Source: http://www.patent-de.com/20030710/EP1159122.html
Timestamp: 2019-09-18 19:39:16
Document Index: 160872666

Matched Legal Cases: ['art 32', 'art 32', 'art 33', 'art 33', 'art 34', 'art 34', 'art 34', 'art 33', 'art 34', 'art 70', 'arts 70', 'art 70', 'arts 70', 'art 33', 'art 70', 'art 70', 'art 70', 'art 170', 'art 170', 'art 170', 'art 170', 'art 170', 'art 170', 'art 190', 'art 190', 'art 190', 'art 190', 'art 190', 'art 364', 'art 362', 'art 363', 'art 364', 'art 363', 'art 363', 'art 364', 'art 364', 'art 364', 'art 364']

ROHR AUS THERMOPLASTISCHEM KUNSTSTOFF - Dokument EP1159122
Dokumentenidentifikation EP1159122 10.07.2003
EP-Veröffentlichungsnummer 1159122
Titel ROHR AUS THERMOPLASTISCHEM KUNSTSTOFF
Anmelder Wavin B.V., Zwolle, NL
Erfinder VAN LENTHE, Dirk, Arjan, NL-7777 RH Schuinesloot, NL;
PRENGER, Hendrik, Jan, NL-7772 SZ Hardenberg, NL;
VISSCHER, Jan, NL-7775 AK Lutten, NL
DE-Aktenzeichen 60003163
EP-Aktenzeichen 009081217
PCT-Aktenzeichen PCT/NL00/00138
WO-Veröffentlichungsnummer 0000053392
IPC-Nebenklasse B29C 47/90 B29C 55/26 B29C 57/04 B29C 47/00
WO 95/25626 has disclosed a method according to the preamble of claim 1 for the production of biaxially oriented plastic tube, also known as a stretched tube. In this method, the stretched tube is of uniform cross section, i.e. has a uniform wall thickness and diameter, over its entire length, and is also uniformly stretched in the axial and tangential (circumferential) direction of the tube over its entire length. A method for providing a tube which has been produced in this way with a socket at one of its ends is known from WO 97/33739.
In US 3,264,383, a method for forming sockets on non-biaxially stretched tubes is presented. The sockets are formed on the tubes by upsetting the tube string emerging from the extruder to the desired distances while the tube string is still in deformable condition, to form a larger wall thickness.
US 5,096,634 discloses a method and apparatus for producing biaxially stretched tubes. The method comprises the steps of extruding thermoplastic tubing in a viscous state, running the tubing over a stretching mandrel to biaxially stretch the tubing and monitoring the molecular orientation in the cooled tubing.
For this purpose, the invention, according to a first aspect, provides a method according to claim 1. When the method according to the invention is carried out, an axial preform part with a different wall thickness from the preceding part of the preform is periodically formed in the section between the extruder die and the tube speed-control means, in practice in particular immediately downstream of the extruder die.
The method according to claim 1 enables a stretched tube of biaxially oriented thermoplastic material to be produced in a continuous process with axial tube parts which have varying wall thicknesses.
In practice, this means, as seen at a point downstream of the expansion mandrel - the stretched tube in each case has a part of great axial length with a uniform first wall thickness and associated diameter, which part is followed by a considerably shorter axial part of the tube in which the wall thickness differs from the said first wall thickness, in particular is of one or more greater values, as seen in the axial direction of the said shorter part. In particular, there is provision for the wall thickness - as seen in the axial direction - to vary between a plurality of values in the latter axial part, so that annular areas which adjoin one another and have different wall thicknesses can be distinguished in the relevant part of the stretched tube.
In a preferred embodiment of the method according to the first aspect of the invention, the stretched tube acquires substantially the same axial stretching over its entire length. To achieve this, in the method according to claim 3 it is in some cases sufficient to keep the advancement speed of the stretched tube downstream of the mandrel, which is determined by the drawing device, constant, so that the ratio of the advancement speed of the stretched tube downstream of the mandrel, on the one hand, and of the preform upstream of the mandrel, on the other hand, remains substantially constant.
In the method according to claim 4, the advancement speed of the preform upstream of the mandrel, which is determined by the tube speed-control means, varies, and for this reason it is then necessary for the advancement speed of the stretched tube downstream of the mandrel, which is determined by the drawing device, to be varied periodically in such a manner that the ratio of the advancement speed of the tube downstream of the mandrel, on the one hand, and of the preform upstream of the mandrel, on the other hand, is kept substantially constant.
If it is assumed that the temperature conditioning substantially consists in cooling the preform, although it is also known from the prior art to supply (relatively small) amounts of heat to the preform upstream of the mandrel, the above-described temperature condition of the preform can in practice be implemented by causing the cooling means, which form part of the temperture-control means, to operate substantially constantly. This can be explained in the following way. In the section between the extruder die and the mandrel it is in fact possible to distinguish between three partial sections. In the first partial section which immediately adjoins the extruder die, it is possible to produce a preform part with a thickened wall by operating as described in claim 1. In the adjoining partial section, the preform is subjected to the action of the temperature-conditioning means, in particular to cooling, and in the adjoining third partial section, there is in fact no significant thermal energy supplied to or removed from the preform.
In the method according to claim 3, a preform part with a thickened wall which is formed in the first partial section will move past the temperature-conditioning means in the second section at the same speed as a preform part with a smaller wall thickness. In relative terms, the thicker preform part will therefore be cooled to a lesser extent and will therefore arrive at the mandrel at a higher average temperature; in particular, the temperature of the core of the said thickened preform part will be higher. Due to the higher temperature, the modulus of elasticity will be lower and the thickened preform part will therefore be easier to deform, in relative terms, a fact which in practice can sufficiently compensate for the wall thickening to avoid the above critical situation.
In the method according to claim 4, the speed of the preform is reduced while a preform part with a thickened wall is being formed. In this case too, the said preform part will form in the first partial section. Due to the reduction in speed, that part of the preform which is situated in the second partial section during this period will be subjected to cooling for a longer time than that part of the preform which has already passed through the cooling and is in the third partial section. When the preform part with a thickened wall is complete, the speed of the preform is increased again and the preform part with a thickened wall will pass through the cooling at the said higher speed and will thus be cooled to a lesser extent. When the thickened preform part then arrives at the mandrel, the said part can be deformed easily, while the thin wall part of the preform which is located immediately downstream thereof is in fact relatively rigid. A combination of the two effects makes it possible to carry out the process successfully in a controllable manner.
It can be seen from the above that, on the basis of the temperature of the preform - within a temperature range which is suitable for obtaining biaxial orientation - and the resultant modulus of elasticity of the plastics material of the preform, it is possible to control the axial stretching of the preform. By causing the preform to be at a higher temperature locally, for example at a thicker part thereof as described above, than other parts of the preform at the time of axial stretching, it is possible to ensure that, given a constant axial stretching force exerted on the preform, the hotter part undergoes greater axial stretching than the cooler parts, even if this hotter part has a greater wall thickness. In a practical embodiment, it is possible for the thinner parts of the preform to be at a temperature of approximately 90°C and for a hotter, optionally thicker, part to be at a temperature in the vicinity of 120°C.
