1. Introduction The plastics processing industry is one of the fastest-growing sectors in the light industry in recent years. With the increasing number of plastic companies, reducing production costs and improving product competitiveness have become common concerns for many plastic product manufacturers. Electricity costs account for a significant proportion of processing costs, and injection molding machines are among the most power-consuming devices. Therefore, many plastic companies have implemented extensive energy-saving technology upgrades for quantitative pump injection molding machines to save energy and improve competitiveness. However, many companies, including many professional energy-saving companies, are at a loss when it comes to variable pump injection molding machines. Therefore, how to achieve energy-saving retrofits for the numerous variable pump injection molding machines on the market has become a hot topic of concern for many plastic companies. Shenzhen Weineng Technology Co., Ltd., through in-depth research on variable pump injection molding machines and combining the latest vector control frequency conversion technology, has successfully achieved energy-saving retrofits for variable pump injection molding machines, achieving energy savings of 10-20%. 2. Introduction to Fixed Quantity Pump and Variable Quantity Pump Injection Molding Machines 1) Fixed Quantity Pump Injection Molding Machines Most of the modified injection molding machines are hydraulic transmission injection molding machines. The power in the hydraulic transmission system is provided by an oil pump driven by a motor. The hydraulic oil pump is a vane pump, and its oil supply is proportional to the speed of the oil pump. When the oil pump motor runs at a constant speed of 50HZ, the oil supply of the oil pump is constant, so it is called a fixed quantity pump. The system diagram is shown below: [align=center] Figure 1 Schematic diagram of the oil circuit system[/align] As shown in Figure 1: A1: Piston area of ​​the rodless chamber of the oil cylinder F: Working load Q1: Working flow rate P1: Working pressure △p: Pressure difference between the two ends of the proportional speed control valve Qy: Flow rate of the proportional relief valve Pb: Oil pump pressure, Qb: Oil pump flow rate Vb: Oil pump displacement n: Motor speed As shown in Figure 1, the system uses a fixed displacement pump for oil supply. The oil pump speed n and oil pump displacement Vb are not adjustable. The flow rates and pressures in the oil circuit are as follows: Oil pump flow rate Qb＝Vb×n is a constant value; Oil pump pressure Pb is set by the proportional relief valve; Working flow rate Q1 is set by the proportional speed control valve; Working pressure P1＝F/A1 depends on the working load F; The general production process of an injection molding machine includes the following steps: mold clamping, injection, melting, pressure holding, cooling, and mold opening. Injection molding machines require different flow rates and pressures at different stages. Flow valves and pressure valves are needed to regulate these requirements. When the required flow rate is low, the oil pump supplies more oil than the actuators need, and the excess hydraulic oil under high pressure flows back through the relief valve. A significant amount of energy is wasted due to the oil's temperature rise. This exacerbates wear on various valves and causes excessively high oil temperatures. Furthermore, additional cooling water circulation is often required to lower the oil temperature, further wasting energy. Therefore, the modification of fixed-displacement pump injection molding machines typically uses a frequency converter to control the pump motor speed and regulate the flow rate. The amount of oil pumped within a specified time precisely meets the needs of each action, minimizing the overflow and saving energy. 2) Variable-displacement pump injection molding machines: Currently, variable-displacement pump injection molding machines mostly use proportional variable pumps as drive elements. These are typical volumetric speed control systems, offering significant energy savings compared to fixed-displacement pump injection molding machines. As shown in Figure 2, this system consists of a load-sensitive proportional variable piston pump, a proportional relief valve, a proportional speed control valve, a pressure feedback valve, and a flow feedback valve. [align=center] Figure 2 Schematic diagram of the oil circuit system[/align] A1: piston area of ​​the rodless chamber of the oil cylinder F: working load P1: working pressure Q1: working flow △P: pressure difference between the two ends of the proportional speed control valve Pb: oil pump pressure Qb: oil pump flow n: motor speed Working principle (1) When the system is in the flow control state, the oil pump pressure Pb is greater than the working pressure P1 and less than the set pressure of the plastic machine, and the proportional relief valve will be reliably closed. When the load changes, the pressure difference △P between Pb and P1 will fluctuate accordingly, and the working flow Q1 will change. The above changes are transmitted to the adjusting piston of the variable pump through the flow and pressure feedback valve. The piston then pushes the swashplate of the plunger pump and changes the oil pump displacement, and finally stabilizes the output flow Qb of the pump. It can be seen that the system realizes pressure adaptive control and basically has no overflow loss, but in order to control the working flow Q1, there is still a certain throttling loss on the speed control valve. (2) When the system is in the pressure control state, the swashplate angle of the variable plunger pump is very small, and only a small amount of hydraulic oil flows through the proportional relief valve to ensure that a certain system pressure is formed. By changing the input signal of the proportional relief valve, the corresponding oil pump output pressure can be obtained. Compared with traditional fixed-displacement pump injection molding machines, this control method reduces overflow losses to a certain extent. 