Precure tread for tire, tires using the same and method of producing tires

Abstract: The precure tread (12) comprises surface grooves (16) facing a tread face (12a), rear grooves (18) facing a crown portion of a base tire (13), and means (20, 21) for communicating the rear grooves (18) to atmosphere. The groove bottom of the rear grooves (18) is above the groove bottom of the surface grooves (16). The tire durability can considerably be improved when this precure tread is applied to a new tire or a retreaded tire.

Description: This invention relates to precure treads for tires, and to methods of producing pneumatic tires using such precure treads. Particularly, the invention provides a considerable improvement of tread durability when such precure treads are applied to, for example, new or retreaded tires for trucks and buses. In tires, such as pneumatic tires for trucks and buses, as the running distance increases, a tread 2 of a tire 1 is gradually worn as shown in Fig. 19 to shallow circumferential main grooves 3 formed on the surface of the tread 2, whereby the road gripping force of the tread 2 as well as the wet performance of the tire are reduced. Therefore, the tire 1 is generally and frequently retreaded at a state that the main groove 3 is left to a certain extent in the tread 2. The retreading of the tire is usually carried out by buffing the remaining tread portion 2a of the worn tire 1 to form a base tire 4 as shown, for example, by dotted lines in Fig. 19, and then bonding a precure tread 6 to a crown portion 4a of the base tire 4 through a cushion rubber 5 by vulcanization as shown in Fig. 20. Moreover, the retreading of the tire is repeated 2 to 3 times or may be repeated more than 3 times though it changes in accordance with the service conditions. In case of repeating the retreading, if the wear resistance of the tread is low or the wet performance at the last worn stage is low, the period between the retreading and next retreading becomes shorter and the retreading becomes uneconomical. In order to sufficiently develop the wet performance and other performances of the tread even at the last worn stage, there is proposed a tire as described in Japanese Utility Model laid open No. 51-95,902. In this proposed tire, a rear groove extending from a position separated outward from the bottom of the surface groove in the radial direction toward the inside in the radial direction is continuously or dis-continuously disposed in the inside of the tread in the circumferential direction of the tire. According to this tire, the rear groove is exposed at the last worn stage, whereby the degradation of the wet performance and other performances can effectively be prevented. That is, when this is applied to a precure tread 8 of a retreaded tire 7 as shown in Fig. 21, the precure tread 8 comprises a plurality of main grooves 3 at its ground contacting portion 8a and a rear groove 9 at its rear portion 8b facing the base tire. When the tread 8 is worn up to a position shown by dot-dash lines M in Fig. 21 during the use of the retreaded tire 7, the rear groove 9 appears at a tread face 8c, and consequently the drainage effect through the rear groove 9 as well as the edge effect at the groove edge of the rear groove are generated to largely improve the wet performance. Furthermore, the precure tread 8 is required to provide sufficient wet performance at the last worn stage, so that it is not sufficient to ensure improved wet performance in the precured tread over a period ranging from initial worn stage to last worn stage. As a result, a ratio of surface groove area occupied in ground contacting area of the tread or so-called negative ratio can be made small to improve the wear resistance and resistance to uneven wear. In the tire of Fig. 21, however, the rear groove 9 forms a closed space through the bonding of the precure tread 8 to the cushion rubber 10, so that gas existing in the rear groove 9 thermally expands through compressive deformation of the tread 8 repeated during the running of the tire and enters an interface between the tread 8 and the cushion rubber 10 due to the increase of the volume of the gas itself. As a result, there is caused a problem of peeling of a portion of the tread 8 near to the rear groove 9 from the cushion rubber 10 as exaggeratedly shown in Fig. 21 to degrade the durability of the tire. Attention is also drawn to the disclosure of GB-A-830933, which describes a method for rebuilding and repairing worn automobile tires in which a pre-vulcanised retreading strip and an unvulcanised intermediate bonding layer is used, wherein the tire with the parts to be united is completely enveloped in an air-tight flexible sheath which conforms to the external and internal surfaces of the tire and from which the air between the surfaces of the tire and the retreading strip which are to be united is forced out through a valve by means of pressure applied externally to the sheath, the bonding layer being caused to flow or run by applied heat to effect a V-shaped bond at the lateral edges of the retreading strip. It is an aim of the invention to advantageously overcome the above-described problems of the conventional technique and to provide a precure tread for a tire capable of considerably improving the durability of the tread and hence the tire by sufficiently preventing the peeling of the tread due to the present of the rear groove, and to provide a method of producing such a pneumatic tire. The present invention in a first aspect provides a precure tread for a tire to be bonded to a crown portion of a base tire and having a ground contacting portion located at a tread side of the tire and