Method Of Manufacturing Game Rackets

Abstract

A method of manufacturing a game racket having improved stiffness and uniformity is disclosed. The racket comprises a shell formed from a molded synthetic resin with a high strength fiber reinforced plastic facing laminated to each face of the shell. The shell is internally recessed in a truss-like pattern to provide lightness with strength. The handle of the racket includes a pair of plastic pallets secured to the handle portion of the shell and covered with a covering material, such as a thin leather strip winding. The finished racket is finally strung in a conventional manner and is then ready for use.

Parent Case Text

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. Patent Application Ser. No. 241,176, filed Apr. 5, 1972, now U.S. Pat. No. 3,840,230.

Description

BACKGROUND OF THE INVENTION

This invention relates generally to tennis rackets or the like and, more specifically, to a racket formed from reinforced plastic materials.

Strung rackets for use in tennis and similar games have long been made from wood and strung with gut or nylon strings. Manufacture of high quality rackets is a complex process. Despite great care in manufacture, the playing characteristics of wood rackets vary greatly due to natural variations in the wood used and manufacturing process variables.

Playing characteristics of wood rackets also vary with changes in temperature and humidity and with age of the racket, which changes may also cause the head to warp due to varying string tension.

Recently, steel and aluminum rackets have been developed in order to obtain greater uniformity. However, it has been found that these metal rackets do not provide the same playing qualities as wood rackets, since sufficient stiffness cannot be provided without excessive weight. Also, metal rackets frequently have short useful lives, due to cracking apparently caused by metal fatigue or stress concentrations.

Rackets are also being manufactured from fiberglass reinforced plastics. These rackets, however, generally are overly flexible and do not provide the desired stiffness for equivalent weight. Also, they lack uniformity and are expensive due to the number of manual manufacturing steps.

No matter which material has been used, there have always been problems in providing sufficient strength in the racket head to resist stresses during stringing of the racket. As stringing progresses, severe stress concentrations result in portions not yet supported by strings. As stringing continues, the locations of stress concentrations shift. The racket frame may severely warp or break due to these stresses, if the frame lacks the strength and stiffness to resist the stresses induced by the very taut strings. Generally, attempts to strengthen the frame by increasing its cross-sectional area have not been successful due to the resulting weight increase and balance shift to the head.

Thus, there is a continuing need for rackets of improved strength, stiffness and playing characteristics.

SUMMARY OF THE INVENTION

It is, therefore, an object of this invention to provide a racket of improved strength and stiffness.

Another object of this invention is to provide a racket having improved resistance to stringing stresses.

Still another object of this invention is to provide a simple and consistent method of manufacturing rackets.

The above objects, and others, are accomplished in accordance with this invention by a racket for tennis or the like which basically comprises a shell formed from a molded synthetic resin with a high strength fiber reinforced plastic facing laminated to each face of the shell. The shell has a truss-like internal structure which is covered by the facing. This internal structure provides high strength with low weight and supports the strings in a desirable manner. Preferably, the shell is formed by injection molding, since this allows rapid production of a highly uniform product. The racket is completed by the addition of a pair of pallets to the handle portion of the shell, after which a surface layer of thin leather strips or the like may be wound around the handle portion to provide a desired gripping surface.

The racket may have any desired shape. In general, an elliptical or round frame is preferred. The method of the present invention is adaptable to a wide variety of frame shapes.

The racket shell may be formed from any suitable resin having the required properties. The shell resin should have high stiffness, good impact resistance, predictable and consistent mold shrinkage, good dimensional stability and high fatigue resistance. For best results, the material should have a flexural modulus in the range of from about 3 .times. 10.sup.5 to about 17 to 10.sup.5 psi and an Izod impact measurement of from about 2 to about 15 ft-lb./in. (notched bar). Suitable resins include polycarbonate and polyphenylene oxide resins, typically available from General Electric Co. under the trademarks Lexan and Noryl, respectively.

The shell resin may contain fillers or additives to improve desired properties. For example, up to 40 volume per cent randomly chopped glass fibers, typically having lengths of from about 0.1 to 0.25 inch may be added to improve strength properties.

The weight of the shell is determined largely by shell thickness, the internal truss pattern and the resin/filler combination selected. Where it is desired to market a line of similar rackets having different weights, varying shell thickness to vary the racket weight has been found to be convenient and effective. Alternatively, the thickness of the truss-like internal webs may be varied. If desired, the internal openings of the shell may be filled with a foamed synthetic resin. In a typical racket of the sort described herein, a reduction of the thickness of the shell in the plane of the racket of 0.05 inch provides approximately one ounce difference in shell weight.

