Patent Publication Number: US-2022212430-A1

Title: Process and apparatus for building tyres for vehicle wheels and tyres obtainable therefrom

Description:
The present invention relates to a process and an apparatus for building tyres for vehicle wheels. The invention also relates to a tyre for vehicle wheels, obtainable by means of the above process and/or apparatus. 
     In particular, the invention may be conveniently used to improve the construction of the tread band and/or other elastomeric components used in the building of a tyre. 
     A tyre for vehicle wheels generally comprises a carcass structure comprising at least one carcass ply having respectively opposite end flaps engaged with respective annular anchoring structures, integrated in the areas usually identified by the name of “beads”, having an inner diameter substantially corresponding to a so-called “fitting diameter” of the tyre on a respective mounting rim. 
     The carcass structure is associated with a belt structure which may comprise one or more belt layers, arranged in radial superposition with respect to each other and with respect to the carcass ply, having textile or metallic reinforcing cords with crossed orientation and/or substantially parallel to the circumferential development direction of the tyre (at 0 degrees). A tread band is applied in a position radially outer to the belt structure, also made of elastomeric material like other semi-finished products making up the tyre. 
     Respective sidewalls of elastomeric material are further applied in an axially outer position on the lateral surfaces of the carcass structure, each extending from one of the lateral edges of the tread band up at the respective annular anchoring structure to the beads. In “tubeless” tyres, an airtight coating layer, usually called “liner”, covers the inner surfaces of the tyre. 
     After the building of the green tyre, carried out by assembling respective components, a moulding and vulcanisation treatment is generally carried out in order to determine the structural stabilisation of the tyre through cross-linking of the elastomeric compositions, as well as to impart a desired tread pattern onto the same, where required, and any distinguishing or information graphic signs at the tyre sidewalls. 
     The terms “radial” and “axial” and the expressions “radially inner/outer” and “axially inner/outer” are used referring to the radial direction of a tyre or of a forming drum (i.e. to a direction perpendicular to the axis of rotation of the tyre or of the forming drum) and to the direction of the axes of rotation of the tyre or of the forming drum. A radial plane of the tyre or of the forming drum contains the respective axis of rotation. 
     The term “elastomeric material” is used to designate a composition comprising at least one elastomeric polymer and at least one reinforcement filler. Preferably, such composition further comprises additives such as, for example, a cross-linking agent and/or a plasticiser. Due to the presence of the cross-linking agent, such material can be cross-linked by heating, so as to form the final manufactured article. 
     The term “to spiral”/“spiraling” means an operation in which at least one continuous elongated element made of elastomeric material, for example in the form of a tape, is circumferentially wound around a geometric axis, to form a plurality of coils respectively approached in the axial direction and/or superimposed in the radial direction. 
     The term “structural component” refers to tyre components which integrate structural reinforcement elements typically in the form of metal, textile or hybrid cords. For example, the carcass ply/plies, the belt layers, the bead cores and some fillers are structural components. 
     The term “elastomeric component” means a component of the tyre made of elastomeric material in the absence of cords or other structural reinforcement elements. The elastomeric material may, however, incorporate binding or reinforcing fillers, for example in the form of dispersed fibres. Elastomeric components are for example the tread band, the sidewalls, the under-layer, the liner, the under-liner, the anti-abrasive elements, the filler inserts or other components in elastomeric material of the tyre. 
     WO2006/046259 on behalf of the Applicant, describes a tyre for vehicle wheels in which the tread band, the sidewalls and/or other structural elements of elastomeric material have a layered structure comprising at least a first component and at least a second component of a material having a composition different from that of the first component. The first and second components have a corrugated interface profile, defining elements of mutual mechanical engagement. 
     WO2006/046162 on behalf of the Applicant describes the construction of a tyre for motor vehicles, in which one or more structural elements in elastomeric material, such as for example a tread band, provided with a respective substrate, are made by laying on the carcass structure or other forming support a continuous elongated element longitudinally divided into a first and a second mutually coupled portion, obtained by extrusion of two different elastomeric materials coming from respective extruders belonging to a common extrusion nozzle. The deposition of the turns takes place so that the first and second materials respectively form a first layer and a second layer superimposed on the first layer. 
