CONNECTING ELEMENT, CONNECTION WITH SUCH AN ELEMENT, AND METHOD FOR MANUFACTURED BY A CONNECTING ELEMENT

Abstract

The present invention provides a connecting element, a fitting comprising such a connecting element, and a method for manufacturing such a connecting element. The connecting element (10) comprises a socket (15) made of thermoplastic material for inserting a tube element, the socket being able to be welded to the tube element along a junction surface, the socket (15) having an internal diameter (D1) of the socket, at least one heated prestressed element (30) embedded in the socket (15), in a first expanding prestressed state in an initial position in which an internal diameter of the at least one prestressed element is equal to or greater than the internal diameter of the socket and around the periphery of the junction surface, in which the at least one prestressed element is capable of assuming a second prestressed state when its temperature exceeds a local melting temperature of the thermoplastic material.At least one prestressed element, in a stable unstressed state, has an outside diameter smaller than the inside diameter of the socket.

Description

[0001] The present invention relates to tube or pipe fittings. More specifically, the present invention relates to a coupling element, a coupling with such a coupling element and a method of manufacturing such a coupling element.

[0002] We already know from the patent application FR 2 748 415 A1 a connecting element designed to allow assembly by induction welding or electro-fusion of at least two parts of thermoplastic materials. The coupling element comprises a block of thermoplastic material, in which elastic heating elements, for example an elastic spiral spring, are embedded in a prestressed state. During the welding operation, the elastic heating member melts the material in the vicinity of the member, which then allows it to move and pass from its prestressed state to its natural state straddling the two parts to be welded.

[0003] Known electrofusion systems make it possible to assemble tubes and fittings made of crystalline or semi-crystalline polymer material, for example such as polyolefins, for example polyethylene, polypropylene or polybutylene, polyamides, fluorines, etc. In these crystalline polymers, the spring can move easily in the material during the melting of the material, thanks to its low viscosity in the molten state, to resume its unconstrained, natural state which positions it astride the interface of the parts to be assembled.

[0004] However, the connection element and the electrofusion process known from document FR 2 748 415 A1 do not make it possible to achieve a tight connection when the parts are made of material of the amorphous or weakly crystalline polymer type, such as PVC, CPVC or ABS. Amorphous or weakly crystalline polymers are in a state characterized by an almost rubbery behavior, therefore a very high viscosity, higher than that of crystalline polymers, once heated beyond their softening temperature. As a result, the elastic member is slowed in its movement by the amorphous or weakly crystalline polymer material. Therefore, the spring cannot return to its natural state, at least it will take a long time during which the amorphous or weakly crystalline polymer can degrade thermally, which does not allow the functional performance of the connection to be achieved.

[0005] In addition, the elastic member cannot move easily in the material and cannot reach a straddling position at the interface of the parts to be assembled, when the parts are made of thermoplastic material of very high viscosity and thermally low stable such as PVC, CPVC, ABS.

[0006] Furthermore, crystalline polymers and amorphous polymers also exhibit different thermal behavior. On the one hand, amorphous or weakly crystalline polymers have a lower thermal stability than that of crystalline polymers, which limits the flexibility of the joining process. On the other hand, when the crystalline polymer, for example of the polyethylene type, is heated, it passes by melting from its solid crystalline state to a non-crystalline liquid state of low viscosity, and expands strongly. This expansion thus develops a pressure in the liquid phase, and thus promotes intimate contact between the materials of the fitting and that of the tube, which, in combination with the low viscosity of the materials of the tube and that of the fitting, facilitates the molecular interpenetration between the materials and therefore a good weld. This ensures the quality of the assembly, both for mechanical strength and sealing. This strong expansion phenomenon is not present for amorphous or weakly crystalline polymers. On this basis, the prior art did not take into consideration the physical specificities linked to the assembly of very viscous polymers, for example weakly crystalline or amorphous, such as PVC, PVC-C and ABS. There was therefore a prejudice against the use of electrofusion with these materials, its implementation in order to obtain satisfactory results seemed impossible, the tests resulting in the impossibility of obtaining real welds.

