Abstract:
A process and an apparatus for building tires for vehicle wheels wherein the process includes the step of assembling components of elastomeric material on a forming support, in which at least one of the components of elastomeric material is manufactured by the steps of: i) dispensing a continuous elongated element of elastomeric material; ii) applying the continuous elongated element in the form of coils disposed in side by side relationship or at least partly superposed, wound up on the forming support, so as to form the at least one component of elastomeric material of the tire; iii) exerting a first pressure on a central region of a portion of the continuous elongated element applied onto the forming support; and iv) exerting a second pressure on side regions of the portion of the continuous elongated element applied onto the forming support.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a national phase application based on PCT/IB2010/052659, filed Jun. 15, 2010, which claims the priority of Italian Application No. MI2009A001238, filed Jul. 13, 2009, and the benefit of U.S. Provisional Application No. 61/213,848, filed Jul. 21, 2009, the content of all of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a process and an apparatus for building tires for vehicle wheels. 
     2. Description of the Related Art 
     A tire for vehicle wheels generally comprises a carcass structure including at least one carcass ply having respectively opposite end flaps in engagement with respective annular anchoring structures, integrated into the regions usually identified as “beads”. 
     Associated with the carcass structure is a belt structure comprising one or more belt layers, located in radially superposed relationship with each other and with the carcass ply and provided with textile or metallic reinforcing cords having a crossed orientation and/or being substantially parallel to the circumferential extension direction of the tire. 
     A tread band is applied at a radially external position to the belt structure, which tread band is made of elastomeric material like other semifinished products constituting the tire. 
     Respective sidewalls of elastomeric material are further applied at an axially external position to the side surfaces of the carcass structure, each extending from one of the side edges of the tread band until close to the respective annular anchoring structure to the beads. 
     After building of the green tire by assembly of respective semifinished products, a vulcanization and molding treatment is generally carried out, which aims at determining the structural stabilization of the tire by cross-linking of the elastomeric compounds and also at impressing the tread band, wound up around the carcass before vulcanization, with a desired tread pattern and the region close to the tire sidewalls with possible distinctive graphic marks. 
     Within the present description and in the following claims, by “elastomeric material” it is intended a composition including at least one elastomeric polymer and at least one reinforcing filler. Preferably, this composition further comprises additives such as cross-linking agents and/or plasticizers. Due to the presence of the cross-linking agents, this material can be cross-linked by heating, so as to form the final article of manufacture. 
     Document WO 2009/040534 in the name of the same Applicant, discloses a process for manufacturing tires for vehicle wheels, which process comprises the steps of: building the carcass structure of a green tire on a first forming drum in at least one carcass-building line, building a crown structure on a second forming drum in at least one crown-building line, shaping the carcass structure into a toroidal conformation, while it is being assembled to the crown structure in at least one assembly and shaping station. The assembly and shaping station is synchronized with the carcass-building line and the crown-building line. Each carcass structure is associated with the respective first forming drum on which it is built up to the end of the assembly and shaping step. The above mentioned stations are operatively associated with units adapted to supply elementary semi-finished products, such as continuous elongated elements of elastomeric material, strips of elastomeric material containing two or more textile or metallic cords, individual textile or metallic of coated with elastomeric material. Once the careen tire has been built, it is cured and molded in at least one vulcanization and molding line separated from the building line. In the above described work stations, said elementary semi-finished products can be wound up into coils disposed in side by side relationship and/or at least partly superposed, such as in the case of the continuous elongated elements for example, contributing to formation of the liner, under-liner, under-layers, fillers present in the regions of the beads, sidewalls, tread band. 
     In this regard, document EP 1 375 118 discloses a green tire, or a tire component, that is manufactured by winding into coils and rolling a rubber strip on a building drum. To this aim, a building device comprises a rigid support or building drum, on which the rubber strip is wound into coils and rolled, an extruder adapted to extrude the rubber strip, a winder for winding the rubber strip on the rigid support, and a flattening roller, adapted to flatten the projection of a step portion produced by a first part of the rubber strip and a second part of the same rubber strip overlapping the first one. Flattening prevents the residual air from remaining entrapped during vulcanization, in the regions close to the step portions between the outer surface of the tire component and the vulcanization mold and avoids the consequent presence of hollows and slits in the finished tire. The surface of the flattening roller is provided with a knurl. The step of spirally winding and rolling the rubber strip and the flattening step can be carried out simultaneously. 
