EP3283271A1

EP3283271A1 - Extrusion die for extruding agri-food materials and extrusion machine comprising such a die

Info

Publication number: EP3283271A1
Application number: EP16719268.1A
Authority: EP
Inventors: Daniel Durand; Michel LAYES; Emmanuel LAVOCAT
Original assignee: Clextral SAS
Current assignee: Clextral SAS
Priority date: 2015-04-13
Filing date: 2016-04-12
Publication date: 2018-02-21
Also published as: FR3034702A1; FR3034702B1; WO2016166102A1

Abstract

The invention relates to an extrusion die (30) which includes at least one insert (31, 32) for shaping a material to be extruded pushed through the die, said insert being tubular, being centred on an axis (X31, X32), and having both an upstream end (31A, 32A), via which the material enters the insert, and a downstream end (31B, 32B), via which the material exits to the outside of the extrusion die. In order for said extrusion die to make it possible, even with a modest flow of extruded material, to obtain large extruded products having a regular surface, it is provided for the material to flow in a cylindrical channel (C31, C32) of the insert, connecting the upstream and downstream ends thereof, which has a substantially constant cross section between a pre-expansion diaphragm located at the upstream end, not included, and the downstream end, included. The diaphragm defines a through-opening (O37, O38), substantially centred on the axis (X31, X32) and having a smaller cross section than that of the channel, so that, before flowing in the channel, the material entering the insert passes through the diaphragm, undergoing a loss of load which, at the outlet of the diaphragm, pre-expands the material by distributing same homogeneously over the entire inner periphery of the channel. Moreover, the extrusion die includes means (310, 320) for controlled heating of the insert, suitable for keeping the temperature of the material pre-expanded by the diaphragm substantially homogeneous along the channel of the insert.

Description

Extrusion die for extruding agri-food materials, as well as an extrusion machine comprising such a die

The present invention relates to an extrusion die for extruding agri-food materials, and an extrusion machine comprising such a die.

The invention is concerned with extrusion machines comprising a sheath, generally thermoregulated, inside which one or more screws, in particular two screws, are rotated on themselves so as to cause a material to be extruded. from an upstream portion of the sleeve to the downstream end of the sleeve where the material is then forced to flow through an extrusion die, provided to shape the extruded material. In such an extrusion machine, the material undergoes both a mechanical transformation, by pressurization and shearing by the screws, and a thermal transformation, by regulating the temperature along the sheath.

Inside the die, the material flows into one or more shaping channels, which open on the outside of the extrusion machine. In these flow channels, the material is no longer worked by the screws so that the temperature of this material is no longer homogenized between the core of the flow of material and its periphery, the latter tending to be less hot than the heart. As long as the extruded product leaving the die is small, that is to say as long as its cross section is much less than about 1 mm ² , the temperature difference can appear between the core and the periphery of the die. material flowing in the die remains limited and has no significant impact on the product: at the outlet of the die, the product forms a bead having a cylindrical or quasi-cylindrical shape, which can be cut in a calibrated manner by means of ad hoc cutting systems, placed just downstream of the die, to produce pellets. On the other hand, for products having a larger size, the material flowing in the channels of the die has, transversely to the direction of flow, a temperature gradient such that the core of the material flow has a significantly lower viscosity. to that of its periphery, which gives to the extruded material a linear speed greater at heart than periphery: it follows that the product leaving the die is no longer cylindrical, but has an ogival shape, incompatible with a calibrated cut.

However, there is demand from the agri-food market for large extruded products, particularly large diameter pellets. These products serve as food for large fish such as tuna, halibut, amberjacks, etc. Similarly, these products are required for large dogs that can to eat them either for food or as treats that dogs take pleasure in chewing and / or chewing.

Currently, to circumvent the problem of the lack of homogeneity of the temperature in the flow of material passing through the die, the large extruded products are obtained by means of the operation of the extrusion machine with very high extruded material flows, by example of the order of several tons per hour. In this way, the channels of the die are "saturated" with material flowing at high speed. However, for obvious economic reasons, the implementation of this provision is excluded if the need for a large product is limited. In addition, this provision does not solve or even accentuate another problem related to large extruded products, ie at the interface between the material flowing in the channels of the die and the inner surface of these channels. friction phenomena occur in a heterogeneous way: the material adheres more in some regions of this interface than in others. Indeed, due to the large cross section of the channels, the flow of extruded material does not flow strictly in the axis of the channels, but tends to flow slightly off-axis. As a result, at the outlet of the channels of the die, the extruded material bead has an irregular surface appearance, in particular cracked, which can induce poorly calibrated cuts, create fines and lead to potentially fragile pellets during subsequent manipulations.

