DK178636B1

DK178636B1 - Annular extrusion head

Info

Publication number: DK178636B1
Application number: DKPA201170322A
Authority: DK
Inventors: Frédéric Demanze; Mickael Guignon
Original assignee: Technip France
Priority date: 2008-12-23
Filing date: 2011-06-23
Publication date: 2016-09-26
Also published as: DK201170322A; BRPI0922470A2; GB2478219A; MY160297A; GB2478219B; FR2940170A1; GB201108739D0; WO2010072955A1; BRPI0922470B1; FR2940170B1

Abstract

The invention relates to an extrusion head (10) inciuding a cylinder (18) and a mandrel (12), said cylinder coaxially covering said mandrel so as to form an an nular chamber (20), said annular chamber having an upstream portion (22) and, opposite said portion, a downstream portion (24) extended by a pair of circular lips (26), said extrusion head (10) inciuding an inlet opening (34) for injecting said soft plastic material which flows into said upstream portion forming a plural ity of flows of soft plastic material, said flows being suitable for joining together SO as to preform a sleeve of a soft plastic material. Said head includes at least one dispersion grating (32) extending circumferentially between said mandrel (12) and said cylinder (18), and radially from said mandrel to said cylinder; and said sleeve of soft plastic material is forced through said at least one dispersion grating (32) SO as to fragment and disperse said soft plastic material.

Description

Title: Annular extrusion head

The present invention relates to an annular extrusion head for forming a tubular sheath made of plastics material.

An envisaged field of application is, in particular, that of flexible oil pipes comprising at least one polymer sheath. These flexible pipes are advantageously of the unbonded type and are described in standards documents published by the American Petroleum Institute API 17 J and API RP 17B.

Polymer or plastics sheaths of these flexible pipes are generally extruded with the aid of an extruder using an extrusion head known as a “crosshead”. Under this tried and tested solution, tubes intended to constitute the innermost layer of oil pipes of the “smooth bore” type and also outer protection sheaths, for example, can both be realized with the same installation.

The extrusion head comprises a mandrel covered with a substantially cylindrical body forming a cylinder or a jacket, so as to form an annular chamber between the inner wall of said body and said mandrel. The annular chamber has an upstream portion, in which the body and the mandrel are joined together, and, opposite thereto, a downstream portion extended by a tool comprising a pair of circular lips which open out onto the outside of the extrusion head. Different mandrels can be envisaged for the realization of these sheaths or tubes and it is generally preferred to use mandrels of the “coat hanger” or “fish tail” type. The extrusion head in this case comprises an inlet opening for injection of said softened plastics material into the inside of the upstream portion and mandrels of this type comprise symmetrical distribution channels which diverge from the point of injection of the softened polymer in the crosshead and come together again along the generatrix diametrically opposite to this point of injection. The softened plastics material is then fit to flow into the upstream portion, forming a plurality of flows of softened plastics materials. Thus the flows re-merge in said downstream portion according to junction zones, so as to preform a sleeve of softened plastics material, which sleeve has weld surfaces corresponding to said junction zones. Mandrels of the aforementioned type have the advantage of being simple, inexpensive and easy to clean between the extrusion runs. Whatever the type of mandrel, the channels have the task of guiding and distributing the molten polymer right around and along the mandrel such that the sheath has the required geometry and homogeneity when leaving the tool.