     Document EP 1 754 592 discloses a method of building a tire, said method allowing elimination of the residual air between the step portions of an overlapped portion of a tire component and the vulcanization mold, so as to avoid formation of hollows, slits, etc. causing a reduction in the tire lifetime. The method comprises the steps of flattening a step formed at an overlapped portion of a tire component and optimizing the cross-section shape of said overlapped portion. The apparatus used for putting this method into practice comprises a building platform, an extruder for extrusion of a rubber strip and a pressure roller. A component of the tire is formed by spirally winding the rubber ribbon-like strip, extruded through the extruder, on the building platform and forming an overlapped portion of this rubber strip. A step of this overlapped portion that is exposed on the outer surface of the tire component is flattened under pressure through the pressure roller that is such disposed as to face the building platform by a specific angle relative to the extension direction of the rubber strip and is heated to the plasticization temperature in order to make the outer tire surface smooth. 
     SUMMARY OF THE INVENTION 
     The Applicant has noticed that, in addition to the air entrapped between the steps defined by the rubber strip and the cavity of the vulcanization mold, as described in documents EP 1 754 592 and EP 1 375 118 mentioned above, also air pockets or more generally gas pockets are formed under the continuous elongated element of elastomeric material already during laying of said element into at least partly superposed coils. During vulcanization, since the radially outermost (tread band) and radially innermost (the liner, for example) surface layers that are cured the first acquire imperviousness features, the air contained in these pockets remains entrapped therein or moves to the outer surface but does not escape therefrom, thus forming surface bubbles. 
     The Applicant has further noticed that the higher the laying temperature of the continuous elongated element is, the more important the just described phenomenon concerning formation of air pockets is, and that said temperature increases on increasing of the laying speed, or on decreasing of the time intervening between ejection of the compound of the continuous elongated element from the extruder and application of said element into coils on the respective forming drum. 
     The Applicant has therefore ascertained that this phenomenon is particularly significant in the processes for manufacturing tires for vehicle wheels like that described in the above mentioned document WO 2009/040594 in which the individual floor to floor times necessary for building the different tire portions such as the crown structure (belt structure and tread band) and carcass structure must be reduced to the minimum for obtaining high production rates. 
     The Applicant has perceived that by exerting pressure in a differentiated manner on the elongated element just laid, the air possibly entrapped between portions of the elongated element at least partly overlapping each other can escape easily and in a more efficient manner, avoiding the persistent presence of air pockets that can generate the above mentioned drawbacks. 
     The Applicant has finally found that, by exerting pressure on a central region of a portion of the continuous elongated element immediately after laying of same and by subsequently exerting pressure on the side regions of the same portion, it is possible to discharge the air possibly entrapped under said continuous elongated element and therefore avoid said air forming pockets and bubbles that would remain entrapped in the cured and molded tire. 
     More specifically, in a first aspect, the present invention relates to a process for building tires for vehicle wheels comprising the step of: assembling components of elastomeric material on a forming support, in which at least one of said components of elastomeric material is made by the steps of: 
     i) dispensing a continuous elongated element of elastomeric material; 
     ii) applying the continuous elongated element in the form of coils disposed in side by side relationship or at least partly superposed, wound up on the forming support in order to form said at least one tire component of elastomeric material; 
     iii) exerting a first pressure on a central region of a portion of the continuous elongated element applied onto the forming support; 
     iv) exerting a second pressure on side regions of said portion of the continuous elongated element applied onto the forming support. 
     It is the Applicant&#39;s opinion that the first pressure directs its force against the continuous elongated element squashing and pressing it in the middle, laterally moving the possible air bubbles entrapped thereby, while the second pressure spreads and smoothes the continuous elongated element on its sides forcing said air bubbles to the outside. 
     In accordance with a second aspect, the present invention relates to a tire for vehicle wheels built following the process as described and/or claimed. 