The facing layers may comprise any suitable high strength fiber in a synthetic resin matrix. Best results are obtained with fibers having Young's modulus of from about 20 .times. 10.sup.6 to about 80 .times. 10.sup.6 psi and density of from about 0.05 to about 0.07 lb/in.sup.3. Therefore, fibers having these properties are preferred. Especially suitable fibers include high strength graphite fibers, such as those available from Union Carbide Corporation under the trademark "Thornel" and high strength organic fibers such as those available from E. I. duPont de Nemours & Co. under the trademark PRD-41.

The high-strength fibers may be imbedded in any suitable synthetic resin. The resin selected should have high impact resistance, good dimensional stability and good peel strength. In general, best results are obtained with epoxy polyimide and phenolic resins.

Preferably, the high-strength fibers are positioned in a substantially parallel arrangement around the racket face and down the handle. In order to provide a uniform, smooth surface and to increase the transverse strength of the facing, it is preferred that a surface layer of a resin impregnated woven fiber be formed over the facing. This over-layer may surround the parallel-fiber main portion, or may merely cover both planar surfaces of the facing. Any suitable fibers may be used in the overlayer. While high-strength fibers, such as high-strength graphite fibers, are preferred, other fibers, such as organic or glass fibers, may be used, if desired.

The handle pallets may be formed from any suitable material, such as wood or plastic. Acrylonitrile-butadiene-styrene (ABS) polymers are preferred, since they are inexpensive, easily injection molded, and are sufficiently flexible to give a desirable handle "feel." If desired, any suitable covering material, such as leather, fabric or plastic sheet or strip material, may be added over the racket handle to improve the grip characteristics. Typically, the circumference of the racket grip (circumference of the pallets plus the leather or other covering) ranges from about 4 to 5 inches. Preferably, eight different pallet molding are prepared, ranging from 4 to 5 inches in circumference in 1/8-inch increments, so that the racket can be sized to meet any player's preference.

BRIEF DESCRIPTION OF THE DRAWING

Further details of the invention will be understood upon reference to the drawing, which illustrates a preferred embodiment of the racket of this invention.

In the drawing:

FIG. 1 is a perspective view of a complete racket;

FIG. 2 is an exploded perspective view illustrating the major racket components;

FIG. 3 is a plan view of the face of the racket;

FIG. 4 is a side elevation view of the racket shown in FIG. 3.

FIG. 5 is an enlarged detail view showing the base of the racket taken on line 5--5 in FIG. 3;

FIG. 6 is an enlarged section view through the racket handle, taken on line 6--6 in FIG. 3;

FIG. 7 is an enlarged section view through the racket throat and frame, taken on line 7--7 in FIG. 3;

FIG. 8 is an enlarged transverse section view through the racket frame, taken on line 8--8 in FIG. 3;

FIG. 9 is a longitudinal detail view of a portion of the frame, taken on line 9--9 in FIG. 3;

FIG. 10 is an enlarged section view through the racket frame, taken on line 10--10 in FIG. 9;

FIG. 11 is a plan view of the facing;

FIG. 12 is a side view of the facing;

FIG. 13 is an enlarged section view through the facing frame, taken on line 13--13 in FIG. 11;

FIG. 14 is a plan view of a handle pallet;

FIG. 15 is a section view of the pallet taken on line 15--15 in FIG. 14; and

FIG. 16 is an enlarged section view through the pallet, taken on line 16--16 in FIG. 14.

DETAILED DESCRIPTION OF THE DRAWING

An overall view of a complete, strung, tennis racket is shown in FIG. 1. The racket basically consists of a shell 10 having a fiber reinforced plastic facing 12 on each face. The racket frame or head portion 14 is strung with a taut gut or nylon string 16 which passes through a plurality of transverse holes 18 in frame 14. A pair of pallets 20 are secured to the sides of handle portion 22 of shell 10 and wrapped with a thin covering 24 to provide a comfortable grip area.

The relationship of the major racket components can more clearly be seen in the exploded view of FIG. 2. Shell 10 is preferably recessed at 26 on each face to receive facing 12, giving a smoothly contoured outer surface to the completed racket. Behind recesses 26 shell 10 is pocketed in a truss-like pattern to provide maximum strength with minimum weight. These truss structures are hidden by facing 12 in the completed racket.

