TWI477382B - Injection molding machine - Google Patents

Description

Injection molding machine

The present invention relates to an injection molding machine.

The injection molding machine shapes the molded article by filling and solidifying the molten resin in the cavity space of the mold device. The mold device is composed of a fixed mold and a movable mold, and a cavity space is formed between the fixed mold and the movable mold when the mold is closed. The mold closing, mold clamping, and mold opening of the mold apparatus are performed by a mold clamping device. As a mold clamping device, a mold clamping device including a mold opening and closing drive unit (for example, a linear motor) that drives a mold opening and closing operation and a mold clamping drive unit (for example, an electromagnet) that drives a mold clamping operation has been proposed (for example, see Patent Document 1).

(previous technical literature) (Patent Literature)

Patent Document 1: International Publication No. 05/090052

When the mold clamping force is released before the mold is opened, the mold clamping force does not immediately drop to 0 (zero) due to the influence of the reaction delay of the mold clamping drive portion. Therefore, the control unit sometimes issues a mold opening command in a state where the mold clamping force is large. When the mold opening command is issued in a state where the mold clamping force is large, the power consumption for the mold opening operation becomes high, or the mold opening operation becomes unstable.

The present invention has been made in view of the above-described problems, and an object thereof is to provide an injection molding machine capable of reducing power consumption for a mold opening operation and stabilizing a mold opening operation.

In order to solve the above problems, an injection molding machine according to an aspect of the present invention is characterized by comprising: The mold opening and closing driving unit drives the mold opening and closing operation; a mold clamping drive unit that drives the mold clamping action; The control unit controls the operation of the mold opening and closing drive unit and the operation of the mold clamping drive unit. The control unit includes a mold clamping force monitoring unit that monitors whether the mold clamping force is below a predetermined value when the mold clamping force is released, and when the mold clamping force monitoring unit detects that the mold clamping force is below a predetermined value, The mold opening and closing drive unit performs a mold opening operation.

According to the present invention, it is possible to provide an injection molding machine capable of reducing the power consumption for the mold opening operation and stabilizing the mold opening operation.

In the following, the embodiments for carrying out the invention will be described with reference to the accompanying drawings. In addition, the moving direction of the movable platen when the mold is closed is performed. The front direction is described below, and the moving direction of the movable platen when the mold is opened is set to the rear.

Fig. 1 is a view showing a state at the time of mold closing and mold clamping in an injection molding machine according to an embodiment of the present invention. In the first drawing, the solid line indicates the state at the time of mold closing, and the double dotted line indicates the state at the time of mold clamping. Fig. 2 is a view showing a state at the time of mold opening of the injection molding machine according to the embodiment of the present invention.

In the figure, 10 is an injection molding machine, Fr is a frame of the injection molding machine 10, Gd is a guide composed of two rails laid on the frame Fr, and 11 is a fixed platen. The fixed platen 11 can be disposed on a position adjustment base Ba that is movable along a guide Gd that extends in a mold opening and closing direction (left-right direction in the drawing). In addition, the fixed platen 11 can be placed on the frame Fr.

A movable platen 12 is disposed opposite to the fixed platen 11. The movable platen 12 is fixed to the movable base Bb, and the movable base Bb is movable on the guide Gd. Thereby, the movable platen 12 can be moved in the mold opening and closing direction with respect to the fixed platen 11.

The rear platen 13 is disposed at a predetermined interval from the fixed platen 11 in parallel with the fixed platen 11. The rear platen 13 is fixed to the frame Fr via the leg portion 13a.

Four connecting rods 14 (only two of the four connecting rods 14 are shown in the drawing) are bridged between the fixed platen 11 and the rear platen 13 . The fixed platen 11 is fixed to the rear platen 13 via a connecting rod 14. The movable platen 12 is disposed in advance along the connecting rod 14. A guide hole (not shown) through which the connecting rod 14 is inserted is formed on the movable platen 12 at a position corresponding to the connecting rod 14. Alternatively, a notch portion may be formed instead of the via hole.

A screw portion (not shown) is formed at a distal end portion (right end portion in the drawing) of the connecting rod 14, and the nut n1 is screwed and fastened to the screw portion, whereby the distal end portion of the connecting rod 14 is fixed to the fixed pressure plate 11. The rear end portion of the connecting rod 14 is fixed to the rear pressure plate 13.

