AU2015203536A1

AU2015203536A1 - Methods and devices for manufacturing biaxially oriented tubing

Info

Publication number: AU2015203536A1
Application number: AU2015203536A
Authority: AU
Inventors: Jan-Mark Bosch; Hendrik Jan Carel Jansen Klomp; Jan Visscher
Original assignee: Hoppmann International BV
Current assignee: Hoppmann International BV
Priority date: 2009-10-19
Filing date: 2015-06-25
Publication date: 2015-07-16
Anticipated expiration: 2030-10-18
Also published as: AU2015203535A1; AU2015203536B2

Abstract

A method for producing a biaxially oriented tube from thermoplastic material, wherein a tube 5 in preform condition is extruded from thermoplastic material using an extruder which is provided with an extruder die head having an inner die member, the inner die member forming a lumen in the tube in preform condition, wherein the tube in preform condition is subjected to a temperature conditioning by one or more tempering devices so that a tempered tube is obtained having an orientation temperature which is suitable for the 0 thermoplastic material, and wherein use is made of an expansion device in the lumen downstream of the extruder, said expansion device including: - a non-deformable expansion part having a gradually increasing diameter to a maximum diameter at a downstream end thereof, which expansion part is contacted by the tube and exerts an expanding force on the tube so as to bring about expansion of the 5 tempered tube in circumferential direction, - a run-off part adjoining the expansion part at the downstream end thereof, wherein the method includes drawing the tempered tube over the expansion device using a drawing device which is arranged downstream of the expansion device and acts on the tube, in such a manner that said tube is drawn over the expansion device and transformed from a 0 tube in preform condition into a biaxially oriented tube with thermoplastic material being oriented in axial direction and in circumferential direction, wherein said biaxially oriented tube is cooled, and the run-off part of the expansion device has a reduced diameter section having a smaller diameter than the maximum diameter of the expansion part, 25 wherein use is made of at least one outer diameter ring member that is arranged at the location of the reduced diameter section of the run-off part of the expansion device and around said reduced diameter section, and wherein the oriented tube passes through the at least one outer diameter ring member while being in contact with said at least one outer diameter ring member, the outer 30 diameter ring member and the reduced diameter section being dimensioned such that seizing of the oriented tube between the run-off part and the at least one outer diameter ring member is avoided, the inside of the oriented tube being radially spaced from the reduced diameter section, and in that the oriented tube is cooled externally while passing over the run-off part by 35 a first external cooling device.

Description

-1 METHODS AND DEVICES FOR MANUFACTURING BIAXIALLY ORIENTED TUBING The present invention relates to methods and devices for manufacturing biaxially oriented tubing of thermoplastic material. 5 The invention relates in general to the issue of establishing production processes and production installations that allow to produce biaxially oriented tubing of thermoplastic material, the oriented tubing having a desired uniformity of the final dimensions of the oriented tubing as well as good strength properties, e.g. as the production of rigid pipes, e.g. o pressure pipes for transportation of water or gas is envisaged. When producing biaxially oriented tubing of thermoplastic material, e.g. pipes of polyvinylchloride, it has proven to be difficult to produce tubing with uniform final dimensions. Such uniformity is desirable, e.g. as biaxially oriented tubing elements, e.g. pressure pipes, 5 e.g. for transportation of water, are interconnected end-to-end, e.g. via socket connections. The invention relates to a method for producing a biaxially oriented tube from thermoplastic material, wherein a tube in preform condition is extruded from thermoplastic material using an extruder which is provided with an extruder die head having an inner die member, the 0 inner die member forming a lumen in the tube in preform condition, wherein the tube in preform condition is subjected to a temperature conditioning by one or more tempering devices so that a tempered tube is obtained having an orientation temperature which is suitable for the thermoplastic material, and wherein use is made of an expansion device in the lumen downstream of the extruder, said expansion device comprising: 25 - a non-deformable expansion part having a gradually increasing diameter to a maximum diameter at a downstream end thereof, which expansion part is contacted by the tube and exerts an expanding force on the tube so as to bring about expansion of the tempered tube in circumferential direction, - a run-off part adjoining the expansion part at the downstream end thereof, 30 wherein the method comprises drawing the tempered tube over the expansion device using a drawing device which is arranged downstream of the expansion device and acts on the tube, in such a manner that said tube is drawn over the expansion device and transformed from a tube in preform condition into a biaxially oriented tube with thermoplastic material being oriented in axial direction and in circumferential direction, 35 wherein said biaxially oriented tube is cooled.

-2 In EP 823 873 a method is disclosed wherein use is made of a rigid expansion device comprising a non-deformable expansion part having an increasing diameter to a maximum diameter at downstream end thereof, which expansion part is contacted by the tube and exerts an expanding force on the preform so as to bring about expansion of the tempered 5 tube in preform condition in circumferential direction. A non-deformable run-off part adjoins the expansion part at the downstream end thereof and generally has a diameter not exceeding the maximum diameter of the expansion part. In EP 823 873, figure 5, the oriented tube is cooled both internally and externally while 0 passing over the run-off part of the expansion device. An outer diameter calibrating ring is arranged spaced a distance downstream from the run-off part of the expansion device. This calibrating ring acts to reduce the diameter of the oriented tube. The calibrating ring is displaceable in axial direction based on control signals obtained via a measuring device that measures the dimensions (diameter, wall thickness, shape) of the oriented tube further 5 downstream. This particular prior art method of EP 823 873 has shown to produce acceptable results with regard to the final dimensions of the oriented tubing. It is noted that biaxially oriented thermoplastic tubes made according to this method, and according to the inventive methods, o are primarily contemplated for pressure applications, e.g. as pressure water transport pipe that delivers water, e.g. potable water, over a long distance. The known tubes are made of polyvinylchloride. Any discussion of documents, devices, acts or knowledge in this specification is included to 25 explain the context of the invention. It should not be taken as an admission that any of the material formed part of the prior art base or the common general knowledge in the relevant art in Australia on or before the priority date of the claims herein. Comprises/comprising and grammatical variations thereof when used in this specification are 30 to be taken to specify the presence of stated features, integers, steps or components or groups thereof, but do not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof. It would be desirable to provide measures that allow for improvements over the prior art or at 35 least to provide a useful alternative.

-3 It would be desirable to provide a method that allows for enhanced strength of the oriented tubing. It would also be desirable to provide a method that allows for enhanced uniformity of the 5 tubing, in particular with respect to wall thickness and cross-sectional shape. It would also be desirable to provide a method wherein no cold liquid, e.g. cooling water, is conveyed to the expansion device. o It would also be desirable to provide a method wherein friction between the expansion device and the tube is reduced, without the measures taken to do so impairing the quality of the oriented tubing and/or the effectiveness of the production process. In accordance with the invention, there is provided a method as in claim 1, wherein the run 5 off part, which is downstream of the expansion part, has a reduced diameter section having a smaller diameter than the maximum diameter of the expansion part, and wherein use is made of at least one outer diameter ring member that is arranged at the location of the reduced diameter section and around said reduced diameter section, and wherein the oriented tube passes through the outer diameter ring member while being in 0 contact with said outer diameter ring member, the outer diameter ring member and the reduced diameter section being dimensioned such that seizing of the oriented tube between the expansion device and the at least one outer diameter ring member is avoided, preferably the inside of the oriented tube being radially spaced from the reduced diameter section, and wherein the oriented tube is cooled externally while passing over the run-off part by a 25 first external cooling device. In an embodiment of this method the non-deformable expansion part causes the entirety of the circumferential expansion of the tube from its preform condition to the expanded condition. Possibly a relatively thin layer of fluid, acting primarily as a lubricant, is present 30 between the expansion device and the tube. For instance the expansion device is embodied as a rigid expansion device, with a run-on part, expansion part, and run-off part of non deformable design, e.g. of metal. Such designs, often indicated as expansion mandrel, are known in the art. 35 In another embodiment of this method the non-deformable expansion part only causes a part of the total expansion of the tube. The other part of the desired expansion is then caused by forming a fluid volume between the expansion device and the tube, e.g. upstream and/or -4 downstream of the region wherein the tube contacts the expansion part. These one or more fluid volumes, e.g. gas filled, are then operated to exert an internal fluid pressure on the tube that causes the other part or parts of the expansion of the tube. 5 In a preferred embodiment the maximum diameter of the non-deformable expansion part governs the final stage of expansion in circumferential direction of the tube. In this embodiment any part of the expansion device - other than the reduced diameter section downstream of said maximum diameter has a diameter at most equal to the maximum diameter of the expansion part. 0 In another possible embodiment a downstream portion, e.g. an end portion, of the run-off part has a greater diameter than the maximum diameter of the expansion part, and a fluid volume with pressurized fluid between the reduced diameter section and the tube is used to cause further expansion of the tube. 5 It is preferred for the run-off part to be of non-deformable design. However it is also envisaged e.g. that the run-off part includes an expandable portion, e.g. an inflatable plug, e.g. at the downstream end thereof, e.g. limiting the downstream end of the reduced diameter section. 0 The reduced diameter section preferably has a diameter that is at least 4 millimetres less than the maximum diameter of the expansion part. Preferably the diameter of the reduced diameter section is about twice the wall thickness of the tube passing over the run-off part of the expansion device less than the maximum diameter of the expansion part. Preferably the 25 reduced diameter section has a diameter of at least 80% of the maximum diameter of the expansion part. The outer diameter ring member is arranged around the reduced diameter section, with the radial spacing between said ring member and the reduced diameter section being more than 30 the projected wall thickness of the tube at said location, so that radial play remains that allows for a possible variation in the wall thickness of the tube during the production process at said location, e.g. to minor disturbances in the process, without the risk that said tube becomes stuck between the ring member and the reduced diameter section of the run-off part. Preferably a radial spacing is maintained between the reduced diameter section and the 35 inside of the oriented tube.

