Patent Description:
Starting from the end of the <NUM>'s processes have been developed for the industrial production of solid wall pipes made of non-plasticised polyvinyl chloride, known as biaxially oriented PVC or PVC-O.

PVC-O pipes are produced by means of a process which allows the orientation of the long molecular chains of PVC obtained from the pipe extrusion process. The orientation in the longitudinal and circumferential directions allows the improvement of the physical properties of the PVC. The orientation is obtained by increasing the temperature to a value greater than the vitreous transition temperature Tg of the PVC (<NUM> - <NUM>), then a large force is applied, both in an axial direction and in a circumferential direction, so as to increase the diameter of the pipe and reduce its wall thickness.

Compared with the production process by extrusion of unplasticized polyvinyl chloride (PVC-U) pipes, the process for the production of PVC-O pipes is much more complex and onerous.

The PVC-O material compared with PVC-U has a high tensile strength, fatigue resistance and impact resistance so, despite the higher production costs compared with PVC-U, the PVC-O pipes, in certain sectors of application, have significant advantages with respect to PVC-U pipes. For example, in the technical sector of pipes for supplying fluids under pressure, relative to the well know PVC-U pipes, the PVC-O pipes are applicable up to operating pressures of <NUM> bar; it should be noted that normally the PVC-U pipes do not exceed operating pressures of <NUM> bar.

Also for systems of conduits with pressures of less than <NUM> bar, at least up to operating pressures of <NUM> bar, the PVC-O pipes, compared with PVC-U pipes, have important advantages.

In fact, under equal operating pressure, the PVC-O pipes have a smaller wall thickness; consequently, they are lighter pipes, characterised by a greater transit section, that is to say, a greater flow rate capacity.

As in PVC-U pipes, in PVC-O pipes the shape of joint between the pipes, which is consolidated and by far the most widespread, is that of the bell integrated with the pipe, that is to say, the enlarged shape of the end of the pipe in which the end of another pipe is inserted to form a conduit. Normally, the bell has in its wide shape a seat in which is housed a gasket made of elastomeric material which guarantees the hermetic seal of the bell joint.

A system for forming the bell is the so-called Rieber system. In the Rieber system the bell is made with the gasket blocked and integral with the wall of the bell, in such a way that it is irremovable and cannot be replaced. The Rieber system comprises forming the bell by means of a metal pad on which the gasket is installed beforehand in a lowered zone.

During shaping of the bell, the gasket remains applied and integral with the wall of the bell, the pad is then extracted from the bell with the final result of a pipe with an integrated bell complete with non-removable gasket.

Unlike PVC-O pipes, in PVC-U pipes the means which render the gasket integral with the wall of the bell is an internal negative pressure action and/or an overpressure action on the outer surface of the bell (for example with pressurised fluid such as, for example, compressed air).

In PVC-O pipes, according to the Rieber system, the action which shapes the bell, and the relative seat of the gasket, is the spontaneous contraction of the molecules oriented on the forming pad and on the gasket. This contraction occurs when the PVC-O material is in a thermal state greater than the vitreous transition temperature Tg of the PVC.

In effect, at temperatures higher than the temperature Tg, the structure of the molecules of PVC releases the large forces which are applied during the process for producing the pipe for achieving the axial and circumferential orientation of the pipe.

The currently known methods for belling PVC-O pipes with the Rieber system, as described and illustrated in the prior art documents <CIT>, <CIT>, <CIT>, <CIT>, <CIT> and <CIT>, have several drawbacks.

A drawback found in the prior art methods is the irreversible collapse of the gasket during insertion of the pipe in the gasket housed on the pad.

The forces induced by the end of the pipe when, during belling, it strikes the gasket, held by a contact flange, results in a considerable deformation of the gasket, which being mainly made of elastomeric material is easily deformable, until adversely affecting the structural condition of the gasket. The pipe having a flat front end, perpendicular to the axis of the pad, impacts the gasket on an edge, with contact surface between the front of the pipe and the gasket decidedly small, generating a contact pressure on the gasket, which is the cause of deformation as well as its subsequent breakage.

Another drawback, due to the high axial compression of the wall of the pipe during the entire step of inserting the pipe in the gasket, is the cancellation or the reduction of the degree of axial orientation of the material in the bell, until, in use, the capacity of resistance to the hydrostatic pressure induced by the fluid circulating in the conduit is adversely affected.

This drawback relates to the resistance generated by the shape of the gasket which protrudes from the cylindrical pad, as well as the friction resistance created by the surface of the elastomeric gasket, which are resistances that are much greater than those induced by the shape and surface of the metal pad. In addition, these resistance effects, caused by the presence of the gasket, are accentuated with the gradual alteration of the shape of the gasket when the wall of the pipe strikes and surmounts the gasket.

In this context, the need has been felt for a method and a machine for belling pipes made of PVC-O, which avoids the deformation of the gasket during belling, as described in the respective independent claims.

The technical characteristics of the invention are clearly described in the claims below and its advantages are apparent from the detailed description which follows, with reference to the accompanying drawings which illustrate a preferred embodiment of the invention provided merely by way of example without restricting the scope of the inventive concept, and in which:.

With reference to <FIG>, the numeral <NUM> denotes a pipe made of thermoplastic material of the PVC-O type to be processed in a machine and by a method according to the invention.

The pipe <NUM> has an axis of symmetry <NUM>.

The pipe <NUM> has a main longitudinal extension in a direction parallel to the axis of symmetry <NUM> from a first end <NUM> to a second end <NUM>.

The first end <NUM> and the second end <NUM> are defined, respectively, by the edge of the pipe <NUM>.

The pipe <NUM> has two end portions E1 and E2, each end portion is positioned at a respective first end <NUM> and second end <NUM>.

The pipe <NUM> has an outer surface <NUM> extending about the axis of symmetry <NUM>.

The pipe <NUM> has an annular cross section.

The pipe <NUM> has an inner chamber <NUM> passing from the first end <NUM> to the second end <NUM>.

The pipe <NUM> has a nominal diameter value "dn" and a nominal wall thickness value "en".

The actual value of the outside diameter at any point of the pipe <NUM> is labelled "de".

The actual value of the wall thickness at any point of the pipe <NUM> is labelled "e".

At the second end <NUM>, the outer surface <NUM> of the pipe <NUM> has a chamfer <NUM> having an inclination relative to a direction parallel to the axis of symmetry <NUM> equal to an acute angle β, in particular the value of the angle is between <NUM>° and <NUM>°, inclusive, more specifically the value of the angle β is equal to <NUM>°.

The chamfer <NUM> is characterised by a longitudinal extension "k", with reference to a direction parallel to the axis <NUM> of symmetry, and by a transversal extension "h" of the first end <NUM>, along a direction at right angles to the direction of the axis <NUM> of symmetry.

The first end <NUM> of the pipe <NUM> is not touched by the chamfer <NUM>. Preferably, the longitudinal extension value "k" of the chamfer <NUM> is correlated with the real minimum external diameter of the pipe <NUM>, called "demin", according to the following relation: k≥<NUM> demin.