     US2006/0096697 proposes a method for manufacturing a tyre in which the construction of the tread band provides that a cylindrical top portion is formed on a substantially cylindrical forming drum by means of a simultaneous spiraling of a continuous elongated element, made with a first non-conductive elastomeric material, and an elongated insert, made with a second electrically conductive elastomeric material, by means of respective applicators, each comprising a belt conveyor. Each belt conveyor continuously feeds the respective elongated element or insert into a predetermined winding position on the surface of the forming drum. The elongated element and insert each come continuously from a respective dispenser, in particular an extruder or a calendar, located upstream of the belt conveyor, after interposition of a festoon through which the control of the outlet speed from the dispenser is carried out. The applicator is supported by a movement unit, and is alternatively movable at least along an axial direction with respect to the forming drum. 
     US 2013/0133811 provides for making a tread band by the simultaneous spiraling of two continuous ribbon-like elements delivered by respective extruders directly onto the deposition surface of a forming drum actuated in rotation. One of the extruders comprises two distinct mixing units, used for processing a first non-conductive elastomeric material and a second conductive elastomeric material, respectively, which flow into the same extrusion nozzle. The mixing unit used for the processing of the second conductive elastomeric material may be selectively activated and deactivated during processing, to determine when needed the dispensing of a ribbon-like conductive insert, coupled side by side along the respective ribbon-like element coming out of the extrusion head. 
     In the manufacture of some elastomeric components, in fact, the coupling of two or more different elastomeric materials may be required. Typically, an elastomeric base material is provided whose composition is designed to impart certain basic properties to the respective elastomeric component, and one or more inserts made with an additional elastomeric material, whose composition is designed to impart desired additional properties to the same elastomeric component. 
     For example, for the construction of the tread band it is known to use an elastomeric base material containing a silica filler, to meet the need to obtain certain basic properties, for example high friction coefficient, abrasion resistance, low hysteresis etc. Since this base material is typically non-conductive, the insertion of at least one insert made of an additional electrically conductive elastomeric material is usually required, to meet the need to give the tread band the ability to discharge electrostatic charges to the ground. 
     The use may be required of one or more further inserts made of a third elastomeric material, different from the elastomeric base material and/or from the additional elastomeric material, for example at each of the axially opposite ends of the tread band, and/or of the radially outer apex of each sidewall, in order to promote a correct coupling between the sidewalls and the tread band, typically made with respective elastomeric materials such that the mutual coupling during the subsequent building steps may be difficult. 
     However, the Applicant has observed that in the building of tyres by using the spiraling of a continuous elongated element, the additional properties sought by the use of the additional inserts may for example be compromised as a result of deformations and displacements undergone by the individual turns formed by the continuous elongated element, for example under the effect of the high pressures induced to the entire structure of the tyre during the moulding and vulcanisation treatment. 
     The Applicant further observed that the greater the quantity of the additional elastomeric material introduced into the elastomeric component in order to impart the required additional property, the greater its negative impact on the performance of the same elastomeric component in relation to other basic properties required. 
     The Applicant has therefore perceived that by implementing suitable measures to optimise the quantity and distribution of the additional elastomeric material in the continuous elongated element, it is possible to obtain qualitative improvements in the product and achieve a simplification of the equipment used for the building and/or application of the continuous elongated element itself, as well as a convenient reduction of the times required for building the tyre. 
     In particular, the Applicant has found that by building during the extrusion a coating or covering layer extending along the continuous elongated element surrounding the profile thereof in cross section, it is possible to impart to the elastomeric component, subsequently obtained by spiraling, desired additional properties dictated by specific design needs without significantly affecting other basic properties typically required of the elastomeric component itself. Furthermore, the coating or covering layer may be made according to a suitably limited thickness, using a reduced amount of additional elastomeric material. The reduced amount of additional elastomeric material required, in addition to reducing the impact on the other basic properties of the elastomeric component obtained, allows the extrusion apparatuses to be significantly simplified and production costs to be reduced. 
     More in particular, in a first aspect thereof, the invention relates to a process for building tyres for vehicle wheels. 
     Preferably, at least one elastomeric component of a tyre is made by applying at least one continuous elongated element according to a plurality of turns around a forming drum rotating around a geometric rotation axis thereof. 
     Preferably, said continuous elongated element is made by the action of extruding a first material through an extrusion nozzle, to form an inner core of said continuous elongated element exiting from an outlet opening of the extrusion nozzle. 
     Preferably, said continuous elongated element is made by the action of, during said extrusion, conveying around the first material, at the extrusion nozzle and upstream of the outlet opening, a second material different from the first material, to form a coating layer of cladding which (entirely) surrounds the inner core. 
     According to another aspect thereof, the invention relates to an apparatus for building tyres for vehicle wheels. 
