602 a nozzle of the injection molding machine to a seat in an exterior wall of a chamber of a shutoff assembly the seat is in fluid communication with the chamber through a first opening extending from the exterior wall to an interior entry wall of the chamber the interior entry wall has a first sealing surface around the first opening an elongate stem of the nozzle is advanced 604 through the first opening into the chamber the stem may be fixed to the nozzle or may be retracted to and from an advanced position as desired by the user thus the stem may be advanced into the first opening after or concurrently with the step of mating the nozzle to the seat similarly the stem may be removed from the first opening before or concurrently with the step of disconnecting the nozzle from the seat molten polymer then flows 606 through the first opening through channels 402 to the second opening in an interior exit wall of the chamber and then to one or more sprue bars or molds in fluid communication with the second opening the molten polymer may flow around a small diameter stem into the chamber or alternatively through one or more channels in the stem such as channels 302 preferably the molten polymer then flows around the movable body through one or more channels 402 that provide fluid communication between the first and second openings when the movable body is in the forward position at the conclusion of injecting polymer into a mold cavity the injection molding system is then decompressed by stopping 608 the flow of the molten polymer through the first.