Preferably, the transversal extension value "h" of the second end <NUM> is correlated with the real value of wall thickness measured at the second end of the pipe <NUM> according to the following relation: h≥<NUM>.

With reference to <FIG>, the numeral <NUM> denotes a pipe made of thermoplastic material of the PVC-O type, processed in the machine and with the method according to the invention, in the state at the end of the processing step performed in the tapering station SV.

All the features described above with reference to <FIG> remain unchanged in the pipe, with the exception of an inner taper <NUM> made at the first end <NUM> of the pipe <NUM>.

The taper <NUM> has an inclination equal to an acute angle "αi", or initial acute angle, and a longitudinal extension equal to "Li", or initial longitudinal extension.

The value of the angle "αi" of the extension "Li"will be defined below. With reference to <FIG>, the numeral <NUM> denotes a pipe made of thermoplastic material of the PVC-O type according to the invention.

The pipe <NUM> of <FIG> constitutes, to all intents and purposes, the finished pipe, according to the invention.

It should be noted that the ends <NUM> and <NUM> of the pipe <NUM> correspond to the ends <NUM> and <NUM> of the pipe <NUM> during the previous processing steps.

The pipe <NUM> has two end portions E1 and E2, each end portion E1 and E2 is positioned at a respective first end <NUM> and second end <NUM>.

At the second end <NUM>, the outer surface <NUM> of the pipe <NUM> has a chamfer <NUM> having an inclination relative to a direction parallel to the axis of symmetry <NUM> equal to an acute angle β, in particular the value of the angle is normally between <NUM>° and <NUM>°, inclusive, more specifically the value of the angle β is equal to <NUM>°.

The chamfer <NUM> is characterised by a longitudinal extension "k", with reference to a direction parallel to the axis <NUM> of symmetry, and by a transversal extension "h" of the second end <NUM>, along a direction at right angles to the direction of the axis <NUM> of symmetry.

The first end <NUM> of the pipe <NUM> does not touch the chamfer <NUM>. Preferably, the longitudinal extension value "k" of the chamfer <NUM> is correlated with the real minimum external diameter of the pipe <NUM>, called "demin", according to the following relation: k≥<NUM> demin.

The inner chamber <NUM> has a first section <NUM>, having a first diameter, and a second section <NUM> has at least a second diameter greater than the first diameter, and a third section <NUM> for connecting the first section <NUM> to the second section <NUM> having a convergent shape from the second section <NUM> to the first section <NUM>.

The second section <NUM> has a second diameter greater than the outside diameter "de".

The convergent trend of the third section <NUM> is inclined relative to a direction parallel to the axis of symmetry <NUM> defined by an angle of convergence ϕ the value of which is within a range of ±<NUM>° starting from the value of the acute angle of inclination β of the chamfer <NUM> relative to a direction parallel to the axis of symmetry <NUM>, that is to say, ϕ= β±<NUM>°.

The third section <NUM> and the second section <NUM> define a bell <NUM> of the pipe <NUM>.

The first section <NUM> of the inner chamber <NUM> extends from the second end <NUM>.

With reference to a direction parallel to the axis of symmetry <NUM>, the first section <NUM> has a length greater than a length of the second section <NUM>.

The inner chamber <NUM> has a fourth section <NUM>, adjacent to the second section <NUM> and leading towards the outside environment at the first end <NUM> of the pipe <NUM>, having a diverging trend starting from the second diameter of the second section <NUM> towards the outside environment.

The fourth section <NUM> has an inclination relative to a direction parallel to the axis of symmetry <NUM> equal to an acute angle "αf", or final acute angle, and a longitudinal extension equal to 'Lf', or final longitudinal extension. The value of the angle "αf" of the extension "Lf" will be defined below.

In other words, the fourth section <NUM> is in the form of an inner taper or first end <NUM> of the pipe <NUM>.

The pipe <NUM> comprises a gasket <NUM>, positioned in a respective housing seat <NUM>.

The gasket <NUM> has a circumferential extension relative to the axis of symmetry <NUM> of the pipe <NUM>.

The gasket <NUM> has an inner surface <NUM>, facing the inner chamber <NUM> of the pipe <NUM>, and an outer surface <NUM>, facing the housing seat <NUM>, as shown schematically in <FIG>.

The gasket is positioned along the second section <NUM> of the inner chamber <NUM>.

The inner surface <NUM> has a protrusion <NUM> designed to define the hermetic seal of the gasket <NUM>.

The outer surface <NUM> has a first portion <NUM> and a second portion <NUM> arranged contiguous to each other and in such a way as to define a cusp <NUM>, as shown in <FIG>.

The first portion <NUM> is also referred to as the rear shoulder of the gasket <NUM>.

The second portion <NUM> is also referred to as the front shoulder of the gasket <NUM>, as shown in <FIG>.

The first portion <NUM> and the second portion <NUM> are inclined relative to a direction parallel to the axis of symmetry <NUM> of the pipe <NUM> by a respective acute angle, indicated in the drawings with θp an θa, as shown in <FIG>.

The value of the acute angle θp of inclination of the first portion <NUM> of the outer surface <NUM> of the gasket <NUM> relative to a direction parallel to the axis of symmetry <NUM> of the pipe <NUM> is preferably between <NUM>° and <NUM>°, in particular equal to <NUM>°.

The value of the acute angle θa of inclination of the second portion <NUM> of the outer surface <NUM> of the gasket <NUM> relative to a direction parallel to the axis of symmetry <NUM> of the pipe <NUM> is preferably equal to <NUM>° and <NUM>°, in particular equal to <NUM>°.

The gasket <NUM> comprises a reinforcing element <NUM> positioned in the gasket <NUM> at the first portion <NUM>.

The reinforcing element <NUM> has a first end 206a and a second end 206b, shown in <FIG>.

Preferably, the reinforcing element <NUM> is at least partly annular in shape. More specifically, the reinforcing element <NUM> is made of metal or plastic material.

Advantageously, the purpose of the reinforcing element <NUM> is to make the relative gasket in use more resistant to the inner hydrostatic pressure and, consequently, also the bell joint <NUM> of the pipe <NUM>.

The reinforcement also guarantees the integrity of the gasket when it is subjected to the forces produced by the loading of the gasket <NUM> on the pad <NUM> of the belling unit <NUM>, as well as to the forces generated during forming of the bell <NUM> when the wall of the pipe impacts and surmounts the gasket <NUM>.

With reference to <FIG>, the reinforcing element <NUM>, relative to a direction parallel to the axis of symmetry <NUM>, comprises an extension "C1", as explained in more detail below.

The invention also comprises a belling machine <NUM> designed to be installed in a plant for producing pipes <NUM> made of thermoplastic material, as schematically illustrated in <FIG>.

The plant comprises a processing unit configured for forming an outer chamfer <NUM> at a second end portion E2 of the pipe <NUM>.

The plant also comprises a belling machine <NUM> configured for making the finished PVC-O pipes <NUM>, as described above.

In particular, the machine <NUM> comprises a belling unit <NUM> which is able to make a bell <NUM> starting from at least one end portion E1 of a PVC-O pipe <NUM>, in particular starting from the first end <NUM> of the pipe <NUM>.

