This invention relates to polyethylene pipe with integrated elec¬ tric resistance thermowelding system. More particularly the invention relates to polyethylene socket-ended pipe, equipped with a cylindrical winding of metallic wire, firmly lodged within the socket, both ter¬ minals of which can be reached from the outside, so that two pipes can be permanently joined by thermowelding through insertion of unexpanded end into said socket and connection of wiring to electric power supply.

It is known that coupling among different pipe sections, as well as with pipe fittings, can be performed through various systems such as welding, flange coupling, threaded joints, etc., depending, among others, from the type of material making up the piping.

For plastic pipes common use is made of bell connections with seal¬ ing ring, of couplings using some suitable adhesive, as well as of welding or heat-fused joints,each one being better suited for specific types of material.

In case of polyethylene, being given its low melting point,as well as its features of fluidity and amalgability, junction by thermowelding is specially advantageous. Both edges to be joined are heated up to a tem¬ perature sufficient for causing some limited melting, then they are coupled, so that material, upon solidification, creates a continuity between the two pieces.

On the other side, since such polymer becomes very easily deformed, there is no convenience for bell type joints with gasket, much used, for instance, with polypropylene and PVC pipe. In case of polyethylene unavoid¬ able ovalizations can easily cause leakage problems.

One system long used for connecting polyethylene pipe sections is

that of butt welding, in which two adjacent ends are approached and then butt welded through the interposition of a heating plate (called: "mir¬ ror") which causes the melting of material on edges of both pipe sections; afterwards the plate is removed and the two ends are joined. However, this system is difficult in actual operation and requires the intervention of skilled labor: as a matter of fact, the pipe edges should be brought to perfectly match in order not to give origin to any joint discontinuance, what quit often results to be practically impossible owing to ovaliza¬ tions along different directions shown by the two facing pipe ends.

Sleeve coupling systems with electrical resistance have been late¬ ly adopted, which doubtlessly offer a better guarantee for connection reliability and call for less assembly labor; they include a polyethylene sleeve whose inside diameter is about equal to the outside diameter of main pipe, which sleeve is fitted on the two pipe ends to be joined, and then thermowelding is performed through a wire coil covering its inside surface: when electric power is applied to said coil, heat produced by Joule effect causes the melting of the external surfaces of both the pipe ends as well as of the sleeve's internal surface. The two fused surfaces are mutually welded, thus forming one whole block.

Such system is however quite costly, as it requires the use of an additional element, whose cost i s not a marginal one. Moreover, such sys¬ tem also calls for considerable attention and time for its implementation.

In order to minimize the number of joints, and therefore the dif¬ ficulties connected to butt weldings or to the use of sleeves, and in order to meet the obvious need of polyethylene pipe for rather long pip¬ ings, it is therefore required to wing the pipe exiting from the extruder in long coils (up to 100 m length), which call for the use of suitable apparatus, as well as for added labor, and are most unsuitable as far as transportation is concerned.

It is therefore an object of this invention to provide a kind of

joint for polyethylene pipe offering a strong reliability even under high pressures and severe operating conditions, but at the same time be¬ ing economic and simple in its implementation, and allowing, advantage¬ ously, the adoption of pipe bars instead of spools.

To this end it is suggested, according to the invention, to directly apply on the pipe the electric thermowelding junction system, without using any additional element; this is made possible by associ¬ ation of the basic principle of bell joint with that of joint with sleeve and electric resistance.

The invention therefore provides, at one end of each pipe bar, a bell type expanded portion, inside which a wire winding is lodged, whose terminals can be reached from outside, so that a simple and quick coupling by thermowelding can be performed through inserting the plain end of one pipe bar in the above-described expanded end of the next pipe bar.

In accordance with the present invention a polyethylene pipe with integrated electric resistance thermowelding system, including a sub¬ stantially cylindrical expanded portion at one of its ends and a resistor formed by a wire coil, showing both terminal at the same end, the axial length of which does not exceed that of said expanded portion, and whose outside diameter practically coincides with the inside one of said ex¬ panded portion, and lodged inside same in such a way that both terminals can be reached from outside, the inside diameter of said resistor being equal, but for a small clearance, to pipe's outside diameter, so that the two ends of a pipe section be complementary.

