CS210670B2 - Method of tube manufacture from screrned polyethylen - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a process for the manufacture of high density crosslinked polyethylene pipes having a molecular weight greater than or equal to 200,000, a substantially powder-containing composition comprising polyethylene and a weakening agent. it exerts a pressure to cause it to penetrate and pass through the heated matrix, or in the matrix, and to cause crosslinking of the material.

The invention also relates to an apparatus for carrying out this method.

Crosslinking or vulcanization by which two adjacent polymer chains join together to form a three-dimensional network under the effect of either radiation or activated carbon is a known process that can be used for polyolefins, vinyl polymers, and elastomers, especially ethylene-propylene elastomers.

In the case of certain polyolefins such as low density polyethylene and ethylene propylene elastomers, when an activated peroxide-activated carbon crosslinker is used, the end product can be obtained without difficulty by gradually forming the initial molded powder mixture, then conventional techniques for converting plastic materials, such as extrusion and injection molding, are performed, and then crosslinking or vulcanization is accomplished, for example, by passing the molded article through an autoclave or fluidized bed. The temperature at which the extrusion or injection molding takes place is considerably lower than that required to induce crosslinking by decomposition of the organic peroxides conventionally used, such as:

dicumylper oxide, di-tert-butylperoxide,

4.4-bls-butyl valerate peroxide

2,5-dimethyl-2,5-di (t-butyl) hexane peroxide;

2,5-Dimethyl-2,5-di- (t-butyl-hexin-3-peroxide).

Between the molding period, during which the plastic has a viscosity allowing it to be converted according to conventional procedures, and between the strengthening period, there is a temperature interval sufficient to ensure that the two operations are well separated and to avoid the risk of premature peroxide decomposition.

Otherwise, when high density polyethylene needs to be thickened, since the mass does not reach a sufficiently low viscosity to be shaped by extrusion or injection than at a temperature between 170 and 220 degrees Celsius, depending on the molecular weight of the high density polyethylene, that is, at a temperature greater than the onset of peroxide decomposition temperature (about 140 ° C for the least active peroxides), so that when high density polyethylene crosslinking is used, the processes used to make low density polyethylene articles are used, or of ethylene-propylene elastomer, a crosslinking reaction occurs in the molded mixture, which would be undesirable due to the fact that, especially in the case of an extruder, the mass would be subjected to shear stresses which would prevent the crosslinking from developing correctly and quality.

It has already been proposed to eliminate this shear stress by using a heated shaping and crosslinking matrix in which the mixture is thickened and bounded to advance to another device under the pressure generated by the piston, but in order to prevent the formation of unfavorable deformations within the mass necessary to form a coating of anti-adherent material such as polytetrafluoroethylene on the surfaces of the matrix with which the material comes into contact.

In this case, good product quality has been found, but this technique does not allow practically continuous production, since it has been found that after a relatively short time of material passing through, a deposit is formed in the crosslinking zone of the matrix which is detrimental to the product quality. This deposit is formed by particles of crosslinked polyethylene adhering to the wall, and is caused by radicals released during the reaction that are sufficiently active to activate the surface of the anti-adhesive material and cause sticking.

Attempts have already been made to inject a considerable amount of lubricant into the matrix upstream of the crosslinking zone, but this technique, while facilitating the process, does not allow removal of deposits in the crosslinking zone due to the unstable liquid flow in the matrix.

In order to solve the problem, the present invention is based on the above method of manufacturing cross-linked polyethylene pipes, and after sintering the powder mixture, it is subjected to pressure along the axis of the matrix and exposed to a temperature lower than after the matrix-shaped material is passed through a bath of molten salt, which is exposed to atmospheric pressure and whose upper part is at a temperature close to the sintering temperature, while its lower part is at a temperature at which the crosslinking agent decomposes and the crosslinked tube is removed from the bath, giving it a velocity (V2) greater than the velocity (V 1) at which the sintered tubular blanks exit the die to induce elongation in the portion before the bath, i.e. before crosslinking.

