ilian
Semi-Pro
I am posting this thread, because I am convinced that injection moulding is the process that has yielded the best tennis racquets. It is a process that truly makes a difference and one that is evident, contrary to most of the other "modern" technologies. Is that a new injection moulding method? It says 1993 on the website - Dunlop had theirs in the early 80'... Check this out (came from here - http://www.patentstorm.us/patents/5176868/description.html):
SUMMARY OF THE INVENTION
The present invention is a thermoplastic frame, particularly for a tennis racquet, which is reinforced with long fibers, oriented at desired angles, which are intimately embedded in thermoplastic resin. A racquet frame according to the present invention has the property of dynamic stiffness, such that it responds with the stiffness of a thermoset resin racquet for power shots, but has a softer response for touch shots and lobs.
A racquet according to the present invention is made by combining a plurality of reinforcing fibers with a thermoplastic material in a flexible form, such as a filaments of thermoplastic material or a thermoplastic powder. In one preferred embodiment, tows composed of commingled or co-wound thermoplastic filaments and reinforcing fibers are braided to form a flexible sleeve. Several such sleeves can be disposed inside one another, and the multi-layer sleeve is placed in a mold, in the shape of a racquet or other implement, with a bladder disposed inside the sleeve. The mold is then closed and heated, until the thermoplastic melts. At the same time, the bladder is pressurized so that the sleeve conforms to the shape of the mold. After the thermoplastic has melted, the mold is cooled to solidify the thermoplastic material, while maintaining internal pressurization, and the solidified implement is removed.
In another preferred method, reinforcement fiber tows are pre-impregnated with thermoplastic in a powder form. The powder prepreg fiber tows are then braided and molded in the same manner as above.
In the case where the implement is a tennis racquet, it may be desirable to include multiple layers of braided sleeve, in order to form a multiple ply frame, and also to select different angles of fiber orientation in the successive plies. Also, it is possible to deploy reinforcement fibers between the braided sleeves extending in the longitudinal direction for additional strengthening. Preferably, such fibers are combined with thermoplastic filaments or powder which would melt to form the embedding substrate.
Stringing holes ma be formed in the head portion of the frame by hot needle injection, which remelts the thermoplastic Hot needle injection is desirable in that holes can be formed without breaking the reinforcement fibers. Alternatively, however, conventional drilling methods may readily be employed.
A tennis racquet according to the invention has the strength of a long fiber thermoset racquet, and like thermoset processes the ability to control the angle of fiber orientation. But, additionally a racquet according to the present invention, unlike known thermoset racquets, has the property of dynamic stiffness. In a thermoset racquet, cross linking occurs upon curing of the resin. In a thermoplastic resin, cross link chains are not formed, such that the resin possesses visco-elastic properties. A visco-elastic polymer, unlike a thermoset polymer, reacts differently to different rates of loading.
This is significant in the case of a tennis racquet, especially wide body tennis racquets with ultra-stiff frames, since the tradeoff for increased power is often lack of touch. A thermoplastic racquet has the ability to behave like a stiffer, more powerful racquet when hitting a hard shot, yet react like a softer racquet when making a touch shot or lob.
A tennis racquet or other implement frame according to the invention has several other advantages. A thermoplastic, fiber reinforced frame has favorable impact strength Thermoplastic resins have a high strain-to-failure rate and therefore thermoplastic composites can take greater impact load before failure A frame member according to the invention also has desirable vibration dampening properties, due to the fact that thermoplastics are inherently able to absorb vibrations. Thus, a long fiber thermoplastic frame according to the invention has the distinct advantage of having the same stiffness properties as a thermoset frame together with the impact and vibration dampening properties of a short fiber reinforced thermoplastic injection molded frame. Another advantage of the invention is that the frame member may be formed so that, taken from the mold, it has a smoother outer surface than thermosetting resins, and the amount of surface standing and finishing of the frame may be reduced. Also, the weight of the frame can be reduced relative to thermoset frames, due to the fact that thermoplastic resins generally have lower densities.
For a better understanding of the invention, reference is made to the following detailed description of the preferred embodiments, taken in conjunction with the drawings accompanying the application.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a front view of a tow of combined fibers and filaments used to make the braided sleeve of FIG. 2;
FIG. 2 is a front view of a section of braided sleeve used to make a frame according to the invention;
FIG. 3 is a sectional view, taken through lines 3--3 of FIG. 2, of the sleeve;
FIG. 4 is an enlarged, front view of a section of sleeve showing one example of a braiding pattern;
FIG. 5 is a front view showing a length of sleeve disposed in a mold for making a tennis racquet; and
FIG. 6 is a front view of a tennis racquet frame made according to the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows a multi-filament tow 10 which may be used in accordance with the present invention. Tow 10 is composed of a plurality of filaments of thermoplastic material 12 together with a plurality of reinforcement fibers 14, such as carbon fibers. In an exemplary embodiment, the carbon tow 14 is a 12K tow (12,000 fibers) of 7200 denier (7,200 gms per 9,000 meter length), and the nylon tow 12 includes a minimum of 136 filaments of nylon so as to have a denier of approximately 4,050. Carbon fibers tows suitable for use in the invention, as well as tows of multi-filament nylon, are commercially available. Preferably, the tow 10 is coated with a thermoplastic-compatible sizing of approximately 0.5-2.0% by weight to promote the wetting of the fibers during the subsequent molding step.
