FR2528318A1 - Tennis racquet frames of hollow thermoplastic extrudate - subsequently shaped and filled, pref. of fibre reinforced polyamide plugged with expanded polyurethane - Google Patents

Abstract

Racquet frame for tennis etc. is made from a hollow extruded thermoplastic profile which is subsequently bent to shape while hot and which receives core and handle components of injection moulding material. Used esp. for mfr. of tennis racquet combining high stiffness and resilience with low mass (wts. of about 270g unstrung, 365-395g when strung) and a relatively uncomplicated mfr. process. The extruded profile is pref. made from a compsn. of e.g. polyamide 11, 12 or 6-12 contg. at least 15% wt. of carbon fibres at least 1, pref. 5-6 mm long and sufficient hollow micro-spheres of aluminium silicate to obtain an overall density of about 0.9. The core passages are opt. filled with an expanded polyurethane resin.

Description

 The present invention relates to a tennis racket frame or similar game and its manufacturing process.

 The design of frozen tennis rackets, for fifty years, has evolved very rapidly over the past ten years.

The expansion of tennis and the evolution in quantity and quality of players poses a problem of material made more sensitive by the transfer of manufacturing to countries with low labor costs and the generalization of new forms of frames, authorized by the absence of regulation.

 Due to the industrialization of wooden rackets, the selection of trunks has become less rigorous, natural drying has been replaced by steam drying and the uncoiling of wood has replaced the assembly of slices sliced in the direction fibers, which results in a decrease in the performance of these rackets.

Gluing, machining, drilling, carried out by semi-automatic means do not allow to obtain a quality equal to that obtained by artisanal ways.

 The already old principle of a frame made from a metal profile is only reliable with hardened steel with a brazed junction between branches, but it does not avoid the need for absolute centering of the bale and the vibrations which are detrimental to the player. It implies an application only for middle quality executives.

 Sandwich structures coated parallel to the plane of the rope used on a plastic neutral fiber or to reinforce a wooden structure work in shear due to the static or dynamic constraints of the rope. They improve the flexural strength of the frames but pose problems of bonding. The cutting of impregnated fabrics does not allow the proper orientation of the fibers to be obtained and results in prohibitive material losses. The possible improvement of the coatings cannot eliminate the problems of shearing. your metal or laminate overlays perpendicular to the stringing plane are illusory due to the drilling and their too often central positioning.

 Known solutions using the molding of continuous glass or carbon fibers, impregnated with thermosetting epoxy resins, involve a long manufacturing process, hence a high cost price. Snowshoes with properly oriented fibers are reserved for the elite.

The use of thermoplastic products quickly implemented makes it possible to reduce the number of operations necessary for the manufacture of a frame. The choice of quality and the addition of cut fibers makes it possible to obtain an anisotropic material of high elasticity modulus, endowed with excellent return characteristics. Polyamides loaded with carbon fibers, forming a "compound", are used - by simple injection, but the density of the compound requires
to reduce the sections of the frame, hence a reduction in the
moment of inertia; - by injection of a lightened compound with a blowing agent
(EUA patent NO 3.981.504 in the name of PPG Industries)
but the loss of modulus in bending does not compensate
possible increase in sections; - by injection on a fusible core (EUA patent

NO 4,287,308 in the name of DUNLOP) which brings the advantage
of a hollow structure with central reinforcements and junction
integrated between branches but results in manufacturing
extremely expensive; - by assembly of injected elements leading to a rib
internal continuous (EUA patent NO 4 194 738 in the name
from HITACHI).

 The injection supposes a limit to the rate of carbon mixed in the compound which does not allow to exploit all the characteristics of the material. During injection, a thermoplastic sheath in contact with the mold changes the distribution of the fibers and the characteristics of the compound.

The injection pressure and the paths through small sections cause the fibers to break such that their residual length does not affect the preservation of the theoretical characteristics. The shape and dimensions of a frame require the welding of material flow and, at the head of the racket, a local loss of resistance also observed between the holes. Despite these drawbacks which, added to design flaws, cause clearly insufficient rigidity, excellent return characteristics have been obtained with full-section rackets injected from non-light compounds which confirm the validity of the material and make it possible to define a new solution.

 According to the present invention, the tennis racket or other frame is characterized in that it is constituted by a profile of thermoplastic material, shaped by hot bending, the core and the handle being over-injected.

 According to another characteristic of the invention, the aforementioned profile has longitudinal compartments which are filled, after extrusion with a more or less dense polyurethane foam which makes it possible to obtain the desired weight for the frame and protects the profile against being crushed during bending.

 The profile is preferably formed by the extrusion of polyamide 11, 12 or 6-12 with low water absorption, into which carbon fibers are incorporated whose length is greater than 1 mm and advantageously of the order of 5 to 6 mm, high resistance type.

 The extrusion operation, unlike injection, keeps a length and a uniform distribution of the fibers inside the frame and, thereby, to exploit all their qualities.