In addition to providing the preceding preforming period of the powder mixture, which allows very favorable direct feed into the matrix, the proposed process ensures accurate separation of the actual molding period by sintering from the crosslinking period. The term "sintering" includes thickening and melting the grains to obtain a compact product. With respect to the use of a molten salt bath to induce a crosslinking reaction, it has the advantage that the product once cross-linked can pass freely through the crosslinking zone in that the product does not need to slide in contact with the walls due to heat exchange. to allow it to advance to the exit, simple mechanical guidance is sufficient which, in the case of a tube, can be achieved by linear contact with the circular surface of the rings carried by the crucible containing the bath and by using in addition the lubricating effect inside the liquid. In order to ensure the calibration of the article, it may be advantageous to have a guide device or guide sleeve in the bath entry area, the useful cross section of which may be constant or variable. If it is a tube, its inner diameter, which decreases at the housing, can be maintained within the desired tolerances by the action of an inert gas, for example nitrogen injected into the interior of the tube.

By using the process of the invention for high density polyethylene products, the absence of any shear stress during crosslinking makes it possible to achieve optimum properties. The elongation of specimens subjected to elongation at rupture is between 400 · and 500 ·%; this result is independent of the time elapsed since the start of production in the plant, since there is no deposition in the crosslinking zone which would have the effect of reducing the quality of the products. For this reason it is possible to proceed with continuous industrial production, since the need for frequent replacement of parts damaged by surface deposits, which is a drawback of the prior art, is eliminated in the process according to the invention.

The method according to the invention can be used for the production of pipes and profile parts as well as for the coating of cables or metal fittings.

According to a preferred method of the invention, the axial pressure acting on the powder mixture is of the order of 100 MPa.

According to another embodiment of the invention, the sintering temperature of the zone is between 150 and 180 ° C, and the crosslinking zone temperature is between 200 and 300 ° C.

According to a further embodiment of the invention, the shaped tube is calibrated before the crosslinking zone during extraction.

By drawing here is meant not only longitudinal stretching or elongation, but also any shaping by longitudinal and radial expansion, which is obtained in particular by the action of a die or die or by the action of a pressurized fluid which can take place inside the mold or without the use of the mold.

Thus, a single device can produce tubes of different diameters at a constant hourly output, regardless of their diameter, profile, etc. It is also possible to produce-body components, reservoirs, packages, corrugated or heat-shaped plates, etc.

The invention also relates to an apparatus for carrying out the method according to the invention, in which, prior to the sintering matrix, a powdered crosslinkable mixture is supplied and an annular compression piston which is coaxial with the matrix and passes through a mandrel arranged coaxially in the matrix; According to the invention, a reservoir containing a bath of molten salt is arranged downstream of the matrix and divided into two compartments, one of which, located at the rear, comprises a profiling device, and downstream of the reservoir is a drawing device carried at a linear speed higher than the inlet speed of the sintered material.

According to a preferred embodiment, the profiling device comprises several groups of profiling rollers.

The device according to the invention may further comprise means for drawing the shaped material between the die exit and the cross-link crucible.

The invention will now be described in more detail with reference to the drawings.

Giant. IA and IB show the assembly of a high density cross-linked polyethylene pipe finishing device; FIG. 2 is an axial sectional view of the forming sleeve used in the apparatus of FIG. IB; FIG. 3 is a sectional view taken along line 3--3 of FIG. Figure 4 is an axial sectional view of another embodiment of the molding sleeve used in the apparatus of Figure IB; Figure 5 is an axial sectional view of one embodiment of the molten salt bath outlet end of Figure IB used for crosslinking the tube material Fig. 6 is a schematic view of a device comprising pulling aids and Fig. 7 is a longitudinal sectional view of the pulling and reinforcing aids used in the device of Fig. 6.