     Preferably, an extrusion assembly is provided, having an extrusion nozzle longitudinally crossed by an outlet channel leading to an outlet opening. 
     Preferably, a first supply unit is provided for introducing a first elastomeric material into a first supply duct which converges axially into the outlet channel of the extrusion nozzle. 
     Preferably, a forming drum is provided, operable in rotation in front of the outlet opening of the extrusion nozzle. 
     Preferably, the extrusion assembly comprises an injection chamber arranged around the outlet channel of the extrusion nozzle and flowing therein through a radial intake slit extending along a closed line around the outlet channel itself. 
     Preferably, the extrusion assembly comprises a second supply unit for introducing a second elastomeric material into a second supply duct leading to the injection chamber. 
     According to a further aspect thereof, the invention relates to a tyre for vehicle wheels. 
     Preferably, a carcass structure is provided, having at least one carcass ply having respective end flaps engaged in respective annular anchoring structures. 
     Preferably, a belt structure extending circumferentially around the carcass structure is provided. 
     Preferably, elastomeric components applied to the carcass structure and the belt structure are provided. 
     Preferably, at least one of said elastomeric components comprises a continuous elongated element wound according to concentric turns around an axis of rotation of the tyre. 
     Preferably, one or more of said turns have an inner core composed of a first elastomeric material and a coating layer composed of a second elastomeric material different from the first elastomeric material, which (integrally) surrounds the inner core. 
     The Applicant believes that the outer surfaces of the elastomeric component thus formed may exhibit the additional properties brought by the elastomeric material which constitutes the coating or covering layer of the spiral-shaped elongated element. At the same time, the elastomeric material constituting the inner core of the continuous elongated element gives the elastomeric component the required basic properties. 
     The Applicant considers it possible to exploit this circumstance in a particularly convenient way when the desired additional properties must be expressed in the behaviour of the elastomeric component in relation to other elements with which it must come into contact during the construction and/or use of the tyre. 
     For example, in a tread band it is possible to provide that at least a part of the elongated element has a high electrical conductivity as an additional property, in order to be able to discharge to the ground the electric charges accumulated by the vehicle while driving. By arranging a coating covering layer or with a minimum or adequately small thickness, it is also possible to effectively promote the dispersion of electrostatic charges without significantly affecting the basic properties, such as road grip, low hysteresis, wear resistance, etc. 
     Furthermore, the cross-sectional areas of the coating or covering layer and of the inner core may be easily modulated with respect to each other according to requirements, during the extrusion of the elongated element. 
     For example, by reducing the thickness and consequently the cross-sectional area of the coating or covering layer and/or the inner core down to a null value, gradually or almost instantaneously during the extrusion, and simultaneously integrating the reduction of the area of one with a corresponding increase in the area of the other, it is also possible to obtain a tread band or other elastomeric component of the tyre having several parts made with respectively different compounds, as well as obtaining more elastomeric components made with respectively different compounds. 
     In particular, it becomes possible to integrate in the axially opposite ends of the tread band portions of radially outer sidewalls having contact surfaces with the remaining portions of the sidewalls made with an elastomeric material compatible or identical to the elastomeric material typically used for building the aforesaid remaining portions of the sidewalls of the tyre, so as to facilitate a satisfactory coupling between the sidewalls and the tread band during building and the subsequent moulding and vulcanisation steps of the tyre. 
     In at least one convenient embodiment, the invention may further comprise one or more of the following preferential features. 
     Preferably, the application takes place simultaneously with the extrusion. 
     Preferably, said application comprises transmitting a transverse distribution movement between the forming drum and the extrusion nozzle, for distributing said turns in mutual approaching relationship. 
     Preferably, the action of extruding the first material in the absence of the second material is also provided. 
     Preferably, the action of extruding the first material in the absence of the second material precedes the action of conveying the second material. 
     Preferably, the action of extruding the first material in the absence of the second material is carried out after the action of conveying the second material. 
     Preferably, the action of interrupting the conveying of the second material is also provided, to extrude the first material in the absence of the second material. 
     Preferably, the action of extruding the second material in the absence of the first material is also provided. 
     Preferably, the action of extruding the second material in the absence of the first material precedes the action of conveying the first material. 
     Preferably, the action of extruding the second material in the absence of the first material is carried out after the action of conveying the first material. 
     Preferably, the action of interrupting the conveying of the first material is also provided, to extrude the second material in the absence of the first material. 
