EP0169668A1

EP0169668A1 - Games racket

Info

Publication number: EP0169668A1
Application number: EP85304469A
Authority: EP
Inventors: Robert Cristopher Haines
Original assignee: Dunlop Ltd
Current assignee: Dunlop Ltd
Priority date: 1984-07-13
Filing date: 1985-06-24
Publication date: 1986-01-29
Also published as: JPS6145780A; GB8417935D0; AU4443085A; KR860000874A

Abstract

The invention relates to hollow frames for games rackets, e.g for tennis, which are in the form of an injection moulding of thermoplastics material reinforced with short, discrete lengths of fibre. Improved stiffness of the frames under conditions of increasing temperature and water content is achieved by using a mixture of fibres of different lengths in the moulding composition. The moulding composition should contain, prior to moulding, at least 40% of reinforcing fibres of length less than 3 mm and up to 60% of reinforcing fibres of length greater than 5 mm, the peren- tages being by weight of the total reinforcing fibre present.

Description

This invention relates to rackets for use in games, for example tennis, squash and badminton and is particularly concerned with the construction of the frames of these rackets.
In our U.K. Patent No. 2,015,886 we have described and claimed a games racket frame in which the frame comprises a head and a shaft, at least the head being a hollow injection moulding of thermoplastics material reinforced with short filament reinforcing material, in which the wall of the moulding which lies at the outer circumference of the head is joined to the wall which lies at the inner circumference of the head by an internal support means and the stringing holes in the head pass through the support means, the walls and support means of the moulding being integrally-formed. By "thermoplastics material reinforced with short filament reinforcing material" is meant a reinforced thermoplastics material in which the reinforcements are in the form of short discrete lengths of fibre-reinforcing material randomly dispersed in the thermoplastics resin matrix.
Games rackets made according to our U.K. Patent No. 2,015,886 have had considerable commercial success. In the commercial constructions the thermoplastics material utilised has been polyamide and the reinforcing fibre carbon fibre or a mixture of carbon fibres and glass fibres.
The present invention aims to provide an even better games racket frame utilising a hollow, injection-moulded structure of thermoplastics material reinforced with short filament reinforcing material but embodying improved physical properties by restriction of the type of reinforcing fibres used.
Rackets moulded from polyamide reinforced with short filament reinforcing material have beeh found to decrease in stiffness with increasing temperature and water content. While this effect is unlikely to have any noticeably deleterious effect under most prevailing conditions, it is possible that p severe conditions of high temperature and high hunidity, for example as might be encountered in the boot (trunk) of a car in a tropical region, a racket might suffer an unacceptable degree of stiffness loss, which could lead to distortion by warping under the tension of the strings.
Commercially-available injection moulding grades of thermoplastics polymers reinforced with short discrete lengths of fibres conventionally contain fibres of lengths up to about 1 mm and in fact reinforced moulding pellets normally available have a major dimension of no more than about 3 mm which of course effectively places that size limit on the length of included reinforcement. It is in fact more difficult and hence more expensive to produce thermoplastics moulding pellets or granules containing fibre reinforcements of longer lengths. We have found, however, by using short filament reinforcing material but of longer length than is conventionally available, that surprisingly effective resistance to such property changes as stiffness loss in the moulded racket frame can be achieved.
We have further found that it is not necessary to increase all reinforcing fibre lengths in order to obtain better reinforced products where there is a risk of exposure to higher than normal temperatures and humidities. This is not only because of the considerably higher cost of moulding compositions containing longer fibres but also-. because we have surprisingly found that the desirable effects of incorporating longer fibre lengths in increasing proportions in blends with shorter conventional fibre lengths are largely achieved by the time a 60:40 proportion and possibly even a 50:50 proportion, by weight is reached, i.e. proportions of long fibres to short fibres.
Thus the present invention provides a games racket frame, the-frame comprising*a head and a shaft, at least the head being a hollow injection moulding formed from a - moulding composition of thermoplastics material reinforced with short filament reinforcing material, as herein defined, in which the reinforcing material in the moulding composition prior to moulding comprises at least 40%(by weight of total reinforcing fibres) of reinforcing fibres of length less than 3 mm and up to 60% (by weight of total reinforcing fibres) of reinforcing fibres of length greater than 5 mm.
The invention also embraces a games racket comprising a strung frame of the invention.
It is preferred that some of the reinforcing fibres in the moulding composition have a length of at least 10 mm but not greater than 15 mm. The fibres may be of any suitable reinforcing material, e.g. carbon fibre, glass fibre or aromatic polyamide (e.g. Kevlar, Registered Trade Mark) fibre or mixtures thereof.
The reinforcing fibres in the moulding composition preferably comprise at least 10% of the longer fibres and, preferably, from 20% to 50% by weight of them.
It will be appreciated that during injection moulding using pellets of thermoplastics material, incorporating reinforcing fibres, the action of moulding causes a reduction in length of the fibres by chopping and shearing into shorter lengths. The lengths specified above for the present invention, as indicated, relate to those pertaining in the moulding composition, i.e. before the injection moulding process has taken place.
In another aspect, the invention provides a games racket frame, the frame comprising a head and a shaft, at least the head being a hollow injection moulding formed from a moulding composition of thermoplastics material reinforced with short filament reinforcing material, as herein defined, in which the moulding composition contained prior to the injection moulding process sufficient reinforcing fibres of at least 5 mm in length to give a moulded product having at least a 15% 'increase in the stiffness, as hereafter defined, of the racket frame.
By at least a 15% increase in stiffness is meant that the racket frame of the invention has a stiffness at least 15% greater than a substantially identical 'control' racket frame which is moulded from a composition whose reinforcement is 100% of the same short fibre composition as is used to constitute the non-long fibre portion used for the frame of the invention, the stiffness being measured by the method described below at conditions of 70°C and 2% moisture content.
The invention is further illustrated with reference to the accompanying drawings in which:

Figure 1 is a diagrammatic representation in side view of a racket frame undergoing loading for the stiffness and Creep tests described below;
Figure 2 is a graph of the stiffness (deflection under load) of carbon-fibre-reinforced nylon tennis racket frames against the proportion of 'long' carbon fibre in the total carbon fibre content of the reinforcement, and
Figure 3 is a graph of the 'Creep' under load of carbon-fibre-reinforced nylon tennis racket frames against the proportion of 'long' carbon fibre in the total carbon fibre content of the reinforcement.

To measure a tennis racket frame stiffness at 70°C and with a 2% moisture content, the frame is first conditioned for 4 hours in boiling water. It is then tested for stiffness in a temperature enclosure maintained at 70°C and the frame is allowed to come up to temperature equilibrium before testing. The racket frame is supported at its extreme ends and a weight of 32 Kg is applied to its mid-point. The deflection of the mid-point of the racket is measured. (The smaller the deflection, the greater the stiffness). This is illustrated in Figure 1 of the accompanying drawings in which a racket frame 10 has a head 11, a shaft 12 and a handle 13. It is supported at its head and handle ends respectively by supports 14 and 15 and a load of 32 Kg is applied to its mid-point.
Figure 2 of the accompanying drawings is a graph of stiffness of carbon fibre-reinforced nylon tennis racket frames i.e. Deflection under load, v. proportion of 'long' carbon fibre in the total carbon fibre. (The total carbon fibre loading was 34% by weight of the reinforced composition used for moulding the racket frame and the 'long' fibres were 10 mm in length when incorporated in the moulding compositions, i.e. prior to the injection moulding process).
It can be seen that increase of proportion of long carbon fibre had no effect on stiffness of frames measured at 20⁰C with 0% moisture content, i.e. the 'Dry as moulded' condition. In contrast a frame with a moisture content of 2% and containing no long fibres suffered a considerable decrease in stiffness, i.e. increase in deflection, when tested at 70°C, i.e. the frame having been brought to 70°C equilibrium and the load applied for 10 minutes while maintained at that temperature. This was an increase from 40 to 101 units. Increasing the proportion of long fibre to 50% reduced the deflection to 71 units whereas increasing further to 100% long fibre only reduced deflection a further 7 units to 64.
Figure 3 of the accompanying drawings is a graph of 'Creep' of carbon fibre reinforced nylon tennis racket frames under a load of 32 Kg against the proportion of long v short carbon in the total fibre content. Again the 'long' carbon fibres were 10 mm in length in the moulding composition prior to injection moulding and the total carbon fibre content was 34% by weight of the nylon moulding composition.
'Creep' was measured as follows:
A load of 32 Kg was applied to the mid-point of the frame as indicated in Figure 1. The deflection 10 seconds after adding the load was noted and then the deflection 10 minutes after adding the load was noted. The difference between the two deflection readings is a measure of 'Creep', i.e. change in deflection with time under constant load. Generally, low Creep in a racket frame is desirable otherwise a racket may distort under the load applied by the strings.
Again it can be seen that at 70° and 2% moisture content, a frame moulded from a composition containing no 'long' fibre reinforcement had a high Creep value of about 16.5 whereas a racket frame in which the reinforcement was 50% long fibre had its Creep reduced to a mere 5.8. Increasing the 'long' carbon fibre proportion to 100% only effected a further reduction of 1.1 units of Creep, i.e. to 4.7.
Racket frames of the invention may incorporate, if desired, the features described in our U.K. Patent No. 2,015,886. Thus, the plastics material used is preferably reinforced with from 10% to 40% by weight of fibres based on total weight of the reinforced moulding composition.
Although polyamides are the preferred plastics material, others, e.g. polycarbonate, acrylonitrile-butadiene-styrene (ABS), acetal resin and poly(phenylene oxide) (PPO) may be used.
