FR2806957A1 - Continuous production of bi=axially oriented plastic tubing includes monitoring axial force applied to extruded blank so force kept constant - Google Patents

Abstract

Continuous production of bi-axially oriented plastic tubing includes extruding a blank, heat treating the blank to orient the plastic, radially expanding the blank into a tube and then gauging and cooling the tube. An axial traction force applied to the tube is monitored so the applied force is kept substantially constant. Independent claims are included for the following: (a) A plastic tube made as above. (b) A production line for plastic tube has an extruder, temperature controlled tank(s) for molecular orientation of the extruded plastic, a radial dilator, a Preferred Features: The traction force is controlled by measuring the force applied to the tube, the internal pressure of the tube and/or depression of the tube at the gauging device. A tube traction device is downstream of the gauging device and cooling devices. Two spaced apart flexible plugs within the expanded blank/tube define a sealed chamber within it.

Description

 METHOD AND LINE FOR CONTINUOUSLY MANUFACTURING PLASTIC TUBES WITH BI-AXIAL STRETCHING, AND PLASTIC TUBE OBTAINED The invention relates to a method of continuous manufacture of plastic tubes with biaxial stretching, according to which a blank is formed by extrusion and is then subjected to combined axial tension and radial expansion.

A-2,753,648 relates to a process of this type, in particular for tubes of relatively large diameters, in particular for diameters greater than 150 mm. This process, which gives satisfaction, is however discontinuous. In some cases, especially for smaller tube diameters, it is desirable that the process be continuous in order to further improve productivity and reduce cost of investment per unit produced. Continuous production of a tube with dual draw -axial problematic as to the realization of the radial expansion of the tube 'in an area of the manufacturing line. The start-up phase of the process is moreover delicate A-2 207 793 discloses a process for manufacturing plastic tubes with continuous bi-axial stretching in which the radial expansion is carried out using a frustoconical mandrel. Traction means of the tube are provided downstream and upstream of this mandrel. The large base of the frustoconical mandrel is extended a same diameter cylinder which enters a calibrator The outer diameter of the large base of the cone frustum of the cylinder extends it determine the internal diameter of the tube f 95/25626 relates to a continuous manufacturing process of the same kind as the previous one, with a mandrel for radial expansion, the diameter of the large base of the mandrel determining the inner diameter of the finished tube. Roughing means of the blank are provided upstream of the mandrel, while a printer is provided downstream of the mandrel.

The use of a mandrel simplifies the operation of starting the line, particularly as to the radial expansion. The blank comes to marry the outer surface <B> frustoconical mandrel and undergoes a dilation </ B> relatively well controlled. However, the friction is <B> relatively important. </ B>

Romanian Patent No. 80960 shows a mandrel of increasing diameter cross section whose branches are provided at their ends with wheels. A <B> plastic tube <B> plastic <B> undergoes on this mandrel a radial expansion, followed by a radial contraction. The <B> cross shape of the cross section of the </ B> mandrel <B> makes </ B> the outer surface of the mandrel not continuous, which can cause marks on the tube and variable stretching and not controlled on the same <B> circumference. </ B>

 WO 90/02644 relates to a method in which the radial expansion of the tube is obtained by internal hydraulic pressure exerted in a closed chamber <B> upstream by a first plug corresponding to the internal diameter </ B> of the blank and, downstream of a calibrator, by another inflatable cap under the effect of a pressure. This uninflated plug allows to start the line in initial diameter and then, by inflation, to reach the <B> nominal internal diameter of the finished tube. </ B>

 WO 97/06940 also uses an inflatable downstream plug and provides for control of tube expansion in response to various parameters which indicate, directly or indirectly, a change in the relative velocity at which the material is supplied to the expansion zone and evacuated from this area. The expansion is carried out by hydraulic pressure in a chamber which is closed by the inflatable plug and a conventional upstream plug.

The use of an inflatable plug in contact with a relatively high temperature plastic tube poses problems as to the choice of the material of this plug. It is indeed necessary that the material has sufficient elasticity to allow inflation of the cap, while being resistant to the temperature and friction imposed by the plastic tube. In addition, to achieve a satisfactory seal without causing excessive friction of the cap against the inner surface of the tube, adjusting the internal pressure of the inflatable cap is difficult.

A first object of the invention, especially, to provide a continuous manufacturing process of plastic tubes with bi-axial stretching allows a simple start avoiding the problems posed by an inflatable cap, and which allows to reduce the friction to which the tube is subjected to steady state of manufacture.

