Discussion of the Related Art and Summarv of the Invention For some time, golfers have been using oversized metal woods. With the advent of low-density head materials, such as titanium and titanium alloys, the popularity of oversized heads has increased. Oversized wood-type heads provide a larger sweet spot at impact and a higher inertia, which provides greater forgiveness at impact than a golf club having a conventional head size.

Unfortunately, there are limitations on how large a golf club head can be manufactured, which is a function of several parameters, including the material, the weight of the head and the strength of the head. Additionally, to avoid increasing weight, as the head becomes larger, the thickness of the walls must be made thinner. As a result, the head face has a tendency to deflect more at impact, and thereby impart more energy to the ball. This phenomenon is typically known as the"trampoline effect."When the club face provides a significant trampoline effect, it can significantly increase the flight distance of the golf ball. If the club results in too large a trampoline effect, the club is likely to violate Rule 4-le, Appendix II of the U. S. G. A. Rules of Golf. Rule 4-le states that the material and construction of the club face"shall not have the effect at impact of a spring." U. S. Patent No. 4,928,965 to Yamaguchi, et al., teaches the matching of the resonance of a golf club head to the resonance of a golf ball. Specifically, Yamaguchi teaches designing a golf club head having a primary natural frequency within the natural frequency region of golf balls available on the market, with the intent of increasing the travel distance of a struck golf ball. Yamaguchi teaches that the primary parameters in adjusting the natural frequency of a golf club head are the spring constant of the head face (k) and the mass distribution of the golf club head.

Unfortunately, at least an example golf club head (Club 1 set forth in Table 2), arguably has a face which has the effect at impact of a spring, based on its coefficient of restitution, in violation of U. S. G. A. rules.

Accordingly, there is needed an improved oversized metal wood which provides the desired larger sweet spot and higher inertia, yet with a structure which clearly complies with U. S. G. A. rules as to the club face.

Applicant has determined, based upon a series of empirical and theoretical models, that the mechanical impedance of a club head is a measurable indicator of whether the club head wilt have a significant trampoline effect.

Mechanical impedance is also a measurable indicator of whether the club head will"have the effect at impact of a spring"in violation Rule 4-1 e. Applicant has further determined that a club head with a mechanical impedance with a natural frequency below a certain limit will have a significant trampoline effect. Additionally, Applicant has determined that a golf club head desirably has a mechanical impedance with a natural frequency greater than approximately 100 Hz above this limit.

One aspect of the invention is a golf club head having a body with a plurality of thin walls including a crown, a sole, a face, and a rear belt which cooperate to define an interior cavity, wherein the cavity has an external volume

of at least 200 cc's. The body has a rib structure including at least one reinforcing member which reinforces at least a portion of one of the plurality of thin walls so that the head has a mechanical impedance with a primary natural frequency which is greater than or equal to a limit selected so that the head will not have a significant trampoline effect, wherein the golf club head in the absence of the rib structure would have a primary natural frequency of less than this limit. Preferably, the head has a primary natural resonance frequency greater than 100 Hz above this limit.

In view of uncertainties as to how the U. S. G. A. would interpret the term"have the effect of a spring"and the ramifications of disqualification for violating this rule, it was previously thought that it would be desirable for the club head to have a primary natural frequency above at least 1,700 Hz to avoid this risk. Since a natural frequency of 1,700 Hz was seen as approaching the aforementioned lower limit, Applicant determined that with the desired safety factor of 100 Hz, the lower limit of natural frequency for a head would be 1,800 Hz.

The U. S. G. A., however, recently issued guidelines describing a procedure that may be performed on a club head to determine whether the club head has the effect at impact of a spring. According to the procedure, a club head is deemed to violate Rule 41e if a golf ball, when fired at the club head, demonstrates a velocity ratio (ball rebound velocity IVOu,) lball incoming velocity (Vin)) which is greater than a velocity ratio baseline plus an allowable test tolerance. The velocity ratio baseline represents a constant coefficient of restitution curve as a function of the club head mass. The baseline velocity ratio for a given clubhead mass is calculated using the following equation: VONt= (-)/ (M=) where es 0.822, M is the mass of the club head, and m is the average mass of the ball population.