Facing 12 extends entirely around frame 14 to provide stiffness and strength to resist impact against a ball in play, and also to strengthen frame 14 against varying stresses in the plane of frame 14 during stringing. Facing extends from frame 14 well down handle 22, adding strength to the racket throat portion 28, which is often weak in rackets made from other materials. Facing 12 also adds desired stiffness to handle 22. Stiffness of the different portions of the racket can easily be varied by varying the cross-sectional area of the facing 12 at different locations to give the racket the desired playing characteristics.

A groove 30 is provided in the outer surface of frame 14 so that the string is recessed as it passes between holes 18. This protects the string against abrasion, should the racket strike the court playing surface during use.

Pallets 20 substantially surround the ends of handle 22. Pallets 20 are securely held in place by sides 32 which extend around the sides of handle 22 and by pins 34 on pallets 20 which enter holes 36 in handle 22. The bases of pallets 20 are preferably recessed at 38 so that the manufacturer's trademark or other emblem may be emplaced there.

Facing 12 may be secured to shell 10 in any suitable manner. Preferably, adhesive bonding, such as with an epoxy adhesive, is used. While pallets 20 are held in place by the interlocking effect of sides 32 and pins 34 together with the leather wrapping, they may also be adhesively bonded to shell 10, if desired.

When assembled as illustrated in FIG. 1, the resulting racket has outstanding strength, stiffness and playing characteristics. These characteristics result in large measure from the configuration and composition of the component parts, which are illustrated in detail in the remaining figures.

The top of shell 10 is shown in FIG. 3 and the side of the shell in FIG. 4. This is a single unitary member which is preferably formed from a polycarbonate resin in a single injection molding step. The truss-like pattern of pockets in shell 10 is arranged so as to permit convenient production in a simple mold.

As best seen in FIGS. 3, 5, and 6, the handle portion 22 can be thought of as a solid polycarbonate member with a plurality of triangular recesses or pockets 40 extending nearly through the handle alternately from the upper and lower surfaces thereof. Pockets extending upwardly in FIG. 3 are illustrated by broken lines 42. This produces a very strong truss-like structure with very light weight, since the webs between adjacent pockets 40 and hidden pockets 42 are thin, and the base 44 of each pocket is thin, as seen in FIG. 6.

As seen in FIGS. 3 and 5, the base of handle 22 is closed by an end wall 48. Also, the sides of handle 22 are grooved in a continuation of groove 30 which surrounds frame 14 to protect the racket strings against abrasion should the playing surface be struck during play. Groove 30 serves to reduce the weight of handle 22 while retaining strength and stiffness. The upper and lower surfaces of handle 22 include a depression or recess 48 sized to receive the handle portion of facing 12. When bonded in place, facing 12 covers and hides the truss arrangement produced by the array of pockets 40 and 42.

As seen in the lower portion of FIG. 3, a depression or hole 36 is formed to receive a locating pin on pallets 20, as further described below. Moving up the handle as seen in FIG. 3, the pattern of pockets changes in throat area 28 to a pattern around frame 14 which accommodates stringing holes. As shown in section in FIG. 7, which illustrates the transition between throat 28 and frame 14, pockets 52 in the upper face of frame 14 and pockets 54 in the lower face of frame 14 alternate around the frame to produce a truss-like web between adjacent pockets. Web 55 is the final handle web before the frame pocket pattern develops. A transverse thickened portion 56 is formed in each inter-pocket web through which a stringing hole 58 may be formed. The stringing holes 58 may either be produced during the molding operation by removable pins positioned in the mold, or may be drilled after molding of the shell. This arrangement in which stringing holes 58 penetrate through solid portions of shell 14 is highly desirable both from a strength standpoint and because only two apertures need be deburred or smoothed to prevent abrading the string. If the holes penetrated through a pocketed area, there would be four surface apertures to be smoothed.

A transverse section through shell 14 is seen in FIG. 8. The frame faces are recessed, with upstanding edges 60, so that when facing 12 is bonded in place the outer facing surface will blend smoothly into edge 60. As described below, the facing is grooved in the frame area, producing a broad U-shaped cross-section. A ridge 62 is provided around frame 14 to fit precisely within the facing groove, assuring excellent bonding between facing 12 and shell 14. As discussed above, a string-protecting groove 30 is provided in the outer edge of frame 14. Also, a groove 64 of generally semicircular cross-section is provided in the inner edge of frame 14 to reduce weight while retaining maximum strength.

