NL2022421B1

NL2022421B1 - A method for producing a biaxially oriented tube from thermoplastic material

Info

Publication number: NL2022421B1
Application number: NL2022421A
Authority: NL
Inventors: Wilhelmus Hendrikus Rijnhart Nicolaas; Lucas Meier Gerrit
Original assignee: Hoppmann Int B V
Priority date: 2019-01-18
Filing date: 2019-01-18
Publication date: 2020-08-18

Abstract

A method for producing a biaxially oriented tube from thermoplastic material, wherein a tube in preform condition is extruded from thermoplastic material and is subjected to a temperature conditioning. Use is made of an expansion device and of a drawing device which is arranged downstream of the expansion device. The expansion device comprises a non-deformable expansion part and a run-on part located upstream of the expansion part and having an upstream sealing member. The expansion device comprises a first gas discharge duct having one or more first inlet ports in the exterior surface of the expansion part of the expansion device, a first inlet port being open or closed or partly closed dependent on whether or not the first inlet port is covered and closed, or partly closed, by the tube. A first gas volume is present between the expansion device and the tube, the first gas volume being limited at an upstream end thereof by sealing contact between the tube in preform condition and the upstream sealing member of the run-of part and a downstream end thereof by sealing engagement between the tube and at least a downstream portion of the expansion part, e.g. near or at the transition to the run-off part. A first gas flow is effected from a first gas supply source via the first gas supply duct and the one or more first gas outlet ports thereof into the first gas volume and the pressure of the gas in the first gas volume is used to cause gradual expansion of the tube already before the tube contacts the expansion part. The first gas flow is maintained at a gas pressure independent constant first gas flow rate.

Description

P33945NL00

A METHOD FOR PRODUCING A BIAXIALLY ORIENTED TUBE FROM THERMOPLASTIC

MATERIAL The present invention relates to methods and devices for manufacturing biaxially oriented tubing, e.g. for water mains or other pressurized medium transportation, of thermoplastic material.

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. 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, e.g. for transportation of water, are interconnected end-to-end, e.g. via socket connections.

In WO2011/049436 a production process for producing biaxially oriented tubing of thermoplastic material, e.g. pipes of polyvinylchloride, according to the preamble of claim 1 is disclosed. Herein a thick-walled 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 inner die member forms a lumen in the tube in preform condition. The tube in preform condition is subjected to a temperature conditioning, so that a tempered tube in preform condition is obtained having an orientation temperature which is suitable for the thermoplastic material. Use is made of an expansion device and of a drawing device which is arranged downstream of the expansion device. The expansion device is connected to the extruder die head via an anchor rod that extends through the lumen of the thick-walled tube in preform condition. The expansion device comprises: - a non-deformable expansion part having a gradually increasing diameter to a maximum diameter at a downstream end thereof, - a run-on part which is located upstream of the expansion part, said run-on part having an upstream sealing member arranged upstream of the expansion part.

The expansion device comprises a first gas supply duct that has one or more first gas outlet ports in the outer surface of the expansion device downstream of the upstream sealing

2. member. This first gas supply duct extends in upstream direction through the anchor rod to the extruder and is connected there to a source of pressurized gas, e.g. an air compressor feeding into a pressurized air storage tank that is connected via a valve arrangement to the first gas supply duct. The expansion device also comprises a first gas discharge duct that has one or more first inlet ports in the exterior surface of the expansion part of the expansion device. This first gas supply duct extends in upstream direction through the anchor rod to a discharge, e.g. into the ambient air. In operation a first inlet port is open or closed or partly closed dependent on whether or not the first inlet port is covered and closed, or partly closed, by the tube, so generally a self-governing situation. A first gas volume is present between the expansion device and the tube. This first gas volume is limited at an upstream end thereof by a sealing contact between the tube in preform condition and the upstream sealing member of the run-of part and at a downstream end thereof by a sealing engagement between the tube and at least a downstream portion of the expansion part, e.g. near or at a transition to the run-off part. During operation a first gas flow is effected from a first gas supply source, e.g.