However, in the case of mandrels of the “coat hanger” or “fish tail” type, a weld surface is generated in the thickness of the sheath, which weld surface forms a visible line at the level of the generatrix diametrically opposite to the point of injection, for it is at this location that the two half-flows of softened material re-merge and fuse together more or less well after each having completed, in opposite directions, a half-circuit of the mandrel. Now, during the extrusion, fractures of the sheath can occur at the level of this weld surface. These sheath fractures are all the more frequent since the material to be extruded is a plastics material with very high molecular weight, such as, typically, PVDF. JP 2001121597 A describes an annular extrusion head for forming a tubular sheath made of plastics. The head comprises a cylinder and a mandrel inside the cylinder. An annular chamber having an upstream and downstream portion is formed between the mandrel and the cylinder. A dispersion grate comprising a crown having two mutually opposing faces and a plurality of axial holes passing through from one face to the other is mounted between the upstream and the downstream portions. A filter having smaller openings than the dispersion grate is mounted against the grate. The softened plastics material is intended to be passed through the dispersion grate to be split into many separate flows and through the filter to be further fragmented. This is done in order to avoid a weld line in the finished plastics sheath. US 3,957,942 A describes a method of extruding a coating or sheath onto an elongated support. The method comprises radially feeding the material into an inlet end of an extrusion head and decompressing, homogenizing, decompressing and compacting the material around a sleeve which surrounds the support. Then the material is expanded radially and then deposited in the form of a sheath on the elongated support. The plastic material flows under pressure and the sheath moves toward the exit pulling with it the elongated support on which it is deposited. In the final portion of the extrusion head the sheath is guided and pressed onto the support to form a homogeneous coating of predetermined thickness. The coated articles may have a tubular or rectangular profile or be in the form of ribbons, wires, or cables, and particularly coated cylinders which may be solid or hollow.

Hence, a problem which arises and which the present invention sets out to resolve is to provide an annular extrusion head by which sheaths or tubes can be realized which are not prone to cracking at the level of the weld surface.

With the aim of resolving this problem, the present invention proposes an annular extrusion head for forming a tubular sheath made of plastics material, said extrusion head comprising a cylinder and a mandrel. Said cylinder coaxially covers said mandrel to form an annular chamber between said cylinder and said mandrel. Said annular chamber has an upstream portion, in which said cylinder and said mandrel are joined together, and, opposite thereto, a downstream portion extended by a pair of circular lips opening out onto the outside of said extrusion head. Said extrusion head comprises at least one inlet opening, for injection of said softened plastics material into the inside of said upstream portion, said softened plastics material being fit to flow into said upstream portion, forming a plurality of flows of softened plastics material. These flows are fit to re-merge towards said downstream portion according to junction zones so as to preform a sleeve of softened plastics material, said sleeve of softened plastics material being intended to be drawn axially into said downstream portion in order to pass continuously through said pair of lips and form said tubular sheath outside said extrusion head. Said annular extrusion head additionally comprises at least one filter having meshes of a diameter less than one millimetre. According to the invention, the annular extrusion head additionally comprises three dispersion grates held spaced apart inside the upstream portion of the annular chamber by means of calibrated spacers, said three dispersion grates extending circumferentially between said mandrel and said cylinder, and radially from said mandrel to said cylinder, where the at least one filter is disposed against one of said three dispersion grates; and said sleeve of softened plastics material is intended to be forced through said three dispersion grates and said filter in such a way as to fragment and disperse said softened plastics material of said junction zones inside said sleeve of softened plastics material.

One characteristic of the invention thus lies in the use of an assembly comprising a dispersion grate and a filter transversely to the flux of softened plastics material so as to cause a disturbance of the flow of this flux at the level of the dispersion grate and, hence, a fragmentation and dispersion of the softened plastics material of the junction zones into the thickness of the sleeve. For example, the filter is disposed downstream of the dispersion grate and forms a crown, whose average diameter of the open meshes ranges between 30 and 1000 pm and advantageously between 450 and 750 pm. In this way, the weld surfaces, which appear in the sleeve of softened plastics material when the flows re-merge and which usually persist in the extruded tubular sheath, are here avoided by virtue of a local mixing of the softened plastics material at the level of the junction zones. Of course, the flux of softened plastics material constituting the sleeve, which flux passes through the dispersion grate, is divided into a plurality of small flows which nevertheless re-merge immediately after having passed through the grate. Materials with very high molecular weight are difficult to extrude in operating conditions of the extruder. Processing aid system agents are thus incorporated in these compositions to facilitate sliding of the material on the walls of the extruder, thus allowing the pressure to be lowered. Now, it has been shown that these processing aid system agents tend to give rise to the sheath fractures. Due to the viscosity of the processing aid system agent, which viscosity is weaker than that of the material to be extruded, they tend to be found in heavy concentration at the level of the junction of the two main material fluxes, thus explaining the defects at the level of the junctions of the two main material fluxes. The local mixing of the plastics material here operates on a scale sufficiently fine to ensure dispersion of the molecules of processing aid system agents in the polymer matrix, in particular by virtue of the filter, the meshing of which is less than 1 mm and ranges between 30 and 1000 pm. The filters are made of interwoven metal wires, defining meshes. These open meshes define passageways characterized by an average diameter, and it is this latter which must be less than 1 mm.

