Filament Wound Fiber Reinforced Thermoplastic Frame for a Game Racquet

This invention relates to a filament wound frame for a game racquet such as a tennis racquet, racquetball racquet, squash racquet, badminton racquet, etc. More particularly, the invention relates to a game racquet frame which is formed by filament winding a tow of fibers and thermoplastic material.

All thermoplastic racquets currently in production are made of layers of braided sleeves of commingled thermoplastic filaments and reinforcing fibers. These 2 dimensional braided sleeves are layered to make a racquet preform and then heated in a mold to produce a game racquet frame. The braiding process adds to the cost of producing a preform, is limited to relatively high wind angles, and produces a bulky, woven pattern that buckles the load-carrying fibers, thereby reducing compressive and shear strengths.

U.S. Pat. No. 5,176,868 describes making a frame from a tow of fibers, for example carbon fibers, and thermoplastic material. The tow can be formed from commingled or co-wound graphite fibers and thermoplastic filaments or from fibers which are coated with thermoplastic powder. However, the only method of forming a racquet preform which is described in the patent is braiding the tow to form a braided sleeve.

Thermoplastic unitapes are available for making composite products. Raw materials in this form are stiff, flat, and cannot be formed without the application of heat. Composite tubes and pipes made with narrow thermoplastic unitapes are produced by the addition of an intense heat source, typically an open torch flame or laser beam, and are filament wound at high speeds to avoid overheating a local area. A thermoplastic composite tube made by this method is not a flexible preform and cannot be formed to a game racquet shape for insertion into the mold.

Filament winding is a well known process for products from filament material such as graphite fiber, glass fiber, etc. In the filament winding process, the filament material is wound around a rotating mandrel to form a tube. The filament material can be in the form of a tow, which is formed from a plurality of filaments or continuous fibers. The filament material is coated with resin before or after winding. The wound tube is flexible and is placed in a mold having the shape of the end product, and the mold is heated to cure the resin. An alternative process is to filament wind reinforcing fibers into a preform shape and load the preform into a mold for resin transfer molding (RTM) or reaction injection molding (RIM). The RTM process is used only for thermoset resins. The RIM process has been explored by injecting braided reinforcements with polyamide monomers to form a tennis racquet frame.

Filament wound pans are normally manufactured by winding discrete layers of material on a mandrel, using a helical or geodesic path. Each layer is made up of a 2-ply balanced laminate formed from a multicircuit winding pattern. The number of circuits required for each layer is calculated from the bandwidth of the material and the wind angle, so that the pattern closes and a constant thickness is achieved in the layer. Different layers may be wound at different angles, but each layer typically consists of a +.alpha..degree. angle ply and a -.alpha..degree. angle ply with respect to the longitudinal axis of the rotating mandrel, resulting from the reciprocating traverse of the carriage in the winding machine.

Conventional filament winding produces discrete layers or lamina of fibers. The layers have a tendency to move relative to each other when the wound article is stressed, during removal from the mandrel and loading of the wound tube in the mold. Also, the physical properties of the article are often limited by the shear properties between layers, called interlaminar shear.

Tennis racquets have been produced by conventional filament winding. For example, EPO patent publication No. 0 470 896 describes forming a frame for a tennis racquet by winding fibers of glass, carbon, or other materials. U.S. Pat. No. 4,871,491 describes filament winding a preform from a tow of carbon fibers and a tow of thermoplastic fibers which are intermixed. Column 13, lines 17-18 refer to using the fiber blend to form racquet frames, but no specific procedure for forming racquet frames is described. Column 13, lines 20-57 refer to filament winding applications. However, no specific preforms are described, and the patent states that the carbon fiber/thermoplastic fiber tow is heated as soon as or soon after it meets the mandrel for melting or fusing the thermoplastic.

The following definitions are used herein:

CIRCUIT: One complete traverse of the fiber feed mechanism of a filament winding machine.

INTERSPERSE: To sequence wind circuits from two or more patterns in an arbitrary manner as selected by the designer. Each circuit is applied individually such that any combination and sequence of wind angles from the patterns may by applied. No discrete layers or lamina exist.

TOW: Untwisted bundle of continuous filaments

PLY: A single pass on a filament winding machine in which fiber is applied in one direction to the axis of the mandrel.

A LAYER is typically formed from a series of winding circuits resulting in a closed pattern of 2-plies; one of +.alpha..degree. and the other of -.alpha..degree..

LAMINA: A single ply or layer in a laminate made up of a series of layers.