The machine <NUM> for belling pipes <NUM> made of thermoplastic material of the PVC-O type according to the invention comprises a unit <NUM> for processing the pipe <NUM> for removal of material, or plastic deformation. The processing unit <NUM> is configured for processing a PVC-O pipe <NUM>, making an inner taper <NUM> of the first end <NUM> of the pipe <NUM>.

The processing unit <NUM> designed to make the inner taper <NUM> of the first end <NUM> of the pipe <NUM> is positioned upstream of a unit <NUM> for heating the pipe to a predetermined heating temperature and a belling unit <NUM> (which will be described in more detail below).

Advantageously, the invention consists in making on the pipe <NUM> partly processed, since it is not yet belled, an inner taper <NUM> to the wall of the pipe <NUM>, such that when the first end <NUM> of the pipe <NUM> strikes against the gasket <NUM>, the taper angle adopted by this taper is equal to or close to the taper value of the first portion <NUM> of the gasket <NUM> designed for being inserted in the pipe <NUM>.

Advantageously, the shape of the first end <NUM> of the pipe <NUM> of this type considerably increases the contact surface in the impact of the first end <NUM> of the pipe <NUM> with the gasket <NUM> and consequently reduces the contact pressure and the local deformation of the gasket <NUM>.

By way of example, the processing unit <NUM> is in the form of a planetary machine tool.

As regards the length of the inner taper <NUM>, this must be optimised as a compromise between the guarantee of maintaining the structural and functional integrity of the gasket <NUM> and a wall thickness in the edge of the bell <NUM>, made at the first end portion E1 of the pipe <NUM>, sufficient to not adversely affect the robustness of the edge of the bell.

The robustness of the edge of the bell <NUM> is necessary to prevent damage during the operations for transporting and installing the pipe <NUM>.

With the type of gasket <NUM> considered, the extension of the inner taper <NUM> is sized in such a way that, in the first impact between the first end <NUM> of the pipe <NUM> and the gasket <NUM>, there is contact between the surface of the taper <NUM> and the rear shoulder <NUM> of the gasket <NUM> which faces, at least partly, the reinforcing ring <NUM>.

In general, if the wall thickness of the finished pipe <NUM>, the dimensions and configuration of the gasket <NUM> allow it, it is advantageous to dimension the extension of the conical surface of the inner taper <NUM> of the pipe <NUM> in such a way that, in the first impact between the first end <NUM> of the pipe <NUM> and gasket <NUM>, there is full contact between the inner taper <NUM> of the pipe <NUM> and the first portion <NUM> of the gasket <NUM> up and beyond the part which faces the reinforcing element <NUM>. Advantageously, these criteria for sizing the inner taper <NUM> of the pipe <NUM> established and applicable for some types of gaskets <NUM> equal to those used for making the Rieber bell in PVC-U pipes can become guidelines for the design of new gaskets optimised and specific for the Rieber belling of PVC-O pipes, given that there are wall thicknesses and diameters of the PVC-O pipe on which the bell will be made with the Rieber system.

These design criteria are aimed at defining shape and sizing of the gasket at least with regard to: taper angle of the first portion <NUM>; extension of the first portion <NUM>; shape of the reinforcing element <NUM>; positioning the reinforcing element <NUM> relative to the elastomeric matrix of the gasket <NUM>.

When determining the geometry of the inner taper <NUM> of the edge to be made on the pipe <NUM> still to be heated, it is necessary to take into account that the wall of the PVC-O pipe at temperatures higher than vitreous transition effects is subjected to diametric and axial contraction effects; these are effects which depend firstly on the degree of circumferential orientation of the pipe <NUM>, on the degree of axial orientation of the pipe <NUM> and on the thickness of the wall.

These parameters are known and characteristic of the pipe <NUM> processed.

The contraction effects will then be more or less significant depending on the thermal state of the pipe <NUM> during its processing.

These effects, when the pipe <NUM> is heated for the belling, change the angle and length of the taper of the inner taper <NUM> originally formed on the cold pipe <NUM>, but just a few experimental tests are sufficient to determine the values of the angle and the length of the taper which must be formed on the cold pipe <NUM> to obtain in the heated pipe for the belling the values of optimum angle and length of the taper.

Not only that, but since the orientation and thickness parameters of the pipe as well as the processing temperatures are fixed, between the geometrical characteristics of the initial taper <NUM> established on the cold pipe <NUM> and those modified during shaping of the bell, direct relations are established which can be easily determined by experimentation.

With the apparatus and the process described below in detail, the Rieber bell integrated with the PVC-O pipe <NUM> is formed.

The bell <NUM> of the pipe <NUM> formed is consistent with the functional requirements for: fitting the pipe, hermetic seal and resistance to the hydrostatic pressure of the joint.

With respect to conventional Rieber bells, this Rieber bell formed in the PVC-O pipe has further qualities.

In particular, the inner taper <NUM> of the pipe <NUM> remains shaped in the edge of the bell in the finished product.

The taper <NUM> of the finished pipe <NUM> originates from the inner taper <NUM> of the pipe <NUM> formed before the heating and belling process steps.

The conical surface of the inner taper <NUM> does not exactly maintain the same dimensions as the surface <NUM> formed before the belling process, since, during the belling process, during the various steps of heating and shaping the bell, the surface is modified due to the effects of spontaneous contraction of the PVC-O pipe generated by the heating to temperatures higher than the temperature Tg, as well as on account of the simultaneous different actions on the edge of the pipe <NUM> induced by the belling apparatuses and by the gasket <NUM>.

They are precisely the particular features of the invention both in terms of the belling apparatus and the processing method which maintain in the first end <NUM> of the finished pipe <NUM> a conical surface having a taper angle and length which can be correlated with simple coefficients of proportionality, with those formed on the cold pipe <NUM> before the heating and belling steps.

For this reason, a taper angle of the inner taper <NUM> of the finished pipe <NUM> which remains on the end edge of the bell <NUM> suitably formed relative to the taper angle of the first portion <NUM> of the gasket <NUM> integrated in the bell <NUM> of the pipe <NUM>.

In effect, the process according to the invention and the belling unit conveniently allow the bell to be formed in a hot state of the pipe at relatively low temperatures (<NUM>-<NUM>), such that the effect of the plastic behaviour of the pipe is reduced, that is to say, insufficient, despite the mechanical actions of the apparatus for forming the bell, for altering the tapered shape of the first end <NUM> of the pipe <NUM>.

In other words, during the production of the PVC-O pipe the following product parameters are determined and stable:.

In these necessarily repeatable conditions, if the making of the bell in the hot state in the processed PVC-O pipe occurs at relatively low and controlled temperatures (approx. <NUM>-<NUM>), the taper <NUM> is maintained of the first end <NUM> of the pipe <NUM>, formed on the pipe <NUM> in the cold state, even if modified in terms of the dimensions of angle and extension, but always with constant and repeatable dimensions, as well as directly correlated with the dimensions of the taper formed in the cold state.

It is precisely the machine and the method according to the invention to allow and make advantageous the processing of shaping of the bell at the indicated temperatures. If the process for making the pipe <NUM> and the bell <NUM> is repeatable, the length Lf and the taper angle αf which are established in the first end <NUM> of the bell <NUM> definitively formed and cooled will be repeatable.