[0007] An object of the invention is to provide a coupling element and a coupling method for polymers of the weakly crystalline or amorphous type.

[0008] To this end, the invention proposes a coupling element comprising a socket made of thermoplastic material for inserting a tube element, the socket being able to be welded to the tube element along a junction surface, the socket having an internal diameter of the socket, and at least one prestressed heating element embedded in the socket, in a first prestressed state in expansion in an initial position in which the internal diameter of the at least one prestressed element is equal to or greater than the internal diameter of the socket and on the periphery of the junction surface, in which the at least one prestressed element is capable of assuming a second prestressed state when its temperature exceeds a local transition temperature from a solid state to a pasty viscous state of the thermoplastic material. The at least one prestressed element, in a stable unstressed state, has an outside diameter smaller than the inside diameter of the socket.

[0009] The thermoplastic material can be an amorphous or weakly crystalline polymer, PVC, PVC-C or ABS. By weakly crystalline is normally meant a percentage or rate of crystallinity of less than 20%. In preferred embodiments, it is possible to define a percentage or degree of crystallinity lower than 10% or even lower than 5% to form the limit of a weakly crystalline material.

[0010] Thus, the present invention provides a coupling element which is adapted to be used for assembly with a tube. During assembly, the elastic heating element can assume a second prestressed state in which the elastic element can be in a final position straddling the tube and the socket of the coupling element. During the retraction movement of the elastic element, the elastic element can entrain the material of the coupling element in the material of the tube, thus creating interpenetrations. Such a connecting element allows an assembly of elements in amorphous or weakly crystalline polymeric thermoplastic material, by combination of local interpenetration of materials, ensuring a mainly sealing role and by overlapping of metal element, mainly playing a role, after assembly, of mechanical strength, between the tube element and the socket. Indeed, the prestressed element, adapted to remain under stress in the final connection position, straddling the socket and the tube element inserted into the socket, will be able to move in the amorphous thermoplastic material, in order to take the final position, for assembly, between the coupling element and the tube.

[0011] The elastic element(s) are elements capable of being heated. The elastic element(s) may be conductive and are intended to be heated by induction.

[0012] Mechanical anchoring and sealing of the connection can be obtained by one or more local weld lines. The welds are obtained by mixing the materials, the prestressed elastic element(s) locally kneading the materials during the passage between the first prestressed state and the second prestressed state. The prestressed element or elements, in addition to allowing softening by heating as well as movement of the materials ensuring good inter-contact, also ensure(s) mechanical stability of the connection after assembly.

[0013] According to one aspect of the invention, the at least one prestressed element is an elastic member, preferably a helical spring closed on itself, sized so as to store elastic energy allowing the elastic member to move into the final position. still under stress straddling the socket and the tube, under the combined effects of the elastic energy, the viscosity and the resistance to friction of the amorphous or weakly crystalline thermoplastic material, as well as the heating power transmitted to every moment by induction to the elastic element or in other words the programming of heating and its interruption making it possible to stop the progression of the prestressed element so that it stops in its position straddling the socket and the tube . In other words, the at least one prestressed element is intended to be heated according to a defined program such that the elastic element stops its progression towards its natural state so as to immobilize astride the socket and the tube.

[0014] These characteristics make it possible to obtain rapid heating by the at least one prestressed element to reach the temperature of sufficient local softening of the thermoplastic material, and allow local softening of the thermoplastic material, while obtaining a restoring force sufficient to allow the elastic member to reach a final connection position while providing mixing and intimate contact despite the virtual absence of pressure generated by the thermal expansion of the materials.

[0015] The present invention also proposes a connector comprising such a connector element and a tube element inserted in the connector element, in which the tube element and the connector element are assembled by combination of local interpenetration of materials, and in which the at least one prestressed element straddles the socket and the tube element, in a final prestressed expanding state, different from the unstressed stable state.

[0016] According to one aspect, the junction surface between the coupling element and the tube comprises a line of welds, on the periphery of the junction surface. By this line of welding, a tight mechanical assembly is obtained, resistant to pressure and temperature, without the need for an additional seal.

