Improved iron type golf club head

Abstract

To provide an iron golf club head achieving improved performance.SOLUTION: Improved performance of an iron golf club is achieved by a specific effective stiffness area ratio between the boundary of the iron golf club head and the face of the iron golf club head. More specifically, the present invention discloses an iron golf club head, the iron golf club head incorporating a variable face profile into specific iron golf club head chassis resulting in an effective stiffness area ratio of between about 4.5 to 5.5 and a coefficient of restitution value as a function of the above effective stiffness area ratio.SELECTED DRAWING: Figure 6

Description

[Technical field]

[0001] The present invention relates generally to an improved iron type golf club head having a specific ratio of effective stiffness area between the boundary of the iron golf club head and the face of the iron golf club head to achieve an improved coefficient of restitution. More specifically, the present invention relates to incorporating an iron golf club having a variable face profile into a specific iron golf club head chassis, resulting in an effective stiffness area ratio of between about 4.5 and 5.5, and achieving a coefficient of restitution value dependent on the effective stiffness area ratio. [Background technology]

[0002] Iron golf club heads, like their counterparts, wood golf club heads, have benefited greatly from the technological advances that the industry has undergone. Although modern metal woods look more different visually to persimmon woods than modern hollow irons do to traditional mashies and niblicks, the advances in technology are arguably game-changing.

[0003] U.S. Patent No. 4,826,172 (Antonious) represents one of the early advances in iron golf club head design and technology by teaching a perimeter weighted iron type golf club head with an improved weight configuration consisting of a concave or cavity back and weight members within the cavity. By moving the mass of the golf club head to the perimeter to create a cavity back, the golf club head can be more forgiving on off-center shots.

[0004] U.S. Patent No. 5,362,047 (Shaw et al.) shows another early example of improving the performance of an iron-type golf club head in which the thickness of the face piece in at least one iron in a set increases from the toe to the heel of the head and in at least one other iron in the set increases from heel to toe. Varying the thickness of the face piece can further improve the performance of the golf club head by reducing material from portions of the striking face that do not require it.

[0005] U.S. Pat. No. 5,971,868 (Kosmatka) shows another early attempt at creating a contoured back surface for a golf club which shows a golf club face with a variable thickness such that areas of the face that experience similar loads with the application of a given force have similar thicknesses, thereby resulting in approximately uniform stresses when similar forces are applied.

[0006] U.S. Patent No. 6,200,228 (Takeda) provides a golf club in which a cavity is formed in the back of the golf club, a rear member is tightly fitted into the cavity, and the bodies are in intimate contact with each other. This backing allows the head to be stronger and the face thickness to be reduced.

[0007] More recently, U.S. Patent No. 7,169,057 (Wood et al.) discloses an iron golf club head having a hollow interior formed from two different casings, where the thickness and hardness of the first casing differs between the long iron golf club head and the short iron golf club head.

[0008] As technology improves in iron-type golf club head design and, as demonstrated by prior patents, gradually evolves away from traditional muscle back constructions toward more cavity back and hollow cavity type constructions, and the striking faces of iron-type golf heads become thinner, opportunities arise to further improve the performance of the striking faces of such golf club heads.

[0009] Therefore, based on the above, there exists a need in the art to develop an improved iron-type golf club head that can take advantage of new modern club head chassis designs. More specifically, there is a great opportunity to design a golf club head with a variable thickness striking face that is a function of modern club head chassis to create an optimal performing golf club head. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] U.S. Patent No. 4,826,172 [Patent Document 2] U.S. Patent No. 5,362,047 [Patent Document 3] U.S. Patent No. 5,971,868 [Patent Document 4] U.S. Patent No. 6,200,228 [Patent Document 5] U.S. Patent No. 7,169,057

[0011] One aspect of the present invention is a face height (H Face ), the striking face further having a substantially planar front surface and a non-planar rear surface. The non-planar rear surface of the striking face defines a VFT profile, the VFT profile further having a thicker central section, a transition section, and a thinner peripheral section. The thinner peripheral section has a face thickness (T Face) and the thick central part is defined as the VFT height (H VFT ) and VFT thickness (T VFT The iron golf club head also has a body portion secured to a rear surface of the front portion. The iron golf club head defines a boundary height (H Boundary ) and boundary thickness (T Boundary ), and the iron golf club head has an ESA ratio between about 4.5 and about 5.5, the ESA ratio being: ESA ratio = (0.5×T Boundary ×H Boundary ) / ((H Face ×T Face )+(0.5×(T VFT -T Face )×H VFT )) The golf club head is defined as follows: COR≧0.0196(ESA ratio)+0.7211 The COR is defined by the relationship:

