EP1590161A1

EP1590161A1 - Method for production of a hollow body by extrusion and blowing of a thermoplastic resin

Info

Publication number: EP1590161A1
Application number: EP04707555A
Authority: EP
Inventors: Michel Lahouati
Original assignee: Eurofeu SA
Current assignee: Eurofeu SA
Priority date: 2003-02-07
Filing date: 2004-02-03
Publication date: 2005-11-02
Also published as: CN1777505A; FR2850899A1; WO2004078458A1; FR2850899B1; US20060141090A1

Abstract

The invention relates to a method for production of a hollow body by extrusion and blowing of a thermoplastic resin, comprising the steps of filling a quantity of resin into an accumulator (1) which comprises an accumulation chamber (12) with an open end, placing a blowing mould (24) with an internal cavity (36) and a constricting section (37) in communication with said open end, axially displacing a core (10, 11) across the constriction point of the internal cavity such as to cover a projecting part of the core with a layer of resin (38), continuing the axial displacement of the core in said internal cavity (36) to axially extend said layer of resin (38) and applying a fluid pressure to the internal surface of the resin layer through said core to obtain a hollow body.

Description

PROCESS FOR THE MANUFACTURE OF A HOLLOW BODY BY EXTRUSION AND BLOWING OF A RESIN

THERMOPLASTIC

The present invention relates to a method for manufacturing a bi-oriented hollow body by extrusion and blowing of a thermoplastic resin and to a device for implementing this method.

The document EP486419 describes such a method which comprises the stages consisting in carrying resin in a malleable state, filling with a quantity of said resin an accumulator comprising an accumulation space delimited between a central core and an outer wall having an opening of end, placing a blow mold with an internal cavity having an open throttle portion in communication with said end opening, axially moving a movable punch from said central core through said accumulation space, said end opening and said throttling portion of the internal cavity, so as to coat with resin a part of the punch projecting from said central core.

In this known process, the resin is then blow molded. This process has proved to be advantageous in terms of productivity and makes it possible to control and homogenize the temperature of the resin, thus avoiding irregularities in weight and size in the hollow body obtained. However, this known method is not suitable for the production of large-capacity hollow bodies, since the preform risks falling off the punch during blowing if it has too great a weight. It is known to use a thermoplastic resin with a high molecular weight to avoid the risk of unhooking of the blank. However, such a resin has a high glass transition temperature, which causes increased energy expenditure in the molding machine. In addition, such a resin does not always have mechanical properties suitable for the application of the hollow body. Finally, this known method makes it possible to manufacture only hollow bodies of simple structure, only the external shape which can be adapted by the choice of the shape of the blow mold.

The present invention aims to manufacture a bi-oriented hollow body by overcoming these drawbacks. For this, the invention provides a method of the aforementioned type, characterized by the steps consisting in: continuing the axial movement of the movable punch in said internal cavity at least to an intermediate level between said open throttle portion and an opposite end wall of said internal cavity, simultaneously pushing resin out of said accumulation space with an exit speed lower than the speed of displacement of the punch, to axially lengthen said resin layer, apply a fluid pressure on said inner surface of the resin layer through said punch to transversely stretch said resin layer up to walls of said internal cavity and obtain a bi-oriented hollow body having a neck corresponding to the throttling portion of the internal cavity, allow said hollow body to cool to a rigid state, retract said punch and eject said hollow body from the mold blowing.

In this process, a layer of resin coats the movable punch at least up to an intermediate level between the throttling portion and the end wall of the mold cavity, so as to form a blank directly in the blow mold. , which allows blowing to take place immediately afterwards, without changing the workstation. The continuity and the speed of execution of these two stages avoid problems of thermal conditioning of the resin. This process works with most of the resins available on the market, such as PVC, polypropylene PP, polyethylenes PE, PET and polyamides PA. One thus obtains a bi-oriented hollow body without welding with a wall devoid of inhomogeneities or other point defects. The use of a coated punch makes it possible to produce a blank of large mass without risking a detachment of the blank, which makes it possible to obtain a hollow body of large capacity and / or with a large wall thickness. The punch fulfills both the function of lengthening the parison and of supporting it.