Other characteristics and advantages of the invention will appear during the description which follows of particular embodiments, given solely by way of nonlimiting examples, with reference to the figures which represent - FIG. 1, a frame of racket according to the invention; - Fig.2, a first form of profile; - Fig.3, another form of profile; - Figs. 4 to 9, the different stages of the
Manufacturing; - Fig.10, a variant of the manufacturing process.

 In Fig.l, the frame 1 has the general shape of an oval, symmetrical with respect to its longitudinal axis but asymmetrical with respect to its transverse axis.

It consists of a folded and curved profile so as to present two branches 5 and 6 joined at their lower part by spacers 2,3, and by a weld 4.

The upper spacer-2 will be called the heart of the racket later. Below the weld 4, there remains a slot 8 which is subsequently filled with polyurethane, the two ends of the arms 5 and 6 being trapped in an injected handle 7. As has been said previously, the profile is hot bent so to take the desired shape. The spacers 2, 3 and 4 are formed by injection of a polyamide similar to that of the frame profile, but which can be a polyamide 6-6.

 According to a characteristic of the invention, the material injected to form the spacers extends in a layer 9 on either side of the frame and passes through holes 10 so as to constitute plastic rivets, the injection being carried out through the holes 10. Thus, the two branches 5 and 6 are very securely joined together and there is no risk of separation even under the most violent efforts, the torsional strength being maximum.

Fig. 2 is a section along line II-II of the
Fig.1 and it shows the structure of a profile 1. This profile is composed of a compound of polyamide 1G, 12 or 6-12 and carbon fibers. It is also possible, in order to obtain a density of the order of 0.9 for the entire frame, to introduce into the compound hollow glass microbeads such as those which are known under the name of "'filite" ( Trademark). During extrusion, two longitudinal cavities 12 are formed inside the profile. These two cavities are subsequently filled with a polyurethane foam, the density of which is chosen as a function of the final weight of the racket. This foam also prevents the profile from being crushed during bending.

Given the position of the cut, a hole 11 allows the passage of a rope (not shown). Preferably, the holes 1 for passage of the strings are seventy in number for a racket with a medium sieve and a single string passes through a hole. This allows, although at the cost of a slight increase in the number of holes (70 against 64) to reduce the diameter and, consequently, to increase the overall resistance of the racket. The edges of the holes 10 are chamfered at 13, on the outside and at 14 on the inside of the frame so that the strings do not bear on sharp angles which would damage them. Note that the frame is made of polyamide, it is not necessary to provide special protection for the strings. It is possible to coextrude with the main profile of the bars 15 or 16 which are intended to ensure better resistance of the profiled during extrusion. They possibly make it possible to tare the racket or to constitute a decoration.

I1 is likewise possible to coextrude, at the periphery of the frame, bands 18 which ensure the decoration and the maintenance of the profile. Finally, the frame presents, on the inner side a cutaway 17. We have indeed noticed that this part of the
framework did not intervene in any way in the efforts in
bending and twisting of the racket after stringing and
the total weight can thus be reduced by a few grams.

Fig.3 shows another profile which is
a section along line III-III of Fig. 1. The same
references designate the same elements as in Fig. 2.

 Abstraction has been made of any strips or bars.

Figs 4 to 9 schematically represent the
different stages of a frame manufacturing process
racket according to the invention.

In Fig. 4, an extruder 19, of the conventional type,
inside which a compound is introduced and melted
polyamide-carbon fibers, pushes through a die
a profile 1 of the type shown in section in Figs 2 and 3.

The profile is then cut by a saw 20 in length
of executives. The sections are then drilled flat (Fig. 5)
and polyurethane foam is introduced into the grooves
12 where it expands, by a press 21. (Fig.6).

These operations are carried out while the profile 1 has cooled and is therefore solid. In order to carry out the hot bending, it is necessary to bring the compound to its softening temperature. This can be obtained, as shown in Fig.7, by immersing a section in a thermostatically controlled bath, for example silicone oil, or by any other means.

 The next operation is the bending shown diagrammatically in Fig. 8. The profile is introduced into a bender made up of two branches 23 and 24, substantially in the extension of one another but articulated one on the other around an axis 25 located near a conformation core 22 which reproduces the internal outline of the frame.

By rotation about the axis 25, the branches 23 and 24 are folded around the core 22. Of course, the drills 28 (Fig.5) are spaced so that after bending, the holes 10 are evenly distributed a the periphery of the frame.

 The following operations which consist of successive injections have been shown diagrammatically in FIG.

The purpose of the first injection is to form the heart of the racket by parallel struts 2, 3 and 4 (Fig. 1)
The heart is overmolded between the branches 5 and 6 by injection, into a mold 25, of a polyamide resin such as a polyamide 6-6, for example. This polyamide has a higher melting point than polyamide 11 or 12 or 6-12 used to form the profile.

The injection of this polyamide therefore causes a fusion or a surface softening of the branches 5 and 6 which is very favorable for good attachment. The connection is completed by the formation of plastic rivets through the holes 10 which are injected during the same operation and which have, on the screws currently used, the advantage of not dissociating from the parts which they join.

The branches 5 and 6 are further secured, at their lower end by an insert performed by over-injection between the branches of a polyurethane foam.

