JP2002192585A - Mold clamping unit for injection molding machine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a mold clamping unit for an injection molding machine capable of being reduced in size. SOLUTION: The mold clamping unit comprises a fixed platen 1 for mounting a fixed mold 5, a movable platen 2 for mounting a movable mold 6 and an end platen 3 having a mini-toggle mechanism 8 provided at the platen 2 to pressurize to clamp the mold provided at the platen 2. One ends of four tie rods 4 are coupled to the platen 1 with nuts 11. Tie rod ball screws 4a formed at the other end of the rods 4 are engaged with the platen 3. The platen 1 is coupled to the platen 3 by a plurality of mold opening/closing ball screws 13. The platen 3 is mounted on a slide 7. The platen 2 is slidably mounted on the slide 7. In mold opening/closing steps, the screws 4a are driven in a state in which a frictionally locking mechanism is released, and the platens 2 and 3 are integrally advanced and retracted. In a mold clamping pressurizing step, the locking mechanism is operated to drive the mechanism 8 in a state in which the platen 3 is fixed to the screw 4a to generate a prescribed molding clamping force.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a plastic injection molding machine, for example, and more particularly to a mold clamping device suitable for driving an electric motor.

[0002]

2. Description of the Related Art As shown in FIG. 13, a conventional electric mold clamping device of an injection molding machine employs a toggle system. In this apparatus, the fixed plate 201 is connected to four tie rods 204 by nuts 211, and the end plate 203 is connected to the same tie rods 204 by nuts 212. Further, a toggle mechanism 208 is provided between the movable board 202 and the end board 203.

The toggle mechanism 208 includes a ball screw 210
, Via the crosshead 209. That is, by rotating the ball screw 210 in the forward direction, the crosshead 209 also serving as a nut of the ball screw 210 is advanced, and the movable die 6 is advanced rightward in FIG. Then, the movable mold 6 and the fixed mold 5 are closed, and a predetermined mold clamping force is generated by the force increasing action of the toggle mechanism 208. Further, by rotating the ball screw 210 in the opposite direction to the above, the movable mold 6 is retracted to the left along with the movable plate 202, and a predetermined mold opening operation is performed.

[0004]

In this conventional example, since a predetermined mold closing stroke is moved by the toggle mechanism 208, the space for the toggle mechanism 208 is increased, the mold clamping device becomes extremely long, and the installation space for the machine is increased. There is a problem that becomes large. Also, since the toggle mechanism turns the link, the vertical turning space in the figure also needs to be large, so that the mold closing stroke cannot be long to fit in the normal vertical space. There was also. In addition, if the mold closing stroke is large, it is possible to mold a deeper product. As described above, the conventional toggle-type mold clamping device has a problem that a machine installation space is large because the machine length is extremely long, and a mold is also required because a space above and below the toggle mechanism needs to be increased. There is a drawback that it is difficult to take a long closing stroke and a deep object cannot be formed. The present invention has been made in view of such circumstances, and has as its object to provide a mold clamping device for an injection molding machine that can be downsized.

[0005]

SUMMARY OF THE INVENTION A mold clamping apparatus for an injection molding machine according to the present invention has been made in order to solve such a problem, and corresponds to a fixed plate having a fixed die attached thereto and the fixed metal fitting. A movable plate having a movable die attached thereto, and an end plate provided with a mold clamping / boosting means disposed between the movable plate and the movable plate, and one end of a tie rod is connected to the fixed plate with a nut to form a ball screw. The end is connected to the end plate, the end plate is mounted on a slide, and the movable plate is slidably mounted on the slide. In the mold closing / mold opening process, the ball screw is connected to a ball screw nut. The movable platen and the end plate are moved forward and backward integrally by driving the mold. In the mold clamping step, the ball screw nut is pressed against the end plate to lock the ball screw nut, and then the mold clamping step is performed. hand Wherein the generating a predetermined mold clamping force by.

In the mold closing / mold opening process, the ball screw nut and the end plate are separated from each other by a biasing force of a spring.

[0007] A means for adjusting the urging force of the spring is further provided.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, an embodiment of a mold clamping device for an injection molding machine according to the present invention will be described with reference to the drawings. FIGS. 1 to 9 show a mold clamping device of an injection molding machine according to a first embodiment of the present invention, FIG. 10 shows a mold clamping device of an injection molding machine according to a second embodiment, and FIGS. FIGS. 11 and 12 show a mold clamping device of such an injection molding machine.

