JP2013082083A - Rotation core control device of screw pull-out mold - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a rotation core control device capable of controlling a servo motor, driving the rotation core of a mold for injection-molding a molding having a screw to reduce a cycle time.SOLUTION: The molding is drawn out, a moving distance to the core origin position (rotation number Dm when returning from a return starting position of the rotation core to the core origin position in reverse rotation and rotation number Dp when returning from the return starting position of the rotation core to the core origin position in normal rotation) is obtained, whether or not Dp is larger than Dm is determined, and when Dp is larger than Dm (that is, in the case of Yes), the rotation core is reversely rotated to the core origin position to finish processing, and when Dp is not larger than Dm (that is, in the case of No), the rotation core is normally rotated to the core origin position to finish the processing (SA01-SA05).

Description

  The present invention relates to a rotary core control device for a screw-off mold used in an injection molding machine.

In a screw-off mold having a structure in which a screw-shaped molded product injection-molded in a mold is extracted from the mold by rotating the core, there is a method of using hydraulic drive or air drive as the rotational power of the core. However, a method using a servo motor has also been devised (see Patent Document 1). The servo motor type rotating core is cleaner than other driving methods and is suitable for molding in a clean environment such as molding of medical products.
Further, since the servo motor type rotating core is also excellent in position controllability, the helical driving motor can be rotated forward (or reverse) to return the helical type to the start angle (see Patent Document 2). ). By returning the spiral mold to the start angle, it is possible to manufacture a high-value-added high-quality molded product in which the screw cutting positions are aligned each time.

JP-A-4-47914 JP-A-6-198692

The prior art described in the background art section has a problem that the cycle time is extended when the core is returned to the original position depending on the rotation direction. The original position is a rotation angle (same phase position) having the same phase as the rotation angle of the core (hereinafter referred to as the screw removal start position) when the screw removal is started.
FIG. 2 is a diagram for explaining the difference in cycle time depending on the rotation direction of the rotating core. When performing the injection molding operation, when the molten resin is injected into the cavity of the mold, the position of the rotating core is positioned at the core original position 100. Thereafter, the molded product in the cavity is cooled, the mold is opened, and the rotating core is rotated in the direction indicated by screw unscrewing 200 to the completion position 120 of the screw unscrewing process. Are separated from the movable mold. (In the normal screwing process, the rotating core is rotated one or more times, but may be less than one rotation.) When performing the next molding operation, the position of the rotating core in the cavity of the mold is set to the core core. It is necessary to return to position 100. To return the position of the rotating core to the core original position 100, there are a reverse rotation direction indicated by reference numeral 202 and a forward rotation direction indicated by reference numeral 204. When returning to the core original position 100 by reverse rotation indicated by reference numeral 202, there is a problem that the cycle time is extended. Cycle time can be shortened by returning to the core original position 100 by the forward rotation shown by the code | symbol 204 rather than the reverse rotation direction shown by the code | symbol 202. FIG.

  On the other hand, when a gear is adopted as the rotating mechanism of the rotating core, backlash may occur when the rotating core is rotated in the reverse direction. FIG. 3 is a diagram for explaining increase / decrease in backlash depending on the rotation direction of the rotating core. In order to avoid backlash, it is desirable that the rotation direction is the same direction (forward rotation) as the rotation direction at the time of screw removal indicated by the screw removal 206 and when returning to the core original position 100. However, depending on the completion position 140 of the screwing process, there are cases where the reverse rotation to the core original position 100 can return to the original position in a shorter time than the forward rotation. Reference numeral 208 denotes a rotation in a direction of rotating backward to the core original position 100. In the reverse rotation direction indicated by reference numeral 208, the cycle time is shortened, but there is a problem that the backlash increases. On the other hand, reference numeral 210 indicates the rotation in the direction in which the rotating core rotates forward to the core original position 100 and returns. When the rotating core returns to the core original position 100 by the rotation of the reference numeral 210 in the forward rotation direction, backlash can be reduced, but there is a problem that the cycle time is extended.

