CA2190533A1

CA2190533A1 - Racket and method for making such a racket

Info

Publication number: CA2190533A1
Application number: CA002190533A
Authority: CA
Inventors: Hugo Sol
Original assignee: Individual
Current assignee: Donnay International SA
Priority date: 1994-05-19
Filing date: 1995-05-16
Publication date: 1995-11-30
Also published as: AU686379B2; EP0759798A1; BE1008451A3; WO1995032029A1; AU2401595A

Abstract

Racket consisting of, on the one hand, a frame (2) formed of a head (3) and a grip (4), whereby this frame (2) is mainly made of composite material and, on the other hand, strings (5) provided in the head (3), characterized in that the frame (2) mainly consists of two parts, a first part (6) which comprises at least a part of the head (3) and which is made of thermoplastic material reinforced with continuous fibres and/or bundles of fibres which extend according to specific orientations, and a second part (7) respectively which is made of thermoplastic material which is reinforced with discontinuous, randomly orientated fibres and/or bundles of fibres.

Description

- 2)~0533;

Racket and method for makin~ such a racket.
The invention concerns a racket and a method for making such a racket.
In particular, it concerns a racket for tennis, squash 10 and badminton games, as well as for any other qames whereby a racket is used, whereby this racket consists of, ~ f~ fo~-7d of ~ h~ n~ ~ g~iF, /~hor~y tho ~rar..o io mà~y made of composite material.
15 Such ~acket, whereby the frama is entirely made of composite\~terial, is known from US 3 . 999 . 756 .
Rackets whic~are made of fibre-reinforced composite materials have ~en replacing the former wooden and 20 aluminium rackets ~ccessfully for quite a while now.
Indeed, composite \~aterials have a very good stiffness/weight ratio ~d strength/weight ratio, which makes it possible to const~t very performing rackets.
25 The superior strength/weight r~i~o of composite materials has allowed for drastic developmen~, such as for example what is called the oversize rack~ and the midsize racket, of which examples are described~ US 3.999.756.
30 Moreover, composite materials allow for a ~e design of the cross section of the frame, whereby ~is cross section can vary. This has allowed for example\l~or the construction of what is called wide body racket~ as described in US 4 . 664 . 380 .
AMENDED SHEEr 2t 9~5~3 `` ``
-- 1 t 1 on the one hand, a frame formed of a head and a grip, whereby this frame iB at least partly made of composite material and, on the other hand strings provided in the head, whereby the frame comprises a first part containing at least a part of the head and being made of thermoplastic material reinforced with fibres and/or bundles of fibres, and a second part which is made of synthetic material.
Such a racket is known from NL-A-9201354. The racket described in this document consists of several parts made of a synthetic material some of which are reinforced with continuous f ibres .

AMENOED SHEEr ljZ
= = =
Rd-~}.e~ lU~I L~ IUI.;~I d rd-;3Ct~t. /
The invention concerns a racket and a~uPL~d for making such a racket-In particular, it cong~r~ racket for tennis, squash10 and badminton gy~ as well as for any other games whereby a r~ is used, whereby this racket con8i8ts of a f~rmed of a head and a grip, whereby the frame is r- y ~d~ of ~ o~it~ -~t~
15 âuoh a racket, whereby the frame i5 entirely made of composite material, is known from US 3.999.756.
~ackets which are made of fibre-reinforced composite materials have been replacing the former wooden and 20 aluminium rackets successfully for quite a while now.
Indeed, composite materials have a very good stiffness~weight ratio and strength/weight ratio, which makes it possible to construct very performing rackets.
25 The superior strength/weight ratio of composite materials has allowed for drastic developments, such as for example what is called the oversize racket and the midsize racket, of which examples are described in US 3.999.756.
30 Moreover, composite materials allow for a free design of the cross section of the frame, whereby this cross section can vary. This has allowed for example for the construction of ~ what is called wide body rackets as described in US 4 . 664 . 380 .
AMENDED SHEET

Woss/32029 21 9 ~53 r ~ 7~ c .