A mold clamping device for an injection molding machine according to a first embodiment will be described with reference to FIGS. FIG. 1 is an overall front view of the mold clamping device according to the first embodiment.
FIG. 2 is a side view of the end board as viewed in the direction of arrow II in FIG. Also,
FIG. 3 shows a detailed view of the ball screw nut portion of the portion III in FIG. 1, FIG. 4 shows a sectional view of the mold opening / closing drive device in FIG. 1, and FIGS. 5 and 6 show the configuration of the mini toggle mechanism 8 in FIG. FIG. 7 shows a driving device configuration diagram of the mold clamping ball screw 10 and the mold opening / closing ball screw 13. FIG. 8 is a front view of the mold clamping device when the mold is closed, and FIG. 9 is a front view of the mold clamping device when the pressure of the mold is increased.

In FIG. 1, a fixed platen 1 is connected to one end of a tie rod 4 by nuts 11 at four corners, and the tie rod is engaged with an end plate 3 by a tie rod ball screw 4a formed at the other end. . This structure is the same for all four tie rods 4, as shown in FIG. On the other hand, a mini toggle mechanism (clamping step-up means) 8 is provided between the movable board 2 and the end board 3, and the crosshead 9 of the drive mechanism of the toggle mechanism is engaged with a ball screw 10 for mold clamping. The end board 3 is fixed to a slide 7, and the movable board 2 is slidably mounted on the slide. The slide 7 slides on the base 19.

Next, in FIG. 3, on the opposite side of the mini-toggle mechanism 8 across the end board 3, the ball screw portion 4a is screwed to the ball screw nut 12, and the ball screw nut is 13.
The bearing box is mounted integrally with the end board 3 by bolts 14. Further, a part of the ball screw nut 12 is inserted into the concave portion of the end plate 3, and is attached to the inner peripheral surface of the end plate 3 by a bearing 42 at one end in the axial direction. The outer of the bearing 42 is in contact with the spring receiver 16 urged by the spring 15. This spring receiver 16
It is separated from the ball screw nut 12. The spring 15 is a compression spring type, and the biasing force can be adjusted by an adjustment bolt 17. The ball screw nut 12 has a stepped shape, and has a nut seat surface 12a on an end surface at an intermediate position in the axial direction. The nut seat surface 12a is provided with the spring support 16 and the bearing 4
By the urging force of the spring 15 via the second member 2, the surface of the end board 3 is separated by a clearance δ (for example, 0.2 to 0.5 mm). A driven pulley 37 is attached to the ball screw nut 12 at the other end in the axial direction by a spline or a key 18. When the toothed pulley 37 rotates, the ball screw nut 12 also rotates. The movement of the toothed pulley 37 in the axial direction is restricted by the nut 20.

Here, in the tie rod / ball screw type mold clamping apparatus as in the present embodiment, when the mold is opened and closed by the same ball screw 4a and the mold is clamped by the mini-toggle mechanism 8, the ball screw nut 12 is fixed to the nut seat. Friction locking can be performed on the surface 12a. Hereinafter, the case of the 1300 tonf machine and the case of the 850 tonf machine will be described with specific numerical values.

In the case of a 1300 tonf machine, the clamping load F per tie rod 4 is 325,000 kg.
f, when the ball screw diameter d is 240 mm and the pitch p of the tie rod ball screw 4a is 70 mm, the ball screw efficiency is η, the reverse rotation torque at the time of mold clamping is Tb, and the average surface diameter per nut seat is D. η = (Tb / (D / 2)) · πD ∴Tb = F · p · η / 2π Here, if η = 0.95, Tb = 3440 kgf
・ It becomes m. 1) On the other hand, assuming that the friction coefficient of the nut seat contact surface is μ and the friction torque of the nut seat surface 12a is TN, TN = F · μ · D / 2 To prevent the ball screw nut 12 from rotating, TN ≧ Tb
Therefore, F · μ · D / 2 ≧ F · p · η / 2π ≧ D ≧ p · η / (π · μ) 2) Here, the friction coefficient of the nut contact surface should be at least 0.1.
05 is secured, D ≧ 423.4 mm 3) When the outer diameter of the nut seat surface 12a is D2 and the inner diameter is D1, D ^ 2−D1 ^ 2 = 0.5 (D2 ^ 2−D1 ^ 2) ∴D2 = (2 · D ^ 2-D1 ^ 2) ^ 1/2 4) where D1 = 420 mm (bearing outer diameter), D = 440
mm, D2 = 459.1 mm ≒ 460 mm, and the outer diameter D2 of the nut seating surface 12a is equal to the screw diameter 240.
1.92 times mm. 5) Regardless of D2, the limit friction coefficient μ that can be reversed
Is μ = p · η / (π · D1) = 0.0504.