The method according to the first aspect of the invention can be carried out in a continuous process, and in this way it is possible to produce a tube from biaxially oriented thermoplastic material with a tube part with a thickened wall at (regular) axial intervals from one another. By then sawing, cutting or suchlike through the tube at the location of the thickened tube parts, it is possible to produce tube sections with, at one or both ends, an end part with a larger wall thickness than the tube body. Furthermore, the invention provides for the said tube sections then to be subjected to a socket-forming operation, in which case an integral socket is formed from an end part with a thickened wall. In a variant - if both end parts are of thicker design - one end part is deformed into a socket and the other end part is used as a spigot. If appropriate, the said spigot is also deformed further, for example is provided with one or more formations, in such a manner that a positively locking socket joint can be obtained.
Particularly in those embodiments in which the end part with a thickened wall - prior to the formation of the socket - has undergone axial stretching which is greater than or equal to the tube body with a smaller wall thickness, the socket obtained has proven to have considerably better properties and a greater load-bearing capacity than the known sockets on such tubes.
The abovementioned measures and other measures provided according to the invention are described in the claims and in the following description and will be explained below, in particular with reference to the drawings. In the drawing:
Figs- 1a and 1b diagrammatically depict a side view, partially in cross section, of an exemplary embodiment of an installation for producing biaxially oriented thermoplastic tubing,
Figs. 5a and 5b diagrammatically depict a side view, partially in cross section, of an exemplary embodiment of an installation for producing biaxially oriented thermoplastic tubing,
Fig. 10 shows a view corresponding to Figures 5a, 5b of a part of a variant of an installation for producing biaxially oriented thermoplastic tubing.
Figures 1a and 1b show, in two partial drawings which should adjoin one another, diagrammatic representations of the most important elements of an installation for producing biaxially oriented thermoplastic tubing in a continuous process.
The extruder die 3 is provided with means for controlling the wall thickness, which are not shown and can be used to produce a uniform wall thickness (in the circumferential direction) of the preform 6 coming out of the extruder die 3. An internal cooling member may be attached to the inner core 5 for internal cooling of the preform.
A heater device 25 is arranged downstream of the tube speed-control means 20. This device 25 comprises a plurality of heater units which are positioned around the path for the preform 6, can be controlled separately and are each directed towards a sector of the circumference of the preform 6. As a result, a separately controllable amount of heat can be fed to each sector of the preform 6, for example six circumferential sectors each of 60°.
At its upstream end, the mandrel 30 has a run-on part 32, which in this case is of substantially cylindrical design. The said run-on part 32 is adjoined by an expansion part 33, the external surface of which substantially corresponds to the surface of a truncated cone with a diameter which increases in the downstream direction. The said expansion part 33 is adjoined by a run-off part 34 of the mandrel 30, which part 34 is of substantially constant diameter, if appropriate tapering slightly in the downstream direction. As a result of being forced over the mandrel 30, the preform 6 changes into a stretched tube 6'.
At the location of the mandrel 30, in particular of the run-off part 34, there is a second external cooling device 40, by means of which the stretched tube 6' is externally cooled. As is generally known for the production of biaxially oriented plastic tube, the stretched tube is cooled after it has passed the expansion part of the stretching mandrel, so that as a result the changes which have been brought about in the plastics material of the tube are frozen.
A second external calibration device 45 is arranged at a distance downstream of the mandrel 30, which calibration device 45 reduces the external diameter of the tube 6'.
The installation also comprises a drawing device 50 which is arranged downstream of the mandrel 30 and of the external calibration device 45. The drawing device 50 is intended to exert a considerable tensile force on the stretched tube 6'. Downstream of the drawing device 50 there is a cutting-to-length device (not shown), for example a sawing, cutting or milling device, in order to cut sections of the desired length from the tube 6' which has been produced.
The preform 6 is forced over the mandrel 30 under the influence of the forces which are exerted on the preform 6 and the tube 6' by means of the drawing device 50 in conjunction with the tube speed-control means 20. By means of the drawing device 50 and the tube speed-control means 20, it is possible to accurately control the advancement speed both at a location upstream of the mandrel 30 (at tube speed-control means 20) and at a location downstream of the mandrel 30 (at drawing device 50).
As a result of passage over the mandrel 30, the molecules of the plastics material are oriented, i.e. stretched, both in the axial direction and in the circumferential direction, which is of great benefit to the properties of the tube 6'.
Downstream of the mandrel 30 there is a unit 60 for measuring wall thickness. This wall-thickness measuring unit 60 may be connected to a control unit which, on the basis of the measured cross section of the stretched tube 6', controls the operation of the drawing device 50, the device 25, and, if appropriate, the distance between the calibration device 45 and the mandrel 30.
The mandrel 30 may be provided with one or more feed ducts which open out in the outer surface of the mandrel 30 and, through the anchor member 31 and the extruder die 3, are connected to pump means (not shown) for supplying a liquid between the mandrel 30 and the preform 6. It is thus possible to form a film of liquid between the preform 6 and the mandrel 30, in particular between the preform 6 and the expansion part 33 of the mandrel 30. It is also possible to form a film of liquid between the run-off part 34 and the tube 6', serving to reduce the friction between the tube and the run-off part and, on the other hand, possibly also as internal cooling for the stretched tube.
It is generally known from the prior art for the installation described above to be operated in such a manner that the preform 6 upstream of the mandrel 30 has, as accurately as possible, a uniform cross section, i.e. wall thickness and diameter, and also has a suitable orientation temperature which is as uniform as possible. Downstream of the mandrel 30, the stretched tube 6' then has a greater diameter and a smaller wall thickness.
In contrast to this known way of operating the installation, according to one aspect of the invention, it is possible, by periodically varying the ratio between the advancement speed of the preform 6, which is determined by the tube speed-control means 20, on the one hand, and the output of the extruder 1, on the other hand, between a first value and a second value, which is lower than the first value, for the extruded preform 6, in the section between the extruder 1 and the tube speed-control means 20, to alternately acquire a first wall thickness - if the said ratio is of the first value - and a second wall thickness - if the said ratio is of the second value - the second wall thickness being greater than the first wall thickness.
In the example presented here, this is effected by keeping the output of the extruder 1 substantially constant and by periodially varying the advancement speed of the preform 6 which is determined by the tube speed-control means 20. In this case, therefore, the ratio between the advancement speed of the preform 6, which is determined by the tube speed-control means 20, on the one hand, and the output of the extruder 1, on the other hand, is kept substantially constant at the first value for a first period, so that a long piece of preform 6 with a first wall thickness "d1" is produced. During a second period, which is considerably shorter than the first period, the speed of the tube speed-control means 20 is set to a lower value, with the result that a preform part having the second, greater wall thickness "d2" is then formed immediately downstream of the extruder die 3, as indicated in Figure 1a by reference numeral 70.
