Miniaturized volumetric extruder using twin conical screws, each provided with a thread, the pitch of which increases to maintain a constant displacement

Abstract

The invention relates to an extruder (1) comprising a first conical screw (2) and a second conical screw (3) which are mounted so as to be counter-rotating in a barrel (4) and which define a volumetric stage (11) within which the threads (5, 6) are interpenetrated and conjugated with respect to each other so as to form, between the barrel (4) and the screws (2, 3), a succession of closed C-shaped chambers (7, 8) in order for the extruder (1) to operate volumetrically. According to the invention, in at least part of the volumetric stage, the pitch (P5, P6) of the thread of said screws (2, 3) increases according to a, preferably quadratic, predefined law of compensation (LP5_11, LP6_11), so that the successive closed chambers (7, 8) each have an individual volume that remains equal to a predetermined constant volume (V2, V3) that forms the displacement of the screw (2, 3) under consideration.

Description

MINIATURIZED VOLUMETRIC EXTRUDER USING TWIN CONICAL SCREWS EACH PROVIDED WITH A THREAD WHOSE PITCH INCREASES TO MAINTAIN A CONSTANT DISPLACEMENT

[0001] The present invention relates to the general field of extrusion, and more particularly to the field of extrusion of rubber-based materials.

[0002] The present invention finds particular application in the manufacture of elements intended to be used in the constitution of tires for vehicle wheels.

[0003] It is known that extrusion operations generally require precise control of the flow rate of the extruded material, to avoid the generation of non-conforming products, and therefore the scrapping of said products.

[0004] However, in practice, it is sometimes difficult to ensure such precise control of the flow rate of the extruded material during transient phases of the extrusion process, such as the start-up, stop-down and then restart phases of the extrusion line. This difficulty is due in particular to the fact that, during such transient phases, the temperature of the extrusion tools on the one hand, and the speed of the moving mechanical parts of the extruder(s) on the other hand, are not stabilized, which causes variations in the rheological properties and behavior of the extruded material.

[0005] To ensure control of the flow rate of the extruded material, it is known to use so-called "volumetric" extruders, i.e. extruders which are provided with moving mechanical members, such as pistons, toothed wheels or interpenetrating twin screws, which are arranged so as to create, within the extruder, one or more chambers which, under the effect of the cyclical movement of said moving mechanical members, will first open and increase their volume to accommodate the incoming material, then close so as to trap a given quantity of said material, and finally contract to mechanically force said captive quantity of material out of the chamber.

[0006] Thus, such extruders are capable of delivering, whatever the pressure prevailing at the outlet of the extruder, a volume of extruded material, called the "displacement" of the extruder, which is constant for each new iteration of the cyclic operation of the extruder, that is, in the example above, a volume of extruded material which is identical with each back and forth movement of the piston, respectively with each revolution of the gear wheels, or with each revolution of the twin screws.

[0007] Installations using such volumetric extruders also often have a multi-stage structure, each stage being formed by an extruder of a chosen type, in order to be able to ensure all the functions represented by feeding the installation with material, plasticizing the material, increasing the pressure of the material, then volumetric dosing of the material at the outlet of the installation.

[0008] Thus, for example, it is known to associate in series, within the same installation, a single-screw extruder, of the Archimedes screw type, on the one hand, which comprises a screw mounted in rotation in a sleeve and which ensures the feeding, the plasticization, and a certain increase in pressure and temperature of the material by shearing, with on the other hand a gear pump, the inlet orifice of which is connected to the discharge orifice of the single-screw extruder, and the counter-rotating toothed wheels of which ensure, in cooperation with the casing of said gear pump, the final increase in pressure and the volumetric operation of the installation.

[0009] However, such installations are particularly bulky and heavy.

[0010] This is particularly true when these installations are intended to extract a rubber-based material. Indeed, in order not to degrade the rubber-based material, it is necessary not to expose it to excessively high temperatures, which means that the gear pump impellers must not rotate at too high a speed. Therefore, if you want to ensure a sufficient flow rate of extruded material, you must opt ​​for a gear pump with a large displacement, and therefore large dimensions.

[0011] Coextrusion installations are also known such as that described in application WO-2017 / 109419 filed by the applicant, in which a first stage formed by a feed screw ensures the feeding of a second stage comprising interpenetrating and mating twin screws which ensure the volumetric operation. If such an arrangement advantageously makes it possible to multiply the number of extrusion paths connected to the same extrusion head while retaining a relatively compact, and to guarantee a relatively high and well-controlled flow rate of each of the extruded materials, installations of this type nevertheless remain dedicated to co-extraction applications which aim to produce complex profiles combining numerous extruded materials, and present, overall, due to the multiplicity of extrusion routes, a relatively large footprint.

[0012] Furthermore, known installations can prove relatively expensive, not only to acquire but also to operate, due in particular to their energy consumption and the complexity of their maintenance.

[0013] The objects assigned to the invention therefore aim to remedy the aforementioned drawbacks and to propose a miniaturized extruder which has a reduced size and weight while maintaining satisfactory volumetric operation which allows excellent control of the flow rate of the extruded material.

[0014] The objects assigned to the invention are achieved by means of an extruder intended to extrude a material, said extruder comprising: - a sheath, - a first screw which is mounted in rotation in the sheath around a first central axis and which is provided with a first thread, - a second screw which is mounted in rotation in the sleeve around a second central axis and which is provided with a second thread, said first screw and second screw being counter-rotating and arranged so that the first thread and the second thread cooperate to convey the material from upstream to downstream of the sleeve, said extruder being characterized in that: - the first screw is conical, so that the top diameter of the first thread decreases along the first central axis, in the upstream-downstream direction, according to a first predetermined conicity angle, - the second screw is conical, so that the top diameter of the second thread decreases along the second central axis, in the upstream-downstream direction, according to a second predetermined conicity angle, in that said extruder comprises a stage called a “volumetric stage” within which the first thread of the first screw and the second thread of the second screw are interpenetrated and conjugated with respect to each other so as to form on the one hand, between the sheath and the first screw, along the first central axis, a first series of successive closed C-shaped chambers and on the other hand, between the sheath and the second screw, along the second central axis, a second series of successive closed C-shaped chambers, so that the rotation of the first and second screws generates a positive displacement of the material captured by the first series of chambers and of the material captured by the second series of chambers, and in that at least part of the volumetric stage forms a stage called "isochoric volumetric stage", within which: - the pitch of the first thread increases along the first central axis, in the upstream-downstream direction, as the top diameter of the first thread decreases, according to a law called the "first compensation law" which allows the progressive increase in the pitch of the first thread to compensate for the taper of the first screw so that, in said isochoric volumetric stage, the individual volume of each of the closed chambers of the first series of closed chambers remains equal to the same predetermined constant nominal volume, called the "first screw displacement", with a maximum tolerance of + / - 2%, preferably + / - 1%, or even + / - 0.5%, and - the pitch of the second thread increases along the second central axis, in the upstream-downstream direction, as the top diameter of the second thread decreases, according to a law called the "second compensation law" which allows the progressive increase in the pitch of the second thread to compensate for the conicity of the second screw so that, in said isochoric volumetric stage, the individual volume of each of the closed chambers of the second series of closed chambers remains equal to the same predetermined constant nominal volume, called the "second screw displacement", with a maximum tolerance of + / - 2%, preferably + / - 1%, or even + / - 0.5%.

[0015] Advantageously, the extruder according to the invention can, on its own, ensure the plasticization of the material, the pressure build-up, and volumetric operation at a precisely controlled flow rate, while occupying a relatively small space.

[0016] The arrangement of the threads according to the invention, the pitch of which lengthens, in the isochoric volumetric stage, as the cone of the screw shrinks, advantageously makes it possible to reconcile the conicity of the screws with the constancy of the volume of each closed chamber while said chamber progresses from upstream to downstream along the central axis. of the screw, as the screw rotates on itself. Thus, the volume of extruded material which is captured by the chamber which opens at the upstream inlet of the isochoric volumetric stage is the same captive unit volume which is transported by each closed chamber of the isochoric volumetric stage which is between the screw and the barrel, and the same volume which is discharged by the chamber which opens at the downstream end of the isochoric volumetric stage. This constant unit volume advantageously corresponds to the volume of extruded material which is expelled at each complete revolution of the screw considered, that is to say to the displacement of said screw.

[0017] Due to the fact that, at each instant considered, the volume of the different closed chambers defined along its central axis by the same conical screw is constant, or quasi-constant taking into account the aforementioned admissible tolerances, that is to say that said volume undergoes neither substantial reduction nor increase, whatever the position that the closed chamber considered occupies along the axis, a regular conveyance of the material contained in the closed chambers is obtained, along the central axis of each screw, from upstream to downstream of the extruder.

