1. The Field of the Invention
This invention relates to sporting goods fabricated from fiber reinforced plastics. More particularly, the present invention relates to improved structures for, and materials and methods of fabricating, golf club heads.
2. The Background Art
The continuing increase in the popularity of the sport of golfing has motivated extensive research and development into improving the fabrication of golf clubs. While golf club heads have traditionally been fabricated from either metal or wood materials, hence the designation "irons" and "woods," modern materials provide significant advantages over these traditional materials.
Among the modern materials which have become popular for the fabrication of golf club heads is fiber reinforced plastic. In many cases, it has become common for one manufacturer to fabricate the golf club head while the club head is assembled to a golf club shaft by another party. Fiber reinforced plastic materials provide several advantages over traditional materials but have also presented several drawbacks.
Several techniques have been used in the art to fabricate golf club heads. Such techniques include compression molding (including lay up, sheet molding compound, and bulk molding compound techniques), resin transfer molding, and injection molding.
In all of these techniques, the material to be molded is inserted into a cavity mold and pressure is applied to force the material against the wall of the cavity. In order to increase the strength of the finished golf club head, fibers can be added to the resin used to form the golf club head. Still, all of these techniques and materials have serious drawbacks.
In compression molding using continuous fiber lay-up, unidirectional tape and/or woven fabric is embedded in an uncured (still fluid) thermoset resin. The fibers are laid in the mold so that the fibers are aligned parallel to and along the mold cavity walls. While this arrangement imparts high strength in the direction of the fibers, it provides very low strength in the direction perpendicular to the direction of the fibers. Thus, the finished golf head has high strength in one plane or direction but very low strength in a direction oriented perpendicular to the plane. Disadvantageously, when the golf club head experiences the repeated high impact loads associated with striking a golf ball and/or the ground, this lack of strength in other than the direction of orientation of the fibers can cause structural failure, including cracks or delaminations between layers of tape and/or fabric, and results in poorer performance of the golf club head.
In compression molding using sheet molding compound, relatively short fibers are suspended in a viscous thermoset resin and are pressed into flat sheets. The flat sheets are placed in the mold cavity. The fibers in the sheet generally are unidirectional and during the molding process the fibers are forced into an alignment parallel with the mold cavity walls. Virtually no fibers are aligned in a direction perpendicular to the mold cavity wall resulting in a lack of strength in the direction perpendicular to the direction of the fibers. Thus, the resulting golf club head has less than desirable strength and stiffness characteristics and results in loss of performance.
In compression molding using bulk molding compound, very short fibers are formlessly suspended in a viscous thermoset resin. Because of the flow of the material during molding, the short fibers align themselves in the direction of the cavity mold wall. Disadvantageously, this results in a weak finished golf club head in any plane because of the very short fibers which are used.
In the resin transfer molding technique, tape and/or woven fabric containing unidirectionally oriented fibers, without resin, is laid into the mold cavity parallel to the mold wall. The mold is closed and thermoset resin is forced into the mold and into the fiber tape/fabric under pressure resulting in no reinforcing fibers being oriented in a direction perpendicular to the cavity wall. Hence, the inherently weak structure may fail during use or lack the stiffness needed for suitable performance.
In the injection molding technique, a single mold can produce high volumes of parts with low labor cost per part when compared to the other plastic golf club head fabrication techniques. Generally, injection molding techniques use thermoplastic, in contrast to thermosetting, materials into which fibers are mixed. During the mixing and injection procedure, the fiber length is reduced due to cutting and breaking and the fibers tend to align in a single direction. The thermoplastic material provides the advantage of being much tougher and less brittle than thermoset resins. Still, the finished golf club head often has significant strength in only one direction. Moreover, golf club heads made using these techniques suffer from problems such as the sole plate screws pulling out from the golf club head material.
It will be appreciated that these described techniques produce golf club heads which do not have high strength and high stiffness in all directions. Low stiffness prevents the incorporation of certain advantageous non-structural or semi-structural features into golf club heads. For example, golf club heads often have face plates inserted into the club head face to provide good appearance and/or higher stiffness and/or increased hardness.
An additional problem with customary injection molding of golf club heads is maintaining the positioning of a face plate within the mold as the molten plastic is injected at high pressure. The forces from the viscous, flowing plastic tend to move the face plate from its desired position. The more flexible the face plate, the more this problem is exacerbated making it difficult to use face plates, especially very thin face plates which are installed for appearance only.
A still further problem with golf club heads fabricated with these described techniques involves "bore-through" shafts. It is desirable to have the cylindrical shaft-receiving cavity be continuous through to the sole of the golf club head so that the bottom of the golf club shaft, when installed, can be viewed through the sole. While this is cosmetically desirable, it presents a difficulty when golf club heads are assembled to golf club shafts. The attachment of a golf club shaft in a bore-through fashion requires that the area of the sole plate at which the shaft cavity opens must be refinished after the club is assembled and usually the entire golf club head must be refinished requiring substantial additional labor.
A further drawback inherent with golf club heads fabricated using these prior techniques is that a foam core which is included in such golf club heads shrink and become loose with age cause a rattling sound when the golf club is used. While the foam core is included to prevent rattle or alter the sound of the club head hitting a golf ball, when the foam core becomes loose it is unsatisfactory and generally results in the golf club being discarded.
In view of the foregoing problems and disadvantages, it would be an advance in the art to provide an improved fiber reinforced golf club head which provides the strength and rigidity needed to provide long and satisfactory service.