A closed hydraulic system for injection molding machine

Abstract

The present invention discloses a closed hydraulic system for an injection molding machine, characterized by comprising an oil tank, a control unit, an energy unit, an equal-area actuator unit, and an unequal-area actuator unit; the energy unit comprising a servo driver, a servo motor, and a bidirectional closed pump; and the control unit comprising a three-position four-way reversing valve, a two-position four-way reversing valve, and a first one-way valve. Advantageously, the control unit, consisting of the three-position four-way reversing valve, the two-position four-way reversing valve, and the first one-way valve, enables sequential operation of the equal-area actuator unit and the unequal-area actuator unit by sequentially switching the valves in the control unit. This makes the closed hydraulic system suitable for use in applications with multiple types of working devices, eliminates the need for a separate hydraulic system, and effectively reduces manufacturing costs.

Description

Technical Field

[0001] The present invention relates to a hydraulic system, in particular to a closed hydraulic system of an injection molding machine. Background Art

[0002] The closed hydraulic system commonly used on injection molding machines is usually as follows Figure 1 As shown, it is mainly composed of energy unit C1, execution unit C2, oil replenishment unit C3, auxiliary unit and working medium. Energy unit C1 is usually composed of servo drive C4, servo motor C5 and bidirectional closed pump C6. Its function is to convert the mechanical energy provided by servo motor C5 into pressure energy of liquid; execution unit C generally refers to equal-area double-rod hydraulic cylinder or hydraulic motor. Its function is to convert the pressure energy of liquid into mechanical energy and drive the load to perform linear reciprocating motion or rotary motion; oil replenishment unit C3 is usually composed of ordinary motor C18, metering pump C17, filter C16, oil replenishment check valve C8, C14 and overflow valve C15. Its function is to replenish the oil leaked during the operation of the closed pump and cooperate with flushing valve C10 to dissipate heat for the closed system oil; auxiliary unit generally includes pressure sensor C7, oil tank C21, oil suction filter C20, cooler C19, system overflow valve C9, system overflow valve C13, flushing valve C10 and overflow valve C12. Their functions are as follows: pressure sensor C7 is used to feedback the working oil circuit The oil pressure is supplied to the controller to achieve closed-loop pressure control of the entire hydraulic system. The oil tank C21 is used to store the oil required by the charge pump C17, recover the oil leaked from the closed pump C6, and recover the oil flushed back through the flushing valve C10. The oil suction filter C20 is used to filter the oil entering the charge pump C17. The cooler C19 is used to cool the oil overflowing from the charge unit C3 and the oil flushed back from the flushing valve C10. The system relief valve C9 is used to limit the maximum working pressure of the D working oil circuit to ensure system safety. The system relief valve C13 is used to limit the maximum working pressure of the E working oil circuit to ensure system safety. The flushing valve C10 uses the oil pressure on the high-pressure side of the working oil circuit to switch, so that the oil on the low-pressure side is flushed and cooled to dissipate heat. The relief valve C12 is used to limit the return oil pressure of the flushing valve C10, establishing the return oil back pressure to maintain the stability of the hydraulic system. The working medium generally refers to hydraulic oil, which is responsible for energy transmission in the hydraulic system.

[0003] The working devices (executor units) of the above-mentioned closed hydraulic system are generally cylinders or hydraulic motors of equal area. The oil inlet and return flow rates of the cylinders or hydraulic motors of equal area are the same, which can ensure the stable operation of the closed pump in the hydraulic system. Based on this, the above-mentioned closed hydraulic system can basically only be used on injection molding machines whose mold opening and closing cylinders are equal-area cylinders to control the mold opening and closing movements. However, due to space and position limitations, the other several non-equal-area working devices on the injection molding machine (such as the injection cylinder, ejection cylinder, core pulling cylinder, etc.) are difficult to be converted into equal areas. In other words, the above-mentioned closed hydraulic system is obviously not applicable to injection molding machines with both equal-area and non-equal-area working devices. That is, for non-equal-area working devices, a separate hydraulic system must be equipped, resulting in a higher overall manufacturing cost of the injection molding machine. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a closed hydraulic system suitable for an injection molding machine with multiple types of working devices.