The fixed mold 15 is attached to the fixed platen 11, and the movable mold 16 is mounted on the movable platen 12. The fixed mold 15 and the movable mold 16 are contacted and separated with the advancement and retraction of the movable platen 12 to perform mold closing, mold clamping and mold opening. Further, as the mold clamping is performed, a cavity space (not shown) is formed between the fixed mold 15 and the movable mold 16, and the molten resin is filled in the cavity space. The mold unit 19 is constituted by the fixed mold 15 and the movable mold 16.

An adsorption plate 22 is disposed in parallel with the movable platen 12. The suction plate 22 is fixed to the slide base Sb via a mounting plate 27, and the slide base Sb is movable on the guide Gd. Thereby, the suction plate 22 can move forward and backward further than the rear platen 13. The adsorption plate 22 may be formed of a magnetic material. Further, the mounting plate 27 may not be provided. At this time, the suction plate 22 is directly fixed to the sliding base Sb.

The rod 39 is disposed to be coupled to the suction plate 22 at the rear end portion and coupled to the movable platen 12 at the front end portion. Thereby, the rod 39 advances with the advancement of the suction plate 22 at the time of mold closing, and advances the movable platen 12, and retreats with the retraction of the suction plate 22 at the time of mold opening, and the movable platen 12 retreats. To this end, a rod hole 41 for penetrating the rod 39 is formed in a central portion of the rear platen 13.

The linear motor 28 is a mold opening and closing drive unit for moving the movable platen 12 forward and backward, and is disposed, for example, between the suction plate 22 coupled to the movable platen 12 and the frame Fr. In addition, the linear motor 28 can also be disposed on the movable platen 12 and the frame. Between the Fr.

The linear motor 28 includes a stator 29 and a movable member 31. The stator 29 is formed to be parallel to the guide Gd on the frame Fr and to correspond to the range of movement of the slide base Sb. The movable member 31 is formed to face the stator 29 at a lower end of the slide base Sb and over a predetermined range.

The movable member 31 is provided with a magnetic core 34 and a coil 35. Further, the magnetic core 34 includes a plurality of magnetic pole teeth 33 that protrude toward the stator 29 and are formed at a predetermined pitch, and the coils 35 are wound around the respective magnetic pole teeth 33. Further, the magnetic pole teeth 33 are formed to be parallel to each other in a direction perpendicular to the moving direction of the movable platen 12. Further, the stator 29 includes a magnetic core (not shown) and a permanent magnet (not shown) formed by extending the magnetic core. The permanent magnet is formed by alternately magnetizing the magnetic poles of the N pole and the S pole. A position sensor 53 that detects the position of the movable member 31 is disposed.

When the linear motor 28 is driven by supplying a predetermined current to the coil 35 of the linear motor 28, the movable member 31 advances and retreats. As a result, the suction plate 22 and the movable platen 12 advance and retreat, thereby enabling mold closing and mold opening. The linear motor 28 performs feedback control based on the detection result of the position sensor 53 so that the position of the movable member 31 becomes a set value.

Further, in the present embodiment, the permanent magnet is disposed on the stator 29, and the coil 35 is disposed on the movable member 31. However, the coil may be disposed on the stator and the permanent magnet may be disposed on the movable member. At this time, since the coil does not move with the driving of the linear motor 28, the wiring for supplying electric power to the coil can be easily performed.

In addition, as a mold opening and closing drive unit, a rotary motor and a rotary motor can be used. Instead of the linear motor 28, the rotary motion of the rotary motor is converted into a linearly moving ball screw mechanism or a fluid pressure cylinder such as a hydraulic cylinder or an air cylinder.

The electromagnet unit 37 generates an adsorption force between the rear platen 13 and the adsorption plate 22. This adsorption force is transmitted to the movable platen 12 via the rod 39, and a mold clamping force is generated between the movable platen 12 and the fixed platen 11.

Further, the fixed platen 11, the movable platen 12, the rear platen 13, the suction plate 22, the linear motor 28, the electromagnet unit 37, the rod 39, and the like constitute a mold clamping device.