The invention forms a preform for a racquet frame by filament winding a tow of fibers and thermoplastic material. The thermoplastic material is in the form of pultruded (or melt impregnated) fibers, thermoplastic powder fusion coated fibers or composite fiber blends of thermoplastic polymeric fibers and reinforcing fibers. The filamentary materials are processed by filament winding into the frame preform by winding over a mandrel, in which neither materials nor mandrel are heated. The unfused preform is loaded into a racquet mold, where heat and pressure are applied to form a game racquet frame. The usage of thermoplastic as the polymeric matrix yields improvements of resin dominated properties including impact, compressive and shear strengths. For composites made with a thermoplastic matrix, the filament winding process improves material properties because the reinforcing fibers are 1) continuous, 2) wound in a manner which places a wide, flat, and thin strip of material, thereby reducing buckling and improving mechanical strength, 3) wound without restriction to angle or through thickness layering resulting in stronger, lighter, and more flexible designs, and 4) wound in a pattern to reduce overlapping area, as compared to braiding, where overlapping is continuous which can result in shear failures. The mechanical stiffness and strength of a thermoplastic composite tube are improved by the filament winding process. The filament winding process yields a preform unlike cutting, piecing, and roll wrapping of unidirectional thermoset prepreg tapes (conventional method) or cutting, piecing, and layering of tubular braided sleeves with either thermoset resin or thermoplastic resin or the non-impregnated preform utilized in RTM or RIM processes.

The preferred embodiment uses an interspersed winding method to form the preform. When a preform is wound by the interspersed winding method, the concept of discrete layers does not apply. Each circuit is applied to the mandrel individually such that any combination and sequence of wind angles may be applied. Adjacent circuits or winds are separated longitudinally along the mandrel. This process may, but does not necessarily, apply a complete series of circuits such that any wind angle eventually covers the entire surface area of the part. The sequence of winding circuits of different wind angles is arbitrary as selected by the designer. This arbitrary sequencing or interspersion of circuits results in a three dimensional interlocking of the fibers. No discrete layers or lamina exist. The interspersed winding method provides a significant amount of interlocking fiber to maintain preform integrity during handling and reduces the tendency of the laminate to fail in interlaminar shear. In addition, an increase in the stiffness of the preform as well as the molded game racquet frame results from winding of filaments with the interspersed method.

The drawings will be explained in conjunction with the illustrative embodiments shown in the accompanying drawings, in which

FIG. 1 is a schematic illustration of a helical-wound filament around a mandrel;

FIG. 2 is a schematic illustration of conventional filament winding;

FIG. 3 is a schematic illustration of a conventional helical wind pattern showing the first of three layers of filaments, the first layer (2- plies) consisting of +.alpha..degree. and -.alpha..degree. angle winds;

FIG. 3a is a cross-sectional view of the first layer of half the mandrel;

FIG. 3b is a schematic illustration of the middle layer of 0.degree. wind angle filaments;

FIG. 3c is a cross-sectional view of the middle layer;

FIG. 3d is a schematic illustration of a conventional helical wind pattern showing the third layer of filaments;

FIG. 3e is a cross-sectional view of all three closed pattern layers;

FIG. 4 is a schematic illustration of an interspersed winding method;

FIG. 5 is a schematic illustration of an interspersed winding pattern showing only the initial few circuits for clarity;

FIG. 5a is a cross-sectional view of the pattern of FIG. 5;

FIG. 6a is a cross-sectional view of a conventional wind pattern of 3 layers of filaments;

FIG. 6b is a cross-sectional view of an interspersed wind pattern to form the laminate;

FIG. 6c is a cross-sectional view of a combination design consisting of a conventional wound inner layer, covered by interspersed wound circuits;

FIG. 7 is an illustration of a tow formed from reinforcing fibers and thermoplastic filaments;

FIG. 8 is an end view of the tow of FIG. 7;

FIG. 9 is an enlarged illustration of a tow formed from reinforcing fibers and powdered thermoplastic:

FIG. 10 is an illustration of a pultruded tow of reinforcing fibers and thermoplastic;

FIG. 11 illustrates the wound tube in during the placement of the initial circuits of material;

FIG. 12 is a plan view of a mold for a tennis racquet frame; and

FIG. 13 is a front elevation view of a tennis racquet frame.

FIG. 1 illustrates a helical-wound filament in which tows 11 are wound over a rotating mandrel 12 covered by an inflatable tubular plastic bladder 17. Filament winding machines are well known and need not be described herein. Such winding machines conventionally include a rotatory driven, elongated mandrel and a filament-feeding carriage which is mounted for longitudinal movement back and forth the length of the mandrel. The winding machine carriage moves along the mandrel in one direction to lay down a filament tow in a constant angle helical path, or geodesic path, and then moves in an opposite direction to lay down the filament at an angle negative to the this previous pass. In conventional multicircuit winding, after the first circuit is completed, the applied filaments are not adjacent and additional circuits must be traversed before the pattern begins to lay filaments adjacent to each other. This helical path is continued for the number of circuits required to close the pattern in both directions of filament traverse. Once this initial pattern becomes closed and the initial layer is completed, then additional layers are wound in a similar manner as selected by the designer.