[0005] The technical solution adopted by the present invention to solve the above technical problems is:

[0006] A closed hydraulic system for an injection molding machine includes an oil tank, a control unit, an energy unit, an equal-area execution unit, and a non-equal-area execution unit;

[0007] The energy unit includes a servo drive, a servo motor and a bidirectional closed pump. The oil tank replenishes oil to the bidirectional closed pump. The servo motor is driven by the servo drive, and the bidirectional closed pump is driven by the servo motor. The control unit includes a three-position four-way reversing valve, a two-position four-way reversing valve and a first one-way valve.

[0008] The first working oil port of the three-position four-way reversing valve is connected to the oil port of the large-area cavity of the non-equal-area execution unit, and the second working oil port of the three-position four-way reversing valve is connected to the oil port of the small-area cavity of the non-equal-area execution unit; the first working oil port and the second working oil port of the two-position four-way reversing valve are respectively connected to the two oil ports of the equal-area execution unit; the oil inlet of the three-position four-way reversing valve and the oil inlet of the two-position four-way reversing valve are connected to form a common oil circuit, which is connected to the oil outlet end of the two-way closed pump through the first working oil circuit, and the oil outlet of the two-position four-way reversing valve and the oil outlet of the first one-way valve are connected to form a common oil circuit, which is connected to the oil inlet end of the two-way closed pump through the second working oil circuit; the common end formed by the oil outlet of the three-position four-way reversing valve and the oil inlet end of the first one-way valve is connected to the oil tank.

[0009] A pressure sensor is provided in each of the first and second working oil circuits, and is electrically connected to the servo driver. The two pressure sensors are used to respectively collect the oil pressure in the first and second working oil circuits and feed the collected oil pressures back to the servo driver. The servo motor then uses an encoder to feed back current speed information to the servo driver. The servo driver then feeds this information back to the controller on the injection molding machine, compares it with a value set within the controller, and adjusts the output value of the servo driver to the servo motor accordingly, achieving dynamic control of the servo motor speed and precise closed-loop control of pressure and flow.

[0010] The oil replenishment unit further comprises an oil replenishment unit, the oil replenishment unit comprises an oil replenishment valve group and an oil replenishment power group, the oil replenishment power group comprises an oil replenishment pump and a three-phase asynchronous motor for driving the oil replenishment pump, the oil replenishment valve group comprises a first relief valve, a second relief valve, a third relief valve, a fourth relief valve, a flushing valve and a second one-way valve;

[0011] The two oil inlets of the flushing valve are respectively connected to the first working oil circuit and the second working oil circuit, the oil outlet of the flushing valve is connected to the oil inlet of the first relief valve, and the oil return end of the first relief valve and the oil return end of the fourth relief valve are connected to form a common oil circuit connected to the oil tank; the oil inlet of the second relief valve is connected to the first working oil circuit, and the oil return end of the second relief valve is connected to the second working oil circuit; the oil inlet of the third relief valve is connected to the second working oil circuit, and the oil return end of the third relief valve is connected to the first working oil circuit; the oil outlet of the second check valve is connected to the first working oil circuit, and the common oil circuit formed by connecting the oil inlet of the second check valve, the oil outlet of the two-position four-way reversing valve, and the oil outlet of the first check valve and the oil inlet of the fourth relief valve are connected to the oil outlet of the charge pump, and the oil inlet of the charge pump is connected to the oil tank. This structure effectively solves the problems of oil leakage and oil heating that can occur during operation of the entire hydraulic system, ensuring stable operation of the entire hydraulic system. If the unequal-area actuator receives less oil than it returns, the excess return oil can overflow back to the tank via the relief valve in the oil replenishment unit. If the unequal-area actuator receives more oil than it returns, the oil replenishment unit can replenish the insufficient oil drawn by the bidirectional closed-loop pump, thereby ensuring the normal operation of the unequal-area actuator.

[0012] An oil suction filter is disposed between the oil inlet of the charge pump and the fuel tank. A first port of the oil suction filter is connected to the fuel tank, and a second port of the oil suction filter is connected to the oil inlet of the charge pump. The oil suction filter is disposed at this location to remove residual contaminants in the fuel tank, thereby protecting the charge pump.