Thus, in view of the temperature and pressure conditions of the softened plastics material, and since the flux is divided as it passes for a relatively short instant through the assembly constituted by the grate and the filter, the diffusion of this softened plastics material at the interfaces of the small flows of the plurality of small flows permits a mixing of the softened plastics material, which does not cause the appearance of a weld surface. On the other hand, that is not the case for the plurality of flows which are formed in the annular chamber after the inlet opening of the extrusion head, for these flows make their way separately for relatively long periods of time around the mandrel, and then re-merge. At the instant at which they precisely re-merge, their temperature and pressure conditions cannot be entirely identical, which could give rise to the appearance of the weld surfaces and hence of the lines which appear on the extruded tubular sheath.

According to a particularly advantageous embodiment of the invention, said at least one dispersion grate comprises a crown having two mutually opposing faces, said crown having a plurality of axial holes passing through said crown from one face to the other. As for the circularly shaped filter, it fits against one of the two opposing faces of the crown. Thus, the crown and the filter are coaxially installed between the mandrel and the cylinder and totally obstruct the annular chamber along a straight section. In this way, the flux of softened plastics material of the sleeve which flows axially into the annular chamber is forced through the axial holes of the crown, and then is fragmented into a plurality of small fluxes as it passes through the meshing of the filter. The mesh size is determined between 30 and 1000 pm to ensure adequate dispersion of the particles of the processing aid system agents, in the order of magnitude of 20 to 100 pm, into the plastics material matrix. In order to sufficiently disturb the flow of softened plastics material in order to sufficiently homogenize this plastics material at the level of the junction zones, said crown preferably has a number of holes ranging between 60 and 140 holes per 100 cm2. For example, the crown has 0.95 holes per square centimeter. In this way, the softened plastics material of the junction zones is fragmented and dispersed into the mass of plastics material of the sleeve, without, however, giving rise to excessive pressure losses inside the annular chamber. Moreover, by virtue of this distribution of holes, the plastics material, whatever it may be, is not denatured, and the material of the tubular sheath which has thus been realized retains its physical properties. Preferably, said plurality of holes corresponds to an open surface representing between 30 and 40% of said opposing faces, which is favorable in order to mitigate the pressure losses. Moreover, said holes are advantageously of cylindrical rotational symmetry and the ratio of their length to their diameter preferably ranges between 2 and 5, by which a first fragmentation of the softened plastics material, and hence then a better dispersion into the mass of plastics material of the sleeve, can be ensured.

According to one particular embodiment of the invention, the extrusion head comprises at least two dispersion grates held at a distance apart and angularly offset one to another so as to form chicanes in said upstream portion. Thus, as regards the crowns, for example, their holes are not respectively spread facing one another, but rather are offset to form chicanes and thus increase the disturbance of the flow. Hence, the softened plastics material of the junction zones is yet more fragmented and dispersed.

According to a particularly advantageous alternative embodiment of the invention, said mandrel has a helical distribution channel, which extends from said upstream portion towards said downstream portion in order to guide the softened plastics material from said upstream portion towards said downstream portion.

Thus, under the effect of the pressure of the softened plastics material at the level of the inlet opening of the extrusion head, the plastics material, in addition to the flows which it forms by borrowing priority passageways, is also guided through the helical distribution channel to be driven towards the downstream portion of the annular chamber and towards the tool. Moreover, said mandrel and said cylinder extend in parallel in a substantially frustoconical manner at the level of the upstream portion of said annular chamber, so as to form a substantially frustoconical annular chamber converging towards said downstream portion of said annular chamber.