a rear portion located at the side of the base tire, in which a plurality of surface grooves are formed in the ground contacting portion and a plurality of rear grooves are formed in the rear portion, characterized in that a groove bottom of said rear groove is located at a level higher than a groove bottom of said surface groove in the radial direction of the tire, and each of said rear grooves is communicated to atmosphere at the side of said ground contacting portion through a passage formed in said precured tread. The term "precure tread" used herein means a tread formed in a mold and then vulcanized therein. The rear grooves may be continuously or discontinuously formed in the circumferential direction of the tire, while the sectional shape of the passages may be circular or venthole-like, rectangular or sipe-like, or of other required shape. The invention in a second aspect provides a precure tread for a tire to be bonded to a crown portion of a base tire and having a ground contacting portion located at a tread side of the tire and a rear portion located at the side of the base tire, in which a plurality of surface grooves are formed in the ground contacting portion and a plurality of rear grooves are formed in the rear portion, characterized in that a groove bottom of said rear groove is located at a level higher than a groove bottom of said surface groove, and an opening portion extending to the surface of the tread is formed in said precure tread above said rear groove and separated from said rear groove through a partition wall. The invention in a third aspect provides a precure tread for a tire to be bonded to a crown portion of a base tire and having a ground contacting portion located at a tread side of the tire and a rear portion located at the side of the base tire, in which a plurality of surface grooves are formed in the ground contacting portion and a plurality of rear grooves are formed in the rear portion, characterized in that a groove bottom of said rear groove is located at a level higher than a groove bottom of said surface groove, and an opening portion of said rear groove extending above said rear groove is formed in said precure tread and terminated in a protrusion formed on the surface of said precure tread. The invention in a fourth aspect provides a method of producing a pneumatic tire, by attaching a precure tread according to the first aspect of the invention to a crown portion of a base tire through an unvulcanized cushion rubber, mounting the resulting assembly onto a rim together with an envelope housing the assembly therein, and then vulcanizing the assembly by heating under pressure, characterized in that said rear grooves are communicated to atmosphere through a passage formed in said precure tread, a wick contacting with the outer peripheral surface of said precure tread, and a guide pipe contacting with said wick. The invention in a fifth aspect provides a method of producing a pneumatic tire, comprising the steps of: (a) attaching a precure tread according to the second or third aspect of the invention to a crown portion of a base tire through an unvulcanized cushion rubber; (b) mounting the resulting assembly onto a rim together with an envelope housing the assembly therein; (c) vulcanizing the assembly by heating under pressure; and (d) communicating said rear grooves to atmosphere after the vulcanization. The invention will be further described by way of example only, with reference to the accompanying drawings, wherein: Figs. 1 to 3 show a first embodiment of a precure tread according to the first aspect of the invention and a pneumatic tire using the same, wherein Fig. 1 is a partly plan view of the tire, Fig. 2 is a sectional view taken along a line II-II of Fig. 1, and Fig. 3 is a partly sectional view in widthwise direction of the base tire used in the manufacture of the retreaded tire of Fig. 1; Figs. 4 and 5 are partly sectional views showing a main part of the precure tread for a tire according to the invention, respectively; Figs. 6 to 13 show second to fifth embodiments of the precure tread according to the invention, wherein Fig. 6 is a partly plan view of the second embodiment, Fig. 7 is a sectional view taken along a line VII-VII of Fig. 6, Fig. 8 is a partly plan view of the third embodiment, Fig. 9 is a sectional view taken along a line IX-IX of Fig. 8, Fig. 10 is a partly plan view of the fourth embodiment, Fig. 11 is a partly plan view of the fifth embodiment, and Figs. 12 and 13 are sectional views taken along lines XII-XII and XIII-XIII of Fig. 11, respectively; Fig. 14 is a sectional view illustrating a method for the production of a tire using the precure tread for tire according to the first aspect of the invention; Figs. 15 and 16 are sectional views in widthwise direction of sixth and seventh precure treads, respectively; Figs. 17 and 18 are diagrammatical views illustrating various modifications in the plan and section forms of the protrusion, respectively; Fig. 19 is a sectional view in widthwise direction of a tire to be retreaded; Fig. 20 is a sectional view in widthwise direction of the conventional retreaded tire; and Fig. 21 is a sectional view in widthwise direction of another conventional tire. When the first precure tread according to the invention is applied to a new tire or a retreaded tire, the rear groove is exposed before the disappearance of the surface groove with the advance of tread wearing. That is, the groove as expected is existent in the tread over a period ranging from the initial worn stage to the last worn stage, so that there is no danger of lowering the drainage and wet performances of the tire even when the tread