[0017] The tube element and the coupling element can be in a thermoplastic material which is a low crystalline or amorphous polymer, for example PVC, CPVC or ABS.

[0018] The present invention also relates to a method of manufacturing such a connecting element, comprising the following steps: placing at least one prestressed element, in particular an elastic heating member, in a first prestressed expanding state, as well as a spacer on a mold core, and injecting a thermoplastic material in order to overmold the at least one prestressed element and the spacer.

[0019] The spacer makes it possible to maintain the position of at least one prestressed element. Other spacers can be provided to maintain the spacing between two prestressed elements, when several prestressed elements are provided.

[0020] According to one aspect, the at least one prestressed element in a first prestressed expansion state is a closed elastic member, and the elastic member and the spacer are introduced into the mold using an expansion cone on which the said at least one prestressed element and the spacer are mounted, the expansion cone having an increasing diameter, with a small diameter less than or equal to the internal diameter of the at least one prestressed element in an unstressed state and a corresponding large diameter to the inside diameter of the socket.

[0021] It will be understood that the process according to the present invention makes it possible to overmold directly and in the same step the elastic heating member(s) and the spacer(s). Advantageously, the use of intermediate thermoplastic parts comprising the prestressed element(s) and the discontinuities resulting from the use of intermediate parts are avoided. Similarly, the thermoplastic material of the fitting can directly fill the prestressed element or elements, which makes it possible to limit sources of leakage or breakage.

[0022] A person skilled in the art understands that the spacers do not participate in the mechanical rigidity or in the sealing.

[0023] Furthermore, the method according to the invention can be used with most existing connection moulds, whether the sockets are perpendicular or parallel to the joint plane.

[0024] According to another aspect, the at least one elastic heating member and the spacer are pushed until they abut against the bottom of the mold using a tubular, and the tubular and the expansion cone are removed before step injection. This step can be done manually or be automated.

[0025] The method may include the step of placing a first spacer between the bottom of the mold and a first prestressed element, and a second spacer between the first prestressed element and a second prestressed element. According to one aspect, the method includes the step of placing a plurality of spacers between a plurality of prestressed elements.

[0026] A limited number of operations are necessary. In addition, the method can use standardized spacers and simple geometries, which makes the method economical.

[0027] The present invention also relates to the use of an amorphous or weakly crystalline polymer material, for example PVC, PVC-C or ABS in such a method, in such a connection and / or in such a connection element.

[0028] Other characteristics and advantages of the present invention will become apparent from the description given below with reference to the appended drawings which illustrate an example of embodiment devoid of any limiting character, among which: The figure 1 illustrates a fitting with a fitting member and a tube according to one embodiment of the present invention; The figure 2 illustrates a coupling element and a tube before assembly according to an embodiment of the present invention; The picture 3 illustrates a heating element according to one embodiment of the present invention. figures 4a, 4b and 5 illustrate a coupling element that can be used with a coupling of the figures 1 and 2 , according to one embodiment of the present invention; The figure 6 is a block diagram of a method of assembling a coupling element according to another embodiment of the present invention, the figure 7 is a block diagram of a method of manufacturing a coupling element according to one aspect of the present invention; The figures 8a to 8c illustrate the process steps of the figure 7 .

[0029] In the drawings, identical or similar elements are indicated with identical or similar reference numerals.

[0030] The figure 1 illustrates a connector 1 according to one embodiment of the invention, for connecting a tube element 3, pipes or other pipes, inserted into a connector element 10. The connector on the right side on the figure 1 shows the assembled fitting, and the figure 2 shows the connection before assembly, around a longitudinal axis X-X'.

[0031] The connector 1 comprises a connector element 10 into which the tube 3 can be inserted to form the connector by assembly, and more precisely welding of the connector element 10 and of the tube 3 according to a welded zone located along a surface junction 20.