The apparatus shown in Figs. IA and IB comprises a vertical portion A in which a powdered mixture comprising polyethylene and a crosslinking agent - distributed in a homogeneous manner. It is tube-shaped and sintered, and a horizontal portion B where it crosslinks masses. ~

The part A having the vertical axis X-X has a horizontal frame 1 which is supported by the pedestal 2 and serves as a support for the matrix 4 of the X-X axis having a bottom 5 with which the shoulder 6A of the threaded rods 6 by means of screws 7, thereby securing the matrix to the frame. The die 4 comprises a cylindrical body 8 parallel to its axis, in which the bore 9 of the die 4 is coaxial with the central bore 3 of the frame 1 and which can be heated by oil circulation, indicated by 8a. The upper end of the housing 4 has a collar 10 through which the rods 6 pass and whose upper side supports a feed hopper 11 which can be cooled by the circulation of water 12, and wherein the diameter of the opening at the bottom corresponds to the bore diameter 9. a ring 11a that is neither cooled nor heated, and whose contact surfaces are grooved to reduce heat transfer.

At the bottom 5 of the die 4 and within the bores 3 of the frame 1, a second die 13 coaxially aligned with the die 4 is positioned vertically and is held in place by adjusting the lower support plate 14 fixed to the tie rods.

6.

This support plate 14 is provided with an aperture 15 whose diameter is somewhat larger than the diameter of the die 13. This assembly thus forms a cylindrical linear guide from the bottom of the feed hopper 11 to the support plate opening 14. The die 13 is also provided with annular heating means, for example formed by external electrical resistors 17.

The frame 1 serves outside the die 4 as a support for two cylinders of the jack 18, 19, which include pistons 20, 21, on whose vertical rods 22, 23, a horizontal plate 24 is fixed by a bolt connection 25, 26. The plate 24 can slide and has a bore 27 at its center which extends vertically into the interior of the annular boss 28 which is integral with the horizontal plate 24 and forms a piston which is cooled by the circulation of water 29 in the vicinity of the plate. which cooperates with the cylindrical part which is formed by the opening of the ring 11a and the upper part of the wall of the bores 3 of the die 4.

The rods 6 support - at their upper end above the plate 24 - a plate 30 on which a jack cylinder 31 comprising a piston 32 integrally formed and a mandrel or vertical punch 33, which slides in engagement with the bore 27 of the piston 28 and is guided by the plate 30 and a horizontal plate 34 formed integrally with the mandrel 33 and sliding along the rods 6.

The mandrel 33 extends into the interior of the die 4 and the die 13, wherein the upward stroke limitation of its lower end is located nearly at the lower end of the die 13. The mandrel 33 also includes a heating element 35, either oil run or electrical resistance.

The device is supplied from a metering tank 37 connected to the hopper 11 by a channel 38 to ensure uniform distribution of powder in the gap between the mandrel 33a and the die

4. The cooling line 12 of the hopper 11 makes it possible to prevent polyethylene, whatever the residence time in the hopper 14, from reaching the temperature at which it becomes a coherent effect of the onset of melting.

The cooled zone is in fact a pre-forming zone whose height is of the order of two times the diameter of the mandrel 33 and in which the mixture is to remain powdered in order to be tube-shaped by the densification effect produced by the piston 28 in the tube portion 36 formed below the die 4. at the input of this matrix.

At the start of the cycle, the piston 28 is in the uppermost position as the mandrel 33. There is no pressure on the piston 32 of the jack 31. As a result of the action of the jack 18, 19, the piston 28 descends, which passes through the powder mixture located in the hopper 11, where it is maintained at ambient temperature through the cooling duct 12. The piston 28 compacts the mixture in the tubular portion 36. for compaction it is of the order of 100 MPa. The mixture is thus shaped and simultaneously compacted. It is then carried into the matrix. The punch 33 is simultaneously subjected to a downward movement under the entrainment of the mass that is subjected in the matrix 4 to the temperature increase caused by the heating elements 8 of the matrix 4 and the heating elements 35 of the mandrel 33. The temperature is controlled to in which a decomposition of the crosslinking agent occurs.

