Patent Publication Number: US-9421725-B2

Title: Molding element comprising cutting means for molding and vulcanizing a tire tread

Description:
This application is a 371 national phase entry of PCT/EP2014/061802, filed 6 Jun. 2014, which claims benefit of European Patent Application No. 13172040.1, filed 14 Jun. 2013, the contents of which are incorporated herein by reference for all purposes. 
     BACKGROUND 
     The present disclosure relates to the field of molds for molding and vulcanizing a tire tread. More specifically, the disclosure relates to the molds used for molding in the tread grooves which are partially or completely covered by an additional cover layer. 
     DESCRIPTION OF RELATED ART 
     It is known practice to design tires the tread of which comprises various rubber compounds. Document WO 03089257 discloses such treads. More specifically, document WO 03089257 discloses a tread comprising grooves the walls of which are covered with a cover layer. The material of which this cover layer is made differs from the rubber compound of which the tread is made. This material notably has much better wet grip than the wet grip of the rubber compound. This allows a very significant improvement in cornering performance on wet ground. 
     One way of manufacturing this tread is notably disclosed in document WO 2006069912. According to this method of manufacture, in a first step, provision is made for the material intended to constitute a cover layer to be injected in the form of one or more inserts into the green tire using an injection nozzle. The insert or inserts is or are then shaped, in a second step, by ribs of a vulcanizing mold so that they cover all or part of the walls of the grooves molded by these ribs. 
     This method of manufacture has its limits, particularly as far as obtaining precision moldings is concerned. Specifically, during the shaping thereof, the insert experiences a significant shear force from the rib in order to convert this insert into a layer of lesser thickness. This shear force may cause cracking within the insert, making it more difficult to control the movements of the material of which this insert is made. The shape and the thickness of the cover layer thus formed may therefore be somewhat haphazard. The advantages afforded by the additional layer to the performance of the tire are then reduced. 
     In addition, in this method of manufacture, it is necessary to make the inserts align with the ribs. This makes manufacture of the tread more complicated. 
     There is therefore a need to improve the incorporation of a cover layer on the walls of grooves of a tire tread. 
     DEFINITIONS 
     A “tire” means all types of elastic tire whether or not subjected to an internal pressure. 
     A “green tire” or “green form” of a tire means a superposition of a plurality of semi-finished rubber products present in the form of strips or sheets, with or without reinforcement. The green tire is intended to be vulcanized in a mold in order to obtain the tire. 
     The “tread” of a tire means a quantity of rubber material bounded by lateral surfaces and by two main surfaces one of which is intended to come into contact with a road surface when the tire is running. 
     A “tread surface” means the surface formed by those points on the tire tread that come into contact with the road surface when the tire is running. 
     A “mold” means a collection of separate molding elements which, when brought closer towards one another, delimit a toroidal molding space. 
     A “molding element” of a mold means part of a mold. A molding element is, for example, a mold segment. 
     A “molding surface” of a molding element means the surface of the mold that is intended to mold the surface of the tire tread. 
     A “blade” of a molding element means a protrusion projecting from the molding surface. In the blades category, a distinction is made between sipe blades which are less than 2 mm wide and ribs which have a width of 2 mm or more. Sipe blades are intended to mold sipes in the tire tread, which means cuts which at least partially close up when they fall within the contact patch where the tire makes contact with the ground. The ribs are intended to mold grooves in the tread, which means cuts which do not close up when they fall within the contact patch in which the tire makes contact with the ground. 
     SUMMARY 
     The invention, in an embodiment, relates to a molding element of a mold for molding and vulcanizing a tire tread, this tread comprising a tread surface intended to come into contact with the ground when the tire is rolling. The molding element comprises a molding surface intended to mold part of the tire tread surface and a blade intended to mold a sipe or a groove in the tread. The molding element comprises two lamella positioned on either side of the blade at a certain distance from this blade, each lamella comprising a bulge at one extremity far away from the molding surface. Indeed, each lamella comprises cutting means extending from the bulge. 
     The cutting means are able to cut a cover layer that covers a green tire. The blade itself will mold a groove in the tread and at the same time guide part of the cover layer into the groove thus molded. 
     The invention, in an embodiment, proposes that the cover layer is cut and some parts of this layer are positioned within the depth of the green tire during one and the same movement of the molding element with respect to the green tire. 
     The cutting means can create a stress concentration at cutting means connection area. That stress causes block chipping during tire rolling and crack initiation risk in the deep of the tread band. By applying a bulge at the base of the cutting means, the inventors have discovered that the stress concentration is reduced during tire rolling. 
     In an alternative form of the embodiment, the lamella extend in a direction of extension X, and the cutting means are continuous along this direction of extension X. Continuous cutting means improve the operation of cutting the cover layer. 
