PURPOSE: To increase the venting effect of a melted resin so as to facilitate the plasticization, by allowing Dulmage section of a screw of a vented injection molder to be all in communication with the outer circumferential surface of the flight of the screw.

CONSTITUTION: A resin material in the molten state enters Dulmage section 6, where it is kneaded by the whirling streams in grooves 6a due to the rotational movement of the screw 4 and by shearing action in the space between the grooves 6a and a heating cylinder 1, and also is heated to a temperature suitable for generation and expansion of volatile gases in the innermost part of the resin. The melted resin flows in a diverged manner along all of the grooves 6a into a thread 4c formed in the venting section. The resin of the diverged flows is expanded by the generated gases and becomes in the foamed state. The resin in the foamed state is dragged along the inner wall of the heating cylinder according to the state of the rotation of the screw 4, is taken away from the grooves 6a, and is sent to the bottom of the thread 4c. The foamed resin mass is moved along the thread 4c.