APPARATUS AND METHOD FOR EXTRUDING RUBBER MIXTURES

Abstract

An extrusion apparatus intended for the manufacture of a coextruded profiled element P for tyres, produced from rubber compounds of various compositions, comprises a profiling device (30) arranged at the outlet of at least two flow channels (34, 35) for rubbery compounds of different composition. The said channels receive the compounds from positive-displacement contrarotating twin-screw extruders (10, 20), the screw flights interpenetrating and having conjugated profiles.

Claims

1-11. (canceled)

12. An extrusion apparatus for the manufacture of a coextruded profiled element P for tires, produced from rubber compounds of various compositions, comprising: a profiling device arranged at the outlet of at least two flow channels for rubbery compounds of different compositions; and positive-displacement contrarotating twin-screw extruders, the screw flights of which are interpenetrating and have conjugated profiles, arranged such that the at least two flow channels receive the rubbery compounds of different compositions from the twin-screw extruders.

13. The extrusion apparatus according to claim 12, wherein each twin-screw extruder is fed by a secondary single-screw extruder.

14. The extrusion apparatus according to claim 13, wherein each twin-screw extruder and a corresponding secondary single-screw extruder are arranged in a common housing.

15. The extrusion apparatus according to claim 14, wherein the corresponding secondary single-screw extruder comprises an outlet end which communicates with a central inlet orifice into an internal chamber of each twin-screw extruder.

16. The extrusion apparatus according to claim 13, wherein each twin-screw extruder and each secondary single-screw extruder has its own drive means.

17. The extrusion apparatus according to claim 13, wherein an axis of rotation of the screw of each secondary single-screw extruder is transverse to an axis of rotation of the screws of each twin-screw extruder.

18. The extrusion apparatus according to claim 12 further comprising: a control unit configured to adjust a rotational speed of the screws of the twin-screw extruders to suit the dimensions of the coextruded profiled element P.

19. The extrusion apparatus according to claim 18, wherein the control unit adjusts the rotational speed of the screws of the twin-screw extruders to suit the operating conditions of an ancillary assembly line.

20. A method for the manufacture of a coextruded profiled element P for tires, produced from rubber compounds of various compositions, comprising the steps of: feeding two compound flow channels with rubbery compounds of different compositions by positive-displacement contrarotating twin-screw extruders, the screw flights of which are interpenetrating and have conjugated profiles; and extruding the coextruded profiled element P through a profiling tool coming from the two compound flow channels.

21. The method according to claim 20 further comprising a step of: adjusting a rotational speed of the screws of the twin-screw extruder to suit the dimensions of the coextruded profiled element P.

22. The method according to claim 20, wherein the flow rate supplied by each twin-screw extruder is greater than 20 kg/min for a speed less than or equal to 40 rpm.

Description

[0035] The following description is based on FIGS. 1 to 6 in which:

[0036] FIG. 1 schematically illustrates a perspective view of an extrusion apparatus according to one exemplary embodiment of the invention;

[0037] FIG. 2 is a view in section on a plane passing through the longitudinal axis X-X of the secondary extruder (40);

[0038] FIGS. 3a and 3b illustrate, by views in cross section, exemplary embodiments of the screws with which the twin-screw extruders of the apparatus of the invention are equipped;

[0039] FIG. 4 is a view in section on a plane passing through the longitudinal axes of the screws of the twin-screw extruders;

[0040] FIG. 5 is a view on a larger scale of part of the apparatus of FIG. 1;

[0041] FIG. 6 is a schematic view in cross section of a profiling device fed by extruders of the prior art.

[0042] The extrusion apparatus depicted in FIG. 1 comprises a first secondary extruder 40 and a second secondary extruder 50, which are extruders comprising an Archimedean screw. These extruders are each equipped, at an inlet end 42, 52, respectively, with a feed chute 41, 51, respectively, feeding a raw rubber compound A and B, the two compounds having different compositions. Each secondary extruder 40, 50 comprises a screw 45, 55 rotationally driven about its longitudinal axis X-X, Y-Y by a geared motor unit 44, 54 inside a barrel 46, 56 of circular cross section. Each rubber compound A, B is mixed, brought up to pressure and temperature, and homogenized before arriving at the outlet end 43, 53 of the extruder.

[0043] The extrusion apparatus also comprises a profiling device 30 having an extrusion orifice 31 the cross section of which gives the geometric shape to the coextruded profiled element. The profiling device for this purpose comprises an upper arch 36 and a lower arch 37 which with an intermediate support 38 delimit two flow channels 34, 35 each for one of the compounds A and B coming from the extruder 40 and from the extruded 50. The channels 34 and 35 open onto the one same extrusion orifice 31 through which the two compounds A and B are discharged. The extrusion orifice 31 is delimited by the walls of the profiling device 30 and allows the assembly made up of the coextruded compounds to be given the desired profile.