When it passes through the external calibration device 45, the thickened part 70 is pressed inwards again (as indicated by a dashed line), resulting in a stretched tube 6' having thickened parts 70 at (regular) axial intervals and, between these thickened parts, in each case a long part of smaller wall thickness d1.
In one practical embodiment, the tube 6' is cut to length downstream of the drawing device 50 at each thickened part 70, and the distance between two thickened parts 70 corresponds to the desired length of the tube sections to be produced by cutting the tube 6' to length. As a result, each tube section then has a tube body and, at one end, a thickened tube part with a greater wall thickness than the tube body. Preferably, the thickened end part of the tube is then subjected to a socket-forming operation, so that a high-quality integral socket can be obtained.
In another variant, the tube 6' is cut to length in such a way that there is a thickened end part at each end of a tube section. It is then possible for one of the ends to be deformed into a socket, while the other end, possibly without further treatment, can be used as a thickened spigot.
In a preferred embodiment of the method according to the first aspect of the invention, the biaxially stretched tube undergoes substantially the same axial stretching over its entire length. Since the advancement speed of the preform 6 upstream of the mandrel 30, which is determined by the tube speed-control means 20, varies, it is therefore necessary for the advancement speed of the tube 6' downstream of the mandrel 30 which is determined by the drawing device 50 to be varied periodically, in such a manner that the ratio between the advancement speed of the tube 6' downstream of the mandrel 30 and of the preform 6 upstream of the mandrel 30 is kept substantially constant during the production of both a thickened part and an unthickened part.
To enable the process to be controlled successfully, it is desirable for the axial stretching of the preform to take place within an accurately defined subsection of the installation. For this purpose, it is possible for the stretched tube 6' to be cooled downstream of the expansion part 33 of the mandrel 30, in such a manner that the cooled tube 6' does not undergo any further axial stretching and the generation of the axial stretching is concentrated in the section between tube speed-control means 20 and the downstream end of the mandrel 30.
The shape of the thickened part 70 shown in Figures 1a and 1b is, of course, only shown by way of example. In fact, it has proven possible for the wall thickness of the thickened part 70 to be controlled accurately and, in this way, for a specific profile to be imparted accurately to the wall of the thickened part 70 as seen in the longitudinal direction of the tube.
Figure 2a shows a longitudinal section through half of the preform 6 at a location immediately after it has passed through the calibration device 10, having a thickened tube part 170 produced by varying the speed of tube speed-control means 20 with respect to the output of the extruder 1.
In Figure 2a, d1 denotes the first wall thickness which is used for a long part of the preform 6. The line 171 is the centre axis of the preform 6. The thickened part 170 has a profile with a plurality of wall thickness values, described by points A, B, C, D, E, F and G.
Figure 2b shows the same thickened part as in Figure 2, but in this case after it has passed over the mandrel 30. This can be seen clearly from the greater diameter and reduced wall thickness of the now stretched tube 6'. It is clear that the internal diameter of the tube 6' is now uniform and the wall thickness profile can be seen on the outside. The points A-G show that stretching has taken place in the axial direction and in the circumferential direction of the thickened part 170 when it passed over the mandrel 30.
Figure 2c shows the part of tube 6' after it has passed through the calibration device 45, which device, incidentally, is optional in the method according to the first aspect of the invention. The external diameter is now uniform once again, while the profile can be seen on the inside.
As described, there is provision for the tube 6' to be cut to length at the thickened part 170, in this case at line 172. Then, the cut-to-length tube section is subjected to a socket-forming operation, during which the thickened part 170 of the said tube section is deformed to form a socket.
Figure 2d shows a possible embodiment of that end of a tube section which is provided with a socket and has been produced as described with reference to Figures 2a, 2b and, if appropriate, 2c.
At one end, the prefabricated tube section with a thickened tube part 170 has a greater wall thickness than the tube body, and the axial stretching of the thickened end part prior to the socket-forming operation is equal to or preferably greater than the axial stretching of the tube body. It will be clear from the preceding text how a tube section of this nature can be produced.
In particular, Figure 2c shows that the end part of the prefabricated tube, as seen from its end face, has a plurality of annular areas which adjoin one another and have a wall thickness which fluctuates from one annular area to the next annular area, the wall thickness, in the case of a plurality of annular areas, being greater than the wall thickness of the tube body.
In an illustration corresponding to Figure 2a, Figure 3a shows another embodiment of a thickened part 190 which has been produced using the method according to the invention. This thickened part 190 has a first zone, indicated by points A-G, which virtually corresponds to the description given with reference to Figure 2a. The line 191 is the centre axis. Further away from the end of the tube section to be produced, shown by line 192, the thickened part 190 has a second zone, between points G and H, with a wall thickness d1 corresponding to the thickness of the preform outside the thickened part 190. This is followed by a third zone, indicated by points H-K, with a greater wall thickness.
It can be seen in Figure 3b that only the first zone of the thickened part 190 has been deformed into a socket. This first zone is deformed in the same way as that described with reference to Figure 2d and has a groove wall 193. The third zone forms an inwardly projecting rim 194. This rim 194 serves to receive a support bush which is introduced into the first zone when the socket is being formed, in order to provide internal support for this zone during heating. When the socket is being formed, this support bush is then pushed further into the tube and then comes to bear against the rim 194. This prevents the support from penetrating too far into the tube and also prevents this support bush from locally overheating the tube.
Figure 4 shows a cross section through part of extruder die 200 which is suitable for use in the method described above and is used to extrude a preform 201 from thermoplastic material. Furthermore, the figure shows a section of an external calibration device 202 arranged downstream of the extruder die 200.
Immediately downstream of the extruder die 200, the preform 201 is also cooled internally by means of an internal cooling member 208, only part of which is shown
As described above, there is provision for the wall thickness of the preform 201 to be changed periodically in order, in this way, to obtain a preform part with a greater wall thickness, as shown in Figure 4. To obtain a preform part with a greater wall thickness than that defined by the gap between the inner core 206 and the outer ring 205, flowable plastic material has to be able to flow from the extruder die 200 to the thicker preform part. For this reason, it is undesirable for a solidified skin to form on the inside of the preform, immediately downstream of the inner core. To counteract this skin formation, an insulating member 210 which is attached to the inner core 206 is provided.
In two partial drawings which are to adjoin one another, Figures 5a and 5b diagrammatically depict the most important elements of an installation for producing biaxially oriented thermoplastic tubing in a continuous process.
Figure 5a shows an extruder 301 having one or more extruder screws 302 with an associated controllable drive, by means of which a flow of molten plastic material is provided, which is fed to an extruder die 303 arranged on the extruder 301.
