Patent Description:
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 <NUM> degrees). A tread band is applied in a position radially outer to the belt structure, also made of elastomeric material like other semifinished 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"/"spiralling" 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.

<CIT> 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.

<CIT> 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.

<CIT> 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 spiralling 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.

<CIT> provides for making a tread band by the simultaneous spiralling 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.

<CIT> relates to rubber strip materials which are used for fabricating a green tire made up of a plurality of tire rubber members. A rubber strip material whose cross section is divided into two or more regions which are made up of different rubber compounds is wound spirally on to a drum in an overlapping fashion so as to build a green tire or a tire member. In <CIT> strip of material is wound in turns each extending in the tire circumferential direction for a preferably unvulcanised tread strip. The strip windings lie adjacent to each other and form two or more zones laterally across the tyre, at least one of which created has a specific resistance similar to the rubber of the breaker layer and forms a conductive path from the breaker layer to road surface.

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 spiralling 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 spiralling, 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, according to claim <NUM>.

According to another aspect thereof, the invention relates to an apparatus for building tyres for vehicle wheels, according to claim <NUM>.

According to a further aspect thereof, the invention relates to a tyre for vehicle wheels, according to claim <NUM>.

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 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 radially outer sidewall portions 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, the elastomeric component formed by said continuous elongated element is a tread band.

Preferably, the second elastomeric material is electrically conductive.

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.

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:.

With reference to the above figures, reference numeral <NUM> indicates an apparatus for building tyres for vehicle wheels, according to the present invention.

The apparatus <NUM> may be conveniently used for building tyres <NUM> (<FIG>) essentially comprising a carcass structure <NUM> having at least one carcass ply <NUM>. An airtight layer of elastomeric material or so-called liner <NUM> may be applied internally to the carcass ply/plies <NUM>. Two annular anchoring structures <NUM> comprising each a so-called bead core 6a bearing an elastomeric filler 6b in radially outer position are engaged to respective end flaps 4a of the carcass ply or plies <NUM>. The annular anchoring structures <NUM> are integrated in the proximity of zones usually identified by the name of "beads" <NUM>, at which the engagement between tyre <NUM> and a respective mounting rim usually occurs.

A belt structure <NUM>, comprising one or more belt layers 8a, 8b, extends circumferentially around the carcass structure <NUM>, and a tread band <NUM> is circumferentially superimposed on the belt structure <NUM>.

The belt structure <NUM> may be associated with so-called "under-belt inserts" <NUM> placed each between the carcass ply/plies <NUM> and one of the axially opposite end edges of the belt structure <NUM>.

Two sidewalls <NUM> are applied in laterally opposite positions on the carcass ply/plies <NUM>. Each side has a radially inner apex 11a joined to the corresponding bead <NUM> and a radially outer end portion 11b possibly joined to a radially outer apex <NUM> carried by the tread band <NUM> at an axially outer end 9a thereof.

With reference to the definitions illustrated above, in the example of tyre shown, the carcass ply/plies <NUM>, the belt layers 8a, 8b, and the bead cores 6a, represent structural components, while the sidewalls <NUM>, the liner <NUM>, the fillers 6a, the under-belt inserts <NUM> and the tread band <NUM> with any radially outer apices <NUM> represent elastomeric components, applied to the carcass structure <NUM> and/or to the belt structure <NUM>.

In a preferred embodiment example described herein, the apparatus <NUM> is used to make the tread band <NUM>. However, the present invention may be conveniently used for spiralling any other elastomeric component, for example sidewalls <NUM>, liners <NUM>, fillers 6a, under-belt inserts <NUM> and/or other elastomeric components required for building the tyre <NUM>.

The apparatus <NUM> comprises an extrusion assembly <NUM> arranged to dispense at least one continuous elongated element <NUM>, preferably in the vicinity of an outer surface S of a forming drum <NUM>. The forming drum <NUM> is adequately supported in the vicinity of the extrusion apparatus, for example by means of a robotic arm 15a. The robotic arm 15a appropriately supports and moves the forming drum <NUM> so as to determine the application of the continuous elongated element <NUM> according to a plurality of turns around the outer surface S of the forming drum <NUM> itself, while the latter rotates around a geometric axis of rotation X thereof. The extrusion assembly <NUM> has at least one extrusion nozzle <NUM> longitudinally crossed by an outlet channel <NUM> leading to an outlet opening <NUM>, through which the continuous elongated element <NUM> is dispensed.