[0018] This ensures volumetric operation of each conical screw while avoiding in particular problems of local overpressure and therefore problems of leakage between the successive chambers of the same screw, which could appear if the captive material of a closed chamber were excessively compressed by seeking to reduce the volume of said chamber without giving said material the possibility of escaping, or even problems of local pressure drop and cavitation, which could appear if one tended to create an expansion of the captive material of a closed chamber by increasing the volume of said chamber, that is to say by increasing the volume accessible to said material, without increasing the quantity of material available in said chamber.

[0019] In this respect, it will be noted that, as will be seen below, it is advantageous to associate with the volumetric stage, upstream of said volumetric stage and within the same first and second screws, a feed stage which will make it possible to work and compress the material in order to ensure feeding of the volumetric stage.

[0020] The taper of the screws has several advantages.

[0021] A first advantage of the conicity is that the upstream portion of the screw, which corresponds to the large base of the truncated cone envelope in which the said screw is fitted, has a large diameter and therefore offers wide access for the introduction of the material into the extruder, which allows the feeding function to be ensured in good conditions.

[0022] A second advantage of the taper is that the projected surface of the thread, considered in a plane normal to the central axis of the screw, decreases along the axis, so that said projected surface is minimal at the downstream end of the tapered screw, which corresponds to the small base of the truncated cone envelope in which said screw is inscribed. Thus, said projected surface of the thread is minimal precisely in the area where the material exerts the highest pressure against the screw, necessary to overcome the pressure at the outlet of the extruder and propel the material through the extrusion die connected to the outlet of the extruder. By minimizing the area of ​​the projected surface that is subjected to the pressure exerted on the screw by the material subjected to the action of the extruder, the resulting axial force exerted by the material against the screw and the bearings that support said screw and allow its rotation in the barrel is reduced.Therefore, it will be possible to safely reduce the size of these bearings, thus gaining in compactness and lightness.

[0023] A third advantage of the taper is to reduce the wetted surface of the screw, that is to say the surface of the screw which is in contact with the material, compared to what this same wetted surface would be within a straight cylindrical screw which would have a constant diameter and an axial length equal to the length of the tapered screw. By reducing the wetted surface, as well as the lever arm which corresponds, at each point of said wetted surface, to the radial distance measured between the central axis of the screw and said point in question, the resistive torque exerted by the material, due to its viscosity, against the rotation of the screw is reduced. An extruder according to the invention therefore requires a relatively low drive torque, which makes it possible to reduce the size, as well as the weight, of the motor(s) and the reducer(s) which drive the first and second screws in rotation.

[0024] For all of these reasons, the invention advantageously makes it possible to implement a compact, lightweight extruder, relatively little subject to mechanical inertia, thermal inertia, and vibrations.

[0025] Advantageously, due to its lightness and compactness, such an extruder can be mounted on a transport device which makes it possible to move and position said extruder dynamically relative to a receiving support on which the desired object is constructed, which makes it possible to produce said object by three-dimensional printing by depositing the extruded material on said receiving support at the desired locations and in the desired quantities.

[0026] Other objects, characteristics and advantages of the invention will appear in more detail on reading the description which follows, as well as with the aid of the appended drawings, provided for purely illustrative and non-limiting purposes, among which:

[0027] Figure 1 illustrates, in a perspective view, a pair of first and second counter-rotating screws twinned according to a first variant of the invention, within which the volumetric stage is preceded by a feed stage in which each of the first and second screws is single-threaded.

[0028] Figure 2 is a top detail view of one screw of the pair of screws in Figure 1.

[0029] Figure 3 illustrates, in a perspective view, a pair of first and second counter-rotating screws twinned according to a second variant of the invention, within which the volumetric stage is preceded by a feed stage in which each of the first and second screws is double-threaded.

[0030] Figure 4 is a top view of one screw of the pair of screws of Figure 3.

[0031] Figure 5 illustrates, in a detailed perspective view, the isochoric volumetric stage of a pair of twin counter-rotating conical screws used in an extruder according to the invention, for example the pair of screws of Figure 1 or that of Figure 3.

[0032] Figure 6 is a top view of the pair of screws of Figure 5.

[0033] Figure 7 is a front view, from downstream, of the pair of screws of Figures 5 and 6.

[0034] Fig. 8 is a sectional view, in a plane containing the first central axis of the first screw and the second central axis of the second conical screw, of the isochoric volumetric stage of an extruder according to the invention, within which the screws of Figs. 5 to 7 cooperate with a barrel to form two series of closed C-shaped chambers.

[0035] Figure 9 is a perspective view of the volumes defined, along the first central axis, by the first closed chamber and the last closed chamber of the first series of C-shaped closed chambers of the volumetric stage, and more particularly of the isochoric volumetric stage, of an extruder according to the invention.

[0036] Figure 10 illustrates, superimposed on the chambers of Figure 9, on the one hand the external fictitious frustoconical envelope inside which the top of the first thread is inscribed, and which therefore corresponds to the overall frustoconical envelope of the first screw, and on the other hand the internal fictitious frustoconical envelope which is inscribed in the bottom of the first thread, and which therefore corresponds to the frustoconical envelope of the core of the first screw, in the isochoric volumetric stage.

[0037] Figure 11 is a top view of the chambers shown in Figures 9 and 10, and of the external fictitious frustoconical envelope.

[0038] Figure 12 is a side view, from upstream of the screw, of the chambers illustrated in Figures 9 to 11.

[0039] Figure 13 illustrates, according to a view folded down in a reference plane which is, by convention, normal to the first central axis and tangent to the upstream axial end of the isochoric volumetric stage of the first conical screw, a principle of dimensioning said isochoric stage of the first conical screw.

[0040] Figure 14 is an example of an extrusion installation using a mobile laying head which carries an extruder with two conical screws according to the invention.

[0041] Figure 15 is an example of dimensioning laws for the thread of a conical screw used by the invention, here in preferential connection with the first variant illustrated in Figures 1 and 2, showing the quadratic increase in the thread pitch in the isochoric volumetric stage, and the reduction in the thread in the feed stage which precedes said isochoric volumetric stage.

[0042] The present invention relates to an extruder 1, intended to extrude a material, and more particularly an extruder 1 of the “twin-screw” type.

[0043] Said extruder comprises, in a manner known per se, a sheath 4, preferably metallic.

[0044] Said extruder 1 comprises, as is clearly visible in figures 1 to 8: - a first screw 2 which is mounted in rotation in the sleeve 4 around a first central axis X2 and which is provided with a first thread 5, - a second screw 3 which is mounted in rotation in the sheath 4 around a second central axis X3 and which is provided with a second thread 6.

[0045] Said first and second screws 2, 3 are counter-rotating and arranged so that the first thread 5 and the second thread 6 cooperate to convey the material from upstream to downstream of the sheath 4, according to an overall advance movement noted here “FWD”.

[0046] For convenience of description, "axial" will denote a direction parallel to the central axis X2, X3 of the screw 2, 3 considered, and "radial" will denote a direction perpendicular to the central axis X2, X3 of the screw 2, 3 considered.

[0047] Note that the first central axis X2 and the second central axis X3 are geometrically intersecting.

[0048] By "counter-rotating" we mean that the first screw 2 and the second screw 3 turn in opposite directions of rotation.

[0049] Furthermore, said first and second screws 2, 3 are synchronous, that is to say advantageously rotate at rotation speeds which are equal to each other in absolute value, although of opposite signs.

[0050] The first thread 5 may, in particular depending on the stage of the first screw 2 considered along the first central axis X2, comprise a single thread, or, as a variant, several threads having the same pitch and angularly offset.

[0051] The term "first channel" 9 denotes the helical groove, or, in the case of a plurality of threads, each of the helical grooves, which is delimited by and between two solid profiles of the first thread 5 which follow one another axially, that is to say which separates two solid profiles of the first thread 5, one of which is immediately adjacent to the other.

[0052] Likewise, the second thread 6 may, depending on the stage of the second screw 3 considered along the second central axis X3, comprise one or more threads, preferably in a number equal to the number of thread(s) of the first thread 5 with which said second thread 6 cooperates.

[0053] The term "second channel" 10 denotes the helical groove, or, in the case of a plurality of threads, each of the helical grooves, which is delimited by and between two solid profiles of the second thread 6 which follow one another axially, that is to say which separates two solid profiles of the second thread 6, one of which is immediately adjacent to the other.

[0054] The direction of the first thread 5 will be opposite to the direction of the second thread 6, that is to say that the first screw 2 may have a left-hand thread 5 while the second screw 3 has a right-hand thread 6, or conversely, the first screw 2 may have a right-hand thread 5 while the second screw 3 has a left-hand thread 6.

