell or cover stock half 44 (made of any suitable thermoplastic material, such as SURLYN®) inside of the female mold cavity 22 with the open side of half 44 facing upward. A standard rubber core ball is then placed inside of the cover stock half and another cover stock half 46 is placed over the top of the ball with its open side facing downward, opposite the open side of half 44. Next, heat is applied to the mold 10 to soften the cover stock. Then, the press is manipulated so that the dies 12, 14 are moved toward one another and forced to mate, with the annular piston 30 entering the recessed area 36. 
     Like Kempshall, when the piston 30 of the present invention enters the recessed area 36 on the opposite mold half, it traps a small amount of overflow cover stock or shell material in the recess 36. If an axial load is continued to be applied, material in the recessed area will be pressurized and will flow back into the mold cavities 20, 22, when the contents of the cavities cool and start to shrink. This back pressure results in the outer surface of the compression-molded two-piece ball being spherical and substantially a mirror image of the inner surface 24 of the mold cavities. 
     Unlike balls made with the Kempshall mold, a two-piece golf ball made with this invention does not include the large deleterious welt described in this application&#39;s BACKGROUND OF THE INVENTION and designated by the reference letter W in this application&#39;s FIG. 8. 
     The present invention includes a unique valving system 48 for preventing or minimizing the Kempshall welt by &#34;draining off&#34; excess extrusion during mating of the dies 12, 14. This draining provides that the amount of &#34;remaining&#34; extruded material that is forced back into the mold cavities 20, 22 is only the amount precisely needed to seal the cover stock halves 44, 46 into an integral cover 16 with a uniform thickness T. 
     If one considers that the idea is to restrict the overflow of cover stock material and/or push material back into the mold 10 to build back pressure and adequately form the golf ball, it seems logical that ideally one would be able to control that pressure. If a valve were attached to the piston 30 or recess 36, one would be able to bleed off excess back pressure and allow the mold halves to close while maintaining the &#34;remaining&#34; pressure at a certain level. By keeping the valve partially open at a predetermined correct setting, one could allow the back pressure to build to the optimum point and then bleed off as any excess occurs. This would maintain enough back pressure in the mold cavities 20, 22 to form the outer shell 16 well but not allow the pressure to build and &#34;cold flow&#34; material into the surface of the ball leaving a permanent set deformation. 
     While a valve in the conventional sense of the word cannot be attached to the mold 10, a unique valve can be used, such as that shown at 48. In the preferred embodiment, the valve 48 comprises an annular gap between the outside diameter of the piston 30 and the vertical sidewall 40 of recess 36. This gap allows bleed off of excess extruded material and excess pressure and maintains the back pressure at a proper level. 
     To achieve the proper valve setting, applicants have found that the inner diameter of the sidewall 40 should be 0.002 to 0.003 inches greater than the outer diameter of the piston 30. Since these dimensions are small, the size of the gap has been exaggerated in the drawings to ease in the understanding of the invention. 
     When the piston 30 enters the recessed area 36 and its leading edge 52 reaches the position shown in FIG. 5, extruded cover material is trapped between the edge 52 and the underlying portion of rim 38 in a pressure chamber, like the prior art mold of Kempshall shown in FIG. 7. However, unlike the trapped material in the Kempshall mold, excess extrusion in the present invention flows into the annular gap. As the dies 12, 14 are closed further together by the hydraulic press and the leading edge 52 moves farther into the recess 36, the trapped material between the edge 52 and rim 38 is forced in two directions. Most is normally pushed into the mold cavities 20, 22 to create the back pressure necessary to compensate for shrinking that is occurring. However, excess material is pushed into the gap. 
     Thus, as shown in FIG. 6, when the die halves are fully closed together and their surfaces 32, 34 bottom out against one another, the level of bled off material in the gap is slightly raised from its FIG. 5 position. Though bled off in this invention, the excess material would not have been removed in Kempshall and would have been forced back into the Kempshall mold cavity during the Kempshall process shown in this application&#39;s prior art FIGS. 7 &amp; 8. 
     By controlling the width of the gap to 0.002-0.003 inches, it has been found that the gap is large enough to permit excess extrusion to flow or be pushed aside but small enough to maintain enough pressure to form a well-shaped golf ball with a substantially uniform thickness T and uniform dimples 28. 
     While a single embodiment of the invention has been disclosed, it should be understood by those skilled in the art that obvious structural modifications can be made without departing from the spirit or scope of the invention. For example, while the valving system 48 has been disclosed as a gap, it can alternatively be a series of one or more notches or overflow channels that extend transversely through the vertical sidewall 40. The notches are located above rim 36, near or contiguous with the bottoming shoulder 34 so that drainage will occur when the level of trapped material exceeds a predetermined height. 
     Accordingly, reference should be made primarily to the appended claims, rather than to the specification, for determining the scope of the invention.