Preferably, the method according to the invention comprises the step consisting in printing an external relief of the projecting part of said punch on an internal surface of said layer of resin, so as to obtain a hollow body having a corresponding internal relief. For example, the projecting part of the punch includes grooves and / or portions having different transverse dimensions and / or a thread. According to a particular embodiment of the invention, said external relief includes at least one threaded punch part to obtain a corresponding thread on the internal surface of said hollow body. The thread obtained on the inner surface of the hollow body has the advantage of offering a pressure-resistant fixing for a plug, a valve or the like accessory to be fixed in the neck of the hollow body.

Advantageously in this case, said movable punch comprises a peripheral bush constituting said threaded punch part and a central rod which can slide axially relative to said peripheral bush. In the step of moving the punch, said peripheral sleeve is brought into said throttling portion of the internal cavity, so as to enclose said layer of resin between said threaded punch portion and a wall of said throttling portion, and , in the retraction step of the punch, said peripheral sleeve is animated with an axial rotational movement so as to disengage said peripheral sleeve from the internal thread obtained in a corresponding throttling portion of the hollow body. The use of a two-part punch with a central rod and a threaded peripheral sleeve allows the movements of the threaded punch part to be controlled independently. The peripheral sleeve surrounds the central rod so that the air gap between the punch and the throttle portion of the mold is reduced when the peripheral sleeve is introduced into the throttle portion. The layer of resin sandwiched between the two parts forms the neck of the hollow body and conforms at the level of its internal surface to the thread carried by the peripheral sleeve. The internal thread can also be formed in another portion of the hollow body, for example in the bottom wall using a corresponding relief on the end part of the central rod.

Advantageously, the end opening of the accumulator and the throttling portion of the blowing mold communicate through an extrusion orifice of an extrusion die and, for example at the end of the step of moving the movable punch, a compaction sleeve is moved around said punch in said extrusion orifice, said compacting sleeve being inserted between said punch and a wall of said extrusion orifice so as to completely evacuate the resin from the extrusion orifice in the internal cavity of the blow mold. This gives a neck of the hollow body without shrinkage.

Preferably, the method according to the invention further comprises the steps consisting in: moving the peripheral sleeve from the throttling portion towards the interior of the internal cavity during the blowing step, so as to fold back a section of said layer of resin between a portion of said layer of resin pressed against the wall of the internal cavity of the blow mold and an end portion of said layer of resin attached to the peripheral sleeve, and press said folded pan against said portion of end of said resin layer attached to the peripheral sleeve at the end of the blowing step. A double-walled neck is thus obtained, having increased rigidity. This neck is provided with an internal thread for fixing a plug or the like. The pressure resistance of the corresponding assembly is further increased. Advantageously, in the step of moving the movable punch _5, the accumulation space is completely emptied through the extrusion orifice. The complete evacuation of the accumulator allows precise control of the quantity of resin which is molded, for precise dimensioning of the walls of the hollow body obtained, at a temperature kept fixed.

The axial displacement of the punch is carried out according to the desired elongation rate. According to a particular embodiment of the invention, the punch is moved substantially to the end wall of the internal cavity. The invention also provides a device for implementing this method, which comprises: a resin accumulator including an outer wall and a central core delimiting between them an accumulation space capable of receiving a thermoplastic resin in a malleable state, an end opening formed through said outer wall, an extrusion piston slidably arranged between said outer wall and said central core for expelling the resin from said accumulation space through said end opening, a blowing and bi-orientation mold with an internal cavity having an open throttling portion which can be placed opposite said end opening and an end wall opposite said open throttle portion, a punch movable axially between a retracted position inside said central core and protruding positions, in which a protruding part of said punch relative to said central core is engaged through said end opening and said throttling portion of the internal cavity, said punch having an axial internal duct opening out of said punch at said projection and a valve for selectively opening and closing said internal duct, drive means controlled to selectively move said extrusion piston and said punch in axial sliding, and a pressure source connected to said inner conduit of the punch, characterized in that said punch can move in said internal cavity at least to an intermediate level between said choke portion and said wall end.