During this same operation, the handle 7 is produced by injection of direct skin polyurethane foam.

The two branches are thus joined by gluing which avoids vibrations and increases torsional rigidity.

 Carbon fibers of the "high strength" type are cut to lengths of 5 to 6 mm. and sized at 5% of the basic thermoplastic. The compounding and extrusion operations are such that the residual length of the fibers is greater than 1 millimeter. Carbon fibers are preferably used for their high modulus of elasticity. For less efficient rackets, it is also possible to use a mixture of carbon fibers and glass fibers, at a rate of more than 15 by weight.

 In the usual technique, the components of the compound are intimately mixed in an extruder which delivers granules. These granules are again extruded to give a profile. According to one characteristic of the invention, the profile is preferably extruded directly, that is to say that polyamide or any other suitable thermoplastic material such as certain qualities of polyurethanes, polycarbonates or polyacetals is introduced into the extruder. provided, however, that their resistance under stress is sufficient, and the carbon and / or glass fibers. Compounding is then carried out directly in it, in a single operation. The carbon fibers can thus retain a greater length.

As mentioned above, the preferred choice of polyamides 11 or 12 or 6-12 results from the criterion of low water absorption. The other thermoplastic materials however have the advantage of having wider softening ranges than those of the polyamides mentioned.

 Another manufacturing process is shown in Fig. 10. It is based on the idea that it is possible to directly shape the frame at the exit of the extruder, while the profile is still in a pasty state. To this end, a mold having a groove 26 of cross section equal to that of the frame is disposed at the outlet of the extruder die 29. This groove guides the profile as it leaves the extruder. 'drawback of this second method, however, comes from the need to drill the holes, after cooling, while the frame has taken its final shape. After bending the profile, a second plate is brought in front of the extruder and a new extrusion-bending occurs. Of course, the groove 26 can be heated and lubricated.

 The present invention makes it possible to obtain tennis racket frames whose elasticity module is of the order of 15,000. For rackets with a medium sieve, the total weight after stringing can vary from ten grams to ten grams from 365 to 395 grams, the weight of the profile alone being approximately 270 grams. As mentioned above, this weight can vary depending on requirements either by introducing hollow microballoons of glass or aluminum silicate into the compound, or by acting on the density of the polyurethane foam introduced into lves. grooves 12, or again by an appropriate choice of the density and dimensions of the strips 15, 16 or 18.

 It goes without saying that many variants can be introduced, in particular by substitution of equivalent technical means, without departing from the scope of the present invention, which is not limited to tennis rackets only but can be applied to squash rackets. , badminton etc .....

 Although in the examples which have been given above the profile is obtained by extrusion of a compound, this characteristic is not at all necessary. As indicated above, the invention aims to keep the length of the carbon fibers as long as possible.

The profile can therefore also be obtained by injection, this injection being followed by hot bending. In fact, the injection of a frame length of a profile does not have the drawback of causing the fibers to break, which was the case in known frame injection methods.

Claims (10)

10 Tennis racket frame or similar game, characterized  in that it consists of a thermoplastic profile  extruded and then bent hot, the core and the handle  being made of injected plastics. 20 Snowshoe frame according to claim 1, characterized  in that it has at least one longitudinal compartment  hollow filled with a taring material before bending. 30 Snowshoe frame according to one of claims 1 or 2,  characterized in that it includes perpendicular bands  in terms of string, coextruded. 40 racket frame according to any one of claims  previous, characterized in that holding bands  arranged at the periphery of the frame are coextruded. 50 racket frame according to any one of claims  previous, characterized in that the profile is  extruded from a mixture of a thermoplastic material  and carbon fibers in a proportion of more than 15% of  carbon fibers. 60 Snowshoe frame according to claim 5, characterized  in that hollow microballoons of aluminum silicate  are included in the mixture to obtain a density  0.9 of said mixture. 70 racket frame according to any one of claims  previous, characterized in that the heart of the racket  is injected and made of a thermoplastic material  forming rivets, the mechanical connection being, moreover,  ensured by a surface fusion of the thermoplastic  contained in the extruded mixture.  80 Method of manufacturing a racket frame according to one  of the preceding claims, characterized in that it  consists of  - form a mixture of a thermoplastic material and  fibers;  - extrude the mixture and allow it to cool;  - cut and pierce it;  - fill the longitudinal compartments;  - reheat the whole to the point of softening  profile;  - to bend it hot;  - inject - the heart of the racket;  - inject the handle.  90 Method of manufacturing an addre according to one of  Claims 1 to 7, characterized in that it consists of:  - form a mixture of thermoplastic material and fibers;  - carry out the extrusion of a profile;  - conform the frame in its final form by guiding  its route;  - drill the holes;  - inject the heart;  - inject the handle. 100 Method for manufacturing a tennis racket frame  or similar game, characterized in that it consists of  - form a mixture of thermoplastic material and  fibers;  - inject a length of frame;  - pierce said length;  - fill the longitudinal cavities;  - reheat the whole to the softening point  mixture;  - hot bend the length;  - form the heart of the racket;  - form the handle.