The object of the invention is a screw (3) for a injection moulding machine (1),in which a hot cylinder (2) with a first compression section (5),is fitted with a pressure reduction section (6) and a second compression section (7) and introduces a noble gas into the pressure reduction section (6). According to the invention, at a part of the screw corresponding to the first compression section (5) a barrier ladder (13) is a double ladder comprising a combination of a main ladder (14) and a secondary ladder (15) with a greater inclination angle than the main ladder (14), is formed and is a ring-shaped dam day (18) with a predetermined width downstream of,i.e. pre-barrier training (13). A sealing structure (20) to prevent the return of resin may be provided for between the Dammsteg (18) and the Pressure Reduction Section (6).
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