[0013] A high-pressure filter is installed at the oil outlet of the charge pump. A first port of the high-pressure filter is connected to the oil outlet of the charge pump. The oil outlet of the two-position four-way reversing valve and the oil outlet of the first check valve are connected to form a common oil circuit, which is connected to the oil inlet of the fourth relief valve and is then connected to the second port of the high-pressure filter. The high-pressure filter prevents contaminants from entering the entire hydraulic system, effectively controlling contamination concentration in the hydraulic system and protecting critical components such as the two-way closed-loop pump.

[0014] The oil return ends of the first relief valve and the fourth relief valve are connected to the oil tank via a cooler. The oil return ends of the first relief valve and the fourth relief valve are respectively connected to a first port of the cooler, and a second port of the cooler is connected to the oil tank. The cooler is used to cool the hot oil flushed back from the first working oil circuit or the second working oil circuit. The hot oil is cooled by the cooler to become cold oil and enters the oil tank for recycling, thereby effectively controlling the heat generation of the entire system during operation and ensuring stable and efficient operation of the entire system.

[0015] The oil drain port of the bidirectional closed pump is connected to the oil tank, so that the leaked hot oil can be cooled in the oil tank and then recycled.

[0016] The flushing valve is a three-position three-way valve with a simple structure and convenient control.

[0017] The non-equal area execution unit is a non-equal area oil cylinder, which can be selected according to different working conditions.

[0018] The equal-area actuator is an equal-area double-rod oil cylinder or a hydraulic motor, which can be selected according to different working conditions.

[0019] Compared with the prior art, the advantages of the present invention are: through a control unit composed of a three-position four-way reversing valve, a two-position four-way reversing valve and a first one-way valve, the sequential operation of the equal-area execution unit and the non-equal-area execution unit is realized by sequential switching of the valves in the control unit. Since the oil intake and oil return volumes of the equal-area execution units are the same, the reversing of the action of the equal-area execution unit is mainly realized by the reversing of the two-way closed pump; and the oil intake and oil return volumes of the non-equal-area execution unit are different, and when working, they will face the situation of less oil intake and more oil return or more oil intake and less oil return, so the reversing of the action of the non-equal-area execution unit is mainly realized by the reversing of the reversing valve (the two-way closed pump keeps the working direction unchanged in this process), thereby realizing that the closed hydraulic system is suitable for use in occasions with multiple types of working devices, does not require the provision of an additional hydraulic system, and effectively reduces the manufacturing cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is the schematic diagram of the closed hydraulic system for traditional injection molding machines;

[0021] Figure 2 This is a schematic diagram of the first embodiment of the present invention;

[0022] Figure 3 This is a schematic diagram of the second embodiment of the present invention. DETAILED DESCRIPTION

[0023] The present invention will be described in further detail below with reference to the accompanying drawings and embodiments.

[0024] Example 1: Figure 2 As shown, a closed hydraulic system of an injection molding machine includes an oil tank 1, a control unit, an energy unit, an equal-area execution unit 2 and a non-equal-area execution unit 3;

[0025] The energy unit includes a servo driver 4, a servo motor 5 and a bidirectional closed pump 6. The oil tank 1 supplies oil to the bidirectional closed pump 6. The servo motor 5 is driven by the servo driver 4, and the bidirectional closed pump 6 is driven by the servo motor 5. The control unit includes a three-position four-way reversing valve 7, a two-position four-way reversing valve 8 and a first one-way valve 9.

[0026] The first working oil port A of the three-position four-way reversing valve 7 is connected to the oil port of the large-area cavity of the non-equal-area execution unit 3, and the second working oil port B of the three-position four-way reversing valve 7 is connected to the oil port of the small-area cavity of the non-equal-area execution unit 3; the first working oil port A and the second working oil port B of the two-position four-way reversing valve 8 are respectively connected to the two oil ports of the equal-area execution unit 2; the common oil circuit formed by the oil inlet P port of the three-position four-way reversing valve 7 and the oil inlet P port of the two-position four-way reversing valve 8 is connected to the oil outlet end of the two-way closed pump 6 through the first working oil circuit D, and the common oil circuit formed by the oil outlet T port of the two-position four-way reversing valve 8 and the oil outlet of the first one-way valve 9 is connected to the oil inlet end of the two-way closed pump 6 through the second working oil circuit E; the common end formed by the oil outlet T port of the three-position four-way reversing valve 7 and the oil inlet end of the first one-way valve 9 is connected to the oil tank 1.