In the case of an 850 tonf machine, the clamping load F per one tie rod 4 is 212500 kgf,
Ball screw diameter d is 200mm, Tie rod ball screw 4
When the pitch p of a is 60 mm and the same calculation is performed, the reverse torque Tb at the time of mold clamping is 1928 kgf · m. 1) In order for the ball screw nut 12 not to reverse, D ≧ 3
62.9 mm. 2) Here, D1 = 360 mm (bearing outer diameter), D = 380
mm, D2 = 399.0 mm ≒ 400 mm, and the outer diameter D2 of the nut seat surface 12a is equal to the ball screw diameter d =
It becomes 2.0 times 200 mm. 3) Regardless of D2, the limit friction coefficient μ that can be reversed
Is μ = p · η / (π · D1) = 0.0504.

In FIG. 4, a tie rod ball screw 4a is driven to rotate by a mold opening / closing motor 40 via a driving pulley 39, a toothed belt 38 and a toothed pulley 37. When the tie rod ball screw 4a rotates, a force is transmitted to the ball screw nut 12 by the ball screw action, the force is transmitted from the nut to the end plate 3 via the bearings 41 and 42, and the end plate 3 is moved in the left-right direction. . At this time, when moving to the right, the separated state between the nut seat surface 12a and the end board 3 is maintained by the urging force of the spring. Further, in the front view of the mold clamping device when the mold is closed in FIG.
This shows a state in which the movable mold 6 and the fixed mold 5 are closed and joined together, and the mini-toggle mechanism 8 is in a position before the mold clamping and pressurization. In the operation after the drawing, the friction lock mechanism operates, and the crosshead 9 is further advanced by a fixed amount a to reach the predetermined mold clamping force generating position XA. The mold opening / closing motor 40 uses a servomotor.

FIG. 9 shows a state in which the crosshead 9 has reached the predetermined mold clamping force generating position XA. In the figure, the main link of the mini-toggle mechanism is in the extended position, and the largest mold clamping force can be generated.

Next, in FIG. 5 showing a configuration diagram of the mini-toggle mechanism 8 in FIG. 1 and FIG. 6 showing an exploded perspective view thereof, the mini-toggle mechanism 8 includes a link A31, a link B32, and a link C.
The link is composed of 33 types of links, and constitutes a vertically symmetrical double link (see FIG. 1). Link A31, Link B
The link 32 and the link C33 are connected via a hole P2 so as to be mutually rotatable. The link A31 has a hole P
0 to the end board 3 so as to be freely rotatable.
The link B32 is rotatably attached to the movable platen 2 via the hole P3. The link C33 is rotatably attached to the crosshead 9 via the hole P1, and the crosshead 9 also serves as a nut of the ball screw 10 for mold clamping. Therefore, when the ball screw 10 for mold clamping is driven to rotate, the crosshead 9 moves forward or backward. When the cross head 9 moves forward, the movable platen 2 also moves forward by the action of the mini-toggle mechanism 8, and the mold can be closed. When the mold is closed, the crosshead 9 is further advanced, and a predetermined mold clamping force can be applied by the force multiplying action of the toggle. When the ball screw 10 for mold clamping is thereafter driven to rotate in the reverse direction, the crosshead 9 retreats, and the mold can be opened by applying a mold release force in the direction opposite to the mold clamping force.

Next, referring to FIG. 7, which shows a driving device configuration diagram of the mold clamping ball screw 10 shown in FIG.
Is screw-coupled to a crosshead 9 also serving as a ball screw nut, attached to the end plate 3 by bearings 51 and 52, and has a driven pulley 47 at the other end. Then, from the mold clamping motor 50 to the drive pulley 49, the toothed belt 4
The ball screw 10 for mold clamping is rotationally driven via the pulley 8 and the driven pulley 47. When the ball screw 10 for mold clamping rotates, a force is transmitted to the crosshead 9 by the ball screw action, and the crosshead 9 is moved in the left-right direction. When the crosshead 9 moves left and right, the mini-toggle mechanism 8
The movable platen 2 is also moved in the left-right direction, and a force multiplying action by the toggle link occurs. The force multiplying action here means that the force transmitted to the movable platen 2 is greatly amplified with respect to the force for moving the crosshead 9 right and left.

