PURPOSE: To increase the sliding contact area between the rear face of a grooved wall at a screw tip and the front face of a valve slider by a structure wherein the rear part in the cylindrical part of the screw tip is made larger in its diameter so as to form a plurality of grooves in a neck part in order to fit the valve slider to the neck part of the screw tip, resulting in formation of the cross-sectional areas of flow passages at the neck part and at the cylindrical part equal to each other.

CONSTITUTION: A screw tip 11 consists of a main body 12 and a mounting part 13. The rear part in the cylindrical part 19 of the main body 12 is made larger in its diameter than the remaining part so as to form a plurality of grooves 22 axially. Countersunk grooves 24 are bored at a necxk part 20 so as to connect them with the respective grooves 22 by smooth curves. An annular valve slider 26 is fitted to the neck part 20 of the screw tip 11 so as to be shiftable back and forth by the amount necessary for forming the flow passage of resin. Because the rear part in the cylindrical part 19 of the screw tip is made larger in diameter, the contact area between the rear face of a grooved wall 23 and the front face of the valve slider 26 is increased. In addition, the countersunk grooves 24 at the neck part 20 uniformly ensures the necessary cross-sectional area of the flow passage for molten resin to pass through the neck part 20.