[0032] The connecting element 10 is well seen on the figures 2 and 4a and 5. In the example illustrated, the coupling element 10 comprises a sleeve 12 forming a socket 15. The socket 15 has an internal diameter D1 substantially equal to the external diameter of the tube 3. The example shown is a straight sleeve with two sockets opposite substantially the same diameter, this example is not limiting and other examples of connection between a socket and a tube can be provided, such as a sleeve, a tee connection, a reduction, a plug, an elbow 10 'as illustrated at figure 4b (90° elbow) or any other example of a fitting commonly used in plumbing.

[0033] The sleeve 12 forms a block in which one or more prestressed elements are trapped. In the example illustrated, the prestressed element is an elastic member 30 which is embedded and fixed in an initial position in which the elastic member is in a prestressed state, in radial expansion. In the example of figures 1 to 5 , there are two prestressed elements per socket. This is only a non-limiting example. A method of manufacturing such a connecting element will be described later with reference to the figure 7 .

[0034] The prestressed element, in particular the elastic member, which can be adapted for the coupling 1 is a helical spring 30 whose ends 32 are mechanically and electrically connected, in order to obtain an elastic toric shape. The ends 32 can for example be welded together.

[0035] Spring 30 is sized to have an outer diameter D less than the inner diameter D1 of socket 15, when spring 30 is in an unstressed, stable state, as seen in Fig. picture 3 .

[0036] The spring can be a stainless steel spring, for example stainless steel, for example AISI 301 or 302.

[0037] As will be explained in more detail with reference to figure 7 , the prestressed element can be heated, for example by induction. Thus heated, the prestressed element can soften the material in which it is trapped, when its temperature locally exceeds the softening point of the thermoplastic material, sufficiently for the prestressed element to be able to move locally in order to reduce its mechanical stresses and to take a constrained lower mechanical stress state. Such a coupling element allows assembly by combination of local interpenetration, made possible by the retraction movement of the spring which causes the material of the coupling element in the material of the tube thus creating interpenetrations of materials.

[0038] On the figures 2, 4a and 5 , that is to say before assembly with the tube 3, the prestressed element 30 - in the embodiment the spring - is embedded in the sleeve 12, in a first prestressed state in expansion in an initial position close to the junction surface 20 and on the periphery of the junction surface.

[0039] After assembly, the elastic member 30 is immobilized in a position straddling the tube 3 and the coupling element 10, in a state in which the elastic member 30 is still prestressed in radial expansion ( figure 1 ).

[0040] The junction surface 20 then comprises a weld line 18, along the junction surface 20, obtained by local interpenetration of the materials of the tube and of the coupling element. Weld line 18 may be circumferential. A person skilled in the art understands that the welded zone is concentrated around the prestressed heating element or elements, where the material is softened in direct contact with the elastic member 30.

[0041] A method of assembly or connection is illustrated in figure 6 , with reference to the connecting element 10 and the tube 3.

[0042] In a first step S101, the prestressed element 30 is heated to reach a temperature above the softening point of the material in which the prestressed element 30 is embedded. During this phase, the prestressed element does not move. The prestressed element is then maintained at a temperature above the softening temperature, making it possible to locally soften the material, which makes it possible to cause the prestressed element 30 to pass from an initial position in which the prestressed element 30 is embedded in a first prestressed expanding state in the socket 15, at an intermediate position, under the effect of the elastic energy of the prestressed element which tends to return to a position without mechanical stress.

[0043] At this stage, a first heating phase is programmed with power injection by induction. This power can be modulated over time, in particular it can be gradually reduced so as not to exceed temperatures liable to cause damage to the material. The prestressed element 30 then passes from the initial expanding position to an intermediate position in an intermediate prestressed state. In the illustrated embodiment, the prestressed element 30 is an elastic element or member, for example a spring closed on itself.

[0044] In a second step S102, the injection of power by induction is cut off. The spring 30 sees its temperature drop and the stored heat continues to locally heat the thermoplastic material for a short time and then stops heating.