-5 Preferably each outer diameter ring member is non-deformable, at least as the diameter of its opening through which the tube passes is concerned, under the influence of the contact with the tube passing through the opening of the ring member. E.g. the ring member is made of a rigid material, e.g. a metal or other thermally conductive material. 5 In a practical embodiment each ring member has an axial dimension less than the diameter of the opening therein for the tube. E.g. a ring member has an axial length of between 0.5 and 5 centimetres. It is however also possible that a ring member is formed as an elongated sleeve, e.g. having a length greater than the diameter of the opening therein for the tube. 0 In a possible embodiment an outer diameter ring member includes one or more internal conduits, e.g. annular internal conduits, through which a cooling fluid is passed, e.g. cooling water, to effect a cooling of the contact surface with the oriented tube. In a possible embodiment the first external cooling device is integrated with the one or more outer 5 diameter ring members, as each ring member has one or more internal conduits through which cooling fluid is passed, e.g. a single outer diameter ring member being used having a length greater that the diameter of the opening therein for the tube, e.g. between 1 and 2 times said diameter. o The external cooling of the tube by the first external cooling device while passing over the run-off section is preferably performed in the absence of internal cooling of the tube while passing over the expansion device, or even more preferably also in the absence of any internal cooling downstream of the expansion device. In this regard referral is made to EP 823 873 wherein not only an external cooling of the 25 oriented tube is performed, but also an internal cooling of the tube is performed by a cooling liquid film between the tube and the run-off part in combination with the passing of the tube through an outer diameter calibrating ring member downstream of the expansion device. It has been found that supplying the cooling liquid to the expansion device is problematic in view of obtaining a uniformly tempered tube in preform condition as the cooling liquid is 30 supplied via a cooling liquid duct in the anchoring rod of the expansion device. It has also been observed that this particular prior art approach may cause deformation of the rather cold inner side of the oriented tube due to the passing through the downstream calibrating ring member, which deformation in cold condition is considered by the present inventors to have a negative effect on the strength of the finally obtained tube. 35 It is noted that the method of the invention allows dispensing with an outer diameter calibration of the oriented tube downstream of the expansion device, which is therefore a -6 preferred embodiment of said method. In order to arrive at a tube with desired dimensions, without using an outer diameter calibration downstream of the expansion device, it has now been found possible to rely on the use of one or more outer diameter ring members at the location of the run-off part in combination with external cooling of the tube at said location, 5 preferably when also using a second external cooling device downstream of the expansion device, and in combination with the so-called snap-back effect. This snap-back effect has already been observed and documented by professionals in the art, and is visible as a reduction of the tube diameter directly downstream of the expansion device. It is believed that this snap-back effect primarily is the result of the tensile force exerted by the o downstream drawing device and the condition of the thermoplastic material of the tube as it leaves the expansion device and is no longer internally supported by said expansion device. In a preferred embodiment use is made of an upstream outer diameter ring member and a downstream outer diameter ring member, said ring members being arranged in series and 5 spaced apart. Through these ring members the oriented tube passes at the location of the reduced diameter section of the run-off part. By providing multiple, preferably two, ring members at spaced apart axial locations along the reduced diameter section of the run-off part, various possibilities are provided for the operator to influence the production process and the finally obtained tubing. 0 Preferably the first external cooling device cools the oriented tube between the upstream and downstream outer diameter ring members. Preferably an intense external cooling is effected here, preferably by the outer surface of the tube being exposed, so not covered by the one or more ring members, and subjected to sprays or jets of cooling liquid, e.g. water. 25 In a preferred embodiment at least one outer diameter ring member, e.g. both an upstream and a downstream ring member, is embodied as a constrictive outer diameter ring member, said ring member exerting a radial constrictive force on the oriented tube passing there through during the production process, thereby reducing the outer diameter of the oriented 30 tube, at least over a short axial length. In a preferred embodiment the upstream outer diameter ring member exerts a constrictive force on the oriented tube which contributes to a sealing engagement of the oriented tube with the expansion device at the transition between the expansion part and the run-off-part. As will be explained in more detail below this approach is e.g. favourable when a fluid, that is a liquid or a gas, is introduced between one 35 or more parts of the expansion device on the one hand and the tube on the other hand.

-7 In a preferred embodiment at least one outer diameter ring member, e.g. an upstream ring member, is displaceable in axial direction. By suitable selection and/or adaptation of the axial position of the one or more outer diameter ring members with respect to the run-off part, e.g. the snap-back effect can be influenced and thus the final dimension of the oriented tube 5 controlled. In particular it is envisaged that the axial displacement of one or more outer diameter ring members is effected in combination with a control, and - possibly automatic adjustment, of the cooling effect of the first external cooling device. In a very practical embodiment the first external cooling device operates with one or more 0 nozzles emitting sprays or jets of cooling liquid, e.g. cooling water. In a preferred embodiment the first external cooling device is adapted and operated to adjust the length and/or location with respect to the expansion device of the stretch of the oriented tube that is affected by the first external cooling device. It has been found that by suitable 5 selection of the length, and preferably also the location, of the affected stretch with respect to the expansion device, the occurrence of the snap-back effect can be influenced, and so the diameter of the tube, without needing to use an outer diameter calibration downstream of the expansion device. o In a preferred embodiment the first external cooling device comprises an upstream shield member and a downstream shield member, said shield members delimiting the stretch of oriented tube that is affected by the first external cooling device, e.g. the sprays or jets of cooling water. Preferably the outer surface of the tube is exposed between said shield members, the device having nozzles spraying or jetting cooling liquid directly onto said 25 exposed surface. Preferably the first external cooling device is effective directly downstream of the transition between the expansion part and the run-off part, in particular when no internal cooling is performed as is preferred. 30 Preferably at least one of the shield members of the first external cooling device, preferably both, is displaceable in axial direction, thereby allowing to adjust the length and/or the location of the stretch of tube that is affected by the spray of cooling liquid. It will be appreciated that by controlling the length and/or position of the shield members during the 35 production process, e.g. automatically or operator controlled, possibly by hand, the cooling of the oriented tube can be controlled, even more when - as is common - the intensity of the cooling spray can be controlled as well.

