MXPA01007690A

MXPA01007690A - Thermoplastic tube

Info

Publication number: MXPA01007690A
Application number: MXPA/A/2001/007690A
Authority: MX
Inventors: Lenthe Arjan Dirk Van; Jan Hendrik Prenger; Jan Visscher
Original assignee: Wavin Bv
Priority date: 1999-03-05
Filing date: 2001-07-30
Publication date: 2002-06-05

Abstract

Method for producing a tube section (6) from thermoplastic material, in which a tube section (6) is extruded by means of an extruder (1) which is provided with an extruder die (3) having an inner core (5), which inner core (5) defines an axial hollow space in the tube section (6), the tube section coming out of the extruder die (3) downstream of the extruder die (3) being internally cooled by means of an internal cooling member and externally cooled by means of an external cooling device. Immediately after the tube section (6) leaves the extruder die (3), the internal cooling member brings about internal cooling of the tube. The external cooling device (15) is positioned downstream of the internal cooling member, so that the external cooling of the tube section is brought about after the internal cooling.

Description

THERMOPLASTIC TUBE DESCRIPTION OF THE INVENTION The invention relates to the production of tube from thermoplastic material, in particular from polyolefin plastic material, such as polyethylene. The invention also relates to the production of plastic tubing in which the thermoplastic material is biaxially oriented, which process is known as the biaxial stretching process. The invention also relates to improvements to the process for the production of extruded tube from thermoplastic material, which process can be part of the production of a biaxially oriented plastic tube. The invention is further related to the production of an improved pipe joint made from biaxially oriented thermoplastic material. The present invention relates in particular to the production of a tube from biaxially oriented thermoplastic material with a receptacle formed integrally at one end, so that tubes of this nature can be coupled together via receptacle joints, in order Thus, a tube is formed, for example to transport water, gas, etc.

WO 95/25626 has described a method according to the preamble of claim 1 for the production of biaxially oriented plastic tube, also known as drawn tube. In this method, the stretched tube is of uniform cross section, ie it has a uniform wall thickness and diameter, over its entire length and is also stretched uniformly in the axial and tangential (circumferential) direction of the tube over its entire length. A method for providing a tube which has been produced in this manner with a receptacle at one of its ends is known from WO 97/33739. Another method for producing tube from biaxially oriented plastic material is known from GB 1 589 052. This method is based on a tube made of thermoplastic material to which has not been subjected to biaxial orientation, tube which has a body of tube with, at one end, and an end part with a wall thickness greater than the body of the tube. The tube is placed in a die and expanded by the internal pressure so that the plastic material of the tube is oriented biaxially. In the process, the end part is deformed to form a receptacle. WO 98/13190 has described another additional method for the production of a tube with an integral receptacle from biaxially oriented thermoplastic material.

Despite all developments in the field of the production of tubes from biaxially oriented thermoplastic material, and in particular in the field of the formation of a receptacle in a tube of this nature, the load tests still show that the receptacle a tube of this nature forms the critical part of the tube. This is because it has been found that the tube ruptures first in the receptacle than in the body of the tube, and therefore the receptacle constitutes an undesirable limitation of the mechanical strength of the tube. The object of the present invention is to propose measures which make it possible to produce a tube of the above type with an integral receptacle at one or both ends. The invention also provides measures for improving the spigot of the tube, which is to be placed inside a receptacle. For this purpose, the invention according to a first aspect, provides a method according to claim 1. When carrying out the method according to the invention, an axial preform part with a different wall thickness starting from the preceding part of the preform is periodically formed in the section between the extruder die and the tube speed control means, in practice in particular immediately downstream of the extruder die.

Surprisingly, in practice it has been shown that it is possible to control the process of biaxial stretching of the preform successfully despite the variation in the thickness of the wall of the preform which will be forced on the mandrel. In particular, it has been shown that it is possible for a preform part with a greater wall thickness to be forced on the mandrel without having undesirable effects on that part of the preform which has a smaller wall thickness and is located between the part of the coarse preform and the stretching device. The method according to claim 1 allows a stretched tube of biaxially oriented thermoplastic material to be produced in a continuous process with axial tube portions which have varying wall thicknesses. In practice, it has proven to be expedient for a maximum wall thickness of operation which is 5-15% greater than the smallest wall thickness of the preform, as seen in the position immediately downstream of the extruder die. It will be apparent that other values are also within the scope of the invention. Preferably, the transition from one wall thickness value to another wall thickness value is gradual. This is beneficial for the stability of the process. In a preferred embodiment, the relationship between the advance rate of the preform, which is determined by the pipe speed control means, on the one hand, and the extruder outlet, on the other hand part, it must be at least a first substantially constant value for a first period and for it to be in one or more values which differ from the first value for a second period, which is considerably shorter than the first period, cycle the which is repeated continuously. In practice, this medium, as seen in the downstream point of the mandrel - the stretched tube in each case has a part of great axial length with a first uniform wall thickness and associated diameter, part which is followed by a part considerably shorter axial length of the tube in which the wall thickness differs from the first wall thickness, in particular it is one or more larger values, as seen in the axial direction of the shortest part. In particular, a wall thickness is provided - as shown in the axial direction - which varies between a plurality of values in the last axial part, such that the annular areas which are adjacent to each other and have different wall thicknesses they can be distinguished in the relevant part of the stretched tube. The method according to the first aspect of the invention can be implemented by periodically varying the extruder outlet, in which case the advance velocity of the preform which is determined by the tube speed control means is kept substantially constant . This requires an extruder which can be adjusted with an adequate interval in terms of its output. However, the method according to the first aspect of the invention can also be implemented, if preferred, by keeping the extruder outlet substantially constant and by periodically varying the advance rate of the preform which is determined by the means of speed control of the tube. In a preferred embodiment of the method according to the first aspect of the invention, the drawn tube acquires substantially the same axial stretching over its entire length. To obtain this, in the method according to claim 3 in some cases it is sufficient to maintain the speed of advance of the drawn tube downstream of the mandrel, which is determined by the stretching device, constant, so that the ratio of the The advancing speed of the drawn tube downstream of the mandrel, on the one hand, and of the preform upstream of the mandrel, on the other hand, remain substantially constant. In the method according to claim 4, the feed speed of the preform upstream of the mandrel, which is determined by the speed control means of the tube, varies and for this reason the pipe advance speed is then necessary. stretched downstream of the mandrel, which is determined by the stretching device, must vary periodically in such a way that the ratio of forward speed of the tube downstream of the mandrel, on the one hand, and of the preform upstream of the mandrel, on the other, are maintained substantially constant. In a variant of the method according to the first aspect of the invention, tube parts with a greater wall thickness are provided which do not have the same level of axial stretching as the interposed tube part with a smaller wall thickness, but rather they have a higher level of axial stretching. For this purpose, in the period during which part of the preform with a greater wall thickness is driven on the mandrel, or during a section of this period, the forward speed ratio of the drawn tube which is determined by the device of stretching, on the one hand, and the speed of advance of the preform which is determined by the tube speed control means, on the other hand, is greater than in the period during which part of the preform with the thickness of smaller wall is driven on the mandrel, such that a part of the tube having a greater wall thickness acquires a higher level of axial stretching compared to a part of the tube with a smaller wall thickness. To allow the method according to the first aspect of the invention, and in particular according to the variant described above, to be successfully controlled, it is desirable that the tube undergoes its axial stretching in a precisely defined section and, outside of this section, it will not be generated in the additional rear axial drawn tube. To accomplish this, an advantageous embodiment of the method according to the first aspect of the invention provides a tube drawn downstream of the expansion part of the mandrel which is cooled in such a way that the cooled tube no longer experiences any axial stretching in the generation of axial stretching and the section is concentrated between the tube speed control means placed in the vicinity of the extruder and the downstream end of the mandrel. Preferably, the axial stretching is carried out between the two tube speed control means which are placed at a distance from each other and both are placed between the extruder and the mandrel. It will be apparent that at the time when a portion of the preform with a thickened wall reaches the upstream end of the mandrel, a possible critical change in the condition of the hitherto stable method occurs, particularly if the thickened wall part of the preform projects inward at that time and therefore has a smaller diameter compared to adjacent parts of the preform. It would then be expected that the preform part with the greater wall thickness could and perhaps would jam in the mandrel, while the thin and still hot part of the preform located immediately downstream of this part can be further stretched in the direction axial, possibly to an unacceptable degree. To solve this problem, in one embodiment of the method according to the first aspect of the invention which is advantageous in practice, the temperature of the preform is controlled in such a way that the part of the preform with a greater wall thickness is on an average at a higher temperature, measured at a position immediately upstream of the mandrel, as compared to the part of the preform with smaller wall thickness which is adjacent to this part immediately downstream and therefore is in advance in the mandrel. Assuming that the conditioning temperature substantially consists of cooling the preform, although it is also known from the prior art to supply (relatively small) amounts of heat to the preform upstream of the mandrel, the temperature condition described above of the preform in practice can be implemented by causing the cooling medium, which is part of the temperature control means, operates substantially constant. This can be explained as follows. In the section between the extruder die and the mandrel it is possible to differentiate between three partial sections. In the first partial section, which is immediately adjacent to the extruder die, it is possible to produce a preform part with a thickened wall when operating as described in claim 1. In the adjacent partial section, the preform is subjected to the action of the temperature conditioner, in particular to cooling, and in the third partial section adjacency, in fact there is no important thermal energy supplied or removed from the preform. In the method according to claim 3, a preform part with a thickened wall which is produced in the first partial section will move by passing the temperature conditioning means in the second section at the same speed as the part of the preform. In relative terms, the part of the thicker preform will therefore cool to a lesser degree and therefore reach the mandrel at a higher average temperature; in particular, the temperature of the core of such part of the thickened preform will be greater. Due to the higher temperature, the modulus of elasticity will decrease and the part of the thickened preform will therefore be easier to deform, in relative terms, a fact which in practice may be sufficient to compensate the wall thickness and avoid the previous critical situation. In the method according to claim 4, the speed of the preform is reduced while a part of the preform will be formed in a first partial section.

Due to the reduction in speed, this part of the preform which is situated in the second partial section during this period will be subjected to cooling for a longer time compared to the part of the preform which has already passed through. cooling and is in the third partial section. When the part of the preform with a thickened wall is completed, the speed of the preform increases again and the part of the preform with a thickened wall will pass through the cooling at a higher speed and therefore will cool to a greater degree . When the thickened part of the preform reaches the mandrel, that part can be easily deformed, while the thin wall portion of the preform which is located immediately downstream thereof is in fact relatively rigid. A combination of the two defects makes it possible to carry out the process successfully in a controllable manner. It can be seen from the above that, based on the temperature of the preform - within a temperature range which is suitable for obtaining biaxial orientation - and the resulting modulus of elasticity of the plastic material of the preform, it is possible to control the stretching axial of the preform. By causing the preform to be at a higher temperature locally, for example, a thicker part thereof as described above, compared to other parts of the preform at the time of axial stretching, it is possible to ensure that, Given a constant axial stretching force exerted on the preform, the hottest part experiences greater axial stretching than the colder parts, even if this hotter part has a larger wall thickness. In a practical embodiment, it is possible that the thinner portions of the preform are at a temperature of about 90 ° C and the hottest part, and optionally the thickest, is at a temperature close to 120 ° C. Surprisingly, it has been shown that it is possible to pass the tube through an external calibration device after the expansion mandrel has passed. In this case, it can be seen that the portion of the tube thickened, or that leaves the mandrel, projects outwardly with respect to the adjacent parts of the tube and is then pressed inwardly by the external calibration device. The method according to the first aspect of the invention can be carried out in a continuous process and in this way it is possible to produce a tube from biaxially oriented thermoplastic material with a tube part with a thickened wall at axial intervals (regular ) each. After stitching, cutting or the like through the tube in the position of the thickened portions of the tube, it is possible to produce sections of tube with, at one or both ends, an end part with a larger wall thickness compared to the body of the tube. Furthermore, the invention provides the tube sections which are then to be subjected to the forming operation of the receptacle, in which case an integral receptacle is formed from an end part with a thickened wall. In a variant - if both end parts are of a thicker design - one part is deformed in one receptacle and the other end part is used as a spike. If appropriate, the spike is also further deformed, for example it is provided with one or more formations, such that a positive locking receptacle gasket can be obtained. In a practical embodiment, the tube sections have a tube body of uniform cross section and wall thickness and with, at one end, an integral receptacle and at the other end, a spike with a wall thickness which is 3- 10% greater than the body of the tube. Particularly in those embodiments in which the end portion with a thickened wall - prior to the formation of the receptacle - has undergone axial stretching which is greater than or equal to the body of the tube with the smallest wall thickness, the receptacle being obtained has been shown to have considerably better properties and a higher load bearing capacity compared to the known receptacles in such tubes.