In view of these guidelines, Applicant has ascertained that to ensure the golf club head will not have the effect of a spring, the golf club should have a primary natural frequency greater than 1,200 Hz. That is, a club head having a mechanical impedance with a primary natural frequency which is tess than or equal to approximately 1,200 Hz would likely violate U. S. G. A. rules under the recently-issued U. S. G. A. testing procedures and allowable tolerances.

To provide a safety factor, preferably, the primary natural frequency of the club head should be no less than 1,300 Hz to more fully ensure compliance with Rule 4-1e.

The golf club head of the present invention is particularly adapted to provide an oversized metal-wood head which provides the desired larger sweet spot and higher inertia, without the undesired excessive trampoline effect.

Specifically, the rib structure desirably prevents the walls from deflecting greater than a given limit. Additionally, tests performed by the applicant indicate that the reduced deflection of the golf club face results in much straighter golf ball flight trajectories and therefore more accurate shots.

A need also exists for a method of manufacturing a metal matrix wood-type head having the desired characteristics described above. Preferably, the method should produce an oversized wood-type head of metal matrix composite having a structure that provides mechanical resistance while also allowing desired mass distribution. The method should desirably produce an oversized wood-type head that has a superior finish.

One aspect of the invention involves a body having a plurality of thin walls including a crown, a sole, a face and a rear belt which cooperate to define an interior cavity. The body has an external volume of at least 200 cubic centimeters and is made of a metal matrix material having a high strength and a low density. For purposes of this application,"high strength"means that the ultimate strength is at least 30 ksi and the yield strength is at least 20 ksi and"low density"means that the density is lower than 3.5 grams per cubic centimeter."Very low density"means that the density is lower than 2.80 grams per cubic centimeter. The head comprises a ribbing structure which reinforces at least a portion of the plurality of thin walls. Furthermore, the ribbing structure desirably comprises a series of at least two spans spaced apart transversely which each extend along an interior surface of the body in a loop configuration to connect the crown, the sole, and the rear belt. Each of said spans desirably has a length extending inwardly which is greater than its width. Additionally, the length is desirably at least two times the width.

The thin walls of the head, particularly the face wall, desirably deflect in the same range of deflection as a steel or titanium head so as to reach the minimum level of primary natural frequency. The ribbing structure advantageously also prevents the walls from reaching the ultimate resistance limit of the material. The head therefore provides no significant loss in ball distance or feel relative to heads made of common materials, such as steel or titanium. Additionally, metal matrix composite materials generally have an ultimate strength and a yield strength that are both on the same order of values. Hence, the head has practically no plastic region of deformation.

The ribbing structure advantageously also acts as resonance means in a metal matrix head. The sound produced by the head upon impact with a ball may be varied by varying the length, width and particular shape of the ribbing structure.

Another aspect of the invention relates to a golf club that comprises a body having a plurality of thin walls including a crown, a sole, a face and a rear belt which cooperate to define an interior cavity. The body comprises a metal matrix whose mass is between 120 grams and 170 grams and at least one mass of higher density between 20 grams and 60 grams. At least one mass is desirably positioned on an outside periphery of the body. Furthermore, the ribbing structure desirably comprises a series of at least two spans spaced apart transversely which each extend along an interior surface of the body in a loop configuration to connect the crown, the sole, and the rear belt. Each of said spans desirably has a length extending inwardly which is greater than its width. Additionally, the length is desirably at least two times the width. It is also intended that the golf head has a primary natural frequency of at least 1,200 Hz, as previously defined.

The loop configuration of the spans advantageously allows the walls to be made thinner so that the head experiences desired deformation upon impact with the golf ball. The desired size of the spans confers increased strength while maintaining flexibility of the walls between the spans. Another advantage of the span size is that they produce vibration after impact so as to achieve a desired sound.

Another aspect of the invention relates to a method of manufacturing a wood-type metal head. The method comprises partially filling a cast with a core to define a space between the cast and the core corresponding to thicknesses of walls of at least a part of a head body, injecting a molten metallic material through the cast to fill the

space to form a head, cooling and removing the head from the cast, and removing the core from the head through at least one opening. Desirably, the core is removed after having made it soluble or after degradation into small pieces by various physical or chemical methods. Advantageously, the method further comprises injecting molten metallic material into slots in the core to produce ribs which form a monolithic block with the head. The method is more adapted to produce a golf club head in a single piece of metal matrix composite.

