NL1011469C2 - Production of biaxially oriented thermoplastic tube, by periodical variation of the advancement speed ratio of the preform which is determined by the speed-control and the output of the extruder of different values - Google Patents

Abstract

A biaxially oriented thermoplastic tube is produced by periodical variation of the advancement speed ratio of the preform which is determined by the speed-control and the output of the extruder of different values. The wall thickness of the preform is periodically changed. Production of a biaxially oriented thermoplastic tube comprises extrusion of tubular preform from thermoplastic material using an extruder which is provided with an extruder die with an inner core. The inner core defines a hollow space in the preform. The preform is temperature conditioned to reach an orientation temperature suitable for the plastics material being used. The tempered preform is forced over a mandrel. The mandrel comprises an expansion part, which brings expansion in the circumferential direction of the preform passing over it. The stretched tube is cooled and the advancement speed of the preform upstream of the mandrel is set by speed control. The periodical variation ratio of the advancement speed of the preform is determined by the speed control and the output of the extruder of different values. The wall thickness of preform is periodically changed. An Independent claim is also included for a biaxially oriented thermoplastic tube having a tube body and at one or both ends, an integrally formed socket. The axial stretching of the plastic material in the socket is equal to the axial stretching of the plastic material in the tube body.

Description

Short designation: Manufacture of thermoplastic plastic pipe.

The invention relates to the production of a tube from thermoplastic plastic material, in particular from polyolefin plastic material, such as polyethylene. The invention also relates to the production of a plastic tube, wherein the thermoplastic plastic material is oriented biaxially, which process is known as the biaxial stretching process. The invention also relates to improvements in the process for the production of extruded pipe from thermoplastic plastic material, which process can form part of the production of biaxially oriented plastic pipe. In addition, the invention relates to providing an improved connection between tubes of biaxially oriented thermoplastic plastic material.

The measures provided according to the invention are described in the claims and the following description and will be explained below, in particular with reference to the drawing. In the drawing: Figs. 1a and 1b schematically show a side view of an exemplary embodiment of an installation for manufacturing pipe from biaxially oriented thermoplastic plastic material, Fig. 2 on a larger scale, detail II in Fig. 1a, Fig. 3 on a larger scale scale a part of the mandrel of fig. 1b, fig. 4 in perspective view the mandrel of fig. 3, and fig. 5 in longitudinal section a connection of two tubes of biaxially oriented thermoplastic plastic material according to the invention.

30

Figures 1a and 1b schematically show, in two sub-drawings to be connected to each other, the main elements of an installation for the production in a continuous process of a tube of biaxially oriented 101 1469 -2-thermoplastic plastic material.

The pipe to be manufactured preferably has a wall strength such that the pipe is dimensionally stable. In particular, it is contemplated to manufacture pipe suitable for assembling piping systems for the transport of liquid or gas, in particular for drinking water, sewage water, natural gas and the like. The pipe is preferably suitable for laying in the ground.

Fig. 1a shows an extruder 1 with one or more extruder screws 2 with an associated controllable drive, with which a flow of molten plastic material is provided, which is fed to an extruder head 3 arranged on extruder 1.

The extruder head 3 has an outer cone 4 and an inner cone 5, which defines with the outer cone 4 an annular outlet from which an extruded tube 6 of thermoplastic material is dispensed in a substantially horizontal direction. The inner cone 5 herein defines an axial cavity in the tube 6.

The extruder head 3 is provided with wall thickness control means, not shown, with which a circumferentially uniform wall thickness of the tube 6 emerging from the extruder head 3 can be realized.

An inner cooling member 10 is attached to the inner cone 3, the construction of which will be explained later with reference to Figure 2. The inner cooling member 10 is designed such that the tube 6 emerging from the extruder head 3 is internally cooled immediately downstream of the extruder head 3.

The tube 6 is externally calibrated with the aid of calibration sleeve 20. This calibration sleeve 20 achieves a slight reduction of the outer diameter of the tube 6. The calibration sleeve 20 is arranged downstream of the inner cooling member 10 at a location where the tube 6 is not passed through a solid part is internally supported. This arrangement has the advantage that the tube 6 can then not get stuck at that calibration sleeve 20, since a reduction of the inner diameter of the tube 6 can take place without problems.

Downstream of the calibration sleeve 20 is a first outside cooling device 30, with which the tube 6 is cooled externally. The outside cooling device 30 comprises, for example, a number of compartments arranged one behind the other and through which water flows through cooling water, through which the tube 6 moves and comes into direct contact with the cooling water. Optionally, the cooling water in each compartment has a different temperature in order to optimize the cooling of the tube 6.

Because the outer cooling device 30 is positioned downstream of the inner cooling member 10 in the extrusion direction, it is achieved that the tube 6 coming out of the extruder head 3 is first only cooled internally (a very slight natural cooling of the outside of the tube is left out of the ambient air). ) and then cooled only externally. This ensures that the tube 6 is not simultaneously subjected to the cooling effect of the inner cooling member 10 and the outer cooling device 30. Depending on the magnitude of the axial distance between the inner cooling member 10 and the outer cooling device 30, there may be some overlap between the cooling operation of the internal and external cooling.

Arranging the inner cooling member 10 and the outer cooling device 30 in an axial direction offset from each other appears to be particularly advantageous with pipes of a thermoplastic material which crystallizes on cooling after the extrusion and consequently exhibits a considerable volume shrinkage. This type of material includes polyethylene (PE), which exhibits a volume shrinkage of up to 30%.

Due to the cooling effect of the inner cooling member 10, a cold wall layer is formed immediately after the extruder head 3 on the inside of the tube 6, which cold wall layer is relatively dimensionally stable. If a cold layer on the outside were then simultaneously formed by an outside cooling, a still warm intermediate wall layer of 101 1469-4 plastic material would have been enclosed between two cold and solid wall layers. When that intermediate layer cools, shrinking cavities can easily form in that intermediate layer and there is also a considerable chance of the occurrence of visible deformations, in the form of pits or dents, in the outside and inside of the tube 6. By first only internal To cool, shrinkage of that intermediate layer can be compensated for by supplying material from the uncooled outer layer of the tube. Once the inner layer has cooled, cooling can then start from the outside.

Downstream of the outer cooling device 30 there is a tube speed control means 40, which engages the cooled outer layer of the tube 6. The tube speed control means 40 is here designed as a pulling device known per se with a plurality of tracks engaging on the tube, which type of pulling device is usual with the extrusion of plastic pipes, and will hereinafter be referred to as first tensile testing machine 40.

Downstream of the first tensile bench 40, a heating device 50 is arranged. This device 50 comprises a plurality of heating units disposed around the path for the tube 6, which are individually controllable and each facing a sector of the circumference of the tube 6.