     In another alternative form of the embodiment, the cutting means comprise a plurality of sharped teeth. 
     With the lamella, there is a space between two consecutive teeth. This space create a gum bridge between two cavities molded by the teeth. This gum bridge limits also the risk of initiating cracks. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other features and advantages of the embodiments of the invention will emerge from the following description, given by way of non-limiting example, with reference to the attached drawings in which: 
         FIG. 1  is a sectional drawing depicting schematically a molding element according to the embodiments of the invention; 
         FIG. 2  illustrates a lamella of the molding element of  FIG. 1 , according to a first embodiment of the invention; 
         FIG. 3  illustrates a lamella of the molding element of  FIG. 1 , according to a second embodiment of the invention; 
         FIG. 4  illustrates a step of a method of manufacturing using the molding element of  FIG. 1 , in which a cover layer is cut by cutting means belonging to the lamella; 
         FIG. 5  illustrates a step of the method of manufacturing using the molding element of  FIG. 1 , in which a blade drives a cut part of the cover layer into the green tire; 
         FIG. 6  illustrates a step of the method of manufacturing using the molding element of  FIG. 1 , in which blade and lamella are totally embedded in the green tire; 
         FIG. 7  is a sectional drawing depicting schematically a tire tread with a raised element partially covered by a cover layer, after using the molding element of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS 
     In the description that follows, elements that are substantially identical or similar will be denoted by identical references. 
       FIG. 1  depicts a molding element  1  for implementing a method of manufacture according to the embodiment of the invention. 
     More specifically, the molding element  1  comprises a molding surface  3  intended to mold part of the tread surface of a tire. The molding element  1  also comprises blades  5  of which just one is depicted here to make the embodiment of the invention easier to understand. The blade in this instance is a rib  5  intended to mold a groove in the tire tread. A “groove” in a tread means a cut in this tread of a width, i.e. the distance separating two lateral walls of this groove, greater than 2 mm. The rib  5  extends heightwise and projects from this molding surface  3  with an height H. The rib  5  also extends lengthwise in a direction of extension X. In the mold, this direction X may be a circumferential direction following the circumference of the mold. As an alternative, the direction of extension X is a transverse direction perpendicular to the circumferential direction of the mold. In another alternative form, this direction of extension is an oblique direction making a non-zero angle with the circumferential direction and with the transverse direction of the mold. 
     The molding element  1  of  FIG. 1  also has two lamella  7  arranged either side of the rib  5 . These lamella extend lengthwise in a direction parallel to the direction of extension X of the rib  5 . A “parallel direction” means that the direction of extension of the cutting means makes an angle of between −5° and +5° with the direction of extension X of the rib. Each lamella comprises a bulge  9  at one extremity far away from the molding surface  3  and each lamella comprises cutting means  11  extending from the bulge. 
       FIG. 2  illustrates lamella  7  comprising cutting means  11  continuously extending along the direction of extension X. 
       FIG. 3  illustrates another embodiment of lamella  7 . In this embodiment, the cutting means  11  comprise a plurality of sharped teeth  13 . 
       FIG. 4  to  FIG. 7  illustrates the different steps of a method of manufacturing a tire tread using a molding element according to the embodiment of the invention. 
       FIG. 4  discloses a step in which the cover layer  12  is cut by the lamella  7 . In this step, the molding element  1  and the green tire  10  move closer to one another. This movement is, for example, initiated by a membrane (not depicted) in the mold. Under the action of a quantity of pressurized steam, this membrane swells and pushes the green tire towards the molding element  1 . 
       FIG. 5  illustrates a step whereby the blade  5  drives part  14  of the cover layer into the green tire. 
       FIG. 6  illustrates an intermediate molding step in which the rib  5  is pushed into the green tire over its entire height. The entirety of the cover layer thus finds itself inside the green tire. Once this step has been performed, it is then possible to vulcanize the green tire, i.e. to convert the rubber material of which the green tire is made from the plastic state to the elastic state. This vulcanizing step may also modify the internal structure of the cover layer. 
       FIG. 7  depicts the result of the various steps of molding and vulcanizing the green tire illustrated in  FIGS. 4  to  FIG. 6 . The part of the tread thus obtained comprises a groove  16  obtained by molding rubber around the rib  5  and two sipes  18  obtained by molding rubber around the two lamella  7 . It will be noted here that all of the walls of the groove, i.e. the lateral walls and the bottom wall flanked by the lateral walls, are covered with the cut part  14  of the cover layer. 
     The embodiments of the invention is not restricted to the examples described and depicted and various modifications can be made thereto without departing from its scope.