-8 It will be appreciated that the one or more displaceable shield members could be construed for a manual adjusting of the axial position thereof. However in a more advanced embodiment - as is preferred - a motorized drive assembly, e.g. including one or more screw 5 spindles, is provided for said one or more displaceable shield members. In a very practical embodiment an outer diameter ring member is integral with a shield member, more preferably the upstream and downstream ring member each being integral with the upstream and downstream shield member. As a result the first external cooling 0 device is effective over the stretch of oriented tubing between both ring members, preferably at least one thereof being movable in axial direction. In a preferred embodiment the run-off part of the expansion device comprises an increased diameter portion, preferably non-deformable, downstream of the one or more outer diameter 5 ring members and delimiting the downstream end of a reduced diameter section, said increased diameter portion having a greater diameter than said reduced diameter section. The method is then performed such that the oriented tube, preferably in a sealing manner, engages or contacts the increased diameter portion. The increased diameter portion then acts as an internal support for the oriented tube, and in a non-deformable embodiment 0 contributes to the uniformity of the dimensions of the tube. The presence of an increased diameter portion is advantageous when a fluid is introduced between one or more parts of the expansion device on the one hand and the tube in preform condition and/or oriented tube on the other hand. 25 The increased diameter portion can have a diameter that is the same as the maximum diameter of the expansion part, or a smaller diameter. However, as indicated above, it is also possible for the increased diameter portion to have a larger diameter than the maximum diameter of the expansion part, preferably a pressurized gas volume being then delimited at 30 its downstream end by said increased diameter portion and the internal gas pressure causing a final stage of circumferential expansion of the tube. During normal production the tube will then also contact the expansion part of the expansion device, thereby effecting an earlier stage of expansion of the tube, possibly preceded by yet another expansion stage effected by internal fluid pressure caused by an upstream fluid volume. 35 In a highly preferred embodiment the run-off part has a single reduced diameter section, and an upstream and downstream outer diameter ring member are arranged at the location of -9 said single reduced diameter section and around said single reduced diameter section. The upstream outer diameter ring member may contribute to or effect a sealing engagement or contact of the tube with the expansion device in a region at or near the maximum diameter of the expansion part. The downstream outer diameter ring member exerts a constrictive force 5 on the tube which may contribute to or effect a sealing engagement of the tube with a downstream located increased diameter portion of the expansion device. This method is highly advantageous when a fluid is introduced between the expansion device, e.g. the run off part thereof, and the tube. The fluid e.g. is a pressurized gas, such as air, the sealing engagement of the tube at both axial locations of the run-off part avoiding an uncontrolled 0 escape of fluid and thus uncontrolled fluctuation of the fluid volume, be it a thin film (e.g. when a liquid is used, e.g. heated water, is used) or an annular volume with significant radial thickness, e.g. an air volume, e.g. for causing some additional circumferential expansion. Downstream of the expansion device, preferably in close vicinity to the expansion device, a 5 further external cooling of the tube is advised to cool down oriented tube to a further degree. For this reason a second external cooling device is arranged, preferably relative close, downstream of the expansion device and is adapted and operated to externally cool the oriented tube. The second external cooling device is controllable independent from the first external cooling device arranged at the location of the run-off part. Preferably the second 0 external cooling device is arranged spaced a distance downstream from the first external cooling device. When the run-off part comprises an increased diameter portion at the downstream end of a single reduced diameter section, it is preferred for a possible second external cooling device 25 to be arranged downstream thereof. Preferably the second external cooling device comprises one or more cooling liquid spray nozzles adapted and operated to spray or jet cooling liquid, e.g. water, onto the exterior of the oriented tube. 30 Preferably the second external cooling device is arranged such that the cooling effect thereof starts at the position where the snap-back effect - wherein the diameter of the tube reduces downstream of the expansion with no more internal support of the oriented tube - takes place. 35 -10 Preferably a dry zone is created between said first and second external cooling device. This is considered to avoid or at least reduce the formation of visual effects, e.g. rings, on the outside of the tube by cooling water. 5 Preferably the second external cooling device is movable in axial direction, e.g. to adjust its position, primarily of the upstream end thereof, to the occurrence of the snap-back effect. Preferably a displacement device, preferably motorized, is associated with the second external cooling device to effect such a motion. o Preferably the second external cooling device comprises an upstream shield member delimiting the upstream end of the stretch of oriented tubing cooled by said second external cooling device, said upstream shield member preferably being movable in axial direction. Preferably the upstream end of the second external cooling device, e.g. the upstream shield 5 member thereof, has a flexible annular lip engaging the oriented tube so that no noticeable constrictive force is exerted by said flexible annular lip on the oriented tube. In an advantageous embodiment use is made of a measuring device for measuring at least one of the outer diameter of the oriented tube, the wall thickness of the oriented tube, and 0 the cross-sectional profile thereof, e.g. the outer diameter and the wall thickness, which measuring device is arranged downstream of the expansion device. A control device is provided to control the first external cooling device and/or the second external cooling device, preferably both when they are both present. 25 The measuring device is linked to the control device so as to control the cooling by said first and/or second external cooling device, e.g. the intensity of the cooling, thereby controlling the snap-back effect - wherein the diameter of the tube reduces - which takes place directly downstream of the expansion device and thereby controlling the diameter of the oriented tube. This can then be done without the need for any further outer diameter calibration 30 downstream of the expansion device, as is preferred. In a preferred embodiment the control device is provided to control the first external cooling device with regard to at least the length and/or location with respect to the expansion device of the stretch of oriented tube that is affected by the first external cooling device. 35 Possibly the control device is adapted such that the length of the stretch of tube that is affected by the first external cooling device is decreased to obtain an increased snap-back -11 effect and thus increased diameter reduction, and wherein said length is increased to obtain a reduced snap-back effect and thus decreased diameter reduction. In a possible embodiment the second external cooling device, or an upstream shield member 5 of the second external cooling device, is movable in axial direction, and the measuring device is linked to a control device provided to control the second external cooling device. The measuring device is linked to said control device of the second external cooling device in order to control the starting point of the cooling of the tube by the second external cooling device, e.g. via controlling the position of the upstream shield member thereof. 0 Even though it is preferred in the invention to dispense with internal cooling of the tube whilst passing over the expansion device, it is preferred to perform the method with the presence of a fluid introduced between one or more parts of the expansion device on the one hand and the tube on the other hand. 5 The fluid can be a liquid e.g. water, e.g. heated water so as to avoid a noticeable cooling action on the inner side of the tube, preferably the fluid supply device including a controllable heater being provided for heating the liquid. o In a preferred embodiment the fluid is a pressurized gas, preferably air, preferably a source for pressurized gas, e.g. air, with associated pressure control assembly being provided. More preferably a heated gas, e.g. heated air, is used, preferably a controllable heater being provided for heating the gas. 25 In a preferred embodiment a fluid is introduced at least between the run-off part of the expansion device and the tube, preferably at least between the reduced diameter section thereof and the tube. Preferably the tube sealingly engages the expansion device in a region at or near the maximum diameter of the expansion part, and preferably the tube sealingly engages the run-off part downstream of the reduced diameter section, preferably the 30 increased diameter portion thereof . As explained above provision of an upstream and a downstream outer diameter ring member is proposed to contribute to or effect said sealing engagements. In a preferred embodiment a first fluid volume is present between the expansion device and 35 the tube at a position upstream of the maximum diameter of the expansion part, and a second fluid volume is present between the run-off part and the tube. The sealing engagement in a region at or near the maximum diameter of the expansion part generally -12 prevents an uncontrolled communication between the two fluid volumes and thus e.g. instability of said fluid volumes and/or mixing of fluids, e.g. upstream pressurized air for the first volume and a liquid, e.g. heated water being used for the second volume. 5 In a possible embodiment use is made of an expansion device with a valve controlled passage in communication with the first fluid volume and with the second fluid volume, wherein the expansion device includes at least one fluid supply conduit which introduces fluid into the first and/or second fluid volume. As mentioned above the sealing engagement at the maximum diameter avoids an uncontrolled communication between said fluid volumes. The 0 valve controlled passage however allows the operator to e.g. equalize pressure in both volumes, or first establish a first fluid filled volume upstream of the maximum diameter sealing region and then let the fluid flow into the second volume downstream of said region. One or more pressure sensors may be provided to sense the actual fluid pressure in a fluid volume. The valve controlled passage, including the valve, can be integrated entirely in the 5 expansion device in the lumen of the tube. However it is also possible for this valve controlled passage to be present outside of the tube and extruder, e.g. as part of the external portion of the fluid supply device. It is also an option that a first fluid supply duct introduces fluid into the first fluid volume and a o second fluid supply duct introduces fluid into the second fluid volume. The fluids may be the same or different, e.g. water and air. In particular it is envisaged in this preferred embodiment to provide the expansion device with one or more fluid supply ducts having one or more ports in the outer surface of the run 25 on part and/or expansion part preferably the run-on part being provided with a sealing member. Possibly, e.g. when a liquid fluid is used that is entrained by the moving tube, one or more fluid discharge ducts are provided in the expansion device, said ducts having one or more 30 discharge ports in the outer surface of e.g. the run-off part, e.g. near the downstream end of the reduced diameter section. In particular when use is made of a fluid between the expansion device and the tube, the fluid forming a film or thicker annular volume at least in the reduced diameter section of the 35 run-off part of the expansion device, then it is considered advantageous to make use of two outer diameter ring members spaced apart from one another and even more advantageous to embody the run-off part with an increased diameter portion delimiting the downstream end -13 of the reduced diameter section, such that the oriented tube sealingly engages said increased diameter portion. This avoids or at least limits any leakage of fluid into the lumen of the oriented tube downstream of the expansion device, and thus avoids undesirable instability of the fluid volume. Most preferably the downstream outer diameter ring member is 5 located somewhat upstream of the increased diameter portion, thereby enhancing the sealing contact between the tube and the increased diameter portion. In a possible embodiment one or more rollers are arranged below oriented tube so as to support said tube, e.g. below the run-off part of the expansion device. 0 In a practical embodiment an upstream replaceable ring is fitted on the expansion device, which ring defines the maximum diameter of the expansion part, possibly the adjoining portion of the run-off part having a smaller diameter, so that a stepped transition is present between the parts. The replaceable ring allows for relatively easy change of the maximum 5 diameter of the expansion device as well as replacement of said portion in case of wear. In a practical embodiment a downstream replaceable ring is fitted at the downstream end of the run-off section, the replaceable ring having a diameter greater than the upstream portion of the run-off part of the expansion device. This allows for relatively easy change of the o diameter of the expansion device at said downstream location as well as replacement of said zone in case of wear. In an embodiment an upstream replaceable ring is fitted at a transition from the expansion part to the run-off part of the expansion device, the replaceable ring defining the maximum 25 diameter of the expansion part. In an embodiment a downstream replaceable ring is fitted at the downstream end of the run off section, the replaceable ring having a diameter greater than the upstream portion of the run-off part. 30 The invention also relates to an installation as described in claim 28. The invention also relates to the combination of an expansion device and at least one outer diameter ring member for use in a method for producing a biaxially oriented tube from 35 thermoplastic material, wherein a tempered tube in preform condition is drawn over the expansion device and passes through the at least one outer diameter ring member, wherein the expansion device comprises: -14 - a non-deformable expansion part having a gradually increasing diameter to a maximum diameter at a downstream end thereof, which expansion part is adapted to be contacted by the tube and to exerts an expanding force on the tube so as to bring about expansion of the tempered tube in circumferential direction, 5 - a run-off part adjoining the expansion part at the downstream end thereof, wherein the run-off part has a reduced diameter section, preferably a single reduced diameter section, having a smaller diameter than the maximum diameter of the expansion part, and wherein the at least one outer diameter ring member is arranged around the reduced 0 diameter section, so that - in use - the oriented tube passes through the outer diameter ring member while being in contact with said outer diameter ring member, the outer diameter ring member and the reduced diameter section being dimensioned such that seizing of the oriented tube between the expansion device and the at least one outer diameter ring member is avoided. 5 The present invention also relates to an expansion device as included in one or more of these installations for performing one or more of the methods of the invention, as an existing installation may be enhanced by replacing the existing expansion device for such a novel expansion device. 0 The present invention also relates to a first external cooling device as included in one or more of these installations for performing one or more of the methods of the invention, as an existing installation may be enhanced by adding said first external cooling device, or replacing the existing external cooling device for such a novel first external cooling device. 25 The present invention also relates to a second external cooling device as included in one or more of these installations for performing one or more of the methods of the invention, as an existing installation may be enhanced by adding said second external cooling device, or replacing the existing external cooling device for such a novel second external cooling 30 device. The methods according to the invention are for instance suitable to produce individual tube elements that are later provided with a socket in a socketing operation, allowing tubes to be connected end to end by inserting an end into a socketed end of another tube, the socket 35 preferably including a sealing ring.

-15 The methods according to the invention may also include the further step of making individual biaxially oriented tubing elements by severing a tubing element from the tube that extends from the extruder, over the expansion device, and beyond the drawing device downstream of the expansion device, e.g. tubing elements having a length between 5 and 15 5 metres, e.g. 6 metres, and the step of providing a socket on an end of each individual tubing element so that individual tubing elements are connectable via a socket connection. The present invention also relates to a biaxially oriented tube obtained with a method according to the invention. In a preferred embodiment the tube is a biaxially oriented tube of 0 polyvinylchloride. In a preferred embodiment the tube obtained is a water or gas transport pipes, e.g. for potable water, e.g. of polyvinylchloride. For example it is envisaged to produce with a method according to the invention a biaxially oriented pipe, e.g. of PVC, having a pressure rating above 8 Bar, e.g. of 12,5 Bar, at 20C, 5 e.g. with an outer diameter between 63 and 630 millimetres. The wall thickness of the biaxially oriented pipe produced with a method according to the invention may lie in practice between, for example, 3 and 10 millimetres. o The invention and preferred embodiments thereof will now be described with reference to the drawings. In the drawings: Fig. 1a, 1b and 1c show schematically an example of an installation for producing biaxially oriented thermoplastic tubing which includes the present invention, Fig. 2 shows schematically in longitudinal section a part of the installation of figures 1 a,b,c, 25 Figs. 3a and 3b show schematically in perspective view and in longitudinal section components of the part of the installation of figure 2, and Fig. 4 shows schematically in longitudinal section a portion of the components of figure 3, Fig. 5 shows schematically in longitudinal section a portion of an installation for producing biaxially oriented thermoplastic tubing according to the invention; 30 Fig. 6 shows schematically in longitudinal section a portion of an installation for producing biaxially oriented thermoplastic tubing according to the invention, Figs 7a and 7b show schematically in longitudinal section a portion of an installation for producing biaxially oriented thermoplastic tubing to illustrate a preferred embodiment of the invention, 35 Fig. 7c shows a detail of a variant of the installation of figures 7a and 7b.

