Clamping Unit of Injection Molding Machine

There are three main parts for the injection molding machine:

  • Clamping unit, injection unit and machine bed.

In this post, we only discuss the clamping unit of the horizontal injection molding machine.

Functions of Clamping Unit

1. Lock the mold and prevent it from being blown open

It works with the mold. While the injection unit injected the melt plastic into the mold cavities, the clamping unit need to lock the mold tight against the pressure inside the mold and prevent the mold from being blown open.

2. Adjust the platen to suit the mold size

Mainly adjust the position of the moving platen (the second platen) to realize the right mold thickness parameter.

3. Eject the plastic product

Ejectors eject the mold platen to eject the plastic products. Empty the mold cavity in preparation for the next cycle.

4. Other functions

Like core puller etc. are just to realize more actions of the mold during the machine is working. This kind of function will have the connection into the controller, so that the controller may centralize its control.

5. Safety function

The safety function is the protection base of the clamping unit. There are mechanical, hydraulic and electrical, to protect workers from being clamped by the CLAMPING UNIT. The clamping force is huge and the human body is quite fragile. Therefore, we must take the protection devices seriously. It is strictly forbidden to destroy the protective device of the machine for the convenience of operation.

Types of Clamping Unit

1. Five-point double toggle clamping unit

Five point double toggle clamping unit

The hydraulic force amply through the toggle mechanism to obtain a highly efficient and excellent results. After years of development, this structure is very mature and the processing requirements are relatively less stringent.

Most of the Chinese injection molding machine clamping units are of this design. 

2. Hydraulic clamping unit

Hydraulic clamping unit

Clamping is achieved by hydraulic force. Some use an oil cylinder. Some may adopt four oil cylinders, which are respectively installed on the four columns.

With the development of this technology, a large number of large two-board machines began to emerge, some of which can achieve some complex actions.

3. Electrical clamping unit

At present, the mature machines are from Japan and South Korea, and the technology focuses on the clamping screw. It has become the technical basis of the full electrical injection molding machine.

Calculation of clamping force

Supporting mold force = projected area of the finished product in the direction of opening and closing the mold (cm²) × number of mold cavities × pressure in the mold (kg/cm²)

When choosing an injection molding machine, we must pay attention to the following points for the clamping unit:

1. Containable

  • The width and height of the mold should be smaller than or at least one side is smaller than the inner distance of the large column.
  • The width and height of the mold should preferably be within the size range of the mold plate.
  • The thickness of the mold should be between the mold thickness of the injection molding machine .

2. Take-able

  • The mold opening stroke must be at least twice the height of the finished product in the direction of opening and closing the mold, includingthe length of the sprue. 
  • The mold supporting stroke must be sufficient to eject the finished product.

3. Lockable

When the raw material is injected into the mold cavity at high pressure, it will generate a force to support the mold. So the clamping unit of the injection molding machine must provide enough “clamping force” to prevent the mold from being opened.

Common faults and solutions of clamping unit of injection molding machine

1. The mold is not adjustable

Reasons and Solutions:

(1) Mechanical level and parallelism are out of tolerance.

– Check with a spirit level. Adjust parallelism and level. (For large models, it has little effect on small models.)

(2) The gap between the pressure plate and the die-adjusting nut is too small.

– Measure with a feeler gauge. Adjust the gap between the pressure plate and the nut, and the gap between the die adjustment nut and the pressure plate (gap ≤ 0.05mm).

(3) The nut is burned

– Check whether the nut can be rotated to generate heat and whether iron powder comes out. Replace nuts.

(4) The I/O board is broken.

– Check whether there is a signal at the output point on the computer page. Repair electronic boards.

(5) The mold adjusting valve core is stuck.

– Remove valve for inspection. Purge valve.

(6) The mold adjusting motor is broken

– Check the oil motor. Replace or repair oil motor.

2. No mold clamping


(1) Check the travel switch in front of the safety door and repair it.

(2) Check the 24V5A power supply in the electric box. Replace the fuse and power supply box.

(3) Check whether the spool is stuck and clean the spool.

(4) Check whether the I/O board has output and whether the solenoid valve is electrified.

(5) Check whether the hydraulic safety switch is pressed and whether the mechanical lock lever baffle is opened.

3. Squeezing noise of mold opening and closing machine


(1) Check whether the lubricating oil pipe is disconnected. If so, reconnect the oil pipe.

(2) If the amount of lubricating oil is small, increase the amount of lubricating oil. It is recommended to oil the 50 die once or add enough lubricating oil manually.

(3) Check whether the mold needs a large clamping force, and reduce the clamping force.

(4) Adjust the current of the amplifier board in disorder. Check whether the current parameters meet the acceptance criteria, and re-adjust the current value.

(5) The parallelism is out of tolerance. Use a dial indicator to check whether the parallelism of the first and second plates is greater than the acceptance standard, and adjust the parallelism.

4. Mould opening is delayed for a few seconds


(1) The starting speed is slow

– Check whether the screw damping is too large, and adjust the screw damping hole.

(2) The middle hole of the damping screw is too large

– Check whether the damping of the Y-hole screw is too large, and replace the damping screw with a thin central hole.