In the embodiment configured as described above, at the start position of the mold closing operation shown in FIG. 1, the nut seating surface 12a of the ball screw nut 12 and the surface of the end plate 3 are separated by the clearance δ. (See FIG. 3), the friction lock mechanism is released, and the mold opening / closing motor 40 is moved from the position shown in FIG.
When the tie rod ball screw 4a rotates in the forward direction by driving (see FIG. 4), the end plate 3, the mini toggle mechanism 8,
The movable platen 2 and the movable mold 6 advance together with the slide 7 in the mold closing direction (rightward in FIG. 1). In the middle of the mold closing operation, the cross head 9 is moved to and stopped at the mold clamping pressure increasing start position (the position in FIG. 8) by the forward rotation of the tie rod ball screw 4a in the mini toggle mechanism 8. FIG. 8 shows a state in which the mold is closed in this manner. When the mold opening / closing motor 40 is further driven in this state, the ball screw nut 12 moves forward against the urging force of the spring 15 (see FIG. 3). Then, the gap δ narrows, and eventually, the nut seating surface 12a of the ball screw nut 12
Contacts the surface of the end board 3. Mold opening / closing motor 40
Is a torque control using a servomotor. When the gap δ becomes 0 (zero) and reaches a predetermined torque, the drive is stopped. In this state, the end board 3 is fixed to the tie rod ball screw 4a. Thereafter, the ball screw 10 for mold clamping is further rotated in the forward direction to move the crosshead 9 to a predetermined mold clamping position and hold it. In the state of FIG. 9 moved to the predetermined mold clamping position, a molten resin is injected into a mold from an injection device (not shown) to perform molding.
When the resin has cooled and solidified in the mold, the mold is opened and the molded product is taken out. In the mold opening operation, first, in the state of FIG. 9, the mold clamping ball screw is rotated in the retreating direction, the crosshead 9 is retracted, the mold clamping force is lowered, and the mold is opened while applying the mold releasing force. . Then, the crosshead 9 is stopped at a position where the release force is not required, the mold opening / closing motor 40 is driven, the ball screw nut 12 is rotated to release the friction lock, and the tie rod ball screw 4a is further removed. By rotating in the backward direction, the end plate 3, the ball screw nut 12,
The mini-toggle mechanism 8, the movable platen 2 and the movable mold 6 are moved together with the slide 7 in the mold opening direction and stopped at a predetermined mold opening completion position.

In the present embodiment, even when the thickness of the mold (the total thickness of the movable mold 6 and the fixed mold 5) is changed, the setting is changed because the friction lock mechanism is employed. Without having to deal with it.

As described above, according to this embodiment, when moving the movable platen 2 forward and backward, the tie rod ball screw 4
The end board 10 and the movable board 2 are integrally moved by the rotation drive of a. On the other hand, when performing the mold clamping pressurization, the mini-toggle mechanism 8 provided between the end plate 10 and the movable plate 2 is moved by the rotation drive of the mold-clamping ball screw 10 to generate a force multiplication effect of the toggle. Therefore, since the end plate 10 and the movable platen 2 move integrally when the mold is opened and closed, the total length of the device is smaller than that of a conventional toggle system in which the end plate does not move at the end of the mold clamping device when the mold is opened and closed. Dramatically shorter. In addition, since the mini-toggle mechanism 8 of the present embodiment moves only a small stroke, the turning space of the link is small, and it is not necessary to take up a large space in the vertical direction. Further, since the mold opening / closing operation is performed by the mold opening / closing ball screw, the mold closing stroke is not restricted by the toggle mechanism.

Next, a mold clamping device for an injection molding machine according to a second embodiment of the present invention will be described with reference to FIG. In the present embodiment, as shown in FIG. 10, a crank mechanism 60 (clamping step-up means) is provided between the movable platen 2 and the end plate 3. Since the mold opening / closing mechanism and the friction lock mechanism in the present embodiment are the same as those in the first embodiment, the description thereof will be omitted. Hereinafter, mold clamping and release will be described.