FIGS. 9 and 10 illustrate a pair of raised areas 66 within groove 30 adjacent to racket throat 28. In the groove area, the angle between the string as it lies within groove 30 between string holes 58 and the string as it lies with holes 58 in this area is an acute angle. This sharp angle as the string changes direction upon entering holes 58 has been found to damage the string. Raised areas 66, properly positioned within groove 30, relieve these severe localized stresses.

Details of the facing members 12 can be seen in FIGS. 11 and 12. The facing generally consists of a frame portion 68 and a handle portion 70. Handle portion 70 may have a constant cross-section, or may taper in thickness and/or width to vary the stiffness characteristics of the completed racket handle. As seen in section in FIG. 13, frame portion 68 has a broad U-shaped cross-section, with raised edges 72 bounding a central groove 74. Ordinarily the thickness of the facing material in the area of groove 74 is about one-half the thickness of the facing in the handle portion 70. It has been found that the raised edges 72 add greatly to the strength of the facing and racket frame 14 without adding appreciably to the weight of the racket. Typically, the facing may be about 0.070 to 0.090 inch thick in the handle 70 portion, with raised edges 72 and groove 74 having thicknesses in the range of about 0.110 to 0.130 inch and about 0.025 to 0.055 inch, respectively. Handle 70 typically may have a width in the range of from about 0.90 to 1.10 inches. The width of the facing in frame portion 68 may typically range from about 0.45 to about 0.65 inch.

Strength and stiffness in the frame area are especially important, since forces in the plane of frame 14 must be resisted during stringing, and ball impact forces in a direction substantially perpendicular to the plane of frame 14 must be resisted during play. As discussed above, groove 74 and raised edges 72 mesh with ridge 62 on shell 10 when facing 12 is bonded to shell 10.

Facing 12 consists primarily of graphite fibers in a resin matrix. Preferably, a thin surface layer 69 comprising resin impregnated fiberglass cloth is applied to the inner, outer or all surfaces of facing 12 in order to improve the transverse strength characteristics of the facing, as discussed above. The inner fiberglass surface layer 69 extends down handle portion 70 only as far as line 71 (FIG. 12) because the increased transverse strength is not required in the handle portion 70 of facing 12.

Details of the handle pallets 20 are shown in FIGS. 14, 15, and 16. FIG. 14 shows the side of pallet 20 which fits against shell handle 22. The outer surface configuration of pallet 20 can be seen in FIG. 2. When installed, pallets 20 substantially surround handle portion 22. Ribs 80 add strength and rigidity to the pallets and rest on the surface of handle 22. Rib 82 includes an outwardly extending pin 34 which engages hole 36 in handle 22 to locate the pallet in the desired position. If desired, pallets 20 may be adhesively bonded to handle 22, though this is not always necessary. After pallets 20 are fitted in place, a winding of thin leather or the like is applied to give a desired gripping surface, as shown in FIG. 1. The racket is then ready for stringing and use.

While the method of manufacturing these rackets has been described in general terms, a preferred embodiment of this method is provided in the following example.

EXAMPLE

A mold is prepared for the racket shell 10 in the configuration described above and shown in the drawing. An injection molding material comprising "Lexan" polycarbonate resin, available from General Electric, filled with 10 percent chopped glass fibers is prepared and injected into the mold. After completion of the molding operation, 0.136-inch diameter holes 58 are drilled through thickened areas 56 to receive the racket strings. Although typical strings are only about 0.05-inch in diameter, the larger sized hole is used in order to accommodate double strings either as part of the basic stringing arrangement or to allow for repairs.

A pair of facing members 12 is prepared by compression molding. Material for the facings is a Type A graphite filament preimpregnated with an epoxy resin, available from the Fiberite Company under the X505 designation. This material is in the form of continuous tows about 57 inches long. Successive tows are laid up in a pre-compaction die and encased in a thin layer (about 0.005 inch thick) of fiberglass cloth. The layup is then placed in the facing forming die, compacted at about 50 psi and cured at about 250.degree.F for about 30 minutes. After minor cleanup and removal of flash, the facings are ready for installation on the shell. The fiberglass casing becomes an integral part of the facing and provides cross-tension strength to the laminate.