[0027] In this specific embodiment, a pressure sensor 10 is provided in each of the first and second working oil circuits D and E, and the pressure sensors 10 are electrically connected to the servo driver 4. The two pressure sensors 10 are used to respectively collect the oil pressure in the first and second working oil circuits D and E, and the collected oil pressure is fed back to the servo driver 4. The servo motor 5 feeds back the current speed information to the servo driver 4 via an encoder. The servo driver 4 feeds back the received information to the controller on the injection molding machine, compares it with the value set in the controller, and adjusts the output value of the servo driver 4 to the servo motor 5 accordingly, realizing dynamic control of the speed of the servo motor 5 and achieving precise closed-loop control of pressure and flow.

[0028] In this specific embodiment, the oil drain port of the bidirectional closed pump 6 is connected to the oil tank 1. The leaked hot oil can be cooled in the oil tank 1 and then recycled.

[0029] In this specific embodiment, the non-uniform area execution unit 3 is a non-uniform area oil cylinder, which can be selected according to different working conditions.

[0030] In this specific embodiment, the equal-area execution unit 2 is an equal-area double-rod oil cylinder or a hydraulic motor, which can be selected according to different working conditions.

[0031] Example 2: Figure 3 As shown, other parts are the same as those of the first embodiment, except that it further includes an oil replenishing unit, which includes an oil replenishing valve group and an oil replenishing power group. The oil replenishing power group includes an oil replenishing pump 11 and a three-phase asynchronous motor 12 for driving the oil replenishing pump 11. The oil replenishing valve group includes a first relief valve 13, a second relief valve 14, a third relief valve 15, a fourth relief valve 16, a flushing valve 17, and a second one-way valve 18.

[0032] The two oil inlet ends of the flushing valve 17 are connected to the first working oil circuit D and the second working oil circuit E respectively. The oil outlet end of the flushing valve 17 is connected to the oil inlet end of the first relief valve 13. The oil return end of the first relief valve 13 and the oil return end of the fourth relief valve 16 are connected to form a common oil circuit connected to the oil tank 1; the oil inlet end of the second relief valve 14 is connected to the first working oil circuit D, and the oil return end of the second relief valve 14 is connected to the second working oil circuit E; the oil inlet end of the third relief valve 15 is connected to The second working oil circuit E is connected, and the return oil end of the third relief valve 15 is connected to the first working oil circuit D. The oil outlet end of the second check valve 18 is connected to the first working oil circuit D. The common oil circuit formed by the oil inlet end of the second check valve 18, the oil outlet port T of the two-position four-way reversing valve 8, and the oil outlet of the first check valve 9 is connected to the oil inlet end of the fourth relief valve 16, and is connected to the oil outlet end of the charge pump 11. The oil inlet end of the charge pump 11 is connected to the oil tank 1. This structure effectively solves the problems of oil leakage and oil heating that may occur during the operation of the entire hydraulic system, ensuring the stable operation of the entire hydraulic system. When the non-equal-area actuator 3 has a low oil inlet and a high oil return, the excess return oil can overflow back to the oil tank through the relief valve in the charge pump unit. When the non-equal-area actuator 3 has a high oil inlet and a low oil return, the insufficient oil suctioned by the two-way closed pump 6 can be replenished through the charge pump unit, thereby ensuring the normal operation of the non-equal-area actuator 3.

[0033] In this embodiment, an oil suction filter 19 is disposed between the oil inlet of the charge pump 11 and the fuel tank 1. A first port of the oil suction filter 19 is connected to the fuel tank 1, and a second port of the oil suction filter 19 is connected to the oil inlet of the charge pump 11. The oil suction filter 19 is disposed at this location to filter out residual contaminants in the fuel tank 1, thereby protecting the charge pump 11.