     In a third aspect, the present invention relates to an apparatus for building tires for vehicle wheels comprising: at least one forming support; at least one assembly device for assembling components of elastomeric material on the forming support; wherein said at least one assembly device comprises: at least one dispensing device for dispensing a continuous elongated element of elastomeric material; at least one application device for applying said continuous elongated element in the form of coils disposed in side by side relationship or at least partly superposed, wound up on the forming support, and forming said at least one component of elastomeric material of the tire; at least one pressure device operatively acting on an applied portion of the continuous elongated element; wherein the pressure device comprises a central roller and two side rollers, each laterally offset relative to the central roller and on the opposite side relative to the other side roller. 
     The present invention, in at least one of the above aspects can have one or more of the preferred features as hereinafter described. 
     Preferably step iv) follows step iii). 
     In this way, the pressure exerted on the central region moves possible air bubbles from the center to the sides of the continuous elongated element while, subsequently, pressures exerted on the side regions cause escape of the entrapped air from the side edges. 
     Preferably, between step ii) and step iii) a first time interval intervenes which is included between about 0 s and about 1 s. 
     In addition, preferably, between step iii) and step iv) a second time interval intervenes which is included between about 0 s and about 1 s. 
     The shorter the time intervening between distribution, application and compression of the continuous elongated element is, the greater the temperature and plasticity of the elastomeric material during the compression steps, and consequently the greater the ease with which the compound is deformed and molded. 
     In addition, preferably, during steps iii) and iv), said portion has an average temperature included between about 90° C. and about 110° C. 
     In a preferred alternative embodiment, along a direction orthogonal to the longitudinal extension of the continuous elongated element, the side regions are partly superposed on the central region according to a superposition width. 
     Preferably said superposition width is included between about 0.5 mm and about 5 mm. 
     Said superpositions ensure that pressure is exerted over the whole surface of the continuous elongated element so as to eliminate the persistent presence of underlying air bubbles. 
     In a preferred alternative embodiment of the process, during step iv) the second pressure is exerted on at least one side edge of the continuous elongated element. 
     In addition, preferably, during step iv), said at least one side edge is submitted to a hammering action. 
     By “hammering” it is intended application of micro-hits on the surface of the continuous elongated element, carried out by means of at least one roller for example, which is provided with a side work surface having raised elements and/or grooves (knurling), 
     Pressure exerted on the edge and hammering have a visual effect on the outer and visible components of the tire such as the tread band and sidewalls for example, because they squash the continuous elongated element wound into coils and create microfractures therein, so that the separation lines between adjacent coils become less visible also after vulcanization. 
     Preferably, step ii) is carried out at a linear application speed of the continuous elongated element included between about 0.1 m/s and about 2 m/s. 
     Compression of the continuous elongated element after application of same takes a fundamental importance for high application speeds, as those stated above, that are necessary for obtaining high production volumes. In fact, corresponding to the high speeds are high laying temperature to which the phenomena of air expansion and bubble generation are more important and frequent. 
     In a preferred alternative embodiment of the apparatus, the central roller has a rotation axis distinct from a rotation axis of the side rollers. 
     Preferably, along an application direction, the central roller is interposed between the application device and the two side rollers. 
     This structure allows pressure to be first exerted in the middle and subsequently on the side regions of the continuous elongated element. 
     Preferably, along an application direction, a first distance between the application device and the central roller is included between about 20 mm and about 200 mm. 
     Preferably, along an application direction, a second distance between the central roller and the two side rollers is included between about 10 mm and about 100 mm. 
     Since the application device too preferably comprises an applicator roller, these distances are intended measured between the contact points of the applicator roller and the rollers of the pressure device with the continuous elongated element applied onto the forming support. 
     Said distances must preferably be maintained within the stated limits for two reasons. 
     First of all, for compressing the continuous element when the compound is still very hot (the smaller the distance is, the shorter the time intervening between application and compression), for the already highlighted reasons; and in addition, for limiting a misalignment between the center line of the application device and the center line of the pressure device. In fact the continuous elongated element is spirally wound on the forming support according to a determined spiraling angle, measured between a longitudinal extension direction of the continuous elongated element and a plane orthogonal to the rotation axis of the forming support. Misalignment between the center line of the application device and the center line of the pressure device and, as regards the individual pressure device, misalignment between the center line of the central roller and the center line of the side rollers allow the just laid portion of the continuous elongated element to be correctly compressed. These misalignments that should be set at each laying cycle with suitable adjustment mechanisms based on the concerned geometries and speeds, can be neglected if the above mentioned distances are of rather small value. 