For its part, EP-A-0 898 890 discloses an extrusion die comprising at least one pair of concentric tubular flow channels. The two channels of the same pair are fed by the same distribution chamber and have concentric outlet orifices, one of these two orifices being central while the other is annular. The extruded product, intended for human or animal food, has the specificity of having a ring shape surrounded by a ring. To obtain this specific shape at the outlet of the die, each channel of the same pair is, on the one hand, provided, at its upstream end, with a pre-expansion diaphragm and, on the other hand, provided with a taper converging towards the downstream. This convergence strangles the flow section of the material, so that the technical teaching of EP-A-0 898 890 is incompatible with the production of large expanded agri-food products such as those mentioned above.

The object of the present invention is to provide an extrusion die that allows, even with a moderate extruded material flow, to obtain extruded food products of large size with regular surface.

For this purpose, the subject of the invention is an extrusion die for extruding agri-food materials, as defined in claim 1. In the extrusion die according to the invention, the incoming material passes, before joining the flow channel of the or each insert of this die, first its upstream diaphragm, that is to say a restriction , located at the upstream end of the channel, the cross section of this channel: the loss of pressure caused by this diaphragm induces an expansion of the material in the flow channel, immediately downstream of the diaphragm. This expansion is described as preexpansion not to be confused with expansion or, more generally, the increase in volume experienced by the material leaving the sector, that is to say from the inside to the outside. outside the downstream end of the channel. The preexpansion of the material resulting from its passage through the diaphragm allows the flow material downstream of the diaphragm to conform to the internal surface of the flow channel evenly over the entire periphery of this surface. In other words, this preexpansion leads to the material coming out of the diaphragm coming to "flatten" homogeneously over the entire inner periphery of the wall of the insert, delimiting the channel. This homogeneous distribution effect of the material over the entire inner periphery of the flow channel of this material is obtained even if the flow rate of the material is modest, for example of the order of 500 kg / hour, or less . Immediately downstream of the diaphragm and up to its downstream end included, the channel of the die according to the invention is cylindrical and has a constant cross section, the contour remains so matched by the flow of pre-expanded material. At the same time, the temperature of the preexpanded material thus flowing in the channel is maintained homogeneous between the core of the material flow and its periphery, under the effect of heating the insert by the controlled heating means. These means make it possible to regulate the temperature of the wall of the insert, delimiting the channel: by contact, this wall of the insert transmits heat to the peripheral region of the material flow of the channel, so that, with the adjustment From the control temperature, the temperature of the material can be kept homogeneous throughout the canal section at any point along the canal. As a result, the viscosity of the preexpanded material flowing in the channel is kept homogeneous over its entire cross section, so that the material offers, at the downstream end of the channel, a homogeneous output speed: the material bead outgoing has a particularly cylindrical shape, for example circular base. Associated with the fact that, due to pre-expansion, the surface of this cylindrical form of the bead is free of irregularities, such as cracks or other similar "skin" phenomena, the cylindrical shape of the bead is easily cut and regular, directly out of the die.

It is understood that all the foregoing technical considerations relating to the invention find a particularly advantageous application in the case where the cross section of the flow channel is important, typically equal to or greater than 1 15 mm ² , or more: thus, the extrusion die according to the invention is used for the production, including moderate flow, of agri-food products Extruded large size, with good surface quality. In practice, the contour of the extruded extrudate at the outlet of the die is not limited to a circle, this contour may indeed have sophisticated shapes, such as a bone shape or a form of fish, or, more generally, animal. Thus, the invention makes it possible to manufacture large extruded agri-food products, with an extrusion machine of modest capacity: the die according to the invention increases the economy of this manufacture and allows a production company to propose products, including sophisticated, high-quality feeds, without having to spend large sums on a machine with a capacity of several tons per hour.