-16 Figures 1 a, 1 b and 1c are not to scale and schematically show consecutive portions of an example of an installation for producing biaxially oriented thermoplastic tubing. The installation is shown to elucidate the present invention, some aspect not being shown in detail, or being elucidated by assuming substitution of a part of this installation for another 5 part, e.g. as explained with reference to figures 5 and 6. The installation comprises an extruder 1 having one or more extruder screws 2 by means of which a flow of thermoplastic material is provided, e.g. of polyvinylchloride (PVC). 0 The thermoplastic material is fed to a die head 3 arranged on the extruder 1. The die head 3 has an outer body 4 and an inner die member 5, which together with the outer body 4 defines an annular passage from which an extruded tube in preform condition 10 of thermoplastic material emerges, as is preferred in a substantially horizontal direction. The inner die member 5 forms a lumen or axial inner cavity in the tube in preform condition 10. 5 As is common in this technology a rather thick walled tube in preform condition 10 is extruded, the wall thickness later being reduced and the diameter being increased by the biaxial orientation process. In an alternative embodiment the die head 3 is an offset die head 3 with an inlet for the 0 extruded material at a lateral side of the die head and with a central axial passage through the die head 3, essentially through the inner die member 5. Preferably the die head 3 is provided with means for controlling and adjusting the annular passage in order to control the wall thickness and/or cross-sectional shape of the tube in 25 preform condition 10 emerging from the die head 3. This type of die head 3 is known in the art. Preferably an appropriate measuring device 6 is arranged directly downstream of the die head 3 and measures the emerging tube in preform condition 10 to provide control signals for the die head 3. 30 As is preferred an external cooling device 8 is arranged downstream of the extruder 1 and the die head 5 to cool and temper the thick walled tube in preform condition 10, e.g. from about 2000C to about 100 C for PVC. The external cooling device 8 may e.g. comprise a number of compartments behind one another through which cooling water is circulated, the tube in preform condition 10 being in direct contact with the cooling water in each 35 compartment. The temperature of the cooling water may vary from one compartment to another. If desired, it can be arranged that the cooling water circulation in each compartment may be switched on or off.

-17 An outer diameter calibrating device 8a may be provided at the upstream end of the external cooling device 8. 5 Downstream of the external cooling device 8 a first drawing device 15, which may also be referred to as a preform speed-control device, is arranged. Preferably said device 15 includes multiple tracks engaging on the exterior of the tube in preform conditions 0, the speed of the tracks being controlled by a suitable track drive system. Such drawing devices 15 are customary in plastic pipe extrusion. 0 In an embodiment not shown here an external heating device for the tube in preform condition is arranged between the external cooling device 8 and the first drawing device 15, said heating device being adapted to heat in an adjustable manner one sector of the circumference of the tube 10, or possibly multiple selected sectors of the circumference of 5 the tube 10, e.g. only a bottom section of the tube 10 and not the remainder of the circumference of the tube 10, prior to reaching the first drawing device 15. It has been found that heating only a bottom section of the tube 10 at this position is beneficial for the uniformity of the wall thickness of the finally obtained tube. This external heating device could comprise one or more infrared heating elements. 0 The figure 1 b further schematically depicts an expansion device 20. The expansion device 20 is held in place by means of an anchoring rod 21 that is at one end fastened to the expansion device 20. The anchoring rod 21 is connected to the die head 3. 25 As is preferred a force sensing assembly 22 is provided to measure the pull force on the anchoring rod 21 during operation of the installation. As is preferred the anchoring rod 21 has one or more internal ducts, e.g. for supply and discharge of fluid; liquid and/or gas (e.g. air); to locations within the lumen of the tube and/or 30 the expansion device 20. Also the anchoring rod 21 may include one or more ducts for electrical wiring, e.g. to connect to one or more sensors (e.g. pressure and/or temperature) in the lumen of the tube and/or the expansion device, or e.g. to control one or more valves or other electronic components, possibly housed within or at the downstream end of the expansion device. 35 In general the expansion device 20 shown here includes - from upstream to downstream end thereof - a run-on part 20a, an expansion part 20b, and a run-off part 20c. The -18 expansion part 20b - as is preferred - has at least one non-deformable or rigid portion with a gradually increasing diameter in downstream direction, e.g. of conical shape, e.g. with the outer surface of a truncated cone, so as to come into contact with the tube 10 and to exert an expanding force on the tube 10 that brings about diametrical expansion of the tube 10. The 5 expansion part 20b has a maximum diameter at its downstream end, the run-off part 20c here has a diameter that does not exceed said maximum diameter, in fact is preferably less over a reduced diameter section as will be explained below. The expansion part 20b and as is preferred also the run-on part 20a and the run-off part 20c 0 here are of rigid, non-deformable embodiment. The run-on part 20a here is of an elongated, generally cylindrical design. The diameter of the run-on part 20a substantially corresponds to the diameter of the lumen within the preform 10 upstream of the expansion device 20. The run-off part 20c here is of a generally cylindrical 5 design. Preferred details of the expansion device 20 or parts thereof will be explained further below. At a distance downstream of the expansion device 20, as is common in this technology, a 0 further drawing device 50 is arranged. This drawing device 50 is adapted to exert a considerable tensile force on the oriented tube 10. In general the passage of the suitably tempered tube 10 over the expansion device 20 under the influence of the tensile force exerted by the drawing device 50 causes the tube 10 to be expanded in diameter as well as stretched in a considerable manner in axial direction, the wall thickness being significantly 25 reduced in the process so that an biaxially oriented tube 10 is obtained. The maximum diameter of the expansion part 20b in this example basically dictates the orientation in circumferential to which the tube in preform condition is subjected. As is preferred an external cooling of the oriented tube 20 is effected soon after the 30 diametrical expansion of the tube 10 has been brought about, preferably -as here - whilst the tube 10 passes over the run-off part 20c, most preferably starting close to, yet not on, the expansion part 20b. For this reason a first external cooling device 60 is provided. This first cooling device 60 preferably includes one or more nozzles spraying or jetting cooling water onto the exterior surface of the oriented tube, preferably with a significant cooling capacity to 35 arrive at an intense external cooling. Other preferred details will be explained below.

-19 As is preferred at least one further or second external cooling device 70 is arranged at a relatively short distance downstream of the expansion device 20. This second external cooling device 70 preferably includes one or more nozzles spraying or jetting cooling water onto the oriented tube 10. 5 As is preferred yet another or third external cooling device 80, preferably embodied with one or more compartments as described with reference to cooling device 8, is arranged downstream of the device 70 and upstream of the drawing device 50 to cool the oriented tube 10 to a final, e.g. ambient, temperature. 0 Downstream of the drawing device 50 the oriented tube 10 may e.g. be cut to individual tube elements with e.g. a sawing, cutting or milling device or the tube, when appropriate may be spooled onto a reel. This equipment is known in the art. 5 It is envisaged, in a preferred embodiment, that no calibration of the outer diameter of the biaxially oriented tube by passing the tube through a sizing opening of a calibration device is effected downstream of the expansion device 20. This is considered to avoid a loss of strength of the finally obtained tube due to the impact of the sizing device on the tube. 0 Here, use is made of an expansion device 20 with a run-on part 20a that is provided with a sealing member 30 that is sealingly engaged by the tube in preform condition 10. The sealing member 30 is arranged at a distance upstream of the expansion part 20b of the expansion device 20. As is preferred the sealing member 30 is arranged at or in the close vicinity of the nose-end of the run-on part 20a. 25 As is preferred, there is no external part of the installation at the height of the sealing member 30 that presses the tube in preform condition 10 onto the sealing member 30 as this would cause a risk of damaging the tube in preform condition, of disturbing the expansion and also entail a risk of seizing of the tube in preform condition between the sealing member 30 30 and any external part. This sealing member 30 and the sealing engagement thereof with the tube in preform condition 10 during the production process is advantageous as it provides a barrier between the zone upstream of the sealing member 30 and the zone downstream of the sealing 35 member 30 within the lumen of the tube in preform condition 10, so that conditions and/or actions can be performed in said zones that are fully or at least largely independent from one another.

-20 As is advantageous for temperature conditioning of the tube in preform condition 10 a liquid circulation compartment 25 is formed in the lumen of the tube in preform condition 10 between a closing member 26 at a distance upstream from the nose end of the run-on part 5 20a on the one hand and the sealing member 30 on the other hand. A liquid of controlled temperature, e.g. water, is circulated through said liquid circulation compartment in direct contact with the inside of the tube in preform condition 10. This allows to establish an effective internal temperature conditioning of the tube in preform condition directly upstream of the expansion device. In practice said internal temperature condition may be effected with 0 hot water, e.g. close to the orientation temperature, e.g. close to the boiling temperature of water when producing biaxially oriented PVC tubing. The closing member 26 may be arranged on the anchoring rod 21, but here is arranged on the inner member 5 of the die head 3. By choosing the location of the closing member 26 the 5 effective length of the compartment 25 can be established. The water to be circulated through the compartment 25 is supplied from a water source, preferably including a pump and a water heater, via one or more ducts in the rod 21. The return flow of water leaving the compartment 25 is discharged via one or more other ducts in 0 the rod 21. Here, use is made of an expansion device 20 having one or more fluid supply ducts 27 (a portion of which is schematically depicted) to form a fluid volume between the expansion device 20 and the tube 10. The fluid may be water, but another liquid or a gas, e.g. 25 compressed air, may also be used as fluid. The use of a gas will avoid any problems associated with the presence of water residue on the inside of the tube downstream of the expansion device. When water or another liquid is used - if desired - provision can be made for a removal of any leaking water downstream of the expansion device 20, e.g. by providing a further compartment 73 within the lumen of the oriented tube 10 that is closed at its end by 30 a closure member 75 and providing a suction means to remove any water. It is noted that in figure 2 the presence of a water film between the run-on part 20a and the upstream portion of the expansion part 20b on the one hand and the tube 10 on the other hand is suggested. However the thickness of the water film relative to the wall thickness of 35 the tube in preform condition and the diameter of the expansion device is exaggerated.