  Accordingly, an object of the present invention is to provide a servo motor that drives a rotating core of a mold for injection-molding a molded product having a threaded portion in consideration of the above-described problems of the prior art. A core control device is provided.

The invention according to claim 1 of the present application is a rotary core control device that controls a servo motor that drives a rotary core of a screw unscrewing mold for molding a molded product having a threaded portion. After the unscrewing process of taking out the molded product having the portion while rotating the rotating core, the rotating core is returned to the core original position that is in the same phase as the core position at the start of the unscrewing process. Therefore, the comparison unit that compares the moving distance when the rotating core is rotated forward from the return start position and the moving distance when the rotary core is rotated backward from the return start position to the core original position, and the comparison An original position return unit that selects a rotation direction with a shorter moving distance as a result of comparison by the unit and rotates the servo motor to rotate the rotating core to the core original position. Screwing mold turning A core controller.
The invention according to claim 2 is characterized in that the backlash amount of the rotating core is added to the number of rotations in the case of reverse rotation from the return start position to the core original position, and the moving distance is obtained. 1 is a rotary core control device for a screw unscrewing die according to 1.
According to a third aspect of the present invention, there is provided a rotary core control device for controlling a servo motor for driving a rotary core of a screw unscrewing mold for molding a molded product having a screw portion. In order to return the rotating core to the core original position which is the same phase position as the core position at the start of the screwing process after performing the screwing process of taking out the molded product having the rotating core while rotating, A forward rotation time calculating unit that calculates a required time when the rotating core is rotated forward from the return start position to the core original position; and the rotating core is reversed from the return start position to the core original position. A result of comparison by the comparison unit, a comparison unit for comparing the required time for the forward rotation and the required time for the reverse rotation, and a comparison unit for calculating the required time for the rotation Less time required An original position return unit for selecting a rotation direction and rotating the servo motor to rotate the rotary core to the core original position, and a rotary core control device for a screw-off mold, It is.
The invention according to claim 4 is characterized in that the reverse rotation time is obtained by adding the backlash amount of the rotating core to the number of rotations when reversely rotating from the return start position to the core original position. Item 4. A rotating core control device for a screw unscrewing die according to Item 3.
According to a fifth aspect of the present invention, when the rotation direction opposite to the rotation at the time of unscrewing is selected, the servo motor is moved in the rotation direction selected beyond the core original position. 4. The screw unscrewing according to claim 1, wherein the screw is rotated in a direction opposite to the selected direction of rotation to the original position of the core of the rotating core. This is a mold rotating core control device.
The invention according to claim 6 is characterized in that the rotating core control device is a control device in the injection molding machine, and the rotation of the screw-off die according to any one of claims 1 to 5 It is a core control device.
The invention according to claim 7 is characterized in that the rotating core control device is a control device provided outside the injection molding machine, and the screw unscrewing die according to any one of claims 1 to 5 This is a rotary core control device.

According to the present invention, the cycle time can be shortened in the rotating core control device of the servo motor type screw-off mold.
According to the present invention, when the backlash amount is known in advance, in addition to the effect of shortening the cycle time, it is possible to provide a rotating core control device for a screw-off mold that can reduce backlash. Alternatively, even when the backlash amount is not known in advance, in addition to the effect of shortening the cycle time, it is possible to provide a rotating core control device for a screw-off mold that can reduce backlash.

It is a schematic block diagram explaining embodiment of this invention. It is a figure explaining the difference in cycle time by the rotation direction of a rotation core. It is a figure explaining increase / decrease in the backlash by the rotation direction of a rotation core. It is a figure explaining the rotation speed to the core original position of a rotation core. It is a figure explaining the flowchart of the process in embodiment of this invention which determines a rotation direction based on the movement distance to a core original position. It is a figure explaining the flowchart of the process in embodiment of this invention which determines a rotation direction based on the required time to a core original position. FIG. 7 is a diagram illustrating a flowchart of processing including a step of rotating the rotating core backward beyond the core original position and further rotating the rotating core forward to the core original position in the flowchart shown in FIG. 6.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
First, a schematic configuration of a known injection molding machine will be described. In addition, the structure of the injection molding machine to which the code | symbol is not provided is not shown in figure. The injection molding machine includes a mold clamping unit and an injection unit on a machine base. The injection part heats and melts the resin material (pellets) and injects the molten resin into the mold cavity. A nozzle is attached to the tip of the injection cylinder, and a screw is inserted into the injection cylinder. A hopper for supplying a resin material into the injection cylinder is attached to the top of the injection cylinder.

