Method and apparatus for forming stable and internally regular bell mouth shapes on the terminal junction section of externally contoured pipes made of thermoplastic material

Abstract: Method and apparatus for forming stable and internally regular bell mouths on the terminal junction section (2) of pipes (20) made of thermoplastic material, preventively heated to the plastic state, comprising the following successive phases and tools: a first mandrel (11) structured with fixed geometry and with smooth external surface, able to be progressively introduced into the terminal section (2) to produce a forced dilation until the pre-forming of a first temporary bell mouth (21'), of maximum dilation, whose inner diameter exceeds the final nominal inner diameter of the definitive bell mouth (21), after heating the terminal section (2) in differentiated form, by means of a furnace (44) which brings the terminal section (2) to an external temperature (TE) exceeding the inner temperature (Tl), sufficient to assure a correct plastic deformation, in spite of the reduced thickness of the pipe (20) and the action of the mandrel (11); a mandrel for the calibration of the definitive bell mouth, able freely to be introduced preventively into the temporary bell mouth (21'), with external diameter equal to the final nominal inner diameter of the definitive bell mouth (21) thereby calibrating it; cooling the definitive bell mouth.

Description: The present invention relates to a method and apparatus for forming stable and internally regular bell mouth shapes on the terminal junction section of externally contoured pipes made of thermoplastic material, of the type operating on a terminal junction section, previously heated to the plastic state and comprising the following successive phases and tools: a device able to produce a forced dilation of the terminal section, operated internally thereto, until pre-forming a first temporary bell mouth shape, of maximum dilation, whose temporary inner diameter exceeds the final nominal inner diameter of the definitive bell mouth shape; a mandrel for calibrating the definitive bell mouth shape, able to be freely introduced preventively into said first temporary bell mouth shape, structured with a fixed geometry and presenting an outer diameter equal to the final inner nominal diameter of the definitive bell mouth shape so as to calibrate it; cooling the definitive bell mouth shape by cooling means operating in correspondence with the calibration mandrel, thereby rapidly bringing the definitive bell mouth shape to ambient temperature. In this technological sector there are difficulties to overcome, linked to the necessary precision that the bell mouth shapes must provide at least internally, in order to obtain an effective junction between multiple sections of pipe, as well as to the problem of the shrinking by the thermoplastic material, after the definitive formation of the bell mouth shape. It is well known that the molecules of the thermoplastic materials that are subjected to dimensional change, remember the previous physical state and tend to return to the original state. Hence when bell mouth shapes are effected, which increase the diameter of the pipe, extruded when hot, the pipe would tend to return to the previous state, i.e. with a smaller diameter than the one formed with the bell mouth shaping operation. Moreover, the aforesaid shrinking varies according to the thermoplastic material in use (polypropylene, PVC, polyethylene, polyolefins in general). To solve the problem of the precision of the internal measurements of the bell mouth shaping, techniques have for instance been adopted whereby the bell mouths are shaped with mechanical mandrels conformed according to the bell mouth shape to be obtained. They are forcedly introduced into the terminal section of pipe to be given a bell mouth shape, previously heated to its plastic deformation temperature, provided the pipe to be treated has sufficient thicknesses so that it will not collapse during the introduction of the mandrel. In the case of bell mouth shapes that comprise internal annular gaskets, for better sealing, radially expandable and subsequently retractable mechanical sectors are provided to allow the formation, under heat, of the gasket seat in the bell mouth. In regard to the aforesaid technique, reference is made to European Patent EP. N.052.581, of 14 October 1981, in the name of the same Applicant, which provides a good description of the wedge shaped actuation means (with inclined planes for constraining each mechanical sector) and the conformation of the mechanical sectors themselves, so shaped as to constitute, when expanded, a homogeneous bearing surface for shaping, from within, the annular seat for the bell mouth gasket. In document DE-3613579 an expandable mandrel is used The expandable portion of the mandrel has at least the same axial length as the bell mouth to be shaped In its contracted position the mandrel has a diameter lower than (or, at most, equal to) the internal diameter of the terminal junction of the pipe which has to be shaped in a bell mouth, so as to be freely introduced in the pipe itself prior to the expansion and after heating the terminal junction at the plastic state. Then, the bell mouth is produced by expanding the mandrel and enlarging its diameter up to the nominal final inner diameter of the bell mouth to be obtained. In document FR-2022958 a mandrel, for forming a bell mouth at one end of a pipe, has a fixed diameter and is