Patent Publication Number: US-2021170661-A1

Title: System and method for extruding complex profiles from elastomer mixtures

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to and the benefit of PCT Patent Application No. PCT/FR2019/051071, filed May 13, 2019, entitled “SYSTEM AND METHOD FOR EXTRUDING COMPLEX PROFILES FROM ELASTOMER MIXTURES” and French Patent Application No. 1854016 filed May 14, 2018 entitled “SYSTEM AND METHOD FOR EXTRUDING COMPLEX PROFILES FROM ELASTOMER MIXTURES”. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The disclosure relates to the field of the extrusion of elastomer compounds intended for the manufacture of tires. More particularly, it relates to the manufacture of co-extruded complex profiled elements based on elastomer compounds, such as tire treads. 
     2. Related Art 
     In the known way, a tire tread is produced in the form of a complex profiled element by co-extruding elastomer compounds, most of which have different compositions, using multiple extruders connected to a common extrusion head. In the known way, an extruder is formed of a cylindrical body or barrel which is stationary, and inside which there is an Archimedean screw that is coaxial with the longitudinal axis of the barrel and driven in rotation about this axis. Its purpose is to homogenize a rubber compound introduced into it and to drive it towards the outlet die of the extrusion head. In the case of a complex product, the outlet die receives multiple elastomer compounds of different compositions arranged side-by-side and which come together before the outlet. The die comprises an outlet orifice the geometric shape of which determines the profile of the co-extruded rubber strip. This profile can be defined between a fixed profiled blade which collaborates with a rotary roller or with a fixed wall. 
     In order to manufacture a tread using co-extrusion, use is generally made of one extruder per type of elastomer compound (what is meant by type is the composition of said compound), the dimensions of the extruder (the diameter of the extrusion screw) being chosen in proportion to the volume of compound extruded. For example, in the case of a tread of which the external part intended for running on represents 80% of the volume and to which is added a sublayer representing approximately 15% of the volume and two lateral edges together representing the remaining 5%, use is generally made of three extruders: a larger first one of which the screw diameter is 250 mm, a second one, for the sublayer, having a screw diameter of 200 mm, and a small third extruder having a diameter of 120 mm. 
     There is now a drive to improve tire performance by using more complex treads which contain an even greater number of components, for example up to seven different components, with very variable volume proportions between the various components. Each component has its own clearly defined properties, for example the tread portion situated on the outside of the profiled strip comprises an elastomer composition containing silica which has good rolling resistance properties, supported by a strip of elastomer of a different composition having shock-absorbing properties, with a small strip of a conductive elastomer compound which is inserted into the previous two strips, all of this supported by a sublayer that has good properties of adhesion to the components situated radially on the inside of the green tire, as well as lateral edges made from a compound different from the previous ones, or even a compound that incorporates mixed offcuts. Thus, because each compound has different properties from another, it is obviously necessary to increase the number of extruders in order to manufacture such a complex tread. In the case of complex treads, it is difficult to master the geometry of the product leaving the extrusion die, because each path (each extruder) is calibrated to its own operating parameters (notably in terms of the supplied flow rate and pressure), but these parameters vary as the mixture passes through the common die, as a function of parameters of the extruders that are feeding into the other paths. This results, on the one hand, in a fairly lengthy time required to bring the installation into service and, on the other hand, in scrap which is furthermore in the form of mixed offcuts, which, being mixed, are therefore difficult to reuse. 
     In order to alleviate this problem, patent application WO2017/109419 in the name of the Applicant proposes an extrusion installation for creating a tread which uses positive-displacement counter-rotating twin-screw extruders with intermeshing screw flights with mating profiles. When such positive-displacement twin-screw extruders are used on each of the paths of the installation, it becomes possible to control the flow rate and, therefore, the geometry of the product right from the very start of the coextrusion operation. Although this operates satisfactorily, it has however been found that such an installation soon becomes complex when the number of paths is high, for example greater than four. 
     SUMMARY OF INVENTION 
     One objective of the disclosure is to overcome the drawbacks recalled herein and to propose an extrusion installation of optimized and economical construction, while at the same time making it possible to obtain a good-quality co-extruded product. 
