Injection device, molding machine, and method for controlling injection device

An injection device comprising:

• a stationary-side frame supporting an injection unit;
• a movable-side frame being movable relative to the stationary-side frame and supporting an extrusion member inserted into the injection unit;
• first and second ball screws each comprising a ball screw shaft and a ball nut for longitudinally moving the movable-side frame relative to the stationary-side frame;
• a first injection drive mechanism comprising a first servomotor configured to relatively rotate the ball screw shaft and the ball nut of the first ball screw;
• a second injection drive mechanism comprising a second servomotor configured to relatively rotate the ball screw shaft and the ball nut of the second ball screw;
• a first force transmission unit configured to transmit a thrust produced by the relative rotation of the ball screw shaft and the ball nut of the first ball screw to the movable-side frame;
• a force detector disposed on the first force transmission unit and configured to produce an output corresponding to a force applied to the first force transmission unit;
• a second force transmission unit configured to transmit a thrust produced by the relative rotation of the ball screw shaft and the ball nut of the second ball screw to the movable-side frame;
• a dummy member disposed on the second force transmission unit and having such a property as to be deformable depending on a force applied to the second force transmission unit;
• and
• a control unit configured to control the first servomotor based on the output of the force detector and control the second servomotor by determining a command to the second servomotor so that the forces applied to the first and second force transmission units are equalized, based on the output of the force detector and spring constants of the force detector and the dummy member.