The freedom regarding the design of the cross sections also allows for a better control of the vibrational behaviour and the mass distribution, so that it is found nowadays that the rackets of better quality are almost 5 always made of composite material.
Until recently, frames of rackets of composite material were made with a matrix of epoxy resin, reinforced with bundles of continuously orientated, long carbon, glass, 10 Revlar or boron fibres. Epoxy is a thermoset which is cured in a mould during the production process by adding heat. During the curing, the fibre reinforcement impregnated with epoxy, called prepeg, is pressed against the hot mould walls as a result of an internal pressure 15 and thus perf ectly adopts the shape of the mould . The inter~al pressure is obtained by means of thin-walled nylon pressure hoses around which the prepegs are wound.
After the curing, the rackets are taken out of the mould, any epoxy flashes are removed, after which the rackets 20 are sanded, sandblasted and painted.
Placing the prepegs in the mould is a time-consuming job because of the complicated connection between the oval, closed racket head and the grip . Also the f; n i ch; n~ of 25 the racket, after it has been taken from the mould, is time-consuming .
In order to avoid these time-consuming operations, one has already tried to make rackets with a discontinuous, 30 randomly orientated short fibre reinforcement in a thermoplastic matrix, among others as described in EP-A-O. 188 . 127 .
The complete racket is hereby made in one piece by means W0 95l321)29 2 ~ q ~3 5 3 3 ~ 6 of ~n injection technique. However, a seriou6 disadvantage of this method is that the short f ibre-reinforced composite in the racket head relaxes because of its viscoelastic behaviour under the influence of the 5 constantly working prestressing forces of the strings.
Thus, the permitted prestress and the size of the string surface are rather limited in such rackets. The structural damping of the rackets, however, is considerably larger than that of rackets with a continuously orientated fibre reinforcement.
~ore recently, one has tried to make complete rackets with continuously orientated fibre reinforcement in a thermoplastic matrix, as described for example in W092/12847. The same method is used hereby as for thermosets with continuously orientated fibre reinforcements, but the above-mentioned impregnated fibre reinforcement is replaced by fabrics of synthetic fibres a~d reinforcement fibres, or by reinforcement fibres powdered with plastic. This thermoplastic matrix saves efforts during the finishing, because the surface quality o~ the rackets, when coming out of the moulds, is better than with thermosets, but the application of the reinforcement fibres in the mould remains a time-consuming occupation.
The present invention aims a racket which combines a large structural damping, which is favourable for the comfort of the player, with a large stiffness in that part of the racket which contains the strings, the latter in order to prevent relaxation of the racket head under the influence of the prestressing forces of the strings and in crder to obtain a large force of impact and precision during the game.
. .

2 11~5~

To this aim, said first part i9 made of thermoplastic material reinforced with continuous fibres and/or bundles of fibres which extend according to ordened orientations, while the second part is made of thermoplastic material reinforced with discontinuous, randomly orientated fibres and/or bundles of fibres AMENDED SHtR

21~ 3~
. ~ .. ...
4~ 1 h i c ~; m t ~ nV~n~ j on ccnc~rn o ~ ~ck~t con ~ ~ 6t,~g of, on the one hand, a frame formed of a head and,y~rip, whereby this frame is mainly made of compos~material and, on the other hand, strings provi,~in the head, 5 characterized in that the frame ma~y consists of two parts, a first part which com~s at least a part of the head and which is ma,~of thermoplastic material reinforced with cont~us fibres and/or bundles of fibres which exte~/ccording to specific orientations, 10 and a secon,s~part respectively which is made of thermopl~c material which is reinforced with d~inuous, randomly orien~ated fibres and/or bundles o f;hr~c 15 The first part preferably extends from the free end of the head in the direction of the grip, so as to cover a part of the head, whereas the second part preferably covers the rest of the frame.
20 Said two parts will preferably overlap over at least three centimetres and at the most up to the free end of the grip .
Further, also a new production techLique is suggested to 25 produce such a racket. As opposed to already existing techniques, this technique makes use of a major physical quality of thermoplastic composites, namely that the matrix or in other words the thermoplastic material can be re-melted by adding heat without its chemical 30 qualities changing.
Also, the inventio~ concerns a method for making the above-mentioned racket, characterized in that it mainly consists i~ forming at least two U-shaped profile halves AMENDED SHEET