The surfaces of the shell and facing are prepared for bonding with a light sandblast and a liquid Freon (a fluorocarbon liquid available from E. I. duPont de Nemours & Co.) wipe just prior to bonding to insure a good "tooth" and a clean interface. The facings are bonded to the shell with an epoxy resin, Epoxy 934, available from the Hysol Chemical Co. About 10 psi mechanical pressure, is applied during bonding. Excess adhesive which may squeeze out is removed immediately.

After the facing adhesive is cured, the racket is inspected, tested, any desired decals are applied and a urethane resin finish coat is applied. The coating is dried at about 120.degree.F for about 8 hours to drive off all volatiles.

A pair of pallets 20 as shown in the drawing are injection molded from Cycolac resin, an acrylonitrile-butadiene-styrene resin available from the Morbon Chemical Division of the Borg-Warner Corporation. The pallets are bonded to the racket with conventional contact cement. The pallets are coated with a grip adhesive, Inco No. 155 from the intercoastal Corp. The leather grip material is installed over the pallets, after which the grip adhesive is activated by applying a solvent through the leather covering.

Finally, any desired labels or identification symbols are applied and the racket is strung in a conventional manner. The racket is found to have excellent playing characteristics and durability.

While certain specific materials, arrangements and conditions are specified in the above description of a preferred embodiment, these may be varied or other materials or steps added where suitable, with similar results as described above.

Other modifications and ramifications of the invention will become apparent to those skilled in the art upon reading the present disclosure. These are intended to be included within the scope of this invention, as defined in the appended claims.

Claims

We claim:

1. A method of manufacturing a game racket which comprises the steps of:

molding a racket shell having a head frame and a handle extension connected to the frame by a throat portion, said shell being substantially planar with two substantially parallel faces, from a material comprising a synthetic resin having flexural modulus of from about 3 .times. 10.sup.5 to about 17 .times. 10.sup.5 psi and a notched Izod impact measurement of from about 2 to 15 ft-lb/in., said shell being formed with a plurality of spaced pockets extending inwardly from the faces;

forming substantially planar facing members having the general shape of said shell from a material comprising high-strength fibers in a synthetic resin matrix, said fibers having a Young's modulus of from about 20 .times. 10.sup.6 to about 80 .times. 10.sup.6 psi and a density of from about 0.05 to about 0.07 lb/in..sup.3 ;

laminating a facing member to each of said shell faces to cover said inwardly extending pockets; and

securing pallet members to the handle extension of said shell to form an easily gripped handle.

2. The method according to claim 1 wherein the walls between said spaced pockets in said shell are formed with thickened transverse portions, and transverse stringing holes are drilled through said thickened portions.

3. The method according to claim 1 wherein said racket shell is molded from a synthetic resin selected from the group consisting of polycarbonate and polyphenylene oxide resins; said resin comprising up to about 40 weight percent chopped glass fibers.

4. The method according to claim 1 wherein said facing members include a surface layer of fiberglass cloth.

5. The method according to claim 4 wherein said facing members are formed by arranging graphite fiber tow pre-impregnated with an epoxy resin in a pre-compaction mold; encasing said tow in fiberglass cloth; placing the resulting composite in a compression mold; and subjecting said composite to heat and pressure for a period sufficient to cure said epoxy resin.

6. A method of manufacturing a tennis racket which comprises the steps of:

injection molding a racket shell from a synthetic resin selected from the group consisting of polycarbonate and polyphenylene oxide resins having a flexural modulus of from about 3 .times. 10.sup.5 to about 17 .times. 10.sup.5 psi and a notched Izod impact measure of from about 2 to about 15 ft-lb/in.; said resin containing up to about 40 weight percent chopped glass fibers; said shell formed with recessed faces in a truss-like configuration in which an array of pockets extends inwardly from the shell faces alternately from opposite faces;

compression molding facing members comprising high strength graphite fibers in an epoxy resin matrix; said fibers having a Young's modulus of from about 20 .times. 10.sup.6 to about 80 .times. 10.sup.6 psi and a density of from about 0.05 to about 0.07 lb/in.sup.3 ; said facings having a surface layer of fiberglass cloth;

drilling transverse stringing holes through said shell, said holes passing through webs between adjacent pairs of said pockets; and

bonding a facing member to each face of said shell with an epoxy adhesive; said facing members covering said truss-like recesses.

7. The method according to claim 6 wherein said facing members are formed by arranging graphite fiber tow pre-impregnated with an epoxy resin in a pre-compaction mold; encasing said tow in fiberglass cloth; placing the resulting composite in a compression mold; and subjecting said composite to heat and pressure for a period sufficient to cure said epoxy resin.