[0012] Another aspect of the present invention is the face height (H Face ), the striking face further having a substantially planar front surface and a non-planar rear surface. The non-planar rear surface of the striking face defines a VFT profile, the VFT profile further having a thicker central section, a transition section, and a thinner peripheral section. The thinner peripheral section has a face thickness (T Face ) and the thick central part is defined as the VFT height (H VFT ) and VFT thickness (T VFT The iron golf club head also has a body portion secured to a rear surface of the front portion. The iron golf club head has a boundary height (H ) between about 35 mm and about 40 mm. Boundary), and a boundary thickness (T Boundary ), and the iron golf club head has an ESA ratio between about 4.5 and about 5.5, the ESA ratio being: ESA ratio = (0.5×T Boundary ×H Boundary ) / ((H Face ×T Face )+(0.5×(T VFT -T Face )×H VFT )) The golf club head is defined as follows: COR≧0.0196(ESA ratio)+0.7211 The COR is defined by the relationship:

[0013] Another aspect of the present invention is the face height (H Face ), the striking face further having a substantially planar front surface and a non-planar rear surface. The non-planar rear surface of the striking face defines a VFT profile, the VFT profile further having a thicker central section, a transition section, and a thinner peripheral section. The thinner peripheral section has a face thickness (T Face ) and the thick central part is defined as the VFT height (H VFT ) and VFT thickness (T VFT The iron golf club head also has a body portion secured to a rear surface of the front portion. The iron golf club head has a boundary height (H ) between about 35 mm and about 40 mm. Boundary ), and a boundary thickness (T Boundary The above iron golf club head is about 380 mm 2 From about 420 mm 2 ESD Boundary and the ESD Boundary teeth, ESD Boundary =0.5×T Boundary ×H Boundary It is defined as:

[0014] These and other features, aspects, and advantages of the present invention will become better understood with reference to the following drawings, description, and claims. [Brief description of the drawings]

[0015] The foregoing and other features and advantages of the present invention will become apparent from the following description of the invention as illustrated in the accompanying drawings, which are incorporated herein and form a part of the specification, and which further serve to explain the principles of the invention and enable one skilled in the relevant art to make and use the invention.

[0016] [Figure 1] FIG. 1 of the accompanying drawings shows a rear view of a golf club head in accordance with an exemplary embodiment of the present invention. [Diagram 2] FIG. 2 of the accompanying drawings illustrates a rear view of a golf club head in accordance with an exemplary embodiment of the present invention with the support member removed. [Diagram 3] FIG. 3 of the accompanying drawings shows a rear cross-sectional view of a golf club head in accordance with an exemplary embodiment of the present invention. [Figure 4] FIG. 4 of the accompanying drawings shows a cross-sectional view of a golf club head in accordance with an exemplary embodiment of the present invention taken along section line 4-4' shown in FIG. [Diagram 5] FIG. 4 of the accompanying drawings shows a cross-sectional view of a golf club head in accordance with an exemplary embodiment of the present invention taken along section line 5-5' shown in FIG. [Figure 6] FIG. 6 of the accompanying drawings shows a toe side view of a golf club head in accordance with an exemplary embodiment of the present invention. [Figure 7] FIG. 7 of the accompanying drawing illustrates a plot showing the relationship between ESA ratio and COR for golf club heads of the present invention along with that relationship for prior art golf clubs. Detailed Description of the Invention

[0017] The following detailed description sets forth the best mode presently contemplated for carrying out the invention. This description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, the scope of which is best defined by the appended claims.

[0018] Various features of the invention are described below, each of which can be used independently of one another or in combination with other features. However, a single inventive feature may not address some or all of the problems discussed above, or may address only one of the problems discussed above. Furthermore, one or more of the problems discussed above may not be fully addressed by any of the features described below.

[0019] Figure 1 of the accompanying drawings illustrates a rear view of a golf club head 100 in accordance with an exemplary embodiment of the present invention. The golf club head 100 may generally have a toe end 102, a heel end 104, a top line 106, and a sole 108 portion of the golf club head 100 common to iron-type golf club heads 100. In addition to these general features and terminology, Figure 1 of the accompanying drawings also illustrates a portion of the golf club head's hosel 110, to which a shaft is adapted to engage (not shown).