[0034] In this specific embodiment, a high-pressure filter 20 is provided at the oil outlet of the charge pump 11. A first port of the high-pressure filter 20 is connected to the oil outlet of the charge pump 11. A common oil path formed by connecting the oil outlet T of the two-position four-way reversing valve 8 and the oil outlet of the first check valve 9 is connected to the oil inlet of the fourth relief valve 16, forming a common oil path that is connected to the second port of the high-pressure filter 20. The installation of the high-pressure filter 20 at this location prevents contaminants from entering the entire hydraulic system, effectively controlling the contamination concentration in the hydraulic system and protecting important components such as the two-way closed-loop pump 6.

[0035] In this specific embodiment, the return oil ends of the first relief valve 13 and the fourth relief valve 16 are connected to the fuel tank 1 via a cooler 21. The return oil ends of the first relief valve 13 and the fourth relief valve 16 are respectively connected to the first port of the cooler 21, and the second port of the cooler 21 is connected to the fuel tank 1. The cooler 21 is used to cool the hot oil flushed back from the first working oil circuit D or the second working oil circuit E. This hot oil is cooled by the cooler 21 to become cold oil and enter the fuel tank 1 for recycling, thereby effectively controlling the heat generation of the entire system during operation and ensuring stable and efficient operation of the entire system.

[0036] In this specific embodiment, the flushing valve 17 is a three-position three-way valve with a simple structure and convenient control.

[0037] Among them, the servo motor 5 drives the bidirectional closed pump 6 to work in forward and reverse directions respectively to control the oil pressure and flow; the bidirectional closed pump 6 can rotate forward and reverse and can work normally; the two dual pressure sensors 10 are responsible for collecting the oil pressure of the two working oil circuits and feeding it back to the servo driver 4; the first one-way valve 9 and the second one-way valve 18 play the role of one-way oil passage to prevent the oil from backflowing back to the replenishment pump 11 when replenishing oil; the second relief valve 14 and the third relief valve 15 are used to prevent the first working oil circuit D and the second working oil circuit E from overloading the pressure and limit the maximum pressure during operation. The first relief valve 13 mainly plays the role of limiting the return oil pressure of the flushing valve 17, establishing the return oil back pressure to keep the hydraulic system stable, and the fourth relief valve 15 plays the role of limiting the pressure of the replenishment pump 11; the flushing valve 17 is when there is a pressure difference between the first working oil circuit D and the second working oil circuit E. 17 is opened by high pressure, and the oil of the oil replenishment pump 11 is replenished into the working oil circuit with lower oil pressure, and then returns to the oil tank 1 through the flushing valve 17 and the first overflow valve 13. Since the oil replenishment pump 11 replenishes filtered cold oil and flushes back hot oil, this circulation process can filter and cool the oil in the working oil circuit; the oil replenishment pump 11 is a fixed-displacement pump driven by a three-phase asynchronous motor 12; the cooler 21 is used to cool the hot oil flushed back from the first working oil circuit D or the second working oil circuit E, as well as to cool the excess oil of the oil replenishment pump 11 and the return oil overflowed through the fourth overflow valve 16; the oil tank 1 is used to provide cold oil to the oil replenishment pump 11 and the two-way closed pump 6 and collect the hot oil returned from the working oil circuit; the three-position four-way reversing valve 7 is used to control the movement and reversing of the non-equal-area actuator 3, and the two-position four-way reversing valve 8 is used to control the movement of the equal-area actuator 2.

[0038] The specific working principle is: when the equal-area execution unit 2 needs to work, the two-position four-way reversing valve 8 is electrically reversed, and the P port of the two-position four-way reversing valve 8 is connected to the A port, and the B port is connected to the T port. Assuming that when the servo motor 5 drives the bidirectional closed pump 6 to rotate forward, the first working oil circuit D is a high-pressure oil circuit and the second working oil circuit E is a low-pressure oil circuit, then the two dual pressure sensors 10 feedback the current pressure information to the servo driver 4, and the servo motor 5 feedbacks the current speed information to the underworld driver 2 through the encoder. The servo driver 4 will receive the information and feedback it to the controller on the injection molding machine, compare it with the value given in the controller, and adjust the output value of the servo driver 4 to the servo motor 5 accordingly. , dynamically controls the servo motor 5 to achieve precise closed-loop control of pressure and flow; when the actuator needs to reverse, the servo motor 5 drives the bidirectional closed pump 6 to reverse, the first working oil circuit D is a low-pressure oil circuit, and the second working oil circuit E is a high-pressure oil circuit. The two dual pressure sensors 10 feedback the current pressure information to the servo driver 4, and the servo motor 5 feedbacks the current speed information to the servo driver 4 through the encoder. The servo driver 4 feeds back the received information to the controller on the injection molding machine, compares it with the value given in the controller, and adjusts the output value of the servo driver 4 to the servo motor 5 accordingly, dynamically controlling the servo motor 5 to achieve precise closed-loop control of pressure and flow. When the equal-area actuator 2 completes its action, the two-position four-way valve 21 is de-energized, the PTAB oil ports of the two-position four-way valve 21 are disconnected, and the oil in the two chambers of the equal-area actuator 2 is sealed;