As a result, a separately adjustable amount of heat can be supplied to each sector of the tube 6, for instance six circumferential sectors of 60 ° each.

The installation further comprises a mandrel 60 which is designed to be undeformable, which is also described here with the term prefabricated. The mandrel 60 is here made of metal. The mandrel 60 is held stationary with respect to the extruder 1 and is here attached to the inner cooling member 10 by means of a pulling member 61 and via that inner cooling member 10 to the extruder 1, in particular to the inner cone 5 thereof.

The mandrel 60 has an upstream portion 62 at its upstream end, which is here essentially cylindrical. An expansion part 63 is connected to that ramp part 62, which has an outer surface substantially corresponding to the surface of a truncated circle cone with an increasing diameter downstream. A drain part 64 of the mandrel 60 connects to said expansion part 63, which part 64 has substantially a constant diameter, possibly slightly tapering in the downstream direction.

At the mandrel 60, in particular at the level of the run-off part 64, there is a second outside cooling device 10, 70 with which the tube 6 is cooled externally. As is well known in the manufacture of biaxially oriented plastic pipe, after passing the expansion part of the stretching mandrel, the stretched pipe is cooled, thereby freezing the effected changes in the plastic material of the pipe.

At a distance downstream of the mandrel 60, a second outer calibrator 80 is disposed, which calibrator 80 achieves a reduction in the outer diameter of the tube 6.

The installation also includes a second tensile testing machine 90, which is disposed downstream of the mandrel 60, and of the outer calibrator 80. The second tensile testing machine 90 is intended to exert a considerable tensile force on the tube 6. Downstream of said second tensile testing machine 90, a cutting device can be placed, for instance a sawing, cutting or milling device, for cutting sections of desired length to the manufactured tube. On the other hand, a reel device for winding the tube 6 manufactured on a reel could also be provided.

The tube 6 emerging from the extruder head 3 is thick-walled. After the tube 6 leaves the extruder head 3 and then has a high temperature, the inner cooling member 10, the first outer cooling device 30, 35 as well as the heating device 50 provide such cooling / local additional heating of the tube 6 that the plastic material is an orientation temperature suitable for its biaxial orientation is 101 1 469-6 before the expansion part 63 of the mandrel 60 is passed.

The tube 6 passing over the mandrel 60 is effected under the influence of the forces exerted on the tube 6 by means of the second tensile machine 90 in cooperation with the first tensile machine 40. With the two tensile benches 40 and 90, the speed of the tube 6 can be controlled both at a location upstream of the mandrel 60 (at tensile bench 40) and at a location downstream of mandrel 60 (at tensile bench 90).

Due to the passage over the mandrel 60, the molecules of the plastic material are oriented both in the axial direction and in the circumferential direction of the tube 6, which is of great advantage for the properties of the tube 6.

Details of the installation according to Figures 1a and 1b will be explained in more detail below, partly with reference to the further figures.

The indoor cooling organ.

20

The inner cooling member 10 is partly recognizable in figure 2. The inner cooling member 10 has a rigid, form-retaining cylindrical outer wall, for instance of metal, with a long central section 11, which has a diameter which is slightly smaller than the diameter of the upstream and downstream end of said middle portion 11 located end portions 12 (only the downstream end portion is visible in Figure 2). The difference in diameter between the portion 11 and the portions 12 is preferably no more than 3 millimeters and at least 0.5 millimeters. In figure 1a that difference is shown excessively large.

The axial length of the end portions 12 is considerably smaller than that of the center portion 11, the length of the middle portion 11 preferably being a multiple of the wall thickness of the tube 6. In practice, it is preferred that this length be one meter or more.

101 1469 -7-

The inner cooling member 10 is provided with a supply channel 13, which opens at one or more mouths 14 located in the surface of the middle section 11, which mouths 14 are located near the downstream end section 12. Furthermore, the inner cooler 10 at the upstream end of the center portion 11 also includes one or more mouths (not shown) connecting to a drain of the inner cooler 10.

The installation furthermore comprises cooling means supply means (not shown), which connect to the supply channel 13 and with which cooling liquid can be introduced between the middle part 11 of the inner cooling member 10 and the tube 6. This cooling liquid then forms a film of liquid and flows, preferably at a high speed, against the extrusion direction towards the mouths of the discharge channel. An internal cooling of the tube 6 is thus realized.

On the one hand, the high speed of the cooling liquid in the liquid film has the advantage that despite the small volume of the liquid film, an effective cooling effect can still be obtained. It is important here that the liquid in the film does not evaporate, because then an undesired pressure build-up in the tube 6 would take place. Another important advantage of the high speed concerns the problem of the formation of air or gas bubbles in the cooling liquid. As is known, water is usually used as a cooling liquid and this cooling water contains air. Air bubbles are then formed when the cooling water is heated and these air bubbles usually rise upwards. If interior cooling is used, in which cooling liquid, hereinafter water, comes into direct contact with the inside of the plastic pipe to be cooled, these air or gas bubbles are very disadvantageous. Due to the presence of an air or gas bubble, the inside of the tube is cooled less at that location than in the surrounding area and therefore becomes less dimensionally stable than the cooler surrounding area. Due to the previously described volume shrinkage of the plastic material upon cooling, the shrinking 1011469-8 material will draw in the already stiff surrounding skin layer of the tube. This creates a well in the inside of the tube at the location of the air bubble, in which well the air bubble is enclosed. This keeps the 5 bubble in that place and the cooling of that area remains poor, with the result that the well becomes even deeper. This leads to a clearly visible pit in the inner surface of the tube, which is not acceptable. Incidentally, bubbles can also be formed by gases released from the extruded tube.

In general, any local disturbance of the interior cooling appears to produce a visible mark on the inside of the tube 6 and for this reason it is important that the interior cooling is very regular.

It is already known to vacuum the bubbles during liquid inner cooling with an extraction tube connecting to the highest point of an inner cooling compartment present in the extruded tube and flowing through with coolant. However, that solution is not always possible and / or satisfactory, in particular because the detrimental effect of the air bubbles occurs very soon after the contact of the tube with the air bubbles and because air bubbles once formed tend to stick to the tube despite the extraction.

For these reasons, when using interior cooling, it is important that the tube is provided with a cool and form-retaining layer on the inside by cooling immediately after leaving the extruder head, as is done with the previously described interior cooling element 10. In particular, this important for the indoor cooling of pipe profiles extruded from plastic material such as PE and PP. It has been found that the same problem has less significance with PVC, for example. It is also important to maintain that cool layer during the entire range in which internal cooling takes place, otherwise the aforementioned pitting could still occur. It will also be clear that it is important to prevent the formation of air bubbles, in particular large air bubbles, or the accumulation of air bubbles.

