The present invention concerns molds and treads for tires. In particular it concerns molds that enable the molding of treads having cavities at least part of which is located below the running surface and which open onto at least one lateral face of the tread.
In this document the terms “axial” and “axially” refer to a direction essentially parallel to the rotation axis of a tire. When these terms are applied to a tread, they refer to the direction parallel to the rotation axis of the tire once the tread is fixed on the tire. In other words, the axial direction is the direction perpendicular both to the thickness of the tread and to the circumference of the tire.
The terms “radial” and “radially” refer to a direction parallel to a vector perpendicular to the axial direction and which intersects the rotation axis of a tire. When these terms are applied to a tread they refer to the directions parallel to a vector perpendicular to the tire's rotation axis, and comprising a point on the tire's rotation axis once the tread is incorporated on the tire. In other words, a radial direction is a direction parallel to the thickness of the tread.
“Running surface” of a tread is understood to mean the surface formed by the points of the tread that come into contact with the ground when the tire is rolling.
“Lateral face” of a tread means any part of the surface of the tread which extends from the axial ends of the running surface to the sidewalls of the tire. When considering a tread before it has been incorporated on a tire, a lateral face consists of the part of the tread's surface that connects one of the axial edges of the running surface to the surface designed to come into contact with the carcass of the tire.
“Pin” is understood to mean any molding element designed to mold a cavity at least part of which is located below the mold face that molds the running surface and which opens onto at least one lateral face of the tread, without any limitation of its geometry.
“Longitudinal axis” of a pin means an axis essentially parallel to the direction in which the dimension of the pin is greatest.
“Mix” or “rubber mix” means a rubber composition containing at least one elastomer and filler.
“Flow” of a mix means the flowing of a quantity of the mix during molding.
“Flow asymmetry” produced during the molding of a cavity, is understood to mean the asymmetry in the flow velocity field between the parts of the mix that surround the pin molding the cavity on either side of the surface perpendicular to the surface of the mold that molds the running surface and comprising the points of the pin located furthest away from the mold face that molds the running surface, as described later.
Here, the term “tire” denotes any type of elastic casing, whether inflated or not and whether or not subjected to an internal pressure during use.
The presence of cavities located under a tire's running surface and opening onto at least one lateral face of the tread confers advantageous properties on the tire, particularly when the maximum thickness of its tread exceeds 15 mm. The cavities contribute towards cooling the shoulders of the tire (by a ventilation effect) and consequently improve its endurance. They also render the pattern of the tread evolutional since they emerge at the tread surface as the wear of the tread progresses, so favoring grip on wet ground, snow and ice.
Molds that enable the molding of treads provided with such cavities have been long known. Two approaches can be distinguished: either mobile pins are caused to penetrate into the uncured rubber mix, or the mix that is to form the tread is made to flow and so envelop the pins during molding.
U.S. Pat. No. 1,604,450 is an example of the first approach: it describes the use of movable pins which penetrate axially into a lateral face of the tread after the uncured tire has been placed in the mold and before the vulcanization of the tire. The pins must penetrate without being deformed but also without displacing, nor deforming the uncured tire, and this imposes constraints in terms of the geometry and thickness (mechanical strength) of the pins.
The molds corresponding to the second approach enable cavities to be molded without moving the pins during the molding phase. U.S. Pat. No. 6,408,910 describes a mold whose pins are permanently attached to the shells provided for molding the sidewalls of a tire. During molding, the internal pressure in the uncured—and therefore plastically deformable—tire exerted by the membrane increases the outer diameter of the uncured tire. This stage, known as “shaping”, forces the material constituting the tread to fill up all the spaces in the mold so as to produce the final tread comprising the cavities. To extract the tire from the mold, the shells are moved apart axially to enable the pins to be extracted from the tread. This process has a drawback: it can only be used for fabrications in which the shaping phase is relatively pronounced, since it must correspond at least to the thickness of the pattern desired, including the cavities under the running surface.
