An injection molding shutoff assembly includes a chamber containing a movable body that is biased to a rear position in the chamber by a magnet in the movable body. In the rear position, the movable body provides a seal and prevents flow of molten plastic through the assembly. When the movable body is in a forward position, one or more passages permit flow of the molten plastic around the movable body and to an exit in the chamber, leading to one or more molds. An injection molding machine nozzle tip has a beveled front end which, when in an engaged position, seals with an exterior seat of the assembly block defining the chamber and protrudes into the chamber to prop the movable body to the forward position. A portion of the assembly block proximate the opening for the nozzle tip is made of ferromagnetic material, while the rest is not.