This temperature will, for example, be between 150 and 180 ° C. When the piston 28 reaches the lower point of its stroke, the pressure on the pistons 20, 21 of the jack 18, 19 is reduced, and since the piston 28 is kept in its lower position, the mandrel 33 rises again with the help of the piston 32. the piston 28 also performs an upward movement and a new cycle is initiated.

Thus, the die 4 forms a sintering area where the mass is compressed into the die 13, due to the downward movement of the piston 28 along the plunger 33, where the temperature is maintained at the same level as before, and from where it exits in the blank. At the exit of the die 13, due to the downward movements of the piston 23, the blank is forced into a housing 15a, for example having a bellows shape where the temperature is maintained at approximately the same level as before, or possibly higher. the zone in which the blank is free from the wall of the sheath 15a can undergo lateral displacements absorbing the impact of its advancement, taking into account a continuous thrust applied to it at the outlet end toward the portion B that forms the amplification zone.

Part B consists essentially of a closed horizontal reservoir 41, preferably cylindrical, which is longitudinally divided by two vertical baffles 42, 43 and comprises a bath of molten salt 44 and is heated, for example, by electrical resistors 46 to a crosslinking temperature, i.e. 200 ° C. In the upper or inlet compartment 47 a tube coupling 48 is inserted, the upper wide end of which is fixed to the reservoir 41 and the other end whose inner narrower diameter corresponds to the outer cross-section of the tube to be reinforced is fixed to the inlet end of the molding sleeve 49. The sintered tube engages with play in the inlet end of the coupling 48, the cross section of which is tapered and, when the direction changes by 90 [deg.], penetrates into the cylindrical bore 52 of the sleeve 48, thereby providing profiling; a condition of the pipe surface such that it is simultaneously subjected to crosslinking at least on the peripheral surface.

The tube then passes through two other compartments 53, 54 in which the crosslinking is completed and where the tube is guided by rings 56 integral with the wall of the reservoir by means of supports 57 and having an inner bore 55 of circular shape that provides sliding and straight contact with pipe. The cross-linked tube extends from the end wall 58 of the reservoir and extends in close contact with the respective ring 59.

The pressure of the gas maintained under pressure inside the bore of the tube and fed from the pipe 61 via an axial guide 62 provided in the mandrel 33 also contributes to the calibration of the tube immersed in the bath 44. The gas used is preferably nitrogen.

The molten salt forming bath 44 is preferably a eutectic mixture of mineral salts, for example nitrates and nitrites, for example a eutectic mixture of the following composition:

- 53 parts by weight KNO3 - 40 parts by weight NaNO2 - 7 parts by weight NaNOs

The partitioning of the reservoir 41 by the baffles makes it possible to create different temperatures in different compartments. The temperature is usually higher, for example between 250 and 300 ° C, in compartment 47 containing the housing 49 in order to achieve rapid conversion of the material in the peripheral region of the pipe. The temperature will be higher in the other compartments 53, 54, for example at most 250 ° C, to avoid the risk of thermal damage to the product.

For processing a 3 mm thick pipe, the length of the reservoir 41 may be approximately 2 m with a pipe circulation velocity, which in this case is 150 m / h.

Reference numeral 63 denotes a device, which is shown schematically and consists in part of two movable beads which are in contact with the cross-linked tube to exert a continuous tension on the tube.

An additional shaping sleeve may be provided outside or between the drawing device 63 and the reservoir 41.

Satisfactory experiments have been carried out using a thermoplastic material consisting of high density polyethylene having a molecular weight of between 300,000 and 500,000, for example polyethylene having a density of 0.956-α and a viscosity index of 2 at a load of 20 kg / cm ² .

Additives which contain, in addition to the organic peroxide, dyes, antioxidants, anti-UV agents, and possibly lubricant210670 1a, which are all known products used in the plastics industry, are incorporated in a homogeneous manner. In particular, structures catalysed at 0.5 percent of di-tert-butyl peroxide were used as crosslinking agents.