3. Energy-saving principle of variable-displacement pump injection molding machines As we can see from the above introduction, compared with fixed-displacement pump injection molding machines, variable-displacement pump injection molding machines reduce a large amount of overflow losses, making the oil pump motor load lighter, reducing the current, and putting it in a light-load state. Variable-displacement pump injection molding machines start from the perspective of hydraulic components, exploring the space for energy-saving modification in the oil circuit, thereby improving the working efficiency of the oil circuit system. However, due to the limitations of its control method, the system cannot completely eliminate throttling and overflow losses. 1) Variable-displacement pump injection molding machines still have waste space. The oil coming out of the oil pump still needs to be regulated by the proportional speed control valve and the proportional relief valve, and there is still a certain power loss in the valves. 2) The speed of the oil pump motor is still not adjustable. Although its load is much lighter than that of a fixed-displacement pump, it is obviously more energy-consuming than adjusting the oil volume by adjusting the speed. Especially when the system is under pressure control, the system needs a large pressure but a small flow rate, and the speed of the oil pump motor cannot be adjusted. If the output flow rate, output pressure, displacement, and speed of the oil pump are represented as Q, P, Vb, and n respectively, then the output power N of the oil pump is expressed by the formula: N = Q * P = Vb * n * P. The injection molding machine adjusts the oil pump displacement Vb through a mechanical device; obviously, its adjustable linear range is limited, and zero displacement output cannot be achieved. Furthermore, the motor always runs at full speed, resulting in significant unnecessary energy loss. 3) The oil pump motor still uses a star-delta start, resulting in a low power factor. 4. Microenergy Company's Energy-Saving System Microenergy Company applies vector frequency conversion technology to the injection molding machine motor drive, achieving precise control of the injection molding machine through speed regulation and pressure PID closed-loop control. The proportional speed control valve in the oil circuit is manually and rigidly adjusted to the maximum flow rate and maintained or removed, while simultaneously cutting off the control of the variable pump piston by the flow and pressure feedback valves. Vector frequency conversion PID control is then used to maintain constant pressure, automatically achieving adaptive pressure control. This system eliminates throttling and overflow energy losses in the oil circuit from the source, enabling the injection molding machine to achieve optimal energy-saving effects. The system is as follows: Working principle: When the oil pump flow rate Qb approaches or reaches the set value of the molding machine, the oil circuit system enters flow control mode, and the inverter's output frequency is proportional to the molding machine's flow rate set value. During this process, the oil pump flow rate Qb = Vb × n always adapts to the needs of the working circuit, eliminating the need for proportional valve speed regulation and excess flow overflow, fundamentally eliminating throttling and overflow energy losses. When the oil pump operates under pressure, as shown in the diagram below: The inverter control system is equipped with an intelligent conversion card. The inverter monitors this pressure change in real time through a pressure sensor and compares it with the molding machine's pressure set value. When the difference is very small, the inverter's intelligent conversion card will promptly switch the inverter to pressure control mode, activating PID control and maintaining a constant system pressure through the PID function. At this time, the inverter's output frequency will be proportional to this pressure difference. Because the difference is very small, the inverter's output frequency and motor speed will rapidly decrease, and the oil pump will only maintain a very small flow output to compensate for various leakage losses in the oil circuit. The electrical system diagram is as follows: The WIN-PSS-IV frequency converter is an open-loop vector inverter with high starting torque, reaching 150% of the rated torque at 0.5Hz; it has fast dynamic response, with a step torque response time of less than 200ms; and a speed stability accuracy of 0.5%. Through the closed-loop vector speed regulation function of the frequency converter, the oil pump speed n of the variable frequency injection molding machine can achieve a linear adjustment range from 0 to the rated speed. Especially when the molding machine is idle or not operating (such as during cooling), the motor and oil pump speeds can drop to 0, with virtually no energy loss. It is worth noting that the volumetric efficiency of the oil pump decreases at low speeds, which to some extent affects the energy-saving effect. To address this, we will improve the situation by increasing the low-frequency gain of the frequency converter. Furthermore, in actual production, flow rates below 20% are extremely rare, so the oil pump speed is unlikely to be excessively low. 5. Cost and Maintenance Analysis Based on our company's energy-saving retrofit examples of variable pump injection molding machines in a plastics factory in Dongguan, a 20% energy saving rate was still achieved after retrofitting a horizontal variable pump injection molding machine with a clamping force of 120 tons using frequency conversion. This demonstrates that the energy saving rate of frequency conversion injection molding machines is higher than that of variable pump injection molding machines. The maintenance analysis is as follows: 6. Conclusion Shenzhen Weineng Technology Co., Ltd. has accumulated valuable experience through years of practical application and developed various energy-saving solutions for the application of frequency converters in injection molding machinery. This time, they have developed a dedicated frequency converter for variable pump injection molding machines, which will undoubtedly lead a new trend in energy-saving retrofitting of plastics machinery.