is worn up to the rear groove, and consequently the wear life of the tread is considerably prolonged. In this case, the surface groove can be arranged in the precure tread without particularly considering the wet performance of the tread at the last worn stage. That is, it is enough to consider only the wet performance ranging from the initial worn stage of the tread to the last worn stage thereof, so that the wear resistance of the tread can largely be improved by effectively reducing the ratio of surface grooves occupied in the tread surface. In case of tires using the above precure tread, since the rear groove is communicated to an atmosphere through the passage formed in the precure tread, gas inside the rear groove freely moves toward the atmosphere, so that even when heat is generated in the tread due to compression deformation of the tread during the running, there is no peeling of the tread resulting from thermal expansion of gas inside the rear groove. Further, the tread is effectively cooled by air flowing in or out in the rear groove, so that the thermal fatigue of the tread and cushion rubber can sufficiently be controlled. In the first method according to the invention, when the precure tread is bonded by vulcanization to the crown portion of the base tire through the unvulcanized cushion rubber, the rear groove of the precure tread is communicated to an atmosphere through the passage formed in the precure tread, the wick contacting with the outer peripheral surface of the precure tread and the guide pipe contacting with the wick, whereby air sandwiched between the precure tread and the unvulcanized cushion rubber and gas produced by the reaction of the unvulcanized cushion rubber are smoothly discharged into the atmosphere, so that the adhesion force between the cushion rubber and the tread is considerably increased to largely improve the durability of the tire. In the second and third precure treads according to the invention, the communication of the rear groove to the atmosphere is obstructed by the partition wall or the protrusion formed on the tread surface. In this case, however, the rear groove can be communicated very simply to the atmosphere at the side of the ground contacting portion of the precure tread by piercing a hole in the partition wall with the opening portion facing the tread surface as a mark or cutting out the protrusion from the precure tread at as-precured tread state or after the bonding to the base tire. Moreover, the partition wall or the protrusion in such a precure tread can be removed in accordance with the wearing of the tread when the precure tread is bonded to the base tire and then mounted onto a vehicle and run over a given distance. In the second method for the manufacture of the tire according to the invention, the vulcanization is carried out by using the above second and third precure treads and then the rear groove is communicated to the atmosphere by the piercing of the partition wall or the removal of the protrusion after the vulcanization, whereby the peeling of tread resulted from thermal expansion of gas in the rear groove can be prevented very effectively. In case of applying the second precure tread, the given piercing position can be simply and rapidly determined owing to the presence of the opening portion facing the tread surface, while in case of applying the third precure tread, all protrusions are cut out by means of a cutter or the like, so that the rear groove can be communicated to the atmosphere very simply and rapidly at small operating steps as mentioned above. In Figs. 1 to 3 are shown the first precure tread according to the invention and a first embodiment of a pneumatic tire using the same. The construction of such a tire will be described below. In Figs. 1 and 2, numeral 11 is the whole of a retreaded tire for a truck or bus, and numeral 12 a precure tread applied for the retreaded tire 11. This precure tread 12 is composed of a vulcanized rubber member and integrally united with a base tire 13 by bonding to a crown portion 13a of the base tire 13, which is obtained by buffing a tread 14 of a used tire shown by a phantom line in Fig. 3 to solid line position, through a sheet-like cushion rubber 15 by vulcanization. In Figs. 2 and 3, numeral 14B is a belt. As shown in Figs. 1 and 2, the precure tread 12 is provided at a ground contacting portion 12b facing a side of a tread face 12a with three main grooves 16 as a surface groove extending zigzag in the circumferential direction of the tire. These main grooves 16 are designed to improve the wear resistance by reducing the groove area in view of the consideration of only the wet performance over a period from the initial worn stage to the last worn stage. Numeral 17 is a sipe, which is obtained by arranging a short sipe in a shoulder portion 12c of the precure tread 12 and both side edge portions of the main groove 16 in the widthwise direction of the tire. Numeral 18 is a rear groove. In the precure tread 12, four rear grooves 18 are formed in a rear portion 12d of the precure tread 12 facing the base tire 13 between the main grooves 16 and between the main groove 16 and the shoulder portion 12c. Each of the rear grooves 18 is directed from the side of the base tire 13 to the side of the tread face 12a and continuously extends in zigzag fashion in the circumferential direction of the tire. Moreover, the main groove 16 is substantially U-shaped in the section and has preferably a relation of D₁₂=D₁₆+(3∼4 mm) , wherein D₁₆ is a depth of the main groove 16 and D₁₂ is a thickness of the precure tread 12. On the other hand, the rear groove is also substantially U-shaped in section and has