     This objective is achieved by the disclosure, which proposes an extrusion installation for the manufacture of a strip of a profiled element, such as a tread, based on elastomer compounds by co-extrusion, and comprising multiple extruders feeding elastomer compounds to an extrusion head, wherein the extrusion head receives a proportion of elastomer compound of between 2 and 25% of the total volumetric throughput of the installation from at least one Archimedean-screw extruder, and the rest from a positive-displacement extruders. 
     What is meant by the total volumetric throughput of the installation is the volume of material that passes through the extrusion head (which comprises the outlet die) in a given space of time. Several different compounds, each having specific physical-chemical properties, constitute the products which, in variable proportions, are involved in the make up of a complex tread. The volume occupied by each of these products with respect to the total volume of the tread varies according to the recipe selected. According to the disclosure, the extrusion head is fed directly from Archimedean-screw extruders, which are not positive-displacement extruders, in respect of a small proportion of the volumetric throughput of the extrusion head, preferably less than 25% of the total volume. The installation also comprises positive-displacement extruders to feed the extrusion head directly with the majority of the volumetric throughput of elastomer compounds of the same head, notably in respect of the remaining volume which represents at most 75% of the total volumetric throughput of the head. 
     This makes it possible to obtain rapid control of the throughput right from the very start-up of the installation, for a compact construction thereof. Indeed, the volumetric throughput of the non-positive-displacement paths is low in comparison with the total volumetric throughput and, even if there are variations in the throughput coming from the non-positive-displacement paths, that variation is negligible, making it possible to obtain an extruded strip that is correctly dimensioned, and is so right from the very start of the run. Furthermore, non-positive-displacement extruders are of simpler construction and occupy less space because they comprise a fairly low number of components (in comparison with positive-displacement extruders) and have nominal dimensions (diameter and length of the extrusion screw) which are small because they are linked directly to the low throughput that the extruder is to supply. 
     As a preference, the flights of the screw of the Archimedean-screw extruder are shallow, the height of the flight being less than 0.2 the value of the diameter of the screw, for a short pitch, the value of which is less than 1.5 the value of the diameter of the screw. Such an extruder of optimized geometry allows better control over the throughput. 
     As a further preference, the length of the screw of the Archimedean-screw extruder is greater than 8 times the value of its diameter. This makes it possible to obtain a stabilized, and therefore better controlled, throughput. 
     Advantageously, the positive-displacement extruders are positive-displacement counter-rotating twin-screw extruders with intermeshing screw flights with mating profiles. Such positive-displacement extruders ensure a throughput that is controlled and constant over time, while at the same time having a compactness that makes them compatible with a coextrusion installation. 
     As a preference, the extrusion head comprises ducts for distributing the elastomer compounds coming from the extruders toward an outlet die, and the Archimedean-screw extruder is connected to the outlet die by a duct of a length less than that of the distribution ducts of the other extruders. The distribution duct that connects the outlet die to the Archimedean-screw extruder is thinner (its cross-sectional area being far smaller than that of the positive-displacement paths) and its location close to the outlet makes it possible to reduce the value of the thrusting pressure that the Archimedean-screw extruder has to apply and to thus optimize pressure drops. 
     Advantageously, the distribution ducts extend in a direction substantially perpendicular to the direction in which the flows of elastomer compounds coming from said extruders flow. Such a solution makes it possible to achieve better cohesion between the two compounds and avoid any problem with the interfacing between them. 
     As a preference, the extruders are arranged on either side of the head with respect to a plane of symmetry passing through the outlet orifice of the die. This solution allows better balancing of the flows coming from the various extruders. 
     Advantageously, the extrusion head is connected directly to the extruders without there being any elastomer-compound transfer duct between the two. Thus, the end of the screw of each extruder substantially reaches the corresponding inlet orifice in the extrusion head, and this greatly reduces the pressure drop. The remaining volume can thus be used to create, in the extrusion head, inlet ducts that are wider than those of the extruders of the prior art, thereby making it possible to reduce the extent to which the compound is heated, and increase the productivity of the machine. 