Advantageously, and according to another particular embodiment of the invention, the extrusion head comprises an inlet opening, so as to form the sleeve of softened plastics material very early in the upstream portion of the annular chamber, in the direction of flow from the upstream portion towards the downstream portion, such that the sleeve of softened plastics material is fully constituted when it meets the downstream portion of the annular chamber.

According to one particular embodiment of the invention, said downstream portion of said annular chamber forms a free space of cylindrical symmetry, which extends axially from said dispersion grate to said pair of circular lips over a distance greater than the axial length of said upstream portion. And what is more, said free space of cylindrical symmetry advantageously has a thickness greater than the thickness of said upstream portion of said annular chamber. As will also be explained in greater detail in the remainder of the description, this available free space is thus sufficiently long and thick to allow perfect homogenization of the softened polymer, which then flows in a turbulent flow pattern after having passed through the grate and before being extruded through the lips.

Other particularities and advantages of the invention will emerge from a reading of the below-stated description of a particular embodiment of the invention, given on an indicative yet non-limiting basis, with reference to the appended drawings, in which: - Figure 1 is a schematic view, in axial section, of an annular extrusion head in accordance with the invention; - Figure 2A is a schematic front view of an element of the extrusion head illustrated in Figure 1; and - Figure 2B is a schematic view, in vertical section, of the element represented in Figure 2A along a plane perpendicular to that of this figure and such as it appears in Figure 1.

Figure 1 illustrates in axial section an annular extrusion head 10 in accordance with the invention. It comprises a mandrel 12 of circular symmetry and of axis A, which mandrel 12 has an axial passage 14 on the inside and an outer surface 16 to the outside. The mandrel 12 is covered with a body 18, forming a cylinder or a jacket, which body has an inner surface 19. The mandrel 12 engaged coaxially inside the body 18 allows a continuous annular chamber 20 to be formed between the outer surface 16 of the mandrel 12 and the inner surface 19 of the cylinder 18. The annular chamber 20 extends axially from an upstream portion 22 towards a downstream portion 24, to end in a tool 26 formed by a pair of coaxial circular lips. It will be noted that the upstream portion 22 and the downstream portion 24 extend axially over similar distances and that the thickness of the annular chamber in the downstream portion 24 has an average thickness greater than that of the upstream portion 22. The advantages of the space available in the downstream portion 24 of the annular chamber 20 will be explained below.

The extrusion head 10 in accordance with the invention thus allows a tubular sheath made of plastics material to be formed on the surface of an object of circular rotational symmetry, which object is engaged through an inlet 28 of the extrusion head 10 and is movable in translatory motion inside the axial passage 14, so as to leave at the outlet 30 of the extrusion head 10 vertically in line with the pair of lips 26. As the object is driven inside the axial passage 14, a sleeve made of softened plastics material is extruded through the lips 26 and is deposited on this object. The annular extrusion head 10 is thus suitable for forming a polymer sheath on the surface of the outermost layer of a flexible tubular pipe in the course of construction.

At the level of the upstream portion 22 of the annular chamber 20, the mandrel 12 and the body 18 are frustoconical in shape, such that the inner surface of the body 18 and the outer surface 16 of the mandrel 12 extend parallelly in mutually facing arrangement to form an upstream portion 22 of the annular chamber 20, of substantially frustoconical shape. The mandrel 12 and the body 18, in the upstream portion 22, are flared towards the inlet 28 of the extrusion head 10. Furthermore, the mandrel 12 has, at the level of the upstream portion 22 of the annular chamber 20, a helical groove 31 recessed around the mandrel 12 in the outer surface 16 and forming a distribution channel. This helical distribution channel 31 is relatively deep to begin with and its depth diminishes as the groove approaches the downstream portion 24. A single distribution channel forming at least three turns is envisaged, for example.