an air compressor unit with an air storage tank, via the first gas supply duct and the one or more first gas outlet ports thereof into the first gas volume. Dependent on whether or not the first inlet port is covered and closed, or partly closed, by the tube, gas flows out of the first gas volume via the one or more first gas inlet ports and the first discharge duct. The pressure of the gas in the first gas volume is used to cause gradual diametrical expansion of the tube already before the tube contacts the expansion part, which contact may bring about a further diametrical expansion of the tube. Also as is disclosed in W02011/049436 it is known to make use of an expansion device having a non-deformable run-off part that adjoins the expansion part of the expansion device downstream of the expansion part. This run-off part has a reduced diameter section having a smaller diameter than the maximum diameter of the expansion part. The run-off part has a downstream sealing member that is downstream of the reduced diameter section. A second gas volume is established between the reduced diameter section run-off part of the expansion device and the tube. This second gas volume is separated from the first gas volume through an upstream sealing engagement of the tube with the expansion device in a transition region. The second gas volume is delimited by a downstream sealing engagement between the tube and the downstream sealing member of the run-off part. The expansion device comprises a second gas duct having a port in the exterior surface of the reduced diameter section of the run-off part. The second gas duct is connected to a second gas source.

-3- In general the method comprises drawing the tempered tube in preform condition over the expansion device using the drawing device. In all the tube is transformed from a relatively thick-walled tube in preform condition into a biaxially oriented tube having a thinner wall thickness and with thermoplastic material in said wall which is oriented in axial direction and in circumferential direction of the tube. As known in the art the biaxially oriented tube is cooled in order to freeze in the obtained biaxial orientation.

In a prior art production process a constant first gas pressure is maintained by means of a first gas pressure control valve regulating the gas pressure in the first gas volume. Also a constant second gas pressure is maintained by means of a second gas pressure control valve regulating the gas pressure in the second gas volume.

In particular in view of the large diameters of biaxially oriented tubing to be produced, e.g.

more than 300 mm OD, e.g. of between 400 and 650 mm OD, stability of the production has become an even more important issue.

The present invention aims to provide measures that enhance stability of the process.

The invention provides for a production method according to the preamble of claim 1, which is characterized in that the first gas flow, that is supplied from the first gas supply source via the first gas supply duct and the one or more first gas outlet ports thereof into the first gas volume, is maintained at a gas pressure independent constant first gas flow rate.

The inventive concept does away with the prior art approach of control of the gradual expansion in the production process on the basis of maintaining a constant gas pressure in the first gas volume. Instead the inventive concept provides control of the gradual expansion in the production process on the basis of maintaining a constant first gas flow rate from the respective source into the first gas volume, which first gas flow is independent of the actual gas pressure in the first gas volume as that pressure is now predominantly governed by the one or more first inlet ports to the first gas discharge duct being open or closed or partly closed dependent on whether or not the first inlet port is covered and closed, or partly closed, by the tube.

In practical embodiments a gas mass flow controller valve, e.g. manufactured by Bronkhorst, may be provided and operated to maintain the gas pressure independent constant first gas flow rate to the first gas volume. It has been observed that local variations in the stiffness of the tube mainly due to variation in temperature of the tube, e.g. between one axial segment

-4- of the tube and another axial segment of the tube, lead to variation of the actual diametrical expansion and thus to variation of the opening and closing of the first inlet ports of the gas discharge duct. It is found that with the inventive supply of gas to the first gas volume at a constant flow rate — instead of the prior art constant gas pressure in the first gas volume - a stabilization of the gradual expansion of the tube and thus of the entire biaxial orientation process is obtained, e.g. reflected in further enhanced accuracy of the diameter and/or of the final wall thickness of the obtained tube.

In a practical embodiment, as shown in exemplary embodiments in W02011/049436, an anchor rod extends from the location of the extruder through the lumen of the tube in preform condition to the expansion device. The anchor rod serves to keep the expansion device in place. In practical embodiments the anchor rod may be more than 10 meters in length between the extruder and the expansion device, even more than 20 meters in order to have sufficient length for equipment involved in cooling the extruded tube in preform condition and tempering the tube to reach the desired temperature profile for the biaxial orientation process.

In a practical embodiment, the first gas supply duct extends in upstream direction through the anchor rod to the location of the extruder and is connected there to a source of pressurized gas, e.g. an air compressor feeding into a pressurized air storage tank that is connected via a valve arrangement to the first gas supply duct. As preferred the valve arrangement comprises said gas mass flow controller valve that serves a maintain a pressure independent flow rate of air, or another gas, into the first gas volume.

In a practical embodiment, as shown in exemplary embodiments in WO2011/049436, the expansion device the first gas supply duct extends in upstream direction through the anchor rod to a discharge end, e.g. into the ambient air.

In a further development of the inventive method it is envisaged that the first gas flow out of the out of the first gas volume via the one or more first gas inlet ports and the first gas discharge duct is throttled by an adjustable throttle valve. This measure allows to stabilize the gas flow out of the first gas volume that takes place via the first gas discharge duct. In a practical embodiment the throttle valve is arranged at the location of the extruder, so at the discharge end of the duct.