Additional advantageous features of the extrusion die according to the invention are specified in the dependent claims.

The invention also relates to an extrusion machine, comprising:

a sleeve inside which is driven at least one drive screw of a material to be extruded, and

an extrusion die, which is as defined above and through which the material leaving the sheath is pushed by the drive screw or screws.

In practice, the invention applies, without limitation, to various extrusion machines, including both single-screw machines and twin-screw machines, the two screws of these are counter-rotating or co-rotating. presses.

The invention will be better understood on reading the description which follows, given solely by way of example and with reference to the drawings in which:

- Figure 1 is a perspective view of an extrusion machine according to the invention;

- Figure 2 is a partial schematic longitudinal section of the extrusion machine of Figure 1, in the plane II of Figure 1;

- Figure 3 is a partial section along the line III-III of Figure 2;

- Figure 4 is a perspective view of a die, shown alone, belonging to the machine of Figure 1; and

FIG. 5 is a view on a larger scale of the box detail V in FIG. 2. FIGS. 1 to 5 are diagrammatically represented an extrusion machine 1. This extrusion machine 1 comprises a sheath 10 of elongated shape, which extends along a geometric axis XX and which is centered on this axis. Inside the sleeve 10, two screws 20 extend parallel to the axis XX, being received in a complementary longitudinal bore of the sleeve, centered on the axis XX. In practice, in a manner known per se, each screw 20 includes for example a central screw shaft 21 on which is mounted a set of screw elements 22. The screws 20 extend on either side of the axis XX, while being interpenetrating, the bore of the sleeve thus having a bilobed transverse profile. These screws 20 are driven in rotation about themselves, around their central axis, by a drive unit, not shown in the figures, in mechanical engagement with the upstream end of the screws, namely that of the right in FIGS. 2, emerging outside the sheath 10.

The screws 20 of the extrusion machine are designed, by their threaded profile, to cause a material to be extruded along the axis XX, from an upstream part of the sleeve 10, in which the ingredient or ingredients of this material are introduced into the aforementioned bore of the sleeve, to the downstream end of the sleeve 10, the terms "upstream" and "downstream" being oriented in the direction of progression of the material inside the sleeve under the action of the screws 20 , this direction of progression being from right to left in FIGS. 1 to 3 and 5. Furthermore, in a manner known per se and as mentioned in the introductory part of this document, the screws 20 are designed to, in addition to cause the material to extrude, shear and pressurize this material, so as to transform it mechanically. This aspect of the extrusion machine 1 being well known in the art, it will not be described here further.

The sheath 10 comprises several modular elements succeeding each other along the axis X-X, which are here five in number, being respectively referenced 1 1 to 15 from upstream to downstream. Each of the elements 1 1 to 15 delimits internally a corresponding portion of the central longitudinal bore of the sleeve 10, these boring portions being in the extension of each other, along the axis XX, in the assembled state of the elements 1 1 to 15, as in the figures. In practice, the elements 1 1 to 15 are assembled in pairs by fastening collars 16.

In the exemplary embodiment considered in the figures, the element 1 1 most upstream allows to introduce, within its central bore portion, one or more agri-food ingredients of the material to be extruded. For this purpose, in a manner known per se and not detailed here, this element 1 1 is provided with a through hole 1 1 A, which, transversely to the axis XX, opens on the outside the central bore portion of Element 1 1. More generally, it is understood that among the elements 1 1 to 15 of the sheath 10, one or more of them allow to introduce, within the longitudinal bore central sleeve 10, the or the solid and / or liquid ingredients, the material to be extruded by the machine 1. Similarly, as non-limiting and known options, steam can be directly injected into the material being extruded into the sheath by at least one of the elements 1 1 to 15 and / or degassing of the material can be provided by at least one of these elements, which is then associated or not with a backflow system of the material using an ad hoc screw.

At its downstream end, the sheath 10 comprises an end plate 17, commonly called a front plate, which is fixedly attached, for example by a clamping collar 18, to the downstream end of the element 15, the most downstream of the sheath 10. In a manner known per se, this plate 17 defines internally a through bore, which is centered on the axis XX and which extends in the extension, along this axis, of the central bore portion of the element 15. As clearly visible in FIG. 3, the free downstream end of the screws 20 is received inside the central bore of the plate 17. This bore of the plate 17 channels the material pushed downstream by the 20. This aspect of the extrusion machine 1 is not limiting of the invention and knowledge of the field, it will not be described here further.