[0044] What is meant by rubber or elastomer is preferably any type of diene or non-diene elastomer, for example thermoplastic elastomer, or a blend of elastomers: natural rubber and synthetic rubber, reinforcing fillers: carbon black and silica, plasticizers: oils, resins and other chemical elements such as sulfur for example.

[0045] The extrusion installation of the invention seeks to create a coextruded profiled element based on various compounds. Such various elastomer compounds are, by way of example, the elastomer or rubber compounds used to create a tread assembly, such as: a first material made of 100% natural rubber, to create a sublayer with one, then a second material to create the tread, made of 100% synthetic rubber. It is possible to add a third or even a fourth material to create the sidewalls, this further material being made of a natural rubber/synthetic rubber compound (containing 20% to 80% natural rubber).

[0046] According to the invention, the extrusion apparatus comprises twin-screw positive-displacement extruders 10, 20 or twin-screw pumps, referred to hereinafter as twin-screw extruders, arranged between the secondary extruder 40, 50 and the profiling device 30. The twin-screw extruders are of the contrarotating type with interpenetrating flights and conjugated profiles and are arranged in such a way that each opens directly into the respective flow channel 34, 35 of the profiling device 30. Each twin-screw extruder 10, 20 is fed by a second extruder 40, 50, the outlet end 46, 56 of the latter being in communication with an inlet orifice into an internal chamber of the twin-screw extruder. The extruder 10 comprises two screws 11, 12 each one rotating about a longitudinal axis parallel to that of the flow channel 34. The extruder 20 comprises two screws 21, 22 each one rotating about a longitudinal axis parallel to that of the flow channel 35.

[0047] A twin-screw extruder is very compact for a given flow rate at a given temperature. Thus, a twin-screw extruder used in the apparatus of the invention has a diameter d for each screw 11, 12 or 21, 22 of 100 mm, a voids ratio (to be understood as meaning the ratio between the diameter of the central part and the outside diameter of the screw) close to 50%, a length-to-diameter ratio L/d equal to 4, a pitch equal to 100 mm, for a flow rate of around 22 kg/min at a speed of around 40 rpm.

[0048] The same is not true of a gear pump extruder of the prior art. In order to better highlight this difference, FIG. 6 illustrates a coextrusion apparatus using gear pumps of the prior art. When the solution of the invention, as visible in FIG. 4, and a gear pumps apparatus of the prior art (FIG. 6) are compared, it can be seen that, for an equivalent size h (which means the height of the inlet section into the flow channels into which the two compounds open in the direction of extrusion), a gear pump, fed by an extruder e1, e2 would need to have pinions p1, p2, p3, p4 of a diameter of 50 mm, and a width of 175 mm with a voids ratio of 20% maximum. A high rotational speed of 80 rpm (namely a tangential speed of 210 mm/s in the case of the twin-screw extruder) would yield only 11 kg/min. By contrast, the mean shearing would be 4 times higher and, in practice, the flow rate values would need to be limited to 5 kg/min in order to limit the heating-up which is liable to degrade or even vulcanize a raw elastomer compound. Furthermore, the gear pump implies lateral leakages or difficulty in achieving seailing at the edges of the pinions given the operating pressures. This problem is solved in the case of a twin-screw extruder since only the intake-side chamber is sealed with respect to the outside (at the geared-motor end).

[0049] More particularly, in the example illustrated in the figures, the twin-screw extruder or pump 10 comprises two screws 11, 12 with parallel axes A-A and B-B which, in the example described, are perpendicular to the axis X-X of the secondary extruder 40. The screws 11, 12 are arranged side-by-side in a housing which exhibits an internal chamber 14 of figure-eight-shaped cross section (FIG. 2). The screws 11, 12 each have a flight the cross section of which is substantially rectangular and constant along the length of the screw, they are arranged in such a way as to intermesh, the profiles of the two screws 11, 12 being conjugated and constituting, with the chamber 14, the means of transferring the rubber compound at constant flow rate between a feed inlet coming from the extruder 40 and its outlet via the flow channel 34. The profiles of the screws 11 and 12 thus exhibit a very small clearance in the central region of the extruder 10, which is to be understood as meaning a region close to a median longitudinal axis passing mid-way between the axes A-A and B-B, which means that the two screws are practically sealed with respect to the compound in this region, whereas they form C-shaped chambers with the periphery of the chamber 14.