In an embodiment the first gas discharge flow monitoring device is provided, e.g. at the location of the extruder, that is operated to monitor the first gas flow out of the first gas

-5. volume via the first gas duct. In an embodiment the method comprises a control routine wherein the monitored first gas flow out of the first gas volume is compared to the constant first gas flow rate into the first gas volume. In an embodiment the throttle valve of the first gas discharge duct is automatically adjusted on the basis of this comparison, e.g. the throttling valve being further opened in case the comparison shows that the gas flow out of the first volume is too much below the constant first gas flow rate into the first gas volume and the throttling valve being further closed in case the comparison shows that the gas flow out of the first volume is too much above the constant first gas flow rate into the first gas volume set. This approach can be done using a comparison done over a time interval, e.g. with multiple measurements being averaged allowing to enhance stability. In an embodiment the expansion device has a first gas pressure sensing duct distinct from the first gas supply duct and from the first gas discharge duct. The first gas pressure sensing duct has a first gas pressure sensing port in the exterior surface of the expansion part of the expansion device and a first gas pressure sensor is in communication with the first gas pressure sensing duct and measures the actual gas pressure in the first gas volume. In a practical embodiment the first gas pressure sensor is located near the extruder. The provision of the first gas pressure sensor and associated duct allows to monitor the actual gas pressure, e.g. allowing to avoid overpressurization that may lead to leakage of gas from the first volume at one of the upstream or downstream end thereof (i.e. the sealing engagement with the upstream sealing member and the engagement with the expansion part). It may also allow for monitoring whether the inventive control of the first gas volume slowly shifts towards an effective gas pressure in the first gas volume that is deemed too high or too low.

In an embodiment, as shown in exemplary embodiments in WO2011/049438, use is made of an expansion device having a non-deformable run-off part that adjoins the expansion part of the expansion device downstream of the expansion part. Herein the run-off part has a reduced diameter section having a smaller diameter than the maximum diameter of the expansion part. The run-off part has a downstream sealing member that is downstream of the reduced diameter section. A second gas volume is established between the reduced diameter section run-off part of the expansion device and the tube. This second gas volume is separated from the first gas volume through an upstream sealing engagement of the tube with the expansion device in a transition region. The second gas volume is further delimited by a downstream sealing engagement between the tube and the downstream sealing member of the run-off part. It is envisaged that the expansion device comprises a second gas duct having a port in the exterior surface of the reduced diameter section of the run-off

-B- part and that this second gas duct is connected to a second gas source, distinct from the first gas source and the flow of gas through the first gas volume. It is envisaged that the pressure of the gas in the second gas volume is maintained at a constant second gas pressure during production of the biaxially oriented tube. It is further envisaged that a second gas flow monitoring device is provided that is operated to monitor any gas flow into and out of the second gas volume via the second gas duct. This inventive approach allows to determine whether or not gas leaks out of or into the second gas volume, e.g. due to the upstream sealing engagement of the tube with the expansion device in a transition region not being as effective. In embodiments corrective action may be derived from said monitoring of any gas flow into and out of the second gas volume, e.g. cooling of the tube when passing over the run-off part being adapted, e.g. increased to enhance the sealing engagement of the tube at the upstream and/or downstream end of the second gas volume.

In an embodiment, as shown in exemplary embodiments in WO2011/049436, an upstream outer diameter ring member is arranged around the tube at an axial location corresponding to the axial location of the reduced diameter section. Preferably said upstream outer diameter ring member is the sole outer diameter that is arranged around the tube at an axial location corresponding to the axial location of the reduced diameter section. Herein the upstream outer diameter ring member is arranged such that the oriented tube passes through the upstream outer diameter ring member while being in contact with said upstream outer diameter ring member. In practical embodiments the upstream outer diameter ring member is arranged in proximity to the transition between the expansion part and the run-off part of the expansion device.

In an embodiment use is made of a first external cooling device that is adapted and operated to cool the oriented tube externally while passing over the run-off part, e.g. the first external cooling device having one or more nozzles that spray a liquid coolant, e.g. water, on the tube, preferably on a segment of the tube directly downstream of the upstream outer diameter ring member with the exterior of the tube remaining dry upstream of said upstream outer diameter ring member.

