FIELD OF THE INVENTION This invention relates to fiber reinforced thermoplastic composite articles and, more particularly, to articles made from continuous cowound fiber reinforced thermoplastic composite tow and a process of producing the same. A preferred embodiment will be described in relation to forming a tennis racquet frame; however, the invention may be applied to other articles.

This application is related to commonly owned U.S. Patent No. 5,176,868, entitled "Long Fiber Reinforced Thermoplastic Frame Especially For A Tennis Racquet," the disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION Continuous fiber-reinforced polymer composites have found a wide range of uses owing to their advantageous strength-to-weight ratio. These composites have been extensively applied in various technologically advanced industries such as sports equipment, transportation, machinery, and aerospace. Composites can be made with either thermoset or thermoplastic materials. However, use of continuous fiber-reinforced thermoplastic composites has been limited by high material costs and manufacturing difficulties as compared to traditional thermosetting materials. This is particularly true for complex and exact geometries such as tennis racquet frames.

Thermoset composite racquet frames are conventionally manufactured using unidirectional tapes. The tapes are composed of reinforcing fibers, such as graphite, glass, or aramid, impregnated with an uncured thermoset resin, such as uncured epoxy. The resultant "prepreg" is compliant and tacky, and thus may easily be formed into any shape at room temperature.

In order to make a tennis racquet frame, plies of unidirectional tape are wrapped about an inflatable bladder to form an elongated tube. The fibers of the tape plies are oriented at specific angles to produce the desired strength and stiffness in the finished racquet. The "preform" tube, which is still flexible, is then shaped by hand to the frame contour and packed into the racquet mold. The tackiness of the resin material acts to hold the plies together during handling. With the application of heat and pressure, the preform conforms to the shape of the mold and the thermoset resin cures and hardens to yield a high strength composite frame.

The process described above cannot, as a practical matter, be used with thermoplastic materials. In contrast to uncured thermoset prepregs, which at room temperature are drapeable and tacky, a thermoplastic prepreg would be very stiff and boardy.

U.S. patent No. 5,176,868 discloses several processes for making tennis racquet frames composed of continuous fiber-reinforced thermoplastic composites. In one preferred embodiment, carbon reinforcing fibers are commingled with flexible thermoplastic resin filaments to form a composite tow. The composite tow is braided to make a sleeve, which is placed in a racquet frame mold. The mold is heated to above the melting point of the thermoplastic filaments, and the sleeve is internally pressurized using an appropriate bladder. The consolidation forces generated by the inflatable internal bladder create resin flow and subsequent impregnation of the surrounding reinforcing filaments. The mold is then cooled, and the thermoplastic material hardens to form a composite racquet frame.

In another embodiment of the aforementioned patent, thermoplastic yarns, composed of many resin filaments, are wound side-by-side with reinforcing fiber tows to yield a flexible cowound composite tow. The cowound tow is then processed in a manner similar to the

commingled tow. In yet another embodiment, reinforcing fibers are impregnated with a thermoplastic powder to form a flexible composite tow.

The cowinding process is desirable, as compared to commingling, in that off-the-shelf textile equipment can be utilized, and in that it is not necessary to perform the commingling operation. However, cowound tows require much higher consolidation pressures and longer processing cycle times to wet out the reinforcing fibers. Unlike commingled materials, cowound resin yarns must melt and flow greater distances to impregnate neighboring reinforcing fiber tows. Powder impregnation processes pose separate problems. For such reasons, commingling has thus far been preferred in the commercial manufacture of tennis racquet frames.

SUMMARY OF THE INVENTION The present invention is an improved process for making tennis racquets and other articles from cowound tow materials. In particular, the invention

Orelates to cowound tow materials that can be processed to form articles under conditions comparable to commingled composite tow materials, and which deliver comparable composite properties at lower cost. In addition to tennis racquets, the cowound tow may be used to form other articles, such as woven into fabrics, may be filament wound into complex or simple preforms, or may be used to make unidirectional tape.

In accordance with the invention, lower molding pressures and cycle times may be used to process cowound materials composed of special low viscosity grade thermoplastic yarns and low filament count reinforcing fibers.

Preferably, low viscosity grade thermoplastic yarns are also wound, or "served", about the cowound core to improve handling. The served tow offers several benefits over unserved or commingled tow:

1. The serve yarn debulks (packs together) the core contents and maintains tow integrity.

2. The serve yarn covers the tow surface and minimizes the amount of reinforcing fiber "fly" (broken filaments that project from the yarn) caused by handling, which may be a skin irritant or an air borne irritant.