An internal cooling member 310, the construction of which will be explained below with reference to Figure 6, is attached to the inner core 303. The internal cooling member 310 is designed in such a manner that the preform 306 coming out of the extruder die 303 is internally cooled immediately downstream of the extruder die 303.
As a result of the cooling action of the internal cooling member 310, a cold wall layer is formed on the inside of the preform 306 immediately downstream of the extruder die 303, which cold wall layer is relatively dimensionally stable. If a cold layer were to be formed on the outside at the same time by means of external cooling, a still hot intermediate layer of plastic material would be enclosed between two cold, rigid wall layers. Cooling of this intermediate layer can then easily result in shrinkage cavities in the intermediate layer, and there is also a considerable risk of visible deformations being formed, in the form of pits or indentations, in the outside and inside of the tube 306' produced. If cooling initially takes place only on the inside, shrinkage of this intermediate layer can be absorbed by material being supplied from the uncooled outer layer of the preform. Once the inner layer has been cooled, cooling from the outside can then begin.
A heater device 350 is arranged downstream of the speed-control means 340. This device 350 comprises a plurality of heater units which are positioned around the path for the preform 306, can be controlled separately and are each directed towards one sector of the circumference of the preform 306. As a result, a separately controllable amount of heat can be supplied to each sector of the preform 306, for example six circumferential sectors each of 60°.
At the mandrel 360, in particular in the area of the run-off part 364, there is a second external cooling device 370, by means of which the stretched tube 306' is cooled externally. As is generally known for the production of biaxially oriented plastic tube, the stretched tube is cooled after it has passed the expansion part of the stretching mandrel, so that as a result the changes which have been brought about in the plastics material of the tube are frozen.
At a distance downstream of the mandrel 360 there is a second external calibration device 380, which calibration device 380 brings about a reduction in the external diameter of the stretched tube 306'.
The installation also comprises a drawing device 390 which is arranged downstream of the mandrel 360 and of the external calibration device 380. The drawing device 390 is intended to exert a considerable tensile force on the tube 306'. A cutting-to-length device, for example a sawing, cutting or milling device, may be located downstream of the said drawing device 390, for the purpose of cutting sections of the tube produced to a desired length. Alternatively, a coiling device could also be provided for the purpose of winding the tube 306' produced onto a reel.
As a result of the passage over the mandrel 360, the molecules of the plastics material are oriented both in the axial direction and in the circumferential direction of the tube 306, which is highly advantageous for the properties of the tube 306'.
Details of the installation shown in Figures 5a and 5b will be explained in more detail below, partly with reference to the further figures.
- The internal cooling member
Part of the internal cooling member 310 can be seen in Figure 6. The internal cooling member 310 has a rigid, dimensionally stable cylindrical outer wall, for example made from metal, with a long central section 311, the diameter of which is slightly smaller than the diameter of end sections 312 lying at the upstream and downstream ends of the said middle section 311 (only the downstream end section can be seen in Figure 6). The difference in diameter between the section 311 and the sections 312 is preferably no more than 3 millimetres and is at least 0.5 millimetre. This difference is exaggerated in Figure 5a.
The axial length of the end sections 312 is considerably shorter than that of the central section 311, the length of the central section 311 preferably being a multiple of the cross-section dimension of the preform 306. In practice, it is preferable for this length to be one metre or more.
Generally, any local disruption in the internal cooling has been found to leave a visible mark on the inside of the tube 306', and for this reason it is important for the internal cooling to be highly regular.
For these reasons, it is important, when using internal cooling, for the preform to be provided with a cool, dimensionally stable layer on the inside by cooling as soon as it leaves the extruder die, as is the case with the internal cooling member 310 described above. This is particularly important for the internal cooling of profiles which have been extruded from plastics material such as polyethylene (PE) and polypropylene (PP). It has been found that in the case of polyvinyl chloride (PVC), for example, this problem is less signficant. It is also important for this cool layer to be maintained throughout the entire path during which internal cooling takes place, since otherwise the abovementioned pitting could still occur. Furthermore, it will be clear that it is important to counteract the formation of air bubbles, in particular large air bubbles or an accumulation of air bubbles.
- Effects of the crystalline composition
In the case of the biaxial stretching process, for example using the installation shown in Figs. 5a and 5b, firstly a thick-walled preform is extruded, which then has to be cooled to a suitable orientation temperature which is significantly lower than the temperature of the preform when it leaves the extruder die 303. For this reason, the internal cooling member 310 and the first external cooling device 330 are active.
This difference may constitute a drawback for the biaxial stretching of the preform and the end result achieved. To solve or reduce this problem, it is conceivable to allow the highly cooled layer of the preform to be heated downstream of the internal cooling of the thick-walled preform coming out of the extruder, so that the small crystals begin to grow. This can be achieved by allowing this layer to be heated by heat transfer from the centre of the wall and/or by bringing the inner side of the preform into contact with a heating medium. In particular, it is possible to provide a compartment downstream of the internal cooling member in the hollow space in the preform, which compartment is filled with hot liquid, for example at a temperature of between 90-100°C.
- Wall thickness control
As described above, the preform is still relatively hot in the section between the extruder and the expansion mandrel, and this causes problems with the operation of such ultrasonic wall-thickness measuring units. Furthermore, in the case of crystalline thermoplastics, the crystallization takes place precisely at the temperatures prevailing in that section, resulting in a considerable change in the density of the thermoplastic, which in turn has consequences for the transmission of the ultrasonic pulse. This effect is also disadvantageous for the operation and reliability of the measurements using the ultrasonic wall-thickness measuring unit. It has been found that the operation improves if a layer of cold liquid lies along the inside of the preform at the location of the ultrasonic wall-thickness measurement, or if the preform is filled with a cold liquid at this location. If the liquid were to be hot, for example water in the vicinity of 100°C, the ultrasonic wall-thickness measurement appears to function considerably less accurately than with a cold liquid. It is assumed that this is because, in particular, the difference in transmission velocity between the preform and the liquid is important for the reflection of the ultrasonic pulse, and in the case of hot liquid this difference is smaller. In known ultrasonic wall-thickness measuring units, one or more ultrasonic transmitter/receivers rotate around the tube. In this embodiment, it is conceivable for a feed for a flow of cold liquid to rotate inside the tube at the same location.
- Formation of differences in wall thickness and orientation
It is also known that, despite these preparatory operations, deviations in the cross section of the preform may still arise as a result of passage over the mandrel. These deviations relate to the wall thickness of the preform as seen in the circumferential direction and, if appropriate, eccentricity of the inner side with respect to the outer side. These deviations are then observed using a second wall-thickness measuring unit 430 arranged downstream of the mandrel. To make it possible to correct these deviations, it is already known to utilize the heater device 350 shown in Figure 5b. As mentioned above, this heater device 350 comprises a plurality of heater units which are arranged in the vicinity of the mandrel 360 and around the preform 306. Each of the said heater units can be used to emit a separately adjustable amount of heat to an associated sector of the circumference of the preform 306 which is moving past. As a result of the added heat, the temperature, and consequently the rigidity, of the plastics material changes. In this way, it is possible to adjust the resistance which the preform 306 undergoes when it passes the mandrel 360 in sectors in the circumferential direction of the preform. This adjustment is known per se.
