To provide a screw, which improves effectively exhaust of a gas in a plasticizing device by a screw provided with a sub-flight.

The screw 2 for plasticizing a raw material M which includes a feed zone ZF, a compression zone ZC, and a metering zone ZM includes a main flight 3 formed continuously spirally in the feed zone ZF, the compression zone ZC, and the metering zone ZM, and having an outer diameter slightly smaller than the inner diameter of an inner hole of a heating cylinder 8, one or more sub-flights 4 formed between the front face 20 and the rear face 21 of the main flight 3 in an optional zone between the feed zone ZF and the metering zone ZM of the main flight 3, and having an outer diameter smaller than the outer diameter of the main flight 3, and a ventilation part 5 provided from a front face part 20a of the main flight with which the sub-flight connects to the main flight 3 in the vicinity of the front.

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