3. Improved tow integrity reduces breakage during subsequent textile operations such as braiding or weaving. 4. Serve yarn provides an extra resin layer on the tow surface which results in less "dry" surface fibers and ultimately a better surface finish in the as molded part.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a front view of a core section of a cowound two of reinforcement yarn and thermoplastic yarns;

Fig. la is a cross-sectional view of the tow of Fig. 1;

Fig. 2 is a front view of a section of cowound core and served with thermoplastic yarns used to make the braided sleeve of Fig. 3;

Fig. 3 is a front view of a section of braided sleeve used to make an article such as a frame;

Fig. 4 is a cross-sectional view, taken through lines 4-4 of Fig. 3, of the sleeve;

Fig. 5 is an enlarged, front view of a section of sleeve showing one example of a braiding pattern; Fig. 6 is a front view showing a length of sleeve disposed in a mold for making a tennis racquet; and

Fig. 7 is a front view of a tennis racquet frame made according to the invention.

DETAILED DESCRIPTION OF THE INVENTION Fig. 1 is an illustration of the cowound core 14 which may be used in accordance with the present invention. Core 14 is composed of both a reinforcement yarn 11 and plurality of thermoplastic yarns 12,13.

Reinforcement yarns may be composed of carbon (graphite) , glass, aramid or other suitable reinforcing fibers, or combinations of more than one type reinforcement fiber to form a hybrid yarn. In accordance with the invention, reinforcement fiber tows having a filament count of 12k (12,000 filaments) or less, where the filaments have diameters of 30 microns or less, should be used. Further, the applicant has found that sizing, which normally is applied by the fiber supplier to enhance wet out and handling, inhibits resin flow into the filament bundle; thus, preferably no sizing is used, unless fiber-matrix adhesion is a problem.

The thermoplastic filaments are made of low- viscosity nylon, preferably nylon-6. As used herein the term "low viscosity" means zero shear rate viscosity in the range of 3,000 poise or less at or above the melt temperature of the material. The most preferred material, for use in making a tennis racquet, is nylon-6 having zero shear rate viscosity as low as 1010 poise at 260 ^β C. Preferably, the thermoplastic filaments are provided in the form of multiple low denier yarns, spaced around each reinforcement fiber yarn, rather than one large yarn.

In an exemplary embodiment, the core reinforcement yarn 11 is an unsized 6K carbon tow (6,000 filaments) of 3600 denier (3,600 gms per 9,000 meter length). Two 430 denier nylon-6 yarns 12, and four 200 denier nylon-6 yarns 13, having a total thermoplastic yarn denier of 1660 (1,660 gms per 9000 meter length), are cowound with reinforcement yarn 11. As shown in Fig. la, preferably the larger yarns 12 are disposed on opposite sides of the carbon tow 11, separated by the smaller nylon yarns 13.

Fig. 2 shows a served or sheathed cowound tow 16 which may be used in accordance with the present invention. Tow 16 is composed of a cowound core 14 of reinforcement yarn 11 and a plurality of thermoplastic yarns 12, 13 served with additional thermoplastic yarns 15. The two served thermoplastic yarns 15 are 100 denier and cross over one another five times per inch.

In this exemplary embodiment, the tow 16 is 65% (by weight) carbon reinforcement and 35% (by weight) thermoplastic resin. However, relative fractions of the fiber/resin materials may vary depending upon the article to be formed. Carbon yarns and thermoplastic yarns suitable for use in the invention are commercially available. Referring to Figs. 3-4, a plurality of tows 16 are braided to form a flexible tube or sleeve 16. Figs. 3 and 5 show an exemplary braiding pattern, in which the tows 16 are oriented at selected angles for making a tennis racquet frame, e.g., in the range of about a 15-30 degrees relative to the sleeve axis 35.

In the example of Fig. 3, the braided sleeve 16 is formed of thirty-six side-by-side tows 16A which are helically wound in one direction, and thirty-six side-by- side cross tows 16B which are helically wound in the opposite direction so as to cross tows 16A, as can be seen more clearly in the example of a suitable braiding pattern depicted in Fig. 5, which shows four intersecting tows similar to tow 16. Tow 40 passes over a pair of crossing tows 36, 37, and then under the next pair of crossing tows 38, 39. This pattern would continue with successive pairs of crossing tows (which are omitted in Fig. 5 for clarity) . Tow 41 passes under crossing tow 36, and then over pair 37, 38, and then under and over successive pairs, in the same pattern as tow 40, but shifted one cross tow to the right. Similarly, tows 42, 43 pass under and over pairs of cross tows, with each successive pattern shifted one cross tow to the right.

Braids can utilize a number of composite tows braided at specific angles at designated diameters. The frame may be comprised of multiple braids of different sizes, e.g., 64 carrier, 72 carrier, and 80 carrier, to form a multi-ply preform.

The foregoing is merely illustrative, and any desirable braiding pattern may be used. Moreover, it is not necessary to initially braid the tows. Braiding, however, produces a flexible sleeve which is easy to manipulate and use in further steps, without unravelling. The object, however, is to arrange the tows at the angle which is desired in the end product.

