Electro-weldable connecting piece having connection terminals and its method of manufacture

A connecting device for connecting two thermoplastic pipes together including a thermoplastic body having a resistive element having a wire mesh structure and connection terminals. Each of the connection terminals has a tubular rivet body made of an electrically conductive material. The tubular rivet body includes a collar integrally formed with a hollow shaft and a flanged portion bent to immobilize the resistive mesh after the connection terminals are riveted to the mesh. This increases the reliability of the connection terminals to the resistive mesh.

BACKGROUND OF THE INVENTION 
The invention relates to a plastic connecting piece (such as a sleeve or a 
T-connection saddle) intended for joining axially or transversely, and by 
heat-welding, two pipes, in particular in the gas industry, the invention 
relating more particularly to the improvement of the connection terminals 
of the electrically conducting resistance element with which said 
connecting device is equipped. 
Such a device, which includes, for the heat-welded joining of two 
polyethylene pipes, an electrical heating resistance element consisting of 
a coiled electrically conducting wire embedded in the thickness of the 
piece in the vicinity of its internal welding surface, is already known 
from publications EP-A-0,149,410 or U.S. Pat. No. 4,684,417. 
The welding operation is then carried out by placing opposite each other 
the pieces to be welded and by connecting the coiled electrical wire to an 
electrical source (such as a rectified source of alternating current) 
which, by the Joule effect, will heat up the resistance element and cause 
the softening until melting of the plastic surrounding the pieces, 
ensuring welding under pressure. 
For the electrical connection of the resistance element, its two wire ends 
are usually each welded to a connection terminal each often emerging into 
a well or a cavity, so as to be able to constitute two pins which will be 
able to be plugged in, inside the said wells, to removable complementary 
sockets. 
Still within the same field, other types of resistance elements can be 
used. 
Thus, publication FR-A-2,654,978 provides the use of a single-wire net or 
of a grid which can be produced from a stretched metal plate. 
However, the external electrical connection of this net or of this grid 
remains the same as the solution mentioned hereinabove. In the first case 
(single-wire net), the two ends of the wire are drawn out in order to be 
welded to the aforementioned pins. In the second case (stretched grid), it 
is advised to use two conducting plates fixed at two opposite ends of the 
grid, and two conducting joint cables welded on one side to the said 
plates and on the other side to the two aforementioned pins. 
However, problems of reliability of the welded joints may then occur. There 
is also the risk that the fusion zone of the plastic body can end up 
damaging these joints, at the moment of the welding. 
Furthermore, especially in the case of the use of a monofilar net, all this 
implies lengthy and tricky implementation operations. 
Another solution is provided in Application EP-A-0,278,553. In this 
application, the connection terminals are applied to a connection piece in 
which a resistance element in the form of a coiled wire is embedded. These 
terminals each consist of two complementary pieces which are fitted into 
each other after having been inserted into the cavities provided for this 
purpose in the connecting piece. The two parts of the one same terminal, 
which are subjected to two opposed direction forces, are thus suitable for 
gripping a thin thickness of plastic where the resistance element passes 
and, once "clipped together", then establish an electrical contact with 
this resistance element. However, the use of such terminals necessitates 
producing tapped holes in the piece in order to insert therein the two 
complementary components of the terminals. On the other hand, the fitting 
of these components one into the other could lead (if the fitting forces 
are not very precisely applied), to a detrimental deformation of the 
connecting piece. 
SUMMARY OF THE INVENTION 
Taking this into account, the object of the invention is especially to 
provide a connecting piece whose connection terminals ensure a connection 
which is reliable and is carried out very rapidly by a simple operation, 
the electrical joint obtained with the resistance element being 
particularly firm mechanically, without appreciable risk of this 
resistance element tearing or of the piece becoming damaged. 
Thus it has been imagined to have recourse to a riveted joint. 