     Preferably, the Archimedean-screw extruders are identical and the positive-displacement extruders are identical to one another. The maintenance of the installation is thus made easier. 
     Advantageously, the extrusion head comprises an assembly of several removable plates positioned side by side. Such a construction involving removable plates makes it possible to obtain an extrusion head that is of simplified and flexible construction. 
     The objective of the disclosure is also achieved with a method for the manufacture of a strip of a profiled element, such as a tread, based on elastomer compounds by co-extrusion, using an installation comprising multiple extruders feeding elastomer compounds to an extrusion head, wherein a proportion of between 2 and 25% of the total volumetric throughput of the installation, coming from at least one Archimedean-screw extruder, and the rest coming from positive-displacement extruders, are passed simultaneously through the extrusion head. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The disclosure will be understood better from the rest of the description, which is supported by the following figures: 
         FIG. 1  is a perspective view of one example of a tread obtained using the installation of the disclosure; 
         FIG. 2  is a schematic perspective view of the extrusion installation of the disclosure; 
         FIG. 3  is a schematic view in longitudinal section of the installation of  FIG. 2 . 
     
    
    
     In the various figures, elements that are identical or similar bear the same reference. Their description is therefore not systematically repeated. 
     DETAILED DESCRIPTION OF THE ENABLING EMBODIMENT 
       FIG. 1  illustrates a complex profiled-element strip  200 , such as a tread for a tire, produced from several elastomer compounds in order to create the various components A, B, C, D, E and F of the tread. The various compounds arrive from the plurality of extruders  10 ,  20 ,  30 ,  40 ,  50  and  60  belonging to an extrusion installation  1  ( FIG. 2 ), which compounds converge on one another in a common extrusion head  2  to create, by coextrusion, a complex profiled-element strip  200 . The complex profiled-element strip  200  is, in the example of  FIG. 1 , a tread for a tire. It comprises a sublayer A and an adjacent layer B situated on top of it, and an external layer C. Two end profiled elements D complete the external layer C, which additionally comprises two conducting inserts E. The assembly is finally completed by two lateral edges F. 
     Such elastomer compounds for creating a tread are, by way of example, compounds based on elastomers or rubber which are used to create a tread assembly, such as: a first material made 100% of natural rubber, to create a sublayer A, then a second material which is made 100% of a synthetic rubber, for example containing silica, to create the external part C of the tread, and a third material not containing silica but having absorbent properties, to form an intermediate layer B, followed by a fourth and a fifth material to create the end parts D and the lateral edges F and made up for example of a compound of natural/synthetic rubber (containing 20% to 80% natural rubber), and a sixth material E which is a conductive compound containing a mixture of natural/synthetic rubber and carbon black. 
     According to the disclosure, the extrusion head  2  of the extrusion installation of the disclosure receives a proportion of elastomer compound of between 2 and 25% of the total volumetric throughput of the installation from non-positive-displacement extruders  10  and  20  which are of the Archimedean-screw extruder type, and the rest from positive-displacement extruders  30 ,  40 ,  50  and  60 . The Archimedean-screw extruders  10 ,  20  comprise a screw which rotates inside a barrel coaxial with the screw, rotationally driven by a geared motor unit. They are more compact in bulk and simpler in construction in comparison with the positive-displacement extruders  30 ,  40 ,  50  and  60 . Furthermore, a complex tread is made up of products in very small volume, notably the conducting insert E and the lateral flanks F of the tread illustrated in  FIG. 1 . The proportion that the volume of these components represents in comparison with the volume of all of the components of the tread or, in other words, the volumetric fraction of the components E and F within the profiled-element strip  200  which incorporates the components A to F, is comprised between 2 and 25%. Such a volumetric fraction is therefore very small in comparison with the total volume of the strip. Now, given that the components that occupy most of the volume of the strip (between 75 and 98%) are supplied by positive-displacement extruders the throughput of which is controlled with precision, the result is that the variation in the throughput of the extruders supplying components E and F does not lead to very significant variations in the total volume of the complex profiled-element strip  200 . This then yields an extrusion installation  1  which is compact and less bulky and which at the same time makes it possible to obtain a co-extruded profiled-element strip of fairly precise dimensions. 
