18 fixed on the movable plate 13 via a lock bolt 19 as shown in fig3 in a closed mold condition as shown in fig3 the outer surface of the vertical ridges 5 a fig2 c is in contact and intersects the inner surface of the thin plate 3 b of the divisions 2 a and 2 b at this time a filter space is formed in cylindrical shape between the core 1 and the cavity 2 with a sprue 24 made to communicate with the space via a gate then pressurized molten resin is injected between the core 1 and the cavity 2 from the sprue 24 at this time the pressure applied to the resin externally releases the air u2014 air inside a number of the vertical groove portions 5 of the outer circumference of the core 1 and air within a number of the groove portions 4 of the divisions 2 a and 2 b u2014 in the direction of the radius from the fine gaps in the individual thin plates 3 a and 3 b causing the air to flow out of the mold via the air bleeding grooves 20 and air discharge holes 21 this allows resin to be filled completely into the groove portions 4 and the vertical groove portions 5 after the hardening of the resin as the movable plate 13 moves downward in fig3 the pair of slide cores 22 opens externally in the direction of the radius it is to be noted that the slide cores 22 are guided so as to be freely slidable on top of the movable plate 13 then the molded product is retrieved externally as an.