Energy saving principle and latest technology of e

2022-07-26
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Energy saving principle and latest technology of electro-hydraulic control system for injection molding Abstract: This paper introduces a variety of energy-saving electro-hydraulic control systems for plastic injection molding, especially the electro-hydraulic injection molding machine system with variable frequency motor-driven variable pump, which was developed in foreign countries in the mid-1990s, analyzes and compares them, and gives the structure and performance of key components

key words: injection molding machine; Energy saving; Electrohydraulic control

1 overview

since the 1980s, the plastic machinery industry has been booming in China. In particular, the annual production capacity of plastic injection molding machines has reached more than million, which not only meets the needs of domestic production, but also earns a large amount of foreign exchange through export. In order to meet the technical requirements of this machine for high output, rapid response, high speed, pressure control accuracy and repetition accuracy, Almost all domestic injection molding machines use liquid, which may turn the large-scale recycling of PLA plastic waste into a profitable and profitable business model. The pressure transmission and electro-hydraulic proportional control technology. In order to further improve the accuracy, foreign countries have also developed closed-loop microcomputer controlled electro-hydraulic systems for process variables: cavity pressure, injection speed, screw speed, etc. China has also made corresponding research in this regard. However, since the 1990s, with the increasingly stringent requirements of the world in environmental protection, such as energy consumption, noise, leakage and other control, energy conservation has become the research focus of injection molding electro-hydraulic system. In order to reduce energy consumption, Germany, Japan and other countries have developed injection molding machines with full electric drive control and put them on the market. Compared with the electro-hydraulic control, the current electric drive still has some shortcomings, such as the inability to achieve centralized control, the difficulty to achieve high-precision pressure control, and the need to pass through the intermediate conversion device to achieve linear transmission. However, it has posed a severe challenge to the electro-hydraulic control. Therefore, the electro-hydraulic control system must make corresponding development in energy-saving technology to avoid being replaced by full electronic control technology

2 current situation of electro-hydraulic control technology for domestic injection molding machines

at present, almost all domestic injection molding machines have applied electro-hydraulic proportional technology. Its technical feature is that the three-way proportional speed regulating valve is used to control the speed, and then the proportional pressure pilot valve is added to it to control the system pressure. Compared with the original system composed of separated elements, although the system can greatly reduce the number of elements and simplify the structure, there is always a flow related energy loss in the working process due to the use of quantitative pump oil supply, especially in the pressure maintaining process, the energy consumption of this part is at the maximum. Such a system is rarely used abroad

3 load sensing electro-hydraulic system for injection molding

in view of the large energy loss of valve controlled electro-hydraulic control system, foreign countries have developed a load sensing electro-hydraulic control system for injection molding that combines variable pump and electro-hydraulic proportional valve. The principle is shown in Figure 1. The movement speed of the actuator will be controlled by the combination of the flow through the proportional throttle valve and the variable displacement pump. The inlet and outlet oil pressure of the proportional throttle valve is fed back to the pilot valve of the variable displacement pump control mechanism, and the variable displacement mechanism of the pump is adjusted through this differential pressure feedback, so that the pump output flow is always consistent with the load requirements. In this way, the energy loss related to the flow can be eliminated by using the quantitative pump and proportional valve control. During the adjustment process of the system, the pressure difference at the inlet and outlet of the proportional throttle valve remains constant, so that the flow through the proportional throttle valve can be continuously proportional controlled by the electrical signal. This pressure difference is usually 1 MPa。 When the operating pressure of the system reaches the set value limited by the proportional relief valve vdb1, the system will change from the flow control state to the pressure control state, and the system only outputs the flow required to maintain the pressure. With the combination of the valve and pump, the mold locking mechanism, injection molding process, back pressure and the movement of the auxiliary actuator can be controlled in open loop or closed loop. In order to control the filling pressure in the plasticizing process, the main overflow valve vdb2 is added to control this pressure together with the proportional pilot overflow valve

Figure 1 load sensing electro-hydraulic control system

4 the electro-hydraulic system using p/q proportional compound pump

adopts the load sensitive control principle combining valve and pump. Although all the energy losses related to flow can be eliminated by using quantitative pump and three-way proportional valve, there are still large throttling losses due to the fixed working pressure difference of proportional throttle valve, especially in the high-speed stage. The method to eliminate this part of energy loss is to use the high response variable displacement pump with direct closed-loop control of pressure and flow as the power source after it was re launched by BMW Group in 2001. The simplified circuit principle is shown in Figure 2