Advantageously, said punch carries at least one groove to obtain a rib of corresponding shape on the interior surface of said hollow body.

According to particular embodiments, said or each of said groove (s) has a closed annular line or a substantially rectilinear axial line or a helical line.

Preferably, said punch comprises at least one threaded punch part to obtain a corresponding thread on the interior surface of said hollow body. Advantageously, the movable punch comprises a peripheral sleeve constituting said threaded punch part and a central rod which can slide axially relative to said peripheral sleeve, and said drive means are capable of axially moving said central rod and said peripheral sleeve out of phase and at driving at least said peripheral sleeve in rotation in a direction of unscrewing the thread of the peripheral sleeve.

According to a particular embodiment of the invention, the central rod is driven in axial rotation by said drive device, a unidirectional coupling being arranged between said peripheral sleeve and said central rod to couple in rotation said peripheral sleeve to said central rod in said unscrewing direction and decoupled in rotation said peripheral sleeve from said central rod in an opposite direction. Preferably, the end opening of the accumulator and the throttling portion of the blow-molding and bi-orientation mold communicate through an extrusion orifice of an extrusion die, a compaction sleeve being arranged. around said punch and axially movable between a retracted position in said central core of the accumulator and a deployed position, in which said compacting sleeve is inserted between said punch and a wall of said extrusion orifice so as to completely evacuate the resin the extrusion orifice in the internal cavity of the blowing and bi-orientation mold. Advantageously, the external wall of the accumulator is provided with a heating means and the central core of the accumulator is provided with an internal circuit intended to circulate a heat transfer fluid. Using these characteristics, the temperature of the resin in the accumulator is regulated from both sides of the accumulation space. The resin can thus be maintained at a homogeneous temperature and optimal for molding. For example, the heating means is an electrical resistance. Any other type of heating means can be provided on or in the outer wall of the accumulator and on or in the central core to effect thermal regulation of the resin simultaneously from the inner peripheral surface and the outer peripheral surface of the space d 'accumulation.

The invention will be better understood, and other objects, details, characteristics and advantages thereof will appear more clearly during the following description of several particular embodiments of the invention, given solely by way of illustration and without limitation. , with reference to the accompanying drawings. In these drawings: - Figure 1 is a partial view in axial section of a device according to a first embodiment of the present invention, the accumulator being associated with a molding station, - Figure 2 is an enlarged detail view of a part of the accumulator of FIG. 1, the accumulator being associated with an injection station,

FIG. 3 is a view similar to FIG. 1, showing an extrusion step by coating a punch,

FIG. 4 is a view similar to FIG. 3, showing a step of bi-orientation with pre-blowing,

FIG. 5 is a view similar to FIG. 4, showing the end of the blowing step,

- Figure 6 is an enlarged detail view of a device according to a second embodiment of the present invention, the accumulator being associated with a molding station, - Figure 7 is a partial view showing an alternative embodiment of the punch,

- Figure 8 is a diagram representing the chronology of operation of the device of Figure 1, - Figure 9 shows an example of a hollow body obtained using the device of Figure 6. We will now describe a molding machine by blow molding according to the first embodiment of the invention and its operation. With reference to FIG. 1, the machine comprises an accumulator 1 which is mounted on a mobile support so that it can be associated with two different work stations. In FIG. 1, the accumulator 1 is associated with a molding station 2.