-~ ~ `21q~533` '' made of thermoplastic material which i5 reinforced with discontinuous, randomly orientated fibres a~d/or bundles of fibres; in forming a thimble-shaped element made of thermoplastic material which is reinforced with 5 continuous fibres andlor bundles of fibres which extend according to lipocific orientations; in applying any of the above-mentioned u-shaped profile halves in a production mould; in applying the above-mentioned thimble-shaped element over a flexible hose which is fit 10 to be put under pressure; in applying the flexible hose with the thimble-shaped element around it i~ the production mould, such that the thimble-shaped element at least partly overlaps the underlying profile half and such that at least one end of the flexible hose can be 15 reached outside the production mould; in applying the second profile half at the height of the already provided U-shaped profile half, such that the part of the flexible hose situated here and the overlapping part of the thimble-shaped element can be enclosed between the two U-20 shaped profile halves; in closing the production mould;in putting the above-mentioned f lexible hose under pressure and simultaneously heating the production mould, such that the above-mentioned thermoplastic material melts, such that both U-shaped profile halves and the 25 thimble-shaped element are melted to one piece; in interrupting the heat and pressure supply after a while;
in taking the thus formed frame, after the production mould has cooled off to a temperature at which the thermoplasti~ material has congealed again, out of the 30 mould; in providing strings.
In order to better explain the characteristics o the invention, the following preferred . on~ ts are described, as an example only without bei~g limitative in ~ME~IDE~ S~t'T

wo g~l32029 r~,D, any way, with reference to the accompanyinq drawings in which:
figure 1 schematically represents a racket according ~o the invention;
figures 2 to 4 schematically represent three variants of a racket according to the invention;
figure 5 shows a 6ection according to line V-V in figure 4 to a larger scale;
figures 6 and 7 show two variants of the section of figure 5;
f igure 8 shows a part of a racket according to the invention, prior to the composition thereof;
figures 9 to 12 show sections according to lines IX-IX to XI I -XI I respectively in f igure 8 to a larger scale;
figure 13 shows yet another part o$ a racket according to the invention, which is meant to be combined with the part of f igure 8;
figures 14 to 16 show sections of lines XIV-XIV to XVI-XVI in figure 13 to a larger scale;
figure 17 shows a part which is used during the manufacture of the racket;
figure 18 shows a section according to line XVIII-XVIII in figure 17 to a larger scale;
figure 19 shows a production mould in which a number of components of the racket to be f ormed are laid down;
figures 20 to 22 show sections of lines XX-XX to XXII-XXII respectively in figure 19 to a larger scale .
As represented in figure 1, the invention concerns a racket 1 consisting of, on the one hand, a frame 2 formed 21 ~3~
W095l320z9 r~l, of a head 3 and a grip 4, whereby this frame 2 is mainly made of composite material and, on the other hand, strings 5 provided i~ the head 3.
5 The invention is special in that the frame 2 is mainly composed of two parts, a first part 6 which at least comprises a part of ~he head 3 and which is m~de of thermoplastic material reinforced with continuous bundles of fibres which extend according to specific 10 orientations, and a second part 7 respectively, which is situated at the height of the grip 4 and is made of thermoplastic material which is reinforced with discontinuous, randomly orientated fibres and/or bundle~
of f ibres .
The parts 6 and 7 are indicated schematically in that they are hatched in opposite directions.
Preferably, the parts 6 and 7 overlap at least over a 20 certain distance, so that an overlap zone 8 is formed.
This Dverlap zone is at least 3 centimetres according to a perpendicular projection on the longitudinal axis 9 of the racket 1, and reaches ~srir-l ly to the free end 10 of the grip 4. The overlap zones 8 on either side of the 25 head 3 are preferably situated symmetrically in relation to the longitudinal axis 9.
In any case, the second part 7, irrespective of the size of the overlap zone 8, pref erably entirely covers the 30 L~ -in;n~ part of the frame 2 next to the first part 6.
Figure l shows a configuration with a short overlap zone 8 . According to f igure 2, however, the overlap zone 8 extends to the free end lO of the grip 4.

W095132029 2 1 ~ a 533 r.~
.