To achieve this object, according to a first embodiment of the invention the continuous manufacturing process of plastic tubes with biaxial stretching consists in producing a blank by extrusion, to bring the blank at a temperature of molecular orientation, to pass this blank around a radial expansion mandrel, to ensure a calibration and cooling, while subjecting the blank to axial traction, and is characterized in that - the radial expansion performed on the mandrel is partial so that the diameter When the blank leaves the mandrel, it is smaller than the nominal internal diameter of the finished tube, and a complementary radial expansion of the blank, up to its nominal diameter, is effected by internal pressure of the fluid. the assembly being such that in steady state of operation, the inner surface of the blank is no longer in contact with the mandrel.

Advantageously, a closed chamber is produced upstream and downstream of the mandrel by means of at least one upstream stopper and a downstream stopper, each stopper sealing, for example comprising at least one disc made of a sufficiently flexible material. to flex and straighten to seal.

Advantageously, the downstream sealing plug is introduced with a larger diameter than the upstream plug by virtue of the initial expansion on the mandrel, thus making it possible to generate a sealed volume.

The fluid under pressure for the complementary expansion of the blank is admitted between two sealing plugs.

A second object of the invention, which may be taken into consideration independently or in combination with the first object, is to provide a method of continuous manufacture of plastic tubes with biaxial stretching which makes it possible to provide axial elongation. practically constant plastic material.

To achieve this other object, according to a second provision of the invention which can be implemented independently or in combination with the preceding provision the continuous manufacturing process of plastic tubes with biaxial stretching consists in producing extruded blank, to bring this blank with a molecular orientation temperature, to ensure a radial expansion of the blank, with calibration and cooling, while subjecting the blank to axial traction, this method being characterized by the fact that the friction force exerted on the blank and the tube, and acting on at least one operating parameter to maintain this friction force substantially constant.

Such a regulation makes it possible to ensure a constant axial orientation (axial elongation), which results in a constant thickness and physical characteristics of the finished tube.

The detection of the friction force can be ensured by measuring the tensile force exerted on the tube. The operating parameter on which it acts to maintain the friction force substantially constant may be constituted by the internal pressure in the tube, and / or by a depression prevailing around the tube in a calibrator. According to another possibility, the friction force is regulated by injection of lubricating liquid, in particular water, between the tube and the wall of the calibrator.

the previous regulation can be combined with an upstream regulation designed to keep the characteristics of the blank constant.

For this, the characteristics of the blank are measured in particular thickness and diameter of the blank and, in response to the measurement results, it is t on the extruder to maintain constant the characteristics of the blank.

According to the first or second arrangement, the traction on the tube is advantageously exerted in one place, downstream of a calibration zone, by at least one printer.

The invention also relates to a line of continuous production of plastic tubes with biaxial stretching, for the implementation of the first process arrangement, comprising - an extruder conventionally equipped for the formation of a blank; at least one molecular orientation of the blank; - A device for progressive radial expansion of the blank comprising an expansion mandrel, held fixed relative to the extruder; - a calibrator and a cooling device of the, the manufacturing line being characterized in that the mandrel has a maximum diameter smaller than the nominal internal diameter of the tube; a plug located upstream of the mandrel; at least one other plug is located downstream of the mandrel for sealing according to the nominal internal diameter of the tube; a pressurized fluid inlet means is provided at a location between the two sealing plugs for admitting pressurized fluid and expanding the tube to its nominal internal diameter.

In such a production line, in steady state, the tube no longer in contact, by its inner wall, with the mandrel Advantageously a coaxial pipe is attached to the extruder the mandrel is attached to the end of this pipe which comprises a channel which opens radially at least one opening, between the two sealing plugs for the injection of pressurized fluid. Advantageously, the bi-orientation portion of the production line comprises means for pulling the tube, in particular a printer, in a single location downstream of the calibrator and cooling device.

The downstream plug may be located downstream of the cooling tanks and be hooked to the mandrel by a flexible connecting element, in particular a cable.

Alternatively, downstream plug is located immediately behind the mandrel, which reduces the size of the sealed volume.

The mandrel has an overall shape of revolution, for example frustoconical, whose large base is turned on the side opposite the extruder; the mandrel is advantageously provided on its surface, rolling members angularly and axially distributed in order to avoid marking the internal surface of the decreasing the friction forces.