The electromagnet unit 37 includes an electromagnet 49 as a mold clamping drive unit formed on the rear platen 13 side, and an adsorption portion 51 formed on the adsorption plate 22 side. The adsorption portion 51 is formed in a predetermined portion of the adsorption surface (front end surface) of the adsorption plate 22, for example, a portion of the adsorption plate 22 that surrounds the rod 39 and faces the electromagnet 49. Further, a groove 45 for accommodating the coil 48 of the electromagnet 49 is formed in a predetermined portion of the suction surface (rear end surface) of the rear platen 13, for example, around the rod 39. The magnetic core 46 is formed inside the groove 45. The coil 48 is wound around the magnetic core 46. A yoke 47 is formed at a portion other than the core 46 of the rear platen 13.

Further, in the present embodiment, the electromagnet 49 is formed separately from the rear platen 13, and the adsorption portion 51 is formed separately from the adsorption plate 22, but the electromagnet may be formed as a part of the rear platen 13, and the adsorption portion may be used as the adsorption portion. A portion of the adsorption plate 22 is formed. Further, the electromagnet and the adsorption portion may be arranged in reverse. For example, the electromagnet 49 can be provided on the side of the adsorption plate 22, and the adsorption portion 51 can be provided on the side of the rear platen 13. Further, the number of the coils 48 of the electromagnet 49 may be plural.

In the electromagnet unit 37, when a current is supplied to the coil 48, the electromagnet 49 is driven to adsorb the adsorption portion 51, whereby a mold clamping force can be generated.

Fig. 3 is a view showing a control system of an injection molding machine according to an embodiment of the present invention. The control unit 60 includes, for example, a CPU, a memory, and the like, and controls the operation of the linear motor 28 and the electromagnet 49 by processing the control program recorded in the memory by the CPU.

The control unit 60 includes a mold opening and closing processing unit 61 that controls the operation of the linear motor 28 and a mold clamping processing unit 62 that controls the operation of the electromagnet 49.

The mold opening/closing processing unit 61 outputs a signal indicating the current supplied to the coil 35 of the linear motor 28 to the linear motor current supply unit 71. The linear motor current supply unit 71 is configured by, for example, an inverter including a plurality of power modules. The linear motor current supply unit 71 supplies a current corresponding to the signal supplied from the mold opening/closing processing unit 61 to the coil 35 of the linear motor 28. A DC power source 80 is connected to the linear motor current supply unit 71. The DC power source 80 is composed of a rectifier 82 that converts an alternating current of the alternating current power source 90 into a direct current diode, a capacitor 82 that smoothes a direct current output from the rectifier 82, and the like.

The mold clamping processing unit 62 outputs a signal indicating the current supplied to the coil 48 of the electromagnet 49 to the electromagnet current supply unit 72. The current supplied to the coil 48 of the electromagnet 49 is feedback-controlled based on the detected value of the mold clamping force sensor (for example, the strain sensor 55) to be described later so that the mold clamping force becomes the target value. The electromagnet current supply unit 72 is constituted by, for example, an inverter including a plurality of power modules. The electromagnet current supply unit 72 supplies the electric power corresponding to the signal supplied from the mold clamping processing unit 62 to the coil 48 of the electromagnet 49. flow. The electromagnet current supply unit 72 has a function of changing the direction and intensity (size) of the direct current flowing to the coil 48 of the electromagnet 49. A DC power supply 80 is connected to the electromagnet current supply unit 72.

The control unit 60 further includes a mold clamping force monitoring unit 64 that monitors whether or not the mold clamping force is equal to or lower than a predetermined value when the mold clamping force is released. For example, the mold clamping force monitoring unit 64 monitors whether or not the mold clamping force is equal to or lower than a predetermined value based on the detection value of the mold clamping force sensor for detecting the mold clamping force. For example, a strain sensor 55 that detects the strain (elongation amount) of the connecting rod 14 that is elongated corresponding to the mold clamping force is used as the mold clamping force sensor. Since the strain of the connecting rod 14 is smaller as the mold clamping force is lower, it is judged whether or not the mold clamping force is equal to or lower than a predetermined value depending on whether or not the strain of the connecting rod 14 is below a predetermined value. As the mold clamping force sensor, a load sensor such as a force sensor that detects a load applied to the rod 39 and a magnetic sensor that detects the magnetic field of the electromagnet 49 can be used instead of the strain sensor 55, and the mold clamping can be performed. The types of force sensors can be varied. For example, the strain sensor can be applied not only to the connecting rod 14, but also to the rod 39. This is because the strain (the amount of contraction) of the rod 39 is proportional to the mold clamping force. Further, the mold clamping force monitoring unit 64 can monitor whether or not the mold clamping force is equal to or lower than a predetermined value based on a positional difference (distance difference) from a position at which the predetermined member is closed. Since the positional difference is smaller as the mold clamping force is lowered, it is judged whether or not the mold clamping force is equal to or lower than a predetermined value depending on whether or not the positional difference is equal to or lower than a predetermined value. The predetermined member may be a member that is displaced in position from the amount of elongation of the connecting rod 14. Examples of the member include a movable platen 11 to which the fixed platen 11, the movable platen 12, the suction plate 22, and the linear motor 28 are fixed. For example, the position difference of the movable member 31 can be detected by the position sensor 53. In order to improve the reliability, the mold clamping force monitoring portion 64 can be based on the mold clamping force Both the detected value of the sensor and the position difference are used to monitor whether the clamping force is below a predetermined value.