More precisely, the invention firstly provides a connecting piece which is 
characterized in that its connection terminals comprise at least one 
tubular blind-rivet body, made of electrically conducting material, acting 
as an electrical connector and including a shaft emerging at the surface 
of the body of the piece, this shaft being connected, at one end, to a 
collar against which is bent over, on the opposite side of the shaft, a 
flanged part immobilizing the resistance element of the piece, between it 
and the collar. 
Of course, the use of rivets is known in mechanical engineering. It is also 
known, from EP-A-0,010,055, to use a rivet as an electric socket. In this 
application, a blind rivet is fixed to an electrically conducting solid 
metal sheet in order to establish an electrical contact with this metal 
sheet. This rivet is installed in a known manner by virtue of a pair of 
tongs exerting on a single side a traction force on the male component of 
the rivet, the female component being held pressed against the metal 
sheet. 
However, EP-A-0,010,055 does not relate to the field of electro-welding. 
This document therefore does not take into account the problems specific to 
electro-weldable thermoplastic pieces and especially the questions: 
a)--of mechanical and thermal stresses which the resistance element and its 
joint have to withstand during the welding; 
b)--of installation of the said terminals within the piece itself. 
With regard to point a), nothing indicates that the riveted joint of 
EP-A-0,010,055 could be suitable for an electro-weldable device, the mole 
so as document EP-A-0,278,553 invites the use of two-piece electrical 
terminals, therefore certainly being able, at least slightly, to act one 
with respect to the other and with respect to the resistance element, if 
the mechanical stresses due to heating during the welding are too great. 
It will also be noted that the rivet of EP-A-0,010,055 is fixed to a solid 
plate, and is therefore relatively rigid, whereas the terminals of 
EP-A-0,278,553 are fixed to a coiled wire. 
Furthermore, as regards point b), if, based on EP-A-0,278,553, it is sought 
to install, instead of terminals having fittable components, the rivet 
described in EP-A-0,010,055, by conventional application, from one side, 
of a force on its male component (the female component being pressed 
against the thin layer of plastic enclosing the resistance element), this 
would certainly run the risk of a disastrous deformation of this layer, or 
indeed of the wire resistance element. 
Now, going precisely counter to the teaching that the person skilled in the 
art would be able to draw from EP-A-0,278,553 and EP-A-0,010,055, the 
invention precisely provides the use of a rivet (and more precisely of its 
female component) as an electrical connector for supplying the resistance 
element of a joint piece for the heat-welded joining especially of 
polyethylene pipes. 
In practice, the female body of the rivet will ensure its connection 
function by means of its shaft which will be able to act as a pin. In 
addition, the connecting device, which is the subject of the present 
invention, will be able to constitute an electro-weldable sleeve for the 
substantially coaxial joining of two pipes by heat-welding. This piece 
will also be able to constitute an electro-weldable branch connector for 
the transverse joining of the pipes. 
Advantageously, and as indicated above, the invention is most particularly 
applicable if the resistance is in the form of a monofilar meshed net, 
which is particularly sensitive to the stresses which can be induced 
during the welding. 
Another problem which arises is therefore that of the electrical contact to 
be established with this particular type of resistance element under 
optimum conditions of effectiveness and in conjunction with the problems 
of mechanical stresses mentioned previously. 
The solution provided here consists in associating with each rivet 
component engaged through a mesh of the resistance element, means for 
tearing the covering layer of the said wire at the location of the 
riveting of the terminal, thus ensuring the electrical contact, the said 
terminals furthermore being, with the resistance element, partly embedded, 
by moulding, in the body of the piece. In practice, and preferably, the 
adopted tearing means will extend around this mesh facing the flange of 
the terminal. For this purpose, and depending on the size of the meshes of 
the net, it will be possible in particular to use two washers for each 
rivet, gripping the net tightly between them, one of these washers (which 
can most particularly be of the "toothed" type). 