The machine 1 also comprises an extrusion die 30 arranged at the downstream end of the sleeve 10. The die 30 is designed to shape the material extruded by the machine 1, this material being forced under the action of the screws 20 , to flow through the die 30. For this purpose, the die 30 comprises tubular inserts for shaping the material passing through the die, these inserts being two in number in the embodiment considered in FIGS. and being referenced 31 and 32.

The inserts 31 and 32 are centered on respective axes X31 and X32, parallel to the X-X axis. As clearly visible in FIGS. 1 and 2, the inserts 31 and 32 are arranged on either side of the axis XX, in other words diametrically opposite along the periphery of the die 30. Of course, as variants not shown, a different number of inserts can be provided within the extrusion die 30: in particular, more than two inserts can be envisaged, distributed substantially uniformly about the axis XX.

As clearly visible in FIG. 5, the tubular wall of each insert 31, 32 has an upstream end 31A, 32A, through which the material passing through the die 30 is admitted inside the tubular wall of the insert. In addition, the tubular wall of each insert 31, 32 has a downstream end 31B, 32B through which the material passing through the die 30 exits the outside of the tubular wall of the insert and is thus evacuated outside the the die, the downstream ends 31 B and 32B of the inserts 31 and 32 being the parts of the die 30, the most downstream. The tubular wall of each insert 31, 32 also has an inner face 31 C, which extends, along the axis X31, X32, from the upstream end 31A, 32A to the downstream end 31B, 32B and which delimits a channel C31, C32 for the flow of the material passing through the die 30, directly connecting the above-mentioned upstream and downstream ends to each other. Each flow channel C31, C32 is cylindrical and centered on the axis X31, X32.

In addition, the tubular wall of each insert 31, 32 has an outer face 31D, 32D, which extends, along the axis X31, X32, between the upstream ends 31A, 32A and downstream 31B, 32B of this tubular wall, opposite its inner face 31 C, 32C. At least a portion of the outer face 31D, 32D of each insert 31, 32, in this case the upstream portion of this outer face, cooperates with a body 33 of the extrusion die 30 for the purposes of fixing the inserts 31 and 32 vis-à-vis this body 33. In the embodiment considered here, as clearly visible in Figure 5, the body 33 comprises a housing 34 which, at the same time, cooperates by complementarity of shapes with the upstream portion of the outer faces 31 D and 32D of the inserts 31 and 32 and which is fixedly attached, for example by a fastening collar 19, to the downstream end of the plate 17. More generally, it is understood that the body 33 of the die 30 fixedly supports the inserts 31 and 32, the latter being preferably related to the body 33, where appropriate interchangeably, but may, alternatively, be permanently integrated, where appropriate in a monobloc manner, the body of the die.

The downstream end of the central bore of the plate 17 is connected to the upstream end

31 A, 32A inserts 31 and 32 from the inside of the housing 34, specifically via a chamber C33 delimited between them a conical diffuser 35 and a central diffuser 36 of the body 33. The diffusers 35 and 36, both centered on the axis XX, are arranged in a concentric manner and are shaped so that the chamber C33 distributes the material, which enters the die 30 centrally on the axis XX, towards the periphery of the body 33, thus distributing this material in the respective upstream ends 31A and 32A inserts 31 and 32. Of course, the specificities of the diffusers 35 and 36 are not limiting of the invention.

The distribution chamber C33 is connected to the flow channels C31 and C32 of the inserts 31 and 32 via the upstream ends 31A and 32A of these inserts, at which respective diaphragms 37 and 38 are provided. Each diaphragm 37, 38 is designed to locally constrict the passage section between the chamber C33 and the corresponding flow channel C31, C32. For this purpose, each diaphragm 37, 38 defines a through opening 037, 038, which is substantially centered on the axis X 31, X 32 of the corresponding insert 31, 32 and whose cross section, that is to say the section in a plane perpendicular to this axis X31, X32, is smaller than the section cross section of the corresponding flow channel C31, C32 and that of the corresponding downstream outlet of chamber C33. The cross section of each opening 037, 038 is preferably circular in profile, centered on the corresponding axis X31, X32.