Upstream of the extrusion nozzle <NUM>, a first supply unit <NUM> is arranged, configured for introducing a first elastomeric material into a first supply duct <NUM> which converges axially into the outlet channel <NUM> of the extrusion nozzle <NUM>.

Preferably, the first supply unit <NUM> comprises a first mixing group 19a, only schematically indicated in that it may be constructed in a known manner, operating on the first elastomeric material. A gear pump <NUM> is operatively interposed along the first supply duct <NUM>, between the first mixing group 19a and the extrusion nozzle <NUM>. The use of the gear pump <NUM>, the actuation speed whereof may be suitably adjusted by means of a programmable logic controller (PLC) associated with an actuator unit <NUM>, allows controlling with suitable accuracy, instant by instant, the flow rate of the first elastomeric material sent to the extrusion nozzle <NUM>.

The extrusion assembly <NUM> further comprises a second supply unit <NUM> configured to introduce a second elastomeric material different from the first elastomeric material into the outlet channel <NUM> of the extrusion nozzle <NUM>.

More particularly, the second supply unit <NUM> preferably comprises a second mixing group <NUM> having a screw <NUM> operatively housed in a mixing chamber <NUM>, and operable in rotation by means of a motor <NUM>. Preferably, the screw <NUM> is also axially movable in the mixing chamber <NUM>, for example on the command of an axial movement unit <NUM>, to facilitate the transfer of the second elastomeric material into a second supply duct <NUM>, extending in a radial direction with respect to the outlet channel <NUM> and leading to an injection chamber <NUM>. The injection chamber <NUM>, preferably having an annular configuration, is arranged around the outlet channel <NUM> of the extrusion nozzle <NUM> and flows therein through a radial intake slit <NUM>, which extends along a closed line around the outlet channel <NUM> itself, entirely surrounding it (<FIG>). The supply duct <NUM> may end with a distributor <NUM> associated with the extrusion nozzle <NUM>. The distributor <NUM> has a conical ring shape having an inner channel for the passage of the above first elastomeric material ending with an outlet section <NUM>. The outlet section <NUM> is slightly spaced from an inlet section <NUM> of the extrusion nozzle <NUM>. The radial slit <NUM> is axially delimited between the outlet section <NUM> of the distributor <NUM> and the inlet section <NUM> of the extrusion nozzle <NUM>. The outlet section <NUM> may have a substantially elliptical, or in any case geometrically similar, albeit possibly larger, peripheral profile with respect to that of the outlet opening <NUM> and therefore of the continuous elongated element <NUM> coming out of it. As exemplified in the embodiment variant shown in <FIG>, the outlet section <NUM> of the distributor <NUM>, and/or the inlet section <NUM> of the extrusion nozzle may have, in a plane orthogonal to the longitudinal axis thereof coinciding with that of the channel <NUM> and of the extrusion nozzle <NUM>, 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 <NUM> and the extrusion nozzle <NUM> along the peripheral extension of the outlet section <NUM> and the inlet section <NUM>. In other words, the radial slit <NUM> may have a variable axial dimension along the peripheral extension thereof. During the operation of the apparatus <NUM>, the first supply unit <NUM> and the second supply unit <NUM> 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 <NUM> of the extrusion nozzle <NUM>.

The use of the first material provides that while the forming drum <NUM> is rotated and suitably moved in front of the outlet opening <NUM>, the first supply unit <NUM> is activated to feed the first material into the outlet channel <NUM> of the extrusion nozzle <NUM>. The first material is therefore extruded through the extrusion nozzle <NUM>, coming out of the outlet opening <NUM> in the absence of the second material, to be simultaneously applied around the outer surface S of the forming drum <NUM>. The rotation imparted to the forming drum <NUM> around its geometric axis of rotation X determines the formation of consecutive turns C, while a transverse movement imposed by the robotic arm 15a to the forming drum <NUM> 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 <NUM>.