     According to a preferred embodiment, a peripheral work surface of the central roller at least partly faces at least one of the peripheral work surfaces of the side rollers. 
     Preferably, along a direction parallel to rotation axes of the central roller and side rollers, the peripheral work surface of the central roller has a superposition width with said at least one of the peripheral work surfaces of the side rollers included between about 0.5 mm and about 5 mm. 
     The relative position between the rollers ensures that the whole surface of the continuous elongated element is submitted to pressure. 
     According to a preferred embodiment, the central roller and each of the side rollers are movable irrespective of each other along a direction substantially orthogonal to the forming support. 
     Preferably, the pressure device comprises spring elements operatively associated with the central roller and the two side rollers, to maintain said rollers into contact with said portion of the continuous elongated element. 
     In addition, preferably, each of the spring elements is associated with one of the rollers in a manner independent of the others. 
     In addition, preferably, the spring elements are pneumatic cylinders. 
     Contact between each of the rollers and the continuous elongated element is ensured under any laying condition and for every speed and laying angle. 
     According to a preferred embodiment, the pressure device comprises: a supporting frame and a central small arm having a central portion thereof hinged thereon. 
     Preferably said central small arm carries the central roller on a first end thereof. 
     Preferably the pressure device further comprises a first spring element mounted on the supporting frame and secured to a second end of the central small arm. 
     More preferably, said pressure device comprises two side small arms positioned on opposite sides of the central small arm, each having a central portion thereof hinged on the supporting frame and carrying one of the side rollers on a first end thereof. 
     In a further preferred alternative solution, said pressure device comprises two second spring elements, each mounted on the supporting frame and secured to a second end of a respective side small arm. 
     The specific structure adopted is of simple and cheap structure and at the same time stiff and reliable, so as to ensure a correct and precise laying and pressing of the continuous elongated element. 
     According to a preferred embodiment, the assembly device comprises a head supporting the application device and pressure device. 
     Alignment between the application device and pressure device is ensured by the fact that they are mounted as a single assembly. 
     Preferably, the central roller has a peripheral contact edge having a radius of curvature in diametrical section that is included between about 0.5 mm and about 3 mm. 
     Preferably, each of the two side rollers has a peripheral contact edge having a radius of curvature in diametrical section included between about 0.5 mm and about 3 mm. 
     In addition, preferably, the diametrical section of each of the two side rollers is asymmetric and said peripheral contact edge is the external one. 
     Said radius of curvature and the position of the peripheral edge are such selected as to exert a predetermined pressure at a predetermined point of the continuous elongated element. 
     Preferably, a peripheral work surface of the central roller has a width included between about 3 mm and about 10 mm. 
     Preferably, a peripheral work surface of each of the side rollers has a width included between about 3 mm and about 10 mm. 
     Preferably, the side rollers have a minimum distance from each other included between about 3 mm and about 10 mm. 
     Preferably, the central roller has a maximum diameter included between about 20 mm and about 80 mm. 
     Preferably, each of the side rollers has a maximum diameter included between about 20 mm and about 80 mm. 
     The geometry and sizes of the rollers enable compression of the whole surface of the continuous elongated element to be compressed and also the steps defined by the superposed adjacent coils to be flattened, thus helping in eliminating the traces of the elongated element on the tread band and/or the sidewalls of the cured and molded tire. 
     Also helping in this second function is a peripheral work surface of the central roller and of each of the side rollers having raised elements for exerting a hammering action on the portion of the continuous elongated element. 