Next, the operation of the injection molding machine 10 having the above configuration will be described. The various operations of the injection molding machine 10 are performed under the control of the control unit 60.

The control unit 60 controls the mold closing process by the mold opening and closing processing unit 61. The mold opening/closing processing unit 61 supplies a current to the coil 35 of the linear motor 28 in the state (opening state) of Fig. 2 to advance the movable platen 12. As shown in Fig. 1, the movable mold 16 abuts against the fixed mold 15. At this time, a gap δ0 is formed between the rear platen 13 and the adsorption plate 22, that is, between the electromagnet 49 and the adsorption portion 51. In addition, the force required to close the mold is very small compared to the mold clamping force.

Next, the control unit 60 controls the mold clamping process by the mold clamping processing unit 62. The mold clamping processing unit 62 supplies a direct current to the coil 48 of the electromagnet 49, and adsorbs the adsorption unit 51 to the electromagnet 49. This adsorption force is transmitted to the movable platen 12 via the rod 39, and a mold clamping force is generated between the movable platen 12 and the fixed platen 11. Since the connecting rod 14 is elastically elongated in proportion to the mold clamping force, the movable member 31 of the fixed pressure plate 11, the movable platen 12, the suction plate 22, and the linear motor 28 is slightly advanced as indicated by the double dotted line in Fig. 1 . The gap δ1 formed between the rear platen 13 and the adsorption plate 22 at the time of mold clamping is smaller than the gap δ0 at the time of mold closing.

The molten resin is filled in the cavity space of the mold device 19 in the mold clamping state. When the resin is cooled and hardened, the mold clamping processing unit 62 adjusts the current supplied to the coil 48 of the electromagnet 49, and releases the mold clamping force. Since the connecting rod 14 is elastically restored as the mold clamping force is released, the movable mold 16 and the suction plate 22 are removed. The movable member 31 of the linear motor 28 is slightly retracted.

Next, the control unit 60 controls the mold opening process by the mold opening and closing processing unit 61. The mold opening/closing processing unit 61 supplies a current to the coil 35 of the linear motor 28 to move the movable platen 12 backward. As shown in Fig. 2, the movable mold 16 is retracted to perform mold opening.

However, when the mold clamping force is released during the mold clamping process, the mold clamping force does not immediately drop to 0 (zero) due to the influence of the reaction delay of the electromagnet 49. The reaction delay of the electromagnet 49 is caused by the influence of the magnetic force remaining in the electromagnet 49 (for example, the magnetic core 46 or the like).

Therefore, the mold opening/closing processing unit 61 prohibits the mold opening operation until the mold clamping force monitoring unit 64 detects that the mold clamping force is equal to or lower than a predetermined value, and when the mold clamping force monitoring unit 64 detects that the mold clamping force is equal to or lower than a predetermined value, The linear motor 28 supplies a current to perform a mold opening operation. Thereby, since the mold opening operation is performed in a state where the mold clamping force is low, the power consumption for the mold opening operation can be reduced, and the mold opening operation can be stabilized.

Next, the mold clamping force release processing by the mold clamping processing unit 62 will be described with reference to Fig. 4 . The following processing is performed after the predetermined mold clamping force is applied to the mold device 19 and before the mold opening.