At the inner cooling member 10, the high flow rate of the cooling liquid ensures that only small air bubbles are formed, which are entrained by the fast-flowing liquid 5 and do not adhere to the inside of the tube.

The formation of air bubbles during indoor cooling can also be reduced by first venting the cooling liquid used for the internal cooling, such as water, before introducing the liquid into the tube to be cooled. The venting can be effected, for example, by first boiling the water and then allowing it to cool, the boiling optionally being possible under a lower atmospheric pressure.

Another solution to counteract the disadvantages of air or gas bubbles in indoor cooling is to use a coolant with a low surface tension. This can be done, for example, by using water as a cooling liquid, in which one or more substances are added to the water which effect a reduction in the surface tension. This can be, for example, the addition of alcohol to the cooling water. Due to the low surface tension, the air bubbles are easily formed, but the air bubbles are very small, which leads to less pitting.

Another solution to avoid the adverse effect of air or gas bubbles is to generate a helically directed flow of the cooling liquid along the inside of the tube to be cooled. This flow prevents air bubbles from accumulating along the top of the inner circumference of the tube. Optionally, a shallow helical profile in the surface 11 could be provided at the inner cooling member 10 to generate that flow.

Yet another measure to avoid the detrimental effect of air or gas bubbles is to improve the wetting of the inner surface of the extruded tube so that the liquid adheres better to that surface 101 1 469 -10- and releases the bubbles more easily .

In combination with the inner cooling member 10 attached to the inner cone 5, it is also conceivable that the inner cone 5 is provided with a cooling in order to initiate the internal cooling 5 of the extruded tube 6 earlier.

It will be clear that the indoor cooling solutions described here are suitable not only for use in the manufacture of biaxially oriented pipe, but also for any other extrusion process of pipe profiles of thermoplastic plastic material. However, in the manufacture of biaxially oriented tube from crystalline thermoplastic plastic material, such as PE, the crystallization, and the associated strong volume shrinkage, take place in a temperature range in the vicinity of the orientation temperature, or stretching temperature, which temperature tube if it passes over the mandrel. 1 2 3 4 5 6 101 1 469

In particular, the first outer calibrating sleeve 20 is disposed 2 downstream of the inner cooling member 10 in connection with the previously described embodiment of the inner cooling member 10, wherein only a thin liquid film is present between the tube 6 6 and the inner cooling member 10 Due to that rigid design of the inner cooling member 10, the tube 6 would thus not be able to constrict there without getting stuck on the inner cooling member 10.

30 - Effects of the crystalline composition.

The biaxial stretching process, in which a tube is extruded and that tube is forced in-line over a stretching mandrel, has already been successfully used in amorphous thermoplastic plastics materials, especially polyvinyl chloride tubes. However, many pipes, for example for drinking water and gas pipes, are made of crystalline thermoplastic materials -11-, in particular of polyethylene and polypropylene. The difference between a composition of the plastic material which can be described as amorphous or crystalline appears to have significant effects on the course and implementation of that biaxial stretching process. It is noted that crystalline materials, such as PE and PP, are in fact two-phase systems, with part of the material being amorphous and part crystalline. The ratio between the amorphous part 10 on the one hand and the crystalline part on the other depends in particular on the cooling of the molten plastic material and in particular on the cooling speed.

In the biaxial stretching process, for example with the installation shown in Figs. 1a and 1b, a thick-walled tube is first extruded, which must then be cooled to a suitable orientation temperature and which is significantly lower than the temperature of the tube when it is the extruder head 3 leaves. For this reason, the inner cooling member 10 and the first outer cooling device 30 are active.

In view of the poor thermal conductivity of thermoplastic materials, it is inevitable in this continuous process, which naturally strives for the highest possible production speed, that the cooling of the plastic material does not take place in the same way everywhere in the cross section of the tube. In particular, rapid cooling will take place on the inside and outside of the tube, which come into contact with a cooling medium, and as a result a lot, but especially very small crystals will be formed there. Inside the tube, the cooling will slow down. As a result, many, but very small crystals are formed on the inside and outside of the tube, while larger crystals are formed inside the tube.

This difference may adversely affect the biaxial stretching of the tube and the final result achieved. To solve or reduce that problem, it is conceivable to allow heating of the strongly cooled layer of the tube downstream of the inner cooling of the thick-walled tube coming from the extruder 101 1469-12, so that the small crystals start to grow. This can be done by allowing heating of that layer by heat transport from the center of the pipe wall and / or by bringing the inside of the pipe into contact with a heating medium. In particular, a compartment can be provided behind the inner cooling member in the hollow space of the tube, which compartment is filled with warm liquid, for instance at a temperature between 90-100 ° C.

The previously described problem that when using internal cooling with an extruded tube from a crystalline thermoplastic, many small crystals are formed on the highly cooled inside, can also be solved by designing the tube 15 with a multilayer wall. The inner wall layer, which is cooled fastest by the inner cooling, is then preferably of an amorphous thermoplastic, while the surrounding layer is extruded from a crystalline thermoplastic. For example, the inner layer 20 is of polyvinyl chloride and the outer layer of polyethylene. Incidentally, the same idea can also be applied to the situation in outside cooling, where it is then advantageous that a wall layer of a crystalline thermoplastic is enclosed by an outer layer of an amorphous thermoplastic.

Combination of the above aspects then yields a tube with an inner wall layer of amorphous material and an outer wall layer of amorphous material with a wall layer of a crystalline thermoplastic in between, for instance a three-layer tube with two (thin) shells of PVC, which have a thicker enclose PE interlayer. Such a tube can be subjected to a biaxial stretching process, for example by forcing the tube coming out of the extruder over an expansion mandrel arranged behind it.

Crystal formation can also be influenced by adding a substance to the plastic material that serves as a seed for the formation of crystals. In the production of biaxially oriented polyethylene tubes, the addition of chalk 101 1469-13 has been found to have a beneficial effect on crystal formation. In particular, a large number of crystals are formed quickly.

It is also noted that an inner wall layer of PVC solves or counteracts the previously described problem of pitting by air bubbles in the cooling water of the inner cooling. In particular, PVC has better heat conductivity than PE and the wetting by coolant, especially water, is also better.

It is noted that extruders for extruding multilayer tubes are well known.

Wall thickness control.

When biaxially stretching a tube over a mandrel, any deviations of the wall thickness in the tube still to be passed over the mandrel appear to have a great influence on the behavior of the tube as it passes over the mandrel and thus on the biaxial orientation achieved. It is already known to arrange a wall thickness measuring device between the extruder and the mandrel, with which the thickness of the pipe wall as well as the shape of the cross-section of the pipe can be measured. Such wall thickness measuring devices are often ultrasonic devices, in which an ultrasonic sound pulse is sent from the outside through the tube wall and the reflection of that pulse determines the wall thickness. The reflection is mainly based on the difference between the sound transmission speed through the pipe wall and the medium contained in the pipe.

