Injection molding machine

1 . An injection molding machine comprising:

a first servomotor;

a first servo amplifier that supplies the first servomotor with power; and

a control device that controls the first servo amplifier to execute a molding cycle, wherein

the first servo amplifier includes a first switching element and a second switching element,

the molding cycle includes a first period in which the first servomotor is rotated and a second period in which the first servomotor is not rotated,

the first servomotor is rotatable by being controlled under first control and stoppable by being controlled under second control or third control,

the first control is control to rotate the first servomotor by performing control to turn on the first switching element and the second switching element in different phases,

the third control is control to stop rotation of the first servomotor by performing control to turn on the first switching element and the second switching element in a same phase,

the second control is control to stop the rotation of the first servomotor by performing control to turn off the first switching element and the second switching element, and

the control device

executes the first control in the first period, and

executes the second control or the third control in the second period.

2 . The injection molding machine according to claim 1 , wherein

the first servomotor is a motor for opening and closing molds, and

the control device executes the third control while injecting an injection material to the molds and keeping pressure to hold the injected injection material in the molds.

3 . The injection molding machine according to claim 1 , wherein

the first servomotor is a motor for injecting an injection material to molds, and

the control device executes the third control while keeping pressure to hold the injected injection material in the molds.

4 . The injection molding machine according to claim 1 , further comprising:

a second servomotor different from the first servomotor; and

a second servo amplifier that supplies the second servomotor with power.

5 . The injection molding machine according to claim 4 , wherein

the first servomotor is a motor for opening and closing molds,

the second servomotor is a motor for taking a molded article off a mold, and

the control device executes the third control on the first servomotor in a period in which the second servomotor is being driven.

6 . The injection molding machine according to claim 1 , further comprising a torque sensor that detects torque generated in the first servomotor.

7 . The injection molding machine according to claim 6 , wherein the control device

executes a test cycle before executing the molding cycle, the test cycle including a third period in which the first servomotor is rotated and a fourth period in which the first servomotor is not rotated,

executes the third control in the fourth period, and

defines, on a basis of a detection value by the torque sensor in the fourth period, a period in which the second control is executed in the second period and a period in which the third control is executed in the second period.

8 . The injection molding machine according to claim 6 , wherein the control device switches the third control to the second control when the torque sensor detects no torque over a predetermined period in a period in which the third control is being executed.

9 . The injection molding machine according to claim 1 , further comprising a speed sensor that detects rotation speed of the first servomotor, wherein

the control device switches the second control to the third control when a motor rotating state is detected from a detection value by the speed sensor in a period in which the second control is being executed.

10 . The injection molding machine according to claim 9 , further comprising:

an angle sensor that detects a rotation angle of the first servomotor; and

a storage device that stores a result of detection by the angle sensor, wherein

the control device

causes the storage device to store a detection value by the angle sensor at time of starting the second control, and

rotates the first servomotor to a rotation angle indicated by the detection value by the angle sensor stored in the storage device when a motor rotating state is detected from the detection value by the speed sensor.

11 . The injection molding machine according to claim 1 , further comprising:

a position sensor that detects rotation speed and a rotation angle of the first servomotor; and

a storage device that stores a result of detection by the position sensor, wherein

the control device

switches the second control to the third control when a motor rotating state is detected from a detection value by the position sensor in a period in which the second control is being executed,

causes the storage device to store the detection value by the position sensor at time of starting the second control, and

rotates the first servomotor to a rotation angle indicated by the detection value by the position sensor stored in the storage device when a motor rotating state is detected from the detection value by the position sensor.