     Preferably, the action of modulating a flow rate of the second material conveyed around the inner core is provided, to modify the thickness of the coating layer applied around the inner core. 
     Preferably, the flow rate of the first material is modulated in conjunction with the flow rate modulation of the second material, to keep the overall flow rate of the first and second material substantially constant through the outlet opening. 
     Preferably, the action of increasing or decreasing a flow rate of the first material in conjunction with a decrease or respectively an increase in the flow rate of the second material is also provided. 
     Preferably, during said extrusion, the action of conveying a third material different from the first material and from the second material around the first material, at the extrusion nozzle and upstream of the outlet opening, is further carried out to form a coating layer which (entirely) surrounds the inner core. 
     Preferably, the action of conveying the third material is carried out in the absence of conveying the second material. 
     Preferably, the action of extruding the first material in the absence of the conveying of second and third material is also provided. 
     Preferably, the action of extruding the third material in the absence of the first material is also provided. 
     Preferably, the action of extruding the third material in the absence of the first and second material is also provided. 
     Preferably, the action of modulating a flow rate of the third material conveyed around the inner core is provided, to modify the thickness of the covering layer applied around the inner core. Preferably, the flow rate of the first material is modulated in conjunction with the flow rate modulation of the third material, to keep the overall flow rate of the first and third material substantially constant through the outlet opening. 
     Preferably, the action of increasing or decreasing a flow rate of the first material in conjunction with a decrease or respectively an increase in the flow rate of the third material is also provided. Preferably, the first supply unit comprises a first mixing group operating on the first elastomeric material and a gear pump operatively interposed between the first mixing group and said extrusion nozzle. 
     Preferably, the second supply unit comprises a second mixing group having a screw operable in rotation in a mixing chamber. 
     Preferably, said screw is axially movable in the mixing chamber, to transfer the second elastomeric material into the injection chamber. 
     Preferably, said first supply unit and second supply unit are operable independently of one another, to cause a selective and controlled feeding of the first material and/or second material towards the outlet channel of the extrusion nozzle. 
     Preferably, a third supply unit is also provided for introducing a third elastomeric material into a third supply duct leading to the injection chamber. 
     Preferably, the third supply unit comprises a respective mixing group having a screw operable in rotation in a mixing chamber and axially movable in the mixing chamber for transferring the third elastomeric material into the injection chamber. 
     Preferably, said first supply duct ends in a distributor associated with said extrusion nozzle, said distributor having a conical ring shape having an inner channel for the passage of said first elastomeric material. 
     Preferably, said radial slit has a variable axial dimension along a peripheral extension thereof. 
     Preferably, the elastomeric component formed by said continuous elongated element is a tread band. 
     Preferably, the second elastomeric material is electrically conductive. 
     Preferably, the second elastomeric material has the same composition as the elastomeric material used for the manufacture of tyre sidewalls. 
     Preferably, in one or more of said turns the coating layer has a different thickness than the thickness of the coating layer present in other turns. 
     Preferably, turns with coating layers having respectively different thicknesses have respectively equal cross-sectional dimensions. Preferably, a plurality of said turns are devoid of the coating layer. 
     Preferably, turns without the coating layer and turns provided with the coating layer have respectively equal cross-sectional dimensions. 
     Preferably, one or more of said turns have a covering layer which (entirely) surrounds the inner core, said covering layer being composed of a third elastomeric material different from the first elastomeric material. 
     Preferably said third elastomeric material is different from the second elastomeric material. 
     Preferably, the third elastomeric material has the same composition as the elastomeric material used for the manufacture of tyre sidewalls. 
    
    
     
       Further features and advantages will become apparent from the detailed description of a preferred but not exclusive embodiment of a process and an apparatus for manufacturing tyres for vehicle wheels, and of a tyre for vehicle wheels obtainable by means of the aforementioned method and/or apparatus, according to the present invention. Such description is given hereinafter with reference to the accompanying drawings, provided only for illustrative and, therefore, non-limiting purposes, in which: 
         FIG. 1  shows a perspective view of an apparatus for building tyres for vehicle wheels, according to the present invention; 
         FIG. 2  shows the apparatus of  FIG. 1  in longitudinal section; 
         FIG. 3  shows an enlarged detail of  FIG. 2 , highlighting a variant embodiment; 
         FIG. 4  shows a detail of the embodiment variant of  FIG. 3  sectioned longitudinally according to an orthogonal plane with respect to  FIG. 3 ; 
         FIG. 5  shows on an enlarged scale a detail of  FIG. 2 , highlighting a continuous elongated element dispensed by the extrusion nozzle; 
         FIG. 6  is an enlarged cross section of the elongated element; 
         FIG. 7  schematically shows a radial section of a tyre for vehicle wheels, according to the present invention; 
         FIG. 8  schematically shows a tread band of the tyre of  FIG. 7 . 