[0020] One feature worth noting here is that Figure 1 of the accompanying drawings shows a cavity back type golf club head 100 that includes a rear opening 112 located at the rear of the golf club head 100. When compared to a traditional blade type golf club head, the cavity back type golf club head 100 generally provides more forgiveness due to the shifting of weight to the perimeter of the golf club head 100. In addition to providing more forgiveness, the cavity back type golf club head 100 also provides a higher Restitution coefficientThe present invention provides an opportunity to improve ball speed of the golf club head 100 by creating "face modes" that can often result in a higher COR, thus providing an improvement over a standard blade-type golf club head.

[0021] Although these types of cavity back iron type golf club heads were previously associated with higher handicap golfers due to the need of players seeking more forgiveness, modern golf clubs have fully embraced this cavity back technology. In fact, modern golf players, regardless of handicap or tour player, often incorporate cavity back irons into their bags, especially for the long irons, as these clubs have transformed into very playable golf clubs that offer benefits to all types of players.

[0022] As more and more effort has been put into meticulously engineering these types of designs, these clubs have developed deeper and more complete cavities, even going so far as to create completely hollow body structures. In these types of performance oriented platforms for iron type golf club heads, there is generally a trend towards thinner and thinner striking faces, which must be combined with stronger and stronger materials. The thinner faces of these types of irons, when combined with those made of stronger materials, especially when built into chassis that produce a higher "faceted" such as a more complete cavity back or hollow body construction, generally achieve a higher face deflection, a higher COR, but with the undesirable effect of increased stress.

[0023] It can therefore be seen that one of the primary objectives of this invention is to create a high performance iron-type golf club head 100 that takes full advantage of the chassis generated "face modes" to reap the benefits that they provide, while balancing the need for performance while managing the increased stresses associated with such a design.

[0024] To achieve the goal of reducing the stresses in these thin-faced iron golf club heads with large unsupported areas of the face that generate "face modes," the present invention utilizes a variable face thickness (VFT) profile 114 on the backside of the striking face 111. The VFT profile 114 may generally be comprised of a thickened center section, a transition area, and a thinned perimeter. In the current exemplary embodiment of the present invention, the thickened center section may generally have a constant thickness. On the other hand, the thinned perimeter may have a generally constant thickness, but may also be tapered. The rear view of the golf club head 100 shown in FIG. 1 is shown as completed, with a support member 120 contacting the rear side of the VFT profile 114. To better illustrate the VFT profile 114, FIG. 2 of the accompanying drawings shows the support member 120 removed. As shown in FIG. 1, the golf club head 100 has a cavity-periphery bulging boundary rib 100A that bulges rearward to form a rear opening (cavity) 112 on the back of the striking face 111, and a support member 120 held at the tip of an arm-shaped member 100B whose base is attached to the portion of the cavity-periphery bulging boundary rib 100A near the toe end 102 and near the sole 108 contacts a VFT profile 114 on the non-flat back surface of the striking face 111 and supports the VFT profile 114.

[0025] 2 of the accompanying drawings shows a rear view of a golf club head 200 in accordance with an exemplary embodiment of the present invention with the support member (shown as 120 in FIG. 1) removed for a clearer illustration of the VFT profile 114. At a glance, it can be readily seen that the VFT profile 214 further comprises the two subcomponents previously discussed. More specifically, the VFT profile 210 further comprises a thickened central portion 210a and a transition portion 210b.

[0026] The thinned perimeter portion 216 of the striking face 111 (previously shown in FIG. 1) shown in FIG. 2 may also be separated into three subcomponents, for example, a heel perimeter portion 216a located near the heel 204 of the golf club head 200, a toe perimeter portion 216c located near the toe 202 of the golf club head 200, and a central perimeter portion 216b juxtaposed between the heel perimeter portion 216a and the toe perimeter portion 216c. In a preferred embodiment of the invention, the entire perimeter portions 216a, 216b, and 216c may be uniformly tapered from the toe perimeter portion 216c to the heel perimeter portion 216a, with the toe perimeter portion 216c being the thickest and the heel perimeter portion 216a being the thinnest. However, in alternative embodiments of the invention, this tapering need not be constant and may only be present in the toe perimeter portion 216c, the central perimeter portion 216b, the heel perimeter portion 216a, or any combination thereof. However, the toe-most portion is thicker than the heel-most portion. In other words, the toe portion of the leading perimeter portion may be said to be thicker than the heel portion of the leading perimeter portion. In other alternative embodiments of the present invention, the entirety of perimeter portions 216a, 216b, and 216c may all be the same thickness without departing from the scope and content of the present invention.