  The mold clamping unit mainly opens and closes the mold. The mold clamping unit includes a fixed platen to which the fixed side mold is attached, a movable platen to which the movable side mold is attached to face the fixed side mold, a mold opening / closing servo motor that moves the movable platen back and forth in the direction of the injection part, A position detector for detecting the rotational position of the servo motor for opening and closing the mold, a rear platen, a toggle link, a ball screw, and a cross head are provided. The rotational drive of the mold opening / closing servomotor is transmitted to the ball screw via a transmission mechanism constituted by a belt or the like. In addition, the mold clamping unit includes a plurality of tie bars that connect the rear platen and the fixed platen, and guide the movable platen to move forward and backward with respect to the fixed platen. The toggle link moves the movable platen back and forth in the fixed platen direction by moving the cross head attached to the ball screw rotated by the mold opening / closing servomotor in the fixed platen direction. The mold clamping unit moves the movable platen in the fixed platen direction by driving the mold opening / closing servo motor, and the mold is closed. After the movable mold and the fixed mold are in contact, the movable platen is further moved in the fixed platen direction. And a predetermined clamping force is generated. Then, mold opening is performed by retracting the movable platen in the rear platen direction.

  The injection molding machine has a structure in which at least the operations of injection, weighing, mold clamping, and mold opening are uniformly controlled by a numerical controller. FIG. 1 is a schematic block diagram illustrating an embodiment of the present invention. A fixed mold 50 is attached to the fixed platen, and a movable mold 52 is attached to the movable platen. In the embodiment of the present invention, the mold composed of the fixed side mold 50 and the movable side mold 52 is used for molding a cap-shaped product having a screw on the inner surface by screw-out molding. The movable mold 52 is provided with a through hole 66.

  A shaft 56 is inserted into the through-hole 66, and a rotating core 54 disposed in a cavity formed by the fixed mold 50 and the movable mold 52 is attached to one end of the shaft 56, and a gear to the other end. A transmission mechanism 58 is attached. The servo motor 60 rotates the rotating core 54 through the transmission mechanism 58 with the shaft 56 as a rotation axis. The servo motor 60 is connected to the servo amplifier 12 and is driven via the servo amplifier 12 based on a command from the servo CPU 13. The servo motor 60 incorporates a position detector 62 that detects its rotational position, and a position feedback signal representing the rotational position of the servo motor 60 detected by the position detector 62 is sent to the servo CPU 13.

  The numerical control device 10 for controlling the injection molding machine has a CNC CPU 20 that is a microprocessor for numerical control, a PMC CPU 17 that is a microprocessor for a programmable machine controller, and a servo CPU 13 that is a microprocessor for servo control. By selecting the mutual input / output via 16, information can be transmitted between the microprocessors.

  The servo CPU 13 is connected to a ROM 14 that stores a control program dedicated to servo control that performs processing of a position loop, a speed loop, and a current loop, and a RAM 15 that is used for temporary storage of data. The servo CPU 13 is connected to a core rotation servo motor 60 that is driven and controlled based on a command from the servo CPU 13, and an output signal from a position detector 62 attached to the servo motor is fed back to the servo CPU 13. So connected. The rotational position of the servo motor 60 is calculated by the servo CPU 13 based on a position feedback signal from a position detector 62 built in the servo motor 60 and is updated and stored in a current position register provided in the RAM 15.