     
    
    
     With reference to the above figures, reference numeral  1  indicates an apparatus for building tyres for vehicle wheels, according to the present invention. 
     The apparatus  1  may be conveniently used for building tyres  2  ( FIG. 7 ) essentially comprising a carcass structure  3  having at least one carcass ply  4 . An airtight layer of elastomeric material or so-called liner  5  may be applied internally to the carcass ply/plies  4 . Two annular anchoring structures  6  comprising each a so-called bead core  6   a  bearing an elastomeric filler  6   b  in radially outer position are engaged to respective end flaps  4   a  of the carcass ply or plies  4 . The annular anchoring structures  6  are integrated in the proximity of zones usually identified by the name of “beads”  7 , at which the engagement between tyre  2  and a respective mounting rim usually occurs. 
     A belt structure  8 , comprising one or more belt layers  8   a ,  8   b , extends circumferentially around the carcass structure  3 , and a tread band  9  is circumferentially superimposed on the belt structure  8 . 
     The belt structure  8  may be associated with so-called “under-belt inserts”  10  placed each between the carcass ply/plies  4  and one of the axially opposite end edges of the belt structure  8 . 
     Two sidewalls  11  are applied in laterally opposite positions on the carcass ply/plies  4 . Each side has a radially inner apex  11   a  joined to the corresponding bead  7  and a radially outer end portion  11   b  possibly joined to a radially outer apex  12  carried by the tread band  9  at an axially outer end  9   a  thereof. 
     With reference to the definitions illustrated above, in the example of tyre shown, the carcass ply/plies  4 , the belt layers  8   a ,  8   b , and the bead cores  6   a , represent structural components, while the sidewalls  11 , the liner  5 , the fillers  6   a , the under-belt inserts  10  and the tread band  9  with any radially outer apices  12  represent elastomeric components, applied to the carcass structure  3  and/or to the belt structure  8 . 
     In a preferred embodiment example described herein, the apparatus  1  is used to make the tread band  9 . However, the present invention may be conveniently used for spiraling any other elastomeric component, for example sidewalls  11 , liners  5 , fillers  6   a , under-belt inserts  10  and/or other elastomeric components required for building the tyre  2 . 
     The apparatus  1  comprises an extrusion assembly  13  arranged to dispense at least one continuous elongated element  14 , preferably in the vicinity of an outer surface S of a forming drum  15 . The forming drum  15  is adequately supported in the vicinity of the extrusion apparatus, for example by means of a robotic arm  15   a . The robotic arm  15   a  appropriately supports and moves the forming drum  15  so as to determine the application of the continuous elongated element  14  according to a plurality of turns around the outer surface S of the forming drum  15  itself, while the latter rotates around a geometric axis of rotation X thereof. The extrusion assembly  13  has at least one extrusion nozzle  16  longitudinally crossed by an outlet channel  17  leading to an outlet opening  18 , through which the continuous elongated element  14  is dispensed. 
     Upstream of the extrusion nozzle  16 , a first supply unit  19  is arranged, configured for introducing a first elastomeric material into a first supply duct  20  which converges axially into the outlet channel  17  of the extrusion nozzle  16 . 
     Preferably, the first supply unit  19  comprises a first mixing group  19   a , only schematically indicated in that it may be constructed in a known manner, operating on the first elastomeric material. A gear pump  21  is operatively interposed along the first supply duct  20 , between the first mixing group  19   a  and the extrusion nozzle  16 . The use of the gear pump  21 , the actuation speed whereof may be suitably adjusted by means of a programmable logic controller (PLC) associated with an actuator unit  22 , allows controlling with suitable accuracy, instant by instant, the flow rate of the first elastomeric material sent to the extrusion nozzle  16 . 
     The extrusion assembly  13  further comprises a second supply unit  23  configured to introduce a second elastomeric material different from the first elastomeric material into the outlet channel  17  of the extrusion nozzle  16 . 