[0050] The rubbery compound arrives via a central inlet orifice 16 coming from the secondary extruder 40. The screws 11, 12 are driven in rotation about their axes A-A and B-B in opposite directions by a geared motor unit 15 having two output shafts each one situated in the continuation of the axes A-A and B-B in order to cause the rubbery compound to advance towards the outlet of the extruder 10 via which it arrives in the channel 34.

[0051] Similarly, the twin-screw extruder or pump 20 comprises two screws 21, 22 with parallel axes C-C and D-D which, in the example described, are perpendicular to the axis Y-Y of the secondary extruder 50. The screws 21, 22 are arranged side-by-side in a housing which exhibits an internal chamber 24 of figure-eight-shaped cross section. The screws 21, 22 each have a flight the cross section of which is substantially rectangular and constant along the length of the screw, they are arranged in such a way as to intermesh, the profiles of the two screws 21, 22 being conjugated and constituting, with the chamber 14, the means of transferring the rubber compound at constant flow rate between a feed inlet coming from the extruder 50 and its outlet via the flow channel 35. The profiles of the screws 21 and 22 thus exhibit a very small clearance in the central region of the extruder 20, which is to be understood as meaning a region close to a median longitudinal axis passing mid-way between the axes C-C and D-D, which means that the two screws are practically sealed with respect to the compound in this region, whereas they form C-shaped chambers with the periphery of the chamber 24. The rubbery compound arrives via a central inlet orifice 26 coming from the secondary extruder 50. The screws 21, 22 are driven in rotation about their axes C-C and D-D in opposite directions by a geared motor unit 25 having two output shafts each one situated in the continuation of the axes C-C and D-D in order to cause the rubbery compound to advance towards the outlet of the extruder 20 via which it arrives in the channel 35.

[0052] The profile and dimensions of the chambers 14, 24 respectively are designed to correspond to those of screws 11 and 12 and of screws 21 and 22, respectively. More particularly, there is a small clearance between the periphery of the flights of the screws and the chamber, this radial clearance being comprised between 0.05 to 0.2 mm.

[0053] In the example illustrated in the figures, the screws 11, 12 and 21, 22, respectively, are identical, they each have two helical flights of trapezoidal cross section and a pitch p that is constant over the length of the screw, they are arranged in such a way as to intermesh, the profiles of the two screws 11, 12 and 21, 22 being conjugated. The width of a void between two screw flight flanks is equal to the width of the flank so that a flank fits into the void of the screw opposite. By way of example, the clearance between two adjacent flanks of screws 11 and 12 is around 0.3 mm.

[0054] The C-shaped chambers formed inside the chambers 14, 24, respectively, are bounded towards the outside by the walls of the chambers 14, 24 respectively, and laterally by the flights of the screws 11 and 12, and of the screws 21 and 22, respectively. In the region in which the flights of the adjacent screws 11 and 12, and screws 21 and 22, respectively, intermesh with one another, the C-shaped chambers are separated by a sealing effect.

[0055] In an alternative form, the flights of the screws 11, 12 and 21, 22 has a pitch that is variable, the pitch in the feed zone, which communicates with the extruders 40, 50, having the smallest value.

[0056] The transfer of each elastomer compound towards the extrusion die or profiling device 30 as performed by the twin-screw extruders 10, 20 is performed at constant flow rate along the flow channels 34, 35.

[0057] The feed pressure for compound coming from one of the extruders 40, 50 is higher than atmospheric pressure and chosen to be high enough to overcome the pressure drop through the extrusion die or profiling device 30.

[0058] In an alternative form of embodiment, the feed pressure at the outlet of one or the other of the extruders 40, 50 is high enough, for example equal to around 200 bar, being chosen so as to overcome the pressure drop experienced by the compound as it passes along the flow channels 34, 35 and through the extrusion die, and is kept substantially constant by the twin-screw extruder or pump 10, 20, respectively.

[0059] In another alternative form of embodiment, the feed pressure at the outlet of one or the other of the extruders 40, 50 is lower, for example around 2 bar and the pressure at the outlet of the twin-screw extruder or pump 10, 20 respectively, is increased by the latter to around 200 bar.

[0060] In yet another alternative form of embodiment, the feed pressure is high and its outlet value is further increased by the twin-screw extruder or pump.