  A ROM 18 storing a sequence program for controlling the sequence operation of the injection molding machine and a RAM 19 used for temporary storage of calculation data are connected to the PMC CPU 17, and an automatic operation program for overall control of the injection molding machine is connected to the CNC CPU 20. A ROM 21 storing various programs such as these and a RAM 22 used for temporary storage of calculation data are connected. In the embodiment of the present invention, a program for controlling the rotation of the rotating core 54 is stored in the ROM 21, and when the CNC CPU 20 executes the program, the servo CPU 13 receives a command from the CNC CPU 20 and rotates the rotating core 54. Control is made.

  The rotating core operating condition storage SRAM 23 is a non-volatile memory that is backed up by a power source, and stores molding data for storing the rotating core operating conditions, molding conditions regarding injection molding work, various set values, parameters, macro variables, and the like. Memory. The display device / MDI unit (manual data input device) 25 includes a display device such as a liquid crystal display device (LCD) and a manual data input device for inputting various data.

  The display device of the display device / MDI unit 25 is controlled by the screen display circuit 24. The display device / MDI unit 25 is connected to the bus 16 via an interface (not shown), and can select a function menu and input various data. A numeric keypad for inputting numeric data, various function keys, a touch panel, and the like are also provided.

Next, the number of rotations up to the core original position of the rotating core will be described with reference to FIG. Here, description will be made in correspondence with the schematic block diagram of FIG. 4A is a diagram viewed from the direction of the rotation axis of the rotating core 54, and FIG. 4B is a diagram illustrating the relationship between the number of rotations of the rotating core 54 and time.
As shown in FIG. 4A, Dp is the number of rotations when returning from the return start position of the rotating core 54 to the core original position 100 by forward rotation, and Dm is reversed from the return start position of the rotating core 54. This represents the number of rotations when returning to the core original position 100 by rotation. The number of rotations up to the core original position 100 of the rotating core 54 corresponds to the moving distance to the core original position 100 of the rotating core 54. Further, the return start position corresponds to the position where the rotating core 54 completes the unscrewing process. Note that the position data of the core original position 100 of the rotating core 54 and the completion position 160 of the unscrewing process are calculated by the servo CPU 13 based on the position feedback signal from the position detector 62 built in the servo motor 60. Are stored in the RAM 15.

  As shown in FIG. 4B, the core original position 100 of the rotating core 54 is in the same phase as the unscrewing start position. In FIG. 4 (b), the position where the rotating core makes one rotation from the screwing start position and the position where the rotating core makes two rotations from the screwing start position are in the same phase. All are in the core position 100. If the rotating core 54 returns to the core original position 100 by the next injection step, a molded product in which the screw cutting positions are aligned each time can be manufactured. As shown in FIG. 4 (b), the moving distance (moving time) from the unscrewing process completion position 160 to the core original position where the rotating core has made one rotation from the unscrewing start position, and the unscrewing process. The movement distance (movement time) for returning from the completion position 160 to the position rotated twice from the unscrewing start position is compared.

After completion of molding, the movable platen is moved backward to open the mold (fixed side mold 50, movable side mold 52), and the rotating core 54 is rotated forward by the servo motor 60 controlled by the rotating core control device. When the product 64 is extracted, the rotating core 54 is rotated in the following operation order.
First, by rotating the rotating core 54 forward from the core original position 100 by a preset number of rotations (corresponding to the movement distance), the molded product 64 is extracted from the mold (movable side mold 52).
In order to return the rotary core 54 to the core original position 100 so that the next injection molding can be executed, the rotary core 54 is rotated forward from the return start position (the screw removal step completion position 160 in FIG. 4). The number of rotations at the time of rotation and the number of rotations at the time of reverse rotation are obtained for each, and the rotation direction with the smaller number of rotations is selected to rotate to the core original position 100. This corresponds to the flowchart of FIG.