The accumulator 1 comprises a tubular outer casing 3 which is fixed at its upper end to a support flange 4. The support flange 4 is part of a turntable known per se and not shown enabling the accumulator 1 to be moved from one work station to another. The outer casing 3 has at its lower end a transverse rim 5 which surrounds and delimits an outlet opening 6 of the accumulator 1. Inside the outer casing 3 is a central core 7 made up of several coaxial parts movable with respect to each other, namely an inner jacket 8, a compacting sleeve 9, a calibrated sleeve 10 and a central hollow rod 11. The inner jacket 8 comprises several individual parts which contain a circuit for making circulate a heat transfer fluid such as thermal oil. The circuit comprises annular conduits 13 arranged near the outer surface of the inner jacket 8. The calibration sleeve 10 and the central hollow rod 11 constitute a coating punch whose function will be explained below. Between the central core 7 and the inner wall of the outer casing 3 is an accumulation space 12 which extends to the outlet opening 6 and which comprises an annular space closed at its upper end 15 by a extrusion piston 14. In FIG. 1, the extrusion piston 14, the inner jacket 8, the compaction sleeve 9, the calibration sleeve 10 and the central hollow rod 11 are shown in a position of withdrawal from the inside the outer casing 3. These different parts can be moved axially towards the outside of the outer casing 3 by a conventional pneumatic drive.

Referring to Figure 2, the central rod 11 has a central duct 17 which is connected at the upper end to a source of pressurized air not shown and which is closed at the lower end by a valve calibrated 18 returned to the closed position by a spring 19.

In FIG. 2, the accumulator 1 is shown associated with the other work station, which is an injection station 16. The production cycle for a hollow body begins at this station, as will now be explained.

At the injection station 16, a screw injection press of known type is used to bring a thermoplastic resin in a malleable state and inject it into the accumulation space 12. In FIG. 2, only shown an end portion of the injection nozzle 20 which fits snugly against the outer casing 3 of the accumulator 1. A predetermined amount of resin 35 is thus injected into the accumulator 1 so as to fill the accumulation space 12. In order to bring and to maintain the resin 35 at the optimum temperature for the bi-orientation molding phase, the temperature in the accumulation space 12 is regulated by means of an electrical resistance 21 and a circulation of fluid in the circuit of the inner liner 8.

Thus, the accumulator 1 makes it possible both to precisely dose the quantity of resin necessary for obtaining a given hollow body and precise thermal conditioning of the material to be molded. This accumulator with double thermal conditioning surface makes it possible to transform all types of plastics in very wide thermoplastic and thermoelastic temperature ranges. In addition, the material is conditioned in the accumulator without retaining internal tension due to the injection. Finally, the thermal conditioning of the material to be molded in the accumulator helps to avoid shrinkage due to premature cooling of the material.

From this situation, the operation of the machine will be explained using the diagram in Figure 8, on which each horizontal box represents a time step equal to about 0.5 s.

In step 22, the accumulator 1 is moved by the rotary support plate to the molding station with bi-orientation 2, visible in Figures 1 and 3 to 5. A cover (not shown) closes the opening 6 during this displacement. In FIG. 1, the material contained in the accumulation space 12 is not shown.

The molding station with bi-orientation 2 comprises an extrusion die 25 fixed on a fixed support plate 26 and a blowing mold 24 consisting of two separate shells 24a and 24b. The shells 24a and 24b are actuated in a transverse movement by a conventional mechanism allowing the opening and closing of the mold 24. The mold 24 contains an internal cavity 36 which has a choke portion 37 of diameter equal to the diameter of the orifice 28 of the extrusion die 25. Step 23, which begins simultaneously with step 22, represents the closing movement of the mold 24. This movement being known, the mold 24 is shown in the closed position in all the figures. Step 27 represents the blocking of the rotary support plate at station 2. The rim 5 is then positioned in an adjusted manner against the upper surface of the extrusion die 25, the accumulator 1 being placed in the axis of the extrusion orifice 28. Step 29 represents the opening of the cover which closed the opening 6.