The golf club heads provide several advantages over current head manufactured of titanium or a metal matrix composite. The volume of the heads may be made large while still maintaining lightness so that weight may be redistributed. The head also provides improved durability and mechanical resistance while also providing improved sound resonance.

Brief Description of the Drainas Figure 1 is a perspective view of a wood-type head configured in accordance with a preferred embodiment of the present invention; Figure 2 is a side view of the head of Figure 1; Figure 3 is a cross-sectional view of the head of Figure 1 taken along the line 3-3 of Figure 2; Figure 4 illustrates a method of assembling the golf club head of Figures 1-3; Figure 5 is a perspective view of a wood-type head configured in accordance with another preferred embodiment of the present invention; Figure 6 is a side view of the head of Figure 5; Figure 7 is a cross-sectional view of the head of Figure 5 taken along the line 7-7 of Figure 6; Figure 8 is an enlarged cross-sectional view of the head of Figure 5 taken along the line 8-8 of Figure 7; Figure 9 is another enlarged cross-sectional view of a portion of the head of Figure 5; Figure 10 is a perspective view in partial cross-section of the head of Figure 5 along the line 10-10 of Figure 7; Figure 11 is a transverse cross sectional view of a mold for manufacturing a golf club head in accordance with the embodiment of Figure 5; Figure 12 is a cross-sectional view of a head after removal from the mold of Figure 11; Figure 13 illustrates the operation of emptying a soluble core from a head in accordance with a method of the present invention; Figure 14 is a bottom view of a preferred configuration of a head configured in accordance with the present invention; Figure 15 is a transverse cross-sectional view of a mold for manufacturing the head of Figure 14; Figure 16 is a transverse cross-sectional view of the head of Figure 14 after removal from the mold and after a finishing operation has been performed.

Figure 17 is a detailed cross-sectional view of the mold configuration of Figure 15 ;

Figure 18 is a detailed cross-sectional view of a mold configuration in accordance with an alternative embodiment of the present invention; Figure 19 is a detailed cross-sectional view of a mold configuration in accordance with yet another embodiment of the present invention; Figure 20 is a perspective view of a wood-type head configured in accordance with another embodiment of the present invention; Figure 21 is a perspective view of a wood-type head configured in accordance with another preferred embodiment of the present invention; Figure 22 is a perspective view of a wood-type head configured in accordance with another preferred embodiment of the present invention; and Figure 23 shows a graph of variation in ball speed as a function of the natural frequency of the head.

Detailed Description of the Preferred Embodiment Figures 1 and 2 are perspective and side views, respectively, of an oversized wood-type golf club head 30 configured in accordance with the present invention. The head 30 consists of a hollow body that has an upper surface or crown 32, a lower surface or sole plate 34, a front surface or strike face 36, and a rear surface or belt 40. The golf club head 30 also includes a heel region 42 and a toe region 44 located at an end of the head 30 opposite the heel region 42. A hosel 45 extends upward from the heel region 42 of the head 30 for connection of a golf club shaft (not shown).

Desirably, the head 30 has an oversized or supersized volume with respect to the conventional established sizes of metal-wood-type heads. Advantageously, the head 30 of the present invention has a volume greater than or equal to 200 cc. Preferably, the volume of the head 30 is greater than or equal to 220 cc, and more preferably, greater than or equal to 250 cc. This volume provides the head with an extremely enlarged sweet spot area. The volume also provides a high level of inertia that renders the head 30 forgiving even for off-centered shots. The size or volume of a metal-wood head is generally measured by the volume that the head (including the hosel) displaces when the head is placed in water or other liquid.

As shown in Figure 1, the head 30 includes an internal rib structure 50 that, in the illustrated embodiment, includes reinforcing members or ribs 51,52 and 53. In the preferred embodiment, the rib structure comprises one central rib 52 extending vertically along an interior surface of the face in a median region and a first and a second lateral rib 51,53 on each side of the central reinforcing member; each being located, respectively, in the vicinity of the heel 42 and toe region 44. Each rib in the rib structure 50 extends along a substantially vertical plane and is attached to the inner surface of the strike face 36. The ribs 51,52 and 53 are spaced apart a predetermined distance sufficient to provide the strike face 36 with a minimum level of deflection which ensures an impermissible level of trampoline effect under U. S. G. A. rules is avoided by restricting the primary natural frequency of the club head to a value greater than a predetermined lower limit.