Finally, taking into account the above, the invention also relates to a 
method of manufacturing a connecting piece of one of the aforementioned 
types, characterized in particular in that: 
the connection terminals are crimped to the resistance element; 
and onto these is overmoulded the body of the connecting piece, causing the 
shaft of each terminal to emerge into a well made on an external surface 
of the body of the piece and having a cross-section greater than that of 
the shaft. 
It will be noted that such a manufacturing method has the advantage of 
being simple to implement, the reliability of the 
terminals/resistance-element electrical connection virtually preventing 
any risk of rupture of this connection, during the overmoulding 
especially. Furthermore, the fixing of the terminals to the resistance 
element before overmoulding the assembly makes it possible to avoid the 
possible risks of deformation of the piece which are connected, on 
installing the terminals on the already-formed body, with its resistance 
element. 
Other characteristics and advantages of the invention will again appear 
from the more detailed description which follows, given with reference to 
the attached drawings which show, in a non-limiting fashion, embodiment 
and utilization examples of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS 
First of all, FIG. 1 shows an assembly piece of the "blind" rivet type 
which will therefore be used as an electrical connection terminal. 
The device, identified in its entirety by 1, comprises an ejectable male 
rivet shank 3 and an elongated female body 5. 
The shank 3 (also called a "stud") is said to be "ejectable" in that it 
breaks off at the location of a preformed rupture neck 7 and is separated 
from the body 5 once the latter has been riveted, the shank then being 
ejected from the riveting apparatus. 
It will be noticed that this shank 3 has a widened part 9 forming the head 
at one end and near the neck 7. 
Regarding the body 5, this is penetrated by an axial passage 11 of 
intermediate cross-section d.sub.1 between the cross-section d.sub.2 of 
the shank and that d.sub.3 of the head 9 so that only the said shank 3 can 
be inserted therein. 
In addition to this, the body 5 has, at an intermediate location along its 
axial length, an external normal collar 13 delimiting a lower tubular 
section 5a (the longer section) and an upper tubular section 5b (the 
shorter section), coaxial one with respect to the other. 
Preferably, (and in fact conventionally), the two parts of the piece 1, 
shank and body, will be produced from metal, such as aluminium typically, 
being thus electrically conducting and relatively malleable under a force 
without rupturing. 
By way of embodiment example, the stud 3 will be able to have a length of 
approximately 40 millimeters with a shank cross-section of the order of 1 
millimeter and a head cross-section (d.sub.3) of the order of 2 
millimeters. As regards the tubular body 5, this will be able to have a 
length of approximately 25 millimeters with a cross-section of the lower 
section 5a of the order of 35 to 40 millimeters and a cross-section of the 
upper section 5b of the order of 25 to 30 millimeters, each section being 
able to have a length of approximately 15 and 8 millimeters, respectively. 
FIG. 2 shows the piece 1 engaged in an orifice 15 made through an 
electrically conducting piece 17, such as a metal plate which is to be 
supplied with electric power via a connection terminal, a role which the 
piece 1 will play once the latter has been riveted to this plate. 
In order to fix the piece 1, the diameter of the orifice 15 will be less 
than the diameter d.sub.4 (see FIG. 1) of the collar 13 which has been 
placed just beneath the plate 17, the orifice 15 of which is only 
penetrated by the upper tubular section 5b. 
The shank 3 has been installed through the body 5 in such a way that its 
head 9 emerges on the side of the upper section 5b. 
Once the piece 1 has been thus arranged, it will suffice, in order to fix 
it, to use a conventional pair of riveting tongs available commercially, 
such as a pair of tongs of the "FACOM" trade mark (registered trade mark), 
model "GESIPA NT X". 
Under the action of the jaws of this pair of tongs which will exert an 
axial traction force on the shank 3 in the direction of the arrow 19 of 
FIG. 2, the head 9 will exert a compressive force on the section 5b which, 
made from a thermoplastically deformable material, will be squashed from 
its free upper end in order to form a flange identified by 5'b in FIG. 3, 
the cross-section of this flange then being greater than the cross-section 
of the orifice 15, thus keeping, by means of this riveting operation, the 
plate 17 between the flange formed and the collar 13. 