In this case, 70 piston impacts per minute are used, and for a total piston stroke of 40 mm, the length of the manufactured tube is approximately 20 mm and the mandrel displacement is 20-25 mm.

Experiments were performed in which the elongation between two marks of a standardized ISO 1/2 sample was measured using an extensometer at a pulling speed of 100 mm / min. The results of the tensile experiments showed the mechanical properties measured on the tube in the function of the extruded length and as a reproduction of the mean value for the five samples were compiled in the following table:

extruded length (m) 100 ALIGN! 500 1000 1500 2000 stress on creep threshold in MPA 1,7658 1,180 1,668 1,864 1,785 breaking load in MPa 29.43 27.47 28.45 26.49 27.47 elongation at break (%) 520 490 460 500 270 burst pressure of tube samples (MPaj 5.3 5.2 5.7 5.0 5.2

Giant. 2. and 3 illustrate the forming sleeve used instead of the sleeve 49 of FIG. 1B and formed by a plurality of groups 64, 65, 66 of concave profiling rollers 67 which give the pipe a section gradually decreasing.

In order to achieve the desired diameter reduction, it is also possible to use the bushings shown in FIG. 4, consisting of a series of cylindrical bore forming rings 68a, 68b with different diameters. These rings consisting of Teflon or polished steel are separated by inserts 69 and held in place by a clamping device 70, optionally supplemented by a clearance compensation system (not shown).

Referring to FIG. 5, the finished product, which, for example, consists of a non-closed profile, instead of exiting the container 41 while passing through the end-side sealing device, is withdrawn through the upper container wall opening 71 to which it is fed by conduit 72.

If the article is not tubular in shape, the mandrel 33 of FIG. IA is not used. · If the cable or fitting is coated, the cable or fitting will replace the mandrel 33.

Referring to the diagram of FIG. 6, the numeral 101 indicates a molding device correspondingly. IA and 1B, which form the tubular blank or panel 102 at speeds v1; the preform passes through the reservoir 103, in which it is subjected to stretching and then crosslinking, and then gradually comes in. to the extractor 104 where it is carried at a velocity v2 greater than velocity v1, to the cooling reservoir 105 and to the extension device 106 at the same speed v2 as before or at substantially the same speed, and into the winder 107.

As shown in Fig. 7, the panel 102 extends from the molding device 101 through the opening of the lower support plate 14. It then passes through a device to suppress impacts caused by discontinuity or discontinuity of the panel advance and exits into the reservoir

103 in a continuous manner. An example of such a device is shown in FIG. 7. In this device, the member 102 changes its direction around a groove lining 108 loosely supported on the yoke 109, which itself rotates in a vertical plane about an axis 110 mounted on the carrier 111. equal to the radius of the member 102.

The member 102 then proceeds along the drive roller 112, also grooved, and connected by the speed converter 113 to the electric motor. 114, shown schematically. The panel then reverses its direction, bringing it to the withdrawal and crosslinking reservoir 103. This reservoir contains a mixture of molten salts of the same type as the reservoir 41 of Fig. 1B, divided into two compartments, namely the pulling compartment 115 and the crosslinking compartment 116 which are separated by a membrane 117 provided with an opening 118 allowing the passage of the member 102.

Inside the reservoir 103 in the compartment 115 there are a plurality of profiling rollers 121a, 121b, 121c and 121d, the profile of which conforms to the geometry of the elongated product, and all of which are rigidly connected to the handling bar 119 mounted on the articulations 120a and 120b. The crosslinking section 116 comprises retaining rollers 122. If desired, the profiling rollers 121a to 121d may be replaced by other suitable profiling devices. The number of roller sets or profiling devices may vary according to the degree of extension desired. Optionally, the retaining rollers may be replaced by any guide and support means, such as channels, rings, etc.