On the other hand, if the process for producing the pipe <NUM> and the belling process is not performed correctly or the pipe <NUM> being processed does not possess the established physical and dimensional characteristics, the regularity of the taper of the inner taper <NUM> will also not be performed correctly.

For example, if the bell were made with a thermal state at temperatures greater than <NUM>, the softening of the material would be such that, in the final shape of the bell, the initial tapered shape of the edge of the pipe <NUM>, formed before heating of the pipe, would be lost, and the degree of orientation of the material in the wall of the bell necessary to guarantee the resistance of the pipe <NUM> to pressure.

Therefore, during production of the belled pipe, the monitoring of the regularity of the conical surface on the edge of the bell makes it possible to identify faults in the process for making the bell, and consequently the diagnostics of the production process and the quality control of the product are facilitated.

Not only during the production of the pipe, but also subsequently, for example during the operations for laying the conduit, the inner taper <NUM> of the first end <NUM> of the pipe <NUM> is an evident visual indicator of the process used for forming the bell.

The bell <NUM> with an end edge with a marked inner taper <NUM> is advantageous for performing the setting up and installation of the conduit. In effect, in the bell <NUM> there is a conical inlet which facilitates the insertion of the pipe into the bell of the other pipe to be formed to be joined in the bell joint. In practice, the conical inlet allows jointing operations even when the operating conditions make the alignment between the axes of the pipes to be joined in the bell joint difficult.

<FIG> identifies the geometrical dimensions which characterise the inner taper <NUM> made on the first end <NUM> of the pipe <NUM> before the process for heating the pipe <NUM>.

The term "αi" denotes a taper angle of the inner taper <NUM> of the first end <NUM> of the pipe <NUM> made before the process for heating the pipe <NUM>. The term "Li" denotes a longitudinal extension of the inner taper <NUM> of the first end <NUM> of the pipe <NUM> made before the heating process.

<FIG> identifies the geometrical dimensions which characterise the inner taper <NUM>, formed at the first end <NUM> of the pipe <NUM>, during formation of the bell <NUM> at the moment of first contact between the first end <NUM> of the pipe <NUM> and the first portion <NUM> of the gasket <NUM> installed on the pad <NUM>.

The term "αp" denotes a taper angle of the inner taper <NUM> of the first end <NUM> of the pipe <NUM> at the moment of impact of the first end <NUM> of the pipe <NUM> with the first portion <NUM> of the gasket <NUM> installed on the pad <NUM>. The term "Lp" denotes a longitudinal extension of the inner taper <NUM> of the first end <NUM> of the pipe <NUM> at the moment of impact of the first end <NUM> of the pipe <NUM> with the first portion <NUM> of the gasket <NUM> installed on the pad <NUM>.

<FIG> identifies the geometrical dimensions which characterise the inner taper <NUM> of the first end <NUM> of the pipe <NUM> completely formed and definitively cooled.

The term "αf" denotes a taper angle of the inner taper <NUM> of the first end <NUM> of the finished pipe <NUM>.

The term "Lf" denotes a longitudinal extension of the inner taper <NUM> of the first end <NUM> of the finished pipe <NUM>.

<FIG> identifies the extension and the localisation of the reinforcing element <NUM> at the first portion <NUM> of the gasket <NUM>.

In this drawing, the extension and the localisation of the reinforcing element <NUM> are compared with the geometrical dimensions <NUM> which characterise the inner taper <NUM> formed at the first end <NUM> of the pipe <NUM>, during formation of the bell, at the moment of first contact between the first end <NUM> of the pipe <NUM> and the first portion <NUM> of the gasket <NUM> installed on the pad <NUM>.

With reference to a direction parallel to the axis <NUM> of the pipe <NUM> there is:.

After defining the values of αp and Lp, the corresponding values of αi and Li which generate αp and Lp such as to comply with the conditions (<NUM>) and (<NUM>) are identified by experimental tests. Unique relations are established of the values of αi and Li with the values of αp and Lp. <MAT> <MAT>.

The final bell <NUM> will maintain an inner taper of the end edge defined by values of αf and Lf which are different from αp and Lp, but correlated by unique relations with the values of αp and Lp. <MAT> <MAT>.

The coefficients m, n, r and s are conditioned by the characteristic parameters of the processed PVC-O pipe such as: diameter of pipe (de); pipe wall thickness (e); degree of circumferential orientation of the pipe; degree of axial orientation of the pipe.

They are known and fixed parameters, the variation of which falls within the normal tolerances of the production processes of that predetermined pipe.

With reference to the condition (<NUM>), the optimum situation is that for which: <MAT>.

A correspondence of the following type therefore applies: αf = r θp.

For the various, but determined pipes of industrial interest to which the invention is intended there is: αf = r θp with <NUM> ≥ r ≥ <NUM>.

As regards the extension Lf of the inner taper <NUM> which remains in the end edge of the bell, for the various but determined pipes of industrial interest to which the invention is intended, it is convenient to select a value of Li such that, as well as generating a value Lp which respects the condition (<NUM>), it generates an Lf value which respects the following condition.

Downstream of the processing unit <NUM> there is the unit <NUM> for heating the pipe to a predetermined heating temperature and the belling unit <NUM> (which will be described in more detail below).

The machine <NUM> comprises a unit <NUM> for cooling the pipe, associated with said belling unit <NUM> for cooling the pipe <NUM> fitted on a forming pad <NUM> (shown in <FIG>, described in more detail below and forming part of the belling unit <NUM>).

The forming pad <NUM> is configured to engage with an end portion E1 of the pipe <NUM> starting from the first end <NUM>.

The belling machine <NUM> therefore comprises a plurality of stations, operating in sequence on the pipe <NUM> (in particular on the end portion E1 of the pipe <NUM> starting from the first end <NUM>).

The station ST1 for receiving the pipe is configured for picking up the pipe <NUM> (suitably cut and chamfered in the end edge <NUM>) from the extrusion line.

The station for receiving the pipe therefore comprises, for this purpose, a pick-up unit <NUM>.

Downstream of the station ST1 for receiving the pipe, the machine <NUM> comprises a taper station SV wherein the unit <NUM> for processing the pipe <NUM> is operatively active at the first end <NUM>.

The machine comprises a pre-heating station ST2, wherein the unit <NUM> for pre-heating the pipe <NUM> is operatively active, in which the pipe <NUM> is positioned after the station for tapering the pipe.

In the pre-heating station ST2, the pipe <NUM> is heated preferably to a temperature lower than the vitreous transition temperature (Tg) of the PVC.

The machine also comprises a heating station ST3, wherein the unit <NUM> for heating the pipe <NUM> is operatively active.

The pipe <NUM> is positioned in the heating station after the pre-heating station.

In this heating station ST3, the pipe <NUM> is heated to a temperature higher than the vitreous transition temperature of the PVC, and in any case higher than the heating temperature.

The presence of a pre-heating station ST2 is optional, as the machine <NUM> may also comprise only one heating station ST3.

Optionally, the machine <NUM> comprises a preheating unit <NUM>, positioned upstream of the heating unit <NUM>, and configured for heating the pipe <NUM> to a predetermined preheating temperature, which is lower than the heating temperature.