Now, the trend in the industry is to try to limit the extent of shaping as much as possible. That is the reason why patent application EP 1 232 852 shows another type of mold, which corresponds to the first approach. The mold proposed comprises a series of crown sectors designed to mold radial cavities or grooves, and a series of shoulder sectors comprising pins designed to mold the axial cavities, both series of sectors being able to move radially. The arrangement of the pins on movable sectors enables the diameter of the circle, on which the pins are arranged, to be increased before the uncured casing is put in place, making it possible to reduce the shapeability of the casing. It is proposed that during the phase of radial penetration into the mix, the pins should be in contact with the crown sectors so as to resist the pressure exerted on the pins by the material that is to be molded. Thus it becomes possible to use relatively long and thin pins, which are susceptible to bending under the pressure exerted by the uncured mix.
Both approaches have proved effective in use; however, treads made in this way can have undesirable properties. In effect, whichever approach is chosen: radial penetration of the pins or radial flow of the mix, the formation of the final tread entails a flow of the mix either side of the pin with the formation of a junction plane that extends between the surface of the pin and the mold face that molds the tire's tread. The use of molds corresponding to the present state of the art creates a junction plane perpendicular to that surface, which can prove fragile, in particular because of its sensitivity to contamination during the molding.
One objective of the present invention is to control the position of the junction zone and its orientation relative to the surface of the tire. It is proposed to do this by means of a mold for molding a tread made of rubber mix, this tread having a running surface that is bounded axially by lateral faces, the mold comprising at least one part for molding the running surface, at least two lateral parts for molding the lateral faces, and at least one pin for molding a cavity within the tread, the pin, viewed in a section plane perpendicular to the surface that molds the running surface and to the axial direction, having a surface with a contour formed of a first contour portion and a second contour portion, these contour portions extending between the point of the contour closest to and the point of the contour furthest from the surface that molds the running surface, the pin being located at least partially below the surface that molds the running surface and being connected to at least one surface that molds at least part of a lateral face of the tread, such that during molding the pin creates two flows of rubber mix which envelop the pin and reunite to form a junction surface, wherein at least one pin has means for rendering the mix flows asymmetrical so that the junction surface, viewed in the said plane, is such that the length of the trace of the junction surface on the plane, measured between the surface of the pin and the surface that molds the running surface, is larger than the minimum distance between the surface of the pin and the surface that molds the running surface.
When several points of the contour are at the same maximum distance from the mold face that molds the running surface, the two points furthest away in a direction perpendicular to the radial direction and to the axial direction are used. The two contour portions extend between each of these two points and the contour point located closest to the surface that molds the running surface. An analogous procedure is used if several contour points have the same minimum distance relative to the mold face that molds the running surface. As an example, for a pin whose surface, in a section plane perpendicular to the mold face that molds the running surface and to the axial direction, is a rectangle whose lengths are parallel to the mold face that molds the running surface and whose widths are perpendicular to that mold surface, the two contour portions coincide with the widths of the rectangle.
The mold of the invention makes it possible to enlarge the junction surface which extends between the surface of the cavity and the mold face that molds the running surface, whatever the degree of shaping; the means proposed, can be implemented in molds with or without shaping.
Several means can be used to obtain asymmetrical flows. In particular a pin geometry can be chosen which can produce asymmetrical mix flows during the penetration of the pin or, in the case of shaping, during the flow of mix around the pin.
In a preferred embodiment, the flows are made asymmetrical by using pins whose first and second contour portions have different length. Preferably, the difference between the lengths of the first and second contour portions is at least equal to 10% of the length of the shorter contour portion.
In another preferred embodiment, the mix flows around a pin are rendered asymmetrical by providing one of the contour portions with a mean roughness greater than the mean roughness of the other contour portion. For example, the roughness of the first portion can be increased by sand-blasting and the roughness of the second portion reduced by polishing. Preferably, the difference between the mean roughnesses Ra of the first and second contour portions is at least equal to 2 microns. Of course, it is possible to combine the effects of geometry and surface roughness by using pins whose first and second portions have both lengths and roughnesses that are different.
In a third preferred embodiment, the flows are made asymmetrical by a chemical coating applied to at least part of at least one of the contour portions so as to produce a difference in the slip velocity of the rubber mix on the portions. For example, the first contour portion can be coated with Araldite® and the second made of steel. The effects of coating and of geometry and/or roughness are advantageously combined by using pins whose first and second portions have both coatings, and lengths and/or roughnesses that are different.
In a fourth preferred embodiment, the mix flows are made asymmetrical by a temperature effect, by producing a mean temperature difference between the surfaces that correspond to the first and second contour portions of each pin. Preferably, the temperature difference is at least 20° C. There is no reason not to combine the effects of temperature and geometry and/or roughness and/or coating of the pin to make asymmetric of the mix flows around each pin in an optimum way.