After passing through the reservoir 103, the member 102 is entrained by an extension 104 of known type, for example a crawler, carried at a speed v2 in the engine group 114a and the variator 113a shown schematically, then enters the cooling tank 105 before winding on the winder 107 an extension 106, which may be of the same type as the extension 104 and rotate at a speed equal to or adjacent to the speed v2. This extension is intended to facilitate the extraction of the pipe, but not to induce a new reduction in cross-section.

The component 102 exits the shaping device 101 in a continuous motion, the stopping times corresponding to the re-rise of the shaping mandrel 33, and is carried continuously toward the other devices of the device by a drive roller 112 whose linear velocity is Vl. that is, the absence of material supply at the stopping time of the member. The diameter of the panel 102 at this time reaches d1 and is maintained due to the gas pressure inside the panel to prevent the panel from collapsing.

Thus, the panel 102 of diameter d1 arrives at the extension section 115 at the speed v1. The effect of the extension 104, which then follows and whose linear velocity v2 is selected higher than the speed v1, is already apparent and the panel 102 is elongated longitudinally. Its diameter is reduced to d1i somewhat less than dl when it comes to the first set of profiling rollers 121a and its diameter reaches d2, substantially less than d1, at the outlet of reservoir 15 due to longitudinal extension. The profiling rollers 121a to 121d support the panel 102 even if its crosslinking has not yet occurred. The temperature in the extension compartment · 115 is chosen in such a way that the half life of the catalyst is still considerable; it is close to the duration which, when molded, is in the molding device, for example of the order of 160 to 190 ° C.

At the level of the aperture 118 of the diaphragm 117, the panel 102 has dimensional characteristics (diameter, thickness) homothetic to those it will have when entering the extension compartment 115. It then enters the crosslinking compartment 116 where the temperature reaches values above 200 ° C and preferably lying between 220 ° C and 250 ° C. The decomposition of the catalyst then causes crosslinking of the material and the elongation phenomenon is stopped by this crosslinking.

The location of the diaphragm 117 in the reservoir 103 is selected as a function of the desired pipe diameter at the outlet of the crosslink compartment 116, this diameter being determined by the ratio v2 / vl. Likewise, the different profiling rollers 121a, 121b, 121c, 121d are selected in the same manner, since their geometry is to be as closely as possible adapted to the profile to be formed if this profile is different from the profile of the member 102 emerging from the forming device. However, in the case that the profile formed remains a smooth tube homothetic to the tubular member 102, the profiling rollers 121a, 121b, 121c, 121d no longer serve for calibration or profiling, but only for support and guidance and can be replaced by all equivalent means. In some cases, the rollers 122, which only serve to support and guide the tube during its cross-linking, can be replaced by any equivalent means for supporting and guiding.

At the outlet of the tank 103 and after passing through the extension 104, the tube is cooled in a tank 105 containing, for example, water. It is then restrained by the generator 106 and wound on the winder 107.

An important characteristic of such an elongated and crosslinked object is its dimensional stability and the absence of elastic or shrink memory.

For example, the polyethylene profile · four times stretched in the extension compartment 115 and · then crosslinked in the crosslink compartment 116, shows no shrinkage other than ordinary heat shrinkage. Crosslinking blocks the stretched strings against each other. The crosslinked product is birefringent, but stable in all dimensions below and above the melting point of the polyethylene crystals in terms of bulk expansion.

In contrast, the same profile, four drawn-in department 115 and then simply cooled instead of cross-linking takes due * elastic memory of its original dimensions when it re · brought to a temperature of about 140 ¹ ° C, a temperature higher than the melting temperature of polyethylene .

The invention will now be explained by means of two exemplary embodiments, in particular with reference to FIGS. 6 and 7.

Example 1

High density polyethylene with a molecular weight of between 300,000 and 500,000 was used to make the tube under the above conditions.

Additives have been added which contain, in addition to stabilizers and colorants of known type, a cross-linking agent which is 2,5-dimethyl-2,5- (di-tert-butylperoxide) -hexin-3. Working conditions for different diameters, based on the part diameter di = 27.5 mm, are compiled in the following table I.