In a further development a gas compartment sealing member is arranged at a distance downstream of the non-deformable run-off part of the expansion device so that a third gas volume is established in the lumen of the tube downstream of the run-off part, which third gas volume is separated from the second gas volume through the downstream sealing engagement between the tube and the downstream sealing member of the run-off part and is separated from the first gas volume. Herein the expansion device comprises a third gas duct

-7- having a port in communication with the third gas volume. The third gas duct is connected to a third gas source. The pressure of the gas in the third gas volume is maintained at a constant third gas pressure, and is set independent of the pressure in the second gas volume. A third gas flow monitoring device is provided that is operated to monitor any gas flow into and out of the third gas volume via the third gas duct. This inventive approach allows to determine whether or not gas leaks out of or into the third gas volume, e.g. due to the sealing engagement of the tube with the downstream sealing member of the run-off part not being as effective and/or the gas compartment sealing member being not as effective. In embodiments corrective action may be derived from said monitoring of any gas flow into and out of the third gas volume, e.g. cooling of the tube when passing over the third gas volume being adapted, e.g. increased to enhance the sealing engagement of the tube at the upstream and/or downstream end of the third gas volume.

In an embodiment the invention envisages that both any flow into and out of the second gas volume and any flow into and out of the third gas volume are being monitored, e.g. allowing to establish whether gas flows from the second into the third gas volume or in opposite direction, e.g. corrective action being based thereon, e.g. adjustment of the gas pressure setting for any or both of the second and third gas volumes.

In an embodiment use is made of a second external cooling device that is adapted and operated to cool the oriented tube externally downstream of the downstream outer diameter ring member at an axial location upstream of or in proximity of the gas compartment sealing member delimiting the downstream end of the third gas volume. For example, this second external cooling device having one or more nozzles that spray a liquid coolant, e.g. water, on the tube, preferably on a segment of the tube directly downstream of the downstream outer diameter ring member. For example this second external cooling device is controlled, at least in part, on the basis of the monitoring of any gas flow into and out of the third gas volume, possibly also the second gas volume. For example monitoring both the gas flow into and out of the second and the third gas volumes may allow to determine whether the sealing by means of the gas compartment sealing member is effective, and if not, possibly, the cooling effect of the second external cooling device is increased so that the tube shrinks harder and seals stronger on the gas compartment sealing member.

In a preferred embodiment the gas compartment sealing member is a flexible member, e.g. a member with a flexible perimeter, e.g. of elastomer material.

-8- In an embodiment at least one outer diameter ring member is mounted so as to be displaceable in axial direction. In an embodiment the downstream outer diameter ring member is mounted so as to be displaceable in axial direction relative to the stationary mounted upstream outer diameter ring member.

In an embodiment the downstream outer diameter ring member is mounted so as to be displaceable in axial direction, and is displaced on the basis of a measurement of the diameter of the tube downstream of the third gas volume.

In an embodiment the biaxially oriented tubing to be produced, has a diameter of more than 300 mm OD, e.g. of between 400 and 650 mm OD. The present invention also relates to an installation for production of a biaxially oriented tube as described herein.

The present invention also relates to biaxially oriented thermoplastic tubing obtained with the method as described herein. The present invention also relates to biaxially oriented PVC tubing obtained with the method as described herein. The present invention also relates to biaxially oriented pressure pipe for water transport obtained with the method as described herein.

The present invention also relates an expansion device adapted for use in a method as described herein and/or in an installation as described herein. The invention will now be discussed 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 according to the present invention, Fig. 2 shows schematically in longitudinal section a part of the installation of figures 1a,b,c. Figures 1a, 1b 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 generally horizontal, and may in practical embodiments have a length from extruder to final drawing device of over 50 meters.

-9- The installation comprises an extruder 1 having one or more extruder screws 2 by means of which a flow of molten thermoplastic material is provided, e.g. of polyvinylchloride (PVC).

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.

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 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 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.

Asis 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 200°C 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 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.

An outer diameter calibrating device 8a may be provided at the upstream end of the external cooling device 8.

-10- 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 condition10, the speed of the tracks being controlled by a suitable track drive system. Such drawing devices 15 are customary in plastic pipe extrusion.

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 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.

The figure 1b schematically depicts an expansion device 20, which will be discussed in more detail below.

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 other end of the anchoring rod 21 is connected at the location of the extruder, here to the die head 3. In an offset die head the rod 21 could also be fastened to an anchoring device arranged at or near the extruder.

Instead of one extruder 1 multiple extruders 1 could be provided to supply molten material to the die head 3.

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.

At a distance downstream of the expansion device 20, as is common in this technology, a 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 reduced in the process so that an biaxially oriented tube 10 is obtained.