As clearly visible in FIG. 5 for the exemplary embodiment considered here, each diaphragm 37, 38 delimits an upstream surface 37A, 38A, which is convergent downstream, for example being conical, and which connects the corresponding downstream outlet. from the chamber 033 to the corresponding through opening 037, 038. Each diaphragm 37, 38 also delimits a downstream surface 38B, 38B, which connects the corresponding through opening 037, 038 to the corresponding flow channel 031, C32, either flaring progressively downstream, or being inscribed in a geometric plane substantially perpendicular to the axis X31, X32 as in the embodiment considered in the figures.

In practice, various embodiments can be envisaged for the diaphragms 37 and 38. Furthermore, in the embodiment considered in the figures, the diaphragms 37 and 38, although functionally similar, are not structurally identical:

- The diaphragm 37 is formed as a separate part of the insert 31, which is arranged completely inside the upstream end 31A of the insert 31, by embedding, for example conical; and

the diaphragm 38 is made in the form of a piece distinct from the insert 32, which is arranged in full overflow from the upstream end 32A of the insert 32, by axial support of the respective end edges of the diaphragm 38 and of the tubular wall of the insert 32.

Of course, as a variant not shown, the diaphragm may be arranged partially inside and partial overhang of the upstream end of the insert. Similarly, rather than providing the diaphragm in the form of a separate part of the insert, a variant not shown is to achieve integrally the insert and the diaphragm, by means of an ad hoc machining of the corresponding single part.

Downstream of each diaphragm 37, 38, the corresponding flow channel 031, C32 has a cross section, that is to say a section in a geometrical plane perpendicular to the corresponding axis X31, X32, which is constant until at the downstream end 31B, 32B of the corresponding insert 31, 32. In other words, more precisely, the cylindrical shape that each channel 031, C32 has has its cross section which is constant between the corresponding diaphragm 37, 38, not included, and the downstream end 31 B, 32B of the corresponding insert, included. In the exemplary embodiment considered in the figures, the channels 031 and C32 are cylindrical with a circular base, so that their Cross-section geometrically constitutes a disk, which is constant along the axis X31, X32, from the downstream surface 37B, 38B of the diaphragm, not included, to the downstream end 31B, 32B, included. According to a preferred dimensioning, associated with obtaining large extruded products such as those mentioned in the introductory part of this document, the diameter of this circular base section, denoted D in FIG. 5, is equal to or greater than 12. mm.

The extrusion die 30 also comprises means 310, 320 for heating in a controlled manner each insert 31, 32, these regulated heating means 310 and 320 being shown only in FIG. 5, schematically. These means 310, 320 comprise a heating element 31 1, 321, such as an electrical resistance or a heat transfer fluid circuit, designed to transmit heat to the tubular wall of the corresponding insert 31, 32. practical and effective embodiment, this heating member 31 1, 312 is attached directly to the outer face 31 D, 32D of the insert, in the downstream portion of this outer face emerging outside the body 33 of the die 30 , As shown in Figure 5. As a variant not shown, the heating member of the regulated heating means 310, 320 is partially or totally integrated with the tubular wall of the insert. In all cases, each heater 31 1, 312 is supplied with a suitable power source, not shown in the figures.

According to a variant, not shown, of embodiment of the regulated heating means 310 and 320, the body 33 of the die 30 defines a circulation chamber of a heat transfer fluid, across which the inserts 31 and 32 extend in such a way that to be at least partly bathed in the fluid of this room. This chamber is provided with an inlet orifice and a heat transfer fluid outlet orifice, so that, for example under the action of a pump, this fluid circulates in a loop in the chamber, sweeping the face external inserts for the part of these arranged in the room. By adjusting the temperature of the coolant, such as water, pressurized water or oil, the inserts 31 and 32 are jointly thermally controlled. In practice, the corresponding thermal regulation is controlled by an apparatus external to the die 30, acting on the coolant circuit connected to the aforementioned chamber.