At any desired moment during the extrusion, the second supply unit <NUM> lends itself to being activated, for example by axial movement of the respective screw <NUM>, for pushing the second material into the injection chamber <NUM>. At the extrusion nozzle <NUM> and upstream of the outlet opening <NUM>, the second material pushed into the injection chamber <NUM> is conveyed through the radial slit <NUM>, around the first material coming from the first supply duct <NUM>, preferably through said distributor <NUM>. The second material is therefore distributed around the first material which is about to be extruded through the extrusion nozzle <NUM>. The second material consequently forms, externally to the continuous elongated element <NUM> dispensed by the outlet opening <NUM>, said coating layer <NUM> which entirely surrounds an inner core <NUM>, defined by the first material. By using the aforementioned concave or non-planar shape of the outlet section <NUM> of the distributor <NUM> and/or of the inlet section <NUM> of the extrusion nozzle <NUM> it is possible to initially create a variable layer thickness, i.e. in the vicinity of the radial slit <NUM> of the coating layer <NUM> 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 <NUM>, becomes substantially constant in proximity of the above outlet opening <NUM>. The dispensing of the second material for the purpose of making the coating layer <NUM> may be maintained for a time necessary to form a desired number of turns C around the forming drum <NUM>. The second supply unit <NUM> 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 <NUM> 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 <NUM> may be advantageously provided, for example by adjusting the axial movement speed of the screw <NUM>, to modify the thickness of the coating layer <NUM> applied around the inner core itself.

By adjusting the actuation speed of the gear pump <NUM>, 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 <NUM>, and therefore the dimensions of the continuous elongated element <NUM> 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 <NUM> may further comprise a third supply unit <NUM> 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 <NUM> leading to the injection chamber <NUM>.

The third supply unit <NUM>, shown schematically in dashed lines in <FIG>, may be made substantially identical to the second supply unit <NUM>.

The third supply unit <NUM> may be activated selectively and independently of the activation of the first supply unit <NUM> and the second supply unit <NUM>. Preferably, the activation of the third supply unit <NUM> is carried out when at least the second supply unit <NUM> remains inactive.

The activation of the third supply unit <NUM> during the extrusion of the first material causes the third material to be conveyed to the injection chamber <NUM>, preferably in the absence of the second material being conveyed, to form a covering layer <NUM> which entirely surrounds the inner core <NUM> formed by the first material itself.

The action of modulating the flow rate of the third material conveyed around the inner core <NUM> may be advantageously provided, by operating on the third supply unit <NUM> in a similar way to that said with reference to the second supply unit <NUM>, to modify the thickness of the covering layer <NUM> 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 <NUM>, and therefore the dimensions of the continuous elongated element <NUM> 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 <NUM> for a number of turns as high as desired, and also for the entire spiralling step of the continuous elongated element <NUM>, alternating the operation of the second supply unit <NUM> and of the third supply unit <NUM>, 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 <NUM> and for the third supply unit <NUM> respectively, in which each separate injection chamber is axially distant from the other along the longitudinal development of the outlet channel <NUM>, it is possible to operate the second supply unit <NUM> and the third supply unit <NUM> simultaneously, making two coating layers with the second material and the third material respectively around the inner core <NUM> defined by the first material, providing the elastomeric component of the tyre formed with the continuous elongated element <NUM> with particular physical features.

The construction of the tread band <NUM> 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>) adjacent to the axial centre line Y of the tread band <NUM>, the use of the second material may be required, for example to determine a desired electrical conductivity between the belt structure <NUM> and the radially outer surface 9b of the same tread band <NUM>, intended for contact with the ground during use of the tyre <NUM>.

A preferential use of the third supply unit <NUM> during the construction of a tread band <NUM> may be aimed at the realization of said radially outer apices <NUM>. In this regard, the third supply unit <NUM> is suitable for being activated during the deposition of the turns C at axially outer portions B (<FIG>) close to each of the respectively opposite axial ends 9a of the tread band <NUM>. The continuous elongated element <NUM> dispensed in these circumstances will be provided with the covering layer <NUM> composed of the third material.