-11- As is preferred an external cooling of the oriented tube is effected soon after the diametrical expansion of the tube 10 has been brought about, preferably —as here — whilst the tube 10 passes over the run-off part, most preferably starting close to, yet not on, the expansion part. 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 arrive at an intense external cooling.

In an embodiment 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.

Optionally 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.

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.

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.

As can be seen- and as is preferred - use is made of at least one external heat exchange or device 110 that is adapted to influence the temperature of the tube in preform condition 10 arriving at the expansion device. For example the device 110 includes infrared heaters and/or a heated air chamber through which the tube 10 passes. This heat exchange may be done in view of the desired temperature profile yet also in view of the sealing contact between the tube in preform condition 10 and the upstream 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 upstream sealing member 30 to influence the temperature of the tube in preform condition 10 from the exterior thereof.

-12- As is preferred a second external heating device 120 may be provided near or overlapping with {a part of) the expansion part of the expansion device 20.

In an arrangement with a first external heating device 110 and a second external heating device 120, each heating device 110, 120 being controllable independently, the first heating device 110 could be used primarily for controlling the sealing engagement with the 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. As can be seen in figure 2 the expansion device comprises: - a non-deformable expansion part 20b having a gradually increasing diameter to a maximum diameter at a downstream end thereof, - a run-on part 20a which is located upstream of the expansion part 20a, said run-on part having an upstream sealing member 30 arranged upstream of the expansion part 20b, - a non-deformable run-off part 20c that adjoins the expansion part 20b of the expansion device downstream of the expansion part 20b, wherein the run-off part has a reduced diameter section 20c1 having a smaller diameter than the maximum diameter of the expansion part 20b and wherein the run-off part has a downstream sealing member 20c2 that is downstream of the reduced diameter section 2001. A first gas supply duct 200 is illustrated, which first gas supply duct 200 has one or more first gas outlet ports 201 in the outer surface of the expansion device 20 downstream of the upstream sealing member 30. A first gas discharge duct 210 is illustrated as well. This first discharge duct 210 has one or more first inlet ports 211 in the exterior surface of the expansion part 20b of the expansion device, a first inlet port 211 being open or closed or partly closed dependent on whether or not the first inlet port 211 is covered and closed, or partly closed, by the tube 10. A first gas volume 215 is present between the expansion device 20 and the tube 10. This first gas volume 215 is limited at an upstream end thereof by sealing contact between the tube in preform condition and the upstream sealing member 30 of the run-on part 20a and a downstream end thereof by sealing engagement between the tube 10 and at least a

-13- downstream portion of the expansion part 20b, e.g. near or at the transition to the run-off part, here formed by a replaceable ring 20d at said transition. A first gas flow is effected from a first gas supply source 220, e.g. at the end of the duct 200 that emerges from the anchor rod, via the first gas supply duct 200 and the one or more first gas outlet ports 201 thereof into the first gas volume 215 and, dependent on whether or not the first inlet port 211 is covered and closed, or partly closed, by the tube 10, out of the first gas volume 215 via the one or more first gas inlet ports 211 and the first discharge duct 210. Herein the pressure of the gas, e.g. air, in the first gas volume 215 is used to cause gradual expansion of the tube 10 already before the tube contacts the expansion part 20b. This expansion may form the majority of the diametrical expansion of the tube, e.g. about all of the diametrical expansion, e.g. to over 80% of the final diameter of the tube. The remainder of the diametrical expansion is then obtained through the tube being forced over the last section of the expansion part 20b, said direct mechanical contact causing said remainder of the diametrical expansion. The first gas flow that is supplied from the first gas supply source 220 via the first gas supply duct 200 and the one or more first gas outlet ports 201 thereof into the first gas volume 215 is maintained at a gas pressure independent constant first gas flow rate as explained above. In an embodiment the first gas flow out of the out of the first gas volume 215 via the one or more first gas inlet ports 211 and the first discharge duct 210 is throttled by an adjustable throttle valve 230.

In an embodiment a control routine is effected by a computerized controller of the installation, or by an operator, wherein the monitored first gas flow out of the first gas volume 215 is compared to the constant first gas flow rate into the first gas volume 215. In an embodiment the throttle valve 230 of the first gas discharge duct 210 is automatically adjusted on the basis of this comparison. A first gas pressure sensing duct 240 is illustrated, which is distinct from the first gas supply duct 200 and from the first gas discharge duct 210. The first gas pressure sensing duct 240 has a first gas pressure sensing port 241 in the exterior surface of the expansion part 20a of the expansion device. A first gas pressure sensor 242 is in communication with the first gas pressure sensing duct and measures the actual gas pressure in the first gas volume 215. The first sensor 242 is preferably arranged at the extruder side end of the duct 240.