Table I

sample speed in lhh second pulling speed in lhh extrusion output power kg / h (the drawing temperature _(° C it department cross-link temperature ° C Outside diameter d2 at outlet / tank 115 [mm] outlet diameter d3 tube on winder mm 1 35.4 140 9.1 185 220 18 17.30 2 35.4 il40 9.1 180 220 17 16.68 3 35.4 180 9.1 175 230 16.5 15 Dec 1 35.4 210 9.1 175 240 15.4 14.50 sample thickness mm weight g / m

1 1.03 60.6 2 1,02 54 3 0.90 48 4 0.90 41.6

Diameter d3 is somewhat smaller than diameter d2; this is not the result of stretching, but simply shrinkage by cooling at the exit of the salt bath in the reservoir 103.

Example 2 dl ~ 27.5 mm d2 = 18.5 mm attenuation temperature = 180 ° C cross-link temperature - 230 ° C

Further experiments were carried out with the same mixture as in Example 1, with different parameters having the following value:

ll = 36.6 m / h v2 = 132 m / h

A tube with a diameter of 16 mm at the beginning and at the end of the extrusion of 2000 m was obtained.

Table II

sample stress stress extension at the creep threshold when breaking when breaking MPa MPa %

1 16.97 20.37 37.86 beginning 2 16.87 17.27 31.42 3 16.87 20.29 39.24 4 16.87 20.51 35.32 end 5 17.36 21.42 36.28 6 16.58 21.98 37.28

The cross-linking rate was 87% at the beginning as at the end and the shrinkage at 120 ° C was 4 ° / o. The shrinkage at 160 ° C was 6%.

The process of the invention is particularly applicable to peroxidic crosslinkable polyolefins and is of particular interest for high molecular weight products; for example, they are below 2.

The invention is also applicable to low density polyethylene which has been made cross-linkable by the addition of peroxide.

Claims (7)

A process for the manufacture of high density cross-linked polyethylene pipes having a molecular weight greater than or equal to 200,000 from a substantially powder-containing composition comprising polyethylene and a cross-linking agent, wherein the composition is compacted and shaped by applying pressure to it causes its penetration and advancement into the heated matrix or. in said matrix, and crosslinking of the material is effected, characterized in that after shaping the powder mixture by sintering, subjected to pressure along the axis of the matrix and subjected to a temperature lower than the temperature at which the decomposition of the crosslinking agent occurs. for. shaped matrix material results in the molten salt bath which is exposed to atmospheric pressure and the upper part is at a temperature close to the sintering, while its lower part is at a temperature at which a crosslinking agent and .rozklad crosslinked tube is pulled out of the bath while ¹ it imparts a velocity (V2) greater than the velocity (V1) at which the sintered tubular blanks exit the die to induce elongation in the portion before the bath, i.e. before crosslinking. 2. The process according to claim 1, wherein the axial pressure acting on the powder mixture is of the order of 100 MPa. 3. The method of claim 2 wherein the sintering zone temperature is between 150 and 180 [deg.] C. 4. The method of items 1 to. 3, characterized in that the temperature of the amplification zone is between 200 and 300 ° C. 5. A method according to any one of claims 1 to 4, wherein the shaped tube is calibrated in front of the amplification zone during extraction. 6. Apparatus for carrying out the method according to claim 1, in which a sintered powder mixture and an annular compression piston coaxial to the matrix are provided upstream of the sintering matrix and through which a mandrel arranged coaxially in the matrix is passed, (103) comprising a molten salt bath and divided into two compartments, one of which (115), located at the rear, comprises a profiling device (121), and behind the reservoir (103) is a drawing device (104) carried at linear speed (V2) before the speed (Vi) of the sintered material enters the reservoir. Device according to claim 6, characterized in that the profiling device (121) is formed by several groups of profiling rollers (121a to 121d).