[0055] More generally, the second screw 3 is preferably the mirror image of the first screw 2, so that the characteristics of one can be deduced identically, by symmetry, from the characteristics of the other.

[0056] According to the invention, the first screw 2 is conical, so that the top diameter D_5C of the first thread 5 decreases along the first central axis X2, in the upstream-downstream direction, according to a first predetermined conicity angle A5.

[0057] Likewise, the second screw 3 is conical, so that the top diameter D_6C of the second thread 6 decreases along the second central axis X3, in the upstream-downstream direction, according to a second predetermined conicity angle A6.

[0058] As can be seen in Figures 2, 4, 6, 8 and 11, the conicity angle A5, A6 corresponds to the angle of inclination formed, in a plane containing the central axis X2, X3 of the screw 2, 3 considered, by the fictitious frustoconical envelope E2, E3 in which said screw 2, 3 is inscribed, a frustoconical envelope E2, E3 which is therefore tangent to the successive vertices 5C, 6C of the thread 5, 6 of the screw 2, 3 considered, relative to the fictitious right cylinder with a circular base, centered on the central axis X2, X3, and in which the screw 2, 3 considered is inscribed.

[0059] Equivalently, the conicity angle A5, A6 corresponds to the half-angle at the top of the fictitious frustoconical envelope E2, E3 in which the screw 2, 3 considered is inscribed, and therefore to the angle formed between the central axis X2, X3 and each generating line of the inclined wall of the fictitious frustoconical envelope E2, E3 in which the screw 2, 3 is inscribed.

[0060] In practice, the first conicity angle A5 is equal to the second conicity angle A6.

[0061] More particularly, the first central axis X2 and the second central axis X3 being geometrically intersecting, the first conicity angle A5 and the second conicity angle A6 are each half the vertex angle formed by the intersection of the first central axis X2 and the second central axis X3, as can be seen in Figure 11.

[0062] Of course, the internal wall of the sheath 4, which cooperates with one of the screws 2, 3, also has a conical profile, combined with the conical profile of said screw 2, 3, that is to say which generally follows the same fictitious frustoconical envelope, being circumscribed to said frustoconical envelope E2, E3. The internal wall of the sheath 4 therefore generally narrows as said sheath 4 is traveled in the upstream-downstream direction FWD, according to the same conicity angle A5, 46 as the screw 2, 3.

[0063] Preferably, the first conicity angle A5 and the second conicity angle A6 are each between 1.8 degrees and 3 degrees, preferably between 2 degrees and 2.5 degrees, even more preferably equal to 2.5 degrees.

[0064] The inventors have in fact noted that, for a given screw length, and therefore for a given size, these conicity angle values ​​correspond to a good compromise between, on the one hand, the resistive forces exerted on the screw 2, 3, which we seek to minimize, and on the other hand the capacity of the screws to receive and be able to exploit a relatively high driving torque.

[0065] In particular, we will seek an optimal compromise between: - a conicity angle A5, A6 which is sufficiently high i) to obtain a significant reduction in the terminal surface of the screw 2, 3, which corresponds to the small base of the frustoconical envelope E2, E3 in which said screw 2, 3 is inscribed, and consequently to obtain a significant reduction in the axial forces which result from the pressure exerted by the extruded material against the screw 2, 3 considered, which makes it possible to reduce the size of the bearings and axial stops which axially support the screw 2, 3, and ii) to create a sufficient center distance, between the first screw 2 and the second screw 3, at the upstream zone 2U, 3U of said screws 2, 3, to be able to have sufficient space to house therein a solid and powerful reducer as well as large diameter shafts capable of driving said screws 2, 3 and of imparting to each of them a high driving torque, and - a taper angle A5, A6 which is sufficiently moderate i) so that the diameter of the screw 2, 3 in the upstream zone 2U, 3U remains sufficiently small to avoid offering the extruded material a strong lever arm relative to the central axis X2, X3 of the screw, and therefore limit the resistive torque that said extruded material opposes to the rotation of the screw 2, 3, and ii) so as not to thin the screws 2, 3 too much at their tip, that is to say so that the cores of the first screw 2 and of the second screw 3 have, up to and including the downstream end 2D, 3D of said screws 2, 3, a thickness of material sufficient to be able to support and transmit without damage, in particular without irreversible deformation in torsion, a high driving torque.

[0066] In view of the aforementioned conicity angle values ​​A5, A6, the apex angle formed by the (fictitious) intersection of the first central axis X2 with the second central axis X3, which is equal to the sum of the two apex angles A5 and A6, and therefore more preferably twice the apex angle A5, will be between 3.6 degrees and 6 degrees, preferably equal to 5 degrees.

[0067] According to the invention, the extruder 1 comprises a stage 11 called the “volumetric stage” within which the first thread 5 of the first screw 2 and the second thread 6 of the second screw 3 are interpenetrated and conjugated with respect to each other so as to form, on the one hand, between the barrel 4 and the first screw 2, along the first central axis X2, a first series of successive closed C-shaped chambers 7 and, on the other hand, between the barrel 4 and the second screw 3, along the second central axis X3, a second series of successive closed C-shaped chambers 8, so that the rotation of the first and second screws 2, 3 generates a positive displacement of the material captured by the first series of chambers 7 and of the material captured by the second series of chambers 8.

[0068] By "interpenetrated", it is indicated that, as is clearly visible in Figures 1, 3, 5, 6, and 8, the first and second threads 5, 6 are arranged so that the top 5C of the first thread 5 arrives substantially at the level of the bottom 6R of the second thread 6, and conversely, the top 6C of the second thread 6 arrives substantially at the level of the bottom 5R of the first thread 5, so that the thread 5, 6 of each screw 2, 3 penetrates the channel 10, 9 defined by the thread 6, 5 of the other screw 3, 2 over the entire radial height of said channel 10, 9.

[0069] For information purposes, a functional radial clearance JRI is provided between the top 5C, 6C of one thread and the bottom 6R, 5R of the other thread, which is certainly not zero to ensure smooth relative movement of one screw 2 relative to the other screw 3, but which is, above all, preferably less than or equal to 0.3 mm to ensure volumetric operation without leakage.

[0070] By "conjugated", it is indicated that, as is clearly visible in Figures 1, 3, 5, 6, and 8, the first thread 5 and the second thread 6 are arranged so that the full axial width of the first thread 5, that is to say the axial width of the full section of the profile of the first thread 5, fills the axial width of the second channel 10 defined by the second thread 6 and delimited axially between two successive flanks 6F of the second thread 6 and, conversely, the full axial width of the second thread 6 fills the axial width of the first channel 9 defined by the first thread 5 and delimited axially between two successive flanks 5F of the first thread 5. Thus, the flanks 5F of the first thread substantially match the flanks 6F of the second thread and vice versa.

[0071] As a guide, to ensure leak-free volumetric operation, a functional axial clearance JAI of less than or equal to 0.3 mm may be provided between the 5F, 6F flank of one thread and the nearest portion of the 6F, 5F flank of the other thread.

[0072] For the same reasons of operation and sealing in operation, a radial clearance JR2 will be provided between the apex 5C, 6C of the thread of the screw 2, 3 and the radially innermost portion of the wall of the sleeve 4, with which the apex 5C, 6C considered of the thread cooperates, which radial clearance JR2 is non-zero and, preferably, equal to or less than 0.1 mm, in particular in the volumetric stage 11.

[0073] Advantageously, as shown diagrammatically in Figures 8, 9, 10 and 11, each of the first and second series of chambers 7, 8 created in the volumetric stage 11 firstly makes it possible to capture the material in a first chamber 7, 8, forming the upstream access to the volumetric stage 11, first chamber that the rotation of the screw 2, 3 will close on the extruded material, in order to maintain a corresponding volume of said extruded material captive inside said closed chamber, which is delimited by the C-shaped space between said screw 2, 3 and the sheath 4, then convey the material downstream 2D, 3D, inside said closed chamber 7, 8, by progressively moving said chamber 7, 8 downstream 2D, 3D, along the central axis X2, X3, according to a global translational advance movement, noted here FWD, thanks to the rotational movement of the screw 2, 3.

[0074] By nature, the closed chambers 7, 8 of the same series of chambers do not communicate with each other, so that each unit volume of extruded material contained in a chamber 7 is isolated from the unit volume of extruded material contained in each of the other chambers 7, in particular in each of the adjacent chambers. Advantageously, the extruded material cannot therefore rise upstream 2U, 3U of the screw 2, 3, whatever the pressure prevailing at the downstream end 2D, 3D of the conical screw 2, 3, at the level where the last chamber 7, 8 of the succession of chambers 7, 8 opens onto the outlet of the extruder 1.

