The invention relates to a device for preventing the backflow of plastic melt for an injection unit of an injection molding machine with a screw cylinder (7) and a plasticizing and injection screw (6), at the front end a screw tip (3) with at least one stop ring (1) and a locking ring (2) and an inner part (4) of the screw tip (3) which largely overlaps the locking ring (2), is arranged. In order to improve the mixing quality of the melt, the invention provides that the inner part (4) is carried out of the screw tip (3) as the mixing member (4), wherein the mixing section (4) of a plurality of radially and axially into the inner part (4) introduced grooves (5), wherein at the screw tip (3) before and after the mixing section (4) is arranged a shearing member, the front of the mixing section (4) arranged shearing section by the stop ring (1) is formed, to which the outside diameter of a maximum of 1, 5 mm is formed smaller than the inner diameter of the cylinder (7), and wherein after the mixing member (4) disposed shear part is formed by a cylindrical portion of the screw tip (3) arranged to form an annular gap to the inner diameter of the cylinder (7) is.
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