Preferably, after the receptacle has been formed, the axial stretching of the receptacle is greater than or equal to the axial stretching of the body of the tube. Additional advantageous embodiments of the method according to a first aspect of the invention are described in the claims and in the description. A second aspect of the present invention relates to a method for producing a tube from biaxially oriented thermoplastic material, which tube has a tube body and, at one or both ends thereof, an integrally formed receptacle, method in which which a prefabricated tube of biaxially oriented thermoplastic material is subjected to a receptacle forming operation. The invention provides a prefabricated tube having an end portion with a greater wall thickness as compared to the body of the tube, the axial stretching of the end portion before the receptacle forming operation is equal to or preferably greater than the drawing axial of the tube body. It will be apparent that a tube of this nature can be made using the method according to the first aspect of the invention. The shape of the receptacle can be complicated, for example, with circumferential rims of different diameters which, inside the tube, form circumferential areas of different diameters. It is also possible for the wall thicknesses of the receptacle, as seen in the longitudinal direction of the tube, which may vary and may be thicker in suitable places, for example with greater load, in comparison with other positions. In a possible embodiment, the end portion of the prefabricated tube - as seen from its end face - has a plurality of annular areas which are adjacent to each other and have a wall thickness which fluctuates from one annular area to the next annular area, in which case in a plurality of annular areas the wall thickness is greater than the wall thickness of the tube body. The wall thickness of the end portion in this manner can be of a plurality of values which differ from the wall thickness of the tube body, based on the receptacle forming operation which is still to be carried out and of the requirements which will be imposed in the receptacle. In a preferred embodiment, an annular area with a larger wall thickness is deformed in comparison with the tube body, during the forming operation of the receptacle, in an outwardly bending slot wall which delimits an internal slot in the tube, which is adapted to accommodate a sealing ring. A third aspect of the invention relates to the production of a tube from a biaxially oriented thermoplastic material, as described in the preamble of claim 21. In this method, at least part of the desired axial drawing of the tube is already has been carried out in advance in the preform, before the preform moves on the expansion mandrel. Then, as it passes over the mandrel, the desired stretch is produced in the circumferential direction, as well as any remaining parts of the axial stretch. In a known method, for example as described in WO 97/10096, two speed control means are placed upstream of the mandrel, in the form of generally known drawing devices, in which case the speed control means in the vicinity of the mandrel imparts a forward speed greater than the preform compared to the other speed control means. This leads to the axial stretching of the preform with a reduction in the wall thickness of the preform. However, in practice this known method of axial stretching has proven to be insufficiently controllable with the result that undesirable variations arise in the preform. Variations of this nature, for example in the cross-sectional shape of the preform, constitute a drawback when the preform subsequently passes over the mandrel. The third aspect of the invention provides an improved control of the axial stretching described above.

According to the third aspect of the invention, in the method according to the preamble of claim 21, the preform, in the section between the speed control means, in which the preform is axially stretched, moves through of a calibration aperture of a calibration device, a calibration device which reduces the external diameter of the preform. As a result, the preform acquires an accurately controllable outer diameter before the preform reaches the downstream speed control means and subsequently passes over the expansion mandrel. In addition, a significant level of axial stretching can be produced in this section combined with a high level of stability and process control capability. A fourth aspect of the invention relates to a method according to the preamble of claim 22 for producing a biaxially oriented tube of thermoplastic material. In this known method, the passage of the preform over the expansion part of the mandrel constitutes a problematic part of the production of the tube. In particular, the preform has shown undesirable deformations during this part of the production process. The fourth aspect of the invention seeks to promote the stability of the preform as it passes over the mandrel.

The invention obtains this object by providing a method according to the preamble of claim 22, wherein an outer surface of the expansion part of the mandrel is provided, in a plurality of positions around the circumference of the expansion part, with elongated slots or reinforcements which extend in the axial direction, and a liquid film that is preferably formed between the expansion portion of the mandrel and the tube. In an advantageous embodiment, the expansion part of the mandrel is provided with axial grooves which are formed at regular angular intervals, preferably between 3 ° and 10 °, on the outer surface of the expansion part, and in which the grooves they are preferably a maximum of 5 millimeters in depth, particularly preferably between 0.5 and 3 millimeters in depth. A fifth aspect of the invention relates to a method for producing a biaxially oriented tube from thermoplastic material, as described in the preamble of claim 25. As is generally known, to force the preform onto the mandrel, it must be exercised a considerable tension force on the stretched tube downstream of the mandrel. When this tension force is exerted, it is fundamentally undesirable to permanently deform or damage the stretched tube.

The fifth aspect of the invention provides the possibility of exerting a high tension force by distributing a plurality of stretching devices which drive the drawn tube at the same speed one below the other, downstream of the mandrel. Furthermore, according to the fifth aspect of the invention, the tube is held internally in the position where a stretching device, placed downstream of the mandrel, preferably acts with the aid of a mechanical support means which, in the position wherein the stretching device acts, it comprises one or more support surfaces which move with the tube and rest against the inside of the tube, supporting means which preferably is attached to the inner core of the extruder. Preferably, the support surfaces of the support means are urged in the direction of advance of the tube. In a variant, it is permissible for the stretched tube to be deformed by the stretching device, specifically, in particular if that part of the tube on which the device acts subsequently is no longer part of the tube which is to be sold. Therefore, for this purpose, it is possible for the stretching device to comprise one or more tube coupling members which can each be moved to and from an axial distance, preferably approximate to the length of the tube which is to be sold on the market, and which acts on part of the tube, so as to deform the tube, and hold the tube securely in that position, each coupling member of the tube an axial displacement mechanism is assigned in order to displace the member and the tube which is fixed therein, in the axial direction. The measures mentioned above and other measures that are provided according to the invention are described in the claims and in the following description, and will be explained in the following, in particular with reference to the drawings. In the drawings: Figures la and Ib diagrammatically show a side view, partly in cross section, of an exemplary embodiment of an installation for producing a biaxially oriented thermoplastic pipe, Figure 2a shows a longitudinal section through a part of the preform immediately after it has passed through the calibration device; Figure 2b shows the part of figure 2a after it has passed over the expansion mandrel, figure 2c shows the part of figure 2b after it has passed through the calibration device downstream of the expansion mandrel, the Figure 2d shows the part of figure 2C after it has been deformed in a receptacle, figure 3a shows an illustration corresponding to figure 2a of another embodiment of the preform, figure 3b shows an illustration corresponding to figure 2d of the part of figure 3a which has been deformed in a receptacle, and figure 4 shows a cross-section through a part of an extruder die according to the invention. Figures 5a and 5b diagrammatically show a side view, partly in cross section of an exemplary embodiment of an installation for producing biaxially oriented thermoplastic tubing, Figure 6 shows detail II in Figure 5a, on an enlarged scale, Figure 7 shows part of the mandrel of the figure 5b on an enlarged scale, figure 8 shows a perspective view of the mandrel of figure 3, figure 9 shows a longitudinal section through a joint between two tubes made of biaxially oriented thermoplastic material according to the invention, and Figure 10 shows a view corresponding to Figures 5a, 5b of a part of a variant of an installation for producing biaxially oriented thermoplastic pipe.

Figures la and Ib show, in two partial drawings which can be joined together, diagrammatic representations of the most important elements of an installation to produce biaxially oriented thermoplastic pipe in a continuous process. The figure shows an extruder 1 with one or more screws 2 extruders and with an associated controllable impeller, which generates a flow of molten plastic material which is fed to a die 3 extruder placed in the extruder 1. The die 3 extruder has an outer ring 4 and an inner core 5 which, together with the outer ring 4, delimit an annular outlet opening from which an extruded tubular preform 6 is made from the thermoplastic material arising in a substantially horizontal direction. In this distribution, the inner core 5 defines an axial space in the preform 6. The die 3 extruder is provided with a means for controlling the wall thickness which is not shown and which can be used to produce a uniform wall thickness ( in the circumferential direction) of the preform 6 that comes from the die 3 extruder. An internal cooling member can be attached to the inner core 3 for internal cooling of the preform. The preform 6 is externally calibrated with the help of an external calibration sleeve 10.

Downstream of the calibration sleeve 10 there is a first external cooling device 15, by means of which the preform 6 is cooled externally. The external cooling device 15 comprises, for example, a certain amount of compartment which are located one behind the other, through which cooling water flows and through which the preform 6 moves, which is placed in direct contact with the cooling water. If appropriate, the cooling water in each compartment is at different temperatures, in order to optimize cooling of the preform 6 in this way. Downstream of the external cooling device 15 there is a speed control means 20 which acts on the cooled outer layer of the preform 6. The tube speed control means 20 can thus be designed as a stretching device which is known per se and has a plurality of tracks acting on the preform, whose type of Stretching device is usual for the extrusion of plastic tubes. A heating device 25 is placed downstream of the tube speed control means 20. This device 25 comprises a plurality of heating units which are placed around the path for the preform 6 and can be controlled separately and each one is directed towards a sector of the circumference of the preform 6. As a result, it can be feeding a quantity of heat controllable separately to each sector of the preform 6, for example six circumferential sectors, each of 60 °. The installation further comprises an expansion mandrel 30, which in this case is non-deformable, also described herein by the dimensionally stable term. The mandrel 30 in this case is made of metal. The mandrel 30 is held in a stationary position with respect to the extruder 1 and in this case is attached to the inner core 5 by means of an anchoring member 31. At this upstream end, the mandrel 30 has an inlet portion 32, which in this case is substantially cylindrical in design. The entrance part 32 is joined by a part 33 of expansion, the outer surface of which substantially corresponds to the surface of a truncated cone with a diameter which increases in the downstream direction. The expansion portion 33 is joined by an outlet portion 34 of the mandrel 30, part 34 which is of substantially constant diameter, if the appropriate taper is made slightly in the downstream direction. As a result of being driven on the mandrel 30, the preform 6 changes to a stretched tube 6 '. In the position of the mandrel 30, in particular of the outlet part 34, there is a second external cooling device 40 by means of which the stretched tube 6 'is cooled externally. As is generally known for the production of biaxially oriented plastic tube, the drawn tube is cooled after the expanding portion of the stretching mandrel passes so that as a result the changes which have occurred around in the plastic material of the tube are freeze A second external calibration device 45 is placed at a distance downstream from the mandrel 30, calibration device 45 which reduces the external diameter of the tube 6 '. The installation also comprises a stretching device 50 which is placed downstream of the mandrel 30 and the external calibration device 45. The stretching device 50 is designed to exert a considerable tension force on the stretched tube 6 '. Downstream of the stretching device 50 there is a cutting device to length (not shown), for example a saw, a cutter or a grinding device, in order to cut sections of the desired length of the tube 6 'which have been produced. The preform 6 leaving the die 3 extruder has a relatively thick wall. In order for this to allow the biaxial stretching to take place. After the preform 6 leaves the extruder die 3, at high temperature, the preform 6 is locally cooled / reheated by means of a first external cooling device 15 and by means of a heating device 25 in such a way that the plastic material is at an orientation temperature which is suitable for biaxial orientation thereof before the preform 6 is driven on the expansion portion 33 of the mandrel 30. The preform 6 is driven on the mandrel 30 under the influence of the forces which are they exert on the preform 6 and the tube 6 'by means of the stretching device 50 together with the tube speed control means 20. By means of the stretching device 50 and the tube speed control means 20, it is possible to precisely control the speed of advance both in an upstream position of the mandrel 30 (in the tube speed control means 20) and in a downstream position of the mandrel 30 (in the stretching device 50). As a result of the passage over the mandrel 30, the molecules of the plastic material are oriented, that is to say, they stretch tanc in the axial direction as in the circumferential direction, which is of great benefit for the properties of the tube 6 '. A unit can be placed to measure the wall thickness between the extruder 1 and the mandrel 30, by means of the unit which can measure the thickness of the preform 6 and the shape of the cross section of the preform 6.