The bearing members are preferably constituted by cylindrical or spherical rollers mounted rotatably Advantageously the last heat-up tank and / or the calibrator are mounted so as to be displaced relative to each other in axial translation, in particular to change their spacing and facilitate the start of manufacture.

The invention also relates to a line of continuous production of plastic tubes with biaxial drawing which, independently of the preceding characteristics or in combination with these characteristics, comprises - an extruder conventionally equipped for the formation of a blank; - At least one molecular orientation temperature of the blank tank; a device for radial expansion of 1 blank, by internal fluid pressure; a calibrator and a tube cooling device; axial drawing means for the blank, this fab-ication line being characterized in that it comprises means for detecting the friction force exerted on the blank and the tube and means for acting on at least one operating parameter of the production line to maintain this friction force substantially constant.

Advantageously, the traction means on the tube are located in one place, downstream of a calibration zone, and comprise at least one printer.

The detection of the friction force can be provided by means for measuring the tensile force exerted on the tube.

Advantageously, means for measuring the characteristics of the blank, in particular the thickness and the diameter of the blank, are provided as well as means for acting on the extruder in response to the measurement results, in order to keep the roughing characteristics constant.

The invention also relates to a plastic tube with bi-axial stretching obtained by the process and / or manufacturing line described above.

The invention consists, apart from the arrangements set out above, in a certain number of other arrangements which will be more explicitly discussed hereinafter with regard to exemplary embodiments described in detail with reference to the accompanying drawings, but which are in no way limiting.

Figure 1 of these drawings is a schematic axial section, with parts outside and parts removed, a production line according to the invention, at the beginning of production.

Figure 2 schematically shows a part of the line after placing the expansion mandrel at the inlet of the calibrator.

FIG. 3 shows, similarly to the figure, the manufacturing line at the beginning of injection of the fluid under pressure.

Figure 4 shows, similarly to the figure the production line after placing a plug downstream of the printer.

Figure 5 shows, similarly to the figure the production line while the steady state of production is established.

Figure 6 shows, similarly to Figure 5, variant embodiment.

Figure 7 is an enlarged elevational view of the radial expansion mandrel.

Figure 8 is a view, along line VIII I of Figure 7, of a mandrel element.

Figure 9 is a top view of the mandrel member of Figure 8.

Figure 10, finally, is an elevational view of an embodiment of upstream cap.

Referring to the drawings, particularly in Figure 1, we can see a manufacturing line 1 implementing the method of the invention. The production takes place continuously upstream (on the left of Fig.l) downstream (to the right of Fig.l).

The production line comprises an extruder 2 schematically shown, fed with thermoplastic material, in particular PVC, from a hopper 3 schematically shown (supply "dry blend"). The extruder 2 is provided at its front end, opposite the hopper 3, a tool 4 extrusion of a blank E tube. The tool 4 comprises one or more lateral inlets for admitting and / or rejecting air under pressure, or more generally a fluid (gas or liquid) under pressure. A shaper (not shown) is further provided in a conventional manner.

A coaxial metal tube 6 is attached to the extruder 2 and extends to the right according to FIG. 1. This tube 6 comprises, towards its closed axial end, at least one radial opening 7. When pressurized air is admitted through the inlet 5, this air is directed by channels (not shown) of the tooling 4 to the tube 6 to exit through the opening 7.

A blank E of tube exits the extruder 2 at a relatively high temperature of the order of 150 C or more, passes through one or more cooling tanks 8, generally containing water, thermoregulated for example at 10-20 C to reduce substantially the temperature of the blank E. A conventional printer (not shown) is generally provided downstream of the cooling tanks 8. This printer, which acts on a blank still hot and therefore deformable, is not really used to pass thrust forces, but only to disconnect extrusion and biaxial stretching functions by damping disturbances and interactions of one over the other.

The bi-orientation part of the blank starts downstream of the printer (not shown) of the extrusion part. This bi-orientation portion comprises a thermoregulating, cooling or heating tank 9, to bring the blank E to a temperature in the molecular orientation temperature range. For PVC, this temperature is in a range of 90 C to 110 C.

At the outlet of the tray 9 is fixed a ring 10 for external plating of the blank on an upstream inner plug 11, shown in FIGS. 2 to 6. This inner plug 11 is located upstream of the orifice 7.