[0027] Thus, when viewed in its entirety at the striking face 211, the golf club head 200 has a variable thickness profile with two subcomponents: 214The peripheral portion 216a may be said to have a thickness of about 1.0 mm, with the two subcomponents being a thick central portion 214a having the greatest thickness, and a transition portion 214b having a variable thickness transitioning from the thick central portion 214a to the peripheral portions 216a, 216b, and 216c. In a preferred embodiment, the peripheral portions 216a, 216b, and 216c may generally be thicker at the toe than at the heel, gradually tapering throughout. To accomplish this, the heel peripheral portion 216a may be flat or tapered, the central peripheral portion 216b may be flat or tapered, and the toe peripheral portion 216c may be flat or tapered, or any combination thereof may be employed, all without departing from the scope and content of the present invention. Finally, in an alternative embodiment of the present invention, as previously discussed, the entire peripheral portions 216a, 216b, and 216c may be constant without departing from the scope and content of the present invention.

[0028] A cross-sectional view of golf club head 200 is provided with the rear of golf club head 200 removed to provide a clearer view of the various dimensions of variable thickness profile 214. Figure 3 of the accompanying drawings shows an enlarged cutaway view of golf club head 300 in accordance with an exemplary embodiment of the present invention, allowing certain additional features to be highlighted.

[0029] First, in this cutaway view of the golf club head 300 shown in Figure 3, the variable thickness profile 314 can be seen more clearly. The thickened central portion 314a shown in Figure 3 allows one to identify the dimension as this dimension is critical to the performance of the golf club head 300 itself. In this embodiment of the invention, the thickened central portion 314a has a width W of between about 15 mm and about 35 mm, more preferably between about 20 mm and about 30 mm, and most preferably about 26 mm. VFT The thickened central portion 314a may have a height H between about 10 mm and about 27 mm, more preferably between about 15 mm and about 22 mm, and most preferably about 18 mm. VFTFigure 3 also shows a center point 333 of the thickened central portion 314a. As shown in FIG. 3 and FIG. 2, the width W of the thickened central portion 314a VFT is its height H VFT Greater than.

[0030] FIG. 3 of the accompanying drawings also shows the dimensions of the thickened central portion 314a as well as a distance H corresponding to the height of the striking face 311 at the center of the face. Face Distance H Face The height of the striking face 311, represented by , may generally be between about 50 mm and about 54 mm, more preferably between about 51 mm and about 53 mm, and most preferably about 52 mm.

[0031] Finally, FIG. 3 of the accompanying drawings also identifies two specific points within the thick central portion 314a, namely, a maximum stress location 332 and a maximum deflection location 334. It is important for the present invention that the thick central portion 314a encompasses the maximum stress location 332 and the maximum deflection location 334, since these locations generally indicate the locations that experience the highest stress and can benefit from the increased material thickness associated with the thick central portion 314a. The maximum deflection location 334 referred to in this embodiment of the present invention generally refers to a point that corresponds to a hot spot of the face, which also happens to be the geometric center of the face of the golf club head. The maximum deflection location 334 can generally be determined by making a physical measurement of the geometric center of the face of the golf club head 300. Alternatively, this location may be determined by trying and understanding the maximum deflection based on a prediction of the actual deflection that the golf club head 300 is expected to experience upon impact with a golf ball via modal analysis. On the other hand, the maximum stress location 332 generally occurs at a location directly behind the impact location of the golf ball. This location is generally slightly heelward of the location of maximum deflection 334 and slightly lower on the face than the maximum deflection location 334. This location is generally estimated to be at the center point of the scoreline in the heel-to-toe direction, approximately 15 mm above the leading edge of the golf club head 300. Again, it is important to reiterate that the thickened central portion 314a encompasses both the maximum stress location 332 and the maximum deflection location 334, which is important to the present invention, because both of these locations experience higher stresses than the rest of the face when impacting a golf ball, and the increased thickness of the thickened central portion 314a is highly beneficial to the durability of the golf club head 300.