Downstream of the mandrel 30 there is a unit 60 for measuring the wall thickness. This wall thickness measuring unit 60 can be connected to a control unit which, based on the measured cross section of the stretched tube 6 ', controls the operation of the stretching device 50, the device 25 and, if appropriate , the distance between the calibration device 45 and the mandrel 30. The mandrel 30 can be provided with one or more feed ducts which open outwardly on the outer surface of the mandrel 30 and, through the anchor member 31 and the extruder die 3 is connected to a pump means (not shown) to supply a liquid between the mandrel 30 and the preform 6. It is therefore possible to form a liquid film between the preform 6 and the mandrel 30, in particular between the preform 6 and the expansion part 33 of the mandrel 30. It is also possible to form a liquid film between the outlet part 34 and the tube 61, which serves to reduce the friction between the tube and the outlet part and, by another part, possibly also as internal cooling for the stretched tube. In a variant, it is possible to introduce a gas, in particular heated air, under pressure between the non-deformable mandrel 30, in particular the expansion part thereof, and the preform 6, in order to obtain, in this way, a film of gas.

It is generally known from the prior art for the installation described above that it operates such that the preform 6 upstream of the mandrel 30 has a uniform cross section, as accurate as possible, i.e., a wall thickness and a diameter and that also has a suitable orientation temperature which is as uniform as possible. Downstream of the mandrel 30, the stretched tube 6 'then has a larger diameter and a smaller wall thickness. In contrast to this known way of operating the installation, according to one aspect of the invention, it is possible to periodically vary the relationship between the forward speed of the preform 6, which is determined by the speed control means 20 of the tube, on the one hand, and the outlet of the extruder 1, on the other hand between a first value and a second value, which is smaller than the first value, for the extruded preform 6, in the section between the extruder 1 and the medium 20 of speed control of the tube, to alternatively acquire a first wall thickness - if the ratio is that of the first value - and a second wall thickness - if the ratio is of the second value - of the second wall thickness which is greater than the first wall thickness. In the example presented here, this is carried out by keeping the extruder 1 output substantially constant and by periodically varying the advance rate of the preform 6 which is determined by the tube speed control means 20. In this case, therefore, the relationship between the feed speed of the preform 6, which is determined by the tube speed control means 20, on the one hand, and the output of the extruder 1, on the other hand, it remains substantially constant at a first value during a first period so that a large part of the preform 6 with a first wall thickness "di" is produced. During a second period, which is considerably shorter than the first period, the speed of the tube speed control means 20 is adjusted to a smaller value, with the result that the part of the preform having the second thickness "d2"of larger wall, then formed immediately downstream of the extruder die 6, as indicated in FIG. 1 by reference numeral 70. The method provides continuous production in which a thickened preform part 70 is preferably obtained at intervals regular During the external calibration 10, the preform 6 acquires a uniform external diameter, so that the thickened preform part 70 projects inwards in that area with respect to the preform part having the first wall thickness., as indicated by the dashed line. The thickened preform part 70 then passes through the external cooling device 15 and reaches the mandrel 30, where the thickened preform part 70 is made to project outwardly by the inlet portion 32 of the mandrel (indicated by a dashed line). ). When it passes over the mandrel 30, the preform 6 and consequently also the thickened preform part 70 is stretched axially and in the circumferential direction, as will be described in greater detail later. When it passes through the external calibration device 45, the thickened portion 70 is pressed in again (as indicated by a dashed line) resulting in a stretched tube 6 'having portions 70 thickened at axial (regular) intervals and , between these thickened parts, in each case a large part of smaller wall thickness di. In a practical embodiment, the tube 6 'is cut to a length downstream of the stretching device 50 in each thickened portion 70, and the distance between two thickened portions 70 corresponds to the desired length of the tube sections to be produced. when cutting the tube 6 'to the length. As a result, each tube section then has a tube body and, at one end, a thickened tube part with a wall thickness greater than the tube body. Preferably, the thickened end portion of the tube is then subjected to a receptacle forming operation, so that a high quality integral receptacle can be obtained. In another variant, the tube 6 'is cut to length such that there is a thickened end portion at each end of a tube section. It is then possible for one of the ends to deform into a receptacle, while the other end, possibly without additional treatment, can be used as a thickened spike. In a preferred embodiment of the method according to the first aspect of the invention, the biaxially stretched tube undergoes the same axial stretching over its entire length. Since the speed of advance of the preform 6 upstream of the mandrel 30, which is determined by the speed control means 20 of the tube varies, it is therefore necessary that the feed speed of the tube 6 'downstream of the mandrel 30 which is determined by the stretching device 50 varies periodically, such that the ratio between the feed speed of the tube 6 'downstream of the mandrel 30 and of the preform 6 upstream of the mandrel 30 is maintained substantially constant during the production of both the thickened part and the non-thickened part. In a variant of the method according to the first aspect of the invention, a thickened portion 70 is provided which does not undergo the same axial stretching as the intermediate portions of the first wall thickness di, but rather for the thickened portion 70 to undergo a axial stretch greater. For this purpose, for the period during which the thickened part 70 is driven on the mandrel 30 or during part of this period, the relationship between the feed speed downstream of the mandrel 30 which is determined by the stretching device 50 and the feed speed upstream of the mandrel 30, which is determined by the tube speed control means 20 is greater than in the period during which a part of the preform which has the first wall thickness is driven on the mandrel 30. To allow the process to be successfully controlled, it is desirable for the axial stretching of the preform to be carried out within a precisely defined subsection of the installation. For this purpose, it is possible for the stretched tube 6 'to be cooled downstream of the expansion portion 33 of the mandrel 30, so that the cooled tube 6' does not undergo any additional axial stretching and the generation of the axial stretching is concentrated in the section between the tube speed control means 20 and the downstream end of the mandrel 30. To control the process, it is further advantageous for the temperature of the preform 6, downstream of the mandrel 30, to be conditioned with the aid of the cooling device 15 and, if appropriate, to a slight degree by a heating device 25 such that the part 70 of the thickened preform is on average at a higher temperature, measured by a position immediately upstream of the mandrel 30, as compared to the immediately adjacent downstream preform part of the first wall thickness which is in advance in the mandrel. 30. As already described, the speed of the preform 6 is reduced while forming the preform part 70 with the wall thickened. As a result of the reduction in speed, that part of the preform which during this period is located in the cooling device 15 will undergo the cooling action for a longer time than that part of the preform which has already passed through the cooling 15. When the part 70 of the preform with a thickened wall has formed, the speed of the preform 6 increases again and the part 70 of the preform will pass through the cooling 15 at this higher speed and through therefore, in relative terms, it will cool to a lesser degree as compared to the immediately downstream part of the preform 6. When the part 70 thickened then reaches the mandrel 30, such part 70 is hot and easy to deform while that part 70 is hot. part of the preform which is located immediately downstream thereof has a thinner wall and is in fact relatively rigid. By a combination of the two -Icts, it is possible to successfully drive the thickened portion 70 on and above the mandrel 30 without the downstream part thereof stretching excessively in the axial direction. Tests have shown that, in the case of PVC, the wall thickness of the thickened preform can be 15% greater than that of the intermediate parts without causing any problems. Preferably, the variation in the thickness of the wall of the preform 6 is always gradual, so that there are no abrupt transitions from one wall thickness to another wall thickness. Incidentally, it is conceivable that the thickened preformed parts are not produced specifically for the subsequent formation of a receptacle but rather, for example, to allow a branch tube to be connected to the stretched tube. The thickened tube part can also be used as a point for a stretching device placed, for example, downstream of the expansion mandrel to be coupled onto the tube, so that a high tension force can be exerted on the tube with the In order to force the preform onto the expansion mandrel. The shape of the thickened part 70 shown in the figures la and Ib is, of course, only shown as an example. In fact, it has been shown that it is possible for the wall thicknesses of the thickened part 70 to be precisely controlled, and in this way, that a specific profile be precisely imparted to the wall of the thickened part 70, as observed in the longitudinal direction of the tube. Figure 2a shows a longitudinal section through the half of the preform 6 in a position immediately after it has passed through the calibration device 10, which has a thickened tube part 170 by varying the speed of the medium 20 of pipe speed control with respect to the extruder outlet 1. In figure 2a, di indicates the first wall thickness which is used for a long part of the preform 6. Line 171 is the central axis of the preform 6 The thickened portion 170 has a profile with a plurality of wall thickness values, described by points A, B, C, D, E, F and G. Figure 2b shows the same thickened part as in Figure 2, but in this case after it has passed over the baboon 30. This can be clearly seen from the larger diameter and reduced wall thickness of the tube 6 'now stretched. It is clear that the internal diameter of the tube 6 'is now uniform and the wall thickness profile can be observed on the outside. The AG points show that the stretching has been carried out in the axial direction and in the circumferential direction of the thickened portion 170 when it passes over the mandrel 30. Figure 2c shows the part of the tube 6 'after it has passed through. of the calibration device 45, device which, incidentally, is optional in the method according to the first aspect of the invention. The outer diameter is now uniform once more, while the profile can be seen inside. As described, there is a provision for the tube 6 'to be cut to length in the thickened portion 170, in this case in the line 172. Afterwards, the tube section cut to the length is subjected to the operation of the formation of the receptacle, during which the thickened portion 170 of the tube section deforms to form a receptacle. Figure 2d shows a possible embodiment of that end of a tube section which is provided with a receptacle and which has been produced as described with reference to Figures 2a, 2b and, if appropriate 2c. At one end, the prefabricated tube section with a thickened tube portion 170 has a wall thickness greater than the body of the tube, and the axial stretching of the thickened end portion prior to the receptacle forming operation is equal to or preferably greater than the axial stretching of the tube body. It will be clear from the preceding text how the tube section of this nature can be produced.

In particular, Figure 2c shows that the end portion of the prefabricated tube, as seen from its end face, has a plurality of annular areas which are adjacent to each other and have a wall thickness which fluctuates from an annular area to the next annular area, the wall thickness, in the case of a plurality of annular areas, is greater than the wall thickness of the tube body. Then, during the receptacle formation operation, in this case the annular area between the points B and E is deformed in an outward projecting slot wall 173 which delimits an internal slot 174 of the tube, which is designed to accommodate a sealing ring (not shown). The slot wall 173 advantageously can have a level of axial stretching compared to the body of the tube with the wall thickness the, in particular if the part 172 of the thickened tube is produced in such a way that in advance it shows a greater level of stretching axially before the formation of the receptacle and in comparison with the adjacent tube body, more or less exceeding the point G. The additional wall thickness of the annular area from which the slot wall 173 is formed makes it possible to ensure that, even as a result of the increase in the diameter of the part during the formation of the receptacle, the final wall thickness of that part is no less than that of the body of the tube. In particular, this is possible without the axial stretching of such part of the tube that is produced or even that it becomes entirely in negative stretch through compression of such part, as is known in the prior art. It will be apparent that the disadvantages discussed with reference to the slot wall 173 also apply to other areas of the receptacle which are formed from the thickened tube portion 170. Finally, therefore, it is possible to produce a tube section from biaxially oriented plastic material which has a tube body and an integral receptacle, the axial drawing of the receptacle is equal to, or preferably greater than that of the tube body . In this case, the wall thickness of the receptacle can also be equal to or even greater than that of the body of the tube. In an illustration corresponding to Figure 2a, Figure 3a shows another embodiment of the thickened portion 190 which has been produced using the method according to the invention. This thickened portion 190 has a first zone, indicated by the points A-G, which virtually corresponds to the description given with reference to FIG. 2a. Line 191 is the central axis. Further away from the end of the tube section to be produced, which is shown by the line 192, the thickened part 190 has a second zone, between points G and H, with a wall thickness di corresponding to the thickness of the preform outside the part 190 thickened. This is followed by a third zone, indicated by the H-K points, with a greater wall thickness. It can be seen in Figure 3b that only the first zone of the thickened part 190 has deformed into a receptacle. This first zone is deformed in the same way as described with reference to figure 2d and having a slot wall 193. The third zone forms a flange 194 projecting inwardly. This flange 194 serves to receive a support bushing which is inserted into the first zone when the receptacle is formed, in order to provide internal support for this zone during heating. When the receptacle is formed, this support bushing is then further pushed into the tube and then supported against the flange 194. This prevents the holder from penetrating too deeply into the tube and also prevents this bushing holder from locally overheating the tube. During the formation of a receptacle at the end part of a biaxially oriented tube, in particular at the thickened end portion as explained above, it is considered advantageous if, during the formation of the receptacle using a receptacle-forming mandrel, the end part does not undergo any compressive tension, i.e., axial compression. This is due to the fact that the compressive tension leads to a reduction in the axial stretching of the end portion which deforms in a receptacle, and this may be disadvantageous. For example, it can be seen in WO 97/33739 that, during the formation of the receptacle, pressure is exerted on the end side of the tube so that compression stress is generated. To control such compression stress during the formation of the receptacle so that the compression tension can be maintained at a low level or even can be avoided altogether, it is possible that the tube is provided, in the vicinity of its end side, with a clamping area which is between the end of that part of the tube which is going to deform in a receptacle. Before the receptacle-forming mandrel is inserted into the tube, the tube is then held and held in the clamping area, while the receptacle-forming mandrel is pressed into the end portion of the tube to that part which it does not deform in a receptacle, part of which is overrunning the clamp, as seen in the insertion direction of the mandrel. As a result of holding the clamping area, an undesirable and uncontrollable compressive tension is avoided in the end part of the tube. If appropriate, lubrication can also be provided between the mandrel and the end portion of the tube in order to reduce the friction therebetween. Preferably, after the receptacle has been formed, the clamping area is removed from the tube, for example by means of a cutting or sawing device.