[0075] Thus, at each complete rotation of the screws 2, 3, the chambers 7, 8 shift downstream 2D, 3D, and therefore advance the extruded material, along the axis X2, X3 of each screw considered, by an axial distance which is equal to the pitch P5, P6 of the thread 5, 6 at the location considered.

[0076] The volume of extruded material which is delivered at the outlet of the extruder 1 at each complete revolution of the screws 2, 3, in other words the total “displacement” of the extruder 1, thus corresponds to the sum of the unit volumes contained respectively in the last closed chamber 7 of the first succession of chambers 7, delimited by the first screw 2, and in the last closed chamber 8 of the second succession of chambers 8, delimited by the second screw 3, that is to say to the sum of the displacement of the first screw 2 and the displacement of the second screw 3.

[0077] This volumetric capacity of the extruder 1 makes it possible to precisely adjust the flow rate of the extruded material by adjusting the rotation speed of the screws 2, 3.

[0078] Advantageously, it will be noted that the fact that each screw 2, 3 generates, with the sheath 4, a multiplicity of chambers 7, 8 which follow each other axially, and which are separated from each other by the thread 5, 6 of the screw 2, 3 considered, makes it possible to globally reinforce the sealing of the extruder 1, and to reduce the sensitivity of this sealing to the wear of said screws 2, 3, by forming as many successive obstacles against possible rises of the extruded material in the direction going from downstream 2D, 3D to upstream 2U, 3U, along the sheath 4, between the sheath 4 and the screw 2, 3 considered.

[0079] According to the invention, and as is clearly visible in figures 1, 3, 5, 6, 8 and 15, at least part of the volumetric stage 11, and more preferably the entirety of the volumetric stage 11, forms a stage called “isochoric volumetric stage” 11 A, within which: - the pitch P5 of the first thread 5 increases along the first central axis X2, in the upstream-downstream direction, as the top diameter of the first thread D_5C decreases, according to a law called the “first compensation law” LP5 11 which allows the progressive increase in the pitch P5 of the first thread 5 to compensate for the conicity of the first screw 2 so that, in said isochoric volumetric stage 11 A, the individual volume of each of the closed chambers 7 of the first series of closed chambers remains equal to the same predetermined constant nominal volume V2, called the “first screw displacement” V2, with a maximum tolerance of + / - 2%, preferably + / - 1%, or even + / - 0.5%, and - the pitch P6 of the second thread 6 increases along the second central axis X3, in the upstream-downstream direction, as the top diameter of the second thread D_6C decreases, according to a law called the “second compensation law” LP6 11 which allows the progressive increase in the pitch P6 of the second thread 6 to compensate for the conicity of the second screw 3 so that, in said isochoric volumetric stage 11 A, the individual volume of each of the closed chambers 8 of the second series of closed chambers remains equal to the same predetermined constant nominal volume V3, called the “second screw displacement” V3, with a maximum tolerance of + / - 2%, preferably + / - 1%, or even + / - 0.5%.

[0080] In other words, each of the closed chambers 7 delimited by the first screw 2 and the sheath 4 will have substantially or even exactly the same individual volume, substantially or even exactly equal to the individual volume of the neighboring chambers 7, and substantially or even exactly equal to the volume of the cylinder capacity of the first screw V2, that is to say here an individual volume equal to V2 + / - 2%, preferably equal to V2 + / - 1%, or even equal to V2 + / - 0.5%.

[0081] Likewise, each of the closed chambers 8 delimited by the second screw 3 and the sheath 4 will have substantially or even exactly the same individual volume, substantially or even exactly equal to the individual volume of the neighboring chambers 8, and substantially or even exactly equal to the volume of the cylinder capacity of the second screw V3, that is to say say here an individual volume equal to V3 + / - 2%, preferably equal to V3 + / - 1%, or even equal to V3+ / - 0.5%.

[0082] From a dynamic point of view, the individual volume of each chamber 7, 8 closed at C thus varies by less than 2%, less than 1%, or even less than 0.5% relative to the individual reference volume constituted by the screw displacement V2, V3, or even remains equal to said individual reference volume, over all the successive axial positions occupied by the closed chamber 7, 8 considered during its transfer from upstream to downstream of the isochoric volumetric stage 11A under the effect of the rotation of the screw 2, 3, and more preferably from the closing of said chamber 7, 8 at the upstream limit 11U of the volumetric stage 11 until the reopening of said chamber at the downstream limit 11D of the volumetric stage 11, here at the outlet of the extruder 1.

[0083] Equivalently, if we consider, rather than a dynamic vision of transfer of a chamber along the axis, a static vision of distribution of the chambers 7, 8 of the same series of chambers at a given instant, this amounts to saying that all the closed chambers 7, 8 of the series of closed chambers 7, 8 delimited by the screw 2, 3 considered all have an individual volume that is substantially or even exactly identical, equal to the reference unit volume, at + / - 2%, preferably at + / - 1%, or even at + / - 0.5%.

[0084] It will be noted that the arrangement proposed by the invention allows the extruder 1 to ensure a volume flow rate precision of less than or equal to 2%, or even less than or equal to 1%, that is to say to deliver a constant volume at + / - 2%, or even + / - 1% per turn of the screw, and this, preferably, for outlet pressures which can be between 200 bar and 500 bar, and for a volume flow rate of between 1 dm 3 / min and 6 dm 3 / min.

[0085] Preferably, the displacement of the first screw V2 is equal to the displacement of the second screw V3.

[0086] Preferably, the first screw displacement V2 and the second screw displacement V3 are each between 8 cm 3 and 50 cm 3 , for example between 10 cm 3 and 30 cm 3 , especially between 10 cm 3 and 20 cm 3 .

[0087] As indicated above, the pitch P5 of the first thread 5 increases progressively, in the isochoric volumetric stage 11 A, and more preferably on the entire volumetric stage 11, along the first central axis X2 of the first screw 2, in the direction from upstream to downstream, as the diameter of said first screw 2 decreases, so that the pitch P5 is strictly greater at the downstream end 11D of the volumetric stage of the first screw 2, at the level of the last chamber 7 forming the outlet chamber of said first screw 2, than said pitch P5 is at the upstream end 11U of the volumetric stage of the first screw 2, at the level of the first chamber 7 forming the inlet chamber.

[0088] This increase in the pitch P5 of the first thread 5 is preferably monotonous, and more preferably linear, along the central axis X2, between on the one hand the value of said pitch P5 considered at the upstream end of the isochoric volumetric stage 11 A, here the upstream end 11U of the volumetric stage 11, a value which is called the “input pitch” P5_in, and on the other hand the higher value of said pitch P5 considered at the downstream end of the isochoric volumetric stage 11 A, here the downstream end 11D of the volumetric stage 11, a value which is called the “output pitch” P5_out.

[0089] This variation of the pitch P5 of the first thread 5, here a continuous enlargement of said pitch P5 along the central axis X2, over the entire isochoric volumetric stage 11 A, and more preferably over the entire volumetric stage 11, of the first screw 2, makes it possible to progressively increase the axial width W9 of the channel 9 defined by the first thread 5, as is clearly visible in Figures 1, 2, 3, 8, 9 and 11, so as to compensate for the corresponding variation, here a continuous narrowing, of the diameter of the first screw 2, in this case a narrowing which affects at least the top diameter of the first thread D_5C, and preferably also the bottom diameter of the first thread D_5R, and this in order to maintain the individual volume of each closed chamber 7 substantially or even exactly constant, while said chamber 7 is progressively transferred downstream by the rotational movement of the first screw 2.

[0090] In other words, the first channel 9 defined by the thread 5 of the first screw has, in the isochoric volumetric stage 11 A, and preferably over the entire volumetric stage 11, a pitch P5 and a width W9 which increase progressively along the central axis X2, and more particularly which increase continuously according to an increasing function of the distance traveled along the central axis X2, so that each portion substantially annular of said first channel 9 which is delimited simultaneously by the first screw 2, by the internal wall of the sheath 4 (or, equivalently, by the frustoconical envelope E2), and by the thread 6 of the second screw 3 which closes the ends of said portion of the first channel 9, so that said portion of the first channel 9 forms one of the chambers 7 closed at C, has a volume which is invariant when said chamber 7 moves from upstream to downstream under the effect of the joint rotation of the first and second screws 2, 3, and, at each instant considered, which is equal to the volume of the neighboring closed chambers 7 of the same series of closed chambers 7.