[0032] FIG. 4 of the accompanying drawings illustrates a cross-sectional view of the golf club head 300 shown in FIG. 3 taken along section line 4-4'. This cross-sectional view allows the thickness of the striking face 411 to be more clearly seen. In this view, it can be seen that the striking face 411 has a substantially planar front surface and a non-planar rear surface, creating a VFT profile. More specifically, the VFT profile is comprised of a thickened central portion 414a transitioning to a peripheral portion 416c as previously discussed. The thickened central portion 414a according to this exemplary embodiment of the present invention will generally have a thickness of between about 1.9 mm and about 2.3 mm, more preferably between about 2.0 mm and about 2.2 mm, and most preferably about 2.1 mm. TVFT 3, the thickness of the perimeter portion is actually the thickness of the central peripheral portion 416b. The central peripheral portion 416b located both above and below the thick central portion 414a may generally have a thickness T Face of about 1.6 mm to about 2.0 mm, more preferably about 1.7 mm to about 1.9 mm, and most preferably about 1.8 mm. The transition regions 414b located above and below the thick central portion 414a also gradually taper from the thick central portion 414a to the thickness of the central peripheral portion 416b.

[0033] Figure 5 of the accompanying drawings shows a cross-sectional view of a golf club head 500 taken along section line 5-5' shown in Figure 3. This section line is taken at a point along the toe portion of the golf club head 300 that results in the highest face height value. Thus, although previously described in Figure 3, H Face However, this cross-sectional view does not provide all the information required in FIG. 3 to achieve accurate measurements because there is no reference point from the side view. More specifically, the face height distance H Faceis measured along the loft of the golf club head 500, which is parallel to the striking face, not parallel to the ground as in FIG. 3. This may be initially suggested. It is important to obtain an accurate definition for measuring the face height distance HFace, which is important to this invention, the effective boundary stiffness area (ESA) of the face (ESA Face ) is an important variable in determining ESA Face is defined by the following equation (1). ESA Face =(H Face ×T Face )+(0.5×(T VFT -T Face )×H VFT Formula (1) Based on equation (1) above, the golf club head 500 according to an exemplary embodiment of the present invention has a ball bearing diameter of approximately 81 mm 2 From about 110 mm 2 More preferably, between about 89 mm 2 From about 100 mm 2 and most preferably between about 96 mm 2 ESA Face It is acceptable to have

[0034] 6 of the accompanying drawings shows a toe side view of a golf club head 600 in accordance with an exemplary embodiment of the present invention, which may indicate the overall size of the golf club head 600. FIG. 6 further illustrates the boundary ESA (ESA Boundary Finally, golf clubs according to the present invention are designed to be able to fit a particular ESA. FACE Against ESA Boudary First and foremost, Figure 6 of the accompanying drawings shows the H Boudary The present invention shows a golf club having a chassis height defined as H Boudaryis measured from the top of the golf club head 600 to the bottom of the club head 600 at the address position. The golf club head 600 may generally have a boundary between about 35 mm and about 40 mm, more preferably between about 36 mm and about 39 mm, and most preferably between about 37 mm and about 38 mm. Boudary Once H is determined, the geometric center of face 634 is Boundary The distance can be determined at least on the face by dividing the center by 2. Once the center of face 634 is determined, the thickness of the chassis boundary, T Boundary can be determined. Boundary is determined as measured rearward from the center of the face 634 at an angle perpendicular to the angle of the striking face itself. As shown in this current exemplary embodiment, T Boundary Generally, the width may be between about 18 mm and about 24 mm, more preferably between about 19 mm and about 23 mm, and more preferably between about 20 mm and about 22 mm.

[0035] Once the HBoundary and TBoundary values ​​are established, the ESA Boundary can be calculated based on the following formula (2): ESA Boundary =0.5×T Boundary ×H Boundary Formula (2) Based on equation (2) above, the golf club head 600 according to an exemplary embodiment of the present invention has a length of approximately 380 mm. 2 From about 420 mm 2 More preferably, between about 390 mm 2 From about 410 mm 2 Between about 400 mm and most preferably 2 ESA Boudary It is good to have.