During this operation, and as indicated hereinabove, the stud 3 will break 
off at the location of its preformed rupture zone 7, the lower shank part 
being ejected from one side, the remainder of this shank and the head 9 
being ejected from the other. 
In this manner, there now only remains, fixed to the plate, a single 
electrically conducting body 5 whose cylindrical hollow shaft 5a will be 
able to be used as a pin for the connection to an associated socket, such 
as, for example, one of the two sockets 21, 23 illustrated in FIGS. 10 and 
12. 
FIG. 4 and the following figures illustrate the application of the 
connection terminal of the invention to the electrical connection of a 
conducting piece produced in the form of a cloth or of a meshed net. 
Such a net used on an electro-weldable connecting piece is in particular 
described in the publication FR-A-2,654,978 (corresponding U.S. Pat. No. 
5,141,580). For any complementary information concerning this net, it will 
be possible to refer to these publications which are introduced by 
reference in the present description. 
Thus, FIG. 4 represents a net portion having interlaced sections, the net 
having the particular feature of being preferably produced from a single 
electrically conducting wire 21 (i.e. ensuring electrical continuity from 
one end to the other of the network). 
The structure of this wire has been recalled in FIG. 5, from which it is 
possible to show that the said wire can in particular consist of a 
conducting cable 23 (for example made of copper) of a few tenths of a 
millimeter in diameter coated with a layer or with a film 25, for example 
based on polyesterimide, and electrically insulating at least up to a 
predetermined temperature (approximately 250.degree. C.). 
FIG. 4 shows the shank 3 inserted into the internal passage 11 of the body 
5, in such a way that its head 9 emerges from the side of the section 5b 
which penetrates one of the meshes of the net. For good integrity of the 
assembly, two larger-diameter metal washers 27, 29 have been added, one 
above the net and the other beneath it. 
After the riveting has been performed, the net or the corresponding meshed 
fabric will thus be encountered again, as shown in FIG. 6, held tightly 
between the two washers 27, 29 the flange 5'b holding it all tightly 
together, between it and the collar 13. 
If the wire of the meshing used is of the type of that of FIG. 5, it will 
therefore be necessary to provide, on the rivet, rupture means for 
tearing, during the riveting, the skin 25 at the location of the placing 
of the rivet, so as thus to ensure the electrical contact. 
These tearing means could, for example, consist of serrations made on the 
"upper" surface 13a (see FIG. 4) of the collar 13. 
However, in this case, it has been preferred to use as the washer 29 a 
toothed washer which is to ensure the same effect. Thus, these tearing 
means extend around the mesh penetrated by the terminal, that is to say 
substantially perpendicular to the shaft 5, between the mesh in question 
and the collar 13, facing the flange 5'b (or indeed its side). 
Let us now turn our attention to FIGS. 7 to 9, in order to describe briefly 
the use, on an electro-weldable plastic sleeve, of such a meshed net thus 
connected to the connection terminal 5 of FIG. 6, towards each of the 
opposite ends of its wire. 
FIGS. 7 and 8 show the tubular shape, in the form of a cylinder of 
substantially circular cross-section, of the net 31 whose two opposite 
ends of the wire constituting it have been identified by 21a and 21b. 
FIG. 9 illustrates diagrammatically the manner in which the 
".OMEGA."-shaped loops of the wire 21 are mutually interlaced in order to 
constitute the meshes of the net, which net is encountered again arranged 
inside the electro-weldable connecting sleeve 33 in FIG. 10. 
Except for this resistance element in the form of a net 31, which is here 
arranged with its cylinder axis 31a substantially coincident with the axis 
35 of the wide central cylindrical orifice 37 which penetrates right 
through the sleeve, the latter has, locally, on two opposite sides of its 
external perimeter, two cavities or wells 39, 41 which are open to the 
outside and, in this case, are substantially perpendicular to the axis 35. 