In practice, even when using this heater device 350, it has emerged that undesirable deviations in the cross-sectional form and wall thickness of the tube forced over the mandrel 360 still arise. This problem, as well as an associated solution, will be explained in more detail with reference to Figures 7 and 8.
Figures 7 and 8 show the mandrel 360 with run-on part 362, expansion part 363 and run-off part 364. The expansion part 363 of the mandrel 360 has an outer surface which substantially corresponds to the surface of a truncated cone.
It can be seen in Figure 8 that a large number of shallow grooves 367 is formed in the outer surface of the expansion part 363. In this figure, for the sake of clarity a number of these grooves 367 are shown on an exaggerated scale. Figure 7 also showns one such groove 367. The grooves 367 extend in the axial direction, i.e. in the direction in which the preform 306 is forced over the mandrel 360. The grooves 367 are preferably distributed over the expansion part at regular angular intervals, preferably of between 3° and 10°.
When the preform 306 is forced over the mandrel, some of the soft plastics material of the preform 306 will move into these grooves 367, as shown in Figure 7, This form of engagement between the preform and the expansion part of the mandrel limits the freedom of movement of the plastics material of the preform in the circumferential direction of the expansion part of the mandrel, which has proven to considerably reduce the abovementioned problem of local deviation of the wall thickness in the tube which is ultimately obtained.
It can be seen from Figure 7, as well as from Figure 5b, that a second film of liquid is formed in a manner known per se between the run-off part 364 of the mandrel 360 and the tube 306'. This second film of liquid is used, on the one hand, to reduce the friction between the tube and the run-off part and, on the other hand, may also serve as internal cooling for the stretched tube.
- Generating the tensile force required
A first problem relates to the transmission of the tensile force to the tube 306' by means of the drawing device 390 positioned downstream of the mandrel 360. In generally known drawing benches, there are a plurality of driven tracks, for example 2, 3 or 4 such tracks, and the transmission of the tensile force from the drawing device to the tube is based on friction between tube and tracks. The friction is determined by the coefficient of friction and the normal force. In this case, the coefficient of friction is determined by the materials coming into contact with one another and is not easy to increase significantly. The normal force is limited by the load-bearing capacity of the tube in order thus to prevent damage. Therefore, the tensile force which can be exerted by means of a drawing device is limited.
The internal support could also be of mechanical design. Fig. 5b diagrammatically depicts one example, in which an internal support device 420 is attached to the mandrel 360, via a anchor member 421, at the level of the drawing device 390. The support device 420 in this case has pressure belts 422 which run with the tube 306' and bear against the inside of the tube 306' opposite the belts of the drawing device 390. As a result, the drawing device 390 can press firmly against the outside of the tube 306' without any risk of the tube 306' being damaged.
In the case of greater tube diameters, the internal support device itself could also be provided with a drive for advancing the tube 306', in which case this device is then supported on the mandrel via a member which can be subjected to compressive loads. This support then leads to a reduction in the tensile force in the connection between the extruder and the mandrel.
- Maintaining properties of the tube produced
A significant problem with polyolefin tubes is that the improved properties obtained through the biaxial stretching process are completely or largely lost even at a low temperature of the tube (40°C for PE). This means that a tube of this nature cannot be stored in the sun without the abovementioned loss occurring, unless special measures are taken to enhance the stability of the tube produced.
It can be seen in Fig. 5b that the run-off part 364 of the mandrel 360 is of a considerable length, which in this case is a multiple of the wall thickness of the tube. In practice, lengths of more than 1 meter may be advantageous, which is possible in particular if a film of water is formed between the run-off part and the tube. The great length of the run-off part 364 makes the tube 306' more stable, since the stretched tube 306' then has a form which is defined by the run-off part 364 for a relatively long period, during which period the effects brought about by the expansion can become stable.
- Connection of biaxially oriented pipes
To connect two tubes of biaxially oriented thermoplastic material, in particular polyolefin plastics material, to one another, an improved connection is therefore proposed, which will be explained in more detail below with reference to Figure 9.
Figure 9 shows those ends of two identical tubes 501, 502 of biaxially oriented polyethylene, for example produced using the method and installation described above, which are to be connected. Each of these tubes 501, 502 is provided at both ends with a socket 503, 504, respectively, a simple design of which, without a sealing ring, is shown in Figure 9.
Figure 9 also shows a plastics connecting-tube body 510, which is provided with two axial ends 511, 512, which each fit into a socket 503, 504 of a tube 501, 502 to be connected. Preferably, the connecting-tube body 510 fits into the socket with a slight clearance, as shown in Figure 9.
To heat the socket which has been pushed over it, the connecting tube body 510 is provided at each of its ends 511, 512 with heater means. These heater means in this case comprise one or more electric heater elements, for example heater wires 515, which in this case are embedded in the connecting-tube body 510 and can be connected to a current source via terminal 516 on the outside of the body 510.
It can also be seen in Figure 9 that the outer surface at each end 511, 512 of the connecting-tube body 510 is profiled in order to create a positive form-locking connection component between the connecting-tube body 510 and the socket of the tube.
- Axial stretching upstream of the mandrel
Figure 10 shows a section of an installation for producing a tube from biaxially oriented thermoplastic material, in this example a section of the variant of the installation shown in Figures 5a, 5b.
Figure 10 shows the temperature-controlled, hollow, tubular preform 306 which has come out of an extruder, and the first speed-control means 340, which is arranged downstream of the extruder and engages on the outside of the preform 306, imparting a controllable first advancement speed to this preform.
Figure 10 furthermore shows a second speed-control means 600 which is arranged at a distance downstream of the first speed-control means 340. The second speed-control means 600 engages on the outside of the preform 306 and is designed to impart a controllable second advancement speed to the preform. The second speed-control means 600 is located upstream of the mandrel (not shown), over which the preform is forced at an orientation temperature which is suitable for the relevant plastics material. In any case, the second speed-control means 600 is located upstream of the expansion part of the mandrel.
It can also be seen from Figure 10 that the preform is moved through a calibration opening of a calibration device 610 in the section between the speed-control means 340 and 600, in which the preform is axially stretched, which calibration device 610 brings about a defined reduction in the external diameter of the preform 306. The reduction in the external diameter and possibly in the wall thickness of the preform 306 is now concentrated at the location of the calibration device 610, as can be seen from Figure 10.