As previously described, the tube is still relatively warm in the range between the extruder and the stretching mandrel, and this presents problems in the operation of such ultrasonic wall thickness measuring devices. Furthermore, with crystalline thermoplastics, crystallization takes place precisely at the temperatures prevailing in that range, and with it a considerable change in the density of the thermoplastic, which in turn has consequences for the transmission of the ultrasonic pulse. This effect is also detrimental to the effect and reliability of the measurements with the ultrasonic wall thickness measuring equipment. It has been found that the effect improves if a layer of cold liquid lies on the inside of the tube at the location of the ultrasonic wall thickness measurement, or if the tube is filled with a cold liquid at that location. If the liquid were to be warm, for example water in the vicinity of 100 ° C, the ultrasonic wall thickness measurement appears to function considerably worse than with a cold liquid. This can probably be explained by the fact that in particular the difference in transmission speed between the tube and the liquid is important for the reflection of the ultrasonic pulse and that difference is smaller with warm liquid. In known ultrasonic wall thickness measuring devices, one or more ultrasonic transmitter / receivers rotate around the tube. In this embodiment it is conceivable that a supply for a flow of cold liquid revolves in the tube in the same place.

In Fig. 1a, reference numeral 100 schematically indicates an ultrasonic wall thickness measuring device, wherein the above described cold liquid layer is realized with the inner cooling member 10 described in detail earlier.

Another consequence of the wall thickness measurement at a location between the extruder 1 and the mandrel 60 is that the temperature of the tube 6 also influences the ultrasonic wall thickness measurement. As described, that temperature can vary in this range, for example because the operation of the indoor and outdoor cooling is adjusted in the start-up phase. In order to reduce the influence of the pipe wall temperature on the measured wall thickness, provision can be made for arranging a pipe wall temperature measuring device near the ultrasonic wall thickness measuring equipment 100 and in a suitable compensation algorithm with which the influence of the temperature in the measured wall thickness is compensated.

35 - emergence of differences in wall thickness and orientation.

In the biaxial stretching process, one of the important aspects is the passage of the tube over the 1011469 -15 stretching mandrel, thereby stretching the tube in axial and radial directions. It is known from the prior art to strive for a treatment of the extruded tube in the range between the extruder and the mandrel in such a way that the tube has the most uniform wall thickness and preferably also the temperature as uniform as possible. the temperature range suitable for the biaxial orientation arrives at the mandrel.

It is also known that despite those preparatory treatments, deviations in the cross-section of the tube can still arise as a result of the passage over the mandrel.

These deviations concern the wall thickness of the pipe viewed in the circumferential direction and possibly eccentricity of the inside with respect to the outside. These deviations are then detected with a second wall thickness measuring device 130 arranged downstream of the mandrel.

In order to be able to correct these deviations, it is already known to make use of the heating device 50 shown in figure 1b. This heating device 50 comprises, as mentioned before, several heating units arranged close to the mandrel 60 and around the tube 6. With each of those heating units, a separately adjustable amount of heat can be delivered to an associated sector of the circumference of the passing tube 6. As a result of the added heat, the temperature and thus the stiffness of the plastic material changes. In this way the resistance which the tube 6 encounters when passing the mandrel 60 can be controlled sector-wise in the circumferential direction of the tube. This control is known per se. In practice, undesirable deviations in the cross-sectional shape and wall thickness of the tube forced over the mandrel 60 still appear to occur even when the heating device 50 is used. This problem and an associated solution 35 will be explained in more detail with reference to Figures 3 and 4.

In Figures 3 and 4, the mandrel 60 can be recognized with run-up part 62, expansion part 63 and run-out part 64. The expansion part 63 of the mandrel 60 has an outer surface 101 1469 -16- which substantially corresponds to the surface of a truncated circle cone.

The mandrel 60 is provided with one or more feed channels 65, which open near the downstream end of the expansion part 63 into the outer surface of the mandrel 60 and connect through the pulling member 61 and the extruder head 3 to pumping means (not shown) for supplying a liquid between the mandrel 60 and the tube 6. Furthermore, the mandrel 60 is provided with one or more discharge channels 66, which extend from a mouth arranged in the run-up part 62 through the pulling member 61 and the extruder head 3 to a discharge. With these channels 65 and 66 and the associated pumping means, a flowing liquid film can be realized between the tube 6 and the mandrel 60, in particular between the tube 6 and the expansion part 63 of the mandrel 60. This formation of a liquid film, for instance a water film, between the tube 6 and the mandrel 60 is known per se. The liquid in the film here flows opposite to the direction of movement of the tube 6 over the expansion part 63. Due to the presence of the liquid film, in fact there is little or no frictional contact between the tube 6 and the expansion part 63. The liquid film does not provide only reduces friction but also cools the surface of mandrel 60 below the melting temperature of thermoplastic. Above that temperature, the coefficient of friction increases very strongly.

In practice, in such a situation known per se with a form-retaining mandrel and a water film 30 between the mandrel and the tube, it appears that when the passage of the tube over the expansion part in the circumference of the tube, local wall thickness differences arise, which were none or only very minor upstream of the mandrel. In other words, it is frequently observed that a zone of the circumference of the tube moving over the mandrel becomes thinner while a thickening of the wall occurs in the adjacent areas. This not only leads to unacceptable deviations in the wall thickness of the manufactured tube, but also to a difference in biaxial orientation.

It has been found that the above problem can be solved / reduced by providing the outer surface of the expansion portion 63 of the mandrel 60 at multiple locations around the circumference of the expansion portion 63 with axially extending elongated grooves and / or ribs. In figure 4 it can be recognized that a large number of shallow grooves 67 are arranged in the outer surface of the expansion part 63. A number of these grooves 67 are shown exaggeratedly large for the sake of clarity. Such a groove 67 can also be recognized in figure 3. The grooves 67 extend in the axial direction, that is, in the direction in which the tube 6 is forced over the mandrel 60. The grooves 67 are preferably distributed at regular angular distances over the expansion part, preferably between 30 ° and 4 °.

When the tube 6 is forced over the mandrel, the soft plastic material of the tube 6 will partly enter those grooves 67, as shown in figure 3. By this interlocking of the tube and the expansion part of the mandrel, the possibility of movement of the plastic material of the tube in the circumferential direction of the expansion part of the mandrel is limited and that appears to greatly reduce the aforementioned problem of local deviation of the wall thickness. .

To achieve the intended effect, a small depth of grooves 67 is sufficient. In practice, 5 millimeters has an upper limit and depths between 0.5 and 3 millimeters are preferred, grooves with a depth of 0.5 mm and a width of 0 .5 mm have already proved effective.