Into each of these cavities emerges the free end of one of the two shafts 
5a of a connection terminal 5 in accordance with the invention. Of course, 
each terminal 5 has been riveted beforehand to the resistive net with 
interposition of the washer 27 and of the toothed washer 29. 
Thus arranged, the two shafts 5a will be able to constitute two electrical 
connecting pins, which are connectable, if care has been taken to give 
them a suitable length and diameter, with two conventional connecting 
sockets, such as the sockets 21 and 23, which are themselves each 
connected to their conducting supply cable, respectively 43 and 45. 
FIG. 10 also shows the two thermoweldable plastic pipes 47, 49 intended to 
be joined together via the sleeve 33. In this case, these pipes have been 
arranged coaxially so that their two ends facing each other are inserted 
quite tightly inside the orifice 37 of the sleeve. 
Let us now turn our attention to FIGS. 11 and 12 in order to show another 
application of the invention. 
First of all, it will be possible to notice in FIG. 11 that the shape of 
the conducting net, referenced in this case by 51, is in the form of a 
flattened ring. 
Such a "resistive ring" can especially be of the type of that described in 
French Patent Application FR-A-92/04415 filed by the present Applicant on 
Apr. 10, 1992. In other words, the component 51 can be produced from the 
meshed cylindrical structure of FIGS. 7 and 8 from which a section will 
have been cut, which section will have been squashed on itself and 
flattened so as to constitute a double-thickness circular ring. 
It is this ring 51 having a single resistive wire which is encountered 
again in FIG. 12. 
This figure more generally shows an electroweldable branch connector 53 for 
the transverse connecting of a pipe 55 with another pipe (not shown). 
In this case, the piece 53 whose body is produced from a heat-meltable 
plastic (such as polyethylene), as was the body of the sleeve 33, has a 
part 57 in the form of a saddle with an internal surface 59 forming a 
cylinder part of semicircular cross-section, so as to be able to straddle 
substantially coaxially the pipe 55. 
In the vicinity of this internal surface 59, and embedded in its plastic 
body, the piece 53 encloses the resistive ring 51 which has, of course, a 
curvature corresponding substantially to that of the saddle 57, the 
central orifice 61 of this resistive element 51 surrounding, at some 
distance, the passage 63 of the shaft 65, which stands up substantially 
half way along the branch connector, perpendicular to the axis of the pipe 
55. In practice, and as is known per se, the passage 63 is intended to be 
prolonged by a coaxial orifice 67 enabling the pipe 55 to communicate 
transversely with another pipe which will be installed and correctly 
connected to the projecting shaft 65. 
As before, and substantially in the region of the two ends of its 
constituent wire, the resistive ring 51 is connected to the then-riveted 
connection terminals 5 preferably with their washers 27, 29. 
As in FIG. 10, the two terminals 5 stand up in order to emerge into two 
transverse external wells 69, 71, each being thus connectable to the 
removable sockets 21, 23. 
In practice, the manufacture of the two pieces 33, 53 will be easy to 
implement. In order to do this, it will suffice, after having chosen the 
most suitable resistance element (such as the resistive cylinder 31 for 
the sleeve 33 and the resistive ring 51 for the transverse T-connection 
connector 53), to fix the two connection terminals 5 by riveting, taking 
care to arrange them at the two locations making it possible to ensure a 
current flow such that it runs, from one of the terminals, at least into 
most of the said resistance element, before reaching the second terminal. 
Once this crimping operation has been completed, the resistance element 
will then be placed, with its terminals, in a moulding machine, known per 
se, and onto these terminals will be overmoulded, by injection, the 
plastic body of the connecting piece. During this operation, the wells 39, 
41 or 69, 71 will be produced with, penetrating through them, the shafts 
5a (of cross-section markedly smaller than that of the said wells). 
It will then suffice to allow the piece to cool until it is completely 
solidified.