According to the most preferred embodiments, which offer optimum play characteristics for most players, the first part 6, to the exclusion of the overlap zones 8, will run on from the head end 11 to a point which, measured 5 according to the perpendicular pro j ection on the longitudinal axis 9, is situated at about 2/3 of the length of the strings measured according to the longitudinal axis 9_ This is the case among others for the examples in f igures 1 and 2, in other words the 10 length L1 amounts to 2/3 of the length L2.
Figure 3 schematically represents a variant in which the first part 6, to the exclusion of the overlap zones 8, runs on to half of the strings 5, measured according to 15 the longitudinal axis 9.
In the case where the racket 1, as represented in figure 4, has two arms 12 and 13 and a reinforcement piece 14 provided in between, which also forms a rounded 20 completion of the head 3, the first part 6, to the exclusion of the overlap zones 8, will preferably never extend any further than those places where the reinfoL. 1 piece 14 and the two arms 12 and 13 come together. Also, figure 4 shows a variant in which the 25 second part 7 runs on right past the place where the arms 12 and 13 ~oin the reinforcement piece 14.
In figures 1 to 4, the frame 2 is each time completely hatched to clearly show the fields over which the parts 30 6 and 7 extend.
In reality, however, the frame 2 will be entirely or almost entirely hollow. It can hereby have different forms of sections, and this form may vary for one and the wogsl3~029 P~~ /ac same racket in different places.
Figures 5 to 7 show a number of embodiments with sections in the shape of an ellipse, a circle and a rectangle 5 respectively.
In the case where the reinforcement piece 14 is al60 made of composite material, this is preferably solid.
According to a variant, it can also be made of another 10 material, f or example aluminium, whereby it matters little in this case whether it is solid or hollow.
The first part 6 is preferably made of different layers, as will be further described in the description of the 15 method.
The second part 7 has randomly orientated reinforcement fibres with a length which can vary from 0.1 mm to 15 cm.
~he length of the f ibres can hereby vary f rom one f ibre 20 to another and also as a function of the technique used to form the second part 7 or the components thereof, as will be further explained in the description.
The cross section of the above-mentioned reinforcement 25 fibres, both of the first part 6 and of the second part 7, preferably amounts to 3 to 20 micrometers. The volume of reinforcement fibres amounts to between 209~ and 8096 of the total composite material volume.
30 The reinforcement fibres are preferably carbon fibres, but also other fibre sorts are possible, such as glass fibres or aramid fibres.
Preferably, the above-mentioned racket 1 is made W0 9S~32029 ~ 1 ~ 0 5 3 3 P.~ ,C

according tQ the new method described in the introduction, which will be explained in further detail hereafter by means of figures 8 to 22.
5 ~ccording to this new method, at least two U-shaped profile halves 15 and 16, in the shape of scale parts, are formed, for example as represented in the figures 8 and 13, which will later form the above-mentioned second part 7. These profile halves 15 and 16 are made of 10 thermoplastic material which is reinforced with discontinuous, randomly orientated fibres and/or bundles of fibres.
The cross sections of the U-shaped profile halves 15 and 15 16 will usually have shapes as represented in figures 9 to 12 and 14 to 16. }3asically, the parts which will form the arms 12 and 13 and the head 3 are elliptic, whereas the parts which are meant to f orm the grip 4 have a rectangular or polygonal shape.
The reinforcement piece 14 will preferably be entirely part of one profile half, in this case the profile half 15. This reinforcement piece 14 can hereby be made, as represented in figure 12, of the same composite material 25 as the rest of the second part 7 and in this case has a solid cross section.
The prof ile halves 15 and 16 can be made in dif f erent ways. According to a first possibility, this can be done 30 by means of an injection techni~ue. In this case, the length of the reinforcement fibres is preferably O.1 to S mm, as such short fibres allow for a smooth injection.
According to another possibility, use can be made of what wo 9sl3202~ P~ l l Dr7 c~ 9 ~ 5 3 3 .

i8 called a thermoforming production techni~ue or a Sheet Moulding Compound (SMC) production technique. In this case, also longer, randomly orientated reinforcement fibres can be used, with a length which may vary between 5 0.1 mm and over 10 cm.
.