  Alternatively, when the speed of the rotating core 54 for returning the rotating core 54 to the core original position 100 varies depending on the rotation direction, the core original position is determined from the rotation speed up to the core original position 100 set in advance. The required time (Tp, Tm) is obtained for each of the forward rotation and the reverse rotation up to 100. This corresponds to the flowchart of FIG. The return start position usually means the completion position 160 of the screw removal process.

Tp = Dp / Vp (Formula 1)
Tp: Time required to return from the return start position of the rotating core to the core original position by forward rotation
Dp: Number of rotations when returning from the return start position of the rotating core to the core original position by forward rotation
Vp: Rotation speed when returning from the return start position of the rotating core to the core original position by forward rotation

Tm = Dm / Vm (Formula 2)
Tm: Time required for returning from the return start position of the rotating core to the core original position by reverse rotation Dm: Number of rotations when returning from the return start position of the rotating core to the core original position by reverse rotation Vm: Rotation speed when returning to the core original position by reverse rotation from the return start position of the rotating core

  Then, a smaller rotation direction is selected from Tp calculated using (Expression 1) and Tm calculated using (Expression 2), and the core is rotated to the original position 100. Here, the required time (Tp or Tm) may be obtained in consideration of acceleration / deceleration. As a result, the rotation direction in which the required time to the core original position 100 is shortened can be automatically selected, and the cycle time can be shortened.

  By the way, in the case where a gear transmission mechanism 58 is provided between the servo motor 60 and the rotating core 54, a backlash of the gear of the transmission mechanism 58 occurs if the gear rotates reversely from the return start position of the rotating core 54. Even if the servo motor 60 returns to the original position, the rotational position of the rotary core 54 may be deviated from the core original position 100. If injection molding is performed in such a state, the accuracy of the screw cut-out position may be deteriorated and the quality of the molded product 64 may be reduced.

Therefore, when the backlash amount (δ) is known in advance, the backlash amount may be added to the reverse rotational speed from the return start position of the rotating core 54 to the core original position 100 (Dm = Dm + δ). When the backlash amount (δ) is not known, when the rotating core 54 is rotated backward from the return start position of the rotating core 54 to the core original position 100, the rotating core 54 is passed through the core original position 100 by a minute distance in the reverse direction. It is rotated by (ΔD), and after the reverse rotation is completed, it is rotated forward to the core original position 100 by a minute distance (ΔD). ΔD is a set value that is at least larger than the backlash, depending mainly on the structure of the molds 50 and 52 (δ ≦ ΔD).
The required time Tm in this case is obtained by the following (formula 3) instead of the above (formula 2). Dm, Vm, and Vp in (Expression 3) are the same as the symbols in (Expression 1) and (Expression 2).

  Tm = (Dm + ΔD) / Vm + ΔD / Vp (Formula 3)

  Then, the smaller rotation direction is selected from Tp calculated using (Expression 1) and Tm calculated using (Expression 3), and the core is rotated to the original position 100. As a result, the rotating core 54 can be stopped at the core original position 100 without backlash. These reverse rotation and forward rotation operations may be performed separately in advance by an operation that rotates by Dm + ΔD and an operation that rotates by ΔD in advance, or only Dm is operated by reducing ΔD = 0 and the position loop gain. May be. By lowering the position loop gain, the rotating core 54 passes through Dm by the inertia due to the rotation at the time of reverse rotation, and then rotates forward and stops toward Dm. Therefore, when reverse rotation and forward rotation are operated individually As with, backlash can be eliminated.

  Besides rotating the rotating core 54 forward from the core original position 100 by a preset number of rotations, the rotating core 54 may be rotated forward from the core original position 100 for a preset time. Note that the core original position 100 does not necessarily coincide with the origin of the servomotor 60 due to the reduction ratio of the transmission mechanism 58.

  The rotary core control device that controls the rotation of the rotary core 54 may be the numerical control device 10 that controls the injection molding machine, or an external rotary core control device such as a sequencer that does not depend on the numerical control device 10. Good. When a rotary core control device outside the injection molding machine is used, the rotary core control device receives a permission signal for driving the core from the numerical control device 10, and controls the drive of the rotary core 54. I do. The rotating core control device sends a core drive completion signal to the numerical control device 10, and upon receiving the completion signal, the numerical control device 10 starts an injection molding operation.