Fig. 4(a) shows changes in the supply current to the coil of the electromagnet with time, and Fig. 4(b) shows changes in the mold clamping force with time based on the electromagnet. FIG. 4, dotted line at time T ₀ the current value is set to 0, the solid line represents the case where the time point T ₀ of the reverse current direction, maintaining the single dotted line represents the current value at time t _1. In either case, the mold clamping processing unit 62 supplies a constant direct current I _{0 to} the coil 48 of the electromagnet 49 until the time t ₀ , thereby generating a predetermined mold clamping force P ₀ (only the dotted line shows the time t _{0 in} Fig. 4). Previous information).

When the predetermined mold clamping force P ₀ is released, the mold clamping processing unit 62 can cut off the supply current to the coil 48 of the electromagnet 49 at time t ₀ as indicated by a broken line in Fig. 4(a). After time t _0, since no current flows in the coil 48 of the electromagnet 49, so as of FIG. 4 (b) are shown in phantom, the clamping force the electromagnet 49 due to the reaction of the delayed slow decline. The reaction delay of the electromagnet 49 is caused by the influence of the magnetic force remaining in the magnetic core 46 of the electromagnet 49 or the like.

Further, when the predetermined mold clamping force P ₀ is released, as shown by the solid line in Fig. 4(a), the mold clamping processing unit 62 can reverse the direction of the direct current at time t ₀ and make the predetermined combination direct current I P mold clamping force of a direction opposite to direction _{0 1} 49 to the coil 48 of the electromagnet. The magnetic field in the direction of the magnetic field remaining in the electromagnet 49 is formed, and the mold clamping force is reduced as indicated by the solid line in Fig. 4(b). Therefore, the waiting time until the mold opening can be shortened.

The greater the intensity (size) of the direct current I ₁ in the opposite direction, the shorter the waiting time until the mold is opened. The shorter the waiting time is, the higher the production efficiency is. On the other hand, the faster the mold clamping force is lowered, the more rapid the fluctuation of the load applied to the mold device 19 or the like is. Therefore, the strength of the DC current I ₁ in the reverse direction is determined in advance by trial or the like in consideration of both the production efficiency and the fluctuation of the load applied to the mold device 19 and the like. Further, the intensity of the direct current I ₁ in the reverse direction may be determined based on the intensity of the direct current I ₀ when the predetermined clamping force P ₀ is generated.

However, when the reverse direction of flow of direct current I ₁ of time becomes longer, the electromagnet 49 and the suction unit 51 will be re-adsorbed, so as of FIG. 4 (b) shown by the dotted line to a single clamping force again increase. In addition, it can be inferred that the increase in the mold clamping force before returning to zero is because the residual magnetic field in the electromagnet 49 is uneven, and the time during which the magnetic field disappears varies with the position.

Therefore, in order to suppress the increase in the mold clamping force, it is possible to determine whether or not the mold clamping force is within a predetermined range (Pmin to Pmax) by the mold clamping force determining unit 66 included in the control unit 60 when the mold clamping force is released. This determination is made based on the detection value of the strain sensor 55 as the mold clamping force sensor and/or the detection value (position difference) of the position sensor 53, and can be repeated for each predetermined time.

When the mold clamping force determining unit 66 determines that the mold clamping force is within the predetermined range, the mold clamping processing unit 62 can stop the supply of current to the coil 48 of the electromagnet 49. The time for suspending the supply of current to the coil 48 of the electromagnet 49 may be in the middle of increasing the mold clamping force, but it is preferable in the middle of the mold clamping force being lowered. Since no current flows in the coil 48 after the supply of current to the coil 48 of the electromagnet 49 is suspended, the mold clamping force decreases at a speed corresponding to the reaction delay of the electromagnet 49.

Next, a modification example of the mold clamping force releasing process by the mold clamping processing unit 62 will be described with reference to FIG. The following processing is performed after the predetermined mold clamping force is applied to the mold device 19 and before the mold opening.

Fig. 5(a) shows the supply current to the coil of the electromagnet as a function of time, and Fig. 5(b) shows the change with time due to the mold clamping force of the electromagnet.

