Injection molding machine which carries out controlling of closing operation of a check ring

An injection molding machine comprising an injection screw having a check ring, rotary drive means for rotating and driving the screw, axial drive means for driving the screw in an axial direction of the screw, rotation amount detection means for detecting a reverse rotation amount of the screw, and resin pressure detection means for detecting a resin pressure, the injection molding machine carrying out closing control of the check ring by rotating the screw in reverse, wherein said injection molding machine comprises:

• resin pressure gradient detection means which rotates the screw in reverse in a state in which the screw is positioned in a predetermined position in the axial direction of the screw, and which detects an gradient of the resin pressure with respect to a reverse rotation amount of the screw in said closing control of the check ring based on the reverse rotation amount of the screw detected by said rotation amount detection means and the resin pressure detected by said resin pressure detection means;
• and
• check ring closing determination means which compares an absolute value of a resin pressure gradient detected by said resin pressure gradient detection means with a predetermined value, and which determines, based on the result of the comparison, that the check ring is closed or not.