TIRE VULCANIZING MOLD WITH OPTIMIZED CLOSING

Abstract

The tire vulcanizing mould includes a plurality of segments for molding the outside of the tire tread. The segments are able to move radially between a position in which the mold is open and a position in which the mold is closed. The segments have frustoconical bearing surfaces that are placed radially on the outside. An axially mobile external ring, which acts on the frustoconical bearing surfaces, is placed radially on the outside of the segments in order to cause them to close and to move back from their position of closure in the mold. The external ring is radially preloaded towards the inside when the mold is in the open position.

Claims

1) A tire vulcanizing mold comprising: a plurality of segments for molding the outside of the tire tread and which are able to move radially between a position in which the mold is open and a position in which the mold is closed, these segments having frustoconical bearing surfaces placed radially on the outside, and an axially mobile external ring which acts on the frustoconical bearing surfaces placed radially on the outside of the segments in order to cause them to close and to move back from their position of closure in the mold, wherein the external ring is radially preloaded towards the inside when the mold is in the open position.

2) The tire vulcanizing mold according to claim 1, wherein the external ring is a shrink-fit assembly.

3) The tire vulcanizing mold according to claim 2, wherein the shrink-fitting pressure is higher than the internal curing pressure.

4) The tire vulcanizing mold according to claim 1, further including a rigid cylindrical tube shrink-fitted around the ring.

5) The tire vulcanizing mold according to claim 1, further including a taut cable wound around the ring.

6) The tire vulcanizing mold according to claim 1, wherein an angle of inclination of the frustoconical bearing surface is between 6 and 20°.

7) The tire vulcanizing mold according to claim 1, wherein a preload is applied to the external ring by creating a cone-to-cone assembly.

8) The tire vulcanizing mold according to claim 7, wherein an angle of inclination of the cone is between 3 and 25°.

9) The tire vulcanizing mold according to claim 7, further including a means for adjusting the amount of preload.

10) The tire vulcanizing mold according to claim 1, further including two plates that are movable axially and supporting shells intended to mold the sidewalls of the tire.

11) A method for moulding a tire using a mold according to claim 1, including the step of applying a preload to the external ring before the mold is closed.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The disclosure will be better understood by virtue of the remainder of the description, which is supported by the following figures:

[0029] FIG. 1 is a view in cross section of the mold according to a first embodiment of the disclosure, the cross section being taken on a vertical plane that passes through the axis of symmetry of the mold;

[0030] FIG. 2 is a view in cross section of the mold according to a second embodiment of the disclosure, the cross section being taken on a vertical plane that passes through the axis of symmetry of the mold.

DETAILED DESCRIPTION OF THE ENABLING EMBODIMENTS

[0031] The mold 1 comprises segments 2 comprising molding elements 3 for molding the radially external surface of the tread and which delimit, with an upper shell 4 and a lower shell 5 an internal cavity 6. The upper shell 4 and the lower shell 5 are each supplemented by a part intended to mold the bottom region of the green tire 7 and 8 respectively. The molding elements 3 are fixed to the segments 2 by snap rings 9. The shells 4 and 5 are each secured to an upper plate and to a lower plate of the press (the plates are not illustrated), which move relative to one another in the axial direction. The mold thus formed exhibits symmetry of revolution about the axis X-X′.

[0032] When the mold 1 is in the closed position, as illustrated in FIG. 1, the molding elements 3 are arranged circumferentially in close contact with each of the shells to form a continuous molding surface. A curing membrane is arranged in the internal cavity 6 and, by means of a heat-transfer fluid, provides the conditions of temperature T1 and curing pressure p1 for curing the green tire arranged inside the mold 1 and pressed firmly by the curing membrane against the molding parts thereof. By way of example, the temperature T1 is comprised between 120 and 160° C. and the pressure p1 between 16 and 24 bar (1.6 and 2.4 N/mm.sup.2).

[0033] An external ring 10 radially on the outside with respect to the segments 2 is mounted with the ability to move axially (understood to mean along the axis X-X′ or parallel thereto), it has a frustoconical internal surface 12 which engages with the frustoconical external bearing surfaces 11 of the segments 2. The external ring 10, radially on the outside with respect to the segments 2 and with the ability to move axially, causes the segments 2 to close by engaging with the frustoconical bearing surface radially on the outside of the segments. The external ring 10 is made to move, for example, by means of an actuating cylinder (not depicted) with an adjustable application force F.

[0034] According to the disclosure, a preload S1, oriented radially towards the inside of the mold 1, is applied to the external ring 10. This preload is permanent and is obtained at the time of manufacture of the ring. In the embodiment of FIG. 1, a cylindrical tube 19 is shrink-fitted in the hot state around the external ring 10. The cylindrical tube 19 is heated and fitted in the hot state over the peripheral or shrink-fitting surface 14 of the body 16 of the external ring 10 until it comes into abutment with a shoulder 15 thereof.