[0039] When the non-equal area execution unit 3 needs to work, the P port of the three-position four-way reversing valve 7 must always keep oil inlet, and the T port must always return oil. That is, during the entire operation of the non-equal area execution unit 3, the two-way closed pump 6 only needs to maintain one direction so that the first working oil circuit D always discharges oil and the second working oil circuit E always returns oil. If the non-equal area execution unit 3 needs to change direction, it will be done through the three-position four-way reversing valve 7, and there is no need to reverse the direction through the two-way closed pump 6. When the left side of the three-position four-way reversing valve 7 is energized, the P port of the three-position four-way reversing valve 7 is connected to the B port, and the A port is connected to the T port, that is, the non-equal area execution unit 3 has oil entering the small cavity and returning oil to the large cavity. At this time, the return oil volume is greater than the oil entry volume, and the excess return oil can be returned to the cooler through the overflow valve 15 for cooling and then returned to the oil tank. The two-way closed pump 6 can operate stably to make the non-equal area execution unit 3 work backward; when the right side of the three-position four-way reversing valve 7 is energized, the P port of the three-position four-way reversing valve 7 is connected to the A port, and the B port is connected to the T port, that is, the non-equal area execution unit 3 has oil entering the large cavity and returning oil to the small cavity. At this time, the return oil volume is less than the oil entry volume, and the insufficient return oil volume can be supplemented by the oil supplement pump 11. The two-way closed pump 6 can operate stably to make the non-equal area execution unit 3 work forward. Since the bidirectional closed pump 6 only needs to maintain one direction to ensure that the first working oil circuit D always discharges oil during the entire operation of the non-equal-area actuator 3, the dual pressure sensors 10 connected to the first oil circuit D only need to feedback the current pressure information to the servo driver 4. The servo motor 5 feedbacks the current speed information to the underworld driver 2 through the encoder. The servo driver 4 will feedback the received information to the controller on the injection molding machine, compare it with the value given by the controller, and adjust the output value of the servo driver 4 to the servo motor 5 accordingly, dynamically controlling the servo motor 5 to achieve precise closed-loop control of pressure and flow.

[0040] After the energy unit and the oil replenishment unit are synchronously started, the bidirectional closed pump 6 drives the execution unit to operate, generating a pressure differential between the first working oil circuit D and the second working oil circuit E. The flushing valve 17 is opened by the oil pressure on the high-pressure side of the working oil circuit. The oil from the oil replenishment pump 11 is first filtered by the high-pressure filter 17 and then replenished into the low-pressure working oil circuit through the one-way valve connected to the low-pressure working oil circuit. Excess oil flows back to the oil tank 1 through the flushing valve 17, the first relief valve 13, and the cooler 21. Excess oil provided by the oil replenishment unit will flow back to the oil tank 1 through the fourth relief valve 15 and the cooler 21. If the first working oil circuit D or the second working oil circuit E is overpressured, the oil pressure on the overpressure side will open the second relief valve 14 or the third relief valve 15, and the overflowing oil will flow into the low-pressure side to prevent system overload.