-21 The one or more supply ducts 27a here have a port in the outer surface of the run-on part 20a and/or the expansion part 20b of the expansion device. Possibly fluid is supplied between the tube and the expansion device directly downstream of the sealing member 30. 5 In this example a discharge duct 27b is provided to discharge the water being entrained with the moving tube 10. As is preferred the first or upstream fluid volume is established over the entire length of the run-on part 20a downstream of the sealing member 30, more preferably also over the 0 upstream portion of the expansion part 20b. Also a second or downstream fluid volume is preferably formed over at least a portion of the run-off part 20c, preferably a sealing engagement of the tube with the expansion device in a region at or near the maximum diameter of the expansion part forming a barrier with an upstream fluid volume. 5 The first or upstream fluid volume, in conjunction with the presence of the sealing member 30, allows to have a relatively long run-on part which is beneficial for the stability of the tube 10 when leading up to the expansion part, the fluid preventing any or at least any excessive frictional contact between the tube in preform condition and the run-on part, more preferably as said contact is concentrated solely on the sealing member 30. 0 In a practical embodiment the sealing member 30 could have a diameter that is between 4 and 20 millimetres greater than the diameter of the downstream located portion of the run-on section. 25 The sealing member 30 forms an effective and reliable seal that prevents the fluid, which is supplied at an elevated pressure that is sufficient for the formation of a fluid volume, from reaching the lumen of the tube in preform condition 10 upstream of the sealing member 30. When the production is performed with the presence of compartment 25, and with - as is preferred - a relatively low pressure of the water in said compartment, it is understandable 30 that the fluid will try to reach said lower pressure zone upstream, the sealing member reliably avoiding this effect. This allows for a stable fluid volume between the expansion device, preferably both the run-on part and the expansion part, and the tube, as any escape of fluid to upstream of the sealing member 30 will cause a pressure drop in the volume and thus instability. 35 As mentioned the fluid is in a practical embodiment a liquid, more preferably when the fluid is used to form a relatively thin layer, that has a primary purpose to serve as a lubricant -22 between the tube and the expansion device. Possibly water is used as fluid, preferably non boiling heated water, wherein a water heater and pump are used to provide and supply the heated water via one or more ducts in the rod 21 at a suitable pressure to the expansion device 20. The pressure is chosen so as to allow the formation of a liquid film between the 5 expansion device and the tube. In another embodiment the fluid is a gas, e.g. air, supplied via a compressor or other pressurized gas source to be introduced between the expansion device and the tube. This is preferred in particular when the expansion of the tube is to be caused, at least partly, by the 0 internal fluid pressure caused by the gas. As can be seen in the drawings - and as is preferred - use is made of at least one external heat exchange device 110 that is adapted to influence the temperature of the tube in preform condition 10 arriving at the sealing member 30, and thereby the sealing contact between the 5 tube in preform condition 10 and the sealing member 30. As is preferred at least one such heat exchange device is an external heat exchange device that is arranged between the drawing device 15 and the location of the sealing member 30 to influence the temperature of the tube in preform condition 10 from the exterior thereof. Preferably said device, here device 110, is arranged directly upstream of the sealing member location. 0 As is preferred a second external heating device 120 here is arranged downstream of the sealing member 30 location, between said sealing member 30 and the expansion part of the expansion device or even overlapping (a part of) the expansion part of the expansion device 20. 25 In an arrangement with a first external heating device 110 directly upstream of the sealing member location and a second external heating device 120 downstream of the sealing member location, each heating device 110, 120 being controllable independently, the first heating device 110 can be used primarily for controlling the sealing engagement with the 30 sealing member 30, and the second heating device 120 in order to influence the tube 10 directly upstream of and/or during the passage of the tube over the expansion part of the expansion device. The heating devices 110, 120 may each include multiple heating elements distributed around the path of the tube, e.g. multiple infrared heating elements. 35 A control device, e.g. electronic, preferably is provided to control the operation of each external heating device 110, 120. For the external heating device 110 the control may be based on a feedback signal representative of the actual sealing engagement of the tube in -23 preform condition 10 with the sealing member 30, e.g. obtained via a force monitoring device that is adapted to monitor the axial force on the sealing member 30 (e.g. with a strain gauge) or obtained via a monitoring device that is adapted to monitor a local deformation in the moving tube caused by the sealing member 30, e.g. a local bulging of the tube as indicated 5 in figure 2, e.g. by measuring the diameter of the preform upstream, at, and downstream of the sealing member. The sealing member 30 is a member 30 having a diameter that is larger than the downstream portion of the run-on part 20a. As is preferred the run-on part 20a has a uniform 0 diameter between the sealing member 30 and the expansion part 20b. As is preferred the sealing member 30 is a separately manufactured annular member fitted on a tubular member of the run-on part. 5 As is preferred the sealing member 30 is a metallic member with no provision to supply a lubricant to the outer surface thereof. In more complex embodiments however the sealing member may be adapted to control the frictional engagement thereof with the tube in preform condition, e.g. provided with a lubrication device, e.g. allowing a gas, e.g. air, to be fed between the sealing portion and the tube in preform condition. In another embodiment the 0 sealing member may be construed to have a variable diameter and an associated control means, e.g. with an outer metallic skin that is expandable under hydraulic pressure, so as to control the sealing engagement with the tube in preform condition. According to the present invention the run-off part 20c, which is downstream of a the 25 expansion part 20b, has a reduced diameter section having a smaller diameter than the maximum diameter of the expansion part 20a. Possibly the reduced diameter section directly adjoins the maximum diameter cross-section, so that a diameter reduction step occurs directly behind said maximum diameter position. This can be clearly seen in figures 2 and 4. 30 Use is made here of at least one outer diameter ring member, here - as preferred - two ring members 90,91, through which the tube 10 passes at the location of the run-off part of the expansion device, here at the location of the reduced diameter section of the run-off part 20c. As is preferred the ring members 90, 91 here are each embodied as a constrictive outer diameter ring member, which means that each ring member 90, 91 exerts a radial 35 constrictive force on the tube 10 passing there through, thereby reducing the outer diameter of the tube 10, at least over a short axial distance. In practice this means that the opening -24 within each ring member 90, 91 has a diameter which is less than the projected outer diameter of the oriented tube 10 at said location during the normal production process. The reduced diameter section here is dimensioned so as to avoid a problem of seizing of the 5 tube between the expansion device 20 and the at least one outer diameter calibrating ring 90, 91. The reduced diameter section preferably has a diameter that is at least 4 millimetres less than the maximum diameter of the expansion part 20b of the expansion device 20. 0 Preferably the diameter reduction is about twice the wall thickness of the tube passing over said section. By providing the reduced diameter section the outer diameter ring members 90, 91 can be arranged around said reduced diameter section, with the radial spacing between said ring 5 members 90, 91 and the reduced diameter section being more than the wall thickness of the tube 10 desired during the production process at said location, so that some radial play remains that allows for possible variations in the wall thickness of the tube during the production process, without the risk that said tube becomes stuck between a ring member 90, 91 and the reduced diameter section of the run-off part of the expansion device. 0 Each ring member 90, 91 may be provided with cooling means for cooling the ring member 90, 91, e.g. with an internal cooling fluid duct, e.g. an annular duct. Each ring member 90, 91 preferably is composed of two semi-circular parts, allowing to place 25 the ring members 90, 91 around the tube 10, e.g. during the start-up phase of the production process, and allowing to remove, e.g. for exchange, the ring members during the production process. Each ring member 90, 91 preferably is made of metal. 30 As indicated above, in order to freeze the orientation of the plastic material, the oriented tube is cooled externally while passing over the run-off part 20c by the first external cooling device 60. 35 The external cooling by first external cooling device 60 of the tube while passing over the run-off section 20c is here performed in the absence of internal cooling of the tube 10 while -25 passing over the expansion device 20, and in fact also in the absence of any internal cooling downstream of the expansion device 20. In order to arrive at a biaxially oriented tube 10 with desired dimensions, as wall thickness 5 and cross-sectional shape, preferably without using an outer diameter calibration downstream of the expansion device 20, it has been found possible to rely on the use of the one or more outer diameter ring members 90, 91 and/or the external cooling of the oriented tube. This is done on the basis of the so-called snap-back effect. This snap-back effect is known in the art and is visible as a reduction of the tube diameter directly downstream of the 0 expansion device 20. In a preferred embodiment the first external cooling device 60 is adapted to adjust the length and/or location with respect to the expansion device 20 of the stretch of the oriented tube 10 that is affected by the first external cooling device 60. It has been found that by suitable 5 selection of the length, and preferably also the location, of the affected stretch with respect to the expansion device, in particular the run-off part 20c, the snap-back effect can be controlled, and so the diameter of the tube 10. Clearly the intensity of the cooling by device 60 can also be controlled and will have an influence on the snap-back effect. 0 In a very practical embodiment the first external cooling device 60 operates with one or more nozzles emitting sprays or jets of cooling liquid, e.g. water, and comprises an upstream shield member 61 and a downstream shield member 62, said shield members 61, 62 delimiting the stretch of oriented tube that is affected by the sprays or jets of cooling liquid. At least one of the shield members, preferably both, is displaceable in axial direction, thereby 25 allowing to adjust the length and/or the location of the stretch of tube that is affected by the cooling liquid. It will be appreciated that by controlling the length and/or position of the shield members, the cooling of the oriented tube can be controlled, even more when - as is common - the intensity of the cooling spray can be controlled as well. 30 In an even more practical embodiment each of the shield members 60, 61 - as depicted here - is integral with a ring member 91, 92. A displacement device 65, here embodied as motorized drive assembly, for axial displacement of at least one of the shield members 61, 61 and/or at least one of the ring 35 members 90, 91 in axial direction along the run-off part 20a is provided. In this example the device 65 includes one or more screw spindles 66, e.g. operated by a common electric motor.