Preferably, the pre-heating unit <NUM> comprises an oven.

Preferably, the heating unit <NUM> comprises an oven.

The oven heats the end portion E1 of the pipe <NUM> in a differentiated manner along the longitudinal direction of the pipe <NUM>.

The heating station ST3 also comprises an inner contact element, which is positioned inside the pipe to support internally the end portion E1 of the pipe <NUM> during the heating, and prevent the diametric contraction of the pipe.

At the end of the heating step the temperature of the pipe <NUM> is approximately <NUM> towards the end and decreases to <NUM> in the zone which will constitute the connecting wall between pipe and bell.

The invention relates to a unit <NUM> for belling pipes T made of thermoplastic material of the PVC-O type (forming part of the machine <NUM>) which comprises a forming pad <NUM> for deforming into the shape of a bell B an end portion E1 of a pipe <NUM> made of thermoplastic material, said pad <NUM> having a longitudinal central axis X1 of symmetry and a region <NUM> for housing an annular gasket <NUM> designed to be coupled, internally, to the pipe <NUM> made of thermoplastic material, see <FIG>.

It should be noted that the inner taper <NUM> of the pipe <NUM> makes it possible to drastically reduce the contact pressure between the rear shoulder <NUM> of the gasket <NUM> and the first end <NUM> of the pipe <NUM>.

As mentioned in the prior art, the problem of collapse of the gasket <NUM> arises during the step in which the pipe <NUM> is inserted in the gasket <NUM> housed on the pad <NUM>.

Subjected to the forces induced by the wall of the pipe <NUM>, the gasket <NUM> is compressed by the first end <NUM> of the pipe <NUM> against a contact element <NUM> and the pad <NUM>.

In the consequent deformation of the gasket <NUM>, the first portion <NUM> tends to adopt increasingly pronounced taper angles, increasing the contrast to the progressive insertion of the pipe <NUM>.

Advantageously, according to the invention, the forces which stress the gasket <NUM> prevent damage to the gasket <NUM>, in particular in the zone of discontinuity which separates the reinforcing element <NUM> from the elastomer body of the gasket <NUM>, that is to say, the boundary zones between the reinforcing element <NUM> and the elastomer body at the first portion <NUM> of the gasket <NUM>.

An annular contact element <NUM> fitted slidably on said pad <NUM> to move along the direction of said longitudinal central axis X1, between an advanced position P1, and a withdrawn position P2.

The unit <NUM> comprises a first heating device <NUM>, configured for heating said annular contact element <NUM> to a temperature (greater than the vitreous transition temperature) and therefore heating by contact the inner surface of the end portion E1 of the pipe <NUM> fitted on the annular contact element <NUM>.

A second heating device <NUM>, configured for heating from the outside said pipe <NUM> made of thermoplastic material fitted on the forming pad <NUM> in a predetermined zone of the pad <NUM> (more specifically, which extends from the zone proximal to the region <NUM> designed for housing the gasket <NUM> up to the first end <NUM> of the pipe <NUM>).

It should be noted that the assembly made up of the second heating device <NUM> and the first heating device <NUM> makes it possible to define a hot (cylindrical) chamber <NUM>.

With reference to the second heating device <NUM>, it should be noted that the device comprises, preferably, an annular heating element <NUM>.

Said annular heating element <NUM> is preferably made of metallic material, preferably an aluminium alloy.

The second heating device <NUM> may comprise a plurality of heating elements <NUM>, positioned in (around, or embedded in) said annular heating element <NUM>, for generating heat from a plurality of different zones.

According to a variant, the second heating device <NUM> may comprise a single electrical resistance, positioned in (around, or embedded in) said annular heating element <NUM>.

The electrical resistance(s) is/are configured for uniformly heating the annular heating element <NUM>.

The second heating device <NUM> heats, from the outside, preferably the end portion E1 of the pipe <NUM> starting from the first end <NUM> up to the region <NUM> designed for housing the gasket <NUM>.

The annular heating element <NUM> is configured for forming with the annular contact element <NUM>, heated by the first device <NUM>, the hot chamber (cylindrical) <NUM>.

This hot chamber <NUM> is preferably sized to contain internally a portion of the end pipe <NUM> fitted on the annular contact element <NUM>.

The wall of the end portion E1 of the pipe <NUM> is therefore contained in the hot chamber <NUM>, with the relative inner surface in contact with the surface of the annular contact element <NUM> and with the adjacent outer surface detached from the inner surface of the annular heating element <NUM>.

It should be noted that the predetermined distance between the outer surface of the wall of the pipe <NUM> and the inner surface of the annular heating element <NUM> allows the insertion in the hot chamber <NUM> of the wall of the pipe without interference (without contact) with the annular heating element <NUM>.

Experimental tests have led to the conclusion that the optimum distance between the wall of the pipe <NUM> and the inner surface of the annular heating element <NUM> is between <NUM> and <NUM>; more preferably between <NUM> and <NUM>; still more preferably, between <NUM> and <NUM>.

The annular heating element <NUM> is positioned in such a way that the distance between the wall of the pipe <NUM> and the inner surface renders negligible the convective effects generated by the hot air, which would disturb the transmission of heat towards the pipe <NUM>.

The transmission of heat from the annular heating element <NUM> to the pipe <NUM> occurs mainly by irradiation.

Preferably, the second heating device <NUM> heats the end portion E1 of the pipe <NUM> in the absence of contact (the pipe is preferably positioned not in contact with the annular heating element <NUM>).

Preferably, the second heating device <NUM> heats the end zone of the pipe <NUM> by irradiation.

Experimentally and advantageously, it has been found that the fact of heating, from the inside the end portion E1 of the pipe <NUM> fitted on the annular contact element <NUM> by contact with the element <NUM> (heated by the first device <NUM>) and from the outside using the heating element <NUM> heated by the second device <NUM>, during the operation for shaping the bell, more specifically, with a homogeneous heating and at a predetermined temperature, it allows the belling of PVC-O to be considerably improved, allowing the formation of bells in pipes made of PVC-O with larger dimensions (diameter and thickness of walls) than those which can be processed according to conventional techniques.

The unit <NUM> according to the invention is therefore able to process PVC-O pipes with large diameters / wall thicknesses.

According to another aspect, the forming pad <NUM> is equipped with a first annular seat S1 for housing at least a part of said gasket <NUM>.

According to yet another aspect, the annular contact element <NUM> is equipped with a second seat S2 for housing the gasket <NUM>, configured to house at least a part of said gasket <NUM>.

Advantageously, the seats S1 and S2 are shaped for receiving respective portions of the gasket <NUM>, in particular portions of the inner surface <NUM> of the gasket <NUM>.

In particular, the seats S1 and S2 are shaped to allow the maximum adhesion between the gasket and, respectively, the forming pad <NUM> and the annular contact element <NUM>.