Whatever their embodiment, the controlled heating means 310 and 320 make it possible to maintain homogeneously, in the cross section of the channels C31 and C32, the temperature of the material flowing in each of these channels. Indeed, the heat provided by these means 310 and 320 to the insert 31, 32 is transmitted to the material flowing in each channel C31, C32, more precisely to the peripheral portion of this material in contact with the inner face 31 C, 32C of the tubular wall of the insert 31, 32. This heat thus transmitted at the periphery of the flow of material in the channel C31, C32 counteracts the tendency that this peripheral part of the material to cool compared to its central region, in other words compared to the core of the flow of material, in the absence of the heating means 310, 320. By balancing, the temperature of the material flowing in the channel C31, C32 is kept homogeneous. For example, a regulation, simple to implement, consists in measuring the temperature of the material leaving the sheath 10, in particular using a probe which is conventionally equipped with the sleeve, and regulating the effect heating means produced by the means 310 and 320 to bring the inserts 31 and 32 to the measured temperature value.

We will now describe the operation of the extrusion machine 1.

The ingredient or ingredients of the material to be extruded are introduced inside the sheath 10, via at least one of its elements 1 1 to 15, and then are driven downstream by the screws 20, while being transformed under the mechanical action of the screws. The material leaving the element 15 is thrust through the plate 17 and the die 30 successively. Inside this die, the material first spreads in the distribution chamber C33 and then flows into the channels C31 and C32, after passing through the diaphragms 37 and 38 via their reduced opening 037 and 038. The pressure drop caused by these diaphragms 37 and 38 induces a pre-expansion of the material in the flow channels C31 and C32, so that downstream of the diaphragms 37 and 38, the mass of pre-expanded material perfectly matches the respective internal faces 31 C and 32 C of these channels: indeed, as the opening 037, 038 of each diaphragm 37, 38 is centered on the axis X31, X32 of the flow channel C31, C32, preexpansion of the material, which occurs immediately downstream of this opening and which is made generally radially to the aforementioned axis, tends to be uniformly distributed around this axis. Thus, the pre-expanded material leaving the openings 037 and 038 is distributed homogeneously over the entire inner periphery of the channels C31 and 032, and all the more in the presence of a circular profile for these openings 037 and 038. This effect of homogeneous distribution on the inner periphery of the channels 31 and 32 is retained from the outlet of the diaphragms 37 and 38 to the downstream end 31 B and 32B inserts 31 and 32 due to the constant cross section of the flow channels 031 and 032. At the same time, the controlled heating means 310 and 320 allow the pre-expanded material to keep a homogeneous temperature all along the flow channels 031 and 032. At the outlet of the die 30, the material is extruded under form of two flanges respectively emerging inserts 31 and 32: each of these strands of extruded material has, on the one hand, a viscosity and thus a homogeneous linear speed between its core and its periphery series and, secondly, a regular outer surface, that is to say free from cracks or similar "skin" phenomena, detrimental to the surface appearance of the extruded product. Such extruded extrudates can, directly at the outlet of the extrusion die 30, be cut in a perfectly calibrated manner, in order to produce pellets.

By providing that the internal diameter of the flow channels C31 and C32 is equal to or greater than 12 mm, each extruded strand may be referred to as a large extruded product, such as those discussed in the introductory part of this document. In this case, to ensure the effective shaping of this flange by the inserts 31 and 32, the length, denoted L in Figure 5, the channel C31, C32, measured axially between the diaphragm 37, 38 not included and the downstream end 31 B, 32B included, is preferably between one and five times the diameter D. It is understood that such a value of the length L, compared to the diameter D whose value is substantial to obtain large extruded products, can to be achieved through the arrangements of the die 30, related to the inserts 31 and 32, the diaphragms 37 and 38 and the controlled heating means 310 and 320.

Furthermore, the die 30 is not limited to the extrusion of large products having a circular base section, as hitherto described with reference to FIGS. 1 to 5. In fact, the cross section of the channels of FIG. flow C31 and C32 may, as a variant not shown, have a profile different from a circle, including sophisticated profiles in the form of animal, symbol, playful pattern, etc. In this case, the qualification of large products is assessed by the minimum transverse dimension of the cross section of the channels C31 and C32, this minimum transverse dimension being equal to or greater than 12 mm. More generally, another preferred dimensional characterization of the invention is that the cross section of the flow channels C31 and C32 is equal to or greater than 1 15 mm ² .