Since this clamping area is subsequently removed, it is also permissible that this area be damaged when it is clamped.

By way of example, a receptacle forming facility that is provided with a receptacle-forming mandrel and with operable clamping means is used to hold and maintain the clamping area of the tube. By way of example, the fastening means comprises teeth which are fixedly housed in the plastic in this area. In an advantageous embodiment, the fastening zone is designed as a thickened annular area of the tube. If appropriate, the securing means forms a type of collar which engages behind the thickened annular area. Figure 4 shows a cross-section through part of the extruder die 200 which is suitable for use in the method described above and is used to extrude a preform 201 of thermoplastic material. In addition, the figure shows a section of an external calibration device 202 placed downstream of the extruder die 200. The extruder die 200 comprises an outer ring 205 and an inner core 206, which between them delimit an annular gap for the plastic material which is supplied by an extruder (not shown). The calibration device 202 is placed closely behind, and virtually against the extruder die 200, in order to prevent the preform 201 from being exposed to outside air for an undesirably long time, which is advantageous from a chemical and thermal point of view . The calibration device 202 has a sleeve 207 which defines the external diameter of the preform 201. The calibration device 202 cools the exterior of the preform and a solidified coating is formed on the outside of the preform 201. Immediately downstream of the die 200 extruder, the preform 201 is also cooled internally by means of an internal cooling member 208, of which only a part is shown. As described in the foregoing, there is provision of the wall thickness of the preform 201 which changes periodically so that, in this manner, a preform portion with a greater wall thickness is obtained, as shown in Figure 4. To obtain a preform part with a wall thickness greater than that defined by the separation between the inner core 206 and the outer ring 205, the flowable plastic material must be capable of flowing from the die 200 extruder to the part of the former. the thickest preform For this reason, it is not desirable for a solidified coating to form inside the preform, immediately downstream of the inner core. To counteract this coating formation, an insulating member 210 is provided which is attached to the inner core 206. The insulating member 210 has a conical outer surface 211 which is adjacent to the outer surface of the inner core 206 and has an outer diameter which decreases in the extrusion direction. During the formation of the thickened portion in the preform 201, the plastic material then bears against the insulating member 210 and the formation of the solid coating in that position is prevented. Preferably, the outer surface 211 of the insulating member 210 is at least partially within the outer ring 205. As a result, the expansion of the preform 201 to obtain a thickened portion in the preform 201 can be carried out even downstream of the external calibration device 202 placed close behind the extruder die 200. In two partial drawings which are adjacent to each other, Figures 5a and 5b diagrammatically show the most important elements of an installation for producing biaxially oriented thermoplastic pipe in a continuous process. The wall thickness of the tube to be produced is preferably such that the tube is dimensionally stable. In particular, it is intended to produce pipe which is suitable for the assembly of working pipe systems to transport liquid or gas, in particular drinking water, waste water, natural gas or the like. Preferably, the tube is suitable for placement in the ground. Figure 5a shows an extruder 301 having one or more extruder screws 302 with an associated controllable impeller, by means of which a flow of molten plastic material is provided, which is fed to an extruder die 303 placed over the extruder 301. die 303 extruder has an outer ring 304 and an inner core 305 which, together with the outer ring 304, delimit an annular outlet from which arises a preform 306 extruded from thermoplastic material in a substantially horizontal direction. In this arrangement, the inner core 305 defines an axial space in the preform 306. The die 303 extruder is provided with a means for controlling the wall thickness (not shown) by means of which a uniform wall thickness can be produced ( in the circumferential direction) of the preform 306 that comes from the die 303 extruder. An internal cooling member 310, whose construction will be explained later with reference to Figure 6, is attached to the inner core 303. The internal cooling member 310 is designed such that the preform 306 exiting the extruder die 303 is cooled internally immediately downstream of the extruder die.

The preform 306 is externally calibrated with the help of the calibration sleeve 320. This calibration sleeve 320 performs a slight reduction in the external diameter of the preform 306. The calibration sleeve 320 is placed downstream of the internal cooling member 310, in a position where the preform 306 is not internally supported by a solid component. This arrangement has the advantage that the preform 306 can not be stuck in the calibration sleeve 320, since a reduction in the internal diameter of the preform 306 can be carried out without problems. Downstream of the calibration sleeve 320 there is a first external cooling device 330, by means of which the preform 306 is cooled externally. The external cooling device 330 comprises, for example, several compartments which are placed one behind the other, through which cooling water flows and through which the preform 306 moves, which comes into direct contact with the Cooling water. If appropriate, the cooling water may be at different temperatures in each compartment, in order to optimize the cooling of the preform 306. Since the external cooling device 330 is placed downstream of the internal cooling member 310, as shown in FIG. in the direction of extrusion, the preform 306 that comes from the die 303 extruder initially cools only internally (in addition to a very light natural cooling of the exterior of the preform from the ambient air) and subsequently only externally cooled. This ensures that the preform 306 is not simultaneously subjected to the cooling action of the internal cooling member 310 and the external cooling device 330. Depending on the axial distance between the internal cooling member 310 and the external cooling device 330, there may be a small overlap between the cooling action of the internal and external cooling. The fact that the inner cooling member 310 and the external cooling device 330 are positioned offset from one another in the axial direction proves to be advantageous in particular for a thermoplastic material which crystallizes upon cooling after extrusion and consequently shows a shrinkage significant volume This type of material includes, for example, polyethylene (PE), which undergoes a volumetric shrinkage which can constitute up to about 30%. As a result of the cooling action of the internal cooling member 310, a cold wall layer is formed within the preform 306 immediately downstream of the extruder die 303, which is a relatively stable wall dimensionally cold layer. If a cold layer is to be formed on the outside at the same time by means of external cooling, an intermediate layer, still hot, of plastic material can be included between two cold rigid wall layers. The cooling of this intermediate layer can easily result in shrinkage cavities in the intermediate layer, and there is also a considerable risk that visible deformation forms in the form of pinholes or indentations on the outside and inside of the tube 306 'that is produced. If the cooling is initially carried out only in the interior, the shrinkage of this intermediate layer can be absorbed by material that is supplied from the uncooled outer layer of the preform. Once the inner layer has cooled, it can then start cooling from the outside. Downstream of the external cooling device 330 there is a speed control means 340 which acts on the cooled outer layer of the preform 306. The speed control means 340 in this case is designed as a stretching device which is known per se and has a plurality of tracks acting on the tube, whose type of stretching device is usually used for the extrusion of plastic tubes. A heater device 350 is placed downstream of the speed control means 340. This device 350 comprises a plurality of heating units which are placed around the path for the preform 306, which can be controlled separately and each is directed towards a sector of the circumference of the preform 306. As a result, a separately controllable amount of heat can be supplied to each sector of the preform 306, for example six circumferential sectors , each of 60 °. The installation further comprises an expansion mandrel 360 which in this case is of a non-deformable design, also described herein by the dimensionally stable term. The mandrel 360 in this case is made of metal. The mandrel 360 is held in a stationary position with respect to the extruder 301 and is here attached to the extruder 301, in particular at its inner core 305, by means of an anchor member 361 on the internal cooling member 310 and via the member 310 of internal cooling. At its upstream end, the mandrel 360 has an inlet portion 362, which in this case is of a substantially cylindrical design. The inlet portion 362 is joined by an expansion portion 363 which has an external surface which substantially corresponds to the surface of a truncated cone with a diameter which increases in the downstream direction. Expansion portion 363 is adjacent by an outlet portion 364 of mandrel 360, part 364 which is of substantially constant diameter, and if appropriate slightly tapered in the downstream direction.

In the mandrel 360, in particular in the area of the outlet part 364, there is a second external cooling device 370 by means of which the stretched tube 306 is externally cooled. As is generally known for the production of a biaxially oriented plastic tube, the drawn tube is cooled after the expansion portion of the stretching mandrel has passed, so that as a result, the changes which have been carried out around of the plastic material of the tube are frozen. At a distance downstream from the mandrel 360 there is a second external calibration device 380, calibration device 380 which is placed around a reduction in the external diameter of the stretched tube 306 '. The installation also comprises a stretching device 390 which is placed downstream of the mandrel 360 and the external calibration device 380. The stretching device 390 is designed to exert a considerable tension force on the tube 306 '. A length cutting device, for example a closing, cutting or grinding device, can be located downstream of the stretching device 390, so that sections of the tube produced to a desired length are cut. Alternatively, the winding device can also be provided for the purpose of winding the produced tube 306 'on a reel.

The preform 306 that comes from the die 303 extruder is thick wall. After the preform 306 leaves the extruder die 303 and then at high temperature, the local cooling / reheating of the preform 306 is carried out by means of the internal cooling member 310, the first external cooling device 330 and by means of the heating device 350, such that the plastic material is in a temperature orientation which is suitable for biaxial orientation thereof before it moves on the expansion portion 363 of the mandrel 360. The preform 306 is passed over the mandrel 360 under the influence of the forces which are exerted on the preform 306 by means of the stretching device 390 together with the speed control means 340. The speed of the preform / tube 306 can be controlled by means of the stretching device 390 and the control means 340 both in a position upstream of the mandrel 360 (at a control speed-340) and in a downstream position of the mandrel 360 (in the stretching device 390). As a result of the passage over the mandrel 360, the molecules of plastic material are oriented both in the axial direction and in the circumferential direction of the tube 306 'which is highly advantageous for the properties of the tube 306'.

The details of the installation shown in Figures 5a and 5b will be explained in greater detail in the following, partially with reference to the additional figures.

The internal cooling member.