 As can be seen in FIG. 10, the inner plug 11 may comprise, from upstream to downstream, a succession of sealing rings, in particular two relatively flexible rings 11a, 11b, for example of elastomeric material, the periphery of which is formed. by a frustoconical lip whose large base is turned downstream. Several discs 11c, lle, lif, for example of polytetrafluoroethylene (PTFE), spaced from each other, parallel to each other and perpendicular to the axis of the blank, are provided downstream of these rings 11a, 11b. The outer diameter of the rings and disks is at least equal to the internal diameter of the blank E.

The succession of rings and discs 11a-11f forms a stack held by a metal core 11g, axially adjustable by means of nuts 11h, 11c screwed onto the tube 6 constituted by a hollow threaded rod.

The example of cap 11 described in detail above is not limiting, any equivalent sealing means may be suitable.

A mandrel 12 of radial expansion, visible in FIG. 2, is fixed, under conditions explained below, to the end of the tube 6 remote from the extruder 2. The mandrel 12 has an overall shape of revolution and is constituted, for example, by a truncated cone. The large base 12a of the mandrel faces downstream. The diameter B of the large base 12a is smaller than the nominal internal diameter D (FIG. 4) of the finished tube T.

Preferably, the diameter B is at least 10% less than the diameter D.

 As illustrated in FIG. 7, the mandrel 12 is advantageously provided on its surface with cylindrical or spherical rollers 13, rotatably mounted on (non-visible) axes carried by the mandrel 12. The rollers 13 are partly housed in cavities at surface of the mandrel and protrude from this surface. The axes of rotation of the rollers 13 are arranged in several parallel planes, perpendicular to the axis of the mandrel 12. The rollers 13, located in a plane, are distributed regularly around the axis of the mandrel. The rollers located in successive planes are shifted staggered to prevent a generatrix of the mandrel 12 is devoid of roller.

Advantageously, the mandrel 12 is constituted by a frustoconical stack of discs 14 of increasing diameter from upstream to downstream. Each disk has at its periphery rollers regularly distributed in the form of a ring. The discs 14 have a central opening 15 with one or more notches 16 for mounting with rotational locking on a shaft 17 of conjugated section. An axial abutment is provided on the shaft 17 to retain the smaller diameter disc. On the opposite side, the shaft 17 has a thread 19 for blocking the stack of disks 14 by means of a nut engaged with this thread.

The mandrel 12 can thus be adjusted to different internal rough diameters by adding or removing discs of appropriate diameters.

cylindrical spherical cylindrical rollers 13 can significantly reduce the friction of the blank against the mandrel and avoid marks on the inner surface of the tube.

Coaxial plug 20 is attached to the mandrel 12 on the side of the large base 12a opposite tube 6. This plug 20 is formed for example by at least one, and preferably several discs 21 spaced, parallel to each other, made of a plastic material to low coefficient of friction such as polytetrafluoroethylene (PTFE) or an elastomeric thermoplastic (TPE), reinforced or not. The discs are made of a sufficiently flexible material to be able to flex and straighten to seal. The plug 20 is provided to ensure a seal at least along a diameter equal to the diameter B of the large base of the trunk of corn 12. Other explanations will be given in this regard about the variant of FIG.

A flexible connecting member 22, for example a cable, is further attached to the center of the mandrel 12 on the side opposite the tube 6. The plug 20 can be installed on the portion of the cable 22 adjacent the mandrel 12. Downstream of the mandrel 12 , the blank enters a calibrator 23, schematically shown. This calibrator 23 is placed in a cooling tank 24 for spraying water under vacuum. The projection of water on the outer surface of the plastic tube leaving the calibrator 23 makes it possible to freeze the material. At the outlet of the tank 24 the tube T in the finished state goes into a printer 25 which exerts the necessary traction force along the entire line for driving the tube and the blank. The printer 25 is conventionally constituted by at least two tracks 26,27 bearing against opposite zones of the tube to drive it from the left to the right according to Fig.l. Each track comprises a flexible element 28 forming an endless loop wrapping around wheels 29 at least one of which is driving, these wheels 29 being rotatably mounted about axes 30 supported by a frame 31 fixed to the ground.

The tensile force exerted by the printer 25 is detected by a sensor 32 (FIG. 1), for example at the mounting of an axis 30 on the frame 31.

The output of the sensor 32 (Fig.l) is connected to a block 33 for adjusting the degree of vacuum in the tank 24 in response to the output signal of the sensor 32, and a block 34 for adjusting the pressure of the air delivered by the orifice 7.