The thickness of the covering layer <NUM> and the overall cross-sectional dimension of the continuous elongated element <NUM> may be modified during the deposition, for example by progressively reducing the dimensions of the inner core <NUM> formed by only the first material during the deposition of the turns C increasingly closer to the axial ends 9a of the tread band <NUM>, possibly until the dispensing of the first material is completely interrupted, so that the turns C closest to the axial ends 9a are exclusively composed of the third material.

In the tyre <NUM> obtained according to the present invention, at least one of the elastomeric components, in the case described the tread band <NUM>, will be formed by the continuous elongated element <NUM> 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 <NUM> composed of the first elastomeric material, completely surrounded by the coating layer <NUM> composed of the second elastomeric material, or by the covering layer <NUM> composed of the third elastomeric material will be identifiable.

For the purposes of making the tread band <NUM>, the first elastomeric material may for example comprise a relatively high quantity of silica, for example greater than <NUM> parts by weight out of <NUM> parts by weight of compound.

For the purposes of making the tread band <NUM>, the second elastomeric material may for example contain an adequate quantity of carbon black, for example greater than <NUM> parts by weight out of <NUM> parts by weight of compound, to obtain an adequate electrical conductivity.

For the purposes of making said radially outer apices <NUM>, the third elastomeric material will have composition and physicochemical properties substantially identical to those of the elastomeric material used in the construction of the sidewalls <NUM>, for an optimal coupling between the radially outer apices <NUM> of the sidewalls <NUM> and the axial ends 9a of the tread band <NUM> during the building of the tyre <NUM>.

By way of example, turns C near the axial centre line Y of the tread band <NUM> may be covered by the coating layer <NUM> formed by the second electrically conductive material, turns C placed at the axial ends 9a of the tread band <NUM> to form the radially outer apices <NUM> may in turn be covered by the covering layer <NUM> composed of the third material, substantially identical to that of the sidewalls <NUM>, and finally a plurality of turns C, distributed between the radially outer apices <NUM> and the areas close to the axial centre line Y of the tread band <NUM>, may be formed by only the first material and without the coating layer <NUM> and/or the covering layer <NUM>.

In one or more of the turns C, the coating layer <NUM> or the covering layer <NUM> may have a different thickness than the thickness of the coating layer <NUM> or the covering layer <NUM> present in other turns C.

The turns C having a coating layer <NUM> or a covering layer <NUM> 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 <NUM> or a covering layer <NUM>.

Since it completely surrounds the inner core <NUM>, the coating layer <NUM> 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 <NUM> also following any deformations suffered by the turns C during vulcanisation or other processing steps after building.

The same applies to the covering layer <NUM> formed by the third material, which may ensure effective covering of the inner core <NUM> 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 <NUM> may not be made as shown in <FIG> and not necessarily, for example in the building of a tread band <NUM>, 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 8a, 8b) 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 <NUM>, the coating layer <NUM> 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 8a, 8b) 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 <NUM> and of the apparatus as a whole.

Claim 1:
Process for building tyres for vehicle wheels, wherein at least one elastomeric component of a tyre (<NUM>) is made by applying at least one continuous elongated element (<NUM>) according to a plurality of turns (C) around a forming drum (<NUM>) rotating around a geometric rotation axis (X) thereof;
wherein said continuous elongated element (<NUM>) is built by the actions of:
extruding a first material through an extrusion nozzle (<NUM>), to form an inner core (<NUM>) of said continuous elongated element (<NUM>) exiting from an outlet opening (<NUM>) of the extrusion nozzle (<NUM>);
during said extrusion, conveying a second material different from the first material around the first material, at the extrusion nozzle (<NUM>) and upstream of the outlet opening (<NUM>), to form a coating layer (<NUM>) which surrounds the inner core (<NUM>),
characterized in that the second material is introduced into a second supply duct (<NUM>) leading to an injection chamber (<NUM>) arranged around an outlet channel (<NUM>) longitudinally crossing the extrusion nozzle (<NUM>) and leading to the outlet opening (<NUM>), said injection chamber (<NUM>) flowing into the outlet channel (<NUM>) through a radial intake slit (<NUM>) extending along a closed line around the outlet channel itself;
wherein said radial slit (<NUM>) has a variable axial dimension along a peripheral development thereof.