     More particularly, the second supply unit  23  preferably comprises a second mixing group  24  having a screw  25  operatively housed in a mixing chamber  26 , and operable in rotation by means of a motor  27 . Preferably, the screw  25  is also axially movable in the mixing chamber  26 , for example on the command of an axial movement unit  28 , to facilitate the transfer of the second elastomeric material into a second supply duct  29 , extending in a radial direction with respect to the outlet channel  17  and leading to an injection chamber  30 . The injection chamber  30 , preferably having an annular configuration, is arranged around the outlet channel  17  of the extrusion nozzle  16  and flows therein through a radial intake slit  31 , which extends along a closed line around the outlet channel  17  itself, entirely surrounding it ( FIG. 2 ). 
     The supply duct  20  may end with a distributor  50  associated with the extrusion nozzle  16 . The distributor  50  has a conical ring shape having an inner channel for the passage of the above first elastomeric material ending with an outlet section  51 . The outlet section  51  is slightly spaced from an inlet section  52  of the extrusion nozzle  16 . The radial slit  31  is axially delimited between the outlet section  51  of the distributor  50  and the inlet section  52  of the extrusion nozzle  16 . The outlet section  51  may have a substantially elliptical, or in any case geometrically similar, albeit possibly larger, peripheral profile with respect to that of the outlet opening  18  and therefore of the continuous elongated element  14  coming out of it. As exemplified in the embodiment variant shown in  FIGS. 3 and 4 , the outlet section  51  of the distributor  50 , and/or the inlet section  52  of the extrusion nozzle may have, in a plane orthogonal to the longitudinal axis thereof coinciding with that of the channel  20  and of the extrusion nozzle  16 , a concave profile or in a different way, such as not to be flat. As a result of such a non-planar profile, a variable distance is created between the distributor  50  and the extrusion nozzle  16  along the peripheral extension of the outlet section  51  and the inlet section  52 . In other words, the radial slit  31  may have a variable axial dimension along the peripheral extension thereof. During the operation of the apparatus  1 , the first supply unit  19  and the second supply unit  23  are adapted to be operated independently of one another, to cause the selective and controlled feeding of the first material and/or second material towards the outlet channel  17  of the extrusion nozzle  16 . 
     The use of the first material provides that while the forming drum  15  is rotated and suitably moved in front of the outlet opening  18 , the first supply unit  19  is activated to feed the first material into the outlet channel  17  of the extrusion nozzle  16 . The first material is therefore extruded through the extrusion nozzle  16 , coming out of the outlet opening  18  in the absence of the second material, to be simultaneously applied around the outer surface S of the forming drum  15 . The rotation imparted to the forming drum  15  around its geometric axis of rotation X determines the formation of consecutive turns C, while a transverse movement imposed by the robotic arm  15   a  to the forming drum  15  determines the distribution of the turns C according to a predefined scheme, for example in mutual approaching relationship along the axial extension of the outer surface S of the forming drum  15 . 
     At any desired moment during the extrusion, the second supply unit  23  lends itself to being activated, for example by axial movement of the respective screw  25 , for pushing the second material into the injection chamber  30 . At the extrusion nozzle  16  and upstream of the outlet opening  18 , the second material pushed into the injection chamber  30  is conveyed through the radial slit  31 , around the first material coming from the first supply duct  20 , preferably through said distributor  50 . The second material is therefore distributed around the first material which is about to be extruded through the extrusion nozzle  16 . The second material consequently forms, externally to the continuous elongated element  14  dispensed by the outlet opening  18 , said coating layer  32  which entirely surrounds an inner core  33 , defined by the first material. By using the aforementioned concave or non-planar shape of the outlet section  51  of the distributor  50  and/or of the inlet section  52  of the extrusion nozzle  16  it is possible to initially create a variable layer thickness, i.e. in the vicinity of the radial slit  31  of the coating layer  32  which subsequently, due to the transverse displacements undergone by the second elastomeric material (and/or by a third elastomeric material as will be seen below) due to the change of section in the path along the extrusion nozzle  16 , becomes substantially constant in proximity of the above outlet opening  18 . The dispensing of the second material for the purpose of making the coating layer  32  may be maintained for a time necessary to form a desired number of turns C around the forming drum  15 . The second supply unit  23  may be subsequently deactivated, so as to interrupt the conveying of the second material and continue the construction of other parts of the elastomeric component to be built with the dispensing of the first material in the absence of the second material. 
     In addition or alternatively, where required, deactivation of the first supply unit  19  may be provided, to extrude the second material in the absence of the first material. This technical measure may be used, for example, to obtain turns C made exclusively with the second material at with certain areas of the elastomeric component to be built, or to make portions of the above elastomeric component with the use of only the second material. 