Moreover, as repre5ented in figures 17 and 18, a thimble-6haped element 17 is made of thermoplastic material which is reinforced with continuously orientated reinforcement 10 fibres or bundles of fibres. These reinforcement fibres are sùrrounded with the matrix material in dif f erent forms, available in specialized trades in a form whereby the reinforcement fibre is enveloped with a thin layer of the thermoplastic material, or a form whereby the fibres 15 are woven or bundled together with filaments of thermoplastic material, or yet another form whereby the fibres are powdered with fine granules of thermoplastic material .
20 The thimble-shaped element 17 is preferably cylindrical, i. e . also in the shape of a hose . This element 17 is made in advance, possibly of different layers which either or not have a different fibre orientation.
Preferably, the element 17 is made by means of a knitting 25 machine or a winding machine.
The thimble-shaped element 17 is, as represented in figures 17 and 18, provided around a flexible hose 18, in particular a hose which is fit to be put under pressure.
30 This hose 18 preferably consists of nylon, rubber or any other material which resists the supplied heat and the supplied pressure, and it can be provided on both ends with connecting pieces 19 and 20 which allow for a pressure supply.

wo 9sl32a29 12 . ~I/DI ~ C
Subsequently, the whole is assembled in a mould. This is illustrated by means of figure l9, which represents a mould half 21.
5 As represented in figure 19, one profile half, in this case the profile half 15, is put in the mould half 21 first, in a recess 22 provided to this end, after which the flexible hose 18 with the thimble-shaped element 17 provided around it is also put in the mould half, such 10 that the thimble-shaped element 17 at least partly overlaps the underlying profile half 15, in order to form the above-mentioned overlap zone 8, such that at least one end, but in this case both ends of the flexible hose 18, and in particular the above-mentioned connecting 15 pieces 19 and 20, can be reached outside the mould.
In a following stage, which is no longer represented in figure 19, but which can be derived from figures 20 to 22, the second profile half 16 is provided on the first profile half 15, such that the part of the flexible hose 20 18 situated here and the overlapping part of the thimble-shaped element 17 can be enclosed between the two U-shaped profile halves 15 and 16. It is clear that one must see to it that the thimble-shaped element 17 overlaps the ends of the profile halves 15 and 16 25 symmetrically.
Subsequently, the mould is closed, in other words the mould half 21 is closed off with a second mould half 23, which is also provided with a recess 24, as represented 30 in figures 20 to 22.
The figures 20 and 21 also show that the profile halves 15 and 16 overlap with their edges over a certain distance D.

~ 9~5~
Wo gsl32029 13 r~
At this stage, the above-mentioned flexible hose 18 is put under pressure and the mould is heated long and hard enough until all thermoplastic material present has become liquid, or at least very viscou6, and a mutual 5 fusion and/or adhesion has been obtained. On the one hand, the thermoplastic material of the thimble-shaped element 17 is melted to one piece and, on the other hand, an adhesion is realized between the thimble-shaped element 17 and the profile halves 15 and 16, as well as 10 a mutual adhesion between the edges of the profile halves 15 and 16 overlapping over the distance D.
The hose 18 is put under pressure by connecting it, as represented in figure 19, via connecting pieces 19 and 20 15 to a pressure source, for example a compressed air source 25, whereby the compressed air can be turned on by means of a suited valve 26.
The heating can for example be provided for by means of 20 electrical heating elements 27.
It is cleaf that, under the inf luence of heat and pressure, the parts 6 and 7 will consolidate entirely and assume the inner shape of the mould.
Subsequently, the heat supply is stopped and the mould is cooled off for a sufficient length of time, whereby the pressure can still be maintained for a while, until the thermoplastic material has stiffened.
When the thermoplastic material has congealed, the mould can be opened and the formed frame can be removed from it .

2 1 ~33 w0 9sl32029 ~ r.

It is clear that this frame can be further finished, such as sanded and/or painted or such, and 6ubsequently can be provided with the strings 5. It is also clear that the grip 4 can be wrapped around or provided with another 5 covering.
~he present invention is by no means limited to the embodiments described by way of example and represented in the accompanying f igures; on the contrary, such a 1~ racket and the above-mentioned method can be made in all sorts of variants while still l. inin~ within the scope of the invention.

Claims

Claims.

1. Racket consisting of, on the one hand, a frame (2) formed of a head (3) and a grip (4) , whereby this frame (2) is mainly made of composite material and, on the other hand strings (5) provided in the head (3), whereby the frame (2) comprises a first part (6) containing at least a part of the head (3) and being made of thermoplastic material reinforced with fibres and/or bundles of fibres, and a second part (7) which is also made of synthetic material, characterized in that said first part (6) is made of thermoplastic material reinforced with continuous fibres and/or bundles of fibres which extend according to ordened orientations, while the second part (7) is made of thermoplastic material reinforced with discontinuous, randomly orientated fibres and/or bundles of fibres.