Fig. 6 illustrates the bottom half 45 of a mold that may be used to make a tennis racquet frame using the braided sleeve 26 of Fig. 2. The mold defines a continuous profile cavity, 49, starting at the butt end 47 of the racquet frame, extending about the throat and head portions of the racquet, and terminating again at the butt end 47. To make the frame, a length of sleeve 26 is provided to extend along cavity 49 with opposite ends of the sleeve terminating at butt end 47. As noted above, preferably two or three sleeve elements are packed inside one another to form a multi-ply layup. The number of plies depends upon the desired wall thickness and weight of the resulting frame.

Prior to packing the sleeve 26 (or multi-ply layup) in the mold, inflatable bladder 46 is inserted through the sleeve 26 (or through the inner sleeve in a multi-ply configuration) , such that the opposite ends 48 of the bladder 46 extend out the opposite ends of the sleeve 26. As shown in Fig. 6, the flexible sleeve 26 is positioned in the mold 45, such that the ends 48 of the bladder 46 project out of the mold.

A throat section 50 may be positioned in the mold, in the customary manner. Throat piece 50 may be an additional section of braided tube, which is disposed around an expandable foam core. The core should be of a

material, such as a heat expandable foam, which can withstand the temperature necessary to melt the thermoplastic.

Because the thermoplastic is not yet coating the fibers and is in filament form, the bulk volume of the main tube assembly is greater than that of the final frame profile. It is thus necessary to pack and close the mold carefully so as to avoid pinching any material. In order to facilitate this, it is preferred to replace the insert plates used in thermoset processes (which use a top plate, bottom plate, and two insert plates) with full length side plates. The main tube assembly is positioned in the cavity of the bottom plate. The throat assembly is then positioned in its respective cavity and the ends of the throat are wrapped around the main tube assembly. Following this the top plate is attached to the bottom plate. Care is needed to ensure that upon closure of the top plate no material is pinched along the inside surfaces of the racquet frame. Once the braided sleeve 26 is inside the mold

45, the mold is closed and the bladder 46 is inflated. The mold is heated to a temperature sufficient to melt the thermoplastic filaments 12, 13 which in the case of nylon would typically be about 450-500° Fahrenheit, while the bladder 46 remains inflated. The bladder may be inflated, in making a tennis racquet, to an internal pressure range of about 100 to 500 psi.

As the thermoplastic material melts, it will flow between the carbon fiber filaments, impregnating the carbon fibers with thermoplastic resin. The pressure exerted by the bladder facilitates the flow of thermoplastic material and ensures that sleeve 16 conforms to the shape of the mold. Thereafter, the mold is cooled, to solidify the thermoplastic material, and the frame can then be removed from the mold.

As explained above, the lower viscosity thermoplastic of the present invention has been found to

provide adequate resin flow for reinforcement fiber wet out at commingled tow processing pressures of about 200 psi. Higher viscosity resin requires undesirably higher consolidation pressures to impregnate the reinforcing fibers for a given cowound construction.

Pressure may be applied in several other ways, including compression molding and autoclaving, and is not limited to inflatable bladders. Moreover, the cowound tows may be used to form other types of articles, e.g., articles which are not hollow.

The heating and cooling cycles need only be long enough to melt the thermoplastic, allowing time for the melted material to flow about the fibers, and then re-solidify. In an exemplary process, the mold is subjected to a 15 minute heat up cycle to 500 degrees F. , 15 minute hold at 500° F, and a 20 minute cooling cycle, while maintaining a pressure of 200 psi. However, the lengths of the cycles depend upon the tool thermal mass, molding temperature, pressure, and viscosity of the material used.

In a preferred embodiment, two braid layers are combined with axially aligned fibers to reinforce the structure. The braid layers are 72 carrier braided at about a 20° angle (relative to the sleeve axis) to form a 0.625 inch diameter sleeve. The inner braid is positioned over a bladder supported by a rigid mandrel to maintain shape. The axial reinforcements are attached to the inner braid layer using a tack welding or other suitable means. The outside braid layer is positioned over the inner braid layer and axial reinforcements. The rigid mandrel is removed from the bladder to facilitate mold packing.

Fig. 7 illustrates a finished racquet, which includes a head portion 54, a throat section 56, a throat bridge 50, and a shaft 58, which are formed in the mold 45. After molding, a handle and grip 60 are secured on the shaft in any known manner. Alternatively, the mold

may be configured to form a handle in the shape of the handle in Fig. 7, in what is know as a molded-in handle. The foregoing represent preferred embodiments of the invention. Variations and modifications of the processes and materials disclosed herein will be apparent to persons skilled in the art, without departing from the inventive concepts disclosed herein. All such modifications and variations are within the scope of the invention, as defined in the following claims.