[0045] This step S102 results in the movement of the spring 30 from the intermediate position in an intermediate prestressed state to the final connection position illustrated in figure 1 . In this final position, the spring straddles the socket and the tube and is in a second prestressed expanding state, different from a stable unstressed state. It is actually a relaxation phase, in which the spring moves from the intermediate position to its final position, under the effect of the residual elastic energy of the spring between the intermediate prestressed state and the second state. prestressed, and the viscosity and frictional resistance of the amorphous or weakly crystalline thermoplastic material.

[0046] The connecting element 10 and the tube 3 are made of an amorphous or weakly crystalline thermoplastic material.

[0047] The injection of power is controlled so that the elastic member is immobilized in a position straddling the tube 3 and the coupling element 10, in a state in which the elastic member is prestressed in radial expansion. The programmed or controlled shutdown of the injection of power by induction can be carried out at a given time or a given position of the elastic member, taking into account the residual elastic energy of the elastic heating member when the injection power is stopped on the one hand, and the characteristics of the thermoplastic material on the other hand, in particular the viscosity of the thermoplastic and the friction with the thermoplastic.

[0048] These characteristics make it possible to obtain both a rapid heating time and a spring return force sufficient to achieve a sufficiently short temperature rise time for the spring. Indeed, the person skilled in the art understands that to heat the spring with a given power and to reach the softening temperature, it takes a certain time. During this temperature rise phase, there is no movement.

[0049] Moreover, these characteristics promote obtaining a sufficiently homogeneous temperature in the softening zone of the thermoplastic. The range of thermal stability of the amorphous or weakly crystalline but not semi-crystalline materials considered is narrow. If the temperature is too low, the material does not soften sufficiently; if it is too high, it may degrade.

[0050] Thus, those skilled in the art understand that the high viscosity of amorphous or weakly crystalline and non-semi-crystalline thermoplastic polymers in their molten state (in relation to the temperature) constitutes a naturally stabilizing effect of the assembly process, unlike crystalline polymers whose the viscosity is much lower. In fact, during fusion, these crystalline polymer materials pass almost instantaneously from a hard state to a liquid state of minimal viscosity. In an assembly process with crystalline polymers, the elastic member resumes an elastically unconstrained position, in which the spring is substantially at rest.

[0051] The variation of the return force of the spring as it relaxes from its initial stressed position to its still stressed final position is a stabilizing factor allowing better control of the final position of the spring.

[0052] Moreover, it is also necessary to take into account the loss of efficiency of the induction phenomenon when the spring moves away from the inductor element. From a certain value of penetration of the spring in the tube, the transfer of energy becomes less efficient. This also constitutes a naturally stabilizing effect of the process.

[0053] The prestressed element, in particular the spring, is also dimensioned to obtain a sufficient restoring force of the spring, a sufficiently short thermal activation time of the spring, and a sufficiently uniform temperature in the melting zone of the thermoplastic.

[0054] The Picture 7 illustrates a method of manufacturing a coupling element 10 according to the present invention and the figures 8a to 8c illustrate the different stages of the process of figure 7 .

[0055] In a first step S201, at least one prestressed element, for example elastic heating member 30, is placed in a first prestressed expanding state and at least one spacer 35 is placed on a mold core 50 of a mold 55.

[0056] As illustrated on the figure 8a the elastic heating member 30 and the spacer 35 are introduced into the mold 55 using an expansion cone 60 on which the at least one elastic member 30 and the at least one spacer 35 are mounted.

[0057] The expansion cone 60 has an increasing diameter, with a small diameter less than or equal to the internal diameter of the torus at rest of the elastic heating member 30 and a large diameter at least equal to the internal diameter D1 of the socket of the connecting element 10 to be manufactured, for example as shown in figure 5 .

[0058] The expansion cone 60 thus allows the radial expansion of the elastic heating member(s) 30 up to their diameter in the coupling element 10, in the first prestressed state in radial expansion.

[0059] The at least one elastic heating member 30 and the at least one spacer 35 can be pushed until they abut against the bottom of the mold 55 using a tubular element 70. Otherwise, the at least one elastic heating member 30 and the at least one spacer 35 can be pushed manually.