a relation of D₁₈=D₁₆x0.4+(3∼4 mm) , wherein D₁₈ is a depth of the rear groove 18. That is, the main groove 16 and the rear groove 18 are exposed to the atmosphere together when the precure tread 12 is worn to a given extent. When the retreaded tire 11 using such a precure tread 12 is mounted onto a truck or bus and actually run, as the running distance increases, the precure tread 12 of the retreaded tire 11 is worn from the side of the tread face 12a to gradually make the depth of the main groove 16 shallow. When the worn quantity of the precure tread 12 reaches about 60% of the thickness D₁₆, the rear grooves 18 appear. In other words, the rear groove is exposed before the complete disappearance of the main groove 16, so that both the main groove 16 and the rear groove 18 contribute to the improvement of the wet performance. Therefore, the retreaded tire 11 using this precure tread 12 can sufficiently develop the drainage and wet performances on wet road surface even at the last worn stage of the tread. Furthermore, the retreaded tire can be used until the rear groove disappears, so that the wear life of the tire can be prolonged. To this end, the depth D₁₈ of the rear groove 18 must extend beyond a position corresponding to the groove bottom 16a (D₁₆) of the main groove 16 as shown in Fig. 4. Preferably, the rear groove 18 has a depth D 18A corresponding to a point 16b which is 0.4 times of the depth D₁₆ of the main groove 16. Further, the groove width W₁₈ of the rear groove 18 is preferably not less than 1 mm. According to the invention, the sectional form of the rear groove 18 is not restricted to only the U-shaped form. For example, as shown in Fig. 5, there may be used a rear groove 18A having a V-shaped form in section in which the groove wall has an angle α with respect to a line segment N perpendicular to the rear face 12e of the precure tread 12, a rear groove 18B having a form in section as shown in which the groove wall is inclined at an angle β with respect to the line segment N in an opposite direction to the case of the rear groove 18A, and the like. In Figs. 1 and 2, numerals 20 and 21 are venthole and sipe formed in the precure tread 12 as a passage for communicating the rear groove to the atmosphere, respectively. The venthole 20 is arranged in each folded portion 18a of the rear groove 18 and preferably has a diameter of not less than 0.5 mm. Moreover, the number of ventholes is preferably not less than 5 on the circumference when the rear groove 18 continuously extends in the circumferential direction. When the rear groove 18 is discontinuously extended in the circumferential direction, 5 or more ventholes are arranged in each of the discontinued groove portions. The sipe 21 arranged in the straight portion 18b of the rear groove 18 preferably has an opening width of not less than 0.5 mm and a length of not less than 3.0 mm along the rear groove 18. Further, the number of sipes is preferably not less than 5 on the circumference when the rear groove 18 continuously extends in the circumferential direction. When the rear groove 18 is discontinuously extended in the circumferential direction, 5 or more sipes are arranged in each of the discontinued groove portions. Since the rear groove 18 is always communicated to the atmosphere at the side of the ground contacting portion through these ventholes and sipes, when the retreaded tire 11 using the precure tread 12 is mounted onto a truck or bus and actually run, even if the gas inside the rear groove 18 is thermally expanded by generating heat from rubber around the rear groove 18 due to the compression deformation of the tread during the running, such a gas is discharged toward the outside through the venthole 20 and/or the sipe 21, whereby the penetration of the expanded gas between the tread 12 and the cushion rubber 15 and hence the peeling of the tread 12 from the cushion rubber 15 are prevented very effectively. At the same time, cooled air is blown from the outside into the rear groove 18 through the venthole and/or sipe to cool the groove walls of the rear groove 18 and the cushion rubber 15, so that the thermal fatigue of the tread 12 and the cushion rubber 15 is sufficiently restrained. The second embodiment of the invention will be described below. Figs. 6 and 7 show the second embodiment of the precure tread according to the invention, wherein the same parts as in the first embodiment are represented by the same numerals. In this precure tread 26, two main grooves 16 are arranged in the tread face 12a to continuously extend in zigzag fashion in the circumferential direction of the tread, and two rear grooves 18 are arranged between these main grooves 16 and in the central part of the rear portion 12d to continuously extend in zigzag fashion in the circumferential direction. Further, only the sipe 21 is arranged in the precure tread 26 over a region ranging from a middle position of the straight portion 18b of the rear groove 18 toward the folded portion 18a thereof. The third to fifth embodiments of the invention will be described below. The same parts as in the first embodiment are represented by the same numerals. Figs. 8 and 9 show the third embodiment of the precure tread according to the invention. In this precure tread 31, many folded rear grooves 28 discontinuously extending in the circumferential direction are arranged in the rear portion 12d of the precure tread between the tread shoulder portion 12c and the main groove 16 adjoining thereto, and the venthole 20 is arranged in the precure tread at each folded portion of the rear grooves 28. Fig. 10 shows a fourth embodiment of the precure tread according to the invention. In