The part of the internal cooling member 310 can be seen in Figure 6. The internal cooling member 310 has a dimensionally stable and rigid cylindrical outer wall, made for example of metal, with a large central section 311, the diameter of which is slightly larger than the diameter of the end sections 312 that are at the upstream and downstream ends of the mid section 311 (only the downstream end section can be seen in Figure 6). The difference in diameter between section 311 and section 312 is preferably no greater than 3 millimeters and is at least 0.5 millimeters. This difference has been exaggerated in Figure 5a. The axial length of the end sections 312 is considerably shorter compared to that of the central section 311, the length of the central section 311 preferably being a multiple of the wall thickness of the preform 306. In practice, it is preferable that This length is one meter or more. The internal cooling member 310 is provided with a feed passage 313, which opens outward in one or more openings 314 that are located on the surface of the central section 311, openings 314 which are located in the vicinity of the downstream end section 312. In addition, the internal cooling member 310 also comprises at the upstream end of the central section 311, one or more openings (not shown) which are adjacent to an outlet passage of the internal cooling member 310. The installation further comprises a feed means (not shown) for cooling liquid, which is connected to the inlet passage 313 and by means of which cooling liquid can be introduced between the central section 311 of the internal cooling member 310 and the preform 306. This cooling liquid then forms a film of liquid and flows, preferably at high velocity, in the direction opposite to the extrusion direction, towards the openings of the outlet passage. In this way, the internal cooling of the preform 306 is carried out. The high speed of the cooling liquid in the liquid film has the advantage, firstly that, despite the small volume of the liquid film, it is still possible to obtain an effective cooling action. In this context, it is important that the liquid in the liquid film does not evaporate, since this would result in an undesirable buildup of pressure in the preform 306. Another important advantage of high speed is related to the problem of forming air bubbles or gas in the cooling liquid. As is known, the cooling liquid used is generally water, and this cooling water contains air. Therefore, when cooling water is heated, air bubbles form and these air bubbles generally rise. If internal cooling is used in which the cooling liquid, referred to in the foregoing as water, comes into direct contact with the interior of the plastic preform to be cooled, air or gas bubbles represent a very considerable drawback. Due to the presence of air or gas bubbles, the interior of the preform cools to a lesser degree in that position where the surrounding area and consequently becomes dimensionally less stable than the colder surrounding area. As a result of the volumetric shrinkage of the plastic material during cooling, as described above, shrinkage of the material will pull the already rigid coating layer of the preform inwardly. As a result, a cavity is formed inside the preform inside the preform in the position where the air bubble was, cavity in which the air bubble is enclosed. As a result, the air bubble remains in place in that position and the cooling of this small area remains poor, so that the cavity becomes even deeper. This leads to a clearly detectable cavity in the inner surface of the stretched tube, which is unacceptable. Incidentally, bubbles may also be formed by gases which are released from the extruded preform. Generally, any local interruption in internal cooling has been found to leave a visible mark inside the tube 306 'and for this reason it is important that the internal cooling be highly regular. When using internal cooling with liquid, it is known in advance that the bubbles are extracted by means of a suction tube which is connected to the highest point of an internal cooling compartment which is present in the extruded tube and through the which flows the cooling liquid. However, this solution is not always possible or satisfactory, in particular since the adverse effect of the air bubbles occurs very quickly after the preform has come into contact with the air bubbles and because once the air bubbles have formed, tend to continue to adhere to the PCA preform suction.

For these reasons, it is important that, when internal cooling is used for the preform, it is provided with a dimensionally stable cold layer on the inside by cooling as soon as it leaves the extruder die, as is the case with the internal cooling member 310 described before. This is particularly important for the internal cooling of profiles which have been extruded from plastic material such as polyethylene (PE) and polypropylene (PP). It has been found that, in the case of polyvinyl chloride (PVC) for example, this problem is less significant. It is also important that this cooling layer is maintained throughout the trajectory during which internal cooling is carried out, since otherwise the aforementioned cavitation may occur. In addition, it will be clear that it is important to counteract the formation of air bubbles, in particular large air bubbles or an accumulation of air bubbles. In the case of the internal cooling member 310, the high flow rate of the cooling liquid ensures that only small air bubbles are formed, which are entrained by the fast flowing liquid and do not adhere to the interior of the preform. The formation of air bubbles during internal cooling can also occur by removing the air first from the cooling liquid, such as water, used for internal cooling before the liquid is introduced into the preform which is to be cooled . The removal of air can be carried out, for example, by first boiling the water and then allowing it to cool. If appropriate, boiling can be carried out at subatmospheric pressure. Another solution to counteract the drawbacks of air or gas bubbles during internal cooling is the use of a cooling liquid with a low surface tension. This can be obtained, for example by using water as the cooling liquid, in which case one or more substances which reduce the surface tension are added to the water. This may involve, for example, the addition of alcohol to the cooling water. Due to the low surface tension, it is easy for air bubbles to form, but the air bubbles are extremely small, which leads to less cavitation. Another solution to avoid is the generation of a helically oriented flow of the cooling liquid along the inside of the preform which is to be cooled. This flow prevents air bubbles from accumulating along the upper side of the inner circumference of the tube. If appropriate, in the case of the internal cooling member 310, a shallow helical profile can be provided on the surface 311 in order to generate this flow.

Another additional measure to avoid the adverse effect of the air or gas bubbles is to improve the wetting of the internal surface of the extruded preform so that the liquid adheres to a greater extent to the surface and the bubbles are released more easily. In combination with the internal cooling member 310 attached to the inner core 305, it is also conceivable that the inner core 305 be provided with cooling in order, in this manner, that the internal cooling of the extruded preform 306 starts even earlier. It will be apparent that the solutions for internal cooling described herein are suitable not only for use in the production of a biaxially oriented tube, but also for any other process for extruding tube sections from thermoplastic material. However, another factor in the production of biaxially oriented tube from crystalline thermoplastic material, such as polyethylene (PE), is that the crystallization and the associated significant volume shrinkage is carried out in a temperature range which is found in the vicinity of the orientation temperature, that is, the stretching temperature, which is the temperature that the preform must have when it passes over the mandrel. The first external calibration sleeve 320 is located in particular at a distance downstream from the internal cooling member 310, in view of the design described before the internal cooling member 310, in which case there is only a thin film of liquid between the preform 306. and the internal cooling member 310. The rigid design of the internal cooling member 310 means that the preform 306 will be incapable of contact here without becoming entangled in the internal cooling member 310.

Effects of crystalline composition The biaxial stretching process, in which the tube is extruded and this tube is driven in line on the drawing mandrel, has already been used successfully for amorphous thermoplastic materials, in particular for pipes made of polyvinyl chloride. Many pipes, for example for drinking water and gas pipes, however, are manufactured from crystalline thermoplastic materials, in particular from polyethylene and polypropylene. The difference between a composition of the plastic material described as amorphous or crystalline has been shown to have significant effects on the progress and execution of the biaxial stretching process. It should be noted that crystalline materials, such as PE and PP, are in fact two-phase systems, in which part of the material is amorphous and part is crystalline. The relationship between the amorphous part, on the one hand, and the crystalline part, on the other hand, depends in particular on the cooling of the molten plastic material and therefore in particular on the cooling rate. In the case of the biaxial stretching process, for example using the installation shown in Figures 5a and 5b, first a thick-walled preform is extruded, which must then be cooled to a suitable orientation temperature which is significantly lower than the temperature of the preform when it leaves the die 303 extruder. For this reason, the internal cooling member 310 and the first external cooling device 330 are active. In view of the poor thermal conductivity of the thermoplastic materials in this continuous process, in which, obviously, the highest possible production speed is desired, it is inevitable that the cooling of the plastic material is not carried out uniformly through the cross section of the preform. In particular, the inner and outer sides of the preform, which are brought into contact with the cooling medium, will undergo rapid cooling and consequently a large amount of crystals, but mainly very small crystals, will form in those areas. Within the preform, cooling will take place more slowly. As a result, a large amount of crystals, but very small crystals, are formed on the inner and outer sides of the preform, while larger crystals are formed within the preform. This difference can be a disadvantage for the biaxial stretching of the preform and the final result obtained. To solve or reduce this problem, it is conceivable to allow the highly cooled layer of the preform to heat downstream of the internal cooling of the thick-walled preform exiting the extruder, so that the small crystals begin to grow. This can be obtained by allowing this layer to be heated by heat transfer from the center of the wall or by placing the inner side of the preform in contact with a heating means. In particular, it is possible to provide a compartment downstream of the internal cooling member in the hollow space in the preform, which compartment is filled with hot liquid, for example at a temperature of between 90-100 ° C. The problem described before, when internal cooling is used for an extruded tube or preform made of a crystalline thermoplastic material, large quantities of small crystals are formed on the intensively cooled inner side, it can also be resolved when designing the tube or preform with a multi-layered wall. In this case, the inner wall layer, which is cooled more rapidly by internal cooling, is preferably made of an amorphous thermoplastic material, while the layer around it is extruded from a crystalline thermoplastic. By way of example, the inner layer is made of polyvinyl chloride and the outer layer is made of polyethylene. Incidentally, the same idea can also be applied to the situation with external cooling, in which case it is advantageous for a wall layer made of crystalline thermoplastic material that is surrounded by an outer layer of an amorphous thermoplastic. By combining the above aspects results in a profile with an inner wall layer made of amorphous material and an outer wall layer made of amorphous material with, among them, a wall layer made of a crystalline thermoplastic material, for example a profile of three layers with two covers (thin) made of PVC, which enclose a thicker intermediate layer of PE. A profile of this nature can be subjected to a biaxial stretching process, for example by driving the profile which leaves the extruder on a downstream expansion mandrel. The crystal formation can also be altered by adding a substance which serves as a nucleus for the formation of crystals to the plastic material. The addition of clay has been shown to have a beneficial effect on crystal formation in the production of biaxially oriented pipes from polyethylene. In particular, a large amount of crystals rapidly form. It should also be noted that the inner wall layer made of PVC solves or counteracts the problem described before pitting caused by air bubbles in the cooling water of the internal cooling. This is because PVC has a better thermal conductivity than PE, and wetting by cooling liquid, in particular water, is also better. It should be noted that extrusion devices for extruding multi-layer tubes are generally known.

Wall thickness control During the biaxial stretching of a preform on a mandrel, any deviation in the wall thickness of the preform which still has to pass over the mandrel has been shown to have considerable influence on the behavior of the preform as it passes over the mandrel and therefore on the mandrel. the biaxial orientation that is obtained. As already known for a unit to measure the wall thickness which is placed between the extruder and the mandrel, which unit can be used to measure the thickness of the wall and the shape of the cross section of the preform. Wall thickness measuring units of this nature are often ultrasonic units in which an ultrasonic pulse is transmitted through the wall from the outside and the reflection of this pulse determines the wall thickness. This is because the reflection is based on the difference between the speed of sound transmission through the wall and through the medium that is located in the preform. As described above, the preform is still relatively hot in the section between the extruder and the expansion mandrel, and this causes problems with the operation of such ultrasonic wall thickness measuring units. Furthermore, in the case of crystalline thermoplastic materials, the crystallization is carried out precisely at the temperatures prevailing in that section, which results in a considerable change in the density of the thermoplastic material which in turn has consequences for the transmission of the thermoplastic material. ultrasonic pulse. This effect is also disadvantageous for the operation and reliability of the measurements using the ultrasonic wall thickness measuring unit. It has been found that the operation improves if a layer of cold liquid lies along the inside of the preform at the position of the ultrasonic wall thickness measurement, or if the preform is filled with a cold liquid in this position. If the liquid is hot, for example water in the vicinity of 100 ° C, the measurement of the ultrasonic wall thickness seems to work in a considerably less precise way compared to a cold liquid. It is assumed that this is due, in particular, to the difference in the transmission speed between the preform and the liquid which is important for the reflux of the ultrasonic pulse and in the case of the hot liquid, this difference is smaller. In known ultrasonic thickness measurement units, one or more ultrasonic transmitters / receivers rotate around the tube. In this mode, it is conceivable for a velocity for a cold liquid flow to rotate inside the tube in the same position. In figure 5a, the number 400 diagrammatically shows an ultrasonic wall thickness measuring unit, the layer described above for the cold liquid is produced using the internal cooling member 310 which has been described in detail in the foregoing. Another consequence of the wall thickness measurement at a position between the extruder 301 and the mandrel 360 is that the preform temperature 306 also influences the ultrasonic wall thickness measurement. As described, in this section, the temperature can vary, for example due to the internal and external cooling action that is adjusted in the starting phase. In order to reduce the influence of the temperature of the wall of the preform on the measured wall thickness, it is possible to distribute a device for measuring the temperature of the wall of the tube in the vicinity of the wall thickness measuring unit 400. Ultrasonic and provide a suitable compensation algorithm, which is used to compensate for the influence of temperature on the wall thickness being measured.