     Further features and advantages will become more apparent from the detailed description of a preferred but not exclusive embodiment of an apparatus for building tires for vehicle wheels in accordance with the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       This description will be set out hereinafter with reference to the accompanying drawings, given by way of non-limiting example, in which: 
         FIG. 1  is a diagrammatic top view of a plant for tire production comprising an apparatus for building tires in accordance with the present invention; 
         FIG. 2  is a diagrammatic side view of an assembly device being part of the apparatus in question and comprising a pressure device in accordance with the present invention; 
         FIG. 3  shows an enlarged portion of the assembly device seen in  FIG. 2 ; 
         FIG. 4 a    is a side view of the pressure device seen in  FIG. 2 ; 
         FIG. 4 b    is a front view of the pressure device seen in  FIG. 2 ; 
         FIG. 5  shows an enlarged portion of the pressure device in  FIG. 4 b    with a forming support carrying a continuous elongated element already laid and spaced apart from the pressure device, for the sake of clarity; 
         FIG. 5 a    is a diagrammatic plan view showing rollers of the pressure device acting on the continuous elongated element; 
         FIG. 6  is a diagrammatic diametrical section of a tire for vehicle wheels obtained with the plant seen in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to the drawings, a plant for tire production comprising an apparatus  2  for building tires in accordance with the present invention has been generally identified with reference numeral  1 . 
     Plant  1  is intended for manufacturing tires  3  ( FIG. 6 ) essentially comprising at least one carcass ply  4  preferably internally coated with a layer of airtight elastomeric material or a so-called “liner”, two so-called “beads”  6  integrating respective annular anchoring structures  7  possibly associated with elastomeric fillers  7   a  and in engagement with the circumferential edges of the carcass ply  4 , a belt structure  8  applied to the carcass ply  4  at a radially external position, a tread band  9  applied to the belt structure  8  at a radially external position, in a so-called crown region of the tire  3 , and two sidewalls  10  applied to the carcass ply  4  at laterally opposite positions, each at a side region of the tire  3 , extending from the corresponding bead  6  to the corresponding side edge of the tread band  9 . 
     Apparatus  2  for building tires preferably comprises a line for building carcass structures  11 , in which a carcass structure comprising at least one of the carcass plies  4  and the annular anchoring structures  7  is formed on a first forming drum  12 ′. Denoted in  FIG. 1  are a plurality of work stations  13  belonging to said line  11  for building carcass structures and each dedicated to formation and/or application of a component of elastomeric material of tire  3  on the first forming drum  12 ′. The first forming drum  12 ′ is sequentially transferred from one station to the following one, by means of robotized arms, not shown, or other suitable devices. 
     By way of example, in a first station  13  liner  5  is made through winding of a continuous elongated element of elastomeric material into coils disposed mutually in side by side relationship and/or at least partly superposed and distributed along the forming surface of the first forming drum  12 ′. In at least one second station  13  manufacture of one or more carcass plies  4  can be carried out, which carcass plies are obtained by laying strip-like elements on the first forming drum  12 ′, in circumferentially approached relationship, said strip-like elements being formed by cutting to size a continuous strip of elastomeric material comprising textile or metallic cords disposed parallel in side by side relationship. A third building station  13  can be dedicated to integration of the annular anchoring structures  7  with said at least one carcass ply  4 , through application of said annular anchoring structures  7  at an axially external position to the flaps of said at least one carcass ply  4  knocked down in the direction of the rotation axis of said first forming drum  12 ′, which flaps will be subsequently turned up around the annular anchoring structures  7 . 
     Apparatus  2  for building tires further comprises a line for building crown structures  14 , in which a crown structure comprising at least the belt structure  8  and tread band  9  is formed on a second forming drum  12 ″. Denoted in  FIG. 1  is a plurality of work stations belonging to said line  14  for building crown structures and each dedicated to forming and/or applying a component of elastomeric material of tire  3  on the second forming drum  12 ″. The second forming drum  12 ″ is sequentially transferred from one station to the subsequent one, by means of robotized arms, not shown, or other suitable devices. 
     At least one building station  15  can be dedicated to manufacture of the annular belt structure  8  obtained by laying strip-like elements in circumferentially approached relationship, which strip-like elements are obtained by cutting to size a continuous strip of elastomeric material comprising preferably metallic mutually parallel cords, and/or by winding a textile or metallic rubberized reinforcing cord into axially approached coils, in the crown portion of tire  3 . By way of example, a work station  15  can be intended for manufacture of the tread band  9  or sidewalls  10 . Tread band  9  and sidewalls  10  are preferably obtained by winding of at least one continuous elongated element of elastomeric material into mutually approached and/or at least partly superposed coils. 
     Apparatus  2  is further provided with an assembly and conformation station  16  operatively associated with line  11  for building carcass structures and line  14  for building crown structures. In the assembly and conformation station  16  the carcass structure is shaped and associated with the crown structure, so as to obtain a green tire. 