Several operations then begin almost simultaneously: step 30 represents the displacement of the extrusion piston 14 to push the resin out of the accumulation space 12 through the opening 6. Step 32 represents the displacement of the parts of the central core 7. Step 33 represents the pre-blowing of a low air pressure through the duct 17. Step 34 represents the transfer of material through the extrusion orifice 28.

More specifically, in step 32, the central rod 11 is first moved, which engages through the extrusion die 25 in the mold 24 while being coated with a regular layer of resin 38. The advance of the central rod 11 takes place at a speed twice the speed of exit of the resin 35 through the extrusion orifice 28, which produces an axial stretching of the resin layer 38 and a molecular orientation corresponding. An end portion of the central rod 11 carries a helical groove 39 on its peripheral surface, which prints a corresponding helical rib on the inner surface of the resin layer 38, as visible in FIG. 3. The pre-blowing slightly delayed of air through the duct 17 of the rod 11 detaches the resin layer 38 from the rod 11, after a certain axial displacement of the latter beyond the throttling portion 37, which avoids excessive cooling fast resin. The resin layer 38 detached from the rod 11 is shown in Figure 4, in which the helical rib 40 is also shown. During the pre-blowing, the resin layer 38 does not come into contact with the peripheral wall of the cavity 36. Late on the central rod 11, the calibration sleeve 10 is also moved towards the extrusion orifice 28 The calibration sleeve 10 enters the air gap between the rod 11 and the peripheral wall of the extrusion orifice 28. The calibration sleeve 10 has an external thread 41, better visible in FIG. 2, which prints a thread corresponding on the inner surface of the resin layer 38. The calibration sleeve 10 moves to the level of the throttling portion 37 of the mold 24, so as to form an internal thread 67 in the neck of the hollow body during manufacture. For example, the ratio between the internal radius of the extrusion orifice 28 and the air gap is approximately 10. While the rod 11 ends its movement up to the bottom wall 42 of the internal cavity 36, the piston 14 and the inner jacket 8 move until they touch the rim 5 to completely empty the accumulation space 12. Finally, the compaction sleeve 9 slides in an adjusted manner between the calibration sleeve 10 and the peripheral wall of the extrusion orifice 28 up to the lower end of extrusion orifice 28, so as to completely expel the resin from the extrusion die 25 and to compress the material in the gap between the calibration sleeve 10 and the throttling portion 37. The end position of the different parts at the end of step 32 is shown in FIG. 5.

From this situation, the blowing step 43 is carried out with a higher air pressure, which transversely expands the resin layer 38 until it contacts the walls of the internal cavity 36 and thus completes the bi-orientation. molecular weight of the material and the formation of a hollow body 50. For example, the blowing ratio, that is to say the ratio between the diameter of the extruded parison and the diameter of the hollow body 50, is approximately 3/4 . Simultaneously, step 44 is carried out of returning the extrusion piston 14 to the withdrawn position and then step 45 of returning the parts of the central core 7 to the withdrawn position. Thus, the parison is supported until its finalization. In step 45, the calibration sleeve 10 is rotated so as to unscrew its external thread 41 from the corresponding thread formed on the internal surface of the resin layer 38. For this, the central rod 11 is coupled to a motor electric rotary numerical control and the calibration socket 10 is coupled to the central rod 11 by a unidirectional ratchet transmission 66, which allows the drive of the calibration socket 10 in the unscrewing direction and also allows the calibration socket 10 to rotate faster than the central rod 11, which avoids forcing the molded thread when removing the calibration sleeve 10. Step 46 represents the closing of the closure shutter of the opening 6. Step 47 represents the cooling of the hollow body 50 to the glass transition temperature of the material and below. Step 48 represents the corresponding plasticization phenomenon of the hollow body 50. Next, step 49 represents the opening movement of the mold 24 to eject the finished hollow body 50. Step 51 represents the unlocking of the turntable and step 52 the displacement of the turntable to bring the accumulator 1 to the injection station 16. In known manner, it is preferably provided with several identical accumulators which work simultaneously in masked time at different positions. In this case, step 53 represents an initialization of the control module of the molding machine, in order to start a new cycle with another accumulator 1 previously filled. As shown in Figure 8, the work cycle at station 2 lasts for approximately 15 s. Steps 52 and 23b are in fact an iteration of steps 22 and 23 which inaugurates this new cycle, which will be executed identically to that which has just been described.