[0091] The above considerations relating to the variation of the pitch P5 of the first thread 5, and therefore of the axial width W9 of the first channel 9, apply of course mutatis mutandis to the pitch P6 of the second thread 6 and axial width W10 of the second channel 10, which, similarly, increase monotonically, along the second central axis X3, so that the pitch P6 of the second thread evolves between a minimum value corresponding to an inlet pitch P6_in at the upstream end of the isochoric volumetric stage 11 A, which preferably coincides with the upstream end 11U of the volumetric stage 11, and a maximum value corresponding to an outlet pitch P6_out at the downstream end of the isochoric volumetric stage 11 A, which preferably coincides with the downstream end 11D of the volumetric stage 11. Thus, the individual volume of each closed chamber 8 is kept substantially constant.

[0092] In all cases, to ensure smooth meshing of the first and second screws 2, 3, the pitch P5 of the first thread 5 is equal to the pitch P6 of the second thread, at each abscissa considered along the bisector of the apex angle which is formed by the intersection of the first central axis X2 and the second central axis X3.

[0093] Of course, as is clearly visible in Figures 2, 4, 5, 6, 8 and 15, as is the case for the axial widths W9, W10 of the channels 9, 10, that is to say of the hollow portions of the first and second threads 5, 6, the axial width of the solid portion of the profile of the first thread 5 increases with the pitch P5 of said first thread 5, and likewise for the axial width of the solid portion of the profile of the second thread 6 which increases with the pitch P6 of said second thread 6, so that the first and second threads 5, 6 remain conjugated along the screws 2, 3, and thus maintain the sealing of the chambers 7, 8, in each occupying the entire width of the channel 10, 9 delimited by the thread 6, 5 of the other screw. In other words, in the isochoric volumetric stage 11 A, and more preferably in the whole of the volumetric stage 11, each of the first and second threads 5, 6 simultaneously increases its pitch P5 and thickens its profile, along the central axis X2, X3, as the diameter of its conical screw 2, 3 decreases.

[0094] Preferably, the first compensation law LP5 11 is an increasing quadratic function of the axial abscissa value considered along the first central axis X2.

[0095] Preferably, respectively, the second compensation law LP6 11 is an increasing quadratic function of the axial abscissa value considered along the second central axis X3.

[0096] Advantageously, the first compensation law LP5 11 and the second compensation law LP6 11 can be expressed in the form of a second-degree polynomial as a function of the axial abscissa.

[0097] Preferably, the first and second screws 5, 6 being images of each other, the first compensation law LP5 11 and the second compensation law LP6 11 will be identical.

[0098] The advantage of quadratic functions is that one can compensate by an increase in the pitch P5, P6 of the thread 5, 6 for a narrowing which is surface, therefore two-dimensional, and which is linked to a joint narrowing of the top diameter D_5C, D_6C of the thread 5, 6 and the bottom diameter D_5R, D_6R of the thread 5, 6.

[0099] Indeed, preferably, in the isochoric volumetric stage 11 A, and more preferably in the entire volumetric stage 11, the core 12 of the first screw 2 and the top of the thread 5 of this same first screw 2 narrow jointly, each according to a conicity angle, and more preferably according to the same conicity angle A5 so that they therefore generally follow, in a radial cutting plane containing the central axis X2, parallel slopes.

[0100] In other words, preferably, at least in the volumetric stage 11, the bottom diameter of the first thread D_5R decreases according to the first taper angle A5, from such that the height of the first thread H5 varies by less than 20% in the volumetric stage along the first central axis X2, preferably by less than 10%, and more preferably by less than 5%. For example, the height of the first thread H5 is constant along the first central axis X2.

[0101] Advantageously, this makes it possible to limit the axial expansion of the pitch P5 of thread 5, at each helical turn of the thread, necessary to maintain the screw displacement V2 constant, but also to maintain a significant first screw displacement V2 and good circulation of the material in the channel 9. Conversely, it will be understood that if the height H5 of the first thread were to decrease, in particular decrease sharply, for example compared to a cylindrical core 12, then it would be necessary to greatly lengthen the pitch P5 of the thread 5 to maintain the cylinder capacity V2 constant, which would require lengthening the screw 2 and would be potentially detrimental to the sealing of the chambers 7.

[0102] For information purposes, it will be possible, for example, to retain, in particular in the volumetric stage 11, a thread pitch P5 which is less than the smallest diameter D_5C_min of the top of said thread 5 in said volumetric stage 11.

[0103] Similarly, in the isochoric volumetric stage 11 A, and more preferably in the entire volumetric stage 11, the core 13 of the second screw 3 and the top of the thread 6 of this same second screw 3 narrow jointly, each according to a conicity angle, preferably according to the same conicity angle A6 in order to follow overall, in a radial cutting plane containing the central axis X2, parallel slopes.

[0104] Thus, the top diameter D_5C, D_6C of the thread 5, 6 of each of the first and second screws 2, 3, i.e. the external diameter of the screw 2, 3, as well as the bottom diameter D_5R, D_6R of said thread 5, 6, i.e. the internal diameter of said screw 2, 3, decrease continuously along the central axis X2, X3 of the screw 2, 3 considered, according to the same predetermined conicity angle A5, A6, while the pitch P5, P6 of the thread 5, 6 increases continuously, in order to compensate for the joint decrease in the thread bottom diameter D_5R, D_6R and the thread top diameter D_5C, D_6C, so that the successive closed chambers 7, 8 that delimit the screw 2, 3 considered each have an individual volume that is substantially constant from one chamber 7, 8 to the other, along the central axis X2, X3.

[0105] Thus, the thread height H5 of the first conical screw 2, called “first thread height H5” is preferably constant along the central axis X2 of said first conical screw 2, at least in the volumetric stage 11.

[0106] The same should preferably apply to the thread height H6 of the second conical screw 3.

[0107] Usually, the thread height H5 of the first screw 2 designates, as can be seen in Figure 8, the distance which separates the midpoint of the bottom 5R of the first channel 9, considered at mid-distance axially of the two flanks 5F which border said channel, on the one hand, from the generating line which is tangent to the vertices 5C of the first thread bordering said first channel 9, that is to say from the line corresponding to the intersection of the frustoconical envelope E2 with said radial section plane, on the other hand. In other words, the thread height H5 is the length of the straight line segment perpendicular to the generator of the frustoconical envelope E2 and which passes through the midpoint of the bottom 5R of the first channel 9.

[0108] The same applies to the height H6 of the thread 6 of the second screw 3, considered perpendicular to the truncated cone envelope E3.

[0109] However, for convenience of calculation in the following, we can also consider the projection H5', H6' of the thread height H5, H6 in a plane normal to the central axis X2, X3 of the screw 2, 3 considered.

[0110] Preferably, in the volumetric stage 11, whatever the angular position which is adopted respectively by each of the first and second screws 2, 3 around its central axis X2, X3 during the counter-rotating movement of said first and second screws 2, 3, the number of chambers 7 of the first series, which are simultaneously in a closed state, as well as the number of chambers 8 of the second series, which are simultaneously in a closed state, is equal to or greater than four, or even equal to or greater than five, lower limit, and preferably less than or equal to twenty, or even less than or equal to twelve, upper limit, for example between four and ten, or between five and eight.

[0111] The inventors have in fact noted that it was necessary to provide an axial succession of several chambers 7, 8 along the same screw 2, 3, here at least four, or even at least five chambers, to obtain satisfactory sealing, and therefore operation satisfactory volumetric performance, including in the presence of high pressures at the outlet of extruder 1.

[0112] Conversely, the inventors have also found that it is preferable to limit the number of chambers 7, 8 along the same screw 2, 3, and more generally the length L2, L3 of said screw 2, 3, typically by providing less than twenty, less than twelve, or even less than ten chambers along the same screw, in particular to avoid the risk of providing too much work to the material, because an excess of work would lead to excessive heating of the material, potentially detrimental to said material, so that the use of long screws 2, 3 multiplying the chambers 7, 8 would require preventively reducing the rotation speed of the screws 2, 3, thus causing a limitation of the maximum useful flow rate of the extruder 1.

[0113] Furthermore, by limiting the length L2, L3 of the screws 2, 3, as well as the number of chambers, the resistive torque opposed by the material is reduced, and therefore the motor torque necessary to drive the screws 2, 3, which makes it possible to limit energy consumption while ensuring good efficiency, and in particular a good mass flow rate, of the extruder 1.

[0114] It will be noted that the above-mentioned dimensioning is particularly suitable for the extrusion of a rubber-based material, since the short screw length limits the residence time of the extruded material in the extruder 1, during which time said extruded material is exposed to the work of the screws 2, 3. This advantageously avoids overheating and therefore deterioration of the rubber-based material.