It has been found experimentally by the Applicant that the presence of the second seat S2 for housing the gasket <NUM> facilitates the insertion of the pipe <NUM> on the pad <NUM> and on the gasket <NUM>, reducing in a certain way (as far as possible) the risk that the gasket <NUM> can undergo serious damage, as a result of the forces originating by contact with the pipe <NUM>. The second seat S2 limits, in effect, the deformation of the gasket which occurs when the pipe <NUM> is fitted on it, receiving a part inside it. Preferably, the second seat S2 has a concave shape towards the distal part of the pad <NUM> (that is, towards the clamp <NUM>).

The first annular housing seat S1 substantially has the same technical effect described above with regard to the second seat S2.

The first annular housing seat S1 therefore contributes to the technical effect of limiting the deformation of the gasket <NUM> during the belling, receiving a part inside it, thereby reducing the risk of excessive deformation of the gasket <NUM>.

According to yet another aspect, the unit <NUM> comprises a control and operating unit <NUM> (electronic, comprising hardware and/or software).

According to yet another aspect, the belling unit <NUM> comprises a first (temperature) sensor <NUM> configured for measuring the temperature at said first heating device <NUM> (more precisely for measuring the temperature of the annular contact element <NUM>).

According to another aspect, the unit <NUM> comprises a second (temperature) sensor <NUM> configured for measuring the temperature at said second heating device <NUM>.

Preferably, but not necessarily, the first sensor <NUM> is a thermocouple. Preferably, but not necessarily, the second sensor <NUM> is a thermocouple.

The control and operating unit <NUM> is configured for adjusting the second heating device <NUM> as a function of a temperature value measured by the second sensor <NUM>, to perform a heating of a portion of the pipe <NUM> at the region <NUM> designed for housing the gasket <NUM> up to the first end <NUM> of the pipe <NUM> to a predetermined temperature.

The control and operating unit <NUM> is configured for adjusting said first heating device <NUM> as a function of a temperature value measured by the first sensor <NUM>, to perform a heating of the annular contact element <NUM> to a predetermined temperature (higher than the vitreous transition temperature).

According to yet another aspect, the apparatus <NUM> comprises a clamp <NUM> for clamping the pipe.

Said clamp <NUM> is provided with clamping jaws (10A, 10B), movable relative to each other between a closed configuration and an open configuration (in particular, preferably, a first and a second jaw).

The clamping clamp <NUM> constrains the pipe <NUM> in a horizontal position in such a way that the longitudinal axis <NUM> of the pipe <NUM> coincides with the axis X1 of the forming pad <NUM>.

According to another aspect, the forming pad <NUM>, the annular contact element <NUM> are supported by a carriage <NUM>.

More specifically, the forming pad <NUM>, the annular contact element <NUM>, the first heater <NUM> and the second heater <NUM> are supported by the carriage <NUM> (movably relative to the frame of the machine).

It should be noted that the annular contact element <NUM> is configured to be able to move relative to the carriage <NUM>, that is, relative to the forming pad <NUM>.

More specifically, the annular contact element <NUM> is supported by the carriage <NUM> with the possibility of movement independently of it. Preferably, the annular contact element <NUM> and the second heater <NUM> are integral with each other (that is, always moved as one) in the movement with respect to the pad <NUM>.

According to another embodiment, the annular contact element <NUM> can be moved independently of the second heating device <NUM> in the relative motion with respect to the pad <NUM>.

This last embodiment offers the advantage of a greater adaptability to variations in the operating conditions; more specifically, when the annular contact element <NUM> withdraws and detaches from the pipe <NUM> the second heating device <NUM> may remain in the heating position, providing a heating contribution on the part of the pipe which, under spontaneous contraction, forms on the gasket and pad.

As a result, the spontaneous contraction effect of the end wall of the pipe is increased.

The carriage <NUM> is configured to be movable between a position P4 close to the pipe <NUM> and a position P5 away from the pipe <NUM>.

The carriage <NUM> is driven by respective actuator means (not illustrated). With reference to the annular contact element <NUM>, it should be noted that, preferably, the annular contact element <NUM> is a hollow cylindrical body (preferably made of metal).

More specifically, it should be noted that the annular contact element <NUM> is mounted on the carriage <NUM> movably relative to the forming pad <NUM>: in other words, the forming pad <NUM> and the annular contact element <NUM> are configured to be able to move independently.

The annular contact element <NUM> is slidable on the forming pad <NUM>; more precisely, the inner surface of the annular contact element <NUM> slides on the outer surface of the forming pad <NUM>.

With reference to the fitting of the gasket on the forming pad <NUM>, the unit <NUM> may comprise means for picking up and moving the gasket <NUM>, not illustrated since it is of the conventional type.

With reference to the gasket <NUM>, it should be noted that it is positioned, before the pipe <NUM> is fitted on the pad <NUM>, in the region <NUM> for housing the pad <NUM>.

The housing region <NUM> is defined by a lowered zone formed on the outer surface of the pad <NUM>.

With reference to the pad <NUM>, it should be noted that the unit <NUM> comprises a heater configured for heating the forming pad <NUM>.

The pad <NUM> may be heated, according to a non-limiting example, by means of an internal circuit for circulating a heating fluid (for example, water), labelled <NUM>.

Preferably, the pad <NUM> is heated to temperatures of between <NUM> and <NUM> (below the vitreous transition temperature of PVC-O).

Still more preferably, the pad <NUM> is heated to a temperature of between <NUM> and <NUM> (or alternatively to temperatures of between <NUM> and <NUM>).

With reference to the annular contact element <NUM>, it should be noted that it is preferably heated (by the first heating device <NUM>) to a temperature of between <NUM>° and <NUM> (more preferably between <NUM>° and <NUM>, even more preferably between <NUM> and <NUM>).

By way of example, the first heating device <NUM> is defined by electrical resistors <NUM>.

The electrical resistors <NUM> are controlled by the control and operating unit <NUM>.

The invention provides a method for processing pipes <NUM> made of thermoplastic material of the PVC-O type according to the invention.

Advantageously, the shape of the end portion E1 which has at the first end <NUM> of the pipe <NUM> the inner taper, considerably increases the contact surface in the impact of the first end <NUM> of the pipe <NUM> with the gasket <NUM> and consequently reduces the contact pressure and the local deformation of the gasket.

Advantageously, the extension of the taper, which is at least equal to an extension which allows in the impact contact with the reinforcing element <NUM>, favours the contact between the first portion <NUM> of the gasket <NUM> without damaging it.

The belling step comprises preparing a forming pad <NUM> designed to deform into a bell shape an end portion E1 of a pipe <NUM>, starting from a first end <NUM>, made of thermoplastic material; preparing an annular contact element <NUM>, fitted on said forming pad <NUM>, and movable along said forming pad <NUM> between an advanced position P1, and a withdrawn position P2; preparing an annular gasket <NUM> on said forming pad <NUM> in a predetermined region <NUM>, said annular gasket <NUM> being designed to be stably located in the bell B to be formed (integrally with the finished pipe <NUM>), positioning said annular contact element <NUM> in the advanced position P1 to make contact with the gasket <NUM> set up in the region <NUM>, preparing a pipe <NUM> having an end portion E1 of the pipe <NUM> heated to a predetermined temperature (greater than the vitreous transition temperature Tg) designed to allow the deformation; moving, up to a predetermined distance, relative to each other, said end portion E1 of the pipe <NUM> and said pad <NUM>, towards each other in the direction of an axis of symmetry <NUM> of the pipe <NUM>, for fitting said end portion E1 on the assembly of said end pad <NUM> and said gasket <NUM> and said annular element <NUM>.