Formation of differences in wall thickness and orientation In the process of biaxial stretching, one of the most important aspects is the passage of the preform over the stretching mandrel, so that the preform is stretched in the radial direction and possibly also in the axial direction. From the prior art, it is known to seek to treat the extruder tube in the section between the extruder and the mandrel, such that the tube reaches the mandrel with a wall thickness which is as uniform as possible and which preferably also at a temperature which is as uniform as possible within the temperature range which is suitable for the biaxial orientation. It is also known that, despite the preparatory operations, the deviations in the cross section of the preform can still arise as a result of the passage over the mandrel. These deviations relate to the wall thickness of the preform as seen in the circumferential direction and, if appropriate, the eccentricity of the inner side with respect to the outer side. These deviations are then observed using a second wall thickness measuring unit 130 distributed downstream of the mandrel. To make it possible to correct these deviations, it is known in advance to use the heater device 350 shown in Figure 5b. As mentioned above, this heater device 350 comprises a plurality of heater units which are distributed in the vicinity of the mandrel 360 and around the preform 306. Each of the heater units can be used to emit an adjustable amount of heat separately. to an associated sector of the circumference of the preform 306 which moves past. As a result of the added heat, the temperature and consequently the rigidity of the plastic material changes. In this way, it is possible to adjust the resistance experienced by the preform 306 when the mandrel 360 passes in sectors in the circumferential direction of the preform. This adjustment is known per se. In practice, even when this heater device 350 is used, undesirable deviations have been generated in the cross-sectional shape and the wall thickness of the forced tube on the mandrel 360 which are still presented. This problem, as well as an associated solution, will be explained in more detail with reference to Figs. 7 and Figs. 7 and 8 show mandrel 360 with inlet portion 362, expansion portion 363 and exit portion 364. The expansion portion 363 of the mandrel 360 has an outer surface which substantially corresponds to the surface a truncated cone. The mandrel 360 is provided with one or more feeding passages 365, in the vicinity of the downstream end of the expansion portion 363, and opens on an outer surface of the mandrel 360 and through the anchor member 361 and the extruder die 303 , which connect to a pump medium (not shown) for supplying a liquid between the mandrel 360 and the preform 306. In addition, the mandrel 360 is provided with one or more outlet passages 366 which extends from an opening placed in the entry portion 362, through the anchor member 361 and die 303 extruder, at an outlet. By means of these passages 365 and 366 and the associated pump means, it is possible to produce a fluid flowing film between the preform 306 and the mandrel 360, in particular, between the preform / tube 306 and the expansion part 363 of the mandrel 360. This formation of a film or liquid, eg, a water film, between the preform 306 and the mandrel 360 is known per se. In this case, the liquid in the film flows in the opposite direction to the direction of movement of the preform 306 on the expansion portion 363. Due to the presence of the liquid film, in fact there is little or no frictional contact between the preform 306 and the expansion part 363. The liquid film not only reduces the friction but also cools the surface of the mandrel 360 below the melting point of the thermoplastic. Above this temperature, the coefficient of friction increases very rapidly. In practice, in such a known situation, with a dimensionally stable mandrel and a water film between the mandrel and the preform it has been found that, when the preform passes over the expansion part, local differences in thickness may not be present. of wall or that are present only to a very slight degree, upstream of the mandrel shape in the circumference of the preform. In other words, it is generally observed that one area of the circumference of the preform moves on the mandrel and moves much thinner, while the adjacent areas there is little or no reduction in the wall thickness. This not only leads to unacceptable deviations in the wall thickness of the tube that is produced, but also to a difference in the biaxial orientation. It has been found that the aforementioned problem can be resolved / reduced by providing the outer surface of the expansion portion 363 of the mandrel 360 with elongated slots or projections extending axially in a plurality of positions around the circumference of the portion 363 of expansion. It can be seen in Figure 8 that a large number of shallow grooves 367 are formed on the outer surface of the expansion part 363. In this figure, for purposes of clarity, several of these slots 367 are shown on an enlarged scale. Figure 7 also shows one such groove 367. The grooves 367 extend in the axial direction, ie in the direction in which the preform 306 is driven on the mandrel 360. The grooves 367 are preferably distributed over the expansion part. at regular angular intervals, preferably between 3o and 10 °. When the preform 306 is driven on the mandrel, part of the soft plastic material of the preform 306 will move in these grooves 367, as shown in Figure 7. This form of coupling between the preform and the expansion part of the mandrel limits the freedom of movement of the plastic material of the preform in the circumferential direction of the expansion part of the mandrel, which has been shown to considerably reduce the above problem of local deviation of the wall thickness in the finally obtained pipe. The shallow grooves 367 are sufficient to obtain the above effect. In practice, 5 millimeters have proven to be the upper limit, while depths between 0.5 and 3 millimeters are preferred; slots with a depth of 0.5 millimeters and a width of 0.5 millimeters have proven to be effective. Part of the water film between the preform and the mandrel will pass through the slots 367, but a film of liquid will remain between the mandrel and the preform in areas in which they lie between the slots 367. Incidentally, it is also conceivable that the liquid is supplied not only via the passage 365, but rather via the passage which is further opened downstream, in the outlet part 364, on the outer surface of the mandrel. In practice, the grooves 367 lead to small longitudinal projections of the inner circumference of the preform that pass over the expansion part of the mandrel. However, these projections are reduced in size considerably by the smooth exit portion of the mandrel. In practice, only a visible impression of the projections remains, which is acceptable. Obviously, if the grooves 367 are replaced by raised projections, a pattern of shallow longitudinal grooves is formed in the tube. This does not present any problem either. It can be seen from figure 7, as well as from figure 5b, that a second liquid film is formed in a manner known per se between the outlet part 364 of the mandrel 360 and the tube 306 '. This second liquid film is used, on the one hand, to reduce the friction between the tube and the outlet part and on the other hand, it also serves as internal cooling for the drawn tube. In a variant which is not shown, the heater device 350, which in a known design comprises infrared radiators, is provided with a means for heating the preform using microwave radiation. In this way, not only the surface of the preform, but also, in particular, the inside of the wall of the preform can be heated.

Generation of the required tension force.

The desired improvement in the properties of the plastic material in the biaxial stretching process is obtained in particular if the extruded tube is stretched to a considerable degree in the axial direction but also in the radial direction. Therefore, in practice, the diameter of the tube will often increase by a factor of two or more when the tube passes over the mandrel. However, at the orientation temperature which is suitable for the biaxial stretching process, the plastic material is reasonably rigid in advance and therefore not easily deformable. Accordingly, considerable forces must be exerted on the tube in order to allow the tube, which is thick walled at the upstream end of the mandrel, to pass over the mandrel. The presence of one or more liquid films between the tube and the mandrel therefore leads to a reduction in the tensile force, but the forces required for the stretching process still remain a problem. A first problem relates to the transmission of the tension force to the tube 306 'by means of the stretching device 390 which is placed downstream of the mandrel 360. In the generally known drawing shelves, there are a plurality of driven rails, for example , 2, 3 or 4 of such rails and the transmission of the tension force from the stretching device to the tube is based on the friction between the tube and the rails. The friction is determined by the coefficient of friction and the normal force. In this case, the coefficient of friction is determined by the materials that come into contact with each other and that is not easy to increase significantly. The normal force is limited by the load-bearing capacity of the tube in order to avoid damage. Therefore, the tension force which can be exerted by means of the stretching device is limited. One measure which allows the tensile force to be increased which can be exerted is the use of a plurality of stretching devices distributed one behind the other, so that the friction between the tube and the stretching devices is distributed over a larger surface area. In this case, the stretching devices must move the tube forward at the same speed, in order to prevent the tracks of one of the stretching devices from sliding on the tube. Since the tube stretched in the position is cooled in advance significantly below the orientation temperature, an additional axial stretching is also undesirable. Another measure is to hold the tube internally in the position of the stretching device 390 so that the stretching device is capable of exerting a greater normal force on the tube than in the absence of this internal support. The internal support may consist, for example, of producing an internal pressure in the tube, for example by using two closing means to form a closed compartment in the tube at the level of the stretching device and by introducing pressurized or liquid gas into the tube. of this compartment. The internal support can also be mechanical design. Figure 5b diagrammatically shows an example, in which an internal support device 420 is attached to the mandrel 360, via an anchor member 421, at the drawing device level 390. The support device 420 in this case has pressure bands 422 which run with the tube 306 'and rest against the inside of the tube 306' opposite the bands of the stretching device 390. As a result, the stretching device 390 can press firmly against the outside of the tube 306 'without any risk of the tube 306' being damaged. In the case of larger tube diameters, the internal support device itself can also be provided with a driver to advance the tube 306 ', in which case this device is then held in the mandrel by means of a member the which can be subjected to compression loads. This support is then directed to a reduction in tension force relative to the extruder and the mandrel. Another possibility to exert the tension force required in the tube during the biaxial stretching process is to base the transmission of the tension force to the tube in a device-like connection between the drawing device and the tube., instead of friction, as described in the above. This can be obtained by allowing the tube to actually deform, and possibly be permanently damaged, in positions which are at an axial distance from one another, through the coupling of the stretching device downstream in the tube. The distance between the coupling points is then preferably slightly greater than the length of the tube sections to be produced. By way of example, the stretching device is coupled to the tube by means of projections which project into or through the wall of the tube.

Maintenance of the properties of the tube that is produced A significant problem with polyolefin tubes is that the improved properties that are obtained through the biaxial stretching process are lost completely or to a large extent even at a low tube temperature (40 ° C for PE). This means that a tube of this nature can not be stored in the sun without the aforementioned loss occurring, unless special measures are taken to improve the stability of the tube that is produced. Look for operations that improve the stability in the tube which can be carried out in line with the production of the tube, instead of downstream or in a separate process in which the tube sections are treated. For this purpose, it is proposed that the crosslinking operation be carried out in line downstream of the expansion portion of the stretching mandrel. It can be seen in figure 5b that the outlet part 364 of the mandrel 360 is of considerable length, which in this case is a multiple of the wall thickness of the tube. In practice, lengths greater than 1 meter can be advantageous, which are possible in particular if a film of water is formed between the outlet part and the tube. The large length of the outlet portion 364 makes the tube 306 'more stable, since the stretched tube 306' then has a shape which is defined by the output portion 364 for a relatively long period, during the period which the effects that are carried out by the expansion become stable. Another way to improve the stability of the tube is to crosslink the plastic material of the tube. This can be carried out in various ways which are known per se. It is also possible for only one or more layers of the tube wall to be subjected to a crosslinking treatment, for example only the layer on the outside of the tube. The stability can also be improved by producing multi-layered tubes, as already described above, in which case the shape of one of these layers is in fact so stable that the less stable layers, for example, the layer PE not reticulated, change shape. This can be obtained, for example, by combining such a PE layer with a PVC layer. It is also conceivable that specific layers of the multi-layer tube are subjected to the crosslinking process so that as a result, one of the layers blocks a change in the shape of another of the layer or layers. Another variant is by using the tube produced in the first place which is cut to a certain length, which results in sections of tube and these sections of tube are then treated in a separate process (batchwise), in order to obtain the desired stabilization. In particular, it is conceivable that a pipe section is pushed on a dimensionally stable internal support and then subjected to heat treatment for a specific period, for example a certain number of hours. During this treatment, the internal support prevents a change in the shape of the biaxially oriented tube section, which form is therefore maintained and a considerable part of the stretching of the plastic material will be maintained. After this treatment, the tube section will be considerably less susceptible to loss of properties obtained by stretching. By subjecting the tube to one or more of the treatments described above, it is possible to obtain a tube of a biaxially oriented plastic material which makes it possible, by means of welding bonding, to form a connection of a tube part with another component which is going to join him Welding joints of this nature are mainly used for polyolefin pipes such as PE pipes. If a tube is now made of biaxially oriented polyethylene or the like, a tube branch holder for making a connection for a branch tube can be welded securely, for example to the same without the tube shape changing undesirably as a result of the heat that is supplied.

Connection of biaxially oriented tubes It is already known to provide tube parts made of biaxially oriented thermoplastic material, in particular PVC, with a result at one end, in order to make it possible to assemble a tube of the tube parts which were placed together. In this arrangement, a receptacle of this nature is known which is provided with an elastic sealing ring which abuts in a sealed manner against the end of the other tube which has been placed therein. In the case of pipes made of biaxially oriented polyolefin, a receptacle gasket of this nature causes problems with respect to the seal, in particular in the long term. These problems arise in particular from the fact that many polyolefins show a significant amount of progressive deformation, that is, the materials begin to yield under load over the course of time. In the case of a receptacle joint as described above, this phenomenon of progressive deformation will cause the contact pressure between the sealing ring and the inserted tube end to gradually decrease, since the tube wall will begin to yield over the course weather. This results in the possibility of leakage, particularly under pressure. In order to connect two tubes of biaxially oriented thermoplastic material, in particular the polyolefin plastic material with one another, an improved connection is therefore proposed which will be explained in more detail below with reference to Figure 9. Figure 9 shows these ends of two identical tubes 501, 502 of biaxially oriented polyethylene, for example produced using the method and installation described above, which are to be connected. Each of these tubes 501, 502 is provided at both ends with a receptacle 503, 504, respectively, a simple design of which, without a sealing ring, is shown in Figure 9. These receptacles 503, 504, as are known per se, they are formed integrally in the tubes 501, 502 and in this case they have a larger internal diameter compared to the adjacent part of the tube. Figure 9 also shows a body 510 of plastic connecting tube which is provided with two axial ends 511, 512, which are each placed within a receptacle 503, 504 of a tube 501, 502 to be connected . Preferably, the connecting tube body 510 is placed within the receptacle with light clearance, as shown in Figure 9. The tubes 501, 502 are attached to the body 510 by the receptacle of each tube that is heated, with the result that that the receptacle shrinks at least in its cross-section and holds firmly at the end of the connecting tube body 510 which is placed inside the receptacle. To heat the receptacle which has been pushed on it, the body 510 of the connecting tube is provided at each of its ends 511, 512 with a heating means. This heating means, in this case, comprises one or more electric heating elements, for example wires 515 heaters, which in this case are embedded in the connecting tube body 510 and can be connected to a current source via the terminal 516 on the outside of the body 510. In a variant, the heating means may comprise one or more elements which can be heated from the outside, for example, elements which can be heated by introduction of microwave radiation and can be distributed or embedded in the body 510 of the tube. To avoid the transition from the receptacle to the adjacent part of the tube that is excessively heated, the heating wires 515 are at a distance from the free end of the body 510 of connection tubes.