In FIG. 2 the first circuit of filament material is indicated by the tow 11a which extends at an angle, typically between + near 0.degree. to +45.degree., to the longitudinal axis 13 of the mandrel. The second circuit of filament material is wound in the opposite direction and is indicated by tow 11b which extends at an angle of minus near 0.degree. to -45.degree. to the axis of the mandrel. Succeeding circuits of filament material, indicated by the tows 11c, 11d, 11e, 11f, etc., are wound in a similar manner until the pattern is closed. These windings form the first and second plies, or the initial layer around the mandrel. Once the pattern is closed with these first and second plies, this initial layer is followed by succeeding layers in a similar manner, until the laminate, consisting of various layers of wound filaments is complete.

FIGS. 3 and 3a illustrate the initial layer of a conventional helical winding pattern in which both the positive angle ply and the negative angle ply, relative to the centerline 13 of the winding mandrel, are closed. In this example, we assume that the winding angle .alpha. is 45.degree. and the diameter of the mandrel is such that a total of 9 circuits have been calculated to close the pattern. A one-half cross-sectional view of the mandrel 12 is given showing the two .+-.45.degree. angle plies of filaments. The length of the arc 14 formed by both the +.alpha..degree. and -.alpha..degree. wind angles is based on the bandwidth and wind angle of the filament.

FIGS. 3 and 3b illustrate the middle layer of a conventional winding pattern where the wind angle .beta. is 0.degree., or parallel, to centerline 13 of the mandrel 12. A one-half cross-sectional view of the mandrel 12 is given showing in this example a total of 12 circuits needed to close this 0.degree. wind pattern. The length of the arc 15 will be equal to the width of the filament.

FIG. 3d illustrates the third layer of this laminate structure where the wind angle .gamma. to the centerline 13 of the mandrel is applied to the mandrel in both positive and negative angle plies, as in FIG. 3. In this example, we have selected .gamma. to be equal to 45.degree., and thus needs 9 circuits to close the pattern. In FIG. 3e, a cross-sectional view of the completed laminate is illustrated consisting of the inflatable bladder, the 1st layer of wound filament 16 wound at angle .+-..alpha..degree.; the 2nd layer of wound filament 18 wound at angle .+-..beta..degree.; and the 3rd layer of wound filament 19 wound at angle .+-..gamma..degree..

The wound filament tube is shaped into a racquet preform and inserted into a mold; heated to flow the thermoplastic resin and then cooled to form a rigid, fiber reinforced frame. It is common in racquet fabrication to insert reinforcement plies of both 0.degree. and/or 90.degree. fiber cut pieces between the layers of filaments at various locations in the hoop and handle areas of the frame.

FIG. 4 illustrates a method of interspersed filament winding for forming a frame for a game racquet. The invention will be explained in conjunction with a frame for a tennis racquet, but the invention can be used to form frames for other types of game racquet such as racquetball racquets, squash racquets, badminton racquets, etc.

As in FIG. 1, a conventional rotatable driven mandrel 12 is covered by an inflatable tubular plastic bladder 17. In an example of interspersed winding, the first circuit of filament material is indicated by tow 20, wound at an angle of +.alpha..degree. to the longitudinal axis of the rotating mandrel, and tow 21, wound at an angle of -.alpha..degree. to the mandrel axis. In the next circuit, tow 22 is also wound at +.alpha..degree. and tow 23 is wound at -.alpha..degree.. The next circuit selected by the designer would be from a different wind pattern. Tow 24 is wound at +.beta..degree. and tow 25 at -.beta..degree.. The angles of successive circuits are arbitrary and contain circuits from some or all of the patterns selected by the designer.

FIG. 5 illustrates an example of a interspersed wound laminate that consists of a combination of .+-.45.degree. and 0.degree. filaments. In this pattern, 16 circuits of .+-.45.degree. filaments are required to achieve a closed pattern and 12 circuits of 0.degree. filaments to close. The designer has chosen to intersperse the circuits in such a manner to alternate these two patterns after every four circuits In FIG. 5a, the resulting laminate cross-section shows the position of the .+-.45.degree. bands 26 and the 0.degree. bands 27.

