Back-flow preventing apparatus

摘要

A back-flow prevention apparatus includes a screw (36) which includes a metering portion and a screw head (67); a seal ring (70) which is removably disposed on a rear portion of the screw head and which has a first resin passageway (73) formed therein; a back-flow prevention ring (43) which is disposed on a front portion of the screw head in such a manner as to be rotatable with respect to the seal ring and which has a second resin passageway (46) formed therein; and a drive unit for selectively rotating the screw in a regular or reverse direction. A first positioning mechanism (72) is disposed between the screw head and the seal ring and adapted to position the seal ring at a predetermined position with respect to the screw head. A second positioning mechanism (75,76) is disposed between the screw head and the back-flow prevention ring and adapted to establish communication between the first resin passageway and the second resin passageway when the screw is rotated in the regular direction and to break the communication between the first resin passageway and the second resin passageway when the screw is rotated in the reverse direction. The second positioning mechanism also positions the back-flow prevention ring with respect to the screw head. The back-flow prevention apparatus can prevent resin movement associated with suck-back.