Fig. 2 electrohydraulic control system using p/q composite proportional pump

the greatest advantage of this system is that there is no energy loss related to flow and pressure

in order to improve the dynamic response speed, the whole control mechanism is composed of differential pressure controlled radial variable piston pump, high-speed proportional valve with position closed loop, pressure sensor and displacement sensor. The flow control is completed by a position closed-loop system composed of a proportional valve and a displacement sensor that detects the eccentricity of the variable displacement pump. The flow loss caused by leakage is compensated by the pressure sensor, so that the pump output flow is linearly proportional to the set value signal within 0 ~ 10V. The control system pressure is completed by the electric closed loop composed of proportional valve and pressure sensor. The separation and conversion of pressure and flow control states are automatically completed by the electronic amplifier according to the set signal. Figure 3 shows the dynamic step response curve of the variable displacement pump when it controls the pressure and flow respectively

Figure 3 dynamic step response curve of p/q proportional compound pump

it can be seen that when the output flow of the system increases from 0 to 90%, the response time is about 50ms; When the output flow is reduced from 90% to 0, the response time is about 30ms, and the dynamic response time of the system pressure is less than 200ms when the load chamber is 4L, which is equivalent to the high-performance proportional valve, so it is completely suitable for the process control of the injection molding machine

with the electro-hydraulic closed-loop p/q proportional compound variable pump, the control oil source can be provided by the auxiliary pump or the oil source of the system itself. When using its own oil source, in order to achieve the desired dynamic characteristics, as shown in Figure 3, a pressure sequence valve must be connected in series on the pressure channel to preload the system to increase the minimum pressure of the system to a certain value (less than 2MPa). When in use, this sequence valve is integrated with the system safety valve and the one-way valve for unloading in a special valve block

5 electro-hydraulic system of constant displacement pump and variable frequency speed regulation motor

in order to further reduce energy consumption and noise, the latest generation of injection molding machine uses the motor-driven hydraulic pump as the power source with narrow speed regulation range, where there is high speed, there is no low speed, or where there is low speed, there is no high speed and adjustable speed, as shown in Figure 4. In order to meet the requirements of high dynamic response of plastic machine, two principles can be adopted. One is to use the low-cost asynchronous motor to form a hydraulic power source with variable pump through variable frequency speed regulation (see Figure 4 (a)), and the other is to use the high response AC servo motor to drive the quantitative pump as the power source (see Figure 4 (b)). Since many pumps have specified the minimum speed (n>200r/min), the first principle that the cooling system must be turned on when the oil temperature exceeds 60 ℃ is adopted to control the flow. When the flow is small, it is controlled by changing the pump displacement; When the flow is greater than a certain minimum value, it can be controlled by changing the motor speed, which has the advantage of reducing noise. When the pressure is controlled, it is carried out by changing the pump displacement. When the second principle is adopted, the cost of the motor is higher, but the cost of the pump is lower. The pressure and flow are controlled by changing the speed of the motor. Figure 5 shows the dynamic response characteristics of its closed-loop control pressure. By adopting these two principles, not only is there no throttling loss at all, there is no need to control the auxiliary energy of the pump variable mechanism, but also can greatly reduce the energy loss under the pressure maintaining condition and idling condition (waiting period). At this time, the motor is in the stopped state, and the noise can be reduced at the same time, so it is very in line with the requirements of environmental protection. The focus of the future research work is to reduce the cost of this system by improving the dynamic rapidity of the variable-frequency variable-speed motor-driven quantitative pump

(a) (b)

Figure 4 principle of controlling hydraulic power by changing motor speed

Figure 5 closed loop pressure response curve of quantitative pump driven by AC servo motor

6 conclusion

the load sensing system composed of variable pump and proportional valve is the most basic energy-saving method in electro-hydraulic control, which has been widely used in foreign construction machinery, but there is still energy loss related to flow; The p/q compound pump and the constant displacement pump driven by the variable frequency variable speed motor do not have any flow saving loss. In contrast, the application of frequency modulation motor and quantitative pump can further reduce idle energy consumption and noise, which is more competitive, but its dynamic rapidity should be further improved

looking forward to the future, it will be a very meaningful work to further study and develop the injection molding electro-hydraulic control system that can recover energy by applying the secondary regulation principle, improve the efficiency of this system, and meet the challenges of electrical and mechanical transmission

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