Some of the water film between the tube and the mandrel will pass through the grooves 67, but in the regions located between the grooves 67, a liquid film between the mandrel and the tube will be maintained. Incidentally, it is also conceivable that the supply of liquid does not take place via channel 65, but via a channel further downstream, in the run-off part 64, in the outer surface of the mandrel 101 1469-18.

The grooves 67 in practice lead to small but visible longitudinal ribs on the inner circumference of the manufactured pipe. In particular, because a biaxially oriented tube is considerably less sensitive to notch effect than a normal extruded tube, these longitudinal ribs are acceptable. Of course, if the grooves 67 are replaced by upright ribs, a pattern of shallow longitudinal grooves in the tube would result. That 10 does not cause any problems either.

In figure 3, as well as in figure 1b, it can be recognized that a second liquid film is realized between the run-off part 64 of the mandrel 60 and the tube 6 in a manner already known per se. The second liquid film serves on the one hand to reduce the friction between the tube and the outlet part and on the other hand it can also serve as internal cooling for the stretched tube.

In a variant not shown, provision is made for the heating device 50, which in a known embodiment comprises 20 infrared radiators, to be provided with means for heating the tube with microwave radiation. This could heat not only the surface of the pipe, but especially the interior of the pipe wall.

25 - generation of required traction.

The improvement of the properties of the plastic material which are sought in the biaxial stretching process is achieved in particular if the extruded tube is stretched to a considerable extent in the axial but also in the radial direction. In practice, for example, the diameter of the tube will often increase by a factor of two or more when it passes over the mandrel.

However, at the orientation temperature suitable for the biaxial stretching process, the plastic material is already quite rigid and not so easily deformable. As a result of this, very considerable forces have to be exerted on the tube in order for the tube, which is thick-walled upstream of the mandrel 101, to pass over the mandrel. The presence of one or more liquid films between the tube and the mandrel does result in a reduction of the tensile force, but the forces in the stretching process 5 nevertheless remain problematic.

A first problem concerns the transmission of the tensile force to the tube 6 with the tensile testing machine 90 placed downstream of the mandrel 60. In well-known tensile testing machines, there are several driven tracks, for example 10 2, 3 or 4, and the tensile force transmission is of the tensile testing machine on the tube based on friction between tube and tracks. The friction is determined by the coefficient of friction and the normal force. The friction coefficient is hereby established by the materials that come into contact with each other and does not simply increase considerably. The normal force is limited by the load-bearing capacity of the pipe to prevent damage. Therefore, the tensile force to be exerted with a tensile testing machine is limited.

A measure to increase the exertable tensile force is to use a number of tensile benches arranged one behind the other, whereby the friction between the tube and the tensile benches is distributed over a larger area. Here, the tensile benches then have to propel the tube at the same speed to prevent the tracks of one of the tensile benches from slipping over the tube. Since the stretched tube has already cooled to well below the orientation temperature at that location, further axial stretching is also undesirable.

Another measure is to internally support the tube at the tensile testing machine 90, so that the tensile testing machine can exert a greater normal force on the pipe than in the absence of said internal supporting material.

The internal support could for instance consist of realizing an internal pressure in the tube, for instance by forming a closed compartment in the tube with two closing means at the height of the tensile testing machine and in that compartment gas or liquid under 101 1469 -20-. pressure.

The internal support could also be mechanical. Fig. 1b shows an example schematically, in which an internal supporting device 120 is attached to the mandrel 60 via a pulling member 121, at the level of the drawing bench 90. The supporting device 120 here has pressure belts 122 traveling along with the tube 6, which face opposite the bands of the tensile testing machine 90 lie against the inside of the tube 6. This allows the tensile testing machine 90 to press firmly against the outside of the tube 6, without the risk of damaging the tube 6.

With larger tube diameters, the internal support device itself could also be provided with a drive to propel the tube 6, the device then supporting on the mandrel via a pressure-loadable member. This support then leads to a reduction in the tensile force in the connection between the extruder and the mandrel.

Another possibility to apply the tensile force required in the biaxial stretching process to the tube is to base the transmission of the tensile force on the tube on a form-fit connection between the pulling device and the tube, rather than on friction as described above. This can be achieved by allowing the tube to deform, possibly permanently damaged, at axially spaced locations by the engagement of the downstream drawing device on the tube. The distance between the engagement points is then preferably slightly more than the length of the pipe sections to be manufactured. For example, the pulling device engages with protrusions on the pipe, which protrude into or through the pipe wall.

thickened areas.

35

As mentioned previously, it is known in an installation for biaxially stretching a tube downstream of the mandrel to arrange a pulling device and upstream of the mandrel a tube speed control means, often also constructed as a tensile machine, so that the speed of the tube before and after the mandrel can be arranged and thus the provision. It is further known that in these biaxial stretching processes the aim is to achieve a wall thickness of the tube which is as uniform as possible.

It has been found that it is possible to operate such an installation in such a way that axial zones thickened at regular axial distances from one another are formed in the pipe 10 with a thicker pipe wall than the intermediate parts of the pipe. Tests have shown that with PVC the wall thickness of those thickened zones can easily be 15% greater than that of the intermediate parts.

Preferably, the thickened zones upstream of the stretching mandrel are realized by periodically changing the tube propulsion speed of the tube speed control means, wherein during a periodic adjustment of that speed the ratio between the tube propulsion speed of the tube speed control means 20 on the one hand and the pulling device on the other hand is kept substantially constant. . As a result, upstream of the mandrel periodically a zone with thicker tube wall is created in the tube yet to be stretched. Also, during that period, the draw remains unchanged because the relationship between the tensile tube propulsion speed and the tube speed control means is maintained. It has been found that the presence of the thickened zones is permissible and does not interfere with the biaxial stretching process. It has further been found that the thicker zones are also possible if those thicker zones still have to undergo external calibration downstream of the mandrel. It can be observed that insofar as these thickened zones protrude outwards, those zones are pressed inwards.

The above zones with greater wall thickness can be used to great advantage as a pipe section from which a plug-in sleeve is later formed on the pipe. Here, the tube is then cut in the middle or along that thicker zone 1011469 -22- and then that thicker zone is subjected to a sleeve-forming process at the end of the cut tube part.

Another application of those thicker zones is the engagement of a pulling device to move the tube.

preservation of properties of the manufactured tube.

A fundamental problem with polyolefin pipes is that the improved properties obtained by the biaxial stretching process are wholly or largely lost at a low temperature of the pipe (40 ° C for PE). This means that such a tube cannot be stored in the sun without the aforementioned loss occurring, unless special measures are taken to increase the stability of the manufactured tube.