[0115] It will also be noted that an extruder 1 according to the invention, when it is dedicated to the extrusion of a rubber-based material, does not require a significant screw length L2, L3 to ensure satisfactory sealing, and in particular can be satisfied with an axial screw length L2, L3, more particularly a threaded axial length, which represents between 4 times and 10 times the maximum diameter of said screws 2, 3, unlike known extruders intended for thermoplastic materials and which, due to the significant fluidity of such thermoplastic materials, must have a great length, typically of the order of 40 times the maximum diameter of the screw.

[0116] The extruder 1 according to the invention can thus be much shorter and lighter than known extruders, while retaining a satisfactory volumetric capacity.

[0117] The length L3 of the second screw 3, which here corresponds to the total threaded length of said second screw 3, is advantageously equal to the length L2 of the first screw 2, which here corresponds to the total threaded length of said first screw 2.

[0118] According to a preferred characteristic which may constitute an invention in its own right, the extruder 1 comprises a stage called the “feed stage” 30 which precedes the volumetric stage 11 and within which: - the first thread 5 is arranged so that, as the apex diameter D_5C of said first thread 5 decreases along the first central axis X2, in the upstream-downstream direction, according to the first conicity angle A5, the pitch P5 of said first thread 5 also decreases, along the first central axis X2, in the upstream-downstream direction, in accordance with a law called the “first compression law” LP5 30, so as to promote compression of the material as it approaches the volumetric stage 11, and - the second thread 6 is arranged so that, as the apex diameter D_6C of the second thread 6 decreases along the second central axis X3, in the upstream-downstream direction, according to the second conicity angle A6, the pitch P6 of the second thread 6 also decreases, along the second central axis X3, in the upstream-downstream direction, in accordance with a law called the “second compression law” LP6 30, so as to promote compression of the material as it approaches the volumetric stage 11.

[0119] Advantageously, said feed stage 30 makes it possible to receive the material, to work it and to pre-compress it in order to ensure the feeding of the volumetric stage, and thus on the one hand to ensure good filling of the first chamber 7, 8 of each screw 2, which makes it possible to optimize the effective cylinder capacity V2, V3 of each screw 2, 3, and on the other hand to limit the pressure gradient between the upstream and downstream of the volumetric stage 11, which avoids leaks and material rising in the direction opposite to the desired FWD advance movement.

[0120] It will be noted that, advantageously, the first screw 2 and the second screw 3 cooperate in a non-volumetric manner within the feed stage 30, which in particular allows the use of a pitch P5, P6 of extended thread 5, 6, and therefore of very wide channels 9, 10, which facilitates the insertion and swallowing of the material in the extruder 1, in particular when said material reaches the extruder in the form of a continuous strip.

[0121] Preferably, the smallest pitch P5, P6 of each screw 2, 3 considered in the feed stage 30, will be strictly greater than the longest pitch P5, P6 of this same screw 2, 3 considered in the volumetric stage 11, and more particularly in the isochoric volumetric stage 11 A.

[0122] For information purposes, and always with the aim of maintaining wide channels favorable to the swallowing of the material, the initial pitch P5, P6 in the feed stage 30 will preferably be equal to or greater than 0.5 times the diameter D_5C, D_6C of the top of the thread 5, 6 considered at the upstream end of said feed stage 30.

[0123] Of course, it will be possible to provide in the sheath 4, opposite the feed stage 30, an inlet orifice, possibly provided with a hopper, to allow the material to enter the sheath 4.

[0124] According to a possible alternative embodiment, illustrated in figures 1, 2 and 15, the first screw 2 and the second screw 3 can be single-threaded in the feed stage 30, and more preferably both in the feed stage 30 and in the volumetric stage 11.

[0125] Advantageously, it is thus possible to have single threads which extend continuously, but by adapting their pitch P5, P6, through the feed stage 30 then the volumetric stage 11 of each screw 2, 3.

[0126] However, preferably, according to another alternative embodiment, illustrated in Figures 3 and 4, in the feed stage 30, the first screw 2 and the second screw 3 are both multi-threaded, preferably bi-threaded, so that the first thread 5 and the second thread 6 each comprise at least two threads which cover the same common axial extent and which are angularly offset from each other around the central axis X2, X3 of the screw 2, 3 in question.

[0127] This multiplication of threads in the feed stage 30 makes it possible in particular to facilitate the attachment of the material, in particular when it is supplied in the form of a strip, and its swallowing by the screws 2, 3.

[0128] This arrangement also improves the working of the material and its pressure build-up to promote the feeding of the volumetric stage 11 located directly downstream of the feed stage 30.

[0129] On the other hand, preferably, the first screw 2 and the second screw 3 are each, in the volumetric stage 11, single-thread.

[0130] The screws 2, 3 may then have a transition zone 31 between the feed stage 30 and the volumetric stage 11, making it possible to switch from a multi-thread upstream to a single-thread downstream, and where appropriate to switch from one core geometry 12, 13 to another core geometry 12, 13, for example to adapt the thread root diameter D_5R, D_6R and / or the taper angle of the core 12, 13.

[0131] Indeed, it will be noted that, preferably, the core 12 of the first screw 2 has, in the feed stage 30, a straight cylindrical shape or a truncated cone shape whose conicity angle is strictly less than the first conicity angle A5.

[0132] Respectively, the core 13 of the second screw 3 preferably has, in the feed stage 30, a straight cylindrical shape or a truncated cone shape whose conicity angle is strictly less than the second conicity angle A6.

[0133] Such an arrangement advantageously makes it possible to maintain a relatively large thread root diameter D_5R, D_6R in the feed stage 30, which makes it possible to transmit a high driving torque to the screws 2, 3, and to keep a “diameter reserve” from which it will then be possible to proceed with the truncated reduction of the core 12, 13 in the isochoric volumetric stage 11 A, without the risk of structurally weakening the screws 2, 3 too much.

[0134] Furthermore, a low or zero taper angle of the core 12, 13 in the feed stage 30 facilitates the reduction of the volume of the channels 9, 10 under the effect of the reduction of the pitch P5, P6 of the thread 5, 6, which therefore promotes the compression of the material and therefore the filling of the volumetric stage 11.

[0135] In the transition zone 31, as seen in Figures 3 and 4, the multiple threads of the feed stage 30 may end in the form of spouts 32, which open the channels 9, 10 to allow the material to enter the chambers 7, 8 of the volumetric stage 11.

[0136] In practice, the respective isochoric volumetric stages 11A of the first and second conical screws 2, 3 can be dimensioned according to the method below, and with reference to figures 11, 12 and 13.

[0137] For the sake of brevity, we will describe the dimensioning of the first conical screw 2, knowing that the dimensioning of the second conical screw 3 will be carried out in a similar manner.

[0138] In absolute terms, it would be possible to provide within the volumetric stage 11, in particular in an axial portion upstream of the volumetric stage 11 which would precede the isochoric volumetric stage 11 A, closed chambers 7, 8 in which no increase in the pitch P5, P6 would be provided, in particular to initially obtain a reinforced compression effect, within the volumetric stage 11, between the first chamber and the second chamber of the series of chambers 7, 8 considered, after the filling of the first chamber by the supply stage 30.However, for convenience of description, a preferred embodiment variant will preferably be considered in the following according to which the entire volumetric stage 11 is concerned by the compensation law LP5 11, LP6 11, that is to say that the isochoric volumetric stage 11A extends to the entire volumetric stage 11, over the entire axial extent of the latter, so that the upstream and downstream axial limits of the isochoric volumetric stage 11A coincide with the upstream 11U and downstream 11D limits of the volumetric stage 11.

[0139] The term “large external diameter” D_5C_max, D_6C_max will be used to designate the top diameter of the thread 5, 6 of the screw 2, 3, considered at the upstream end 11U of the isochoric volumetric stage 11 A, which thus forms the diameter of the large, circular base of the truncated cone-shaped envelope E2 of said isochoric volumetric stage 11 A.

[0140] Similarly, the "small external diameter" D_5C_min, D_6C_min, which is strictly less than the large external diameter D_5C_max, D_6C_max, will be designated as the top diameter of the thread 5, 6 of the screw 2, 3, considered at the downstream end 11D of the isochoric volumetric stage 11 A, which thus forms the diameter of the small, circular base of the truncated cone-shaped envelope E2 of said isochoric volumetric stage 11 A.

[0141] Advantageously, the large external diameter D_5C_max of the first screw 2 will be equal to the large external diameter D_6C_max of the second screw 3, and the small external diameter D_5C_min of the first screw 2 will be equal to the small outside diameter D_6C_min of the second screw 3.

[0142] To determine the compensation law LP5 11, the radius of the top of the thread to be used to obtain the desired cylinder capacity V2 will first be determined, along several different abscissas along the isochoric volumetric stage 11 A, and more preferably at least at the upstream end 11U and at the downstream end 11D of the volumetric stage 11 A, and preferably at one or more additional abscissas between these ends, then the points thus defined will be interpolated by means of a second-degree polynomial law, which will constitute the compensation law LP5 11 making it possible to define the top radius of the thread 5 along any abscissa of the isochoric volumetric stage 11 A.