[0036] ESA Face and ESA BoundaryWith ESA defined and established, a ratio between the two numbers can be made to generate the ESA ratio. This illustrates one of the key features of the present invention, which describes one way to quantify the face modes experienced by the club head 600. The ESA ratio is defined by equation (3) below: ESA ratio = ESA Boundary / ESA Face Formula (3) Golf club heads according to the present invention may generally have an ESA ratio between about 4.5 and about 5.5, more preferably between about 4.7 and about 5.3, and more preferably between about 4.8 and about 5.2. Alternatively, equation (3) can be combined with equations (1) and (2) to create a more detailed definition of the ESA ratio, as shown in equation (4) below. ESA ratio = 0.5×T Boundary ×H Boundary / ((H Face ×T Face )+(0.5×(T VFT -T Face )×H VFT )) Formula (4)

[0037] FIG. 7 of the accompanying drawings shows a plot of ESA ratio versus coefficient of restitution (COR) for several embodiments of the present invention as well as four different prior art clubs. In FIG. 7, as shown, the ESA ratio is shown along the X-axis and the COR of the golf club head is shown along the Y-axis. Focusing on three of the prior art golf club heads, it can be seen that the ESA ratio values ​​of these three prior art data points are not between 4.5 and 5.5, are not between about 3.4 and about 4.25, and are not about 4.0. Another fourth prior art data point has an ESA ratio value between 4.5 and 5.5, but does not achieve a sufficiently high COR of more than 0.828 for a given ESA ratio value. To capture the area of ​​improved performance achieved by the present invention, a formula is provided in FIG. 7, where y≧0.0196x+0.7211 helps to further narrow the range of performance values ​​not previously achieved within the aforementioned ESA ratio values. To put the above relationship into the context of values ​​relevant to the current discussion, equation (5) is given below. COR≧0.0196(ESA ratio)+0.7211 Formula (5)

[0038] Combining this discussion, it can be said that golf club heads currently in accordance with the present invention have ESA ratios of between about 4.5 and about 5.5, and COR values ​​greater than 0.0196 (ESA ratio) + 0.7211.

[0039] Finally, no discussion of thin-faced irons made herein would be complete without acknowledging the importance of durability. The durability of a golf club is generally related to the amount of stress it experiences upon impact with a golf ball. Typically, for iron-type golf club heads made of steel, the durability of the golf club drops significantly at any number above about 2.2 GPa. In other words, the golf club heads of the present invention are intended to have stresses of less than about 2.2 GPa, more preferably less than about 2.1 GPa, and most preferably less than about 2.0 GPa.

[0040] Other than in the working examples, or unless otherwise indicated, all numerical ranges, amounts, values, and percentages in the preceding specification, such as amounts of materials, moments of inertia, center of gravity locations, lofts, draft angles, various performance ratios, etc., may be understood as if preceded by the term "about" even if the term "about" does not explicitly appear in the value, amount, or range. Accordingly, unless otherwise indicated, the numerical parameters set forth in the foregoing specification and appended claims are approximations that may vary depending upon the desired properties sought to be obtained by the present invention. At the very least, and without attempting to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should be construed at least in light of the number of reported significant digits and by applying ordinary rounding techniques.

[0041] Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, the numerical values ​​set forth in the specific examples are reported as precisely as possible. However, such numerical values ​​inherently contain certain errors resulting from the standard deviation found in their respective testing measurements. Further, when numerical ranges of various scopes are set forth herein, it is to be noted that any combination of these values ​​inclusive of the recited values ​​may be used.

[0042] Of course, it will be understood that the foregoing relates to exemplary embodiments of the invention and that modifications can be made without departing from the spirit and scope of the invention as set forth in the following claims.