rotated around its longitudinal axis during the forced insertion in a pipe having circular or helicoidal external shapes, to reduce stresses and to limit the risk of making the pipe bend around or collapse along its axis during belling. Techniques have also been studied that provide for exerting inward pressure from the exterior, on the pre-heated terminal section of the pipe, countered by an internal mandrel, but these techniques are not applicable to the case at hand, which aims at applying a precise bell mouth shape without material shrinking, also for corrugated pipes, which clearly cannot be treated under heat, with pressure by fluid or mechanical means from the exterior and which notoriously have rather low thicknesses, in favour of the corrugations and/or external fins or contours. In order to solve the problem of the shrinking by the thermoplastic material, in 1983 the Applicant perfected an apparatus that provides a shaping chamber defined internally by a mandrel that presents the final shape and nominal measurements of the bell mouth to be obtained, and externally by moulds whereon the terminal section of pipe to be treated is preliminarily made to adhere, by means of pressurised air coming from the interior of the mandrel. The terminal section is thus expanded beyond its required nominal size, keeping it for a certain time in contact against the aforesaid external moulds, then letting the bell mouth so formed retract spontaneously and calibrate itself with precision on the underlying mandrel, whereon conduits for the outflow of the pressurised air are provided. It has been observed that in such cases the shrinking by the thermoplastic material was absent or in any case highly reduced after the complete formation of the bell mouth shape and an adequate precision was obtained, since an internal calibration of the bell mouth shape was always present. Unfortunately, this technique too is clearly not applicable to pipes made of thermoplastic material that are corrugated or externally provided with fins and/or contouring. Therefore an apparatus has been realised by the same Applicant, as per EP 096.3833 (European patent application no. 98830360.8, of 11 June 1998) and as per the foreword to the description. In this patent documentation, the device able to produce the forced dilation is a mandrel with mechanical sectors, radially movable from a first retracted position, wherein it can freely be introduced into the terminal section, to a second expanded position, wherein it dilates the terminal section until forming the temporary bell mouth shape with greater than nominal diameter. This apparatus, although it is valuable to realise bell mouths on corrugated or contoured pipes, can nonetheless be further improved. It has been noted that this technique causes, on the inner surface of the bell mouth, some impressions due to contact with the lateral extremities of the radially moving sectors, during their passage from the retracted position of minimum size to that of maximum expansion or maximum size. Therefore the aim of the invention, as it is characterised in the enclosed claims, is to eliminate the aforementioned drawback, providing additional advantages and retaining those already present. The invention retains the advantage of being applicable to any type of thermoplastic pipe available on the market, including corrugated ones and those with external contouring (for instance, of the kind called ultrarib), notoriously with reduced thickness, realising stable bell mouths, in particular if made of polypropylene and the like (polyolefins). It further provides the advantage of allowing very precise bell mouth shapes, with perfectly regular inner surfaces and without any kind of impression, even when the dilation of the terminal section beyond the nominal size of the bell mouth to be obtained is effected with mandrels of the kind with radially movable mechanical sectors. Not the least advantage is also to provide a considerable rapidity of execution, a phase having been provided whereby the calibration operation is accelerated, by means of air pressure from the exterior, which can also be used effectively for corrugated pipes, which as is well known have never been subjected to said external pressures, due to the danger that they may implode. Further features and advantages shall be made more readily apparent from the content of the detailed description that follows, made with reference to the accompanying drawings, which represent an embodiment provided purely by way of non limiting example, wherein: Figure 1 shows a schematic embodiment of the furnace useful to conduct the first necessary phase of heating the terminal junction section of the pipe whereon the bell mouth operation is to be performed; Figure 2 shows a schematic embodiment of the apparatus able to execute the phase aforesaid whereby the terminal section is dilated; Figure 3 shows a schematic embodiment of the apparatus able to execute the aforesaid calibration phase of the terminal section; Figures 4 and 5 schematically show an advantageous alternative embodiment of the apparatus and of the related method for the dilation and calibration of the terminal section; Figure 6 shows a section of a corrugated pipe according to the invention. With reference to the aforementioned figures, the subject method is of the type operating on a terminal junction section (2) and comprising the following successive phases: forced dilation of the terminal section (2), operated internally thereto, until pre-forming