this precure tread 41, a rear groove 18 continuously extending in zigzag fashion in the circumferential direction is arranged in the rear portion 12d located at the central portion of the precure tread, and the venthole 20 is arranged in the precure tread at the folded portion 18a of the rear groove 18. Figs. 11 to 13 show a fifth embodiment of the precure tread according to the invention. In this precure tread 51, a rear groove 18 continuously extending in zigzag fashion in the circumferential direction is arranged in the rear portion 12d located at the central portion of the precure tread, and a wide sipe 21 is arranged in the precure tread at the straight portion 18b of the rear groove 18. In order to confirm the effect of the afore-mentioned precure treads according to the invention, there were provided three test tires (tire I∼III) and then a durability test was made with respect to these tires. The durability test will be described below. The test tire I is the first embodiment, wherein only 4 ventholes each having a diameter of 2 mm are arranged on the circumference for communicating each of the rear grooves to the atmosphere, and the test tire II is a modified example of the test tire I , wherein 12 ventholes are arranged on the circumference. Further, the test tire III is also a modified example of the test I, wherein only 5 ventholes are arranged on the circumference. These test tires were subjected to a durability test by means of an in-door type drum testing machine under usual durability test conditions. As a result, the test tire I was broken at the running distance of 3304 km, and the test tire II was broken at the running distance of 5292 km, and the test tire III was broken at the running distance of 5180 km. As seen from the above, the test tires II and III have substantially the same durability, while in the test tire I, the effect of communicating to the atmosphere is insufficient. Further, when each of the test tires II and III is mounted onto a vehicle and actually run, the vehicle can normally be run even when the worn quantity of the tread is 75%∼92%, whereby the wear life of the tire can largely be improved. On the other hand, it has been confirmed that the durability is more improved by enhancing the fatigue resistance of the cushion rubber. As mentioned above, according to the invention, the wet performance on wet road surface can sufficiently be developed even at the last worn stage of the tread and also the wear life can largely be improved without damaging the wet performance. Moreover, the groove area of the surface grooves can be reduced over a period extending to the last worn stage, whereby the substantial ground contacting area of the tread can be increased to realize the improvement of the wear resistance of the tread. Fig. 14 is a sectional view illustrating the first method for the manufacture of tires using the above first embodiment of the precure tread. According to this method, air sandwiched between the precure tread 12 and unvulcanized cushion rubber when the precure tread 12 is attached to the outer peripheral surface of the unvulcanized cushion rubber, and reaction gas produced from the cushion rubber when the precure tread 12 is bonded to the base tire 13 by vulcanization are sufficiently discharged from the rear grooves 18, whereby the adhesion force of the precure tread 12 to the cushion rubber 15 and hence the tire durability can be improved. In the first method, the precure tread 12 is first attached to the crown portion 13a of the base tire 13 through the unvulcanized cushion rubber 15a to form an attached assembly 61. Then, a wick 62 extending in the widthwise direction of the assembly 61 and inward in the radial direction over the arranged position of the unvulcanized cushion rubber 15a is attached to at least one portion of the assembly 61 on the outer periphery thereof, while a protection liner (not shown) is attached to the remaining portion of the outer periphery of the assembly 61 other than the position of the wick 62. Next, the assembly 61, the wick 62 and the protection liner are enveloped with an envelope 63 to contact a top of a guide pipe 64 attached to the envelope 63 with the outer periphery of the wick 62. Thereafter, the assembly 61 is lined with an inner tube 65 and mounted into a rim 66 together with the envelope 63. Then, pressurized air of, for example, about 8 kg/cm² is supplied to the inside of the inner tube 65 through a valve 67. In this case, the attachment of the inner end portion of the envelope 63 in the radial direction to the rim can be carried out by sandwiching another flange member 68 between the envelop and the rim flange to airtightly seal the envelope 63 and the assembly 61, whereby the entry of air or other gas therebetween can sufficiently be prevented. After the resulting rimmed structure 69 is placed in a vulcanizer 70, the other end portion of the guide pipe 64 is led from the vulcanizer 70 to the outside thereof and then the vulcanizer 70 is closed. Thereafter, the assembly 61 is vulcanized by supplying a heated pressurized fluid to the inside of the vulcanizer 70. Such a vulcanization can be carried out at a temperature inside the vulcanizer of about 130°C, for example, by supplying steam to the inside of the vulcanizer under a pressure of about 2 kg/cm² and further feeding air until the inner pressure of the vulcanizer reaches about 6 kg/cm². When the precure tread 12 is bonded by vulcanization to the base tire 13 as mentioned above, air sandwiched between the assembly 61 and the envelope 63 moves toward the wick 62 under the influence of the pressure applied to the envelope 63 and then is discharged to the atmosphere