Verfahren zur Herstellung eines biaxial ausgerichteten Thermoplast-Rohres, welches die folgenden Schritte umfasst: Extrudieren eines rohrförmigen Vorformlings aus einem Thermoplastmaterial unter Verwendung eines Extruders (1), der mit einer Extruderdüse mit einem Innenkern (5) versehen ist, wobei der Innenkern (5) einen Hohlraum in dem Vorformling (6) definiert, wobei das Verfahren des Weiteren die folgenden Schritte umfasst: thermisches Konditionieren des Vorformlings (6) in der Weise, dass der Vorformling (6) eine Ausrichttemperatur erreicht, welche für das jeweils verwendete Kunststoffmaterial geeignet ist, und Aufziehen des temperierten Vorformlings (6) unter Kraftaufwand über einen Dorn (30), welcher ein Dehnteil (33) aufweist, das dessen Dehnung in Umfangsrichtung des über den Dorn aufgezogenen Vorformlings (6) in der Weise herbeiführt, dass ein gestreckter Rohr (6') mit Kunststoffmaterial, das in axialer Richtung und in Umfangsrichtung erhalten wird, woran sich eine Abkühlung des gestreckten Rohres (6') anschließt, wobei eine Vorschubgeschwindigkeit des Vorformlings (6) auf der Zuführseite des Dorns mit Hilfe einer Einrichtung zur Steuerung der Geschwindigkeit (20) eingestellt wird, welche auf den Vorformling (6) auf der Zuführseite des Dorns einwirkt, und wobei eine einstellbare Vorschubgeschwindigkeit des Rohres (6') auf der Austragseite des Dorns (30) mit Hilfe einer Ziehvorrichtung (50) eingestellt wird, welche auf den gestreckten Rohr (6') auf der Austragseite des Dorns einwirkt,
dadurch gekennzeichnet, dass - durch periodische Veränderung des Verhältnisses zwischen der Vorschubgeschwindigkeit des Vorformlings (6), welche mittels der Einrichtung (20) zur Steuerung der Geschwindigkeit festgelegt wird, einerseits und dem Austrag aus dem Extruder (1) andererseits zwischen einer Vielzahl unterschiedlicher Werte - die Stärke der Wandung des Vorformlings (6) periodisch verändert wird.
Verfahren nach Anspruch 1, bei welchem das Verhältnis zwischen der Vorschubgeschwindigkeit des Vorformlings (6), welche mittels der Einrichtung (20) zur Steuerung der Geschwindigkeit festgelegt wird, einerseits und dem Austrag aus dem Extruder (1) andererseits über einen ersten Zeitraum im Wesentlichen auf einem ersten Wert konstant gehalten wird, so dass der Vorformling dann eine erste Wandungsstärke erhält, und über einen zweiten Zeitraum auf einem oder mehrere Werte eingestellt wird, die von dem ersten Wert verschieden ist bzw. sind, wobei der zweite Zeitraum beträchtlich kürzer als der erste Zeitraum ist.
Verfahren nach Anspruch 1 oder 2, bei welchem der Austrag aus dem Extruder (1) in periodischen Abständen verändert wird und bei welchem die Vorschubgeschwindigkeit des Vorformlings (6), welche mittels der Einrichtung (20) zum Steuern der Geschwindigkeit festgelegt wird, im Wesentlichen konstant gehalten wird.
Verfahren nach Anspruch 1 oder 2, bei welchem der Austrag aus dem Extruder (1) im Wesentlichen konstant gehalten wird und bei welchem die Vorschubgeschwindigkeit des Vorformlings (6), die mittels der Einrichtung (20) zum Steuern der Geschwindigkeit festgelegt wird, in periodischen Abständen verändert wird.
Verfahren nach Anspruch 4, bei welchem die Vorschubgeschwindigkeit des gestreckten Rohres (6') auf der Austragseite des Dorns (3a), welche mittels der Ziehvorrichtung (50) festgelegt wird, in periodischen Abständen in der Weise verändert wird, dass der Verhältnis zwischen der Vorschubgeschwindigkeit des gestreckten Rohres (6') auf der Austragseite des Dorns (3a) einerseits und des Vorformlings (6) auf der Zuführseite des Dorns (30) andererseits im Wesentlichen konstant gehalten wird.
Verfahren nach Anspruch 2, bei welchem innerhalb des Zeitraums, während dessen oder während eines Teils desselben ein Teil des Vorformlings (6) mit einer Wandungsstärke, die größer ist als die erste Wandungsstärke, die gerade unter Kraftaufwand über den Dom (30) gezogen wird, das Verhältnis zwischen der Vorschubgeschwindigkeit des gestreckten Rohres (6'), welche mittels der Ziehvorrichtung (50) festgelegt wird, einerseits und der Vorschubgeschwindigkeit des Vorformlings (6'), welche mittels der Einrichtung (20) zur Steuerung der Geschwindigkeit festgelegt wird, andererseits größer ist als in dem Zeitraum, während dessen ein Teil des Vorformlings (6) mit der ersten Wandungsstärke, die gerade unter Kraftaufwand über den Dorn (30) gezogen wird, so dass ein Rohrteil mit der größeren Wandungsstärke eine stärkere Streckung in axialer Richtung erfährt als ein Rohrabschnitt mit der ersten Wandungsstärke.
Verfahren nach einem oder mehreren der vorhergehenden Ansprüche, bei welchem der gestreckte Rohr (6') auf der Austragseite des Dehnteils (33) des Dorns (30) in der Weise gekühlt wird, dass der gekühlte Rohr keine weitere Streckung in axialer Richtung erfährt und die Ausbildung der Streckung in axialer Richtung auf den Abschnitt zwischen einer Einrichtung (20) zum Steuern der Geschwindigkeit für den Vorformling (6) und dem austragseitigen Ende des Dorns (30) und/oder auf den Abschnitt zwischen einer Vielzahl von Einrichtungen zur Steuerung der Geschwindigkeit für den Vorformling konzentriert wird, welche auf der Zuführseite des Dorns angeordnet sind.
Verfahren nach einem oder mehreren der vorhergehenden Ansprüche, bei welchem der Vorformling (6) auf der Austragseite der Extruderdüse (3) einer Kalibrierung des Außendurchmessers des Vorformlings (6) unterzogen wird, so dass der Vorformling einen gleichmäßigen Außendurchmesser in diesem Bereich erhält und ein Abschnitt des Vorformlings mit einer größeren Wandungsstärke einen kleineren Innendurchmesser als die angrenzenden Teile des Vorformlings mit einer kleineren Wandungsstärke aufweist.
Verfahren nach einem oder mehreren der vorhergehenden Ansprüche, bei welchem der Vorformling (6) auf der Austragseite der Extruderdüse (3) einer Kalibrierung des Innendurchmessers des Vorformlings (6) unterzogen wird, so dass der Vorformling einen gleichmäßigen Innendurchmesser in diesem Bereich erhält und ein Teil des Vorformlings mit einer größeren Wandungsstärke einen größeren Außendurchmesser als die angrenzenden Teile des Vorformlings mit einer kleineren Wandungsstärke aufweist.