The hollow body 50 obtained by the process which has just been described has a regular wall thickness, a helical rib 40 on its internal surface, which reinforces its resistance to pressure, and an internal thread in its neck. Other shapes of ribs can be obtained in a similar manner, by adapting the layout of the groove or grooves on the central rod 11. For example, a plurality of parallel peripheral annular grooves makes it possible to obtain a plurality of parallel annular ribs in the hollow body 50, and parallel axial grooves make it possible to obtain axial ribs in the hollow body 50.

In step 32, the ratio between the speed of the central rod 11 and the speed of exit of the resin 35 through the extrusion orifice 28 controls the rate of axial elongation of the resin layer 38 and can be chosen according to the desired properties. This rate is equal to 2 in the example described above.

With reference to FIG. 6, a second embodiment of the manufacturing method according to the invention and a corresponding variant of the molding machine will now be described. The same numbers are used to designate elements identical or analogous to those of the first embodiment.

As visible in FIG. 6, in the blowing mold 24, the internal cavity 36 has a shoulder face 54 at right angles to the wall of the throttling portion 37. FIG. 6 also shows annular conduits 55 for the circulation of a heat transfer fluid in the extrusion die 25 and in the throttling portion 37, in order to regulate the temperature of the resin in these areas. During the blowing, the pressure injection being carried out through the end of the central rod 11 which is at the bottom of the mold 24, the resin layer 38 is pressed against the walls of the cavity 36 from the bottom to the top of the mold. The right half of FIG. 6 represents the resin layer 38 substantially as it is obtained during the blowing step 43 in the first embodiment. In the second embodiment, the calibration sleeve 10 and the compacting sleeve 9 continue to be moved together towards the interior of the mold 24 during the blowing. Thus, a section 56 of the resin layer 38, which is adjacent to an end portion 58 hooked to the calibration sleeve 10, is driven at a distance from the shoulder face 54 and thus folds back towards a lower portion 57 of the resin layer 38, which is hooked to the peripheral wall of the cavity 36. The panel 56 remains more flexible than the rest of the resin layer 38 because the absence of contact with the mold 24 and the coating punch slows down its cooling.

The left half of FIG. 6 represents, at number 56a, the face as it is approximately positioned when the sockets 9 and 10 arrive at the end of the race. In this embodiment, the compaction sleeve 9 also sweeps the throttling portion 37 of the blow mold 24 and the threaded part of the calibration sleeve 10 enters the main cavity of the mold 24. Finally, the blowing is finished with higher pressure, which folds the folded pan against the end portion 58, as shown in Figure 56b, forming a bend of material. This gives a neck with a double wall and an internal thread. The rest of the process is identical to the first embodiment. The hollow bodies obtained by the methods described above can have many applications, for example for water treatment, filtration, packaging of chemical, food, pharmaceutical or cosmetic products. Large capacity hollow bodies, for example 200 liters, can be produced. One can in particular manufacture hollow bodies resistant to high internal pressures, due to the quality of their walls and the presence of reinforcing ribs on their internal surface, for example an aerosol can body intended to contain a pressure of 30 to 35 bar. The thickness of the wall is adjusted by the dimension of the air gap existing around the central rod 11 in the extrusion orifice 28.

