Nonreturn valve for the screw of a plasticising unit of an injection-moulding machine

A nonreturn valve (23) for a screw (20) of a plasticising unit of an injection-moulding machine, comprising two half-shells (24a, 24b), which can be arranged in a portion (21) of the screw (20) with reduced screw diameter, wherein in each of the half-shells (24a, 24b) one or more channels (25) are provided for melt to flow through from the screw (20) into the space in front of the screw, wherein the channels (25) lie substantially parallel to the longitudinal axis of the nonreturn valve (23) or respectively to the longitudinal axis of the screw (20), wherein in each of the channels (25) there are provided a ball (26) and, following the ball as seen in the direction of flow of the melt, a pin (27), wherein the pin (27) is arranged substantially transversely to the direction of flow of the melt, wherein the channels (25), as seen in the direction of flow of the melt have respectively a first channel portion (28a) with a channel diameter smaller than the ball diameter, and a second channel portion (28b) with a channel diameter greater than the ball diameter, and wherein ball (25) and pin (27) are arranged in the second channel portion (28b).