The block 33 may control, if necessary, a water injection to reduce the friction of the tube against the calibrator 23 if the force measured by the sensor 32 is too large.

A sensor j is advantageously provided at the outlet of the extruder 2 for measuring the diameter of the blank. Several other sensors h, for example ultrasonic, are distributed around the periphery of the blank, in the vicinity of the extruder 2 outlet, to measure the thickness of this blank. The sensors j and h are connected to a regulating block K able to act on the operating parameters of the extruder 2 to maintain the dimensions of the blank, at the outlet of the extruder 2, as constant as possible according to values measured by the sensors j and h.

The tray 24 and the calibrator 23 are mounted adjustable in the axial direction, as well as the 9. These elements are mounted movable in translation, for example on 1s fixed to the ground (not shown on drawing), with locking means in a desired position.

distance L between the downstream face of the 9 and the upstream face of the tray 24 is thus adjustable.

steady state of operation, a downstream plug .4) hooked to the cable 22, bears tightly against the inner surface of the tube T. The plug may be constituted by disks 36 spaced apart, parallel to each other, perpendicular to the tube and made a material such as PTFE or an elastomeric thermoplastic (TPE), not reinforced The plug t provided to achieve a seal according to the internal diameter D of the finished tube.

the foregoing description, it appears that the production line is composed of two different parts, with the minimum of interactions: - an extrusion part of the blank, consisting of a conventional extrusion line (extruder, tool 4, shaper not shown, cooling tanks 8, printer not shown); a bi-orientation part of the blank with temperature trays 9, expansion device (12; 6, 7, 11, 20, 20a), calibrator 23, cooling device 24, printer 25 and saw (not shown ).

This being the case, the operation of the manufacturing line implementing the method of the invention is as follows.

Starting the line is illustrated by Figures 1 to 4.

According to Fig.l a blank E comes out of the extruder 2, in a relatively pasty state, and crosses the entire line. To ensure this crossing of the line, it is possible to use an auxiliary pulling tube which is introduced downstream upstream in the line to the vicinity of the outlet of the extruder is attached, by any suitable means, the blank to this pulling tube which, driven by the printer 25, itself drives the blank through the trays 8, 9 and 24.

The mandrel 12 of radial expansion is not yet installed (Fig.l) and there is no pressurized air injection through the inlet 5 and the outlet port 7. The tank 9 is moved back to the upstream while the tray 24 is advanced downstream so that the distance L is maximum.

The mandrel 12 and the plug 20 are then put in place. For this, a slot is made along a generatrix in the wall of the blank in an area between the tray 9 and the tray 24. The edges of the slot are spread apart to set up, upstream of the orifice 7, the plug 11 as illustrated in FIG. Then the mandrel 12 is fixed to the end of the tube 6, for example by screwing, and the cable 22 is introduced along the entire length of the blank so as to be able to recover the end of this cable downstream of the container or bins. cooling 24 of the bi-oriented tube, possibly by cutting with a saw (not shown) a downstream part of the blank.

The arrival of material continuing, the slot made in the blank, for the introduction of the plug 11 and the mandrel 12, moves downstream and disappears. At this time the blank undergoes its first expansion in contact with the mandrel 12 (Fig.2). There is still no injection of pressurized air through port 7.

 When the blank is properly closed downstream of the mandrel 12, it controls a movement of the tray 9 downstream, as illustrated by an arrow in Fig.3, so that the ring 10 comes to be placed around the cap there . An injection of air is then provided through the inlet 5 and the orifice 7, with gradual increase of the pressure so that the blank takes off slightly from the mandrel 12 as illustrated in FIG.

As the air pressure increases, the blank completely deviates from the mandrel 12 as illustrated in FIG. 4 and comes into contact with the calibrator 23. The internal diameter of the tube then reaches its nominal value D greater than the diameter B of the large base mandrel 12.

The downstream plug 35 is put in place the end of the cable downstream of the cooling tank or tanks 24 and thus provides additional sealing.

alternatively, it determines with the downstream plug 20 a chamber in which there is an air pressure allowing, after the calibrator, the additional cooling of the tube shaped.

 When the steady state is established, it controls the movement of the tray 24 to the tray 9, as shown by an arrow in Fig.5, to reduce the space between these two tanks and increase the distance between the inner surface of the blank and the mandrel 12.

preferably, a pull on the tube is exerted only downstream of the tray 24. The measurement of the tensile force achieved by the sensor 32 (Fig.l) is a measure of the friction of the blank and the tube in the line, in particular in the calibrator 23. The regulation carried out from this measurement of force by the sensor 32 makes it possible to work with a traction force and therefore a substantially constant friction force, which corresponds to a substantially constant axial orientation (axial elongation). for the material of the tube T.