In FIG. 7, an alternative embodiment of the central rod 11 has been shown, in which the latter has two portions l ia and 11b having a reduced diameter relative to the remainder of the rod 11, to form by coating a parison having a staggered thickness and thus obtain a hollow body having a peripheral wall staggered as to its thickness and / or its diameter. The thinned portions 1a and 11b thus make it possible to obtain an extra thickness of the walls at the bottom and at the top of the hollow body 50, which are the areas where the greatest pressure is exerted when the hollow body is used as a pressurized tank. . Example FIG. 9 shows a hollow body 60 obtained using the device according to the second embodiment described and used as a reservoir for a portable fire extinguisher 61. The hollow body 60 is made of a polymer resin crosslinked by ionic bonds known under the registered trademark Surlyn® and manufactured by the company DuPont®. This material has excellent transparency, high scratch resistance, a wide range of processing temperatures and very good resistance to organic solvents. The wall 62 has a substantially uniform thickness e being between 3 and 5 mm, to contain a pressure of 55 bar. Its internal surface carries a helical rib 63. The neck 64 of the hollow body 60 has a double wall and an internal thread for screwing an expulsion device 65.

Claims

1. A method of manufacturing a bi-oriented hollow body (50) by extruding and blowing a thermoplastic resin, comprising the steps of: bringing said resin (35) into a malleable state, filling with a predetermined quantity of said resin an accumulator (1) comprising an accumulation space (12) delimited between a central core (7) and an outer wall (3) having an end opening (6), regulating the temperature of the resin in said accumulator from the two faces of the accumulation space, place (22) a blow mold (24) with an internal cavity (36) having a throttle portion (37) open in communication with said end opening, move axially (32) a movable punch (10, 11) from said central core through said accumulation space, said end opening and said throttling portion of the internal cavity, so as to coat with a layer of resin (38 ) a pa rtie the punch projecting relative to said central core, continue (32) the axial displacement of the movable punch in said internal cavity (36) at least to an intermediate level between said open throttle portion and an end wall (42 ) opposite of said internal cavity, by simultaneously pushing (30) said predetermined quantity of resin out of said accumulation space with an exit speed lower than the speed of movement of the punch, to axially lengthen said resin layer (38), apply (43) a fluid pressure on said inner surface of the resin layer through said punch to transversely stretch said resin layer to the walls of said internal cavity and to obtain a bi-oriented hollow body (50) having a corresponding neck at the throttling portion of the internal cavity, allow said hollow body to cool (47) to a rigid state, retract (45) said punch and ejecting (49) said hollow body from the blow mold.

2. Method according to claim 1, characterized by the step consisting in printing an external relief (39, 41) of the projecting part of said punch (11) on an internal surface of said layer of resin (38), so as to obtain a hollow body having a corresponding internal relief (40, 67).

3. Method according to claim 2, characterized in that said external relief includes at least one threaded punch part (10) to obtain a corresponding thread (67) on the internal surface of said hollow body.

4. Method according to claim 3, characterized in that said movable punch comprises a peripheral sleeve (10) constituting said threaded punch part and a central rod (11) which can slide axially relative to said peripheral sleeve, and that, in the step of moving the punch (32), said peripheral sleeve (10) is brought into said throttling portion (37) of the internal cavity, so as to enclose said layer of resin (38) between said punch portion threaded and a wall of said throttling portion, and that, in the retraction step (45) of the punch, said peripheral sleeve (10) is animated with an axial rotation movement so as to disengage said peripheral sleeve from the thread interior (67) obtained in a throttling portion of the hollow body.

5. Method according to claim 4, characterized by the steps consisting in: moving the peripheral sleeve (10) from the throttling portion (37) towards the interior of the internal cavity (36) during the blowing step (43 ), so as to fold a section (56) of said resin layer (38) between a portion (57) of said resin layer pressed against the wall of the internal cavity of the blow mold and an end portion (58 ) of said layer of resin attached to the peripheral sleeve, and press said folded panel (56a, 56b) against said end portion (58) of said layer of resin attached to the peripheral sleeve at the end of the blowing step .