outside the vulcanizer 70 through the guide pipe 64 contacting at its top with the wick 62, while air sandwiched between the unvulcanized cushion rubber 15a and the precure tread 12 and reaction gas produced from the cushion rubber and reach the wick 62 arranged on at least one portion of the assembly 61 in the circumferential direction and contact with the tread 12 through the rear grooves 18, ventholes 20 and/or sipes 21 and then discharge to the atmosphere through the guide pipe 64. In this way, the inner pressure of the rear groove can be always maintained at a pressure equal to atmospheric pressure. Therefore, the poor adhesion between the precure tread 12 and the cushion rubber 15 due to the fact that air and gas remain in the rear grooves under a pressure higher than the atmospheric pressure is completely eliminated and sufficiently strong adhesion can be realized. When the precure tread 12 is bonded by vulcanization as mentioned above, it is preferable that in order to prevent the entry of the cushion rubber into the rear groove during the vulcanization, the groove width of the rear groove 18 is 2∼5 mm, and the inner pressure of the vulcanizer is within a range of 2∼6 kg/cm², and the cushion rubber has a Mooney viscosity of not less than 40 when it is preheated for 1 minute and cured at 130°C for 4 minutes. Thus, there can advantageously be prevented the lowering of the drainage performance at the last worn stage due to the fact that the lower end portion of the rear groove or an inward portion of the rear groove in the radial direction of the tire is buried with the cushion rubber 15, and the occurrence of cracking in the cushion rubber due to the fact that the cushion rubber flows in a complicated manner. In the manufacture of tires according to the first method, there is danger that the main groove 16 is thermally deformed in connection with the size and form of the main groove 16. In this case, after a given time has passed since the beginning of the vulcanization, air is supplied into the main groove through the guide pipe 64 under a pressure lower than the inner pressure of the vulcanizer, for example, a pressure of about 5 kg/cm², whereby the thermal deformation of the main groove 16 can sufficiently be removed without exerting on the adhesion force between the precure tread 12 and the cushion rubber 15. The comparison test for the durability and tread adhesion force will be described below with respect to a tire manufactured according to the first method of the invention and a tire manufactured without communicating the rear groove to the atmosphere. Test tires: a) Invention tire Each of the rear grooves is opened to the tread face of the tire through 12 ventholes having a diameter of 2 mm. b) Comparative tire All ventholes are omitted from the invention tire. Test method: As to the tire durability, a drum test was carried out with respect to the test tire to measure the running distance until the occurrence of the peeling between belt layers or the peeling at the retreaded interface. As to the tread adhesion force, the peeling resistant force between tread and cushion rubber per width of 1 inch was measured by means of a tensile testing machine. Test results: The results are shown in the following Table 1 by an index on the basis that the comparative tire is 100. The larger the index value, the better the property. Comparative tire Invention tire Tire durability 100 300 Tread adhesion force 100 120 As seen from the above results, both the tire durability and tread adhesion force can largely be improved according to the first method of the invention. According to the first method of the invention, when the precure tread 12 is bonded by vulcanization to the base tire 13 through the cushion rubber, the rear groove is communicated to the atmosphere through the passage arranged in the precure tread 12, the wick 62 contacting with the precure tread 12 and the guide pipe 64 contacting with the wick 62 to discharge air and reaction gas to the outside, whereby the tire durability can largely be improved while increasing the adhesion force of the tread 12. Moreover, the rear grooves 18 are indirectly connected to the guide pipe 64 through the wick 62, so that undesirable gases exerting on the adhesion performance can be completely and concentrically discharged through the single guide pipe 64. Fig. 15 is a sectional view in widthwise direction of a sixth embodiment of the precure tread according to the invention. In this precure tread 71, the main grooves 16 and the rear grooves 18 are arranged likewise the precure tread 12, while at least one opening portion 72 opened to a tread face 71a is arranged above each of the rear grooves 18 and separated from the rear groove 18 through a partition wall 73. In this case, the opening portion 72 has a diameter a of 0.5∼4.0 mm and a depth h of 0.5∼8.0 mm, and the partition wall 73 has a thickness b of 0.5∼6.0 mm. These diameter, depth and thickness can properly be selected within the above ranges, if necessary. In the precure tread 71, the reason why the diameter a of the opening portion 72 is within a range of 0.5∼4.0 mm, preferably 2.0∼3.0 mm, is based on the fact that the flowing of gas is sufficiently smooth and the required rigidity of the island portions in the tread is ensured, while the reason why the depth h of the opening portion 72 is within a range of 0.5∼8.0 mm, preferably 1.0∼2.0 mm, is based on the fact that the durability of the mold pin is ensured and the deviation from center in the communication between the opening portion 72 and the rear groove is made small. Further, the reason why the thickness b of the partition wall 73 is within a range