It can also be seen in figure 9 that the outer surface of each end 511, 512 of the connection tube body 510 are profiled in order to create a component positively locking connection between the connection tube body 510 and the tube receptacle. The connecting tube body advantageously has an internal diameter which is substantially equal to the internal diameter of the part of each tube which is outside the receptacle. The connection shown can also be used for biaxially oriented tubes which have undergone crosslinking treatment or which have a multi-layer tube wall as explained above.

Axial stretching upstream of the mandrel Figure 10 shows a section of an installation for producing a tube from biaxially oriented thermoplastic material, in this example a section of the variant of the installation shown in figure 5a, 5b. Figure 10 shows the temperature-controlled hollow tubular preform 306 which comes from an extruder, and the first speed control means 340, which is placed downstream of the extruder and engages on the outside of the preform 306 imparting a first Controllable feedrate to this preform. Figure 10 further shows a second speed control means 600 which is placed at a distance downstream of the first speed control means 340. The second speed control means 600 engages on the outside of the preform 306 and is designed to impart a second controllable feed rate to the preform. The second speed control means 600 is located upstream of the mandrel (not shown), on which the preform is driven at an orientation temperature which is suitable for the related plastic material. In any case, the second speed control means 600 is located downstream of the expansion part of the mandrel. In a modality which is known per se, the first speed control means 340 and the second speed control means 600 are each designed with a plurality of endless tracks, for example, two tracks as shown in the document. WO 95/25626, which are supported against the preform. The speed control means 340 and 600 is then also provided with a track drive with controllable speed. In the installation, one or more third speed control means are also provided, which are located downstream of the mandrel and which engage a stretched tube so as to define a third speed of advance of the tube.

An installation of this nature makes it possible to produce biaxially oriented pipes in various forms. For example, the second speed control means may be used to adjust the speed of the preform which varies between a speed lower than that of the first speed control means and a speed greater than that of the third speed control means. In particular, it is possible for the preform 306 to be axially stretched, which adapts the wall thickness reduction of the preform 306, in the section between the first and second speed control means 340 and 600. In this case, the second speed is then greater than the first speed. The axial stretching of the preform 306, which is produced in this section, may correspond to the desired axial stretching of the tube or may form part of this stretching, in which case the rest of the axial stretching is placed further downstream in the installation, for example during the passage over the mandrel. This has the advantage, for example, that the behavior of the preform as it passes over the mandrel is stable, so that the process can be controlled successfully. It can also be seen from Figure 10 that the preform moves through a calibration aperture of a calibration device 610 in the section between speed control means 340 and 600, in which the preform is axially stretched, device 610 of calibration which is placed around a defined reduction in the outer diameter of the preform 306. The reduction in the external diameter and possibly the wall thickness of the preform 306 is now concentrated in the position of the calibration device 610, as can be seen from figure 10. As a result of passing through the calibration device, the preform acquires a defined external diameter, which is advantageous for the coupling of the second tube speed control means 600 in the preform and improves the stability of the process. By means of speed control, preferably in combination with the calibration device 610, and a suitable mandrel, it is possible, for example, to ensure that the stretch sum in the axial direction and the circumferential direction is approximately 5. In tests in which the drawn polyethylene tubes biaxially subjected to internal pressure have shown that at this value, there is no progressive deformation phenomenon in the plastic material. At a lower value, progressive deformation is observed. One possible explanation is that at a value of 5, the plastic molecules are roughly straight and therefore no longer elongate. At a higher level of stretching it would simply lead to more or less straight molecules sliding past one another. Preferably, the axial stretching ratio and the stretching ratio in the circumferential direction has the ratio 3: 2. In addition to the aspects described in the associated claims, the present application also relates to several additional aspects, which are described in the following paragraphs. A method for producing a tube section from thermoplastic material, in which a section of tube is extruded using an extruder which is provided with an extruder die having an inner core, inner core which defines an axial hollow space in the tube section, in which the tube section comes from the extruder die, downstream of the extruder die, is cooled internally by means of an internal cooling member, and is cooled externally by means of an external cooling device , in which the internal cooling member internally cools the tube immediately after the tube section has left the extruder die, in which the internal cooling member has a dimensionally stable outer wall with an axial length which is a multiple. of the cross-sectional dimension of the tube section, and in which the cooling liquid is pressed between the ex-wall Dimensionally stable terior and tube section, such that a fast-flowing film of liquid occurs between the tube section and the dimensionally stable outer wall, the liquid flows in the countercurrent direction, ie, contrary to the direction of extrusion, and the liquid film preferably has a maximum thickness of 3 millimeters. A method is provided for producing a tube section from thermoplastic material in which the tube section is extruded using an extruder which is provided with an extruder die having an inner core, inner core which defines an axial hollow space in the tube section, in which the section of tube coming from the extruder die, downstream of the extruder die, is cooled internally by means of an internal cooling device which comprises an internal cooling member located inside the extruded tube , and externally cooled by means of an external cooling device, the internal cooling member is designed to produce direct contact between the cooling liquid and the tube section, the internal cooling device comprises an air removal means for eliminating the air of the cooling liquid, by means of which the cooling liquid removes the air and s e feeds the internal cooling member. A method is provided for producing a tube section of thermoplastic material, in which a section of tube is extruded using an extruder which is provided with an extruder die having an inner core, inner core which defines an axial hollow space in the section of tube, in which the section of tube coming from the extruder die, downstream of the extruder die, is cooled internally by means of an internal cooling device which comprises an internal cooling member located inside the extruder tube, and externally cooled by means of an external cooling device, the internal cooling member is designed to produce direct contact between a cooling liquid and the tube section, the internal cooling member is designed to produce a helical flow of the liquid cooling along the inner wall of the tube section. A method for producing a tube section from thermoplastic material, in which a tube section is extruded using an extruder which is provided with an extruder die having an inner core, inner core which defines an axial hollow space in the tube section, in which the section of tube coming from the extruder die, downstream of the extruder die, is cooled internally by means of a cooling liquid which is brought into direct contact with the pipe section, and cooled externally by means of external cooling device, a cooling liquid with a low surface tension is used, the cooling liquid preferably being water in which one or more additives have been added which reduce the surface tension. A method for producing a tube section from a polyolefin plastic material, in which the tube section is extruded using an extruder which is provided with an extruder die having an inner core, inner core which defines a space axial hollow in the tube section, in which the section of tube coming from the extruder die, downstream of the extruder die, is cooled internally by means of an internal cooling device which comprises an internal cooling member attached to the inner core , and externally cooled by means of an external cooling device, a heating means is present in the hollow space in the tube section downstream of the internal cooling member, for the purpose of increasing the temperature of the layer inside of the tube section which has been cooled by the internal cooling member, the heating means preferably is a liquid, if appropriate with an added substance which reduces the surface tension to a temperature between 90 and 100 ° C. A method for producing a tube section having a wall layer made from crystalline thermoplastic material, in which a tube section is extruded using an extruder which is provided with an extruder die having an inner core, inner core which defines an axial hollow space in the tube section, in which the tube section coming from the extruder die, downstream of the extruder die, is cooled internally by means of an internal cooling device which comprises a cooling member internally located in the tube and externally cooled by means of an external cooling device, a multi-layer tube which is extruded with at least one wall layer of amorphous thermoplastic material inside the wall layer consisting of crystalline thermoplastic material, the crystalline wall layer is made, for example, of polyethylene and the amorphous wall layer is made, by e example, polyvinyl chloride. A method for producing a biaxially oriented tube from thermoplastic material, in particular polyolefin plastic material, comprises extruding a preform from a thermoplastic material using an extruder which is provided with an extruder die having an inner core, the inner core defines an axial hollow space in the preform, and then push the preform over a mandrel, which comprises an expansion part which carries out the expansion of the tube in the circumferential direction, the extruder die is provided with a medium to control the par thickness of the preform that comes from the extruder die, and an ultrasonic device for measuring the wall thickness, which is placed along the outside of the tube that is provided between the extruder die and the mandrel, for the purpose of measuring the wall thickness and the shape of the cross section of the extruded preform, a cold liquid layer that is produced inside the preform at the position of the wall thickness measuring device, the temperature of the cold liquid layer is preferably at most 50 ° C. A method for producing a biaxially oriented tube from thermoplastic material, in particular polyolefin plastic material, comprising the expression of a preform from thermoplastic material using an extruder which is provided with an extruder die having an inner core, the inner core defines an axial hollow space in the preform, and then impel the preform onto a mandrel in the axial direction, mandrel which comprises an expansion part which carries out the expansion of the preform in the circumferential direction , the preform is driven on the mandrel by means of a speed control means which is engaged in the preform upstream of the mandrel and by means of a pulling device which is placed downstream of the mandrel, the preform is heated such that it can be controlled by the circumferential sector upstream of the mandrel, this heating Controllable by circumferential sector is carried out by microwave radiation. A method for producing a biaxially oriented tube of thermoplastic material, in particular polyolefin plastic material comprising extruding a preform from a thermoplastic material using an extruder which is provided with an extruder die having an inner core, the inner core defines an axial hollow space in the preform, and then drives the preform over the mandrel in the axial direction, which mandrel comprises an expansion part which carries out the expansion of the preform in the circumferential direction and a current output part downstream of the expansion part, exit portion of substantially constant cross-section, the preform is driven on the mandrel by means of a speed control means which engages in the preform upstream of the mandrel, and by means of a device of stretching which is placed downstream of the mandrel, and the exit part has a length axi to which is a manifold of the wall thickness of the oriented tube. A method for producing a biaxially oriented tube having a wall layer made of polyolefin plastic material, comprising extruding a preform from a thermoplastic material using an extruder which is provided with an extruder die having an inner core, the inner core defines an axial hollow space in the preform, and then drives the preform over the mandrel in the axial direction, which mandrel comprises an expansion part which carries out the expansion of the preform in the circumferential direction, and a outlet part downstream of the expanding part, part of outlet which is substantially constant in its cross section, the preform is driven on the mandrel by means of a speed control means which engages the preform upstream of the mandrel and by means of a stretching device which is placed downstream of the mandrel, a preform of multiple layer e is extruded, which incorporates a plurality of wall layers with different properties, at least one of which is made from polyolefin plastic material, at least one of the wall layers is subjected, for example, to crosslinking treatment, preferably an inner or outer wall layer, or both, which preferably contain additives that promote crosslinking. A method to produce a biaxially oriented tube from polyolefin plastic material, which comprises extruding a preform from a thermoplastic material using an extruder which is provided with an extruder die having an inner core, the inner core defining an axial hollow space in the preform and then driving the preform onto a mandrel in the axial direction, mandrel which comprises an expansion part which carries out the expansion of the tube in the circumferential direction and an outlet part downstream of the expansion part, outlet part which is of substantially constant cross-section , the preform is driven on the mandrel by means of a speed control means which is engaged in the preform upstream of the mandrel and by means of a stretching device which is placed downstream of the mandrel, the tube is subjected to a crosslinking treatment downstream of the mandrel expansion part, preferably only one wall layer adjacent to the core. or outside, or both of the tube, is subjected to a reticulate treatment. A connection of the two tubes of biaxially oriented thermoplastic material, in particular polyolefin plastic material, in which the tubes, at their ends which are oriented towards each other, are each provided with an integrally formed receptacle, which preferably it has a larger internal diameter than the adjacent part of the tube and in which a connecting tube body is provided, which has two axial ends which are each placed inside a receptacle of a tube which is to be connected, and wherein the receptacle of each tube is heat shrunk onto that end of the connecting tube body which is securely placed inside the receptacle. The connection according to the previous paragraph, in which a connecting tube body is provided, at each of its ends with a heating means for heating the receptacle which has been pushed on it, for example one or more electrical heating elements, for example heating wires, or one or more elements which can be heated from the outside, for example metal elements which can be heated by induction. The connection according to one or more of the previous paragraphs, in which the heating means is at a distance from the free end of the body of the connection pipe. The connection according to one or more of the preceding paragraphs, in which the outer surface of each end of the connection tube body is profiled in order to create a dimensionally stable connection component between the connection tube body and the connection tube body. tube receptacle. The connection according to one or more of the preceding paragraphs, in which the connecting tube body consists substantially of plastic material. The connection according to one or more of the preceding paragraphs, in which the internal diameter of the connecting tube body is substantially equal to the internal diameter of that part of each tube which is outside the receptacle.