[0061] FIGS. 3a and 3b illustrate two preferred embodiments of the flights of the screws of the twin-screw extruders 10, 20. The flights of the screws 11, 12 in FIG. 3a are of substantially trapezoidal shape with flanks 11a, 12a of concave shape. The flights of the screws 11, 12 in FIG. 3b are of substantially trapezoidal shape with flanks 11b, 12b of convex shape. These flights ensure better sealing of the C-shaped chambers and therefore better efficiency of the twin-screw extruder.

[0062] The screws 11, 12 and 21, 22 of the twin-screw extruders 10 and 20 are of the single-flight type or, in an alternative form of embodiment, may exhibit several flights. The flights preferably have a constant pitch. In an alternative form of embodiment, for example when an increase in pressure is desired, the pitch may be variable.

[0063] As best visible in FIG. 2, the first secondary extruder 40 and the first twin-screw extruder 10 are arranged in a common housing 40. As a result, the secondary extruder 40 works and increases the pressure of the first compound A and then transfers it directly to the twin-screw extruder 10 via a central orifice 16. The same is true of the secondary extruder 50 and the second twin-screw extruder 20 which are arranged in a common housing 50, the pressurized compound being transmitted from the first to the second via the central orifice 26.

[0064] The two twin-screw extruders 10 and 20 are arranged in parallel by being juxtaposed (it is to be understood that they are on the same side, as viewed in relation to a vertical plane passing through the outlet orifice 31) to converge into the profiling device 30. The longitudinal axis A-A (or respectively B-B) of the twin-screw extruder 10 makes an acute angle with the axis C-C (or respectively D-D) of the twin-screw extruder 20. The coextruded profiled element P is obtained by coextrusion by passing two separate streams of compound, a first one coming from the positive-displacement twin-screw extruder 10 via the flow channel 34 and a second coming from the positive-displacement twin-screw extruder 20 via the flow channel 35, through the profiling device 30 passing in the direction of the arrow F and being pulled off by a motorized roller (not illustrated). In an alternative form of embodiment, the profiling device collaborates with a rotary roller (not illustrated) on which the coextruded product P is placed.

[0065] The apparatus also comprises a control unit 16 which is able to adjust the rotational speed of the screws 11, 12 and of the screws 21, 22 of the twin-screw extruders 10 and 20 according to the dimensions of the coextruded profiled element P. The dimensions of the coextruded profiled element P, notably the width and thickness, are preestablished and measured using a sensor 70, for example an optical sensor. The sensor 70 is connected to the control unit 60 to which it transmits the measurement signal so as to allow the dimensions of the profiling device to be optimized at the start of the operation. Once optimization has been achieved, the sensor 70 checks the dimensions of the coextruded profiled element and its compliance with preestablished dimensions. In an alternative form of embodiment, a weight sensor measures the weight of the coextruded profiled element P, and makes even finer adjustments to the tolerances on the coextruded product.

[0066] The control unit 60 operates the geared motor units 15 and 25 according to the desired dimensions of the coextruded profiled element P or according to the operating conditions (variable speed, stop-start conditions).

[0067] The twin-screw extruders 10 and 20 convey the compound through the peripheral C-shaped chambers in such a way that the material contained in the chambers advances by the magnitude of one pitch of the screw flight per revolution. As a result, adjustments to the rotational speed of the screws of each twin-screw extruder 10, 20 are in direct proportion with variations in its flow rate. This allows the control unit 60 to adjust the flow rate of each extruder according to dimensions of the coextruded product and also to keep it constant during operation.

[0068] The control unit 60 is also connected to the geared motor units 44 and 54 of the secondary extruders 40 and 50 and is able to control the rotational speed of the screws 42 or 52. This control is performed in synchronism with control of the geared motor units 15, 25 on the basis of experimentally preestablished relationships. In an alternative form of embodiment, the rotational speed of the screws of the secondary extruders is established on the basis of data received from pressure sensors situated in the arch of each secondary extruder.

[0069] The control unit 60 may also be connected to the pull-off means that pull off the strip of coextruded profiled element P in order to adjust the speed thereof.

[0070] Other alternative forms and embodiments of the invention may be envisaged without departing from the scope of these claims.

[0071] Thus, the twin-screw extruder may be fed directly with a rubber compound. For that, the screws are modified to exhibit an inlet and feed portion which conveys and works the compound before it arrives in the C-shaped metering chambers.

[0072] The apparatus of the invention may comprise several twin-screw extruders arranged in parallel to create a coextruded profiled product based on several non-vulcanized compounds of different composition.

[0073] In one alternative form of embodiment, the profiling device comprises one or several flow channels fed directly by a single-screw extruder and other channels fed by (two or several) positive-displacement twin-screw extruders according to the invention.