The method also comprises the following steps: heating the annular contact element <NUM> to a predetermined temperature (greater than the vitreous transition temperature) in such a way as to heat by contact the inner surface of the portion of the end pipe <NUM> fitted on the annular contact element <NUM> and, simultaneously, heating from the outside said pipe <NUM> made of thermoplastic material fitted on the forming pad <NUM>, in a predetermined zone of the pad <NUM> which extends from the area proximal to the region <NUM> for housing said gasket <NUM> up to the first end <NUM> of the pipe <NUM>.

Moving said annular contact element <NUM> from the advanced position P1 to the withdrawn position P2, to release (not make contact with) the annular contact element <NUM> from the gasket <NUM> and from the pipe <NUM>.

According to yet another aspect, the annular contact element <NUM> is provided with a second seat S2 for housing a portion of the gasket <NUM> and the step of positioning said annular contact element <NUM> in the advanced position P1 comprises a step of receiving at least a portion of the gasket <NUM> inside the second housing seat S2.

According to yet another aspect, the forming pad <NUM> is provided with a first annular seat S1 for housing the gasket <NUM>, facing radially towards the outside of the pad <NUM>, and the step of fitting said end portion E1 of the pipe <NUM> on the assembly of said pad <NUM> and said annular gasket <NUM> comprises a step of receiving at least a portion of the gasket <NUM> inside the first seat S1 for housing the forming pad <NUM>.

According to another aspect, the step of preparing an annular gasket <NUM> on said forming pad <NUM> in a predetermined position comprises a step of fitting said gasket <NUM> on the forming pad <NUM> and moving the annular contact element <NUM> from the withdrawn position P2 to the advanced position P1 to make contact with the gasket <NUM> and move it to the predetermined position of said forming pad <NUM>.

According to yet another aspect, before the step of moving, up to a predetermined distance, relative to each other said end portion E1 of the pipe <NUM> and the pad <NUM>, the method comprises a step of clamping the pipe by closing a clamp <NUM>.

According to another aspect, the method comprises a step of releasing the pipe <NUM> by opening the clamp <NUM> for a predetermined time, after the steps of: moving, up to a predetermined distance, relative to each other said end portion E1 of the pipe <NUM> and said forming pad <NUM>, towards each other in the direction of a longitudinal central axis <NUM> of the pipe <NUM>; positioning said annular contact element <NUM> in the advanced position P1, for making contact with the pipe <NUM> and the gasket <NUM> of the pipe; heating to a predetermined temperature, from the inside and, simultaneously, heating to a predetermined temperature, from the outside, the end portion E1 of a pipe <NUM> made of thermoplastic material in the zone which extends from the housing region <NUM> in which said gasket <NUM> is positioned up to the first end <NUM> of the pipe <NUM>, for a predetermined time.

According to yet another aspect, the step of releasing the pipe by opening the clamp <NUM> is carried out before, or partly superposing on, a step of moving said annular contact element <NUM> from the advanced position P1 to the withdrawn position P2, so as to release it from the gasket <NUM> and from the pipe <NUM>.

According to another aspect, the method comprises, after the step of releasing the pipe <NUM> by opening the clamp <NUM> for a predetermined time, a step of further clamping the pipe <NUM> by closing the clamp <NUM>. According to yet another aspect, the method comprises, after the step of moving said annular contact element <NUM> from the advanced position P1 to the withdrawn position P2, to release from the gasket <NUM> of the pipe <NUM>, a step of moving the annular contact element <NUM> from the withdrawn position P2 of disengagement towards the advanced position P1 of engagement, up to an intermediate position P3 of engagement of the annular contact element <NUM> with an end part <NUM> of the pipe <NUM> between the gasket <NUM> and the end edge <NUM> of the pipe <NUM>.

Advantageously, this allows a bell B to be obtained wherein the gasket <NUM> has a desired final internal diameter, since the end of the pipe <NUM> is stressed by the annular element <NUM>, when it is still in a malleable phase (not completely cooled), allowing the pipe <NUM> and seat of the gasket <NUM> to be modelled so that, once the finished pipe <NUM> has cooled, the gasket <NUM> has the correct final diameter and can correctly receive the coupling of another pipe <NUM>.

In the process for forming the bell, when the wall of the pipe <NUM> has formed completely on the gasket <NUM> and on the pad <NUM> and the step of cooling and final stabilising of the bell has still not been activated, the response to the mechanical stress of the wall of the bell, even though mainly elastic, is, in general, elasto-plastic.

In particular, the plastic behaviour is accentuated in the zone of the bell adjacent to the front shoulder of the gasket <NUM> up to the end edge of the bell.

In effect, this zone has been subjected to the transmission of the heat of the hot chamber and, thanks also to this heating, has maintained a residual plasticity.

If during this step of the process for forming the bell an advancing movement of the annular contact element <NUM> towards the edge of the bell of the pipe <NUM> is activated, moving the annular contact element <NUM> to a predetermined height at which the edge of the bell is pressed, a beneficial release of the radial pressure action of the wall of the bell is applied on the gasket <NUM>.

During this step, the gasket <NUM> is compressed and flattened by the wall of the bell towards the pad <NUM>.

The gasket <NUM> is elastic; so, even if the inner wall of the bell in the zone of the gasket expands, the gasket <NUM> always maintains the adhesion to the inner wall of the bell, that is to say, it recovers elastically part of the previous flattening. Simultaneously, the progressive cooling of the wall of the bell of the pipe <NUM> continues, so that at the end of the contact action with the edge of the pipe <NUM> of the annular contact element <NUM>, that is, when the annular contact element <NUM> is moved from the intermediate position P3 to the withdrawn position P2, the elastic relaxation of the gasket <NUM> is partly preserved, since the mechanical response of the part of the bell to the stress induced by the annular contact element <NUM> is not completely elastic, but elasto-plastic.

This partial relaxation of the gasket <NUM> is sufficient to ensure that, after extracting the pad <NUM> from the definitively cooled bell, the internal diameter of the pipe <NUM> at the gasket is the one desired (necessary for the correct functionality of inserting a further pipe in the bell with a guarantee of the seal).

<FIG> are briefly described below which illustrate, in detail, the method according to the invention.

<FIG> show the steps of the belling cycle.

<FIG> illustrates the step of positioning the pipe <NUM> inside the clamp <NUM>.

As is evident, the pipe <NUM> is positioned with its axis <NUM> aligned with (coinciding with) the axis X1 of the forming pad <NUM> (coinciding with the axis of the contact element <NUM>).

<FIG> illustrates the step of closing the clamp <NUM>. The pipe <NUM> is locked between the jaws (10A, 10B) of the clamp <NUM>.

<FIG> illustrate the advancing of the carriage <NUM> towards the pipe <NUM>, in different advancing positions, respectively. During these steps, the pipe <NUM> is progressively inserted on the forming pad <NUM> (the pipe is fitted on the forming pad <NUM>, inserted in the gasket <NUM> and lastly in the annular contact element <NUM>, that is to say, inserted in the hot chamber <NUM>).

