Mold clamp control method for injection molding machine having toggle-type mold clamping mechanism

A mold clamp control method for an injection molding machine having a toggle-type mold clamping mechanism,

• the toggle-type mold clamping mechanism bending/extending a toggle link by retreating/advancing a crosshead in a mold opening/closing direction by a driving section which has a servomotor, and thereby mold-opening/closing and mold-clamping a movable mold attached to a movable plate, with respect to a fixed mold attached to a fixed plate,
• the mold clamp control method including:
• a low-pressure mold clamping step that performs position hold control by which the crosshead is held in a set holding position in a state where the toggle link of the toggle-type mold clamping mechanism has been bent, when injection-filling is started;
• and
• a compression-press step that performs speed and position control by which the crosshead is advanced toward a set advancement position from the set holding position in a state where a first output upper limit value has been provided to a rotary torque of the servomotor of the driving section of the toggle-type mold clamping mechanism,
• advancement of the crosshead being continued in at least part of the compression-press step in a state where a generated output of the servomotor of the driving section of the toggle-type mold clamping mechanism is maintained at the first output upper limit value,
• the mold clamp control method further including:
• a low-pressure mold clamping protection step in which, in at least part of the low-pressure mold clamping step, a second output upper limit value is provided to the rotary torque of the servomotor of the driving section of the toggle-type mold clamping mechanism, and when a mold opening force caused to act on the crosshead by the movable plate has exceeded a position holding force of the crosshead due to the second output upper limit value in the servomotor of the driving section, the crosshead retreats to a position at which the mold opening force and the position holding force become substantially equal, whereby the movable plate moves in the mold opening direction, wherein the low-pressure mold clamping protection step occurs during the injection-filling;
• and
• a low-pressure mold clamping return step in which, when the mold opening force has become smaller than the position holding force, the crosshead that has retreated in the low-pressure mold clamping protection step is advanced to the set holding position, whereby the movable plate is moved in the mold closing direction.