Pipe bar according to the invention can be connected one to an¬ other by inserting the unexpanded end of one into the complementary end of the other and by supplying electric power to the interposed resistor, in the amount required for heat-fusing the plastic material; it is clear that such a type of coupling is considerably more economic and easier to

implement than the conventional ones.

Pipe bars according to the invention can moreover be connected, by exploiting the proposed coupling system, to any fitting or piping com¬ ponent equipped with complementary ends.

The terminal expanded portion can be manufactured separately and then fixedly connected to a plain pipe bar, but much more advantageously it can be formed by one socket integral with the pipe itself, obtained by working one of its ends.

Axial length of the socket and of the resistor housed therein can vary according to pipe's NP, that is nominal pressure for which the pipe has been designed.

According to a preferred embodiment of the invention, the resistor is made up by one compact helical coil of wire covered by a thin poly¬ ethylene insulating layer; this makes the handling of resistor easier dur¬ ing the manufacture of the joint and, besides, while assembling the pipe the polyethylene covering melts thus adding continuity to the joint.

Two lugs can be joined to the resistance terminals at the edge of the socket, in order to ensure quick and steady connection to elec¬ tric power supply. Lugs are preferably housed In two hollows provided on the bell's Inside surface, adjacent to its edge. Alternatively a circular groove can be provided internally on the bell's edge, supplying the space required for lodging both lugs.

Said lugs are oriented parallel to pipe axis, but it is also pos¬ sible to fasten them to the socket edge, so that orientation of their ends Is perpendicular to said axis.

Another solution can be that of providing, instead of lugs, a couple of plugs passing through the thickness of bell wall and protruding outside, suitable for connection to female terminals.

An alternative solution, falling within the basic concept of this

invention, provides for placing the electric winding outside the pipe's plain end, instead of inserting it within the socket end.

Therefore, in accordance with another embodiment of the invention, a polyethylene pipe with integrated electric resistance thermowelding system is provided, including a substantially cylindrical expanded por¬ tion at one of its ends and a resistor formed by a wire coil, showing both terminals at the same end, the axial length of which does not ex¬ ceed that of said expanded portion, and whose inside diameter practically coincides with the outside one of said pipe, and being assembled around the plain end of the pipe, in such a way that said terminals be turned opposite the pipe edge, being the outside diameter of said resistor equal, but for a small clearance, to the inside diameter, of said expanded por¬ tion, so that the two ends of a pipe section be complementary.

It clearly appears that also in this case the coupling between two pipes, or between a pipe and a fitting, is performed in the same way.

This invention will now be described, for illustrative but not limitative purposes, with specific reference to a preferred embodiment, shown in the accompanying drawing.

In the figure, that shows a partially cutaway, axono etric view of the pipe joint according to the invention, there is generically shown by 1 the end of a first pipe section, and by 2 the complementary one of a second, analogous section.

End 2 is a plain end of unworked pipe, while end 1 is provided with bell 3 and resistor 4, made up of a helical coil of wire. The re¬ sistor 4 looks like a compact block, since the wire is coated with a thin polyethylene layer making up its insulation, which makes it easier, as already said, the thermowelding of the two parts.

By 5 there is pointed out one of the twin hollows for housing each of the lugs (not shown).

Upon assembling ends 1 and 2, as shown in figure, and supplying suitable electric power to resistor 4 through lugs, the connection of the two pipe sections is quickly obtained by thermowelding, without any possibility of mistake.

Pipe shown is produced by a suitable bell-forming operation, which is performed directly after extrusion, through on-line process. Based on the times necessary for the bell-forming operation, which comprises an oven heating phase and an expanding phase, provision should be made for one or more parallel lines, connected to the extrusion one.

After expanding, the cylindrical resistor is inserted into the bell- shaped end, and then blocked in its housing by material's shrinkage, due to cooling. As a last phase, the pipe undergoes some quick cooling, for instance in a tank; said cooling blocks the socket in its shape, thus avoiding that it lose the shape obtained by the expansion, owing to poly¬ ethylene's strong thermic memory.

In such a way a latent radial shrinkage originates in the bell; therefore when assembling the pipes, upon heating, a pressure is devel¬ oped, due to "reshrinkage" of bell, that contributes to guarantee a per¬ fect seal to the joint.

This invention has been described with specific reference to some of its specific embodiments, but it is understood that several changes and modifications can be made by the skilled in the art, without departing from its general scope.