During these steps, the carriage <NUM> is moved from the far position P5 to the close position P4.

<FIG> illustrates a step wherein the carriage <NUM> is in the close position P4 and the annular contact element <NUM> in the advanced position P1.

During this step, the clamp <NUM> is in the open configuration, that is, the respective jaws (10A, 10B) are open.

The drawing shows the start of the step of extending the bell being formed, wherein the jaws 10A, 10B of the clamp <NUM> are kept open for a predetermined time.

<FIG> illustrates the continuation of the step of extending the bell.

During this step, the clamp <NUM> is in the open configuration, the carriage <NUM> in the close position P4 and the annular contact element <NUM> starts the movement from the advanced position P1 towards the withdrawn position P2.

Starting from the step of inserting the pipe <NUM> in the gasket <NUM>, until the end of the extension step, the first heating device <NUM> is activated, for heating the inside of the pipe and, simultaneously, the second heating device <NUM> is activated, for heating from the outside the pipe <NUM> made of thermoplastic material. Both the heating devices (<NUM>, <NUM>) contribute to heating the end portion E1 of the pipe <NUM> which extends from the seat <NUM> of the gasket <NUM> to the first end <NUM> of the pipe <NUM>.

<FIG> illustrates the end of the step of extending the bell.

During this step, the clamp <NUM> is in the closed configuration.

<FIG> illustrates the step wherein the annular contact element <NUM> is moved towards the pipe <NUM> (towards the advanced position P1), that is to say, it is moved to an intermediate position P3 (between the advanced position P1 and the withdrawn position P2) in which it engages with the edge of the bell of the pipe <NUM>.

In this way, the annular contact element <NUM> strikes and compresses the edge of the bell.

The annular contact element <NUM> is kept in this intermediate position for engaging with the edge of the pipe <NUM> for a predetermined time, to allow an elastic relaxation of the gasket <NUM> and the fitting of the edge of the pipe <NUM>.

This step defines an elastic relaxation of the gasket <NUM> and the fitting of the edge of the pipe <NUM>.

<FIG> illustrates the end of elastic relaxation of the gasket <NUM> and the fitting of the edge of the pipe <NUM>.

The annular contact element <NUM> is moved away from the pipe <NUM>, towards the withdrawn position P2.

<FIG> illustrates the annular contact element <NUM> in the withdrawn position P2 of disengagement (it does not engage in any way the gasket <NUM> and/or the pipe <NUM>).

The bell is still hot and, during this step the cooling (using the cooling unit <NUM>) of the first end E1 of the pipe <NUM> starts.

<FIG> illustrates the bell <NUM> of the pipe <NUM> made, with the clamps <NUM> still closed. During this step, the carriage <NUM> is moved towards the far position P5, for disengaging the pad <NUM> from the pipe <NUM>.

<FIG> illustrates the clamp <NUM> in the open configuration. During this step, the pipe <NUM> (with the bell <NUM> made and the gasket <NUM> inserted inside the pipe) is extracted from the unit <NUM>.

<FIG> illustrate, with greater precision in detail (in terms of time), what occurs during the process for forming the bell of the pipe <NUM> according to the invention.

In <FIG> the carriage <NUM> is moved towards the pipe <NUM>; in <FIG> the annular element <NUM> is moved towards the withdrawn position P2.

As shown in <FIG>, the first impact of the end edge of the pipe against the shoulder of the gasket occurs with the surface of the end edge of the flared pipe according to a taper angle αp comparable to the angle θp of inclination of the shoulder <NUM> of the gasket <NUM>.

In <FIG> the annular element <NUM> is moved towards the close position P1, more specifically until reaching a predetermined intermediate contact position P3 with the edge of the pipe <NUM> (prior to being moved again towards the withdrawn position P2.

It should be noted that, according to the method described and the unit <NUM> described above, the gasket <NUM> is locked permanently inside the pipe <NUM> (for the shape coupling and for the bond between the materials of the pipe <NUM> and the gasket <NUM> which are established during the processing). With the machine and the method described, and according to the accompanying claims, in particular with the prior embodiment, before the heating process, of the inner taper <NUM> of the first end <NUM> of the pipe <NUM>, an evident reduction in the axial compression stresses is obtained in the wall of the first end portion E1 designed to define the bell <NUM> of the finished pipe <NUM>.

Internal actions which are generated during the entire step of inserting the pipe <NUM> in the gasket <NUM> which cancel out or reduce considerably in the first end portion E1 designed to define the bell <NUM> of the finished pipe <NUM> the degree of original axial orientation of the PVC-O pipe.

The presence of these stresses is highlighted and correlates with the increase in thickness of wall in the bell <NUM> of the finished pipe <NUM> relative to the thickness of the wall of the pipe <NUM> not formed in the form of a bell.

Indeed, this thickening is basically a permanent deformation generated by axial compression loads.

In fact, according to the invention, during the entire step of inserting the pipe <NUM> in the gasket <NUM> the maintaining of the shape of the gasket <NUM> installed in the pad <NUM> is substantially obtained, so the resistance which axially contrasts the wall of the pipe which generates its thickening is not accentuated and, consequently, the capacity of resistance to the hydrostatic pressure of the bell <NUM> is not adversely affected.

Indirectly, the belling unit <NUM> according to the invention, which is already advantageous for achieving the correct shape of the final part of the bell <NUM>, that is to say, the locking of the gasket <NUM> in the wall of the bell <NUM>, also contributes to limiting the phenomenon of generation of axial compression stresses which adversely affect the degree of orientation of the material in the bell <NUM>.

In effect, the unit <NUM> designed for shaping perfectly the final part of the bell <NUM> on the gasket <NUM> makes it possible to limit the state of heating of the pipe necessary to start and complete the process for shaping the bell. The colder pipe is elastically more rigid and less subject to undergo permanent plastic deformations of axial compression during insertion of the pipe <NUM> in the pad <NUM>, in the gasket <NUM> and in the flange <NUM>. Advantageously, the belling unit <NUM> and the method described, and according to the appended claims, allow a bell to be obtained in a PVC-O pipe, according to the RIEBER method, even with considerable thicknesses of walls.

The belling unit <NUM> and the method are extremely efficient, and allow a bell to be made in a PVC-O pipe of very high quality (both as regards the dimensional and constructional features of the bell, and with regard to the coupling between the gasket and the pipe and the relative gasket during coupling of the pipe with other pipes).

Claim 1:
A method comprising the following steps:
- a step of feeding pipes (<NUM>) made of thermoplastic material of the PVC-O type into a station (ST1) for receiving the pipe (<NUM>);
- a step of heating a first end portion (E1) of the pipe (<NUM>), in a heating station (ST3);
- a step of belling the first end portion (E1) of the pipe (<NUM>), after the step of heating the first end portion (E1) of the pipe (<NUM>);
the method being characterised in that it also comprises:
- a step of making an inner taper (<NUM>) at a first end portion (E1) of the pipe (<NUM>), in a tapering station (SV), said inner taper (<NUM>) being performed before the heating step.