1.  Field of the Invention
The present invention relates to a tire mold for vulcanizing and molding a tire and, particularly, to a piece type tire mold comprising a plurality of pieces for forming a tread pattern on a side in contact with the tread forming portion of a tire and a method of producing the same. 
2.  Description of the Prior Art
To form a tire, a vulcanizing mold (to be referred to as “tire mold” hereinafter) is used in which pressure is applied to the inside of the molded green tire to pressure contact the outer surface of the above green tire to the inner wall of the heated mold in order to vulcanize the crude rubber by heat and pressure.  One of such tire molds is a piece type tire mold 50 shown in FIGS.  12(A) and 12(B) (as disclosed by Japanese Laid-open Patent Application No.  11-198145).  This tire mold 50 comprises upper and lower molds 51 and 52 in contact with a tire side portion and a sector mold 55 which consists of a holder 3 and a plurality of pieces 54 fixed to the holder 53 and arranged in the circumferential direction of the tire.  A plurality of projections 54k for forming the tread pattern of the tire are formed on the inner sides of the above pieces 54. 
Several types of the pieces 54 are prepared according to the tread pattern of a tire and generally produced by a die cast method for pouring a molten metal into a metal mold at a high temperature and a high pressure and casting it.  They are machined to a required size, combined together based on the above tread pattern and fitted in the above holder 53. 
However, in the above die cast method, equipment becomes bulky and the cast product cannot be removed from the mold when an undercut or complex blade is buried in the pieces. 
In the piece type tire mold 50 having the above constitution, in order to discharge air and gas produced during vulcanization to the outside of the mold, air vent through holes called “vent holes” are formed in the above piece 54 for air ventilation.  However, a rubber material enters the above vent holes during the vulcanization and molding of a tire, and a large number of projections called “spew” are formed on the surface of the tire.  Therefore, the work of removing these spews is required after molding.  There is proposed a spewless mold (as disclosed by Japanese Laid-open Patent Application No.  4-223108) in which back blocks obtained by dividing the tread pattern of a tire for each pitch are assembled together and air is discharged to the outside of the mold through a gap between adjacent back blocks without forming vent holes.  However, the above spewless mold has problems that it is difficult to ensure appropriate air ventilation at an appropriate position and that air ventilation cannot be carried out completely in a portion far from the above gap. 
Then, it is conceivable that a fine slit which communicates with the piece dividing face is formed in each piece to facilitate air ventilation from a gap between pieces or that a slit communicating with a vent hole on the rear side of each piece is formed to escape air from a portion other than the gap between pieces.  However, it is difficult to form a slit having a width of 0.1 mm or less in the piece mechanically and it is troublesome to form a slit because an obstacle to processing such as a blade or groove is existent on the tire inner side of the piece. 
For example, milling has limits in the strength of a cutter and milling depth because the width of a slit to be machined is very small and takes a long time.  Therefore, it is not practical. 
Discharge processing requires the manufacture of electrodes and has a limit in processing depth due to restrictions such as the removal of chips and electrode curvature. 
Laser processing making use of laser light from a CO2 laser or YAG laser has a limit in processing depth from the relationship with the focusing distance of laser light when the width of a groove is 0.1 mm or less. 
It is an object of the present invention which has been made in view of the above problems of the prior art to provide a method of producing a tire mold piece, which is capable of producing a piece accurately even when it has a complex shape and of forming a slit having a width of 0.1 mm or less in the piece easily as well as a piece type tire mold which comprises a plurality of the pieces.