PURPOSE: To ensure that the center of a mold is accurately opened by causing the mold to be opened while it is completely clamped in between the platens of an injection molding machine.

CONSTITUTION: A molten material is passed through the passage 11 of a hot spool assembled in such a manner as to be incorporated in the nozzle of an injection molding machine and allowed to reach its end. A bottomed hollow plus 12 having a plurality of through holes has been screwed into that end. The molten material is allowed to join together after it passes through the holes once while forming a disc storage 13 before being injected into a cavity 6. A bar heater 14 is buried in this hot spool and a nozzle sleeve 15 is attached on the periphery thereof. The heater moves forward together with the hot spool 10 by means of a circular projection 17 at the injection position (a), whereas it moves backward up to a horizontal projection 9 by means of a spring 18 at the cooling position (b), so that heat radiation toward the hot spool 10 may be prevented.