  As the servo motor 60 in the embodiment of the present invention, a servo motor with an encoder may be used as described with reference to FIG. 1, or the encoder is not a servo motor 60 but a mold (movable side mold). 52), and the rotation of the gear of the transmission mechanism 58 may be detected.

FIG. 5 is a diagram illustrating a flowchart of processing in the embodiment of the present invention in which the rotation direction is determined based on the distance to the core original position. Hereinafter, it demonstrates according to each step.
[Step SA01] The molded product is extracted. More specifically, the molded product 64 is extracted from the mold (movable side mold 52) by rotating the rotating core 54 forward from the core original position 100 by a preset number of rotations.
[Step SA02] The movement distance (Dm and Dp) to the core original position is obtained. Dm is the rotation speed when returning from the return start position of the rotating core to the core original position by reverse rotation, and Dp is the rotation speed when returning from the return start position of the rotating core to the core original position by forward rotation. is there. The number of rotations corresponds to the moving distance.
[Step SA03] It is determined whether or not Dp is greater than Dm. If Dp is greater than Dm (that is, Yes), the process proceeds to Step SA04. If Dp is not greater than Dm (that is, No), the process proceeds to Step SA05. Transition.
[Step SA04] The rotating core is reversely rotated to the core original position, and the process is terminated. More specifically, the rotation is reversed by the moving distance Dm, and the process is terminated.
[Step SA05] The rotating core is forwardly rotated to the core original position, and the process is terminated. More specifically, the forward rotation is performed by the moving distance Dp, and the process is terminated.

FIG. 6 is a diagram illustrating a flowchart of processing in the embodiment of the present invention in which the rotation direction is determined based on the required time to the core original position. Hereinafter, it demonstrates according to a step.
[Step SB01] The molded product is extracted. More specifically, the molded product 64 is extracted from the mold (movable side mold 52) by rotating the rotating core 54 forward from the core original position 100 by a preset number of rotations.
[Step SB02] The required time (Tm and Tp) to the core original position is obtained. Tm is the time required for returning from the return start position of the rotating core to the core original position by reverse rotation, and Tp is the time required for returning from the return start position of the rotating core to the core original position by forward rotation. is there. More specifically, Tp is calculated using (Expression 1), and Tm is calculated using (Expression 2).
[Step SB03] It is determined whether Tp is greater than Tm. If Tp is greater than Tm (that is, Yes), the process proceeds to Step SB04. If Tp is not greater than Tm (that is, No), the process proceeds to Step SB05. Transition.
[Step SB04] The rotating core is reversely rotated to the core original position, and the process is terminated. More specifically, reverse rotation with a moving distance of Dm is performed.
[Step SB05] The rotating core is rotated forward to the original position of the core, and the process is terminated. More specifically, the forward rotation with the moving distance of Dp is performed, and the process is terminated.

FIG. 7 is a flowchart of a process including the steps of rotating the rotating core in the direction opposite to screw unscrewing beyond the core original position in the flowchart shown in FIG. 6, and further rotating the rotating core in the screw unscrewing direction to the original position. FIG.
[Step SC01] The molded product is extracted. More specifically, the molded product 64 is extracted from the mold (movable side mold 52) by rotating the rotating core 54 in the screwing direction from the core original position 100 by a preset number of rotations.
[Step SC02] The required time (Tm and Tp) to the core original position is obtained. Tm is the time required to return from the return start position of the rotating core in the opposite direction to screw removal and return to the original position of the core, and Tp is rotated from the return start position of the rotary core to the screw removing direction. This is the time required to return to the child position. Tp is calculated using (Equation 1), and Tm is calculated using (Equation 3).
[Step SC03] It is determined whether or not Tp is larger than Tm. If Tp is larger than Tm (that is, Yes), the process proceeds to Step SC04. If Tp is not larger than Tm (that is, No), the process proceeds to Step SC06. Transition.
[Step SC04] The rotating core is rotated in the direction opposite to the screw removal beyond the core original position. More specifically, the movement distance is Dm + ΔD and the rotation opposite to the screw removal is performed.
[Step SC05] The rotating core is rotated in the screwing direction to the original core position, and the process is terminated. More specifically, the rotation is performed in the screwing direction by a movement distance of ΔD.
[Step SC06] The rotating core is rotated in the screwing direction to the core original position, and the process is terminated. More specifically, the moving distance is rotated by Dp in the screwing direction, and the process is terminated.
In the above description, the rotation direction at the time of screwing is described as the forward direction, but the screwing direction may be defined as the reverse direction.