-14- The run-off part 20c has reduced diameter section 20c1 having a smaller diameter than the maximum diameter of the expansion part. The run-off part has a downstream sealing member 20c2 that is downstream of the reduced diameter section 20c1.

A second gas volume 250 is established between the reduced diameter section run-off part 2001 of the expansion device and the tube 10. The second gas volume is separated from the first gas volume 215 through an upstream sealing engagement of the tube 10 with the expansion device in a transition region 20d. The second gas volume 250 is delimited by a downstream sealing engagement between the tube and the downstream sealing member 20c2 of the run-off part. A second gas duct 260 is illustrated having a port 261 in the exterior surface of the reduced diameter section 20c1 of the run-off part.

The second gas duct 260 is connected to a second gas source 265. The pressure of the gas in the second gas volume 250 is maintained at a constant second gas pressure. In an embodiment a second gas flow monitoring device 263 is provided that is operated to monitor any gas flow into and out of the second gas volume 250 via the second gas duct

260. It is shown that an upstream outer diameter ring member 90 is arranged around the tube at an axial location corresponding to the axial location of the reduced diameter section 20c1, wherein the upstream outer diameter ring member 90 is arranged such that the oriented tube passes through the upstream outer diameter ring member while being in contact with said upstream outer diameter ring member. A gas compartment sealing member 20e is arranged at a distance downstream of the non- deformable run-off part 20c of the expansion device so that a third gas volume 280 is established in the lumen of the tube downstream of the run-off part 20c. The third gas volume 280 is separated from the second gas volume 250 through the downstream sealing engagement between the tube and the downstream sealing member 20c2 of the run-off part.

-15- A third gas duct 290 has a port 291 in communication with the third gas volume 280. The third gas duct is connected to a third gas source 295. The pressure of the gas in the third gas volume 280 is maintained at a constant third gas pressure. A third gas flow monitoring device 293 is provided that is operated to monitor any gas flow into and out of the third gas volume 280 via the third gas duct. A downstream outer diameter ring member 91 is arranged around the tube at an axial location corresponding to the axial location of the third gas volume 280. The downstream outer diameter ring member is arranged such that the oriented tube passes through the downstream outer diameter ring member while being in contact with said downstream outer diameter ring member. The first external cooling device 60 is adapted and operated to cool the oriented tube externally while passing over the run-off part in a region between the upstream and the downstream outer diameter ring members 90, 91. The mentioned ducts all extend through or along the anchoring rod 21 for supply and discharge of gas (e.g. air).

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.

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 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 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 explained.

The expansion part 20b and as is preferred also the run-on part 20a and the run-off part 20c here are of rigid, non-deformable embodiment.

-18- As is preferred, there is no external part of the installation at the height of the upstream 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 upstream sealing member 30 and any external part.

This upstream 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 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.

Asis preferred the sealing member 30 is a separately manufactured annular member fitted on a tubular member of the run-on part.

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 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.

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.

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 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

-17- 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 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.

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 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.

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 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.

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.

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

-18- 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 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.

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.

A displacement device 65, here embodied as motorized drive assembly, for axial displacement of at least one of the ring members 90, 91 in axial direction along the run-off part 20a is provided. In this example, the device 85 includes one or more screw spindles 66, e.g. operated by a common electric motor.

Asis 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 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 first external cooling device 80 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.

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 downstream of the second external cooling device 70.

-19- 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.

With the ring members 20, 91 both suitably dimensioned as constrictive ring members, the effect can be obtained that the ring member 90 may contribute to the sealing engagement of the tube with the expansion device in the region at or near the maximum diameter of the expansion part 20b. This avoids uncontrolled escape or leakage of fluid from the one volume to the other volume.

The ring member 91 may contribute to the sealing engagement of the oriented tube with the increased diameter portion 20c2. This avoids or at least limits any leakage of fluid into the 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 20c2, thereby enhancing the sealing contact between the tube and the increased diameter portion 20c2.

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 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 are arranged below tube 10 so as to support said 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 is fitted at the transition 20d between the expansion part 20b and the run-off part 20c of the expansion device, the replaceable ring 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 20c2 is formed by a downstream replaceable ring. 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.