In FIG. 1, one end of a crank 61 is attached to the end board 3 via a crank attachment portion 62,
One end of the rod 63 is attached to the movable platen 2 via a rod attachment portion 64. The other ends of the crank 61 and the rod 63 are pin-connected. The crank 61 is rotated by a mold clamping motor (not shown), whereby the distance between the movable platen 2 and the end plate 3 can be changed.
Mold clamping and mold release can be performed.

Next, a mold clamping device for an injection molding machine according to a third embodiment of the present invention will be described with reference to FIGS. In the present embodiment, as shown in FIG. 11, a sealing hydraulic mechanism 70 (clamping pressure increasing means) is provided between the movable platen 2 and the end platen 3. Since the mold opening / closing mechanism and the friction lock mechanism in the present embodiment are the same as those in the first embodiment, the description thereof will be omitted. Hereinafter, mold clamping and release will be described.

In FIG. 11, a mold clamping cylinder 71 is provided on the end board 3, and this mold clamping cylinder 71 is provided.
The inner space is defined by a main ram 72 and a main seal 73 as two pressure chambers 74A and 74B. Each pressure chamber 74A, 74B has a pressurizing piston 75A,
75B is provided so as to be movable in the axial direction, and the liquid is sealed by piston seals 76A and 76B. Further, a pressure detection sensor 77 is provided in each of the pressure chambers 74A and 74B.
A and 77B are provided, and the detection results are output to the mold clamping motor 78 and the release motor 79, respectively.

The pressurizing piston 75A is moved in the axial direction by the rotation of the ball screw 80 for clamping. That is, the mold clamping motor 78 drives the mold clamping ball screw 80 via the drive pulley 81, the toothed belt 82, and the ball screw nut 83 to rotate. When the mold clamping ball screw 80 rotates, the pressurizing piston 75A is moved in the axial direction by the ball screw action.

The pressurizing piston 75B is moved in the axial direction by the rotation of the release ball screw 84. That is,
A release ball screw 84 is driven to rotate by a release motor 79 via a bevel gear 85 and a ball screw nut 86. When the release ball screw 84 rotates, the pressurizing piston 75B is axially moved by the ball screw action.

When the main ram is moved in the direction in which the movable platen 2 is pressed (rightward in FIG. 12) (during mold clamping), the pressurizing piston is projected into the pressure chamber by the mold clamping motor, and the amount of projection is Then, the pressurizing piston is retracted from the pressure chamber by the release motor according to. When the main ram moves the movable platen 2 in the pulling direction (left direction in FIG. 12) (at the time of release), the pressurizing piston is retracted from the pressure chamber by the mold clamping motor.
The pressurizing piston is made to protrude into the pressure chamber by the release motor in accordance with the retracted amount. As described above, the interlocking of the pressurizing pistons ensures smooth operation of the main ram, and prevents the sealed liquid from being negatively pressured in the pressure chamber. This interlocking is performed by pressure control based on the detection result of the pressure detection sensor.

In this embodiment, a double-acting cylinder is used, but a single-acting cylinder and a ball screw (both not shown)
Can be used in combination.

[0030]

According to the present invention, a mold clamping device for an injection molding machine comprises:
A fixed plate on which a fixed die is mounted, a movable plate on which a movable die is mounted corresponding to the fixed bracket, and an end plate provided with a mold booster between the movable plate, and one end of a tie rod are disposed. The other end where the ball screw is connected to the fixed plate with a nut is connected to the end plate, and the end plate is attached to a slide, and the movable plate is slidably mounted on the slide. In the closing and mold opening processes, the movable platen and the end plate are moved forward and backward integrally by driving the ball screw through a ball screw nut. In the mold clamping and pressurizing process, the ball screw nut is pressed against the end plate. After the ball screw nut is locked, a predetermined mold clamping force is generated by the mold clamping and boosting means, so that the length from the end plate to the fixed plate can be shortened, and the conventional toggle method is used. It can significantly reduce the device length as compared to. If the apparatus length can be shortened, the space for installing the injection molding machine can be reduced accordingly, and there is a great effect that the injection molding machine can be set in a smaller building or the extra space around the injection molding machine can be effectively used. Further, since the ball screw used for the mold opening / closing operation is formed in a tie rod, the number of components can be reduced, and the cost and the size of the device can be reduced. Further, since the end board can be locked at an arbitrary position, it is possible to cope with a change in the dimension of the mold without changing any setting, and the versatility can be improved.