Various arrangements and options to the die 30 and the machine 1 described so far are also possible. For example, the elements 1 1 to 15 of the sleeve 10 may be provided thermoregulated.

Claims

1. Extrusion die (30) for extruding agri-food materials, comprising:

at least one insert (31, 32) for shaping a material to be extruded pushed through the extrusion die, which insert:

- is tubular, being centered on an axis (X31, X32),

has an upstream end (31A, 32A) through which the material enters the insert,

has a downstream end (31B, 32B) through which the material leaves outside the extrusion die, and

defines a flow channel (C31, C32) of the material, centered on the axis and connecting the upstream and downstream ends of the insert to each other, and

a diaphragm (37, 38) for preexpansion of the material, which is situated at the upstream end of the insert and which delimits a through opening (037, 038), this through opening being substantially centered on the axis (X31 , X32) and having a cross section smaller than that of the channel (031, 032), so that, before flowing in the channel, the material entering the insert passes through the diaphragm undergoing a pressure drop therein which, at the outlet of the diaphragm, preexpans the material by distributing it homogeneously over the entire inner periphery of the channel,

characterized in that the channel (031, 032) of the insert is cylindrical and has a cross-section which is substantially constant between the diaphragm (37, 38), not included, and the downstream end (31 B, 32B) of the insert, included,

and in that the extrusion die (30) further comprises means (310, 320) for controlled heating of the insert (31, 32), which are adapted for, along the channel (031, 032) of the insert, to maintain substantially homogeneous the temperature of the material pre-expanded by the diaphragm (37, 38).

2. - extrusion die according to claim 1, characterized in that said cross section of the channel (031, 032) of the insert (31, 32) is equal to or greater than 1 15 mm ² .

3. - extrusion die according to one of claims 1 or 2, characterized in that the minimum transverse dimension of said cross section of the channel (031, 032) of the insert (31, 32) is equal to or greater than 12 mm.

4. Extrusion die according to any one of the preceding claims, characterized in that said cross section of the channel (C31, C32) of the insert (31, 32) is circular.

5. Extrusion die according to claim 4, characterized in that the diameter of said section is equal to or greater than 12 mm.

6. - extrusion die according to any one of the preceding claims, characterized in that the section of the through opening (037, 038) of the diaphragm (37, 38) is circular profile.

7. - extrusion die according to any one of the preceding claims, characterized in that the length (L) of the channel (031, 032) of the insert (31, 32), measured between the diaphragm (37, 38); ) not included and its included downstream end (31 B, 32B) is between one and five times the minimum transverse dimension of said cross section of this channel.

8. - extrusion die according to any one of the preceding claims, characterized in that the diaphragm (37, 38) is formed as a separate part of the insert (31, 32), which is arranged partially or totally inside the upstream end (31A, 32A) of the insert.

9. - extrusion die according to any one of claims 1 to 7, characterized in that the diaphragm (37, 38) is formed as a separate part of the insert (31, 32), which is arranged in overflow, partial or total, of the upstream end (31A, 32A) of the insert.

10. - extrusion die according to any one of claims 1 to 7, characterized in that the diaphragm and the insert are monobloc.

1 1. - Extrusion die according to any one of the preceding claims, characterized in that the regulated heating means (310, 320) comprise a heating member (31 1, 312) attached to the outer face (31 D, 32D). of the insert (31, 32).

12. Extrusion die according to claim 1 1, characterized in that the heating member (31 1, 312) is electric.

13. - extrusion die according to any one of claims 1 to 10, characterized in that the controlled heating means comprise a heat transfer fluid circulation chamber, through which the insert extends to be bathed in this heat transfer fluid.

14. - extrusion die according to any one of the preceding claims, characterized by a plurality of inserts (31, 32) which are respectively associated with diaphragms (37, 38) and corresponding regulated heating means (310, 320). and in that the extrusion die (30) further comprises a body (33) which bears the inserts and defines a material distribution chamber (C33) in the respective upstream ends (31A, 32A). inserts.

15. - Extrusion machine (1), comprising:

a sleeve (10) inside which is driven at least one screw (20) for driving a material to be extruded, and

- An extrusion die (30), which is according to any one of the preceding claims and through which the material exiting the barrel is pushed by the one or more drive screws.