-26 As is preferred the ring members 90,91 and shield members 61, 62, as well as the associated displacement device 65, are mounted on a mobile support 68 (here with axial linear guides 69) allowing to displace said components in axial direction, e.g. to a retracted 5 position downstream of the position of the expansion device 20, e.g. in order to allow access to the expansion device e.g. when replacing the expansion device and/or during start-up of the installation. As is preferred a second external cooling device 70 is arranged spaced downstream from the 0 first external cooling device 60 and the expansion device 20. The second external cooling device 70 preferably comprises one or more nozzles emitting sprays or jets of cooling water onto the exterior of the oriented tube 10. Preferably a dry zone is created between the external cooling devices 60 and 70 on the 5 outside of the tube 10. This is considered to avoid or at least reduce the formation of visual effects, e.g. rings, on the outside of the tube by cooling water. Preferably the second external cooling device 70 comprises an upstream shield member 71 delimiting the upstream end of the stretch of oriented tubing 10 affected by the external 0 cooling device 70. As is preferred the upstream shield member 71 is movable in axial direction by an associated displacement device, or is coupled to an axially mobile ring member 91 or shield member 62. The upstream shield member 71 preferably has an easily flexible annular lip 72 engaging the 25 oriented tube 10 so as to avoid any scratching or deformation of the oriented tube. Here use is made of a measuring device 130 for measuring at least the outer diameter of the oriented tube 10, and preferably also the wall thickness and/or cross-sectional profile, which measuring device 130 is arranged downstream of the expansion device 20, here 30 downstream of the second external cooling device 70. Also use is made of a control device (not shown), e.g. an electronic device, which is linked to the measuring device in order to obtain input signals that allow to control the first external cooling device 60 and/or the second external cooling device 70. 35 -27 For instance the device 60 is controlled with regard to at least the length and/or location with respect to the expansion device of the stretch of oriented tube that is affected by the first external cooling device 60, and/or the intensity of the cooling. 5 For instance the second external cooling device 70 is controlled with regard to the axial position of the upstream shield member 71 and/or the intensity of the cooling. By control of the external cooling of the tube by the cooling device 60 and/or device 70 the snap-back effect can be controlled, and thus the diameter of the finally obtained tube. This 0 can then be done without the need for any further outer diameter calibration downstream of the expansion device. In a practical embodiment this control device is adapted such that the length of the stretch of tube that is affected by the first external cooling device 60 is decreased to obtain an 5 increased snap-back effect and thus increased diameter reduction, and wherein said length is increased to obtain a reduced snap-back effect and thus decreased diameter reduction. In a practical embodiment the axial position of the upstream shield member of the second external cooling device 70 is chosen or adjusted to be in the region where the snap-back effect occurs. 0 As is preferred provisions are made for the presence of a first or upstream fluid volume upstream of the maximum diameter of the expansion part of the expansion device and a second or downstream fluid volume between the reduced diameter section of the run-off part 20c of the expansion device and the oriented tube 10. 25 In order to supply fluid, e.g. water, preferably gas, more preferably air, most preferably heated, to the second fluid volume, one or more dedicated supply ducts can be provided having a port in the run-off part exterior surface. As an alternative, or in combination therewith, a communication passage can be made, preferably a valve controlled passage, 30 that communicates with both the first and the second fluid volume. Such a passage allows e.g. to equalize the pressure in the two fluid volumes when desired and/or to introduce fluid into one volume via a supply duct having a port at the other volume. While not depicted here, the skilled person will appreciate that the passage could extent between ports in the outer surface of the expansion device, e.g. on at the run-on part and one at the run-off part, with 35 interposition of a valve, e.g. an electrically operated valve, e.g. mounted at the rear end of the expansion device.

-28 The presence of two outer diameter ring members 90, 91 spaced apart from one another is advantageous, even more advantageous when the run-off part 20c is embodied with an increased diameter portion 20c1 delimiting the downstream end of the reduced diameter section. Preferably each ring member 90, 91 being mobile in axial direction relative to the 5 reduced diameter section. With the ring members 90, 91 both suitably dimensioned as constrictive ring members, the effect can be obtained that the ring member 90 contributes to the sealing engagement of the tube with the expansion device in the region at or near the maximum diameter of the 0 expansion part 20b. This avoids uncontrolled escape or leakage of fluid from the one volume to the other volume. The ring member 91 contributes to the sealing engagement of the oriented tube with the increased diameter portion 20c1. This avoids or at least limits any leakage of fluid into the 5 lumen of the oriented tube downstream of the expansion device 20, and thus avoids undesirable instability of the fluid volume. Most preferably the downstream ring member 91 is located closely upstream of the increased diameter portion 20c1, thereby enhancing the sealing contact between the tube and the increased diameter portion 20c1. o Possibly the expansion device 20 has one or more discharge ducts 28 for the fluid, e.g. with a port near the downstream end of the reduced diameter section of the run-off part 20c, which is advantageous when use is made of a liquid that is entrained with the tube 10 from an upstream port of a supply duct to said discharge duct port. 25 In a preferred embodiment the installation is provided with both supply means for a liquid fluid to one or both fluid volumes between the expansion device and the tube and supply means for gaseous fluid to one or both fluid volumes, and with an arrangement of ducts and one or more valves allowing to selectively feed fluid to one or both fluid volumes. For instance in a start-up phase fluid is only fed to the first volume, e.g. first heated water and 30 later gas, such as air. The ring members 90, 91 may be absent during the start-up phase, facilitating the first passage over the run-off part. Later the ring members 90, 91 are fitted or made operative and a fluid is fed to the second volume, e.g. heated air. The excellent sealing engagement in the region of the maximum diameter of the expansion 35 part, also allows for a reliable operation with the first fluid volume not being a film to prevent friction, but an internal pressurized zone within the tube that causes gradual expansion of the tube to an internal diameter less than the maximum diameter in order to maintain the reliable -29 sealing engagement. The use of an internal pressurized zone to cause gradual expansion in a production process for biaxially oriented thermoplastic tubing is known in the art, e.g. from WO 90/02644. However in the known installations operating according to this approach use is made of an inflatable plug to delimit the downstream end of the pressurized zone, which 5 inflatable plug presses the expanded tube against a surrounding sizing sleeve to obtain a seal that avoids pressure loss in the pressurized zone. This approach has shown to come up with less favourable results, e.g. with regard to uniformity of dimensions of the finally obtained tube and stability of the production process. One aspect is e.g. that the inflatable plug is deformable, and thus does not dictate the obtained orientation in the manner as the 0 non-deformable expansion part. In the embodiment depicted here the expansion part of the expansion device 20 has a stepped design with a first conical surface increasing in diameter in downstream direction, adjoining a cylindrical surface of a first diameter, followed by a second conical expansion 5 surface increasing in diameter in downstream direction. As is preferred the diameter of the sealing member 30 is greater than the first diameter of the expansion part in this stepped design. The expansion part could have multiple steps. In an embodiment one or more rollers 125 are arranged below tube 10 so as to support said 0 tube, e.g. below the run-off part of the expansion device or, with preference, downstream of the expansion device e.g. to avoid interference with any of the rings 90, 91. In this practical embodiment an upstream replaceable ring 20b1 is fitted at the transition between the expansion part 20b and the run-off part 20c of the expansion device, the 25 replaceable ring 20b1 forming the maximum diameter of the expansion part 20b. This allows for relatively easy change of the maximum diameter of the expansion device as well as replacement of ring in case of wear. In this practical embodiment the increased diameter portion 20c1 is formed by a downstream 30 replaceable ring which is fitted at the downstream end of the run-off section, the replaceable ring having a diameter greater than the upstream portion of the run-off part of the expansion device. This allows for relatively easy change of the diameter of the expansion device at said downstream location as well as replacement of said zone in case of wear. 35 Fig. 5 shows schematically in longitudinal section a portion of an installation for producing biaxially oriented thermoplastic tubing according to the invention.

-30 Figure 5 shows the portion wherein the thermoplastic tube is passing over the expansion device 100. This expansion device 100 may e.g. be integrated in the installation that is described with reference to figures 1a,1b, and 1c, or in an installation including at least an extruder, one or more tempering devices for the tube in preform condition, a drawing device 5 downstream of the expansion device as well as a cooling device for the oriented tube. When seen in conjunction with figures 1a-c the expansion device 100 depicted in figure 5 then replaces the expansion device 20. As is preferred in combination with the expansion device 100 a first external cooling device 0 60 is used, here, as preferred, having one or more features of the external cooling device 60 as has been discussed herein before. As is preferred a second external cooling device, preferably having one or more of the features of external cooling device 70, is arranged downstream of the expansion device 100. It will be appreciated that a control device for these first and/or second external cooling device, preferably having one or more of the 5 features of the control device as discussed herein before, is also present. By suitable control of the first and/or second external cooling device the snap-back effect, that occurs at a short distance downstream of the expansion device can be controlled, and thereby the diameter of the oriented tube, this without - as is preferred - making use of any 0 external diameter calibrating device downstream of the expansion device. The expansion device 100 is embodied to cause expansion of the tube from the preform condition into a biaxially oriented tube based on internal fluid pressure in a relative large fluid volume 101 inside the lumen of the tube between the tube and the expansion device. 25 The expansion device 100 here includes an upstream sealing portion 103 that fits sealingly into the yet unexpanded tube in preform condition, e.g. the sealing portion having one or more of the features of the sealing member 30. The upstream sealing portion 103 delimits the volume 101 at its upstream end. 30 The expansion device 100 also includes a downstream sealing portion 105 that fits sealingly in the expanded tube 10 and delimits the fluid volume 101 at its downstream end. As is preferred the sealing portion 105 is non-deformable, e.g. of a metal. 35 The expansion device 100 includes one or more fluid supply ducts 106 that allow to introduce fluid, e.g. pressurized gas, e.g. air, into the fluid volume 101.

-31 The figure 5 shows the presence of a downstream outer diameter ring member 91 that is arranged a short distance upstream of the downstream sealing portion 105. The ring member 91 exerts a constrictive force on the tube, thereby contributing to the sealing engagement between the tube and the sealing portion 105. As is highly preferred, there is no part of the 5 installation at the same axial location as the portion 105 that contacts the tube on the exterior so as to press it against the sealing portion 105. This avoids any risk of seizing of the tube between such a part of the installation and the sealing portion 105, as well as prevents undesirable damage to the tube. 0 As is preferred a further, upstream outer diameter ring member 90 is arranged at a distance upstream of the ring member 91. As will be appreciated the ring members 90, 91 are preferably integrated with shield members of the first external cooling device 60. The cooling by first external cooling device 60 also contributes somewhat to the sealing 5 engagement between the tube 10 and the portion 105. However its main purpose is to freeze the biaxial orientation, as well as to control the snap-back effect, as is preferred in combination with the use of the second external cooling device 70. Figure 6 shows schematically in longitudinal section a portion of an installation for producing 0 biaxially oriented thermoplastic tubing. The figure 6 is used to elucidate the invention. Figure 6 shows the portion wherein the thermoplastic tube is passing over the expansion device 200. This expansion device 200 may e.g. be integrated in the installation that is described with reference to figures 1a,1b, and 1c, or in an installation including at least an 25 extruder, one or more tempering devices for the tube in preform condition, a drawing device downstream of the expansion device as well as a cooling device for the oriented tube. When seen in conjunction with figures 1a-c the expansion device 200 depicted in figure 5 then replaces the expansion device 20. 30 The expansion device 200 is embodied to effect expansion of the tube by a hybrid process of expansion by internal fluid pressure and expansion caused by contact with a non-deformable expansion part of the device 200 In this example the expansion device includes a run-on part 200a including a sealing portion 35 201 that sealing engages the non-expanded tube in preform condition. Downstream thereof, spaced from the sealing portion 201, the expansion device 200 has a non-deformable expansion portion 200b with a gradually increasing diameter to a maximum diameter.