The three ribs stiffen the face such that the measured primary natural frequency of the head is higher than or equal to a lower limit of 1,200 Hz. Applicant has determined that a club head with a primary natural frequency less than 1,200 Hz will likely have the effect of a spring at impact with a golf ball in violation of U. S. G. A. rules. Preferably, the primary natural frequency of the club head is higher than or equal to 1,300, and more preferably, higher than or equal to 1,700 Hz. Given the desired structure of the club, Applicant has ascertained that, in the absence of the rib structure, the measured primary natural frequency of the club head would result in an impermissible level of trampoline effect under U. S. G. A. rules. The primary natural frequency corresponds to the first or lowest frequency at which the mechanical impedance of the body shows a corresponding local minimum value. The definitions and conditions for the measurement of the impedance are given in U. S. Patent 4,928,965, which content is hereby incorporated herein by reference. More specifically, the measuring method consists of applying the conditions of measures for the golf club head of the invention alone attached to a vibrator through a fixation rod of 12.5 mm, as shown in Figure 3C of U. S.

Patent 4,928,965. This measure of frequency was found to be sensitive to the diameter of the rod. Therefore, in the experiment, the diameter of 12.5 mm was chosen because it is about half of the diameter of the ball which, it so happens, corresponds approximately to the diameter of the average amount of surface contact during the time of impact between the golf ball and the golf club face.

The applicant has noticed that a rib structure as previously described behind the face of an enlarged metal- wood head both cuts down the trampoline effect and stabilizes the ball flight. The rib structure is regarded as the best way to eliminate the deflection of the face which creates the trampoline effect, while at the same time saving mass.

While the deflection of the club face could be reduced by increasing the thickness of the face until the determined limit of primary natural frequency is achieved, this approach is undesirable in that it leads to an increase of the overall weight of the golf club head. As the strike face is larger for the oversized metal-wood heads, this approach would be even less desirable for oversized metai-wood heads than for conventional wood-type heads.

Importantly, the rib structure has also a beneficial influence on the trajectory of the ball. In particular, tests have shown that off-center hits produce a smaller spin rate modification compared to on-center hits (backspin or side spin, depending on the type of off-center hit) than for off-center hits with golf club heads without such rib structure.

The particular orientation of the ribs 51-53 may be varied to suit various circumstances. In the embodiment illustrated in Figure 1, the ribs 51-53 are generally oriented vertical and parallel to each other. It is contemplated that the spacing between the ribs 51-53 could be varied so that the ribs converge or diverge from the strike face 36 toward the belt 40.

Preferably, the number of ribs is between 2 and 20, and more preferably 3 and 8. The desired number of ribs depends on various factors, such as, for example, the volume of the head 30, the thickness of the walls of the head 30, the type of material used to manufacture the head 30, and the desired deflection characteristics of the head 30.

Figure 3 is a cross-sectional view of the head 30 taken along the fines 3-3 of Figure 2. As shown, the ribs 51-53 have a length L, and a width W,. The width W, is substantially constant along the length of the ribs 51-53, but tapers to a point at the tip. Alternatively, the width W, could vary along the rib so that the ribs have an overall tapered

shape. In the preferred embodiment, the length L, of the ribs 51-53 is greater than the width W,. Preferably, the length is at least two times its width and, most preferably, it is between 3-15 times its width.

Such an arrangement provides an appropriate stiffness to the head 30 while reducing the number of ribs required to maintain the required level of frequency as previously defined. Generally, the walls of the head 30 vibrate less upon impact as the number of ribs are increased and the size of the ribs, such as their length and width, is increased. As a consequence, the primary natural frequency, as measured, increases accordingly.

As a matter of example, the face thickness may be about 0.140 inches if made of titanium-base alloy. Three ribs having a length L, of 0.59 inches and a width of W, of 0.05 inches woutd allow a maximum face deflection of about 0.30 millimeters or lower and a primary natural frequency of about 2,500 Hz. The same head having a face thickness of 0.140 inches and no ribs would experience a face deflection of about 0.8 millimeters and a primary natural frequency of about 1,650 Hz.