Claims

1. In the iron golf club head, Face height (H Ｆａｃｅ a forward portion having a striking face defining a The striking face further has a substantially planar front surface and a non-planar rear surface; the non-planar rear surface of the striking face defines a VFT profile, the VFT profile further having a thickened central portion, a transition portion, and a thinned peripheral portion; The thin peripheral portion is the face thickness (T Ｆａｃｅ ) and The thick central part is the VFT height (H ＶＦＴ ) and VFT thickness (T ＶＦＴ ) the forward portion defining a body portion secured to a rear surface of the front portion; a VFT width (W VFT ) of the thick central portion is greater than the VFT height (H VFT ) of the thick central portion; the body portion having a cavity perimeter bulging boundary rib (100A) bulging rearwardly to form a cavity on the rear surface of the striking face; The cavity periphery bulging boundary rib (100A) has a boundary height (H Ｂｏｕｎｄａｒｙ ) and boundary thickness (T Ｂｏｕｎｄａｒｙ ) and a support arm (100B) having a base attached to the toe-side and sole-side portions of the cavity periphery bulging boundary rib (100A) has a retaining member (120) at the tip thereof, which contacts the VFT profile portion of the non-flat back surface of the striking face to support the VFT profile portion, thereby increasing the allowable stress of the striking face; The iron golf club head has an ESA ratio between 4.5 and 5.5, the ESA ratio being: ESA ratio = (0.5 × T Ｂｏｕｎｄａｒｙ ×H Ｂｏｕｎｄａｒｙ ) / ((H Ｆａｃｅ ×T Ｆａｃｅ )+(0.5×(T ＶＦＴ -T Ｆａｃｅ )×H ＶＦＴ )) It is defined as The golf club head is COR≧0.0196 (ESA ratio) +0.7211 1. An iron golf club head having a COR defined by the following relationship:

2. The above iron golf club head is 81 mm ２ From 110 mm ２ ESA between Ｆａｃｅ and ESA Ｆａｃｅ teeth, ESA Ｆａｃｅ =(H Ｆａｃｅ ×T Ｆａｃｅ )+(0.5×(T ＶＦＴ -T ＶＦＴ )×H ＶＦＴ 2. The golf club head of claim 1, defined by:

3. The above ESA Ｆａｃｅ is 89 mm ２ From 100 mm ２ 3. The iron golf club head according to claim 2, wherein the diameter of the iron golf club head is between 100 mm and 150 mm.

4. The above ESA Ｆａｃｅ is 96 mm ２ 4. The iron golf club head according to claim 3, wherein

5. The iron golf club head is 380 mm ２ From 420 mm ２ ESA Ｂｏｕｎｄａｒｙ and the ESA Ｂｏｕｎｄａｒｙ teeth, ESA Ｂｏｕｎｄａｒｙ =0.5×T Ｂｏｕｎｄａｒｙ ×H Ｂｏｕｎｄａｒｙ 3. The iron golf club head of claim 2, defined as:

6. The above ESA Ｂｏｕｎｄａｒｙ is 390 mm ２ From 410 mm ２ 6. The iron golf club head according to claim 5.

7. The above ESA Ｂｏｕｎｄａｒｙ is 400 mm ２ 7. The iron golf club head according to claim 6, wherein

8. The thick central portion has a VFT thickness (T ＶＦＴ ), the VFT height (H ＶＦＴ ), and the VFT width (W ＶＦＴ 2. The iron golf club head of claim 1, further comprising:

9. The VFT thickness (T ＶＦＴ ) is between 2.0 mm and 2.2 mm, and the VFT height (H ＶＦＴ ) is between 10 mm and 27 mm, and the VFT width (W ＶＦＴ 9. The iron golf club head according to claim 8, wherein the distance between the center of the shaft and the center of the shaft is between 20 mm and 30 mm.

10. The VFT thickness (T ＶＦＴ ) is 2.1 mm, and the VFT height (H ＶＦＴ ) is 18 mm, and the VFT width (W ＶＦＴ 10. The iron golf club head according to claim 9, wherein the distance between the shaft and the shaft is 26 mm.

11. In the iron golf club head, Face height (H Ｆａｃｅ a forward portion having a striking face defining a The striking face further has a substantially planar front surface and a non-planar rear surface; the non-planar rear surface of the striking face defines a VFT profile, the VFT profile further having a thickened central portion, a transition portion, and a thinned peripheral portion; The thin peripheral portion is the face thickness (T Ｆａｃｅ ) and The thick central part is the VFT height (H ＶＦＴ ) and VFT thickness (T ＶＦＴ ) the forward portion defining a body portion secured to a rear surface of the front portion; a VFT width (W VFT ) of the thick central portion is greater than the VFT height (H VFT ) of the thick central portion; the body portion having a cavity perimeter bulging boundary rib (100A) bulging rearwardly to form a cavity on the rear surface of the striking face; The cavity perimeter bulging boundary rib (100A) has a boundary height (H) between 35 mm and 40 mm. Ｂｏｕｎｄａｒｙ ), and the boundary thickness between 18 mm and 24 mm (T Ｂｏｕｎｄａｒｙ ) and a support arm (100B) having a base attached to the toe-side and sole-side portions of the cavity periphery bulging boundary rib (100A) has a retaining member (120) at the tip thereof, which contacts the VFT profile portion of the non-flat back surface of the striking face to support the VFT profile portion, thereby increasing the allowable stress of the striking face; The iron golf club head has an ESA ratio between 4.5 and 5.5, the ESA ratio being: ESA ratio = (0.5 × T Ｂｏｕｎｄａｒｙ ×H Ｂｏｕｎｄａｒｙ ) / ((H Ｆａｃｅ ×T Ｆａｃｅ )+(0.5×(T ＶＦＴ -T Ｆａｃｅ )×H ＶＦＴ )) It is defined as The golf club head is COR≧0.0196 (ESA ratio) +0.7211 1. An iron golf club head having a COR defined by the following relationship:

12. The boundary height (H Ｂｏｕｎｄａｒｙ ) is between 36 mm and 39 mm, and the VFT thickness (T ＶＦＴ 12. The iron golf club head of claim 11, wherein the distance between the center of the shaft and the center of the back of the club is between 19 mm and 23 mm.

13. The boundary height (H Ｂｏｕｎｄａｒｙ ) is between 37 mm and 38 mm, and the VFT thickness (T ＶＦＴ 13. The iron golf club head of claim 12, wherein the distance between the center of the shaft and the center of the back of the club is between 20 mm and 22 mm.

14. The iron golf club head is 380 mm ２ From 420 mm ２ ESA Ｂｏｕｎｄａｒｙ and the ESA Ｂｏｕｎｄａｒｙ teeth, ESA Ｂｏｕｎｄａｒｙ =0.5×T Ｂｏｕｎｄａｒｙ ×H Ｂｏｕｎｄａｒｙ 14. The iron golf club head of claim 13, defined as:

15. The above ESA Ｂｏｕｎｄａｒｙ is 390 mm ２ From 410 mm ２ 15. The iron golf club head according to claim 14.

16. The above ESA Ｂｏｕｎｄａｒｙ is 400 mm ２ 16. The iron golf club head according to claim 15,

17. In the iron golf club head, Face height (H Ｆａｃｅ a forward portion having a striking face defining a The striking face further has a substantially planar front surface and a non-planar rear surface; the non-planar rear surface of the striking face defines a VFT profile, the VFT profile further having a thickened central portion, a transition portion, and a thinned peripheral portion; The thin peripheral portion is the face thickness (T Ｆａｃｅ ) and The thick central part is the VFT height (H ＶＦＴ ) and VFT thickness (T ＶＦＴ ) the forward portion defining a body portion secured to a rear surface of the front portion; a VFT width (W VFT ) of the thick central portion is greater than the VFT height (H VFT ) of the thick central portion; the body portion having a cavity perimeter bulging boundary rib (100A) bulging rearwardly to form a cavity on the rear surface of the striking face; The cavity perimeter bulging boundary rib (100A) has a boundary height (H) between 35 mm and 40 mm. Ｂｏｕｎｄａｒｙ ), and the boundary thickness between 18 mm and 24 mm (T Ｂｏｕｎｄａｒｙ ) and a support arm (100B) having a base attached to the toe-side and sole-side portions of the cavity peripheral bulging boundary rib, the support arm (120) at the tip of the support arm contacts the VFT profile portion of the non-flat back surface of the striking face to support the VFT profile portion, thereby increasing the allowable stress of the striking face; The iron golf club head is 380 mm ２ From 420 mm ２ ESA Ｂｏｕｎｄａｒｙ and the ESA Ｂｏｕｎｄａｒｙ teeth, ESA Ｂｏｕｎｄａｒｙ =0.5×T Ｂｏｕｎｄａｒｙ ×H Ｂｏｕｎｄａｒｙ 1. An iron golf club head, characterized in that:

18. The above ESA Ｂｏｕｎｄａｒｙ is 390 mm ２ From 410 mm ２ 18. The iron golf club head according to claim 17.

19. The above ESA Ｂｏｕｎｄａｒｙ is 400 mm ２ 20. The iron golf club head according to claim 18, wherein:

20. The thick central portion has a VFT thickness (T ＶＦＴ ), the VFT height (H ＶＦＴ ), and the VFT width (W ＶＦＴ 20. The iron golf club head of claim 19, further comprising:

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