a first temporary bell mouth (21'), with maximum dilation, whose temporary inner diameter (D12) is greater than the final inner nominal diameter (DN) of the definitive bell mouth (21); calibration of the definitive bell mouth (21) on a calibration mandrel (12) which can be freely introduced beforehand into the first temporary bell mouth (21'), whilst the mandrel (12), structured with fixed geometry, presents an outer diameter equal to the final inner nominal diameter (DN) of the definitive bell mouth (21); cooling the definitive bell mouth (21) through the use of cooling means (9, 99) operating in correspondence with the calibration mandrel (12), thereby rapidly bringing the definitive bell mouth (21) to ambient temperature. It is instead characterised in that the phase whereby the first bell mouth (21') is forcedly dilated is effected by means of at least a first mandrel (11) with smooth outer surface, able to be progressively introduced into the terminal section (2), after heating the terminal section (2) to the plastic state, but in differentiated form. A first quantity of heat (Q1) is transferred to the terminal section (2) thereby bringing it to an external temperature (TE) exceeding the internal temperature (TI), the latter being as low as possible in order to prevent the collapse of the thermoplastic material and yet sufficient to assure a correct plastic deformation, without any breaks in the molecular links of the thermoplastic material constituting the terminal section (2), during the dilation of the first mandrel (11) and in spite of the reduced thickness of the pipe (20). In parallel, the apparatus with multiple operating phases, which also constitutes the subject of the invention, is of the kind operating on the terminal junction section (2) previously heated to the plastic state and comprising at least: a device able to preform on the terminal section (2), operating internally thereto, the first bell mouth (21'); the calibration mandrel (12), able to be introduced freely and preventively into the first bell mouth (21'), after the complete intervention by said device, in such a way as to calibrate it exactly; cooling means (9, 99). It is characterised instead in that the device able to pre-form the first bell mouth (21') comprises at least said first mandrel (11), with maximum diameter equal to the temporary diameter (D12), as well as a furnace (44) able preventively to heat the terminal section (2) to the plastic state, but in a differentiated manner, to an external temperature (TE) greater than the internal temperature (TI). With reference in particular to Figure 1, the pipe (20) is held axially locked by the pair of rollers (100): the upper rollers are driving rollers and make the pipe (20) rotate around its axis, to make more homogeneous the preventive heating. This preferably takes place with hot air in dual circulation (external and internal to the pipe), according to a known patent in the Applicant's name. In the case at hand, it is noted that the subject pipe (of the ultrarib type) is heated more intensely in the external part of the wall of the pipe, appropriately regulating the quantity of hot air, by means of the small shutter (101), that laps the pipe both externally and internally to the pipe, where a holed conduit (102) is provided which conveys a calibrated quantity of air, that is recirculated, driven by an agitator (103), actuated by a related motor (104). A further characteristic of the method rests in the fact that the dilation phase can be progressive and take place in at least two distinct phases, for instance with a pair of mandrels (11), structured with fixed geometry, the first with a lesser diameter than the second (whose diameter is (D12)), and acting consecutively to one another. An advantageous execution of the dilation method, after the aforesaid preliminary heating, provides for the dilation phase to be operated by means of a mandrel (11') of the type with radially movable mechanical sectors (6), set in their totally expanded, hence totally smooth, configuration. Their length, measured along the longitudinal axis (5) of the mandrel (11') is equal to the length of the bell mouth (21,21'). In this case it is very advantageous to place the mandrel (11') aligned with and forward of the calibration mandrel (12), the latter operating after the complete intervention of the mandrel (11') of the type with mechanical sectors (6), when it is in the minimum size configuration, contained inside the pipe (20) to be treated (see Figs. 4 and 5). The related apparatus comprises a single composite mandrel (112) designed on a single axis coaxial to the pipe (20) to be treated and presenting, as first mandrel, the mandrel (11') of the type with radially movable mechanical sectors (6) and as second mandrel the calibration mandrel (12). Also provided are organs (3) for imparting a relative motion between composite mandrel (112) and pipe (20), operating along the longitudinal axis (5) of the composite mandrel (112) and in combination and synchrony with organs (4) driving the expansion and retraction of the radially movable mechanical sectors (6). During the dilation phase (see Figs. 2 and 4), the pipe is held by clamps (105), fastened to the frame. According to the subject method, a pre-set further quantity of heat (Q2) is simultaneously transferred inside the terminal section (2) by the first mandrel (11) or (11'), which is a heating mandrel, and outside the terminal section itself (2), so as to reduce its circumferential stresses