Moreover, in the downstream portion 24 and towards the upstream portion 22 of the annular chamber 20, a dispersion grate 32 and an annular filter (not represented) are interposed, the latter being fitted to the dispersion grate 32 towards the downstream portion 24, and the structures of which will be described in greater detail below. In particular, the dispersion grate 32 will be detailed with reference to Figures 2A and 2B, followed by the functioning thereof.

Furthermore, the annular extrusion head 10 has an inlet opening 34 which is apparent in the figure and through which is injected a softened plastics material, of the thermoplastic polymer type, including, for example, polypropylene, polypropylenediene ethylene and various compatibilizing agents. Another example of the type of thermoplastic polymer suitable for use is a fluorine-based polymer, for example PVDF. The annular extrusion head 10 has another inlet opening, which does not appear here in the figure. The extrusion installation is a traditional installation, with an endless screw encased in a packing or heating cylinder. The endless screw ends in an injection nose directly connected to the inlet opening 34.

Thus, the softened plastics material is injected under pressure through the inlet opening 34, and it then flows precisely inside the distribution channel 31. In this way, the mass of softened plastics material divides into a plurality of flows, and, in particular, into two basic flows, along the distribution channel 31, which tends to drive the mass of softened plastics material, according to a circumferential component and an axial component, towards the downstream portion 24 of the annular chamber 20. The two aforementioned basic flows remerge, in particular, in the helical groove 31, according to junction zones, and the softened plastics material of these junction zones then pursues its path around the mandrel 12.

Moreover, the softened plastics material flows likewise according to other flows outside of the helical groove 31 and between the outer surface 16 of the mandrel 12 and the inner surface 19 of the body 18, which surfaces are brought closer together. Under the effect of the injection pressure of the softened plastics material, the flows of this latter between the two aforementioned surfaces tend to re-merge likewise in junction zones towards the downstream portion 24 of the annular chamber 20. It will also be noted that, due to the distribution channel 31, a helical motion favorable to the mixing of the different flows and to the formation of a sleeve of softened plastics material can be imparted to the total mass of softened plastics material situated between the two aforementioned surfaces. Furthermore, by virtue of the frustoconical shape of the annular chamber 20 in the upstream portion 22, the mass of softened polymer tends to compress, from the inlet opening 34 towards the downstream portion 24.

Thus the sleeve of softened and compact plastics material, in this case constituted in continuous form, is forced through the dispersion grate 32, which latter will be described below with reference to Figures 2A and 2B, and through the filter.

The dispersion grate 32 represented in front view in Figure 2A is constituted by a crown 36, having two opposing faces, a front face 35 and a rear face 37. According to one particular embodiment, the crown 36 has an inner diameter and an outer diameter ranging between 200 and 500 mm, for example an inner diameter of 332 mm and an outer diameter of 400 mm. The crown 36 which is represented here comprises three circumferential rows of holes 38, an inner row 40, an outer row 42, and an intermediate row 44 situated between the two other rows. These correspond in each case to three different diameters, 350 mm, 369 mm and 388 mm. The holes have a diameter ranging between 8.5 and 5.5 mm, for example 7 mm. They are spaced evenly apart and their density is close to one hole per square centimeter. Thus the percentage of open surface in relation to the surface of the faces 35, 37 is close to 35%.

According to a particularly advantageous embodiment of the present invention (not represented), the dispersion grate 32 is rotationally mounted in the annular chamber 20, around the axis of symmetry A. Geared rotational driving means are in this case installed in the thickness of the body 18.

Furthermore, it will be noted in Figure 2B that the crown 36 has a substantial thickness ranging between 20 mm and 40 mm, for example 25 mm. Also, the length of the holes is substantially equal to 3.5 times their diameter.

As regards the filter, it is annular in shape and has the same dimensions as the crown 36 so as to be able to fit against the whole of the surface of the rear face 37 of this crown. This filter is cut, for example, from a gauze of woven metal wires. The metal wires are made of stainless steel, for example, and have a diameter ranging between 0.63 and 0.40 mm. The wires of the gauze are, for example, contiguous in the direction of the warp and/or contiguous in the direction of the weft, and the weave is plain or twilled. These wire gauzes thus define nominal openings less than 1 mm in size.