[0143] For this purpose, the displacement V2 of the first screw 2 is first decided, that is to say the volume of extruded material that said screw 2 must expel each time said screw 2 performs a complete rotation around its central axis X2. This displacement V2 corresponds in practice to the individual volume V2, invariant, of the different chambers 7 in C of the first series of closed chambers 7. Said closed chambers 7 correspond to the different portions of the first channel 9 which follow one another along the first central axis X2 and which are each on the one hand included between said screw 2 and the internal wall of the sleeve 4, and on the other hand closed by the second thread 6 of the other screw 3 which penetrates into the first thread 5 so as to locally close the first channel 9.

[0144] We then refer to the truncated envelope E2 of the isochoric volumetric stage 11 A of the first screw 2 to calculate: - on the one hand the entry radius R2_in, which corresponds to the radius of the vertex 5C of the first thread 5 at the upstream end 11U of the isochoric volumetric stage 11 A, and which is therefore half the large external diameter D_5C_max, which corresponds to the diameter of the large base of the truncated cone envelope E2, - and on the other hand the exit radius R2_out of the isochoric volumetric stage 11 A, which corresponds to the radius of the vertex 5C of the first thread 5 at the downstream end 11D of the isochoric volumetric stage 11 A, and which is therefore half the small external diameter D_5C_min, which corresponds to the diameter of the small base of the truncated cone envelope E2.

[0145] To do this, we consider: - the length L1 1 A of the isochoric volumetric stage 11 A, which is, like the cylinder capacity V2, chosen by the designer, said length L2 being measured along the first central axis X2, that is to say along the straight line which carries the height of the frustoconical envelope E2 (and which therefore corresponds, in a section plane containing the first central axis X2, to the bisector of the angle at the apex of the frustoconical envelope E2), - the total length L tot which corresponds to the height measured between the large base of the first truncated envelope E2 and the apex S2 of the cone of said first truncated envelope E2, - the conicity angle A5 of the first screw 2, also chosen by the designer, preferably in the ranges of values ​​indicated above.

[0146] By simple trigonometry in a radial section plane containing the first central axis X2, and as illustrated in figure 7, we have: R2_in = L tot * tan (A5) R2_out = (L tot - L2) * tan (A5)

[0147] Knowing the chosen height H5 of the first thread 5, and therefore the projection H5' of said height in a plane normal to the central axis X2, we deduce therefrom a value Rl in called "apparent half-entry center distance", which, in the plane of the large base of the first frustoconical envelope E2, which plane is normal to the first central axis X2, corresponds to the radial distance measured between on the one hand the central axis X2 and on the other hand the intersection Ml of the plane of the large base of the first frustoconical envelope E2 with a straight line XI which corresponds to the median axis of the extruder 1 and which runs equidistant from the first central axis X2 and the second central axis X3, in the plane which contains both the first central axis X2 and the second central axis X3: Rl in = R2_in - (H5' / 2)

[0148] Similarly, in the plane of the small base of the first truncated envelope E2, we deduce the value Rl out from the apparent half-exit center distance: Rl out = R2_out - (H5' / 2)

[0149] In the plane of the large base of the first truncated envelope E2, we then identify, as illustrated in figure 13, the circular segment called "segment circular truncation input » 20_in, which corresponds to the domain between on the one hand the arc chord C20 which passes through the intersection point Ml and which is perpendicular to the radius from the first central axis X2 and on the other hand the circular arc which is delimited by this arc chord C20 and whose radius corresponds to the input radius R2_in.

[0150] The area A20_in of this circular segment with input truncation 20_in is: A20_in = V2 * (R2_in) 2* (alpha_in - sin(alpha_in)) where alpha in represents the angle covered by the circular arc that delimits the input truncation circular segment 20_in, and is therefore: alpha in = 2 * Arccos (RI in / R2_in)

[0151] Similarly, we consider the area of ​​the output truncation circular segment A20_o where alpha out represents the angle covered by the arc of the circle which delimits the circular segment 20_out, and is therefore: alpha out = 2 * Arccos (RI out / R2_out)

[0152] In the plane of the large base of the truncated cone shape E2, the area A7_in of the inlet chamber 7 is considered to be the difference between the area of ​​the ring between the bottom 5R of the first thread 5 and the top 5C of the first thread on the one hand, and the truncation area which is occupied by the thread 6 of the second screw 3 which penetrates into the channel 9 of the first screw 2.

[0153] Said truncation area, of substantially oval shape, is equal, in view of the symmetrical arrangement of the first screw 2 and second screw 3, to twice the area A20_in of the aforementioned circular truncation segment 20_in.

[0154] Thus, to the extent that, at the upstream end 11U of the isochoric volumetric stage 11 A, the top 5C of the first thread is located at a radius which corresponds to the entry radius R2_in, and the bottom 5R of the first thread at a radius located at a height H5' set back from the top 5C of the thread, we obtain:

[0155] Similarly, in the plane of the small base of the truncated cone shape E2, we can define the area A7_out of the outlet chamber 7, at the 2D downstream end of the first screw:

[0156] We then define the inlet pitch P5_in, considered at the axial abscissa of the upstream end 2U of the first screw, as being the ratio between the desired cylinder capacity V2 and the area A7_in of the inlet chamber as defined above: P5_in = V2 / A7_in

[0157] In an identical manner, we define the output pitch P5_out at the downstream end 2D of the first screw 2, as being the ratio of this same cylinder capacity V2, desired to be constant, by the area of ​​the output chamber A7_out (which is, in fact, smaller than the area A7_in of the inlet chamber): P5_out = V2 / A7_out

[0158] Once these two extreme step values ​​P5_in, P5_out are fixed, we can repeat the operation on one or more intermediate abscissas, and apply to the obtained point cloud a regression law, preferably a second-degree polynomial law, which will define the compensation law LP5 11.

[0159] The invention of course also relates to an installation 100 which, as can be seen in FIG. 14, comprises an extruder 1 according to any one of the characteristics described above, for delivering a material, preferably a rubber-based mixture.

[0160] The installation 100 comprises a frame 101. This frame advantageously forms a fixed reference, and may correspond to the floor of the building hosting the installation, or to a chassis possibly fixed to the building.

[0161] The installation 100 also comprises a receiving support 102, such as a tray, a drum or a toroidal core, which is intended to receive the material extruded by the extruder 1.

[0162] Said receiving support 102 is carried by the frame 101, and can be mounted to move relative to the frame 101. For example, in the case of a drum or a core forming a form of revolution around a main axis Y102, said drum or said core can be mounted in rotation, preferably in motorized rotation, relative to the frame 101, around said main axis Y 102.

[0163] The installation 100 further comprises a robotic transport device 103, such as a Cartesian robot, as illustrated in FIG. 14, or an anthropomorphic robotic arm, which carries the extruder 1 and which is arranged so as to be able, while the extruder 1 delivers the extruded material, to move said extruder 1 relative to the receiving support 102, in order to be able to place the extruded material in different locations of the receiving support 102, according to a predetermined desired arrangement.

[0164] The lightness and compactness of the extruder 1 according to the invention make it possible to use said extruder 1 within a mobile laying head 104, mounted on the robotic transport device 103.

[0165] The robotic transport device 103 which carries the extruder 1 is interposed between the frame 101 and the extruder 1 so as to be able, while the extruder delivers the extruded material, to move said extruder relative to the frame 101 and relative to the receiving support 102, according to a movement which is advantageously distinct, and controllable separately, from any specific movement which drives the receiving support 102 relative to the frame 101.

[0166] Thus, the robotic transport device 103 will preferably be able to move the extruder 1 in translation along at least one axis, preferably at least two axes, or even three orthogonal axes in order to position the extruder 1 in the frame of reference of the frame 101.

[0167] The robotic transport device 103 may for example comprise for this purpose at least one, preferably two, motorized translation plates 106, 107, for example two horizontal motorized translation plates 106, 107, which cross perpendicularly.

[0168] Furthermore, the robotic transport device 103 may preferably move the extruder 1 in rotation along at least one axis, two axes, or even three axes to orient the extruder relative to the receiving support 102 in pitch, roll and / or yaw.

[0169] The laying head 104 will comprise a die, connected to the output of the extruder 1, in order to give the extruded material a suitable shape, for example the shape of a flattened ribbon.

[0170] The laying head 104 may also comprise an applicator member 105, such as a pressure roller 105, arranged to press the extruded material which leaves the extruder 1 through the die against the receiving support.