-20- The gas discharge duct 210 provides for the relief of gas pressure from the volume 215 as the corresponding inlet port 211 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 211 is fully covered and closed by the tube 10. So the cooperation of the tube 10 with the inlet port 211 achieves in a very attractive manner a control of the degree of expansion that is caused in the tube 10 due to the internal gas pressure in volume 215. Effectively the position of the inlet port 211 on the sloping exterior face of the expansion part of the device 20 controls where the tube 10 will contact said face, assuming that the gas pressure in volume 215 is sufficient to cause the tube 10 to expand.

It is noted that a group of multiple inlet ports 211 connected to a common gas discharge duct 210 could be arranged distributed around the circumference of the expansion part 20b 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.

In another embodiment, multiple inlet ports 211, each associated with a corresponding discharge duct 210, are provided at differing diameter positions in 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 that are associated with the discharge ducts, so that a selected inlet port and associated discharge duct can be made effective to relief gas pressure when the tube does not cover and close said inlet port, whereas one or more non-selected inlet ports 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 before reaching the non-deformable expansion part.

Claims

-21- CONCLUSIONS

A method of producing a biaxially oriented tube of thermoplastic material, wherein a tube is extruded in a preformed condition from thermoplastic material using an extruder having an extrusion die head having an internal die element, the inner die element being the tube in preformed condition forms a hollow space, the tube in preformed condition being subjected to a temperature conditioning, so that a tempered tube in preformed condition is obtained having an orientation temperature suitable for the thermoplastic material, using an expansion device and of a pulling device disposed downstream of the expansion device, the expansion device comprising: - a non-deformable expansion member having a diameter that gradually increases to a maximum diameter at a downstream end thereof, - a folding part located upstream of the expansion part, the ramp part having an upstream closing element arranged upstream of the expansion part, the expansion device including a first gas supply channel, the first gas supply channel having one or more first gas outlet openings in the outer surface of the expansion device downstream of the upstream closure member, the expansion device comprising a first gas outlet channel, the first outlet channel having one or more first inlet openings in the outer surface of the expansion portion of the expansion device, a first inlet opening being opened or closed or partially closed depends on whether the first inlet opening is covered and closed, or partially closed, by the tube, a first volume of gas being located between the expander and the tube, the first volume of gas being limited at an upstream end thereof d for sealing contact between the tube in preformed condition and the upstream sealing element of the ramp and at a downstream end thereof by sealing engagement

-22- between the tube and at least a downstream portion of the expansion part, e.g. near or at the transition to the downstream part, whereby a first gas flow is established from a first gas supply source via the first gas supply channel and the one or more first gas outlet openings thereof the first gas volume into and, depending on whether the first inlet opening is covered and closed, or partially closed, through the tube, the first gas volume out through the one or more first gas inlet openings and the first discharge channel, whereby the pressure of the gas in the first gas volume is used to effect gradual expansion of the tube already before the tube comes into contact with the expansion member, the method comprising drawing the tempered tube over the expansion device in preformed condition using the pulling device, and wherein the tube is transformed from a tube in preformed condition to a biaxially oriented one tube of thermoplastic material oriented axially and circumferentially of the tube, the biaxially oriented tube being cooled, characterized in that the first gas flow flowing from the first gas supply source through the first gas supply channel and the one or more first gas outlet openings thereof the first volume of gas is supplied in is maintained at a constant first gas flow rate independent of gas pressure.

The method of claim 1, wherein the first gas flow is throttled out of the first volume of gas through the one or more first gas inlet ports and the first gas discharge tube by an adjustable throttle valve.

A method according to claim 2, wherein the method comprises a control routine in which the monitored first gas flow is compared with the first gas volume out of the first gas volume against the constant first gas flow rate in the first gas volume, preferably automatically turning the throttle valve of the first gas discharge channel on the basis of this comparison. adjusted.

A method according to any one of claims 1 to 3, wherein the expansion device has a first gas pressure sensing channel different from the first gas supply channel and from the first gas discharge channel, the first gas pressure sensing channel having a first gas pressure sensing opening in the outer surface of the expansion portion of the expansion. -

-23.

device and wherein a first gas pressure sensor is in communication with the first gas pressure sensing channel and measures the current gas pressure in the first gas volume.