caused on the wall of the terminal section (2) by effect of the dilation induced by the first mandrel (11) and in such a way as to favour the subsequent calibration phase. The related apparatus presents means (8) for heating the mandrel (11) or (11') to a temperature (T) greater than that (TI) of the interior of the terminal section (2), the latter being also heated from the exterior by means (8) to a temperature (T') greater than (TE), so as to transfer heat to the terminal section (2). The calibration phase advantageously starts immediately thereafter, with the terminal section (2) in the hot condition, it being immediately subsequent the dilation phase, without any removal of heat. As soon as the calibration phase begins, a progressive and spontaneous reduction of the diameter of the terminal phase (2) also starts, from the dilated configuration characteristic of said first temporary bell mouth (21'), to the definitive one determined by the calibration mandrel (12), thanks to the quantity of heat still present on the terminal section (2) itself. It is also possible to accelerate the spontaneous reduction in diameter of the terminal section (2) by means of the pressure applied by a compressed fluid acting from the exterior on the wall of the terminal section (2). The related apparatus in this case comprises, in combination with the calibration mandrel (12), means (55) able to exert compressed fluid pressure acting from the exterior on the outer wall of the terminal section (2) during the calibration phase, also to improve the regularity of the inner surface of the definitive bell mouth (21). The apparatus and the method further provide for the diameter (DN) of the calibration mandrel (12) to be computed as a function of the constitution of the thermoplastic material. Lastly, with reference to the corrugated pipe, it is important to stress that, after each undulated projection (23) of its corrugated surface there is an isolated annular channel (24), the latter placed in communication with the exterior through a hole (25), in such a way as to uniform both the interior and the exterior of the corrugated pipe to the same pressure, thereby eliminating at the root any danger that the pipe may implode, regardless of the pressure exerted on the corrugated pipe itself.

Claims: Method for forming stable and internally regular bell mouths, on the terminal junction section of externally contoured pipes made of thermoplastic material, of the type operating on a terminal junction section (2), preventively heated to the plastic state and comprising the following successive phases: forced dilation of the terminal section (2), operated internally thereto, until pre-forming a first bell temporary bell mouth (21'), of maximum dilation, whose temporary inner diameter (D12) is greater than the final inner nominal diameter (DN) of the definitive bell mouth (21); calibration of the definitive bell mouth (21) on a calibration mandrel (12) which can be freely introduced preventively into the first bell mouth (21'), said mandrel (12) structured with fixed geometry, presenting an outer diameter equal to the final nominal inner diameter (DN) of the definitive bell mouth (21); cooling the definitive bell mouth (21) through the use of cooling means (9, 99) operating in correspondence with the calibration mandrel (12), in such a way as to bring the definitive bell mouth (21) to ambient temperature rapidly; and wherein the phase of forced dilation of the first bell mouth (21') is operated by means of at least a first mandrel (11) with smooth outer surface, able to be progressively introduced into the terminal section (2), after heating to the plastic state, but in differentiated form, the terminal section (2) whereto a first quantity of heat (Q1) is transferred thereby bringing it to an external temperature (TE) exceeding the internal temperature (TI), the latter being as low as possible in order to prevent the thermoplastic material from collapsing and at the same time sufficient to assure a correct plastic deformation, without breaks in the molecular links of the thermoplastic material constituting the terminal section (2), during the dilation action of the first mandrel (11) and in spite of the reduced thickness of the pipe (20). Method according to claim 1, characterised in that the dilation phase is progressive and takes place in at least two distinct phases. Method according to claim 1, characterised in that the dilation phase is operated by means of a mandrel (11') of the type with mechanical sectors (6) radially movable and set in their totally expanded configuration, the length of said mechanical sectors (6), measured along the longitudinal axis (5) of the mandrel (11') being equal to the length of the bell mouth (21,21'), said mandrel (11') of the type with mechanical sectors (6) being aligned with and forward of the calibration mandrel (12), the latter operating after the complete intervention of the mandrel (11') of the type with mechanical sectors (6), when it is in the minimum size configuration, contained within the pipe (20) to be treated. Method according to any of the previous claims, characterised in that a pre-set additional quantity of heat (Q2) is simultaneously transferred to the interior of the terminal section (2), by the first mandrel (11, 11') which is a heating mandrel, and to the exterior of the same terminal section (2), in such a way as to reduce its circumferential stresses caused on the wall of the terminal section (2) by effect of the dilation