Figure 4 shows a conventional method to produce the golf club head as described above. The golf head may be produced in two separate pieces 100 and 200. A first piece 100 includes the strike face 36, the crown 32, the hosel 45 and the ribs attached to the inner surface of the strike face. The first piece may be obtained either by casting or forging process. The other piece 200 includes the sole plate 34 and the belt 40 and may be produced by either casting, stamping a plate or forging, depending upon the material used for the piece. Then both pieces are assemble by welding or other suitable mechanical means. In an alternative, the ribs may be produced separately and then welded to the inner surface of the striking wall. In all cases, it is preferred that the ribs are further attached to another wall as, for example, the inner surface of the crown, as shown in Figure 2.

The body may be made of various metallic materials; among them are titanium, steel, aluminum, metal matrix and alloys thereof. However, in the case of the head described above, titanium or titanium alloys are preferred, as they facilitate the manufacture of larger heads having the desired mechanical properties. Titanium may also be easily cast or forged according to the method described in Figure 4.

In accordance with another feature of the present invention, the head 30 is manufactured of a metal matrix composite material. The metal matrix composites are materials that constitute at least two distinct constituents, including a main material or matrix, such as a metal or an alloy that preferably has a low density, and a reinforcing material, such as a ceramic material. Such composites comprise one or more matrix material metals such as, preferably, aluminum, titanium, beryllium or magnesium, in either a pure form or in an alloyed form. A preselected percentage of ceramic materials or alloying elements are added to the matrix materials in order to enhance the properties of the matrix material metal to increase its strength and resistance. Some examples of alloy elements are silicon, germanium, cobalt, and silver. Typical ceramics are boron carbide, silicone carbide, titanium diboride, titanium carbide, aluminum oxide and silicone nitride, as well as various blends thereof.

An example of a preferred metal matrix composite consists of a metallic matrix material and a reinforcing material boron carbide. The matrix material is preferably taken from the group consisting of aluminum, titanium, alloys of aluminum, and alloys of titanium. Preferably, the matrix material is selected from an aluminum alloy, which has a

lower density to thereby allow for an increased head size without increasing the weight beyond a desired weight range. Preferably, the metal base material is selected from an aluminum alloy 380 which comprises 8.5% silicone and 3.5% copper. The boron carbide preferably represents 4-20% of the weight of the composite. The boron carbide has a particular size in the range of 2 to 19 microns.

A metal additive is preferably added to provide a chelating opportunity for the aluminum alloy. Such an additive metal is preferably taken from the group consisting of silicon, iron, aluminum and titanium. The additive material is added in a small amount on the order of 0.01 to 0.4 weight percentage.

Depending upon the ratio of boron carbide to aluminum, as well as upon the particular aluminum alloy used as the matrix material metal, the resulting material has a density of approximately 2.69 grams per cubic centimeter. The resulting material also has an ultimate strength range from 30 to 104 ksi and a yield strength of 20 to 98 ksi.

Furthermore, the resulting material is extremely fracture resistant and more predictable than other composites. The material is also lighter and stronger than the aluminum alloy materials that do not have ceramic agents.

A preferred metal matrix composite made from the aforementioned materials is known under the trademarked name of Boralyn and is manufactured by the Alyn Corporation. Its compositions and method of manufacture thereof are described in U. S. Patent No. 5,486,223 whose content is hereby incorporated by reference herein.

Figures 5-10 show another preferred embodiment of the present invention in which the rib structure 50 includes ribs 54,55,56 and 57. The ribs 50-57 are annular shaped with a hollow center so as to converse weight while providing sufficient stiffness. The loop configuration of the ribs 54-57 confers appropriate mechanical resistance to the golf club head 30 in the direction in which a golf ball is shot. The ribs also desirably ensure homogenous deformation of the structure of the golf club head 30. The ribs also facilitate transfer and distribution of the impact force or shock waves from the face 36 to the rear surface 40 of the head 30.

Figure 8 is an enlarged cross-sectional view of a portion of the rib 54 from Figure 7. The length L, and width W, of a rib 54 may be varied along the length of the rib 54 in order to achieve the desired resonance at impact.