2. Racket according to claim 1, characterized in that the second part (7) entirely covers the remaining part of the frame (2) next to the first part (6).

3. Racket according to claim 1 or 2, characterized in that the first part (6) and the second part (7) overlap.

4. Racket according to claim 3, characterized in that the first part (6) and the second part (7) overlap over at least 3 cm.

5. Racket according to claim 3 or 4, characterized in that the first part (6) and the second part (7) overlap up to the free end (10) of the grip (4).

6. Racket according to any of claims 3 to 5, characterized in that the racket (1) has two arms (12, 13) and a reinforcement piece (14) provided in between, and in that the first part (6), to the exclusion of the overlap zones (8), extends maximally to those places where the reinforcement piece (14) and the two arms (12, 13) come together.

7. Racket according to any of claims 3 to 6, characterized in that the first part (6), to the exclusion of the overlap zones (8), extends from the head end (11) of the racket (1) to a point which, measured according to the perpendicular projection on the longitudinal axis (9), is situated at about 2/3 of the lengths of the strings (5) measured according to the longitudinal axis (9).

8. Racket according to any of the preceding claims, characterized in that the first part (6) has different layers of fibres.

9. Racket according to any of the preceding claims, characterized in that it is provided with a reinforcement piece (14) and in that the frame (2) is hollow to the exception of this reinforcement piece (14).

10. Racket according to any of the preceding claims, characterized in that the volume of fibres, both for the first part (6) and the second part (7), amounts to 20 to 80% of the total composite volume.

11. Racket according to any of the preceding claims, characterized in that the fibres consist of carbon fibres, glass fibres or aramid fibres.

12. Racket according to any of the preceding claims, characterized in that the thickness of the fibres is 3 to 20 micrometers.

13. Method for making a racket as described in claim 1, characterized in that it mainly consists in forming at least two U-shaped profile halves (15,16) made of thermoplastic material which is reinforced with discontinuous, randomly orientated fibres and/or bundles of fibres; in forming a thimble-shaped element (17) made of thermoplastic material which is reinforced with continuous bundles of fibres which extend according to ordened orientations; in applying any of the above-mentioned U-shaped profile halves (15) in a production mould; in applying the above-mentioned thimble-shaped element (17) over a flexible hose (18) which is fit to be put under pressure; in applying the flexible hose (18) with the thimble-shaped element (17) around it in the production mould, such that the thimble-shaped element (17) at least partly overlaps the underlying profile half (15) and such that at least one end of the flexible hose (18) can be reached outside the production mould; in applying the second profile half (16) at the height of the already provided U-shaped profile half (15), such that the part of the flexible hose (18) situated here and the overlapping part of the thimble-shaped element (17) can be enclosed between the two U-shaped profile halves (15,16); in closing the production mould; in putting the above-mentioned flexible hose (18) under pressure and simultaneously heating the production mould, such that the above-mentioned thermoplastic material melts, such that both U-shaped profile halves (15,16) and the thimble-shaped element (17) are melted to one piece; in interrupting the heat and pressure supply after a while;

in taking the thus formed frame (2), after the production mould has cooled off to a temperature at which the thermoplastic material has congealed again, out of the mould; in providing strings (5).

14. Method according to claim 13, characterized in that the profile halves (15, 16) are made by means of injection and in that for the discontinuous fibres, fibres are used with a length of 0.1 to 5 mm.

15. Method according to claim 13, characterized in that the profile halves (15, 16) are made by means of what is called a thermoforming production technique or a Sheet Moulding Compound (SMC) production technique, and in that fibres are used with a length between 0.1 mm and 10 cm.

16. Method according to any of claims 13, 14 or 15, characterized in that the thimble-shaped element (17) is formed by winding or knitting.

17. Method according to any of claims 13 to 16, characterized in that the thimble-shaped element (17) is composed of different layers with different fibre orientations.

18. Method according to any of claims 13 to 17, characterized in that the racket (1) to be formed is provided with two arms (12, 13) and a reinforcement piece (14) extending in between, and in that this reinforcement piece (14) has been previously connected to one of the profile halves (15) or formed in one piece with it.

19. Method according to any of claims 13 to 18, characterized in that the flexible hose (18) consists of a material which can resist the supplied heat and pressure and which is provided on both ends with connecting pieces (19, 20).