Verfahren nach einem oder mehreren der vorhergehenden Ansprüche, bei welchem der Vorformling (6) in der Weise temperiert wird, dass ein Teil des Vorformlings mit größerer Wandungsstärke im Mittel eine höhere Temperatur aufweist, die an einer Stelle unmittelbar zuführseitig von dem Dehndorn (33) gemessen wird, als ein auf der Austragseite unmittelbar angrenzender Teil des Vorformlings mit einer kleineren Wandungsstärke, der sich bereits auf dem Dorn befindet.
Verfahren nach einem oder mehreren der vorhergehenden Ansprüche, bei welchem in jedem Fall eine Reihe aus einer Vielzahl von Teilen mit einer größeren Wandungsstärke, die sich vergleichsweise nah bei einander befinden, auf dem Vorformling (6) geschaffen wird, woran sich ein beträchtlich längerer Abschnitt des Vorformlings mit einer gleichmäßigen kleineren Wandungsstärke anschließt.
Verfahren nach einem oder mehreren der vorhergehenden Ansprüche, bei welchem der gestreckte Rohr (6') in dem Abschnitt zwischen dem austragseitigen Ende des Dorns (30) und der Ziehvorrichtung (50) einer Kalibrierung des Außendurchmessers des Rohres unterzogen wird.
Verfahren nach einem oder mehreren der vorhergehenden Ansprüche, bei welchem der gestreckte Rohr (6') auf der Austragseite des Ziehvorrichtung (50) an der Stelle eines Rohrteils mit einer größeren Wandungsstärke, oder direkt daneben, unterteilt wird, was zu Rohrabschnitten führt, die an einem oder an beiden axialen Enden einen Endabschnitt mit einer größeren Wandungsstärke als der Rest des Rohrabschnitts aufweisen, der eine gleichmäßige kleinere Wandungsstärke aufweist.
Method for producing a biaxially oriented thermoplastic tube, comprising the extrusion of a tubular preform from thermoplastic material using an extruder (1) which is provided with an extruder die having an inner core,(5) the inner core (5) defining a hollow space in the preform (6), which method furthermore comprises temperature conditioning of the preform (6), so that the preform (6) reaches an orientation temperature which is suitable for the plastics material being used, and forcing the tempered preforms (6) over a mandrel (30), which mandrel (30) comprises an expansion part (33), which brings about expansion in the circumferential direction of the preform (6) passing over it, in such a manner that a stretched tube (6') with plastics material which is stretched in the axial direction and in the circumferential direction is obtained, followed by cooling of the stretched tube (6'), an advancement speed of the preform (6) upstream of the mandrel being set by means of a speed-control means (20) which acts on the preform (6) upstream of the mandrel, and an adjustable advancement speed of the tube (6') downstream of the mandrel (30) being set by means of a drawing device (50) which acts on the stretched tube (6') downstream of the mandrel,
characterized in that - by periodical variation of the ratio of the advancement speed of the preform (6) which is determined by the speed-control means (20), on the one hand, and the output of the extruder (1), on the other hand, between a plurality of different values - the wall thickness of the preform (6) is periodically changed.
Method according to claim 1, in which the ratio of the advancement speed of the preform (6), which is determined by the speed-control means (20), on the one hand, and the output of the extruden (1), on the other hand, is kept substantially constant at a first value for a first period, so that the preform then acquires a first wall thickness, and is set to one or more values which differ from the first value for a second period, which is considerably shorter than the first period.
Method according to claim 1 or 2, in which the output of the extruder (1), is varied periodically and in which the advancement speed of the preform (6), which is determined by the speed-control means (20), is kept substantially constant.
Method according to claim 1 or 2, in which the output of the extruded (1) is kept substantially constant and in which the advancement speed of the preform (6), which is determined by the speed-control means (20), is varied periodically.
Method according to claim 4, in which the advancement speed of the stretched tube (6') downstream of the mandrel (30), which is determined by the drawing device (50), is varied periodically in such a manner that the ratio of the advancement speed of the stretched tube (6') downstream of the mandrel (30), on the one hand, and of the preform (6) upstream of the mandrel (30), on the other hand, is kept substantially constant.
Method according to claim 2, in which, in the period duxing which a part of the preform (6) with a wall thickness which is greater than the first wall thickness is being forced over the mandrel (30) or for part of this period, the ratio of the advancement speed of the stretched tube (6'), which is determined by the drawing device (50), on the one hand, and the advancement speed of the preform (6), which is determined by the speed-control means (20), on the other hand, is greater than in the period during which a part of the preform (6) with the first wall thickness is being forced over the mandrel (30), in such a manner that a tube part with the greater wall thickness acquires a greater axial stretching than a tube part with the first wall thickness.
Method according to one or more of the preceding claims, in which the stretched tube (6') downstream of the expansion part (33) of the mandrel (30) is cooled in such a manner that the cooled tube does not undergo any further axial stretching and the generation of the axial stretching is concentrated in the section between a speed-control means (20) for the preform (6) and the downstream end of the mandrel (30) and/or in the section between a plurality of speed-control means for the preform which are arranged upstream of the mandrel.
Method according to one or more of the preceding claims, in which the preform (6) downstream the extruder die (3) is subjected to calibration of the external diameter of the preform (6), so that the preform acquires a uniform external diameter in that area and a preform section with a greater wall thickness has a smaller internal diameter than the adjoining parts of the preform with a smaller wall thickness.
Method according to one or more of the preceding claims 1-7, in which the preform (6) downstream of the extruder die (3) is subjected to calibration of the internal diameter of the preform (6), so that the preform acquires a uniform internal diameter in that area and a preform part with a greater wall thickness has a greater external diameter than the adjoining parts of the preform with a smaller wall thickness.
Method according to one or more of the preceding claims, in which the prefom (6) is tempered in such a manner that a preform part with a larger wall thickness on average is at a higher temperature, measured at a location immediately upstream of the expansion mandrel (33), than an immediately adjoining downstream preform part with a smaller wall thickness which is already on the mandrel.
Method according to one or more of the preceding claims, in which in each case a series of a plurality of parts with a larger wall thickness which are located relatively close together is created in the preform (6), followed by a considerably longer preform section with a uniform, smaller wall thickness.
Method according to one or more of the preceding claims, in which the stretched tube (6'), in the section between the downstream end of the mandrel (30), and the drawing device (50), is subjected to calibration of the external diameter of the tube.
Method according to one or more of the preceding claims, in which the stretched tube (6') downstream of the drawing device (50) is divided at the location of or next to a tube part with a larger wall thickness, resulting in tube sections which at one or both axial ends have an end part with a greater wall thickness than the remainder of the tube section, which has a uniform, smaller wall thickness.