     Tires built by apparatus  2  are sequentially transferred to a vulcanization unit line  17  integrated into plant  1  from which cured and molded tires  3  are obtained. 
     As mentioned above, in accordance with the present invention, at least one of the components of only elastomeric material of tire  3 , such as liner  5 , fillers  7   a  and/or other parts of elastomeric material of beads  6 , sidewalls  10 , tread band  9 , underliner, underbelt layer, underlayer of the tread band, abrasion-proof elements and/or others, is obtained by an assembly device denoted as a whole at  18  ( FIG. 2 ). 
     The assembly device  18  comprises a dispensing device in the preferred form of an extruder (not shown), producing a continuous elongated element  19  of elastomeric material. The extruder is provided with a cylinder into which elastomeric material is introduced. The cylinder heated to a controlled temperature, just as an indication included between about 60° C. and about 100° C., operatively houses a rotating screw, by effect of which the elastomeric material is pushed along said cylinder to an outlet orifice of the extruder. 
     Through the outlet orifice, the continuous elongated element  19  is dispensed at a desired linear speed, corresponding to a so-called “target value” of the volumetric flow rate, just as an indication included between about 10 cm 3 /s and about 60 cm 3 /s, and at a temperature just as an indication included between about 90° C. and about 110° C. 
     An application device  20 , operating downstream of the extruder, carries out application of the continuous elongated element  19  coming from the extruder, onto a forming support  12 . The forming support  12  can be said first forming drum  12 ′ or said second forming drum  12 ″. 
     During application, the forming support  12 , supported in overhanging by one of said robotized arms for example, is driven in rotation and suitably moved in front of the application device  20  for distributing the continuous elongated element  19  into approached and/or at least partly superposed coils, wound around such a forming support  12 , so as to form liner  5  for example, or any other component of elastomeric material of the tire being processed. 
     The application device  20  comprises ( FIG. 2 ) at least one roller or other applicator member  21  acting in thrust relationship towards the forming support  12 , for instance by effect of a pneumatic actuator  22 , for applying the continuous elongated element  19  onto the forming support  12  itself. 
     Operatively interposed between the extruder and the application device  20  is a conveyor  23 , the function of which consists in bringing the continuous elongated element  19  coming out of the extruder onto the forming support  12  and until the application device  20 . 
     In the preferred embodiment herein illustrated, the conveyor  23  comprises a conveyor belt  24  defined by a cogged rubber belt or a metal belt, passing over rollers  24   a ,  24   b . The conveyor belt  24  on the upper part has a forward stretch supporting the elongated element  19 . 
     The continuous elongated element  19  coming out of the conveyor belt  24  at the end roller  24   a  is continuously laid on support  12  by the applicator roller  21 . In particular, the applicator roller  21  presses the continuous elongated element  19  sliding under and against it, against the forming support  12  for determining adhesion of same. The applicator roller  21  therefore rotates in the opposite direction relative to rotation of the forming support  12 . 
     Downstream of the applicator member  21  there is a pressure device  27  which is preferably mounted on a head  28  also carrying the application device  20 . The applicator member  21  and pressure device  27  are further operatively supported relative to conveyor  23 . The head  28  is in fact mounted on a structure holding the conveyor  23 . 
     The pressure device  27 , better shown in  FIGS. 3, 4   a ,  4   b  and  5 , comprises a substantially box-shaped supporting frame  29  formed with an upper wall  30  and a pair of side walls  31   a  defining an inverted-U shape. A bottom wall  31   b  connects the two side walls  31   a  and is secured to head  28 . 
     Respective fixed arms are secured to lower portions of the side walls  31   a , which arms each have a first end  32   a  integral with the supporting frame  29  and a second end  32   b . The second ends  32   b  of the two fixed arms  32  face each other for supporting a central roller  33   a  and two side rollers  33   b , as described in detail in the following. 