     According to one advantageous aspect of the disclosure, the Archimedean-screw extruders  10 ,  20  are dimensioned in such a way that their operational throughput is as constant as possible. As is best visible in  FIGS. 2 and 3 , the extruder  10 ,  20  comprises a screw  15 ,  25  which rotates about an axis  11 ,  21  inside a barrel  16 ,  26 , driven by a geared motor unit (not depicted). The dimensions of the screws  15 ,  25  are designed so that their screw flights are shallow and so that the pitches of the screws are fairly short. Thus, the screw flight of each screw  15 ,  25 , has a height less than 0.2 times the diameter of the screw and a short pitch, preferably less than 1.5 times the diameter of the screw. The length of each screw  15 ,  25  is chosen so that it is greater than 8 times the diameter of the screw. 
       FIG. 2  illustrates an extrusion installation  1  of the disclosure, comprising multiple extruders  10 ,  20 ,  30 ,  40 ,  50 ,  60  for creating a profiled product, by co-extrusion, in a common extrusion head  2 . The profiled product obtained is a tread  200  and comprises multiple components, six in the example illustrated, produced from elastomer materials coming from different extruders. The extruders  10 ,  20 ,  30 ,  40 ,  50 ,  60  have been illustrated schematically, depicting only their respective screws, but it will be appreciated that, in a way that is generally known, the screw of each extruder rotates inside a barrel equipped with an elastomer-compound feed inlet and with an outlet which opens into the extrusion head  2 . The extruders  10  and  20  are of the Archimedean-screw type, as previously described, whereas the extruders  30 ,  40 ,  50  and  60  are positive-displacement counter-rotating twin-screw extruders with intermeshing screw flights with mating profiles. Each positive-displacement extruder comprises two parallel screws rotating in contrary directions inside a barrel of figure-eight cross section, so as to form C-shaped sealed chambers with the interior walls of said barrel. Each positive-displacement extruder is fed by a feed extruder  70 ,  80 ,  90 ,  100  which is an extruder of the Archimedean-screw type of axis  71 ,  81 ,  91 ,  101  each perpendicular to respective axis  31 ,  41 ,  51 ,  61  of the positive-displacement extruder screws. Such positive-displacement extruders are of the type described in patent application WO2017/109419 in the name of the Applicant. 
     The extruders  10 ,  20 ,  30 ,  40 ,  50 ,  60  are arranged on each side of the extrusion head  2  and the longitudinal axes of the screws  10 ,  20  and the central axis of each of the positive-displacement extruders  30 ,  40 ,  50 ,  60  (what is meant by central axis is an axis parallel to the longitudinal axis of the screws of the twin-screw extruder and passing through the center of the outlet opening of said extruder) are situated in the plane of symmetry P of the tread  200 . The plane of symmetry P is a vertical plane that passes through the center of gravity of the strip, when the product is a symmetrical product or, in the case of an asymmetric product, through the center of inertia thereof. What is meant by extruders situated on either side of the extrusion head is extruders facing one another by being arranged on either side of a vertical plane perpendicular to the plane of symmetry P and passing through the center of the extrusion head  2 . Such an arrangement makes it possible to balance the flows from the various extruders that pass through the extrusion head. In a variant (not illustrated in the drawings), the various extruders are arranged on either side of the extrusion head  2  without their respective longitudinal axes lying in the plane of symmetry P. In yet another variant, certain extruders are arranged on the sides of the extrusion head  2 , or even in the opposite part to the outlet-die part thereof. 
     In operation, when each extruder is fed with an elastomer compound, the various compounds extruded by the extruders  10 ,  20 ,  30 ,  40 ,  50  and  60  pass along the distribution ducts provided for that purpose in the extrusion head  2  without mixing and converge towards a die  3  which gives the product, in this instance the tread  200 , its final shape. 