[0060] Once the at least one elastic heating member 30 is in a first prestressed expanding state and the at least one spacer 35 is in place on the mold core 50 of a mold 55, the expansion cone 60 and the tubular element 70 (if present) are removed from the mold 55 ( fig. 8b ).

[0061] The at least one elastic heating member 30 is in the first state prestressed in the expanded position. Thus, the elastic heating member 30 exerts a radial force which presses it against the core of the mold 50. This generates friction which prevents any axial movement of the at least one spacer 35 after it has been placed in the mold 55.

[0062] In the illustrated embodiment, the prestressed elements are provided as being two elastic heating elements 30, separated by an additional spacer 36. This example is not limiting and the type and number of prestressed element and additional spacers may vary depending on the design of the connecting element 10 to be manufactured by the method described.

[0063] In a second step S202, the thermoplastic is injected in order to overmold the at least one prestressed element, here the elastic heating member 30, and the spacer 35.

[0064] During injection, the thermoplastic fills the cavity of the mold 55, including the elastic heater(s) 30 ( Fig. 8c ).

[0065] After the holding and cooling phases, the mold can open automatically or be opened manually, in order to allow the ejection or the recovery of the coupling element 10 thus obtained.

[0066] The spacer 35 is preferably cylindrical on its internal face. It can be made of thermoplastic, identical or not to the thermoplastic constituting the connecting element 10.

[0067] The method thus allows the manufacture of a thermoplastic coupling element, in which at least one elastic member is embedded, in a controlled position and in a state of prestressed radial expansion fixed by the thermoplastic. The method of manufacturing a thermoplastic coupling element comprises a limited number of steps.

[0068] A person skilled in the art understands that the overmolded spacer or spacers 35, 36 do not contribute to and do not harm the mechanical rigidity and the tightness of the coupling element 10 thus obtained. Thus, advantageously, the shapes and materials of the spacer(s) can vary to give them other functions.

[0069] For example, the spacer 35 can be used as a good connection indicator. To do this, it can for example use independently or combine the following elements: (1) Be made of a thermoplastic whose coefficient of thermal expansion is greater than that of the main thermoplastic material of the fitting. Thus, during the connection, the spacer will, by expanding, come out of the connection in a visible manner. (2) Otherwise the spacer may have a conical shape, so as to accentuate its movement out of the connection during connection. (3) A split shape may allow the spacer to come loose from the fitting when connecting. (4) A thermoplastic with irreversible thermochromic properties can ensure that the spacer changes color irreversibly when it reaches a given temperature, upon connection.

[0070] Alternatively to the manufacturing process described with reference to the figure 7 using one or more spacer(s), such a connecting element can be manufactured using a positioner. The positioner is of hollow cylindrical shape, so that its internal diameter is equal to the diameter of the socket of the coupling element. The positioner has one or more circumferential grooves on its outer face, which make it possible to position and hold the elastic heating element or elements. The positioner can be made of a thermoplastic, identical or not to the thermoplastic constituting the connecting element.

[0071] Manufacturing is done in the following steps: In a first step, the elastic element or elements are expanded, for example by means of a cone, then positioned in the grooves of the positioner.

[0072] The elastic element(s) may optionally be heated to penetrate into the positioner, so as to be flush with its inner surface, which is the diameter of the socket. This heating step is optional.

[0073] The positioner and the elastic element(s) being now integral, the assembly is introduced onto the core of the mold and pushed only to abut against the bottom of the mold. The thermoplastic is then injected in order to overmold the elastic heating element(s) as an example of a prestressed element, and the positioner. During injection, the thermoplastic fills the mold cavity, including the elastic heating element(s).

[0074] After the holding and cooling phases, the mold can open automatically or be opened manually, in order to allow the ejection or the recovery of the coupling element thus obtained.

[0075] The method thus allows the manufacture of a thermoplastic coupling element, in which at least one elastic member is embedded, in a controlled position and in a state of prestressed radial expansion fixed by the thermoplastic. The method of manufacturing a thermoplastic coupling element comprises a limited number of steps.