With reference to Figure 1, it will thus be understood that the preformed sleeve of softened plastics material in the upstream portion 22 of the annular chamber 20 is driven continuously through the holes 38 of the crown 36. Thus, in contact with the front face 35 of the crown 36, the flux of softened plastics material of the sleeve divides into as many small flows as there are holes 38. In this way, the softened plastics material is fragmented into as many parts as there are holes 38. These parts are then driven through at least one dispersion filter, comprising mesh diameters ranging between 30 and 1000 pm, which mesh diameters ensure an effective dispersion of the particles of compatibilizing agent in the PVDF matrix such that these different small fluxes of plastics material are dispersed and mixed at the outlet on the rear face 37 of the crown. In this way, the plastics material of the junction zones founded on the flows of softened plastics material in the upstream portion 22 is fragmented and dispersed into the mass of the plastics material during passage through the crown 36. Thus this device avoids the local over-concentration of compatibilizing agent in the PVDF matrix by adequate dispersion of the agent by virtue of the dispersion filter, which latter comprises a fine meshing with mesh diameters ranging between 30 and 1000 pm. Thus the junction zones, which generally form interfaces where the material is not under a single phase but under two phases, are at least partially destroyed following passage through the crown 36.

In this way, those junction zones which constitute weld surfaces of a nature to fragilize the plastics material of the finally extruded tubular sheath are then destroyed during the extrusion. The tubular sheath covering the flexible pipe thus appears without weld line or weld surface and then has homogeneous mechanical properties throughout the plastics material.

According to another embodiment of the invention (not represented), the extrusion head no longer comprises one dispersion grate, but three dispersion grates held spaced apart inside the upstream portion of the annular chamber by means of calibrated spacers. Moreover, these dispersion grates are respectively angularly offset in relation to one another, so as to form chicanes in said upstream portion. In this way, the sleeve of softened plastics material which passes successively through the three dispersion grates, thereby forming chicanes, is even more fragmented and dispersed, in particular at the level of the junction zones.

Moreover, by virtue of the aforementioned available space in the downstream portion 24 of the annular chamber 20, after the dispersion grate 32 in the direction of flow of the polymer and of the sleeve of softened plastics material, and in particular for polymer compositions of very high molecular weight, the aforementioned small flows of the flux of softened plastics material reunite and merge more easily together to form just a single phase, which can then flow through the tool between the lips of the pair of lips 26. The conditions for optimal mixing are obtained by adjusting the pressure inside the annular chamber 20, which pressure determines the dwell time of the softened plastics material inside the downstream portion 24 of the annular chamber 20, and take the form of a turbulent flow pattern. Thus, by maintaining the pressure in the downstream portion 24 at a value ranging between 30 and 100 bar, for example between 35 and 50 bar, the mixing of the plurality of flows is optimized and the dwell time inside this upstream portion 22 ranges between 5 and 30 seconds, for example between 15 and 20 seconds.

By way of example, the conditions of use of the annular extrusion head forming the subject of the invention, for cladding a six inch carcass of a flexible pipe with a PVDF type material over a 7 mm thickness and at a cladding speed of 0.64 m/min, will be described.

In order to do this, the extrusion screw has a Gina-type profile, medium head K. The temperature of the hopper is 70°C, whilst the material flow rate is 540 kg per hour. The feed pressure is 500 bar, whilst at the end of the screw the pressure of the softened polymer is 277 bar and its temperature 250°C.

Moreover, the die comprises a mandrel of the coat hanger type and the extrusion head comprises three dispersion grates, of the type of those previously mentioned, and, what is more, between each of the grates, three crown-forming circular filters, for which the diameter of the meshes is respectively 710 pm, 480 pm and 710 pm. The first filter is fitted against the face of the first grate orientated towards the upstream portion 22, whilst the two others are interposed between the grates. Thus the diameter of the meshes of the filters which fit facing the holes is close to one- tenth of the diameter of these holes. The pressure prevailing in the downstream portion 24 of the annular chamber 20 is then 78 bar.