induced by the first mandrel (11, 11') and as to favour the successive calibration phase. Method according to claim 4, characterised in that the calibration phase starts with the terminal section (2) in the heated state, it being immediately consecutive to the dilation phase, without any subtraction of heat. Method according to claim 5, characterised in that as soon as the calibration phase starts, a progressive and spontaneous reduction in diameter of the terminal section (2) also starts from the dilated configuration characteristic of said first temporary bell mouth (21'), to the definitive one determined by the calibration mandrel (12), thanks to the quantity of heat still present on the terminal section (2) itself. Method according to claim 6, characterised in that the spontaneous reduction in diameter of the terminal section (2) is accelerated by means of compressed fluid pressure acting from the exterior on the wall of the terminal section (2). Method according to claim 7, whose pipe (20) to be treated is of the corrugated type, where each undulated projection (23) is involved by an isolated annular channel (24), characterised in that each said annular channel (24) is put in communication with the exterior, through at least a hole (25). Apparatus for forming stable and internally regular bell mouths on the terminal junction section of externally contoured pipes made of thermoplastic material with reduced thickness, of the type operating on a terminal junction section (2) previously heated to the plastic state and comprising at least: a device able to pre-form on the terminal section (2), operating internally thereto, a first temporary bell mouth (21') whose temporary inner diameter (D12) exceeds the final inner nominal diameter (DN) of the definitive bell mouth (21); a calibration mandrel (12), able to be introduced freely and preventively into the first bell mouth (21'), after the complete intervention of said device, structured with fixed geometry and presenting an outer diameter equal to the nominal and final inner diameter (DN) of the definitive bell mouth (21), thereby calibrating it; cooling means (9, 99), operating in correspondence with the calibration mandrel (12), in such a way as to bring the definitive bell mouth (21) to ambient temperature rapidly; and wherein the device able to pre-form the first bell mouth (21') comprises at least a first mandrel (11) with smooth outer surface, able to be progressively introduced into the terminal section (2), as well as a furnace (44) able preventively to heat the terminal section (2) to the plastic state, but in differentiated form, to an external temperature (TE) exceeding the internal temperature (TI), the latter being as small as possible in order to prevent the collapse of the thermoplastic material and at the same time sufficient to assure a correct plastic deformation, without breaks in the molecular links of the thermoplastic material constituting the terminal section (2), during the dilation of the first mandrel (11) and in spite of the reduced thickness of the pipe (20). Apparatus according to claim 9, characterised in that at least two mandrels (11) are provided, structured with fixed geometry, the first one with a smaller diameter than the second one, with maximum outer diameter equalling the inner temporary diameter (D12) of the first temporary bell mouth (21'). Apparatus according to claim 9, characterised in that said first mandrel is a mandrel (11) of the type with mechanical sectors (6) movable radially and set in their totally expanded configuration, which has a maximum external diameter equal to the temporary inner diameter (D12) of the first temporary bell mouth (21), the length of said mechanical sectors (6), measured along the longitudinal axis (5) of the mandrel (11') being equal to the length of the bell mouth (21,21), said mandrel (11') being aligned to and forward of the calibration mandrel (12), to form a single composite mandrel (112) devised on a single axis coaxial to the pipe (20) to be treated, organs (3) being provided for imparting relative motion between composite mandrel (112) and pipe (20), operating along the longitudinal axis (5) of the composite mandrel (112) and in combination and synchrony with organs (4) for actuating the expansion and retraction of the mandrel (11') of the type with radially movable mechanical sectors (6). Apparatus according to claim 9 or 10 or 11, characterised in that at least the first mandrel (11, 11') presents means (8) for heating the mandrel itself to a temperature (T) exceeding the temperature (TI) of the interior of the terminal section (2), the latter also being heated from the exterior by means (88) to a temperature (T') exceeding (TE); thereby transferring heat to the terminal section (2). Apparatus according to claim 9 or 10 or 11 or 12, characterised in that it comprises, in combination with said calibration mandrel (12), means (55) able to exert compressed fluid pressure acting from the exterior on the outer wall of the terminal section (2) during the calibration phase, to accelerate said phase and improve the regularity of the inner surface of the definitive bell mouth (21). Apparatus and method according to any one of the previous claims, characterised in that the diameter (DN) of the calibration mandrel (12) is computed as a function of the constitution of the thermoplastic material.

Inventors: SAVIOLI LEOPOLDO
Applicants: SICA SPA, SICA S.P.A.
Application year: 1999
Main classification: B29C57
IPC codes: B29B13, B29C57