-32 Downstream of said portion 200b the expansion device 200 includes a run-off part 200c, here with an increased diameter portion also acting as downstream sealing portion 204. An upstream or first fluid volume 210 is present between the upstream sealing portion 201 5 and the location of contact of the tube with the expansion part 200b. This fluid volume 210 is filled with a pressurized fluid, preferably a gas, here air, via supply duct 206 so as to effect gradual expansion of the tube due to internal fluid pressure. This expansion is such that the tube 10 is expanded from its preform condition to a diameter such that the tube 10 still contacts the conical face of expansion part 200b during normal production of the biaxially 0 oriented tube. This contact causes a further expansion of the tube 10 due to the forces exerted by the expansion part 200b on the tube. As can be seen the tube now sealingly engages the expansion device in the region at or near the maximum diameter of the expansion part 200b. 5 A downstream or second fluid volume 220 is present between the location of contact between the tube at or near the maximum diameter of the expansion part 200b on the one hand, and the location of contact between the tube and the downstream sealing portion 204 on the other hand. 0 This fluid volume 220 is filled with a pressurized fluid, preferably a gas, here gas, via supply duct 207 so as to effect gradual expansion of the tube due to internal fluid pressure. So a further expansion is effected by said fluid pressure, generally to expand the tube so that it can pass over the sealing portion 204 which has a greater diameter than the maximum diameter of the expansion part 202. 25 As is preferred a communication passage 208 with a control valve 209 is provided in the fluid supply device, here schematically shown, allowing to bring the fluid volumes 210 and 220 in communication, thus allowing to equalize the pressure in both volumes on command. As shown here the supply duct 207 is placed in series with said passage 208, however the 30 supply duct could also be a distinct supply duct, the passage 208 forming a controllable connection between the duct 206 and the duct 207. As is preferred a downstream outer diameter ring member 91 is provided, preferably such that said ring member 91 contributes to the sealing engagement of the tube with the sealing 35 portion 204. Further preferred details of said ring member 91 have been disclosed herein before and may be used in combination with the expansion device 200.

-33 As is preferred an upstream outer diameter ring member 90 is also provided, said ring member 90 being arranged downstream of the maximum diameter of the expansion portion 202. 5 As is preferred a first external cooling device 60 is employed for cooling the tube when the tube passes between the expansion part and the downstream sealing portion 204. As is preferred the first external cooling device includes one or more of the features of the cooling device described herein. 0 As is preferred the one or more ring members 90, 91 are integrated with one or more shield members of the first external cooling device 60. When desired the downstream sealing portion 204 can be embodied with a gradually increasing diameter and the tube contacting said face of the sealing portion in a manner that 5 a further circumferential expansion is caused by said contact, thus the portion 204 acting as an expansion part to effect the final expansion of the tube. The downstream sealing portion 204 can also be embodied as an expandable portion, e.g. an inflatable portion, e.g. as an inflatable plug as is known in the art, allowing to vary the o diameter thereof, e.g. to facilitate start-up of the installation with the diameter of portion 204 in the start-up phase being reduced, e.g. to at most the maximum diameter of part 200b. In combination with an expandable downstream sealing portion, the rigid run-off part as shown in figure 6 could e.g. be of reduced axial length, e.g. just sufficient for contact with the tube directly downstream of the maximum diameter of the part 200b, e.g. cylindrical having the 25 same diameter as the maximum diameter. A slender rod could extend between the expandable portion 204 and the non-deformable body including at least the part 200b to hold the expandable portion in position. 30 In figures 7a and 7b an installation is shown that largely corresponds to the installation shown in figure 6. Parts have the same or similar structure and function have been denoted with the same reference numerals. Figure 7c shows a detail of a variant of said installation. In the expansion device 200 now also a gas discharge duct 250, which duct 250 has an inlet 35 port 250a in the exterior surface of the expansion part of the expansion device, as can be seen in the enlarged detail of figure 7a.

-34 In figure 7a the situation is shown wherein the inlet port 250a is covered and closed by the tube 10, so that said port 250a is not in communication with the gas volume 210. In figure 7b the same installation is shown, but now the tube 10 has expanded some more 5 under the influence of the gas pressure in volume 210. As can be seen in the enlarged detail, the port 250a is now not covered by the tube 10 and thus in communication with the gas volume 210. The gas discharge duct 250 in figure 7b provides for the relief of gas pressure from the 0 volume 201 as the corresponding inlet port is fully or at least partly open and thereby the expansion of the tube 10 caused by internal gas pressure is controlled. This relief of gas pressure stops when the inlet port 250a is fully covered and closed by the tube 10 (as in figure 7a). 5 In practice an equilibrium situation may be reached wherein the port 250a remains partly open, so that a circulation of gas through the volume 210 is present. So the cooperation of the tube 10 with the inlet port 250a achieves in a very attractive 0 manner a control of the degree of expansion that is caused in the tube 10 due to the internal gas pressure in volume 210. Effectively the position of the inlet port 250a on the sloping exterior face of the expansion part of the device 200 controls where the tube 10 will contact said face, assuming that the gas pressure in volume 210 is sufficient to cause the tube 10 to expand. 25 The provision of duct 250 with inlet port 250a also provides for an automatic safety against the tube being expanded by gas pressure excessively, e.g. to a greater diameter that the expansion part prior to reaching said expansion part, which situation in reality would lead to a stoppage of the production. 30 It is noted that a group of multiple inlet ports 250a connected to a common gas discharge duct could be arranged distributed around the circumference of the expansion part and at the same radial distance to a central longitudinal axis of the expansion part, so as to avoid that the tube would over-expand somewhere along its circumference. 35 In another embodiment, shown in figure 7c, multiple inlet ports 250a, 250b, each associated with a corresponding discharge duct 250, 260, are provided at differing diameter positions in -35 the exterior surface of the expansion part, said differing diameter positions having different radial distances from a central longitudinal axis of the expansion part (so in axial direction of the expansion device one inlet port behind the other inlet port). In this embodiment it is envisaged to provide one or more operable valves 270, 271 that are associated with the 5 discharge ducts 250, 260, so that a selected inlet port and associated discharge duct can be made effective to relief gas pressure (here port 250a) when the tube does not cover and close said inlet port, whereas one or more non-selected inlet ports (here port 260a) and associated discharge ducts are made ineffective. This allows to provide control over the internal diameter of the tube as it expands by the internal gas pressure in the fluid volume 0 before reaching the non-deformable expansion part. In figure 7a also a temperature sensor 280 is shown at the sealing member 201. This sensor 280 allows to measure the temperature of the preform in said region. This sensor 280 may e.g. be coupled to the first and/or second external heat exchange devices that are used to 5 influence the sealing engagement of the preform with the sealing member 201 in order to assist in suitable operation thereof.

Claims

1. A method for producing a biaxially oriented tube from thermoplastic material, wherein a tube in preform condition is extruded from thermoplastic material using an extruder which is 5 provided with an extruder die head having an inner die member, the inner die member forming a lumen in the tube in preform condition, wherein the tube in preform condition is subjected to a temperature conditioning by one or more tempering devices so that a tempered tube is obtained having an orientation temperature which is suitable for the thermoplastic material, and wherein use is made of an expansion device in the lumen 0 downstream of the extruder, said expansion device including: - a non-deformable expansion part having a gradually increasing diameter to a maximum diameter at a downstream end thereof, which expansion part is contacted by the tube and exerts an expanding force on the tube so as to bring about expansion of the tempered tube in circumferential direction, 5 - a run-off part adjoining the expansion part at the downstream end thereof, wherein the method includes drawing the tempered tube over the expansion device using a drawing device which is arranged downstream of the expansion device and acts on the tube, in such a manner that said tube is drawn over the expansion device and transformed from a tube in preform condition into a biaxially oriented tube with thermoplastic material being 0 oriented in axial direction and in circumferential direction, wherein said biaxially oriented tube is cooled, and the run-off part of the expansion device has a reduced diameter section having a smaller diameter than the maximum diameter of the expansion part, 25 wherein use is made of at least one outer diameter ring member that is arranged at the location of the reduced diameter section of the run-off part of the expansion device and around said reduced diameter section, and wherein the oriented tube passes through the at least one outer diameter ring member 30 while being in contact with said at least one outer diameter ring member, the outer diameter ring member and the reduced diameter section being dimensioned such that seizing of the oriented tube between the run-off part and the at least one outer diameter ring member is avoided, the inside of the oriented tube being radially spaced from the reduced diameter section, 35 and wherein the oriented tube is cooled externally while passing over the run-off part by a first external cooling device. -37

2. Method according to claim 1, wherein use is made of an upstream outer diameter ring member and a downstream outer diameter ring member, the ring members being arranged spaced apart in series at the location of the reduced diameter section of the run-off 5 part and around said reduced diameter section, the oriented tube passing through said ring members.

3. Method according to claim 2, wherein the first external cooling device cools the stretch of oriented tube between the upstream and downstream outer diameter ring 0 members.

4. Method according to any one of claims 1 - 3, wherein at least one outer diameter ring member is embodied as a constrictive outer diameter ring member, said ring member exerting a radial constrictive force on the oriented tube passing there through, thereby, at 5 least during the passage through the ring member, reducing the outer diameter of the oriented tube.

5. Method according to any one of claims 1 - 4 , wherein an upstream outer diameter ring member is embodied a constrictive outer diameter ring member, said ring member 0 exerting a constrictive force on the oriented tube that contributes to a sealing engagement of the oriented tube with the expansion device in a region at or near the maximum diameter of the expansion part.

6. Method according to any one of claims 1 - 5 , wherein at least one outer diameter ring 25 member is mounted so as to be displaceable in axial direction relative to the run-off part.