     Depending on the requirements, the action of extruding the first or second material in the absence of the other material may precede or follow the action of extruding the same other material. 
     The action of modulating the flow rate of the second material conveyed around the inner core  33  may be advantageously provided, for example by adjusting the axial movement speed of the screw  25 , to modify the thickness of the coating layer  32  applied around the inner core itself. 
     By adjusting the actuation speed of the gear pump  21 , also the flow rate of the first material may be advantageously modulated, possibly in conjunction with the variation of the flow rate of the second material. More specifically, it is possible to increase or decrease the flow rate of the first material in conjunction with a decrease or, respectively, an increase in the flow rate of the second material. For example, an increase in the flow rate of the second material may correspond to an equal decrease in the flow rate of the first material, so as to keep the overall flow rate, i.e. the sum of the flow rates, of the first and second material through the outlet opening  18 , and therefore the dimensions of the continuous elongated element  14  in cross section, substantially constant. 
     Flow modulation may also be used to gradually change from a condition in which only one material is extruded in the absence of the other, to an operating condition in which only the other material is extruded. 
     According to a possible preferential variant, the apparatus  1  may further comprise a third supply unit  34  configured to introduce a third material, different from the first material and, preferably but not necessarily also from the second material, into a third supply duct  35  leading to the injection chamber  30 . 
     The third supply unit  34 , shown schematically in dashed lines in  FIG. 2 , may be made substantially identical to the second supply unit  23 . 
     The third supply unit  34  may be activated selectively and independently of the activation of the first supply unit  19  and the second supply unit  23 . Preferably, the activation of the third supply unit  34  is carried out when at least the second supply unit  23  remains inactive. 
     The activation of the third supply unit  34  during the extrusion of the first material causes the third material to be conveyed to the injection chamber  30 , preferably in the absence of the second material being conveyed, to form a covering layer  36  which entirely surrounds the inner core  33  formed by the first material itself. 
     The action of modulating the flow rate of the third material conveyed around the inner core  33  may be advantageously provided, by operating on the third supply unit  34  in a similar way to that said with reference to the second supply unit  23 , to modify the thickness of the covering layer  36  formed by the third material. 
     It is also possible to carry out the action of extruding the first material in the absence of conveying the second and/or third material, as well as extruding the second/third material in the absence of the first and/or third/second material. 
     The flow rate of the first material may be modulated in conjunction with the flow rate modulation of the third material, by increasing or reducing a flow rate of the first material in conjunction with a decrease or, respectively, an increase in the flow rate of the third material. For example, an increase in the flow rate of the third material may correspond to an equal decrease in the flow rate of the first material, so as to keep the overall flow rate, i.e. the sum of the flow rates, of the first and third material through the outlet opening  18 , and therefore the dimensions of the continuous elongated element  14  in cross section, substantially constant. 
     In a preferred embodiment which provides the second material equal to the third material, it becomes possible if required to create the continuous coating layer around the inner core  33  for a number of turns as high as desired, and also for the entire spiraling step of the continuous elongated element  14 , alternating the operation of the second supply unit  23  and of the third supply unit  34 , where one is placed in the operating (injection) step while the other is in the material loading step. 
     In a further preferred embodiment (not illustrated), providing a separate injection chamber for the second supply unit  23  and for the third supply unit  34  respectively, in which each separate injection chamber is axially distant from the other along the longitudinal development of the outlet channel  17 , it is possible to operate the second supply unit  23  and the third supply unit  34  simultaneously, making two coating layers with the second material and the third material respectively around the inner core  33  defined by the first material, providing the elastomeric component of the tyre formed with the continuous elongated element  14  with particular physical features. 
     The construction of the tread band  9  or other elastomeric component may require the use of only the first material, normally for a prevailing part of the elastomeric component itself. However, in some portions of the elastomeric component, for example in an axially central portion A ( FIG. 6 ) adjacent to the axial centre line Y of the tread band  9 , the use of the second material may be required, for example to determine a desired electrical conductivity between the belt structure  8  and the radially outer surface  9   b  of the same tread band  9 , intended for contact with the ground during use of the tyre  2 . 
     A preferential use of the third supply unit  34  during the construction of a tread band  9  may be aimed at the realization of said radially outer apices  12 . In this regard, the third supply unit  34  is suitable for being activated during the deposition of the turns C at axially outer portions B ( FIG. 6 ) close to each of the respectively opposite axial ends  9   a  of the tread band  9 . The continuous elongated element  14  dispensed in these circumstances will be provided with the covering layer  36  composed of the third material. 