In a fifth preferred embodiment, at least one pin for molding a cavity has additional means for causing it to rotate essentially about its longitudinal axis. This rotation can be carried out during the penetration of the pin into the mix (if the pins are mobile), during the shaping phase (if there is one) and/or when the pin is surrounded by mix, before the vulcanization of the tread.
All the embodiments described above are applicable in molds with or without shaping. A last preferred embodiment concerns more particularly molds without shaping whose mobile pins penetrate into the mix forming the tread after the tire blank or the tread has been inserted into the mold. In this preferred embodiment at least one pin, preferably of cylindrical geometry, is mounted so as to be both axially and radially mobile relative to the mold, so that it can carry out a translation movement in a plane forming an angle smaller than 90° with a radial direction. The mix flows around the pin during its penetration into the mix reunite after the pin has passed and form a junction surface which is inclined relative to a plane perpendicular to the mold face that molds the running surface. Consequently, the mean length of the trace of the junction surface on the this plane, measured between the surface of the pin and the mold face that molds the running surface, is larger than the minimum distance between that pin and the mold face that molds the running surface. It is possible to combine the effect of penetration along a direction forming an angle smaller than 90° relative to a direction perpendicular to the molding surface, with the effects of temperature and/or geometry and/or roughness and/or coating and/or rotation of the pin essentially about its longitudinal axis, to enlarge the junction surface extending between the surface of the pin and the mold face that molds the running surface.
The invention also concerns a tread made of rubber mix, this tread having a running surface bounded axially by lateral faces and comprising at least one cavity at least part of which is located below the running surface and which opens onto at least one lateral face of the tread, the tread being provided with a junction surface that extends between the surface of at least one cavity and the running surface, wherein the length of the trace of the junction surface in a section plane perpendicular to the running surface and to the axial direction, measured between the cavity and the running surface, is longer than the minimum distance between the surface of the cavity and the running surface. Preferably, the difference between the length of the trace and the minimum distance between the surface of the cavity and the running surface is greater than 20% of the minimum distance.
The invention will be better understood thanks to the description of the drawings, in which:
FIGS. 4 to 7 are schematic representations, in a radial section plane AA′ of the mold of
FIGS. 8 to 11 are schematic representations of variants of pins according to the invention, viewed in section.
The pin 50 of the prior art does not produce asymmetrical mix flows during the molding of the cavity 18: the velocity fields characterizing the flow of the mix are symmetrical with respect to the axis perpendicular to the mold face 16 that molds the running surface 15 and passing through the point 60 of the pin located furthest away from that face. Consequently, the junction surface 30 formed between the parts of the mix that have flowed round the pin 50 during molding is essentially perpendicular to the mold face 16 that molds the running surface 15. Its length is essentially equal to the minimum distance 20 separating the surface of the pin 50 from the face 16.
FIGS. 4 to 7 are schematic representations, in a section plane AA′ of the mold 1 of
Satisfactory results are obtained when the mix flows are made asymmetrical over almost the whole of the pin, possibly except at the ends of the pin.
Whereas
Likewise, it is understood that the principle of the invention can be applied regardless of whether it is desired to mold cavities on only one lateral face of the tread or on both lateral faces. The cavities on opposite sides can be arranged symmetrically or not. The crown sectors 2 can cover the full width of the tread or only part thereof.
The number of pins 5 and their precise geometry are determined depending on the result desired in the finished tread. Each sector can have several pins or, on the contrary, some sectors can have no pins at all.
It is also possible for the pins 5 to be mounted on shoulder sectors that can move radially, as described in the document EP 1 232 852.
Pins can be arranged axially or in a direction oblique to the tire's axis. Moreover, the radial cross-section of at least one pin can vary along the direction of the pin's largest dimension.
The molds according to the invention enable treads to be molded, which may or may not be annular and of finite length or, on the contrary, of quasi-infinite length, continuous and flat. In this way not only treads intended for the production or retreading of tires can be molded, but also rubber caterpillar tracks.
Number | Date | Country | Kind |
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04/03907 | Apr 2004 | FR | national |
Number | Date | Country | |
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60586874 | Jul 2004 | US |