     In particular, a central small arm  34  is hinged, at a central portion thereof, on the second ends  32   b  of the fixed arms  32  around a first articulation axis “X-X”. A first end  34   a  of the central small arm  34  has a fork rotatably carrying the central roller  33   a . E second end  34   b  of the central small arm  34 , opposite to the first one  34   a , is hinged on a spring element  35  in turn mounted on the supporting frame  29 . Preferably, as in the embodiment shown, the spring element  35  is a pneumatic cylinder. An end of the pneumatic cylinder  35 , belonging to the rod  35   a  of the cylinder  35  itself is hinged on said central small arm  34  around a second articulation axis “Y-Y” and an opposite end, belonging to the body  35   b  of the cylinder  35 , is hinged on a bracket integral with the upper wall  30  of the supporting frame  29 . 
     Two side small arms  36  are positioned on opposite sides of the central small arm  34 . The lateral small arms  36  are parallel to each other and cross the central small arm  34 . Each of the lateral small arms  36  has a central portion thereof hinged on the second ends  32   b  of the fixed arms  32  and on the central small arm  34  around the first articulation axis “X-X”. A first end  36   a  of each of the lateral small arms  36  rotatably carries one of the side rollers  33   b . A second end  36   b  of each of the lateral small arms  36 , opposite to the first one  36   a , is hinged on a respective spring element  37  in turn mounted on the supporting frame  29 . 
     Preferably, as in the embodiment shown, each of the two spring elements  37  is a pneumatic cylinder an end of each of the pneumatic cylinders  37  belonging to the rod  37   a  of cylinder  37 , is hinged on the respective lateral small arm  36  around a third articulation axis “Z-Z” and an opposite end, belonging to the body  37   b  of the cylinder  37 , is hinged on a bracket integral with the upper wall  30  of the supporting frame  29 . Each of the pneumatic cylinders  37  is associated with one of rollers  33   b  in a manner independent of the others. 
     The central roller  33   a , two side rollers  33   b  and the applicator roller  21  have rotation axes that are substantially parallel to each other. The central roller  33   a  is substantially aligned with the applicator roller  21  along a trajectory or application direction of the continuous elongated element on the forming support  12 . The two side rollers  33   b  are coaxial to each other and offset towards opposite sides of the central roller  33   a  ( FIGS. 5 and 5   a ). 
     The central roller  33   a  has a rotation axis “A-A” distinct from the rotation axis “B-B” of the side rollers  33   b  and the rotation axis “C-C” of the applicator roller  21 . In particular, along said application direction, the central roller  33   a  remains positioned between the applicator roller  21  and the two side rollers  33   b  ( FIG. 3 ). 
     The central roller  33   a  has ( FIG. 5 a   ) a maximum diameter “Da”, intended as the diameter of the radially outermost portion, included between about 20 mm and about 80 mm. Each of the side rollers  33   b  has a maximum diameter “Db” included between about 20 mm and about 80 mm. 
     Immediately after laying carried out by the applicator member  21 , the central roller  33   a  exerts a first pressure “P 1 ” on a central region  38  of the just laid portion of the continuous elongated element  19 , and subsequently the two side rollers  33   b  exert respective second pressures “P 2 ” on side regions  39  of the same portion ( FIGS. 5 and 5   a ). The central  33   a  and side  33   b  rollers roll against the continuous elongated element  19 , that has already adhered to the forming support  12  and rotate in opposite ways relative to the rotation direction of said forming support  12 . 
     Downstream of the pressure device  27 , therefore, the continuous elongated element  19  laid on the forming support  12  has a central strip compressed by the central roller  33   a  and two side strips compressed by the side rollers  33   b  ( FIG. 5 a   ). 
     The pneumatic cylinders  35 ,  37  push rollers  33   a ,  33   b  against the continuous elongated element  19  and, through the central small arm  34  and lateral small arms  36 , maintain rollers  33   a ,  33   b  in contact with the continuous elongated element  19 . Due to the described structure, the central roller  33   a  and each of the side rollers  33   b  are movable independently of each other along a direction substantially orthogonal to the peripheral surface of the forming support  12 . 
     The portion of the continuous elongated element  19  that has just come out of the extruder and has been submitted to said pressures “P 1 ”, “P 2 ”, has an average temperature “t m ” substantially equal to or not much lower than the exit temperature and preferably included between about 90° C. and about 110° C. 