Procédé de production d'un tube en matière thermoplastique à orientation biaxe, comprenant l'extrusion d'une préforme tubulaire à partir d'une matière thermoplastique à l'aide d'une extrudeuse (1) équipée d'une filière d'extrusion comportant un noyau interne (5) qui définit un espace creux dans la préforme (6), procédé qui comprend, en outre, un conditionnement thermique de la préforme (6), afin que celle-ci atteigne une température d'orientation adaptée pour la matière plastique utilisée, et le passage de force de la préforme (6) tempérée sur un mandrin (30) comprenant une partie d'élargissement (33) qui provoque un élargissement dans la direction circonférentielle de la préforme (6) passant sur elle, de façon à obtenir un tube étiré (6') dont la matière plastique est étirée dans la direction axiale et dans la direction circonférentielle, opérations qui sont suives par un refroidissement du tube étiré (6'), une vitesse d'avancement de la préforme (6) en amont du mandrin étant déterminée à l'aide d'un moyen de réglage de vitesse (20) qui agit sur la préforme (6) en amont du mandrin, tandis qu'une vitesse d'avancement réglable du tube (6') en aval du mandrin (30) est déterminée au moyen d'un dispositif de traction (50) qui agit sur le tube étiré (6') en aval du mandrin,
caractérisé en ce que - grâce à une variation périodique du rapport de la vitesse d'avancement de la préforme (6), qui est déterminée par le moyen de réglage de vitesse (20), d'une part, et de la sortie de l'extrudeuse (1), d'autre part, entre plusieurs valeurs différentes - l'épaisseur de la paroi de la préforme (6) est modifiée périodiquement.
Procédé selon la revendication 1, dans lequel le rapport de la vitesse d'avancement de la préforme (6), qui est déterminée par le moyen de réglage de vitesse (20), d'une part, et de la sortie de l'extrudeuse (1), d'autre part, est maintenu sensiblement constant à une première valeur pendant une première période, afin que la préforme acquière alors une première épaisseur de paroi, et est fixé à une ou plusieurs valeurs qui diffèrent de la première valeur pendant une seconde période considérablement plus courte que la première période.
Procédé selon la revendication 1 ou 2, dans lequel la sortie de l'extrudeuse (1) est modifiée périodiquement, et dans lequel la vitesse d'avancement de la préforme (6), qui est déterminée par le moyen de réglage de vitesse (20), est maintenue sensiblement constante.
Procédé selon la revendication 1 ou 2, dans lequel la sortie de l'extrudeuse (1) est maintenue sensiblement constante, et dans lequel la vitesse d'avancement de la préforme (6), qui est déterminée par le moyen de réglage de vitesse (20), est modifiée périodiquement.
Procédé selon la revendication 4, dans lequel la vitesse d'avancement du tube étiré (6') en aval du mandrin (30), qui est déterminée par le dispositif de traction (50), est modifiée périodiquement de telle façon que le rapport de la vitesse d'avancement du tube étiré (6') en aval du mandrin (30), d'une part, et de la préforme (6) en amont du mandrin (30), d'autre part, soit maintenu sensiblement constant.
Procédé selon la revendication 2, dans lequel, pendant la période au cours de laquelle une partie de la préforme (6) présentant une épaisseur de paroi supérieure à la première épaisseur de paroi est contrainte à passer sur le mandrin (30) ou pendant une partie de cette période, le rapport de la vitesse d'avancement du tube étiré (6'), qui est déterminée par le dispositif de traction (50), d'une part, et de la vitesse d'avancement de la préforme (6), qui est déterminée par le moyen de réglage de vitesse (20), d'autre part, est plus grand que pendant la période au cours de laquelle une partie de la préforme (6) présentant la première épaisseur de paroi est contrainte à passer sur la mandrin (30), de telle façon qu'une partie du tube présentant la plus grande épaisseur de paroi acquière un étirement axial plus important qu'une partie du tube présentant la première épaisseur de paroi.
Procédé selon l'une au moins des revendications précédentes, dans lequel le tube étiré (6') en aval de la partie d'élargissement (33) du mandrin (30) est refroidi de telle façon que le tube refroidi ne subisse plus aucun étirement axial supplémentaire et que la production de l'étirement axial soit concentrée dans la portion située entre un moyen de réglage de vitesse (20) pour la préforme (6) et l'extrémité aval du mandrin (30) et/ou dans la portion située entre plusieurs moyens de réglage de vitesse pour la préforme qui sont disposés en amont du mandrin.
Procédé selon l'une au moins des revendications précédentes, dans lequel la préforme (6) est, en aval de la filière d'extrusion (3), soumise à un calibrage de son diamètre extérieur, afin d'acquérir un diamètre extérieur uniforme dans cette zone et qu'une portion de la préforme présentant une plus grande épaisseur de paroi ait un diamètre intérieur plus faible que les parties adjacentes de celle-ci qui présentent une épaisseur de paroi plus faible.
Procédé selon l'une au moins des revendications 1 à 7 précédentes, dans lequel la préforme (6) est, en aval de la filière d'extrusion (3) soumise à un calibrage de son diamètre intérieur, afin d'acquérir un diamètre intérieur uniforme dans cette zone et qu'une partie de la préforme présentant une plus grande épaisseur de paroi ait un diamètre extérieur plus grand que les parties adjacentes de celle-ci qui présentent une épaisseur de paroi plus faible.
Procédé selon l'une au moins des revendications précédentes, dans lequel la préforme (6) est tempérée de telle façon qu'une partie de la préforme présentant une plus grande épaisseur de paroi en moyenne soit à une température, mesurée en un endroit situé immédiatement en amont de la partie d'élargissement (33), plus élevée qu'une partie aval immédiatement adjacente de la préforme, qui présente une épaisseur de paroi plus faible et qui est déjà sur le mandrin
Procédé selon l'une au moins des revendications précédentes, dans lequel, dans chaque cas, une série de plusieurs parties présentant une plus grande épaisseur de paroi qui sont situées relativement près les unes des autres est créée dans la préforme (6), série qui est suivie par une portion de préforme sensiblement plus longue et présentant une épaisseur de paroi uniforme plus faible.
Procédé selon l'une au moins des revendications précédentes, dans lequel le tube étiré (6') est, dans la portion située entre l'extrémité aval du mandrin (30) et le dispositif de traction (50), soumis à un calibrage de son diamètre extérieur.
Procédé selon l'une au moins des revendications précédentes, dans lequel le tube étiré (6') est, en aval du dispositif de traction (50), divisé à l'endroit ou à proximité d'une partie de tube présentant une plus grande épaisseur de paroi, ce qui se traduit par des portions de tube qui, au niveau de l'une au moins de leurs extrémités axiales, ont une partie d'extrémité présentant une plus grande épaisseur de paroi que le reste de la portion de tube, qui a une épaisseur de paroi uniforme plus faible.