6. Method according to one of claims 1 to 5, characterized in that said end opening (6) of the accumulator and said throttling portion (37) of the blowing mold communicate through an orifice extrusion (28) of an extrusion die (25), and that a compaction sleeve (9) is moved around said punch (10, 11) in said extrusion orifice, said compacting sleeve being inserted between said punch and a wall of said extrusion orifice (28) so as to completely evacuate the resin from the extrusion orifice in the cavity internal (36) of the blow mold.

7. Method according to one of claims 1 to 6, characterized in that, in the displacement step (32) of the movable punch, the accumulation space (12) is completely emptied through the orifice d 'extrusion (28).

8. Method according to one of claims 1 to 7, characterized in that said punch (11) is moved substantially to the end wall (42) of the internal cavity.

9. Device for implementing the method according to claim 1, comprising: a resin accumulator (1) including an outer wall (3) and a central core (7) delimiting between them an accumulation space (12) suitable to receive a predetermined quantity of thermoplastic resin

(35) in a malleable state, an end opening (6) formed through said outer wall, an extrusion piston (14) slidably arranged between said outer wall and said central core for expelling said predetermined amount of resin of said accumulation space through said end opening, a blowing and bi-orientation mold (24) with an internal cavity

(36) having an open choke portion (37) which can be placed opposite said end opening and an end wall (42) opposite said open choke portion, a punch (10 , 11) axially movable between a retracted position inside said central core (7) and protruding positions, in which a protruding part of said punch relative to said central core is engaged through said end opening and said portion d constriction of the internal cavity, said punch comprising an inner axial duct (17) opening out of said punch at the level of said protruding part and a valve (18, 19) for selectively opening and closing said inner duct, means for 'drive controlled to selectively move said extrusion piston and said punch in axial sliding, and a pressure source connected to said inner conduit of the punch, characterized in that the external wall (3) of the accumulator is provided with a heating means (21) and that the central core (7) of the accumulator is provided with an internal circuit (13) intended to make circulate a heat transfer fluid and that said punch (11) can move in said internal cavity (36) at least to an intermediate level between said choke portion (37) and said end wall (42).

10. Device according to claim 9, characterized in that said punch (11) carries at least one groove (39) to obtain a rib (40) of corresponding shape on the inner surface of said hollow body (50).

11. Device according to claim 10, characterized in that said or each of said grooves (39) has a closed annular line or a substantially rectilinear axial line or a helical line.

12. Device according to one of claims 9 to 11, characterized in that said punch comprises at least one threaded punch part (10) to obtain a corresponding thread on the inner surface of said hollow body.

13. Device according to claim 12, characterized in that said movable punch comprises a peripheral sleeve (10) constituting said threaded punch part and a central rod (11) which can slide axially relative to said peripheral sleeve, and that said drive means are able to move said central rod and said peripheral sleeve axially out of phase and to drive at least said peripheral sleeve in rotation in a direction of unscrewing the thread of the peripheral sleeve.

14. Device according to claim 13, characterized in that the central rod is driven in axial rotation by said drive device, a unidirectional coupling being arranged between said peripheral sleeve and said central rod to couple in rotation said peripheral sleeve to said central rod in said unscrewing direction and decoupling in rotation said peripheral sleeve from said central rod in an opposite direction.

15. Device according to one of claims 9 to 14, characterized in that said end opening (6) of the accumulator and said throttling portion (37) of the mold blowing and bi-orientation communicate through an extrusion orifice (28) of an extrusion die (25), a compaction sleeve (9) being arranged around said punch (10) and movable axially between a retracted position in said central core (7) of the accumulator and a deployed position, in which said compacting sleeve is inserted between said punch and a wall of said extrusion orifice so as to completely evacuate the resin from the extrusion orifice in the internal cavity (36) of the blowing and bi-orientation mold.