A method according to any one of claims 1 to 4, wherein use is made of an expansion device having a non-deformable drain portion connecting downstream of the expansion portion of the expansion device to the expansion portion, the drain portion having a reduced diameter section. which has a smaller diameter than the maximum diameter of the expansion section, and wherein the downstream section has a downstream shut-off element located downstream of the reduced diameter section, whereby a second volume of gas is established between the reduced diameter section of the downstream part of the expansion device and the tube, wherein the second gas volume is separated from the first gas volume by an upstream occlusive engagement of the tube with the expander in a transition zone and the second gas volume is defined by a downstream occlusive engagement between the tube and the downstream occluder member from the downstream portion, the expansion device comprising a second gas channel having an opening in the outer surface of the reduced diameter section of the downstream portion, the second gas channel being connected to a second gas source and the pressure of the gas in the second gas volume at a constant second gas pressure is maintained, and wherein a second gas flow monitoring device is provided which is operated to monitor each gas flow in and out of the second gas volume through the second gas channel.

The method of claim 5, wherein an upstream outer diameter ring element is disposed around the tube at an axial location corresponding to the axial location of the reduced diameter section, the upstream outer diameter ring element being disposed such that the oriented tube passes through the upstream outer diameter ring element passes while in contact with the upstream outer diameter ring element.

A method according to any one of claims 1 to 6, wherein use is made of a first external cooling device arranged and operated to externally cool the oriented tube as it passes over the downstream portion.

The method of claim 5, wherein a gas compartment closure element is spaced from and downstream of the non-deformable drain portion of the expansion device so that a third volume of gas is created in the cavity of the expansion device.

The tube downstream of the downstream portion, the third volume of gas being separated from the second gas volume by the downstream occlusive engagement between the tube and the downstream sealing member of the downstream portion, and wherein the expander comprises a third gas channel having an opening is in communication with the third gas volume, wherein the third gas channel is connected to a third gas source and wherein the pressure of the gas in the third gas volume is maintained at a constant third gas pressure, and wherein a third gas flow monitor is provided which is operated to monitor each gas flow in and out of the third gas volume through the third gas channel.

The method of claim 8, wherein a downstream outer diameter ring element is disposed around the tube at an axial location corresponding to the axial location of the third volume of gas, the downstream outer diameter annulus element being arranged such that the oriented tube passes through the downstream outer diameter - ring element passes while in contact with the downstream outer diameter ring element, and wherein the first external cooling device is arranged and operated to externally cool the oriented tube while it passes over the downstream part in a zone between the upstream and downstream outer diameter passing ring elements.

10. Plant for producing a biaxially oriented tube of thermoplastic material, wherein a tube in a preformed condition of thermoplastic material is extruded using an extruder having an extrusion die head having an inner die element, the inner die element being the tube in preformed condition forms a hollow space, the tube in preformed condition being subjected to temperature conditioning so that a tempered tube in preformed condition is obtained having an orientation temperature suitable for the thermoplastic material, the installation comprising an expansion device and a pulling device disposed downstream of the expansion device, the expansion device comprising:

- 25.- a non-deformable expansion part having a diameter that gradually increases to a maximum diameter at a downstream end thereof, - a ramp-up part located upstream of the expansion part, the ramp-up part having an upstream closure element upstream of the expansion part wherein the expansion device comprises a first gas supply channel, the first gas supply channel having one or more first gas outlet openings in the outer surface of the expansion device downstream of the upstream closure member,

wherein the expansion device comprises a first gas outlet channel, the first outlet channel having one or more first inlet openings in the outer surface of the expansion portion of the expansion device, a first inlet opening being opened or closed or partially closed, depending on whether the first inlet opening is covered and closed, or partially closed, by the tube, wherein, in operation, a first gas volume is located between the expander and the tube, the first gas volume being limited at an upstream end thereof by sealing contact between the tube in preformed condition and the upstream closure member of the ramp and at a downstream end thereof by sealing engagement between the tube and at least a downstream portion of the expansion member, e.g. near or at the transition to the ramp, wherein, during operation, a first gas flow is established v from a first gas supply source through the first gas supply channel and its one or more first gas outlet openings into the first gas volume and, depending on whether the first inlet opening is covered and closed, or partially closed, through the tube, the first gas volume out through the one or more first gas inlet openings and the first discharge channel, the pressure of the gas in the first gas volume being used to effect gradual expansion of the tube even before the tube comes into contact with the expansion member, the action of pulling the expansion device over the expansion device tempered tube in preformed condition when using the pulling device, and wherein the tube is converted from a tube in preformed condition to a biaxially oriented tube with thermoplastic material oriented axially and circumferentially of the tube, the biaxially oriented tube is cooled, characterized in that

The installation is arranged such that the first gas flow supplied from the first gas supply source via the first gas supply channel and the one or more first gas outlet openings thereof into the first gas volume is maintained at a constant first gas flow rate independent of gas pressure.