It is preferable to strive for stability-enhancing treatments of the pipe that can be accomplished in-line with the manufacture of the pipe, rather than afterwards in a separate process in which pipe sections are treated. To this end, it is proposed to perform in-line cross-link treatment downstream of the expansion portion of the stretching mandrel.

In Fig. 1b it can be recognized that the run-off part 64 of the mandrel 60 has a considerable length, which here is a multiple of the wall thickness of the tube. In practice, lengths of more than 1 meter can be advantageous, which is particularly possible if a water film is formed between the drain part and the pipe. Due to the great length of the run-off part 64, the tube 6 becomes more stable, because the stretched tube 6 then has a shape defined by the run-off part 64 for a relatively long period, during which period the effects caused by the expansion can take a stable state.

Another way of improving the stability of the pipe is by cross-linking the plastic material of the pipe. This can be done in various ways known per se.

10114 β P

-23-

It may also be provided that only one or more layers of the pipe wall are subjected to a cross-link treatment, for example only the layer on the outside of the pipe.

Stability can also be improved by manufacturing multilayer pipes, as already described above, in which one of those layers is then in fact so stable in shape that a change in shape of less stable layers, for example an uncrosslinked PE-10 layer, is prevented. . This can be done, for example, by combining such a PE layer with a PVC layer. It is also conceivable that specific layers of that multilayer tube are subjected to the cross-link process, so that one of the layers thereby blocks a change of shape of the other layer or 15 layers.

Another variant is to first cut the manufactured pipe and thus make pipe sections and then treat these pipe sections in a separate (batch) process to achieve the desired stabilization. In particular, it is conceivable that a tube section is slid onto a form-retaining internal support and then subjected to a heat treatment for a certain period, for instance a number of hours. In that treatment, the internal support prevents a change in the shape of the biaxially oriented tube section, which shape is thus maintained, and a substantial part of the stretch of the plastic material will be maintained. After that treatment, the tube section will be considerably less sensitive to loss of the stretch properties.

By subjecting the tube to one or more of the aforementioned treatments, a tube of biaxially oriented plastic material can be obtained, which allows a connection to be made via a heat-seal connection with a tube part or other part to be connected thereto. Such welded joints are mainly used with polyolefin pipes, such as PE pipes. If a pipe is now made of biaxially oriented polyethylene or the like, a pipe saddle for making a connection of a branch pipe can for instance be welded to it without the pipe being changed undesirably by the heat supply.

5 connection of biaxially oriented pipes.

It is already known to provide pipe parts of biaxially oriented thermoplastic plastic material, in particular PVC, with an insertion sleeve at one end in order to be able to assemble a pipe from pipe parts inserted in each other. It is known here to provide such an insert sleeve with an elastic sealing ring which rests in a sealing manner against the end of the other tube inserted therein.

With biaxially oriented polyolefin pipes, such a sleeve connection presents sealing problems, especially in the longer term. These problems arise in particular from the fact that many polyolefins exhibit a clear creep behavior, that is to say that the material will yield over time under a load. With an insert sleeve connection as described above, the behavior between the sealing ring and the inserted tube end will gradually decrease as a result of this behavior, as the wall of the tube will deviate over time. This creates a leakage opportunity, especially under pressure.

For the interconnection of two tubes of biaxially oriented thermoplastic plastic material, in particular polyolefin plastic material, an improved connection is therefore proposed, which will be explained in more detail below with reference to Figure 5.

Figure 5 shows the ends of two identical tubes 201, 202 of biaxially oriented polyethylene to be joined, for example manufactured by the method and installation described earlier. Each of these pipes 201, 202 is provided at both ends with a plug-in sleeve 1011469, respectively 203, 204, of which a simple embodiment, without sealing ring, is shown in figure 5.

As is known per se, said plug-in sleeves 203, 204 are molded in one piece onto the tubes 201, 202 and here have a larger inner diameter than the neighboring part of the tube.

Figure 5 further shows a plastic connecting tube body 210, which is provided with two axial ends 211, 212, each of which inserts an insertable tube 201, 202 into an insert sleeve 203, 204 of 10. Preferably, the connecting tube body 210 fits lightly into the insert sleeves, as shown in Figure 5.

The fixing of the tubes 201,202 to the body 210 is effected by heating the insertion sleeve of each tube, as a result of which that insertion sleeve shrinks at least in cross section and clamps onto the end of the connecting tube body 210 inserting therein.

For heating the insert sleeve slipped over it, the connecting tube body 210 is provided with heating means at each end 211, 212 thereof. These heating means here comprise one or more electric heating elements, for example heating wires 215, which are embedded here in the connecting tube body 210 and can be connected to a power source via connection 216 on the outside of the body 210.

In a variant, the heating means may comprise one or more elements which can be heated from the outside, for instance elements to be heated via induction or microwave radiation, which are arranged on and / or embedded in the tubular body 210.

To prevent the transition from the insertion sleeve to the neighboring portion of the tube from being excessively heated, the heating wires 215 are spaced from the free end of the connecting tube body 210.

In Figure 5, it is further recognized that the outer surface of each end 211, 212 of the connecting tube body 210 is profiled to provide a form-retaining connection component between the connecting tube body 210 and the insert sleeve of the tube.

The connecting tube body advantageously has an inner diameter which is substantially equal to the inner diameter of the part of each tube located outside the insertion sleeve.

The shown joint is also applicable to biaxially oriented pipes which have been subjected to a cross-link treatment and / or have a multilayer pipe wall, as explained above.

101 1469

Claims (41)