[0076] This invention is not limited to the exemplary embodiment described above but it encompasses all variants thereof. In particular, a characteristic illustrated and / or described in combination with other characteristics can be provided independently or in combination with other characteristics illustrated independently or in combination with other characteristics, and this respectively independently or in arbitrary combination.

Claims

1. Fitting element (10) comprising - a socket (15) made of thermoplastic material for inserting a tube element (3), the socket being able to be welded to the tube element according to a joining surface (20), the socket (15) having an internal diameter (D1) of the socket, - at least one heated prestressed element (30) embedded in the socket (15), in a first prestressed state in expansion in an initial position in which an internal diameter of the at least one prestressed element is equal to or greater than the internal diameter of the socket and on the periphery of the joining surface, in which the at least one prestressed element is capable of taking a second prestressed state when its temperature exceeds a local softening temperature of the thermoplastic material, - in which the at least one prestressed element, in a stable unstressed state, has an external diameter (D) less than the internal diameter of the socket.

2. Connecting element according to claim 1, wherein the thermoplastic material is an amorphous or weakly crystalline polymer, for example PVC, PVC-C or ABS.

3. Element according to claim 2, wherein at least one prestressed element is suitable for induction heating.

4. Element according to claim 3, wherein at least one prestressed element (30) is an elastic member, preferably a helical spring closed on itself, dimensioned so as to store elastic energy enabling the elastic member to move into the final fitting position straddling the socket and the tube, under the combined effects of the elastic energy, viscosity and frictional resistance of the amorphous or weakly crystalline thermoplastic material, and the heating power transmitted at each instant by induction to the elastic member.

5. Fitting (1) comprising a fitting element (10) according to any one of the preceding claims and a tube element (3) inserted into the fitting element, in which the tube element (3) and the fitting element (10) are welded, and in which at least one prestressed element (30) is straddling the socket (15) and the tube element (3), in a final prestressed expanding state, different from the unstressed stable state.

6. Fitting according to claim 5, wherein the joining surface between the fitting element and the tube comprises a line of welds, on the periphery of the joining surface (20).

7. Fitting according to claim 5 or 6, wherein the tube element (3) and the fitting element (10) are made of a thermoplastic material which is a weakly crystalline or amorphous polymer, for example PVC, PVC-C or ABS.

8. Method of manufacturing a connecting element according to any one of claims 1 to 4, comprising the steps: - Placing at least one prestressed element (30), in particular a heated elastic element, in a first prestressed expanding state, as well as at least one spacer (35) on a mold core (50), - Injecting a thermoplastic material in order to overmold the at least one prestressed element (30) and the at least one spacer (35).

9. Method according to the preceding claim, wherein the at least one prestressed element (30) in a first prestressed expanding state is a closed elastic member, and the elastic member (30) and the at least one spacer (35) are introduced into the mold by means of an expansion cone (60) on which said at least one prestressed element and the at least one spacer are mounted, the expansion cone (60) having an increasing diameter, with a small diameter less than or equal to the inner diameter of the at least one prestressed element (30) in an unstressed state and a large diameter corresponding to the inner diameter of the socket (15).

10. Method according to the preceding claim, wherein at least one heating elastic element (30) and at least one spacer (35) are pushed until they butt against the bottom of the mold by means of a tube (70), and the tube (70) and the expansion cone (60) are removed before the injection step.

11. Method according to any one of claims 8 to 10, comprising the step of placing a first spacer (35) between the bottom of the mold and a first prestressed element (30), and a second spacer (36) between the first prestressed element and a second prestressed element.

12. A method according to any one of claims 8 to 11, comprising the step of placing a plurality of spacers (35, 36) between a plurality of prestressed elements.

13. Use of an amorphous or weakly crystalline and non-semi-crystalline polymer material, for example PVC, PVC-C or ABS in a process according to any one of claims 8 to 12, in a fitting according to any one of claims 5 to 7 and / or in a fitting element according to any one of claims 1 to 4.