of 0.5∼6.0 mm, preferably 2.0∼3.0 mm, is based on the fact that the sufficient pressure resistance of the partition wall and the easiness of the subsequent piercing operation are simultaneously established. After the precure tread 71 is bonded by vulcanization to the base tire 13, the whole or a part of the partition wall 73 is removed by a drill or other piercing means with the opening portion 72 as a mark to properly communicate the rear groove 18 to the opening portion 72, whereby the desired tire is obtained. Even when heat is generated in the tread during the running of the tire, gases inside the rear groove can freely be discharged to the outside of the tire likewise the aforementioned tire 11, so that the peeling of tread due to the thermal expansion of gas inside the rear groove can be eliminated very effectively to largely improve the tire durability. Moreover, the removal of the whole or a part of the partition wall 73 can be carried out by the piercing operation before the adhesion of the precure tread 71 to the base tire 13, or by the wearing of tire surface during the running under load after the adhesion to the base tire 13. Fig. 16 is a sectional view in widthwise direction of a seventh embodiment of the precure tread according to the invention. In this precure tread 81, at least one rear opening portion 82 is arranged to communicate with the upper end of the rear groove 18, while the end of the rear opening portion 82 is terminated in a protrusion 83 disposed on the tread face 81a. In the precure tread 81, the diameter d of the rear opening portion 82 can be selected within a range of 0.5∼4.0 mm, preferably 2.0∼3.0 mm, having regard to the flowability of gas and the rigidity required in the island portions of the tread. Further, the penetration depth e of the rear opening portion 82 into the protrusion 83 is about 0.5 mm when the protrusion 83 can be cut out in the same plane as the tread face in connection with the subsequent cutting accuracy of the protrusion, and is preferably about 1.0 mm. Moreover, the protruding height f of the protrusion 83 can be selected within a range of 3.0∼10.0 mm having regard to the cutting easiness and the yield of material, while the thickness g of sidewall of the protrusion 83 can be selected within a range of 0.5∼2.0 mm having regard to the pressure resistance and the yield of material. The plane form of the protrusion 83 can properly be selected from forms shown in Figs. 17a to 17e and other forms, if necessary. On the other hand, the sectional form of the protrusion may take a base portion necked form as shown in Figs. 18a and 18b for facilitating the subsequent cutting operation. In case of using the precure tread 81 for the manufacture of tires, the precure tread 81 is first attached to the crown portion 13a of the base tire 13 through the unvulcanized cushion rubber 15a likewise the case of using the precure tread 71. The attached assembly is mounted onto a rim 66 together with the envelope 63 and vulcanized in a vulcanizer by heating from the outside of the envelope 63 under pressure. After the vulcanization, the protrusion 83 protruding from the tread face 81a of the vulcanized assembly is removed by cutting out with a hand cutter or the like. According to the third method, many protrusions 83 can efficiently be removed by means of the cutter, so that the number of steps required for communicating the rear groove 18 to the atmosphere can considerably be reduced as compared with the aforementioned case of piercing a through-hole in the partition wall 73. When the time required for cutting out 100 protrusions 83 is compared with the time required for forming the through-holes in 100 partition walls 73, the former case is 5 minutes, while the latter case is 30 minutes. Even in the resulting tire, the rear groove 18 is communicated to the atmosphere during the use, so that the tire durability can largely be improved likewise the use of the precure tread 71. The removal of the protrusions 83 in the precure tread 81 can be carried out before the adhesion to the base tire 13, or by the wearing during the running of the tire under a load. Although the invention has been described with reference to the illustrated embodiments, the partition wall 73 in the precure tread 71 may be located at the same level as the tread face or a level near thereto, and also the lateral section of the opening portion 72 and the rear opening portion 83 may take sipe-like or other proper form, if necessary. As mentioned above, according to the invention, the rear groove is communicated to the atmosphere at least in the use of the tire, so that the peeling of tread due to the thermal expansion of gas inside the rear groove can sufficiently be prevented to largely improve the tire durability. In the first precure tread and the tire using the same, the wet performance can sufficiently be developed even at the last worn stage of tread, and also the wear life can considerably be prolonged. Further, the wear resistance of tread can advantageously be improved by reducing the negative ratio of the surface groove. In the first method for the manufacture of tires according to the invention, gas inside the rear groove can freely be discharged to the atmosphere, so that the adhesion force between the precure tread and the cushion rubber can be increased and the tire durability can largely be improved. In the second and third precure treads and the second method for the manufacture of tires using the same, the peeling of tread due to the thermal expansion of gas inside the rear groove can effectively be prevented, and also the rear groove can simply and easily be communicated to the atmosphere.