When modification of the embodiment shown in FIG. 5, the predetermined clamping force is released P _0, flowing to the solenoid coil 48 of the strength of the DC current t ₀ of 49 slowly reduced from time, at time t ₁₁ becomes zero after change The direction of the direct current causes the intensity of the direct current to increase slowly. Thus, such that the predetermined mold clamping force is generated in the P direction opposite to the direction ₀ of DC current I ₁₁ to the coil 49 of the electromagnet 48, before opening the mold, flowing to the solenoid coil 48 of the direct current of reversing the direction of 49 1 More than once. Whenever the inversion is performed, the maximum value of the direct current intensity can be set small (I ₁₁ >I ₁₂ >I ₁₃ >I ₁₄ >I ₁₅ ). The inversion time (t ₁₂ <t ₁₃ <t ₁₄ <t ₁₅ ) may be before the re-adsorption of the electromagnet 49 and the adsorption portion 51 is started, that is, immediately before the mold clamping force starts to increase.

By inverting the direction of the direct current flowing to the coil 48 of the electromagnet 49, the influence of the magnetic field unevenness remaining in the electromagnet 49 can be reduced, and the mold clamping force can be further reduced, and the production efficiency can be further improved.

Further, when the current supply time is long and the mold clamping force is increased again, the mold clamping force can be lowered again by the direction of the direct current flowing to the coil 48 of the electromagnet 49. Therefore, the degree of freedom in pattern setting is higher and the control is easier.

In the above, the embodiment and the like of the present invention are not limited to the above-described embodiments, and various modifications and changes can be made to the above-described embodiments and the like without departing from the scope of the invention.

For example, the example of FIG. 4 and indicated with a solid line at time t ₀ 49 flowing to the solenoid coil 48 in the direction of the DC current reversal, but can also be t ₀ temporarily stop the current supply to the electromagnet 49 at time After the predetermined time elapses, the DC current I ₁ in the reverse direction is supplied to the coil 48 of the electromagnet 49.

10‧‧‧ Injection molding machine

15‧‧ ‧ fixed mode

16‧‧‧moving

28‧‧‧Linear motor (mould opening and closing drive unit)

31‧‧‧moving parts for linear motors

35‧‧‧Linear motor coil

48‧‧‧ coil of electromagnet

49‧‧‧Electromagnet (Molding drive unit)

51‧‧‧Adsorption Department

55‧‧‧ strain sensor

60‧‧‧Control Department

61‧‧‧Mold opening and closing processing department

62‧‧‧Molding Processing Department

64‧‧‧Molding force monitoring department

66‧‧‧Molding force judgment department

Fig. 1 is a view showing a state at the time of mold closing and mold clamping in an injection molding machine according to an embodiment of the present invention.

Fig. 2 is a view showing a state at the time of mold opening of the injection molding machine according to the embodiment of the present invention.

Fig. 3 is a view showing a control system of an injection molding machine according to an embodiment of the present invention.

Fig. 4 is a graph showing changes with time in the supply current to the coil of the electromagnet and a change in the mold clamping force based on the electromagnet.

Fig. 5 is a graph showing changes with time in the supply current to the coil of the electromagnet and a change in the mold clamping force based on the electromagnet.

80‧‧‧DC power supply

90‧‧‧AC power supply

82‧‧‧Rectifier

84‧‧‧ capacitor

72‧‧‧Electromagnetic current supply unit

35, 48‧‧‧ coil

71‧‧‧ Current supply unit for linear motors

53‧‧‧ position sensor

60‧‧‧Control Department

61‧‧‧Mold opening and closing processing department

64‧‧‧Molding force monitoring department

62‧‧‧Molding Processing Department

66‧‧‧Molding force judgment department

55‧‧‧ strain sensor

Claims (3)

An injection molding machine comprising: a mold opening and closing drive unit that drives a mold opening and closing operation; a mold clamping drive unit that drives a mold clamping operation; and a control unit that controls an operation of the mold opening and closing drive unit and the combination In the operation of the die driving unit, the control unit includes a mold clamping force monitoring unit that monitors whether the mold clamping force is equal to or lower than a predetermined value when the mold clamping force is released, and when the mold clamping force monitoring unit detects that the mold clamping force is below a predetermined value. The mold opening operation is performed by the mold opening and closing drive unit. The injection molding machine according to the first aspect of the invention, wherein the mold clamping force monitoring unit monitors whether the mold clamping force is equal to or lower than a predetermined value based on a detection value of a mold clamping force sensor. The injection molding machine according to the first or second aspect of the invention, wherein the mold clamping force monitoring unit monitors whether the mold clamping force is equal to or lower than a predetermined value based on a positional difference from a position at which the predetermined member is closed.