A tube of biaxially oriented thermoplastic material which contains material having a stretching ratio, in the axial direction and in the circumferential direction, with respect to the preform from which the tube is manufactured, the sum of the stretching ratio in the axial direction and in the circumferential direction it is between 4 and 6, preferably between 4.5 and 5.5, particularly preferably it is about 5. A tube of biaxially oriented thermoplastic material, for example polyethylene, plastic material which has a ratio of stretched, in the axial direction, and in the circumferential direction, with respect to the preform from which the tube is produced, the sum of the stretching ratio in the axial direction and in the circumferential direction is between 4 and 6, preferably between 4.5 and 5.5, particularly preferably it is about 5 and the drawing ratio in the axial direction is in a at a ratio of 3: 2 with respect to the stretching ratio in the circumferential direction.

Claims

1. A method for producing a biaxially oriented thermoplastic tube, comprising extruding a tubular preform from thermoplastic material using an extruder which is provided with an extruder die having an inner core, the inner core defining a hollow space in the preform, method which further comprises conditioning the temperature of the preform, so that the preform reaches an orientation temperature which is suitable for using the plastic material and driving the preform hardened on a mandrel, which comprises an expansion part , which carries out the expansion in the circumferential direction of the preform that passes over it, in such a way that a stretched tube with plastic material is obtained which is stretched in the axial direction and in the circumferential direction, followed by cooling of the stretched tube, a feed speed of the preform upstream of the mandrel is set It is provided by means of a speed control means which acts on the preform upstream of the mandrel, and an adjustable feed speed of the tube downstream of the mandrel which is established by means of a stretching device which acts on the stretched tube. downstream of the mandrel, characterized in that - by periodic variation of the ratio of the feed rate of the preform which is determined by the speed control means, on the one hand, and the exit of the extruder, on the other, between a plurality of different values - the wall thickness of the preform is periodically changed.

2. The method as described in claim 1, wherein the rate of advance of the preform, which is determined by the speed control means, on the one hand and the extruder outlet on the other, is substantially maintained constant to a first value for a first period so that the preform then acquires a first wall thickness and is set to one or more values which differ from the first value for a second period, which is considerably shorter than the first period .

3. The method as described in claim 1 or 2, wherein the extruder outlet is periodically varied and in which the advance rate of the preform, which is determined by the speed control means, is substantially maintained constant.

4. The method as described in claim 1 or 2, wherein the outlet of the extruder is maintained substantially constant and in which the advance rate of the preform, which is determined by the speed control means, is varied periodically

5. The method as described in claim 4, wherein the advancing speed of the drawn tube downstream of the mandrel, which is determined by the stretching device, is periodically varied in such a way that the ratio of the forward speed of the drawn tube downstream of the mandrel, on the one hand and of the preform upstream of the mandrel, on the other, remains substantially constant.

6. The method as described in claim 2, wherein, in the period during which a part of the preform with a wall thickness which is greater than the first wall thickness is driven on the mandrel or on the part of this period, the ratio of the forward speed of the drawn tube, which is determined by the stretching device, on the one hand and the advance speed of the preform, which is determined by the speed control means, on the other hand greater than the period during which a part of the preform with the first wall thickness is driven on the mandrel, such that a part of the tube with the greater wall thickness acquires a greater axial stretch compared to a part of the tube with the first wall thickness.

7. The method as described with one or more of the preceding claims, in which the tube stretched downstream of the expansion part of the mandrel is cooled in such a way that the cooled tube does not undergo any additional axial stretching and the generation of axial stretching is concentrated in the section between the speed control means for the preform and the downstream end of the mandrel or is the section between a plurality of speed control means for the preform which is placed upstream of the mandrel.

8. The method as described in one or more of the preceding claims, wherein the preform upstream of the extruder die is subjected to calibration of the external diameter of the preform, so that the preform acquires a uniform external diameter in this area and a a preform section with a larger wall thickness, having a smaller internal diameter compared to the adjacent parts of the preform with a smaller wall thickness.

9. The method as described in one or more of the preceding claims 1-7, wherein the preform downstream of the extruder die is subjected to calibration of the internal diameter of the preform, so that the preform acquires a uniform internal diameter in that area and a preform part with a larger wall thickness having a larger external diameter than the adjacent parts of the preform with a smaller wall thickness.

10. The method as described in one or more of the preceding claims, in which the preform is tempered such that the part of the preform with a larger wall thickness on average is at a higher temperature, measured in a position immediately upstream of the expansion mandrel, as compared to an immediately adjacent downstream preform part with a smaller wall thickness which is in advance in the mandrel.

11. The method as described in one or more of the preceding claims, wherein, in each case a plurality of parts with a larger wall thickness which are located relatively close to each other, are generated in the preform, followed by a considerably larger preform section with a smaller, uniform wall thickness.

12. The method as described in one or more of the preceding claims, wherein the drawn tube, in the section between the downstream end of the mandrel and the stretching device, is subjected to calibration of the outer diameter of the tube.

13. The method as described in one or more of the preceding claims, in which the drawn tube downstream of the drawing device is divided into the position of, or close to, the part of the tube with a larger wall thickness, which it results in tube sections which, at one or both axial ends, have an end portion with a greater wall thickness as compared to the remainder of the tube section, which has a smaller, uniform wall thickness.

14. Method for producing biaxially oriented thermoplastic tube, tube which has a tube body and, at one or both ends thereof, an integrally formed receptacle, in which a prefabricated tube of the biaxially oriented thermoplastic material is subjected to a forming operation of receptacle, characterized in that the prefabricated tube has an end portion with a greater wall thickness as compared to the body of the tube, end portion which is subjected to the forming operation of the receptacle, the axial stretching of the end part before the receptacle forming operation is equal to or preferably greater than the axial stretching of the tube body.

15. The method as described in claim 14, wherein the end portion of the prefabricated tube, as seen from its end face, has a plurality of annular areas, which are adjacent to each other and have a wall thickness of which varies from the annular area to the adjacent annular area, the wall thickness of a plurality of the annular areas is greater than the wall thickness of the tube body.

16. The method as described in claim 15, wherein the annular area with a larger wall thickness as compared to the tube body is deformed, during the receptacle forming operation, in a groove projecting out of the wall of the receptacle, which delimits an internal slot in the receptacle, slot wall which is designed to house a sealing ring.

17. The method as described in claim 15 or 16, wherein the annular area with an - 0 03 - larger wall thickness projects inwardly with respect to the internal diameter of the tube body subsequent to the operation of the receptacle formation .

18. Biaxially oriented thermoplastic tube, tube which has a tube body, and at one or both ends, an integrally formed receptacle, characterized in that the axial stretching of the plastic material in the receptacle is substantially equal to the axial stretching of the plastic material in the body of the body. tube.

19. Biaxially oriented thermoplastic tube, which tube has a tube body, and at one or both ends thereof, an integrally formed receptacle, the receptacle has an outward projecting slot wall which delimits an internal slot in the tube, which it is designed to house a sealing ring, characterized in that the slot wall has a wall thickness which is greater than or equal to the adjacent parts of the receptacle which have a smaller diameter.

20. Biaxially oriented thermoplastic tube, which tube has a tube body, and at one end, an integrally formed receptacle and, at the other end, a spike designed to be placed inside the receptacle of a similar tube, characterized in that the spike has a thickness of wall greater than the body of the tube.

21. Method for producing a biaxially oriented thermoplastic tube, comprising extruding a tubular preform from a thermoplastic material using an extruder which is provided with an extruder die having an inner core, the inner core defining a hollow space in the preform, method which further comprises temperature conditioning of the preform, so that the preform reaches an orientation temperature which is suitable for the plastic material to be used and urges the hardened preform onto a mandrel, which comprises a part of expansion, which carries out the expansion in the circumferential direction of the preform that passes over it, followed by cooling of the drawn tube, a speed control means which acts on the preform that is placed at a distance from each other, between the extruder and the expansion part of the mandrel, speed control means which maintains each or, an associated feed speed of the preform, such that the preform, in the section between the speed control means, is axially stretched, which reduces the wall thickness of the preform, and an adjustable feed rate of the tube downstream of the mandrel is established by means of a stretching device which acts on the drawn tube downstream of the mandrel, so that a stretched tube is obtained with plastic material which has been stretched in the axial direction and in the circumferential direction, characterized in that the preform, in the section between the speed control means, in which the preform is stretched axially, moves through a calibration aperture of a calibration device, calibration device which reduces the external diameter of the preform.

22. Method for producing a biaxially oriented pipe from the thermoplastic material, in particular polyolefin plastic material, comprising extruding a preform from thermoplastic material using an extruder which is provided with an extruder die with an inner core, the core internal defines an axial hollow space in the preform and then impel the preform on a mandrel dimensionally stable in the axial direction, mandrel which comprises an expansion part, which carries out the expansion of the tube in the circumferential direction, the preform is driven on the mandrel by means of a speed control means which acts on the preform upstream of the mandrel and by means of a stretching device which is placed downstream of the mandrel, the expanding part of the mandrel has an outer surface which substantially corresponds to the surface of a truncated cone, characterized in that the outer surface of the expanding part of the mandrel is provided, in a plurality of positions around the circumference of the part. of expansion, with slots or elongated reinforcements which extend in the axial direction, and a liquid film that is preferably formed between the expansion part of the mandrel and the tube.

23. The method as described in claim 22, wherein the liquid in the film flows over the expansion part in the direction opposite to the direction of movement of the tube, in which liquid is pressed between the mandrel and the tube preferably in the vicinity of the end downstream of the expansion part or downstream of this end, and in which the liquid is collected and discharged upstream of the expansion part.

24. The method as recited in claim 22 or 23, wherein the expansion part is provided with axial grooves which are formed at regular angular intervals, preferably between 3o and 10o on the outer surface of the expansion part and in which the slots preferably have a maximum depth of 5 millimeters, particularly preferably between 0.5 and 3 millimeters in depth.

25. Method for producing a biaxially oriented pipe from thermoplastic material, in particular polyolefin plastic material, comprising extruding a preform from thermoplastic material using an extruder which is provided with an extruder die with an inner core, the core The interior defines an axial hollow space in the preform, and then drives the preform on a mandrel in the axial direction, which mandrel comprises an extension part, which carries out the expansion of the preform in the circumferential direction, the preform is driven on the mandrel by means of a speed control means which acts on the preform upstream of the mandrel by means of a stretching device which is placed downstream of the mandrel, characterized in that the tension force is exerted by means of drawing at least one stretching device which acts on the outside of the tube.

26. The method according to claim 25, in which a plurality of stretching devices are placed one behind the other and urges the tube at the same speed.

27. The method as described in claim 25 or 26, wherein the tube is supported internally in a position where a stretching device acts, preferably with the aid of a mechanical support means which, in the position where it acts The stretching device comprises one or more support surfaces which move with the tube and rest against the inside of the tube, the support means which preferably is attached to the inner core of the extruder, and in which is further provided preferably for the support surfaces of the support means to be driven in the direction of advance of the tube.

28. The method as recited in claim 25, wherein a drawing device comprises one or more tube coupling members, each of which can be moved an axial distance to and from, acting on part of the tube in a manner which deform the tube and clamp the tube in that area, an axial displacement mechanism is associated with each tube coupling member in order to displace the member and the fixed tube therein, in the axial direction.