Figure 9 is an enlarged view of a portion of the rib 54 shown in Figure 7. As shown, the dimensions of the ribs may be varied by providing local sections that have an enlarged shape, or bulge 53, as illustrated by the dashed lines. The bulged areas are used to adjust more independently the stiffness, as well as the sound of the head 30. The size of the bulge 53 may be varied to produce desired vibrations at impact in order to achieve the desired resonance.

Figures 11-13 illustrate a preferred method of manufacturing the head 30. The method generally includes casting by injection that involves various steps described beiow. Although the method is particularly desirable to produce a body made of metal matrix head, the desirability of the procedure is not limited to this specific material.

Figure 11 is a transverse cross sectional view of a mold 66 in which a removable core 68 is positioned. As used herein, a"removable"core is a core that may be soluble or that may be degraded into small solid particles and then removed from a body through a relatively small opening.

As shown in Figure 11, the core 68 and the mold 66 define a cavity 70 that is located between the core 68 and an internal wall of the mold 66. The cavity 70 has a size and shape that corresponds to the desired size and shape of the head 30 that is to be manufactured. Preferably, the core 68 includes a series of slots 72 which each have a shape complementary to the shape of the ribs 52 that are to be produced. That is, the core 68 includes transverse slots 72 that each have an annular shape. Each slot 72 communicates with the cavity 70.

Referring to Figure 11, a cog or dowel 74 is connected to and communicates with the core 68. The dowel 74 preferably maintains the core 68 in the correct position within the cavity 70 during the manufacturing process. The dowel 74 preferably extends through a hosel bore 76 located in the mold 66. The shape of the hosel bore 76 corresponds to the shape of the hosel 45 on the head 30 to be produced.

The mold 66 and dowel 74 are preferably made of a material that has a high thermal conductivity and that is resistant to high temperatures.

During manufacturing of the head 30, a metal matrix composite material is injected into the cavity 70 through an opening 80 in the mold 66. The injection of the metal matrix composite is preferably preformed at a temperature range of approximately 800 and 1,200°C and at a high pressure that preferably varies between 6,000 and 15,000 psi. The relatively high pressures advantageously facilitate both a fast injection of the metal matrix material and a complete distribution of the molten metal throughout the thin cavity 70 and into the slots 72 within the core 68. The injection time is preferably around 0.5 seconds or less.

The metallic material is then allowed to solidify within the mold 66. The material forms into the shape of the head 30. As shown in Figure 12, after the metal matrix material solidifies within the cavity 70, the mold 66 is removed to produce a head 30. When the mold 66 is removed, the head has an injection gate 82 that was formed by the material in the opening 80. The injection gate 82 is preferably removed by machine cutting or grinding. The dowell 74 is also retracted from the mold 66 at this time, as illustrated by the arrow in Figure 9. The head 30 may then be submitted to a finishing process for smoothing.

In the preferred embodiment, the core 68 is made of a salt which is soluble under water or other proper solvent. For example, the core 68 may be made of sodium chloride. Alternatively, the core 68 may consist of a material that is flaky or friable when exposed to heat treating, chemical treating or mechanical attack, such as ultra- sound and the like. As another example, the core 68 could also be a bound sand core which consists of a mixture of sand and a binder, such as clay or a thermoset resin. Such a core 68 can advantageously be broken down after die casting the head 30, such as by reheating the core 68 at a certain temperature in order to degrade the binder of the core 68. When exposed to these conditions, the sand becomes somewhat fluid and can thus be extracted through a small opening. The use of a fusible core is also contemplated. As shown in Figure 13, the core is removed by pouring the material out through the hosel 45.

One advantage associated with the use of a removable core 68 by destruction or by making the core 68 liquid, is that this process allows the production of a unitary hollow body of large volume with thin walls, as well as the production of a ribbing structure 50 for reinforcement. A head 30 with this structural arrangement has increased

mechanical properties, an enhanced finished appearance, and is less expensive to produce than the traditional process of investment casting distinct pieces and then assembling the pieces by welding. The above-described process also does not have the difficulties associated with welding together separate pieces of metal, such as the lower mechanical properties along the weld junctions.