     In the example illustrated in the figures, the extruders  10 ,  20 ,  30 ,  40 ,  50 ,  60  are arranged with their longitudinal axes  11 ,  21 ,  31 ,  41 ,  51 ,  61  mutually parallel, and perpendicular to the lateral walls  22 ,  23  of the extrusion head  2 , three extruders  10 ,  30  and  50  being situated to the right of the vertical midplane of the extrusion head and the other three  20 ,  40  and  60  to the left of this plane. The extruders on the right and those on the left face one another; they are situated in pairs at one and the same height, their longitudinal axes being situated in one and the same plane. However, the extruders on the right could be arranged at a different height from those on the left. In a variant, the longitudinal axes of the extruders are not parallel, but at an angle to one another, it being possible for the angle formed by the longitudinal axes of two adjacent extruders to differ from that formed by the longitudinal axes of two other adjacent extruders. In yet another variant, it is possible to envisage a different number of extruders on the right compared with those situated on the left of the head. 
     The extrusion head  2  is made up of a stack of several plates, six in the example illustrated in  FIG. 5 :  2   a ,  2   b ,  2   c ,  2   d ,  2   e ,  2   f  parallel to one another and parallel to the lateral walls  22 ,  23  of the extrusion head and perpendicular to the longitudinal axes of the extruders  10 ,  20 ,  30 ,  40 ,  50 ,  60 . The plates  2   a  to  2   f  have a length equal to or slightly smaller than that of the lateral walls  22 ,  23  of the head and between them define transverse distribution ducts  5   a ,  5   b ,  5   c ,  5   d ,  5   e  which carry the elastomer compound coming from the extruders of the installation towards the outlet die  3 . The plates  2   a  to  2   f  are removable and are held together and attached to the lateral walls of the extrusion head for example by a screw-fixing (not depicted). The installation of  FIG. 2  is able to create the six-component tread  200  of  FIG. 1 . 
     In the example illustrated in  FIG. 3 , the extrusion head comprises a distribution duct  5   d  which is common to the extruders  40  and  50 . 
     According to one advantageous aspect of the disclosure, the extrusion head  2  is interchangeable and is connected directly to the extruders  10 ,  20 ,  30 ,  40 ,  50 ,  60  without there being any elastomer-compound transfer duct between the two. Thus, the choice of making two extruders collaborate with one another is made by fitting the appropriate extrusion head, and then in operating the extruders according to the geometry of the product that is to be obtained. The quantity of product sent to the die is regulated by adjusting the rotational speed of the extruder screws. Furthermore, the extruders  10 ,  20 ,  30 ,  40 ,  50 ,  60  deliver directly into the extrusion head  2 , thereby markedly limiting the pressure drops. 
     According to another advantageous aspect of the disclosure, the Archimedean-screw extruders  10 ,  20  are identical to one another and the positive-displacement extruders  30 ,  40 ,  50 ,  60  are identical to one another. The maintenance of the installation is thus made easier. 
     Furthermore, a “facing one another” arrangement is advantageous because it reduces the size of the dimensional tooling, notably of the extrusion head, providing for better ergonomics and at the same time making it possible to reduce the weight of same. 
     In operation, an extrusion head  2  fitted with a die  3  able to create a complex tread  200 , based on multiple elastomer compounds of different compositions by co-extrusion of elastomer compounds originating from the extruders  10 ,  20 ,  30 ,  40 ,  50 ,  60  is selected and introduced into a support provided for that purpose in the installation  1 . The extrusion head is locked in place using the quick-fit means (which are not illustrated in the figures). The extrusion operation is performed. 
     Other variants and embodiments of the disclosure can be envisaged without departing from the scope of its claims. 
     Thus, more than six extruders can be arranged on either side of the extrusion head, or even on the other faces of the extrusion head that are not assigned, one to the outlet of the compounds towards the extrusion die and the other to the grasping of the head so that it can be moved around, at least two of them working with different compounds and certain others working with one and the same elastomer compound. 
     Moreover, it is possible to use other types of positive-displacement extruders, for example of the geared pump or piston pump type.