Claims

1. A closed hydraulic system for an injection molding machine, characterized in that It includes a fuel tank, a control unit, an energy unit, an equal-area execution unit and a non-equal-area execution unit; The energy unit includes a servo drive, a servo motor and a bidirectional closed pump. The oil tank replenishes oil to the bidirectional closed pump. The servo motor is driven by the servo drive, and the bidirectional closed pump is driven by the servo motor. The control unit includes a three-position four-way reversing valve, a two-position four-way reversing valve and a first one-way valve. The first working oil port of the three-position four-way reversing valve is connected to the oil port of the large-area cavity of the non-equal-area execution unit, and the second working oil port of the three-position four-way reversing valve is connected to the oil port of the small-area cavity of the non-equal-area execution unit; the first working oil port and the second working oil port of the two-position four-way reversing valve are respectively connected to the two oil ports of the equal-area execution unit; the oil inlet of the three-position four-way reversing valve and the oil inlet of the two-position four-way reversing valve are connected to form a common oil circuit, which is connected to the oil outlet end of the two-way closed pump through the first working oil circuit, and the oil outlet of the two-position four-way reversing valve and the oil outlet of the first one-way valve are connected to form a common oil circuit, which is connected to the oil inlet end of the two-way closed pump through the second working oil circuit; the common end formed by the oil outlet of the three-position four-way reversing valve and the oil inlet end of the first one-way valve is connected to the oil tank; The first working oil circuit and the second working oil circuit are respectively provided with a pressure sensor, and the pressure sensor is electrically connected to the servo driver; The oil replenishment unit further comprises an oil replenishment unit, the oil replenishment unit comprises an oil replenishment valve group and an oil replenishment power group, the oil replenishment power group comprises an oil replenishment pump and a three-phase asynchronous motor for driving the oil replenishment pump, the oil replenishment valve group comprises a first relief valve, a second relief valve, a third relief valve, a fourth relief valve, a flushing valve and a second one-way valve; The two oil inlets of the flushing valve are respectively connected to the first working oil circuit and the second working oil circuit, the oil outlet of the flushing valve is connected to the oil inlet of the first relief valve, and the oil return end of the first relief valve and the oil return end of the fourth relief valve are connected to form a common oil circuit connected to the oil tank; the oil inlet of the second relief valve is connected to the first working oil circuit, and the oil return end of the second relief valve is connected to the second working oil circuit; the oil inlet of the third relief valve is connected to the second working oil circuit, and the oil return end of the third relief valve is connected to the first working oil circuit; the oil outlet of the second check valve is connected to the first working oil circuit, and the common oil circuit formed by connecting the oil inlet of the second check valve, the oil outlet of the two-position four-way reversing valve, and the oil outlet of the first check valve and the oil inlet of the fourth relief valve are connected to the oil outlet of the charge pump, and the oil inlet of the charge pump is connected to the oil tank.

2. A closed hydraulic system for an injection molding machine as claimed in claim 1, characterized in that An oil suction filter is provided between the oil inlet end of the oil replenishment pump and the oil tank. The first port of the oil suction filter is connected to the oil tank, and the second port of the oil suction filter is connected to the oil inlet end of the oil replenishment pump.

3. A closed hydraulic system for an injection molding machine as claimed in claim 2, characterized in that The oil outlet of the oil replenishing pump is provided with a high-pressure filter, the first port of the high-pressure filter is connected to the oil outlet of the oil replenishing pump, the oil outlet of the two-position four-way reversing valve and the oil outlet of the first one-way valve are connected to form a common oil circuit, which is connected to the oil inlet end of the fourth overflow valve and formed into a common oil circuit connected to the second port of the high-pressure filter.

4. A closed hydraulic system for an injection molding machine as claimed in claim 1, characterized in that The return oil ends of the first overflow valve and the fourth overflow valve are connected to the oil tank through a cooler, and the return oil ends of the first overflow valve and the fourth overflow valve are respectively connected to the first port of the cooler, and the second port of the cooler is connected to the oil tank.

5. A closed hydraulic system for an injection molding machine as claimed in claim 1, characterized in that The oil drain port of the bidirectional closed pump is connected to the oil tank.

6. A closed hydraulic system for an injection molding machine as claimed in claim 1, characterized in that The flushing valve is a three-position three-way valve.

7. A closed hydraulic system for an injection molding machine as claimed in claim 1, characterized in that The non-equal area execution unit is a non-equal area oil cylinder.

8. A closed hydraulic system for an injection molding machine as claimed in claim 1, characterized in that The equal-area execution unit is an equal-area double-rod oil cylinder or a hydraulic motor.

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