Referring to Figure 14, the above-described process also makes it impossible to provide weights plugs 84 on the head 30 during the die casting operation. The weights plugs 84 may be positioned on the head 30 to maximize the inertia of the head 30 during use. A wood-type head made of a metal matrix of very low density, such as 2.7 grams per cubic centimeters and below, provides the ability to redistribute a mass of 0 to 80 grams by providing the head 30 with weight plugs 84 of higher density. In a preferred example, the metal matrix part is between 120 and 170 grams and it comprises at least one plug of higher density between 20 and 60 grams.

Figure 14 illustrates an oversized head 30a having two weight plugs 84 located on the outside surface of the head 30. Preferably, the weight plugs 84 are located on the external surface rather than the internal surface of the head 30, as external weight plugs maximize the moment of inertia of the head 30. In the illustrated example, the weight plugs 84 are located in both the toe and heel of the head 30 in a region that extends substantially across the sole plate 34 and the belt 40.

Figures 15-19 illustrate a preferred method of manufacturing by casting the head 30a with the weight plugs 84. As shown in Figure 15, the weight plug 84 include relief structures 86 that are positioned in small recesses 88 of complementary shape in the mold 66. The relief structures 86 maintain the plugs 84 in a static position during the operation of injection. The weight plugs 84 preferably have a tenon shape with sloped sides 90 that converge outwardly in order to assist in the fixation of the weight plugs 84 in the completed head 30.

Figure 17 is a close-up cross-sectional view of a portion of the mold 66 and core 68. A weight plug 84 is positioned in the mold 66 adjacent the core 68. During an injection step, the metallic material is injected into the mold 66 so that it flows through the cavity 70, as shown by the arrows in Figure 13. The material bonds with the weight plug 84. As shown in Figure 14, the mold 66 is removed from the head 30 after injection and curing. The relief structures 86 are removed, such as by grinding, after the head 30a has been removed from the die mold 66.

Figure 18 illustrates an alternative embodiment of the golf club head 30a where the weight plugs 84 include retaining structures 89 that are attached to the core 68. The retaining structures 89 facilitate fixation of the weight plugs 84 during injection. In the illustrated embodiment, the retaining structures 89 consist of pins 90 that communicate with the core 68. A gap 92 is maintained between the weight plug 84 and the core 68 to ensure a good bond between the weight plug 84 and the bonding material.

Figure 19 illustrates yet another embodiment of the weight plugs 84. In this embodiment, the pins 90 that have a tenon shape to improve the bonding of the plug with the head 30 after casting.

Figures 20 to 22 illustrate several examples in which the reinforcing members may have various configurations. Figure 20, for example, shows a cross-shaped rib structure 580 extending behind the strike face.

Figure 21 shows another design of the reinforcing structure 581 with a W-shaped configuration behind the face.

Figure 22 is an example with a honeycomb configuration behind 582 the face. Of course, other various designs may be imagined without departing from the scope of the present invention.

Figure 23 represents a graph showing the influence of the natural frequency expressed in Hertz along the X- axis on the evolution of the golf ball speed expressed by mph along the Y-axis. The curve shows the exponential influence of the trampoline effect on the ball speed between 1,200 Hz and 2,800 Hz. At and below 1,600 Hz, the trampoline effect will impact ball speed. At and below 1,200 Hz, the impact on ball speed is deemed based on current guidelines to sufficiently increase ball speed that it will be deemed in violation of U. S. G. A. rules. Above 2,500 Hz, the benefit of the trampoline effect is effectively eliminated and, even at about 2,000 Hz, the effect is very minor. This graph has been obtained by simulation and modal analysis tests after head designs were simulated using a finite element analysis software,"ABAQUS,"well known in the art of structure calculation. Very close results to these would reasonably be expected by prototyping and measuring.

Although the foregoing description of the preferred embodiment of the preferred invention has shown, described, and pointed out certain novel features of the invention, it will be understood that various omissions, substitutions, and changes in the form of the detail of the apparatus as illustrated as well as the uses thereof, may be made by those skilled in the art without departing from the spirit of the present invention. Consequently, the scope of the present invention should not be limited by the foregoing discussion, which is intended to illustrate rather than limit the scope of the invention.