Thus, in extrusion conditions, the sheath has no surface defect and no weld line is apparent.

Furthermore, and according to another embodiment of the invention, the crown of the dispersion grate has holes over a defined annular sector, for example ranging between 40° and 90°. Thus, by adjusting the defined annular sector into a position diametrically opposite the inlet opening, no weld zone is apparent.

Claims

An annular extruder head (10) for forming a tubular sheath made of plastics material, said extruder head comprising a cylinder (18) and a mandrel (12), said cylinder covering the mandrel coaxially to form an annular chamber (20) between the barrel and mandrel the annular chamber having an upstream portion (22) in which the barrel and mandrel are interconnected and, conversely, a downstream portion (24) which is elongated by a pair of circular lips (26) which open onto the extruder head. outside, the extruder head (10) comprising at least one inlet opening (34) for injecting the plasticized plastic material into the interior of the upstream portion (22), said plasticized plastic material being adapted to flow into the upstream portion forming a plurality of streams of plasticized plastic material which is adapted to be reunited in the direction of the downstream portion (24) in connection zones in such a manner as to form a sleeve a. f plasticized material, said plastic material socket being intended to be axially pulled into the downstream portion (24) to pass continuously through the pair of circular lips (26) to form the tubular sheath outside the extruder head, further comprising at least one filter having meshes less than one millimeter in diameter, characterized in that it further comprises: - three spreader tines held apart within the upstream portion of the annular chamber by means of calibrated spacers extending three spreader tines in the circumferential direction between the mandrel (12) and the cylinder (18) and radially from the mandrel in the direction of the cylinder, - and that at least one filter is positioned against one of the three spreading rakes, - and that the sleeve of plasticized plastic material is intended for being forced through the three spreading rakes (32) and the filter in such a way that the plastic zones of the connecting zones are softened iale is fragmented and dispersed in the sleeve of plasticized plastic material.

Extruder head according to claim 1, characterized in that one of the spreader ridges (32) comprises a crown (36) having two mutually opposite faces (35, 37), the crown having a plurality of axial holes (38), which passes through the wreath from one side to the other.

Extruder head according to claim 2, characterized in that the wreath (36) has a number of holes (38) lying in the interval between 60 and 140 holes per 100 cm 2.

Extruder head according to claim 2 or 3, characterized in that the number of holes (38) corresponds to an open surface representing between 30 and 40% of the facing surfaces.

Extruder head according to any of claims 2 to 4, characterized in that the holes (38) are cylindrically rotationally symmetrical and the ratio of the length of their diameter is in the interval between 2 and 5.

Extruder head according to any one of claims 1 to 5, characterized in that the three spreader blades (32) are held at a distance from one another and rotated relative to each other in such a way as to create harassment in the upstream part (24).

Extruder head according to any one of claims 1 to 6, characterized in that the mandrel (12) has at least one helical distribution channel (31) extending from the upstream part (22) in the direction of the downstream part (24) in order to guide the plasticized plastic material from the upstream portion towards the downstream portion.

Extruder head according to any one of claims 1 to 7, characterized in that the mandrel (12) and the cylinder (18) extend parallel in a substantially cone-shaped manner at the level of the upstream part (22) of the annular chamber (20), such that a generally cone-shaped annular chamber is formed which converges towards the downstream portion (24) of the annular chamber.

Extruder head according to any one of claims 1 to 8, characterized in that the downstream portion (24) of the annular chamber (20) forms a free space of cylindrical symmetry extending axially from the spreader (32) to the pair of circular lips (26). over a distance greater than the axial length of the upstream portion (22).

Extruder head according to claim 9, characterized in that the free space of cylindrical symmetry has a thickness greater than the thickness of the upstream portion (22) of the annular chamber (20).