The friction force is maintained substantially constant by the block 34 (FIG. 1) which acts on the internal pressure in the blank and in the tube, as a function of the quantity detected by the sensor 32. It is also possible to act on the degree of vacuum in the tank 24, around the tube, or by injecting water of lubrication between the tube and the internal wall of the calibrator 23.

This regulation is effective if the dimensional characteristics of the blank are constant. To improve the production, it is also possible to regulate the blank at the outlet of the extruder 2 by means of the sensors j and h (FIG. 1) which, by the detected values, allow the block K to modify the conditions. operating the extruder 2 to maintain substantially constant dimensional characteristics of the blank.

FIG. 6 illustrates an embodiment variant according to which the downstream plugs 20 and 35 of the preceding embodiment form no more than a single plug 20a located immediately downstream of the mandrel 12. This plug 20a preferably also consists of parallel discs 21a, perpendicular to the axis of the tube. The discs must have sufficient flexibility to be able to bend and ensure in a first step the sealing according to the diameter B corresponding to the large base of the mandrel 12. The discs 21a of the cap 20a, by their elasticity, must be able to recover and then ensure sealing according to the internal nominal diameter D of the tube.

The solution of FIG. 6 has the advantage of a reduced volume for the closed chamber between the upstream plug 11 and the downstream plug 21a. This results in less inertia with respect to pressure variations and temperature variations that can be controlled. As the volume of air stored is smaller, in the event of a leak or incident, the risks are also reduced.

In the foregoing description, the fluid used to generate the internal pressure of the blank is air.

 Of course, this fluid could be a liquid, especially hot water.

Claims (2)

1. A method of continuous manufacture of plastic tubes with biaxial stretching, in which a blank (E) is produced by extrusion, this blank is brought to a temperature of molecular orientation, and the blank is subjected to expansion. radial, it provides a calibration and cooling, while subjecting the blank to an axial tension, characterized in that one detects (32) the friction force exerted on the blank and the tube, and that acts (33,34) on at least one operating parameter to maintain this frictional force substantially constant. Process according to Claim 1, characterized in that the friction force is detected by measuring (32) the tensile force exerted on the process according to Claim 1 or 2, characterized in that the operating parameter on which it acts to maintain the substantially constant friction force is constituted by an internal pressure in the tube, and / or by a depression prevailing around the tube in a calibrator (23). 4. Method according to one of claims 1 to 3, characterized in that one measures (j, h) the characteristics of the blank (E) and, in response to the results of measurements, it acts on the extruder (2) to keep the characteristics of the blank constant. 5. Method according to one of the preceding claims, characterized in that the traction on the tube is exerted in one place (25) downstream of a calibrator (23) and a cooling device. 6. Method according to one of the preceding claims, characterized in that a closed chamber is produced upstream and downstream with the aid of at least one upstream plug (11) and at least one downstream plug (21, 35, 21a), each plug may comprise at least one disc made of a sufficiently flexible material to be able to bend and straighten out to seal. 7. Line of continuous manufacture of plastic tubes with bi-axial stretch comprising - an extruder (2) conventionally equipped for the formation of a blank (E); at least one molecular orientation tray (9) of the blank; a radial expansion device; - a calibrator (23) and a cooling device (24) of the tube, characterized in that it comprises means (32) for detecting the friction force exerted on the blank and the tube, and means (33) , 34) for acting on at least one operating parameter of the production line to maintain this friction force substantially constant. 8. Production line according to claim 7, characterized in that it comprises a block (34) which acts on the internal pressure in the blank and in the tube. 9. Production line according to claim 7 or 8, characterized in that it comprises means (32) for measuring the tractive force exerted on the tube. 10. Production line according to one of claims 7 to 9, characterized in that it comprises measuring means (j, h) characteristics of the blank, in particular the thickness and diameter of the blank, and means (K) for acting on the extruder in response to the measurement results, for keeping the characteristics of the blank 11 constant. Production line according to one of claims 7 to 10, characterized in that it comprises pulling means (25) of the tube, in particular a printer, in a single location downstream of a calibrator (23) and the cooling device (24). Plastic tube obtained by the process according to one of claims 1 to 6.