7. Method according to any one of claims 1 - 6 , wherein the first external cooling device comprises one or more cooling liquid spray nozzles adapted to spray or jet cooling liquid onto the exterior of the tube. 30

8. Method according to any one of claims 1 - 7 , wherein the first external cooling device is adapted and operated to adjust the length and/or location with respect to the run-off part of the expansion device of the stretch of the oriented tube that is affected by the first external cooling device. 35 -38

9. Method according to any one of claims 1 - 8, wherein the first external cooling device comprises an upstream shield member and a downstream shield member, said shield members delimiting the stretch of oriented tube that is affected by the first external cooling device, and wherein at least one of the shield members is displaceable in axial direction, 5 thereby allowing to adjust the length and/or the location of the stretch of tube that is affected by the first external cooling device.

10. Method according to claim 9, wherein an outer diameter ring member is integral with a shield member of the first external cooling device. 0

11. Method according to any one of claims 1 - 10, wherein the run-off part includes an increased diameter portion - downstream of the one or more outer diameter ring members and delimiting the downstream end of a reduced diameter section of the run-off part - said increased diameter portion having a greater diameter than said reduced diameter section, 5 and wherein the oriented tube sealingly engages the increased diameter portion.

12. Method according to claim 11, wherein the run-off part has a single reduced diameter section, an upstream and downstream outer diameter ring member being arranged at the location of said single reduced diameter section, the upstream outer diameter ring member 0 contributing to a sealing engagement of the oriented tube with the expansion device in a region at or near the maximum diameter of the expansion part, the downstream outer diameter ring member exerting a constrictive force on the oriented tube which contributes to a sealing engagement of the oriented tube with the increased diameter portion of the run-off part. 25

13. Method according to any one of claims 1 - 12, wherein a second external cooling device is arranged downstream of the expansion device, which second external cooling device is adapted and operated to externally cool the oriented tube, said second external cooling device being controlled independent from the first external cooling device, said 30 second external cooling device including one or more cooling liquid spray nozzles adapted to spray or jet cooling liquid, e.g. water, onto the exterior of the oriented tube.

14. Method according to claim 13, wherein said second external cooling device is arranged spaced a distance downstream from the first external cooling device, wherein a dry 35 zone being maintained between the first and second external cooling devices during operation. -39

15. Method according to either claim 13 or 14, wherein the second external cooling device is located to with its upstream end in a region where the snap-back effect - wherein the diameter of the tube reduces - takes place downstream of the run-off part. 5

16. Method according to any one of claims 13 - 15 , wherein the second external cooling device includes an upstream shield member delimiting the upstream end of the stretch of oriented tubing affected by said second external cooling device, said upstream shield member being movable in axial direction. 0

17. Method according to any one of preceding claims 1 - 16 , wherein a measuring device for measuring at least one of the outer diameter, the wall thickness, and the cross-sectional profile of the oriented tube, is arranged downstream of at least the first external cooling device, and wherein a control device is provided to control the first external cooling device and/or a second external cooling device downstream from the first external cooling device, 5 and wherein said measuring device is linked to said control device so as to control the cooling by said first external cooling device and/or said second external cooling device.

18. Method according to claim 17 when dependent upon claim 8, wherein the control device is adapted and operated to control the first external cooling device with regard to at 0 least the length and/or location with respect to the expansion device of the stretch of oriented tube that is affected by the first external cooling device.

19. Method according to claim 18, wherein the control device is adapted such that the length of the stretch of tube that is affected by the first cooling device is decreased to obtain 25 an increased snap-back effect and thus increased diameter reduction of the oriented tube, and wherein said length is increased to obtain a reduced snap-back effect and thus decreased diameter reduction of the oriented tube.

20. Method according to claim 17 when dependent upon claim 13, wherein the second 30 external cooling device is mounted to be movable in axial direction, and wherein the measuring device is linked to a control device provided to control the second external cooling device, and wherein the measuring device is linked to said control device of the second external cooling device in order to control the position thereof. 35

21. Method according to any one of claims 1 - 20, wherein no internal cooling of the oriented tube is performed when passing over the run-off part, and wherein no internal cooling of the oriented tube is performed downstream of the expansion device. -40

22. Method according to any one of claims 1 - 21 , wherein a fluid is introduced between the reduced diameter section of the run-off part of the expansion device and the oriented tube, and wherein the oriented tube sealingly engages the run-off part downstream of the 5 reduced diameter section.

23. Method according to claim 22, wherein the fluid is a pressurized gas.

24. Method according to either claim 22 or 23, wherein a first fluid volume is present 0 between the expansion device and the tube upstream of the sealing engagement of the tube in a region at or near the maximum diameter of the expansion part, and wherein a second fluid volume is present between the reduced diameter section of the run-off part and the oriented tube. 5

25. Method according to claim 24, wherein use is made of an expansion device with a valve controlled communication passage extending between the first and second fluid volumes, and wherein the expansion device includes at least one fluid supply duct which introduces fluid into the first and/or second fluid volume. 0

26. Method according to either claim 24 or 25, wherein a first fluid supply duct introduces fluid into the first fluid volume and a second fluid supply duct introduces fluid into the second fluid volume.

27. Method according to any one of claims 1 - 26, wherein the expansion device has a 25 run-on part which is located upstream of the expansion part, which run-on part is provided with a sealing member that is sealingly engaged by the tube in preform condition, said sealing member being arranged at a distance upstream of the expansion part and having a diameter that is greater than the run-on part downstream of the sealing member, the sealing member forming an effective seal with the tube in preform condition, and wherein a fluid is 30 introduced between the run-on part and the tube in preform condition, the sealing member preventing the fluid from reaching the lumen of the tube upstream of the sealing member.

28. Installation for producing a biaxially oriented tube from thermoplastic material, which installation includes: 35 - an extruder with an extruder die head having an inner die member adapted to extrude a tube in preform condition from thermoplastic material, the inner die member forming a lumen in the tube, -41 - one or more tempering devices for temperature conditioning of the tube in preform condition, so that a tempered tube in preform condition is obtained having an orientation temperature which is suitable for the thermoplastic material, - an expansion device in the lumen downstream of the extruder, said expansion device 5 including: - a non-deformable expansion part having an increasing diameter to a maximum diameter at downstream end thereof, which expansion part is contacted by the tube and exerts an expanding force on the tube so as to bring about an expansion of the tempered tube in circumferential direction, o - a run-off part downstream of the expansion part, - a drawing device which is arranged downstream of the expansion device and is adapted to act on the tube, in such a manner that said tube is drawn over the expansion device and is transformed from a tube in preform condition into a biaxially oriented tube with thermoplastic material which is oriented in axial direction and in circumferential direction of the tube, 5 - a cooling device that is adapted to cool the biaxially oriented tube, wherein the run-off part has a reduced diameter section having a smaller diameter than the maximum diameter of the expansion part, o and wherein at least one outer diameter ring member is arranged at the location of the reduced diameter section and around said reduced diameter section, and wherein the outer diameter ring member is arranged such that the oriented tube passes through the ring member while being in contact with said ring member, the outer diameter 25 ring member and the reduced diameter section being dimensioned such that seizing of the oriented tube between the run-off part and the at least one outer diameter ring member is avoided, the inside of the oriented tube being radially spaced from the reduced diameter section, 30 and wherein the installation includes a first external cooling device that is adapted to cool the oriented tube externally while passing over the run-off part.

29. Installation according to claim 28, wherein the installation includes an upstream outer diameter ring member and a downstream outer diameter ring member, the ring members 35 being arranged spaced apart in series at the location of the reduced diameter section of the run-off part and around said reduced diameter section, the oriented tube passing through said ring members. -42

30. Installation according to claim 29, wherein the first external cooling device is adapted to cool the stretch of oriented tube between the upstream and downstream outer diameter ring members. 5

31. Installation according to any one of claims 28 - 30, wherein at least one outer diameter ring member is embodied as a constrictive outer diameter ring member, said ring member being adapted to exert a radial constrictive force on the oriented tube passing there through, thereby, at least during the passage through the ring member, reducing the outer 0 diameter of the oriented tube.

32. Installation according to any one of claims 28 - 31, wherein an upstream outer diameter ring member is embodied a constrictive outer diameter ring member, said ring member exerting a constrictive force on the oriented tube that contributes to a sealing 5 engagement of the oriented tube with the expansion device in a region at or near the maximum diameter of the expansion part.

33. Installation according to any one of claims 28 - 32 , wherein at least one outer diameter ring member is mounted so as to be displaceable in axial direction relative to the 0 run-off part.

34. Installation according to any one of claims 28 - 33 , wherein the first external cooling device includes one or more cooling liquid spray nozzles adapted to spray or jet cooling liquid onto the exterior of the tube. 25

35. Installation according to any one of claims 28 - 34 , wherein the first external cooling device is adapted and operated to adjust the length and/or location with respect to the run-off part of the expansion device of the stretch of the oriented tube that is affected by the first external cooling device. 30

36. Installation according to any one of claims 28 - 35 , wherein the run-off part includes an increased diameter portion - downstream of the one or more outer diameter ring members and delimiting the downstream end of a reduced diameter section of the run-off part - said increased diameter portion having a greater diameter than said reduced diameter section 35 such that the oriented tube sealingly engages the increased diameter portion. -43

37. Installation according to claim 36, wherein the run-off part has a single reduced diameter section, wherein upstream and downstream outer diameter ring members are arranged at the location of said single reduced diameter section, the upstream outer diameter ring member being adapted to contribute to a sealing engagement of the oriented 5 tube with the expansion device in a region at or near the maximum diameter of the expansion part, the downstream outer diameter ring member being adapted to exert a constrictive force on the oriented tube which contributes to a sealing engagement of the oriented tube with the increased diameter portion of the run-off part. 0

38. Installation according to any one of claims 28 - 37, wherein a second external cooling device is arranged downstream of the expansion device, which second external cooling device being adapted to externally cool the oriented tube, said second external cooling device being controlled independent from the first external cooling device, said second external cooling device comprising one or more cooling liquid spray nozzles adapted to spray 5 or jet cooling liquid, e.g. water, onto the exterior of the oriented tube.

39. Biaxially oriented thermoplastic tubing obtained with the method according to any one of preceding claims 1 - 27. 0

40. Biaxially oriented PVC tubing obtained with the method according to any one of the preceding claims 1 -27.

41. Biaxially oriented pressure pipe for water transport obtained with the method according to any one of preceding claims 1 - 27. 25 HOPPMANN INTERNATIONAL B.V. WATERMARK PATENT & TRADE MARKS ATTORNEYS 30 P35903AU02