1. A method for manufacturing a tubular profile of thermoplastic plastic material, wherein a tubular profile is extruded with an extruder, which is provided with an extruder head with an inner cone, the inner cone defining an axial hollow space in the tubular profile, wherein the extruder head emerges from the extruder head. tube profile downstream of the extruder head is internally cooled with an inner cooling member and externally cooled with an outside cooling device, characterized in that the inner cooling member effects an internal cooling of the tube immediately after the tube profile exits the extruder head and the outside cooling device is downstream of the inner cooling member so that the external cooling of the tubular profile is effected after the internal cooling. 2. Process according to claim 1, wherein the thermoplastic material is of the type which crystallizes during cooling after extrusion, resulting in a considerable volume shrinkage, in particular polyethylene or polypropylene. 1 101 1 469 A method for manufacturing a tubular profile of thermoplastic plastic material, wherein a tubular profile is extruded with an extruder, which is provided with an extruder head with an inner cone, which inner cone defines an axial hollow space in the tubular profile, wherein it is extruder head tubing coming downstream of the extruder head is internally cooled with an inner cooling member and externally cooled with an outdoor cooling device, characterized in that the inner cooling member effects an internal cooling of the tube immediately after the tubular profile leaves the extruder head, the inner cooling member forming a form-retaining outer wall, having an axial length that is a multiple of the cross-sectional dimension of the tubular profile -28-, and wherein coolant is forced between the form-retaining outer wall and the tubular profile, such that a rapidly flowing liquid film is formed between the tubular profile and the form-retaining outer wall dt realized, wherein the liquid flows in the counter-flow direction, i.e. against the extrusion direction. The method of claim 3, wherein the liquid film is at most 3 millimeters thick. 10 5. Method for manufacturing a tubular profile of thermoplastic plastic material, wherein a tubular profile is extruded with an extruder, which is provided with an extruder head with an inner cone, the inner cone defining an axial hollow space in the tubular profile, wherein the extruder head emerges. tubing profile downstream of the extruder head is internally cooled with inner cooler, which includes an inner cooler contained in the extruded tube, and externally cooled with an outer cooler, the inner cooler being arranged to realize direct contact between a coolant and the tubular profile, with characterized in that the inner cooling device comprises venting means for venting the cooling liquid, with which the cooling liquid is vented before it is supplied to the inner cooling member. 6. Method for manufacturing a tubular profile 30 of thermoplastic plastic material, wherein a tubular profile is extruded with an extruder, which is provided with an extruder head with an inner cone, the inner cone defining an axial hollow space in the tubular profile, the projecting coming out of the extruder head Tubing profile downstream of the extruder head is internally cooled with inner cooler, which includes an inner cooler contained in the extruded tube, and externally cooled with an outer cooler, 101 1469-29- wherein the inner cooler is adapted to realize direct contact between a coolant and the tube profile, characterized in that the inner cooling member is adapted to realize a helical flow of the cooling liquid along the inner wall of the tube profile. 7. Method for manufacturing a tubular profile of thermoplastic plastic material, in which a tubular profile is extruded with an extruder, which is provided with an extruder head with an inner cone, which inner cone defines an axial hollow space in the tubular profile, wherein the extruder head emerging tubing profile downstream of the extruder head is internally cooled with a coolant directly contacted with the tubing, and externally cooled with an outdoor cooling device, characterized in that a low surface tension coolant is used. 20 The method of claim 7, wherein the cooling liquid is water to which one or more surface tension reducing additives have been added. 25 9. Method for manufacturing a tubular profile from a polyolefin plastic material, wherein a tubular profile is extruded with an extruder, which is provided with an extruder head with an inner cone, the inner cone defining an axial hollow space in the tubular profile, whereby it is extracted from the extruder head coming tubing downstream of the extruder head is internally cooled with inner cooler, which includes an inner cooler attached to the inner cone, and externally cooled with an outer cooler, characterized in that a heating medium is provided downstream of the inner cooler in the hollow space of the tubular profile , for increasing the temperature of the layer cooled on the inside of the tubular section by the 101 1469-30 cooling element. 10. A method according to claim 9, wherein the heating medium is a liquid, optionally with an addition of a surface tension-reducing substance, at a temperature between 90-100 ° C. 11. Method for manufacturing a pipe profile 10 with a wall layer of crystalline thermoplastic plastic material, wherein a pipe profile is extruded with an extruder, which is provided with an extruder head with an inner cone, which inner cone defines an axial hollow space in the pipe profile, wherein the tubular profile emerging from the extruder head downstream of the extruder head is internally cooled with an inner cooler, which includes an inner cooler contained in the tube, and externally cooled with an outer cooler, characterized in that a multilayer tube is extruded with at least one wall layer of amorphous thermoplastic plastic material on the inside of the wall layer consisting of crystalline thermoplastic plastic material. The method of claim 11, wherein the crystalline wall layer is made of polyethylene and the amorphous wall layer is made of polyvinyl chloride. 1 101 1469 Method for manufacturing a biaxially oriented tube from thermoplastic plastic material, in particular polyolefin plastic material, comprising extruding a tube made of thermoplastic plastic material with an extruder having an inner cone, the inner cone 35 defines an axial void in the tube, and then forcing the tube over a mandrel, which mandrel includes an expansion member that effects circumferential expansion of the tube, thereby forcing the tube over the mandrel with a tube speed control means upstream of the mandrel engaging the tube and with a pulling device disposed downstream of the mandrel, the extruder head being provided with means for controlling the wall thickness of the tube emerging from the extruder head and between the extruder head and the mandrel an ultrasonic arranged along the outside of the tube wall thickness measuring device is provided for measuring the wall thickness and shape of the cross section of the extruded tube, characterized in that a cold liquid layer is realized on the inside of the tube profile at the location of the wall thickness measuring device. A method according to claim 13, wherein the temperature of the cold liquid layer is at most 50 ° C. 15. Method for manufacturing a biaxially oriented tube from thermoplastic plastic material, in particular polyolefin plastic material, comprising extruding a extruder head with an inner cone, extruding a tube from thermoplastic plastic material, the inner cone forming a defines axial cavity in the tube, and then forcing the tube in an axial direction over a form-retaining mandrel, which mandrel includes an expansion member that effects circumferential expansion of the tube, forcing the tube over the mandrel with a tube velocity control means upstream of the mandrel engaging tube 30 and with a pulling device disposed downstream of the mandrel, the expansion portion of the mandrel having an outer surface substantially corresponding to the surface of a truncated circle cone, characterized in that it outer surface of the expansion part v The mandrel is provided with elongated grooves and / or ribs extending in axial direction at several locations around the circumference of the expansion part. 1011469 -32- The method of claim 15, wherein a liquid film is realized between the expansion portion of the mandrel and the tube. The method of claim 16, wherein the liquid in the film flows over the expansion section opposite to the direction of movement of the tube. 18. A method according to claim 17, wherein near the downstream end of the expansion part and / or its downstream end liquid is pressed between the mandrel and the tube and the liquid is collected and discharged upstream of the expansion part. 15 Method according to one or more of claims 15-18, wherein the expansion part is provided with axial grooves, which are formed at regular angular distances, preferably between 30 ° and 4 °, in the outer surface of the expansion part. 20 A method according to any of claims 15-19, wherein the grooves are at most 5 millimeters deep, preferably between 0.5 and 3 millimeters. 