5. Crawling of mold opening and locking


(1) The two-plate guide rail and the Colin column are heavily worn

– Check the two-plate guide rail and Colin column. Replace the two-plate copper sleeve and Colin column. Add lubricating oil.

(2) Improper adjustment of the speed and pressure of mold opening and clamping. When the flow rate is set to 20 and the pressure is 99, the second mold clamping plate should not crawl.

– Adjust the flow proportional valve hole or pilot valve hole, and the linear current value of the proportional valve.

(3) There is air in the pipes and cylinders.

– Exhaust.

6. The mold does not open


(1) Increase the speed of mold opening and locking. If the pressure flow is too small and not adjusted properly, check whether the speed of mold opening and locking and the pressure is appropriate. Increase the pressure and speed of mold opening and locking.

(2) The zero position of the clamping electronic scale is changed.

– Check whether the clamping and straightening machine ends at the zero position after twisting, and re-adjust the zero position of the electronic scale.

(3) Check for reverse hinges.

7. In automatic production, the mold adjustment becomes tighter or looser


(1) Internal leakage of the solenoid valve for mold adjustment.

– Check whether the solenoid valve is “O” type or whether the solenoid valve is powered by 24V when it is not working, replace the solenoid valve.

(2) Check whether there is a mold adjustment action when manually playing other actions, and whether the valve is stuck.

8. When other actions work after mold clamping, the mold is fully automatically opened slowly.


(1) The oil plate is leaking

– Check the express clamping valve or replace it. Also, replace the oil plate.

(2) If the mold opening valve leaks, start the oil pump and stop the mold clamping. Press the injection table or injection action to check whether the second plate moves backward. Replace the mold opening oil valve. It is normal that the opening and locking the mold are without movement.

9. Only mold opening action during mold clamping


(1) Wrong wiring.

– Check for 24VDC to valve and wiring. Connect the wiring.

(2) The valve is stuck or the wrong valve core is installed.

– Check whether the valve core is installed incorrectly or blocked, reinstall the valve core or clean it. Under normal circumstances, the action of opening and locking the mold is not moving.

10. Poor mold clamping


(1) A and B holes are not adjusted properly.

When the system flow is set to 20 and the pressure is 99, observe whether the clamping action is crawling, readjust or change the valve.

(2) There is air in the oil circuit.

Listen and check if there is sound of air or air bubbles in the oil circuit. Exhaust.

(3) The ramp-up and ramp-down adjustment of the magnifying board is improper.

Observe whether the current value of the ammeter is proportional to the rise and fall or the rotation speed, and adjust the amplifier board.

11. Clamping can not afford high pressure, exceeding the stroke


(1) The limit switch is out of limit.

Check whether the mold adjustment is appropriate, adjust the appropriate mold thickness. Check if the motor is in normal condition.

(2) The hydraulic limit exceeds the stroke.

Check whether the stroke position of the electronic ruler is appropriate and whether the mold adjustment is appropriate. Properly adjust the mold forward.

12. Manual with mold-opening termination, semi-automatic without mold-opening termination


(1) The mold opening valve leaks.

Activate after the manual shooting table, observe whether the second plate is backward, and replace the mold opening valve.

(2) Check the maximum stroke and pressure flow of the electronic scale.

13. No thimble action


(1) The thimble limit switch is broken.

Use a multimeter to check if the 24V proximity switch is illuminated. Replace the thimble limit switch.

(2) Valve is stuck.

Use a hexagon key to press the thimble valve core to see if it can move, and clean the pressure valve.

(3) The thimble limit rod is broken.

After stopping, take out the limit rod by hand and replace the limit rod.

(4) The thimble switch is shorted.

Use a multimeter to check the 0 voltage of the thimble switch to ground, and replace the thimble switch.

(5) The position of the electronic ruler is not set properly.

14. Thimble out of control when semi-automatic.


(1) The ejector board is broken.

– Check whether the circuit is normal, the normal voltage is DV24V. Repair the thimble plate.

(2) The wire is broken

– Check the connection wire of the switch and the connection wire of the I/O board thoroughly. Check wiring and rewire.

(3) Check whether the mold is out of position.

(4) Check whether the oil cylinder piston rod sealing ring is damaged.

15. Mould opening is loud

Reasons and Solutions

(1) The proportional linearity is poor, and the pressure flow at the opening and closing time position is poorly adjusted.

– Check the ramp up and ramp down in the parameters, and adjust it in the parameters.

(2) Poor lubrication of clamping machine hinge.

– Check the lubrication of the Colin column, the second plate sliding foot and the machine hinge. Increase the lubrication and the number of oiling.

(3) The clamping force of the mold is too large.

– Check the clamping force when the mold is stressed, and reduce the clamping force according to the user’s product. Check if the time position is appropriate.

(4) The deviation of the parallelism of the first and second plates.

– Check the parallelism of the head board and the second board. Adjust the parallel error of the second board and the head board.

(5) The setting position of the mold opening at slow speed is too small and the speed is too fast.

– Check whether the position of slow mold opening to fast mold opening is appropriate, and whether the slow mold opening speed is too fast. Lengthen the slow mold opening position and reduce the slow mold opening speed.

16. Semi-automatic has 2 times of clamping action.


(1) The clamping spool is not completely reset

– Check whether the next action is too continuous after the clamping action is completed.

(2) Increase the delay time of the next action.

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