[0035] In the examples illustrated in the figures, the ring is of the heated ring type and contains pressurized steam. The external ring 10 of FIG. 1 is thus created by assembling a body 16 that has a cylindrical external surface 14 and a frustoconical sleeve 17 welded to the body 16, a chamber for the circulation of heat-transfer fluid 18 being formed between the two. Similarly, the external ring 10 illustrated in FIG. 2 is created by welding together a body 20 and a frustoconical sleeve 17 defining a chamber for the circulation of heat-transfer fluid 18 between the two.

[0036] In the example illustrated, the cylindrical tube 19 has a thickness of 15 mm and the external ring has an external diameter of the shrink-fitting surface 14 of 957 mm at ambient temperature. When a shrink-fit tube made of steel and a tensile stress of 150 N/mm.sup.2 are chosen, a pre-heating band diameter of 956.3 mm is obtained, which will be easy to fit over the ring when heated to a temperature of around 180° C.

[0037] In an alternative form, the external ring 10 is cooled using liquid nitrogen or dry ice in order to shrink it and fit it inside the cylindrical tube 20.

[0038] In another alternative form of the disclosure, the external ring 10 is shrink-fitted by winding a taut metal cable around the shrink-fitting surface 14 of the ring. The cable is wound in a helix at a determined pitch over all or part of the height of the said shrink-fitting surface 14. Individual annular hoops may also be created so that they are uniformly distributed over the height of the shrink-fitting zone 14. The shrink-fitting pressure needs to be comprised between 4 and 6 N/mm.sup.2.

[0039] In another alternative form, resin-coated wires may be used in place of the metal cables.

[0040] FIG. 2 illustrates the mold 1 according to a second embodiment of the disclosure, in which elements similar to those of FIG. 1 have kept the same reference numeral. As can be seen in FIG. 2, stress is imparted to the external ring 10 by producing a cone-to-cone assembly in which the cones are moved using an adjusting screw. To do that, the external ring 10 comprises a body 20 having a frustoconical external surface with angle of inclination a. A frustoconical annulus 22 having an internal surface inclined by the same angle α as the body 20 is fitted over the external surface of the latter. The body 22 has a protruding part 21 which fits over a part of corresponding shape belonging to the body 20, the body 20 and the annulus 22 being fixed together using a screw 23. The screw 23 is used to adjust the axial position of the annulus 2 with respect to the body 20 and therefore to adjust the amount of preload applied to the ring 10

[0041] By way of example, the body 16, 22 of the external ring 10 is made of a steel that can be welded, such as 25CrMo4, having the following characteristics at ambient temperature: a minimum tensile strength of 460 N/mm.sup.2, a minimum elastic limit of 250 N/mm.sup.2 and an elongation at break in excess of 14%. The same is true of the material of the sleeve 17 which, in addition, has a nitriding treatment of its frustoconical surface which collaborates with that of the segments 2. The cylindrical tube 19 and the frustoconical annulus 22 are made from a steel having mechanical properties equivalent to those of the body of the external ring 10.

[0042] The way in which the mold 1 works is as follows: having placed a green tire inside the internal cavity 6, the mold 1 is closed through a progressive advancing movement of the external ring 10 due to the closure force F coming from the press. The frustoconical internal surface 11 of the ring progressively engages with the frustoconical bearing surfaces 12 of the segments 2, which causes the segments and the internal cavity of the mold 1 to close through axial movement of the upper plate of the press. When the mold 1 is opened, the external ring 10 advances axially and the segments 2 retreat radially and are then driven axially at the same time as the upper plate in order to move away from the lower plate.

[0043] FIG. 1 illustrates the mold 1 in the closed position during the vulcanization operation. The resultant of the forces due to the internal pressure p1, which is the pressure that the curing membrane in exerts on the green tire, at the interface between the ring and the segments, is represented as P1 in FIG. 1. The resultant P1 has a horizontal component P2 and a vertical component P3. The clamping force F of the press needs to be higher than the vertical component P3 in order for the mold 1 to remain closed during curing. The direction of the force S1 supplied by the preload of the ring 10 is the opposite of that of the force P2 due to the internal pressure of the mold 1. This prevents the segments from opening up during vulcanization, thereby preventing molding defects from occurring on the tire. What is more, the clamping force F supplied by the press is lower, because it needs to overcome only the force P3.

[0044] The embodiments described hereinabove are of course nonlimiting and a person skilled in the art will be able to envisage other equivalent means that allow stress to be imparted to the external ring of the mold 1. Thus, in place of the shrink-fitted cylindrical tube it is possible to use a split cylindrical tube combined with means of circumferential clamping around the external surface of the ring.

[0045] It is moreover possible to envisage an assembly using several successive bands which may be identical or different.

[0046] The solution of the disclosure is just as applicable to the molds used for manufacturing tires as it is to annular treads for the retreading of tires.