If the ball screw nut and the end plate are separated from each other by the biasing force of a spring in the mold closing / mold opening process, the components can be simplified, and the cost and the size of the apparatus can be reduced. Can contribute.

If a means for adjusting the biasing force of the spring is further provided, the separated state in the mold closing / mold opening process becomes more reliable, and a smooth operation can be ensured.

[Brief description of the drawings]

FIG. 1 is a front view of a mold clamping device of an injection molding machine according to a first embodiment of the present invention.

FIG. 2 is a view as viewed from an arrow II in FIG. 1;

FIG. 3 is a partially enlarged sectional view of a part III in FIG. 1;

FIG. 4 is a cross-sectional view illustrating the mold opening / closing drive device of FIG.

FIG. 5 is a front view of the mini-toggle mechanism of FIG. 1 with a part cut away.

FIG. 6 is an exploded perspective view of the mini-toggle mechanism of FIG.

FIG. 7 is a sectional view showing a driving device of the mini-toggle mechanism of FIG. 1;

8 is a front view showing a state in which the mold clamping device of the injection molding machine in FIG. 1 is closed when the mold is closed.

9 is a front view showing a state where the mold clamping device of the injection molding machine in FIG.

FIG. 10 is a front view of a mold clamping device of an injection molding machine according to a second embodiment of the present invention.

FIG. 11 is a front view of a mold clamping device of an injection molding machine according to a third embodiment of the present invention.

FIG. 12 is a sectional view showing a driving device of the sealing hydraulic mechanism of FIG. 11;

FIG. 13 is a front view of a mold clamping device of a conventional injection molding machine.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Fixed board 2 Movable board 3 End board 4 Tie rod 4a Tie rod ball screw 5 Fixed mold 6 Movable mold 7 Slide 8 Mini toggle mechanism 9 Crosshead 10 Ball screw for mold clamping 11, 20 Nut 12, 83, 86 Ball screw nut 12a Nut seat surface 13 Bearing box 14 Bolt 15 Spring 16 Spring support 17 Adjustment bolt 18 Spline or key 19 Base 31 Link A 32 Link B 33 Link C 37, 47 Driven pulley 38, 48, 82 Toothed belt 39, 49, 81 Drive Pulley 40 Mold opening / closing motor 41, 42, 51, 52 Bearing 50, 78 Mold clamping motor 60 Crank mechanism 61 Crank 62 Crank attachment part 63 Rod 64 Rod attachment part 70 Sealing hydraulic mechanism 71 Mold clamping cylinder 72 Main ram 73 Main seal 74A , 7 B pressure chamber 75A, 75B pressurizing piston 76A, 76B piston seal 77A, 77B pressure sensor 79 release motor 80 mold clamping ball screw 84 release ball screw 85 bevel gear P0, P1, P2, P3 hole δ clearance

 ────────────────────────────────────────────────── ─── Continuing from the front page (72) Inventor Gen Hamada 1st character highway, Iwazuka-cho, Nakamura-ku, Nagoya City, Aichi Prefecture Mitsubishi Heavy Industries, Ltd.Industrial Equipment Division (72) Inventor Takeshi Takeshi Iwada F-term (reference) 4F202 AR01 CA11 CL00 CL01 CL22 CL32 CL39 CL46

Claims (3)

[Claims] 1. A fixed plate having a fixed die attached thereto, a movable plate having a movable die attached thereto corresponding to the fixed metal fitting, and an end plate provided with a mold clamping / boosting means between the movable plate and the movable plate. One end of a tie rod is connected to the fixed plate with a nut, the other end with a ball screw is connected to the end plate, and the end plate is mounted on a slide and the movable plate is slidably mounted on the slide. In the mold closing / mold opening process, the movable platen and the end plate are integrally moved forward and backward by driving the ball screw through a ball screw nut, and the ball screw nut is moved in the mold clamping / boosting process. A mold clamping device for an injection molding machine, wherein a predetermined mold clamping force is generated by the mold clamping / boosting means after the ball screw nut is locked by being pressed against the end plate. 2. The mold clamping device for an injection molding machine according to claim 1, wherein the ball screw nut and the end plate are separated from each other by a biasing force of a spring in a mold closing / mold opening process. 3. The mold clamping device for an injection molding machine according to claim 2, further comprising means for adjusting the urging force of said spring.