     Distance “d 1 ” between the rotation axis “C-C” of the applicator roller  21  and the rotation axis “A-A” of the central roller  33   a  is preferably included between about 20 mm and about 200 mm. Distance “d 2 ” between the rotation axis “A-A” of the central roller  33   a  and the rotation axis “B-B” of the side rollers  33   b  is preferably included between about 10 mm and about 100 mm ( FIGS. 3 and 5   a ). 
     As a result, a first distance “ΔS 1 ” measured along the application direction between the application device  20  and central roller  33   a , intended as the distance measured between the contact point of the applicator roller  21  with the continuous elongated element  19  and the contact point of the central roller  33   a  with the continuous elongated element  19 , is included between about 20 mm and about 220 mm. 
     A second distance “ΔS 2 ” measured along the application direction between the central roller  33   a  and the two side rollers  33   b , intended as the distance measured between the contact point of the central roller  21  with the continuous elongated element  19  and the axis passing by the two contact points between the side rollers  33   b  and the continuous elongated element  19 , is included between about 10 mm and about 110 mm. 
     The periods of time intervening between the action of the applicator roller  21 , the action exerted by the central roller  33   a  and the action exerted by the side rollers  33   b  on the same portion of continuous elongated element  19  depend on the above stated distances and the linear application speed “V” that is preferably included between about 0.1 m/s and about 2 m/s, more preferably between about 0.5 m/s and about 1.5 m/s. 
     Between laying of a portion of the continuous elongated element  19  and the pressing action carried out by the central roller  33   a  on the same portion there is a first time interval “ΔT 1 ” included between about 0 s and about 1 s. In addition, between the pressing action carried out by the central roller  33   a  and the pressing action carried out by the two side rollers  33   b  there is a second time interval “ΔT 2 ” included between about 0 s and about 1 s. 
     In the embodiment shown, the peripheral work surface  40   a  of the central roller  33   a  has, in diametrical section, an arched and symmetric peripheral contact edge ( FIGS. 5 and 5   a ) which has a radius of curvature “r a ” preferably included between about 0.5 mm and about 3 mm. Also the peripheral work surface  40   b  of each of the side rollers  33   b  is arched and symmetric ( FIG. 5 ) and has a radius of curvature “r b ” preferably included between about 0.5 mm and about 3 mm. 
     In accordance with an alternative embodiment not shown, said peripheral work surface  40   b  of each of the side rollers  33   b  is asymmetric and the edge in contact with the continuous elongated one  19  is the outer edge. 
     In the embodiment shown in the accompanying drawings, the peripheral work surface  40   a  of the central roller  33   a  partly faces the peripheral work surfaces  40   b  of the two side rollers  33   b . In other words, in a front view as the one in  FIG. 5 , the peripheral work surface  40   a  of the central roller  33   a  is partly superposed on the peripheral work surfaces  40   b  of both the side rollers  33   b . The facing or superposition widths “Δ 1   a ”, measured along a direction parallel to the rotation axes of the rollers, are included between about 0.5 mm and about 5 mm. 
     Width “La” of the peripheral work surface  40   a  of the central roller  33   a  is included between about 3 mm and about 10 mm. Width “Lb” of the peripheral work surface  40   b  of each of the side rollers  33   b  is included between about 3 mm and about 10 mm. 
     In addition, the two side rollers  33   b  are mutually spaced apart by a minimum distance “d m ”, measure parallel to the rotation axes, included between about 3 mm and about 10 mm. 
     The central roller  33   a  and each of the side rollers  33   b  act on a common region of the continuous elongated element  19 . As a result, the continuous elongated element  19  has two parallel bands compressed both by the central roller  33   a  and the side rollers  33   b  ( FIG. 5 a   ). The central region  38  and side regions  39  and, consequently, the aforesaid central strip and side strips are partly superposed at said bands according to superposition widths “Δls” preferably included between about 0.5 mm and about 5 mm. 
     Due to the width and position of the side rollers  33   b , the side edges  41  of the continuous elongated element  19  are compressed and preferably also squashed through an hammering action. To this aim, preferably, the peripheral work surface  40   a ,  40   b  of the central roller  33   a  and/or of the side rollers  33   b  has a knurling defining raised elements  41  delimiting corresponding grooves. 
     Alternatively, smooth rollers are used on softer and adhesive compounds for example, where surface working of the rollers could cling to the continuous elongated element of elastomeric material.