10 Numerical control device 12 Servo amplifier 13 Servo CPU
14 ROM
15 RAM
16 bus 17 PMCCPU
18 ROM
19 RAM
20 CNCCPU
21 ROM
22 RAM
23 SRAM for saving rotating core operating conditions
24 Screen display circuit 25 Display device / MDI unit

DESCRIPTION OF SYMBOLS 50 Fixed side metal mold | die 52 Movable side metal mold | die 54 Rotating core 56 Shaft 58 Transmission mechanism 60 Servo motor 62 Position detector 64 Molded product 66 Through-hole 68 Flow direction of resin at the time of injection

100 Core position 120,140,160 Completion position of unscrewing process

Claims (7)

In a rotary core control device for controlling a servo motor that drives a rotary core of a screw-off mold for molding a molded product having a threaded portion,
After driving the servo motor and performing a screwing step of taking out a molded product having a screw part while rotating the rotating core,
In order to return the rotating core to the core original position that is in phase with the core position at the start of the screwing process, the moving distance when the rotating core is forwardly rotated from the return start position, and the return A comparison unit that compares the distance traveled when reversely rotated from the start position to the core original position;
An original position return unit that selects a rotation direction with a shorter moving distance as a result of comparison by the comparison unit and rotates the servo motor to rotate the rotating core to the core original position;
A rotary core control device for a screw-off mold, comprising:   2. The screw removal die according to claim 1, wherein the backlash amount of the rotating core is added to a rotation speed when the backlash amount is reversely rotated from the return start position to the core original position to obtain a moving distance. Rotating core control device. In a rotary core control device for controlling a servo motor that drives a rotary core of a screw-off mold for molding a molded product having a threaded portion,
After driving the servo motor and performing a screwing step of taking out a molded product having a screw part while rotating the rotating core,
When the rotating core is rotated forward from the return starting position to the core original position in order to return the rotating core to the core original position that is in the same phase position as the core position at the start of the screwing process. A forward rotation time calculation unit for calculating the required time;
A reverse rotation time calculation unit for calculating a required time when the rotating core is reversely rotated from the return start position to the core original position;
A comparison unit that compares the time required for the forward rotation and the time required for the reverse rotation;
An original position return unit that selects a rotation direction with a smaller required time as a result of comparison by the comparison unit and rotates the servo motor to rotate the rotating core to the core original position;
A rotary core control device for a screw-off mold, comprising:   4. The screw remover according to claim 3, wherein a reverse rotation time is obtained by adding a backlash amount of the rotating core to a rotational speed when the backlash amount is reversely rotated from the return start position to the core original position. 5. Mold rotating core control device.   When the rotation direction opposite to the rotation at the time of unscrewing is selected, the original position return unit is rotating after rotating the servo motor in the selected rotation direction beyond the core original position. 4. The rotating core control device for a screw-off mold according to claim 1, wherein the rotating core is rotated in a direction opposite to the selected direction of rotation to a core position of the core. 5. .   The rotary core control device for a screw-off mold according to any one of claims 1 to 5, wherein the rotary core control device is a control device inside an injection molding machine.   6. The rotary core control device for a screw-off mold according to claim 1, wherein the rotary core control device is a control device provided outside the injection molding machine.

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