1. Field of the Invention
The present invention relates to a method and an apparatus for manufacturing a pneumatic tire using a rigid core in the toroidal shape.
2. Prior Art
There is known a method for manufacturing a pneumatic tire by forming a green tire through sequential application of tire constituting members such as unvulcanized rubber ribbon, wire, cord, etc. on a rigid core having an outer surface corresponding to an inner surface of a product tire, as disclosed, for example, in the JP 11-115420 A2. In this case, by transporting the green tire with the rigid core into a vulcanizing mold and closing the vulcanizing mold so as to form a vulcanization space where the green tire is sealed and stored between it and the rigid core, and then by supplying a vulcanizing medium into the vulcanizing mold and the rigid core so as to vulcanize the green tire so that a pneumatic tire is produced.
When a pneumatic tire is to be produced using such a rigid core, there are cases where the rigid core is subjected to an excessive gripping force from the vulcanizing mold and broken upon vulcanization. That is, when the green tire is transported into the vulcanizing mold together with the rigid core, the temperature of the rigid core is a normal temperature, about 20° C., for example, or if it has been pre-heated, the temperature is about 100°, while the temperature of the vulcanizing mold is held at a temperature somewhat lower than 170° C., which is the vulcanization temperature, because its heat capacity is large. After the vulcanizing mold is brought into contact with the inner end of the rigid core by closing the vulcanizing mold in this state, if the vulcanizing medium is supplied into the vulcanizing mold and the rigid core, the rigid core has its temperature rise from the temperature at transportation to a vulcanizing temperature by about 150° C. or 70° C. and is subjected to large thermal expansion. But the vulcanizing mold has its temperature hardly rise and is not substantially expanded thermally, and moreover, its movement is strongly regulated by a large clamping force, which exerts an excessive gripping force to a contact portion of rigid core with the vulcanizing mold and might break the rigid core.
In order to avoid occurrence of the above mentioned excessive gripping force, a gap somewhat narrower than the thermal expansion of the rigid core may be provided between the inner end of the rigid core and the vulcanizing mold when the vulcanizing mold is closed. However, in this case, during the period till the gap is lost due to thermal expansion of the rigid core, the green rubber might protrude out of the gap and generate a burr.
And such problem is more remarkable when the thermal expansion rate (linear expansion rate) of the rigid core is larger than that of the vulcanizing mold, for example, when the vulcanizing mold is made of steel, while the rigid core is formed by an aluminum alloy or the like.