The tows 10 may be formed by co-winding a tow of reinforcement fibers 14 and a tow of thermoplastic filaments 12, that is, by bringing a tow of fibers side-by-side with a tow of filaments, as shown in FIG. 1. It is also possible to commingle the tows of fibers and thermoplastic filaments or to use a powder prepreg process, as described further on. Generally, however, in any of the methods the desired ratio is about 38% by weight resin and 62% by weight reinforcement fiber.
Referring to FIGS. 2-3, a plurality of tows 10 are braided to form a flexible tube or sleeve 16. FIGS. 2 and 4 show an exemplary braiding pattern, in which the tows 10 are oriented at selected angles, e.g., in the range of about a 15°-30° relative to the sleeve axis 35.
In the example of FIG. 2, the braided sleeve 16 is formed of sixteen side-by-side tows 10a which are helically wound in one direction, and sixteen cross tows 10b which are helically wound in the opposite direction so as to cross tows 10a. As can be seen more clearly in the example of a suitable braiding pattern depicted in FIG. 4, which shows four intersecting tows 10, 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. 4 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., 24 carrier, 28 carrier, and 32 carrier, to form a multi-ply frame, where the higher number carriers are positioned toward the outside, where larger diameters are preferred.
In a preferred embodiment, 12K (12,000 filaments) tow of carbon fiber is combined with nylon thermoplastic filaments to achieve a 62% (by weight) fiber-reinforced structure. The inner braid has 24-carrier braid braided at a 25 degree angle (relative to the sleeve axis) to form a 0.562 inch diameter sleeve. A second 28-carrier braid, at a 22 degree angle and 0.625 inch diameter, is positioned over the inner braid. Before installing the outer braid, the second braid is reinforced with axially aligned fibers typically positioned at the 12 o'clock position of the racquet frame, or what will be the outwardly facing surface in the plane of the strings. The axial reinforcement fibers may be provided as a unidirectional woven tape which is approximately 25 mm wide and 400 mm long. This unidirectional tape may be attached to the second braid using mono-filament nylon which spiral wraps around the second braid. A 32-carrier braid, at a 20° angle and 3/4 inch diameter, is positioned over the previous two braids to form the outer braid.
Preferably, all three braids are positioned over a rigid 0.562 inch mandrel. This will facilitate forming the proper overall size prior to packing in the mold. This will also assist in controlling weight.
The mandrel is then removed and the bladder is inserted inside the inner carrier braid to form the main tube assembly.
SUMMARY OF THE INVENTION
The present invention is a thermoplastic frame, particularly for a tennis racquet, which is reinforced with long fibers, oriented at desired angles, which are intimately embedded in thermoplastic resin. A racquet frame according to the present invention has the property of dynamic stiffness, such that it responds with the stiffness of a thermoset resin racquet for power shots, but has a softer response for touch shots and lobs.
A racquet according to the present invention is made by combining a plurality of reinforcing fibers with a thermoplastic material in a flexible form, such as a filaments of thermoplastic material or a thermoplastic powder. In one preferred embodiment, tows composed of commingled or co-wound thermoplastic filaments and reinforcing fibers are braided to form a flexible sleeve. Several such sleeves can be disposed inside one another, and the multi-layer sleeve is placed in a mold, in the shape of a racquet or other implement, with a bladder disposed inside the sleeve. The mold is then closed and heated, until the thermoplastic melts. At the same time, the bladder is pressurized so that the sleeve conforms to the shape of the mold. After the thermoplastic has melted, the mold is cooled to solidify the thermoplastic material, while maintaining internal pressurization, and the solidified implement is removed.
In another preferred method, reinforcement fiber tows are pre-impregnated with thermoplastic in a powder form. The powder prepreg fiber tows are then braided and molded in the same manner as above.
In the case where the implement is a tennis racquet, it may be desirable to include multiple layers of braided sleeve, in order to form a multiple ply frame, and also to select different angles of fiber orientation in the successive plies. Also, it is possible to deploy reinforcement fibers between the braided sleeves extending in the longitudinal direction for additional strengthening. Preferably, such fibers are combined with thermoplastic filaments or powder which would melt to form the embedding substrate.
Stringing holes ma be formed in the head portion of the frame by hot needle injection, which remelts the thermoplastic Hot needle injection is desirable in that holes can be formed without breaking the reinforcement fibers. Alternatively, however, conventional drilling methods may readily be employed.
A tennis racquet according to the invention has the strength of a long fiber thermoset racquet, and like thermoset processes the ability to control the angle of fiber orientation. But, additionally a racquet according to the present invention, unlike known thermoset racquets, has the property of dynamic stiffness. In a thermoset racquet, cross linking occurs upon curing of the resin. In a thermoplastic resin, cross link chains are not formed, such that the resin possesses visco-elastic properties. A visco-elastic polymer, unlike a thermoset polymer, reacts differently to different rates of loading.