FIGS. 6a through 6c illustrate a cross-sectional view showing three possible patterns that would be applicable in filament winding of game racquets. In FIG, 6a, a conventional wound tube is given with the laminate thickness of t.sup.1. This prior art wound tube has three layers, as discussed in FIG. 3. In FIG. 6b, a wound tube consisting of a series of circuits wound with an interspersed pattern, as discussed in FIG. 5, is given with the laminate thickness of t.sup.2. In order to provide additional strength for the string holes which will be drilled in the completed frame, it may be desirable to form a completely closed inner or outer layer, or both, of filaments wound in a conventional manner. In FIG. 6c, a combination design is presented of an inner layer formed by a conventional helical wind followed by a series of circuits wound using an interspersion of two patterns with a resulting laminate thickness of t.sup.3. In order to control the final weight of the preform laminate the total thickness 28 would be the same for each option, that is t.sup.1 =t.sup.2 =t.sup.3.

FIGS. 7 and 8 illustrate a tow 34 which may be used to filament wind or preform for a racquet frame. The tow comprises a plurality of commingled fibers 35 which include reinforcing fibers and thermoplastic fibers or filaments. The reinforcing fibers are preferably graphite fibers, but other fibers can also be used, either alone or in combination, such as glass, aramid, boron, etc. Any conventional thermoplastic material can be used for the thermoplastic fibers. Exemplary materials include Nylon 6, Nylon 6/6, and Polypropylene.

The tow can include any desired number of reinforcing and thermoplastic fibers, for example, 3000, 6000, and 12,000, or up to 50,000 or more. The ratio of reinforcing fibers to thermoplastic fibers can also be varied depending on the physical properties which are desired. The tow 34 is preferably shaped like a flat ribbon which has width W (FIG. 7) and a thickness T (FIG. 8) which is substantially less than the width.

FIG. 9 illustrates another tow 36 which comprises a plurality of reinforcing fibers 37 of graphite or the like, each of which are coated with thermoplastic powder 38. The reinforcing fibers can be coated by any conventional coating technique, for example, fusion coating. The powder coated tow 36 is also preferably shaped like a flat ribbon like the tow 34.

FIG. 10 illustrates a pultruded tow 40 which includes reinforcing fibers 41 of graphite or the like and a thermoplastic matrix 42. The tow is formed by conventional pultruding technique and is also preferably shaped like a flat ribbon.

The tows of reinforcing fiber/thermoplastic which are illustrated are commercially available and have been used to produce other products. However, such tows have not previously been used to make frames for game racquets. Tows of commingled fibers and thermoplastic filaments are available from Cytec Industries under the name Comingled Ultramid.TM./G30-500 Hybrid Yarn. Tows of thermoplastic powder coated fibers are available from Custom Composites Materials Inc. under the name Towflex.TM.. Tows of pultruded fibers and thermoplastic are available from Polymer Composites Inc. under the name Fiber Rod.TM..

Such tows are dry and flexible, and the wound preform is also dry and flexible. However, the filament wound preform maintains its structural integrity after removal from the mandrel and during subsequent handling.

Referring to FIGS. 11 and 12, the wound tube 44 and the bladder 17, inside the tube, are removed from the mandrel and placed in a mold 45 which has a mold cavity 48 in the shape of a frame for a tennis racquet. Alternatively, the tube can be wound directly on the mandrel without a bladder, and the bladder can be inserted into the tube after the tube is removed from the mandrel. The mold cavity 48 includes a hoop-shaped head portion 49 and a handle portion 50 in which both ends of the wound tube are positioned. A separate yoke piece (not shown) is inserted into the yoke portion 51 of the cavity and connected to the adjacent portions of the wound tube.

The ends of the bladder extend outside of the mold and are connected to a pressure manifold 52 in accordance with conventional racquet molding technique. The mold is closed and heated while the wound tube is forced against the surface of the mold cavity by pressurizing the bladder.

The tows which are used to form the wound tube are preferably formed from graphite fibers. However, other fibers can also be used, either alone or in combination, such as glass, aramid, boron, polymeric, etc to form a hybrid wind.

The arbitrary interspersion of wound circuits provided by the interspersed winding technique results in a three dimensional interlocking of the fibers. The interlocked fibers maintain the structural integrity of the wound tube after the tube is removed from the mandrel, particularly if the tube is formed from dry fiber which are not prewet with resin. The interlocked fibers also reduce the tendency of the tube to fail in the interlaminar shear mode because no discrete layers exist. Additionally, this interlocking of continuous fibers increases the stiffness of the molded game racquet.

FIG. 13 illustrates the molded racquet frame 53 after it is removed from the mold. The frame includes a head portion 54, a handle portion 55, a throat portion 56, and a yoke 57. The racquet is completed in the conventional manner be drilling string holes in the head and yoke, wrapping a grip around the handle, and stringing the head.

The winding angle can also be changed at a selected position along the mandrel to achieve selected properties of the racquet laminate.

While in the foregoing specification, a detailed description of a specific embodiment of the invention was set forth for the purpose of illustration, it will be understood that many of the details herein given may be varied considerably by those skilled in the art without departing from the spirit and scope of the invention.