1 2 3 4 5 6 7 8 9 10 11 1011 469 21. Method for manufacturing a biaxially 2 oriented tube from thermoplastic plastic material, 3 in particular polyolefin plastic material, comprising 4 with an extruder, which is provided with an extruder head 5 with an inner cone, extruding a tube from 6 thermoplastic plastic material, the inner cone 7 defining an axial hollow space in the pipe, and 8 then forcing the pipe in the axial direction over a form-retaining mandrel 9, which mandrel comprises an expansion part, 10 effecting expansion of the pipe in the circumferential direction, 11 whereby the the mandrel forcing the tube is realized with a tube speed control means upstream of the mandrel engaging the tube and with a pulling device disposed downstream of the mandrel, wherein the pipe is circumferentially controlled heating upstream of the mandrel, characterized in that, that the circumferential sectors can be controlled in a heating manner takes place by microwave radiation. 5 22. Method for manufacturing a biaxially oriented tube from thermoplastic plastic material, in particular polyolefin plastic material, comprising extruding a tube from thermoplastic plastic material with an extruder comprising an extruder head 10, the inner cone forming a defines axial hollow space in the tube, and then axially forcing the tube over a mandrel, which mandrel includes an expansion member that effects circumferential expansion of the tube, thereby forcing the tube over the mandrel a tube speed control means upstream of the mandrel engaging the tube and with a pulling device disposed downstream of the mandrel, characterized in that the tensile force is applied with at least one pulling device engaging the outside of the tube. 23. A method according to claim 22, wherein a plurality of pulling devices are arranged one behind the other, which drive the tube at the same speed. A method according to claim 22 or 23, wherein the tube is internally supported at the location of the engagement of a pulling device 30. 1 2 3 4 5 101 1469 Method according to claim 24, wherein the tube 2 is supported internally by means of mechanical 3 supporting means, which move one or more with the tube at the location of the engagement of the pulling device and 5 against the inside of support surfaces adjacent the tube. -34- The method of claim 25, wherein the support means is attached to the inner cone of the extruder. A method according to claim 25 or 26, wherein a drive of the support surfaces of the support means is provided in the direction of movement of the tube. 28. A method according to claim 22, wherein the pulling device comprises one or more, each axially displaceable tube engaging members, which engage a part of the tube under deformation of the tube and grip the tube there, including at each tube engaging member an axial displacement mechanism 15 belongs to axially displace that member and the tube secured therein. 29. A method for manufacturing a biaxially oriented tube from thermoplastic plastic material, in particular polyolefin plastic material, comprising extruding a tube from thermoplastic plastic material with an extruder comprising an extruder head, wherein the inner cone is defines axial hollow space in the tube, and then forcing the tube axially over a mandrel, which mandrel includes an expansion member that accomplishes expansion of the tube circumferentially, forcing the tube over the mandrel with a tube velocity control means upstream of the mandrel engaging the tube 30 and having a drawing device disposed downstream of the mandrel, characterized in that the oriented tube is provided with thickened zones regularly spaced axially apart, said zones having a greater wall thickness 35 than the div located between those zones and of the oriented tube. A method according to claim 29, wherein the wall thickness of the thickened zones is preferably 10-30% greater than that of the intermediate parts of the pipe. 31. A method according to claim 29 or 30, wherein the 5 thickened zones upstream of the mandrel are realized by periodically changing the tube propulsion speed of the tube speed control means, wherein during a periodic adjustment of that speed the ratio between the tube propulsion speed of the tube speed control means on the one hand and of the pulling device, on the other hand, is kept substantially constant. 32. A method according to any one of claims 29-31, wherein an outer tube calibrator is arranged downstream of the mandrel which effects a reduction of the outer diameter of the tube. 33. A method for manufacturing a biaxially oriented tube from thermoplastic plastic material, in particular polyolefin plastic material, comprising extruding a tube from thermoplastic plastic material with an extruder having an extruder head, the inner cone 25 defines an axial void in the tube, and then axially forcing the tube over a mandrel, which mandrel includes an expansion member that effects expansion of the tube in the circumferential direction, and a bleed portion downstream of the expansion member, which bleed portion in has substantially a constant cross section, forcing the tube over the mandrel is accomplished with a tubular velocity control means acting upstream of the mandrel on the tube and with a pulling device disposed downstream of the mandrel, characterized in that the bleed portion has an axial length which is a multiple of the wall thickness of the oriented tube. 101 1469 -36- 34. Method for manufacturing a biaxially oriented tube with a wall layer of polyolefin plastic material, comprising an extruder comprising an extruder head with an inner cone, extruding a tube of thermoplastic plastic material, the inner cone having an axial hollow space in the tube, and then forcing it in an axial direction over a mandrel, the mandrel comprising an expansion member that effects expansion of the tube in the circumferential direction, and a bleed portion downstream of the expansion member, said bleed portion being substantially a constant cross-section wherein the forcing of the tube over the mandrel is accomplished with a tube speed control means upstream of the mandrel engaging the tube and with a pulling device disposed downstream of the mandrel, characterized in that a multilayer tube is extruded into which multiple wall layers having different properties are included of which there is at least one of polyolefin plastic material. 20 The method of claim 34, wherein at least one of the wall layers is subjected to a cross-link treatment. A method according to claim 34 or 35, wherein an inner and / or outer wall layer contains cross-linking promoting additives. 1 1011469 A method for manufacturing a biaxially oriented tube from polyolefin plastic material, comprising extruding an extruder head with an inner cone, extruding a tube from thermoplastic plastic material, the inner cone having an axial hollow space in the tube defines, then, axially forcing the tube axially over a mandrel, which mandrel includes an expansion member that effects expansion of the tube in the circumferential direction, and a bleed portion downstream of the expansion member, which bleed portion is substantially a constant cross-section having forcing the tube over the mandrel with a tube speed control means upstream of the mandrel engaging the tube and with a pulling device disposed downstream of the mandrel, characterized in that the tube is downstream of the expansion portion of the mandrel subject to cross-link treatment. The method of claim 37, wherein only a wall layer adjacent to the inside and / or outside of the tube is subjected to a cross-link treatment. 39. Connection of two tubes of biaxially oriented thermoplastic plastic material, in particular polyolefin plastic material, characterized in that the tubes at their turned ends each have a one-piece molded insert sleeve, preferably with a larger inner diameter than the neighboring part 20, and that a connecting tube body is provided with two axial ends, each of which extends into an insert sleeve of a tube to be connected, and that each tube with its insert sleeve is shrunk onto the end of the connecting tube body by heating thereof. 25 The connector of claim 39, wherein the connector tube body is provided at each end thereof with heating means for heating the insert sleeve slipped over it. 30 A connection according to claim 40, wherein the heating means comprise one or more electric heating elements, for example heating wires. 35 A compound according to claim 41, wherein the heating means comprise one or more externally heatable elements, for example metal elements to be heated via induction. 43. Connection according to one or more of claims 40-42, wherein the heating means are spaced from the free end of the connecting tube body. 44. A joint according to any one of claims 39-43, wherein the outer surface of each end of the connecting tube body is profiled to provide a form-retaining connecting component between the connecting tube body and the insert sleeve of the tube. 45. A joint according to any one of claims 39 to 44, wherein the connecting tube body consists essentially of plastic material. A connection according to any of claims 39-44, wherein the connection tube body has an inner diameter 20 substantially equal to the inner diameter of the portion of each tube located outside the insertion sleeve. 101 1469