This is significant in the case of a tennis racquet, especially wide body tennis racquets with ultra-stiff frames, since the tradeoff for increased power is often lack of touch. A thermoplastic racquet has the ability to behave like a stiffer, more powerful racquet when hitting a hard shot, yet react like a softer racquet when making a touch shot or lob.
A tennis racquet or other implement frame according to the invention has several other advantages. A thermoplastic, fiber reinforced frame has favorable impact strength Thermoplastic resins have a high strain-to-failure rate and therefore thermoplastic composites can take greater impact load before failure A frame member according to the invention also has desirable vibration dampening properties, due to the fact that thermoplastics are inherently able to absorb vibrations. Thus, a long fiber thermoplastic frame according to the invention has the distinct advantage of having the same stiffness properties as a thermoset frame together with the impact and vibration dampening properties of a short fiber reinforced thermoplastic injection molded frame. Another advantage of the invention is that the frame member may be formed so that, taken from the mold, it has a smoother outer surface than thermosetting resins, and the amount of surface standing and finishing of the frame may be reduced. Also, the weight of the frame can be reduced relative to thermoset frames, due to the fact that thermoplastic resins generally have lower densities.
For a better understanding of the invention, reference is made to the following detailed description of the preferred embodiments, taken in conjunction with the drawings accompanying the application.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a front view of a tow of combined fibers and filaments used to make the braided sleeve of FIG. 2;
FIG. 2 is a front view of a section of braided sleeve used to make a frame according to the invention;
FIG. 3 is a sectional view, taken through lines 3--3 of FIG. 2, of the sleeve;
FIG. 4 is an enlarged, front view of a section of sleeve showing one example of a braiding pattern;
FIG. 5 is a front view showing a length of sleeve disposed in a mold for making a tennis racquet; and
FIG. 6 is a front view of a tennis racquet frame made according to the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows a multi-filament tow 10 which may be used in accordance with the present invention. Tow 10 is composed of a plurality of filaments of thermoplastic material 12 together with a plurality of reinforcement fibers 14, such as carbon fibers. In an exemplary embodiment, the carbon tow 14 is a 12K tow (12,000 fibers) of 7200 denier (7,200 gms per 9,000 meter length), and the nylon tow 12 includes a minimum of 136 filaments of nylon so as to have a denier of approximately 4,050. Carbon fibers tows suitable for use in the invention, as well as tows of multi-filament nylon, are commercially available. Preferably, the tow 10 is coated with a thermoplastic-compatible sizing of approximately 0.5-2.0% by weight to promote the wetting of the fibers during the subsequent molding step.
The tows 10 may be formed by co-winding a tow of reinforcement fibers 14 and a tow of thermoplastic filaments 12, that is, by bringing a tow of fibers side-by-side with a tow of filaments, as shown in FIG. 1. It is also possible to commingle the tows of fibers and thermoplastic filaments or to use a powder prepreg process, as described further on. Generally, however, in any of the methods the desired ratio is about 38% by weight resin and 62% by weight reinforcement fiber.
Referring to FIGS. 2-3, a plurality of tows 10 are braided to form a flexible tube or sleeve 16. FIGS. 2 and 4 show an exemplary braiding pattern, in which the tows 10 are oriented at selected angles, e.g., in the range of about a 15°-30° relative to the sleeve axis 35.
In the example of FIG. 2, the braided sleeve 16 is formed of sixteen side-by-side tows 10a which are helically wound in one direction, and sixteen cross tows 10b which are helically wound in the opposite direction so as to cross tows 10a. As can be seen more clearly in the example of a suitable braiding pattern depicted in FIG. 4, which shows four intersecting tows 10, 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. 4 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., 24 carrier, 28 carrier, and 32 carrier, to form a multi-ply frame, where the higher number carriers are positioned toward the outside, where larger diameters are preferred.
In a preferred embodiment, 12K (12,000 filaments) tow of carbon fiber is combined with nylon thermoplastic filaments to achieve a 62% (by weight) fiber-reinforced structure. The inner braid has 24-carrier braid braided at a 25 degree angle (relative to the sleeve axis) to form a 0.562 inch diameter sleeve. A second 28-carrier braid, at a 22 degree angle and 0.625 inch diameter, is positioned over the inner braid. Before installing the outer braid, the second braid is reinforced with axially aligned fibers typically positioned at the 12 o'clock position of the racquet frame, or what will be the outwardly facing surface in the plane of the strings. The axial reinforcement fibers may be provided as a unidirectional woven tape which is approximately 25 mm wide and 400 mm long. This unidirectional tape may be attached to the second braid using mono-filament nylon which spiral wraps around the second braid. A 32-carrier braid, at a 20° angle and 3/4 inch diameter, is positioned over the previous two braids to form the outer braid.
Preferably, all three braids are positioned over a rigid 0.562 inch mandrel. This will facilitate forming the proper overall size prior to packing in the mold. This will also assist in controlling weight.
The mandrel is then removed and the bladder is inserted inside the inner carrier braid to form the main tube assembly.
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