[0171] The laying head 104, and more particularly the extruder 1, will preferably be fed by a continuous strip of material coming from a storage unit or a production unit.

[0172] The installation finally relates to an extrusion process using an extruder 1, or an installation 100, according to the invention.

[0173] In particular, the invention relates to the use of an extruder 1 according to the invention, or of an installation 100 according to the invention, for extruding a rubber-based mixture, for example for manufacturing a part of a vehicle wheel tire, in particular a part of a pneumatic tire.

[0174] Thus, the extruder 1, and more generally the installation 100, can be arranged to place a strip of raw rubber on a drum or on a toroidal core.

[0175] Preferably, on this occasion, the extruder 1 will provide a mass flow rate greater than or equal to 1 kg / min, for example between 1 kg / min and 6 kg / min, for a rotation speed of each of the first and second screws 2, 3 which is less than or equal to 300 rpm, for example between 10 rpm and 300 rpm.

[0176] These performances will preferably be achievable while the pressure at the outlet of the extruder, at the outlet of the last chamber 7, 8 and just at the inlet of the die, is between 150 bar and 500 bar, for a material temperature between 80°C and 150°C.

[0177] Of course, the invention is in no way limited to the embodiment variants described above, the person skilled in the art being able in particular to isolate or freely combine one or other of the aforementioned characteristics, or to substitute equivalents for them.

Claims

CLAIMS 1. Extruder (1) intended to extrude a material, said extruder (1) comprising: - a sheath (4), - a first screw (2) which is mounted in rotation in the sheath (4) around a first central axis (X2) and which is provided with a first thread (5), - a second screw (3) which is mounted in rotation in the sheath (4) around a second central axis (X3) and which is provided with a second thread (6), said first screw (2) and second screw (3) being counter-rotating and arranged in such a way that the first thread (5) and the second thread (6) cooperate to convey the material from upstream to downstream of the sheath (4), said extruder (1) being characterized in that: - the first screw (2) is conical, so that the top diameter (D_5C) of the first thread (5) decreases along the first central axis (X2), in the upstream-downstream direction, according to a first predetermined conicity angle (A5), - the second screw (3) is conical, so that the top diameter (D_6C) of the second thread (6) decreases along the second central axis (X3), in the upstream-downstream direction, according to a second predetermined conicity angle (A6), in that said extruder (1) comprises a stage called "volumetric stage" (11) within which the first thread (5) of the first screw (2) and the second thread (6) of the second screw (3) are interpenetrated and conjugated with respect to each other so as to form on the one hand, between the barrel (4) and the first screw (2), along the first central axis (X2), a first series of successive closed C-shaped chambers (7) and on the other hand, between the barrel (4) and the second screw (3), along the second central axis (X3), a second series of successive closed C-shaped chambers (8), so that the rotation of the first and second screw (2,3) generates a positive displacement of the material captured by the first series of chambers (7) and of the material captured by the second series of chambers (8), and in that at least part of the volumetric stage forms a stage called “isochoric volumetric stage” (11 A), within which:, - the pitch (P5) of the first thread (5) increases along the first central axis (X2), in the upstream-downstream direction, as the top diameter of the first thread (D_5C) decreases, according to a law called the “first compensation law” (LP5 11) which allows 1' progressive increase in the pitch (P5) of the first thread (5) to compensate for the conicity of the first screw (2) so that, in said isochoric volumetric stage (HA), the individual volume of each of the closed chambers (7) of the first series of closed chambers remains equal to the same predetermined constant nominal volume (V2), called "first screw displacement" (V2), with a maximum tolerance of + / - 2%, preferably + / -1%, or even + / -0.5%, and - the pitch (P6) of the second thread (6) increases along the second central axis (X3), in the upstream-downstream direction, as the top diameter of the second thread (D_6C) decreases, according to a law called the "second compensation law" (LP6 11) which allows the progressive increase in the pitch (P6) of the second thread (6) to compensate for the conicity of the second screw (3) so that, in said isochoric volumetric stage (HA), the individual volume of each of the closed chambers (8) of the second series of closed chambers remains equal to the same predetermined constant nominal volume (V3), called the "second screw displacement" (V3), with a maximum tolerance of + / - 2%, preferably + / - 1%, or even + / - 0.5%.

2. Extruder according to claim 1 characterized in that the first compensation law (LP5 11) is an increasing quadratic function of the axial abscissa value considered along the first central axis (X2), and, respectively, the second compensation law (LP6 11) is an increasing quadratic function of the axial abscissa value considered along the second central axis (X3).

3. Extruder according to claim 1 or 2 characterized in that the first conicity angle (A5) and the second conicity angle (A6) are each between 1.8 degrees and 3 degrees, preferably between 2 degrees and 2.5 degrees, even more preferably equal to 2.5 degrees.

4. Extruder according to one of the preceding claims, characterized in that, in the volumetric stage (11), whatever the angular position which is adopted respectively by each of the first and second screws (2, 3) around its central axis (X2, X3) during the counter-rotating movement of said first and second screws (2, 3), the number of chambers (7) of the first series, which are simultaneously in a closed state, as well as the number of chambers (8) of the second series, which are simultaneously in a closed state, is equal to or greater than four, or even equal to or greater than five, lower limit, and preferably less than or equal to twenty, or even less than or equal to twelve, upper limit, for example between four and ten, or between five and eight.

5. Extruder according to one of the preceding claims, characterized in that the cylinder capacity of the first screw (V2) and the cylinder capacity of the second screw (V3) are each between 8 cm 3and 50 cm 3 , for example between 10 cm 3 and 30 cm 3 , especially between 10 cm 3 and 20 3 cm.

6. Extruder according to one of the preceding claims, characterized in that, at least in the volumetric stage (11), the bottom diameter of the first thread (D_5R) decreases according to the first taper angle (A5), so that the height of the first thread (H5) varies by less than 20% along the first central axis (X2) in the volumetric stage.

7. Extruder according to one of the preceding claims, characterized in that it comprises a stage called the “feed stage” (30) which precedes the volumetric stage (11) and within which: - the first thread (5) is arranged so that, as the top diameter (D_5C) of said first thread (5) decreases along the first central axis (X2), in the upstream-downstream direction, according to the first conicity angle (A5), the pitch (P5) of said first thread (5) also decreases, along the first central axis (X2), in the upstream-downstream direction, in accordance with a law called the “first compression law” (LP5 30), so as to promote compression of the material as it approaches the volumetric stage (11), and - the second thread (6) is arranged so that, as the top diameter (D_6C) of the second thread (6) decreases along the second central axis (X3), in the upstream-downstream direction, according to the second taper angle (A6), the pitch (P6) of the second thread (6) also decreases, along the second central axis (X3), in the upstream-downstream direction, in accordance with a law called the “second compression law” (LP6 30), so as to promote compression of the material as it approaches the volumetric stage (11).

8. Extruder according to claim 7 characterized in that the core (12) of the first screw (2) has, in the feed stage, a straight cylindrical shape or a frustoconical shape whose conicity angle is strictly less than the first conicity angle (A5), and, respectively, the core of the second screw (3) has, in the feed stage (30), a straight cylindrical shape or a frustoconical shape whose conicity angle is strictly less than the second conicity angle (A6).

9. Extruder according to claim 7 or 8 characterized in that, in the feed stage (30), the first screw (2) and the second screw (3) are both multi-threaded, preferably double-threaded, so that the first thread (5) and the second thread (6) each comprise at least two threads which cover the same common axial extent and which are angularly offset from each other around the central axis (X2, X3) of the screw (2, 3) in question.

10. Extruder according to one of the preceding claims, characterized in that the first screw (2) and the second screw (3) are each, in the volumetric stage (11), single-threaded.

11. Installation (100) comprising an extruder (1) according to one of claims 1 to 10 for delivering a material, preferably a rubber-based mixture, the installation also comprising a receiving support (102), such as a tray, a drum or a toroidal core, which is intended to receive the material extruded by the extruder (1), as well as a robotic transport device (103), such as a Cartesian robot or an anthropomorphic robotic arm, which carries the extruder (1) and which is arranged so as to be able, while the extruder (1) delivers the material, to move said extruder (1) relative to the receiving support (102), in order to be able to place the extruded material at different locations of the receiving support (102), according to a predetermined desired arrangement.

12. Use of an extruder (1) according to any one of claims 1 to 10, or of an installation (100) according to claim 11, for extruding a rubber-based mixture.

13. Use according to claim 12 characterized in that the extruder (1) provides a mass flow rate greater than or equal to 1 kg / min, for example between 1 kg / min and 6 kg / min, for a rotation speed of each of the first and second screws (2, 3) which is less than or equal to 300 rpm, for example between 10 rpm and 300 rpm.

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