The hollow racket frame may, if desired, have one of the constructions described and claimed in 2,015,886. Thus the frame may have an integrally-moulded internal support means in the form of a row of centrally-disposed hollow pillars which join the outer wall to the inner wall of the head. The pillars provide convenient, integrally-formed stringing holes through the racket frame. Alternatively, support means between the outer and inner walls of the head may be provided in the form of abutments extending from the sidewalls of the moulding. (By 'sidewalls' is meant those portions of the frame that constitute one or other of the two visible faces of the racket when it is viewed from the front or rear at right angles to the plane that will contain the strings).
The actual dimensions of the hollow frame will depend of course on the type of racket, e.g. whether for tennis, squash or badminton. Similarly: the wall thickness will be governed by strength and weight requirements for the particular game. The average skilled man of the art will readily be able to decide suitable dimensions for his particular requirements. As an example only, a useful wall thickness may be 2.5 mm for a tennis racket.
The transverse sectional shape of the frame may be any desired shape, for example, circular, oval or rectangular. The latter may be preferred as its box-like section can give very high stiffness and strength to weight ratios. If it is desired, a longitudinally-extending groove or channel may be formed in the outer face of the wall of the frame which is to lie on the outer circumference of the head. The strings of the racket may then be recessed in this groove to safeguard them from abrasion. The transverse sectional shapes referred to above, therefore, include those shapes when modified by incorporation of such a groove or channel.
Hollow moulded racket frames of the invention may be made, if desired, by the methods described in our U.K. Patent No. 2,105,886, i.e. methods utilising the making of a suitably-shaped low-melting point core, the injection of the reinforced thermoplastics material around that core and the melting out of that core after the moulding has set.

Claims

1. A games racket frame, the frame comprising a head and a shaft, at least the head being a hollow injection moulding formed from a moulding composition of thermoplastics material reinforced with short filament reinforcing material, as herein defined, characterised in that the moulding composition prior to moulding comprised at least 40% (by weight of total reinforcing fibres) of reinforcing fibres of length less than 3 mm and up to 60% (by weight of total reinforcing fibres) of reinforcing fibres of length greater than 5 mm.

2. A games racket frame according to Claim 1, characterised in that the moulding composition contained at least 10% (by weight of the total reinforcing fibres) of the fibres of length greater than 5 mm.

3. A games racket frame according to Claim 2, characterised in that the moulding composition contained from 20% to 50% (by weight of the total reinforcing fibres) of the fibres of length greater than 5 mm.

4. A games racket frame according to Claim 1, 2 or 3, characterised in that some of the reinforcing fibres are at least 10 mm in length prior to use in the injection moulding process.

5. A games racket frame according to any one of the preceding claims, characterised in that the reinforcing fibres prior to moulding had a maximum length of 15 mm.

6. A games racket frame according to any one of the preceding claims, characterised in that the reinforcing fibres are of carbon fibre, aromatic polyamide fibre, glass fibre or mixtures thereof.

7. A games racket frame according to any one of the preceding claims, characterised in that the thermoplastics moulding composition comprises a polyamide.

8. A games racket frame according to any one of Claims 1 to 6, in which the thermoplastics moulding composition comprises polycarbonate, acrylonitrile-butadiene-styrene copolymer, acetal resin or poly(phenylene oxide).

9. A games racket frame according to any one of the preceding claims, characterised in that the moulding composition contained prior to the injection moulding process sufficient reinforcing fibres of at least 5 mm in length to give a moulded product having at least a 15% increase in the stiffness, as herein defined, of the racket frame.