Claims: A precure tread (12) for a tire to be bonded to a crown portion (13a) of a base tire (13) and having a ground contacting portion (12b) located at a tread side of the tire and a rear portion (12d) located at the side of the base tire, in which a plurality of surface grooves (16) are formed in the ground contacting portion (12b) and a plurality of rear grooves (18) are formed in the rear portion (12d), characterized in that a groove bottom of said rear groove (18) is located at a level higher than a groove bottom of said surface groove (16) in the radial direction of the tire, and each of said rear grooves (18) is communicated to atmosphere at the side of said ground contacting portion (12b) through a passage (20, 21) formed in said precured tread. A precure tread (71) for a tire to be bonded to a crown portion (13a) of a base tire (13) and having a ground contacting portion (12b) located at a tread side of the tire and a rear portion (12d) located at the side of the base tire, in which a plurality of surface grooves (16) are formed in the ground contacting portion (12b) and a plurality of rear grooves (18) are formed in the rear portion (12d), characterized in that a groove bottom of said rear groove (18) is located at a level higher than a groove bottom of said surface groove (16), and an opening portion (72) extending to the surface (71a) of the tread is formed in said precure tread above said rear groove (18) and separated from said rear groove through a partition wall (73). A precure tread (81) for a tire to be bonded to a crown portion (13a) of a base tire (13) and having a ground contacting portion (12b) located at a tread side of the tire and a rear portion (12d) located at the side of the base tire, in which a plurality of surface grooves (16) are formed in the ground contacting portion (12b) and a plurality of rear grooves (18) are formed in the rear portion (12d), characterized in that a groove bottom of said rear groove (18) is located at a level higher than a groove bottom of said surface groove (16), and an opening portion (82) of said rear groove (18) extending above said rear groove is formed in said precure tread and terminated in a protrusion (83) formed on the surface (81a) of said precure tread. A method of producing a pneumatic tire, by attaching a precure tread (12) as claimed in claim 1 to a crown portion (13a) of a base tire (13) through an unvulcanized cushion rubber (15a), mounting the resulting assembly onto a rim (66) together with an envelope (63) housing the assembly therein, and then vulcanizing the assembly by heating under pressure, characterized in that said rear grooves (18) are communicated to atmosphere through a passage (20, 21) formed in said precure tread, a wick (62) contacting with the outer peripheral surface of said precure tread, and a guide pipe (64) contacting with said wick. A method of producing a pneumatic tire, having the steps of: (a) attaching a precure tread (71, 81) as claimed in claim 2 or 3 to a crown portion (13a) of a base tire (13) through an unvulcanized cushion rubber (15a); (b) mounting the resulting assembly onto a rim (66) together with an envelope (63) housing the assembly therein; (c) vulcanizing the assembly by heating under pressure; characterized in (d) communicating said rear grooves (18) to atmosphere after the vulcanization.

Inventors: GOTO NAOYUKI, ISHII MICHIO, IWATA TOYOMI, KURODA MASAHIRO, NAKAYAMA SHOHACHIRO, UEDA HIROSHI, GOTO, NAOYUKI, ISHII, MICHIO, IWATA, TOYOMI, KURODA, MASAHIRO, NAKAYAMA, SHOHACHIRO, UEDA, HIROSHI
Applicants: BRIDGESTONE CORP, BRIDGESTONE CORPORATION
Application year: 1989
Main classification: B29D30
IPC codes: B60C11, B29D30