     The thickness of the covering layer  36  and the overall cross-sectional dimension of the continuous elongated element  14  may be modified during the deposition, for example by progressively reducing the dimensions of the inner core  33  formed by only the first material during the deposition of the turns C increasingly closer to the axial ends  9   a  of the tread band  9 , possibly until the dispensing of the first material is completely interrupted, so that the turns C closest to the axial ends  9   a  are exclusively composed of the third material. 
     In the tyre  2  obtained according to the present invention, at least one of the elastomeric components, in the case described the tread band  9 , will be formed by the continuous elongated element  14  wound according to concentric turns C around the geometric axis of rotation X of the tyre itself. 
     In one or more of the turns C, the inner core  33  composed of the first elastomeric material, completely surrounded by the coating layer  32  composed of the second elastomeric material, or by the covering layer  36  composed of the third elastomeric material will be identifiable. 
     For the purposes of making the tread band  9 , the first elastomeric material may for example comprise a relatively high quantity of silica, for example greater than 30 parts by weight out of 100 parts by weight of compound. 
     For the purposes of making the tread band  9 , the second elastomeric material may for example contain an adequate quantity of carbon black, for example greater than 50 parts by weight out of 100 parts by weight of compound, to obtain an adequate electrical conductivity. 
     For the purposes of making said radially outer apices  12 , the third elastomeric material will have composition and physico-chemical properties substantially identical to those of the elastomeric material used in the construction of the sidewalls  11 , for an optimal coupling between the radially outer apices  12  of the sidewalls  11  and the axial ends  9   a  of the tread band  9  during the building of the tyre  2 . 
     By way of example, turns C near the axial centre line Y of the tread band  9  may be covered by the coating layer  32  formed by the second electrically conductive material, turns C placed at the axial ends  9   a  of the tread band  9  to form the radially outer apices  12  may in turn be covered by the covering layer  36  composed of the third material, substantially identical to that of the sidewalls  11 , and finally a plurality of turns C, distributed between the radially outer apices  12  and the areas close to the axial centre line Y of the tread band  9 , may be formed by only the first material and without the coating layer  32  and/or the covering layer  36 . 
     In one or more of the turns C, the coating layer  32  or the covering layer  36  may have a different thickness than the thickness of the coating layer  32  or the covering layer  36  present in other turns C. 
     The turns C having a coating layer  32  or a covering layer  36  having a respectively different thickness, may have cross-sectional dimensions equal to each other and/or with respect to the turns C without a coating layer  32  or a covering layer  36 . 
     Since it completely surrounds the inner core  33 , the coating layer  32  created with the second material may advantageously be made with thin thickness, without for example risking to interrupt the electrical continuity through the tread band  9  also following any deformations suffered by the turns C during vulcanisation or other processing steps after building. 
     The same applies to the covering layer  36  formed by the third material, which may ensure effective covering of the inner core  33  also following any deformations or crushing imposed on the individual turns C. 
     The pattern of deposition of the turns C of the continuous longitudinal element  14  may not be made as shown in  FIG. 8  and not necessarily, for example in the building of a tread band  9 , for the single turn C to simultaneously have a radially outer part exposed to the radially outer surface of the tread and a radially inner part in contact with a structural component (for example belt strip  8   a ,  8   b ) in conductive material. 
     Frequently the single turn C may be positioned over other turns C so that, for example in the building of the tread band  9 , the coating layer  32  completely covering the outer surface of each single turn C allows contact and electrical conductivity between the turn C positioned in contact with the structural component (belt strip  8   a ,  8   b ) in conductive material with that positioned radially outer thereto, and so on up to the radially outer surface of the tread, promoting maximum flexibility of the building operations, not forced to respect particular schemes to favour, for example, electrical conductivity. 
     The use of the second material and/or third material in less quantity allows the impact thereof on the basic performance features required of the elastomeric component to be reduced. Furthermore, the reduced quantity of the second material allows an advantageous containment of the dimensions and costs of the second supply unit  23  and of the apparatus as a whole. 
     The need for a continuous supply of the second and/or third material to the second supply unit  23  for operating purpose is also eliminated. In fact, the periods of inactivity of the second supply unit  23  and/or of the third supply unit  34  may be exploited to restore the supply of second and/or third material inside the mixing chamber  26 , without having to interrupt the building.