Abstract This paper describes the modification of an injection molding machine using frequency converter technology. By controlling the flow rate and pressure of the hydraulic system, the operating speed of the hydraulic pump changes accordingly with the working conditions of the injection molding machine. This achieves real-time control of the working flow rate and pressure of the hydraulic system, avoiding energy waste and reducing production costs. Keywords Injection molding machine, frequency converter, hydraulic system, energy saving Injection molding is one of the main molding methods for producing various products from thermoplastics. Compared with other plastic molding methods, injection molding has advantages such as the ability to mold complex and dimensionally precise products, strong adaptability to plastics, short molding cycle, high production efficiency, and ease of automation. Globally, injection molding accounts for about 30% of the total plastic molding volume, while the annual production volume (number of units) of injection molding machines accounts for more than 50% of the total annual output of plastic processing machinery. The hydraulic system is an important component of the injection molding machine. Currently, most injection molding machines at home and abroad use hydraulic systems to complete injection molding processes such as plasticizing, mold closing, injection, pressure holding, and mold opening. Therefore, the demand for electricity is extremely high, and energy consumption has become one of the main costs in injection molding production. In recent years, some small and medium-sized injection molding enterprises in the Pearl River Delta region have used frequency converter technology to carry out energy-saving renovations on the hydraulic systems of injection molding machines, significantly reducing the power consumption in injection molding production, thereby significantly reducing the production cost of injection molded products and bringing considerable economic benefits to the enterprises. 1 Working Characteristics of Injection Molding Machines 1.1 Working Cycle The injection molding process of an injection molding machine is a cyclical process that begins with the mold closing action. In this cyclical process, the hydraulic system must complete a series of actions: mold closing, mold opening, overall movement of the injection seat, pressure holding, and ejection of the product, etc. The working cycle of an injection molding machine is shown in Figure 1. [align=center] Figure 1 Working Cycle of Injection Molding Machine[/align] 1.2 Working Parameters The main working parameters of an injection molding machine that are closely related to the hydraulic system in a working cycle include injection pressure, injection rate, clamping force, and holding pressure. Injection pressure is an important factor in overcoming the flow resistance of molten material flowing through the nozzle, runner, and mold cavity, ensuring mold filling stability, and ensuring the accuracy and pass rate of injection molded products. When the screw diameter is constant, the magnitude of the injection pressure is determined by the hydraulic system. Injection rate is a parameter characterizing how quickly the melt fills the mold cavity. It directly affects the quality and productivity of the product, and its magnitude is controlled by the working oil flow rate of the injection cylinder. Clamping force is the maximum clamping force that the clamping mechanism of the injection molding machine can apply to the mold. It, along with the injection volume, constitutes a parameter of the nominal injection molding machine specification. For hydraulic-toggle clamping devices, the clamping force is determined by both the hydraulic system and the toggle mechanism; for fully hydraulic clamping devices, the clamping force is determined by the injection molding machine's hydraulic system. Holding pressure is one of the important injection molding process parameters. The selection of holding pressure and holding time directly affects the quality of the injection molded product. Like injection pressure, holding pressure is determined by the hydraulic system. 2. Injection Molding Machine Hydraulic System The hydraulic system is an important component of the injection molding machine. It must meet the different pressure and speed requirements of each stage of the entire work cycle. The hydraulic system should have good operational stability, high repeatability and sensitivity, low noise, and energy efficiency. The hydraulic system mainly consists of a hydraulic pump, valves, cylinders, and circuits. In injection molding production, the working flow and pressure provided by the hydraulic system to the injection molding machine should be constantly changing according to the needs of the injection molding process. Therefore, the power consumed by the hydraulic system of the injection molding machine should also change constantly with the changes in working flow and pressure. Figure 2 is a schematic diagram of the change in the working flow of hydraulic oil provided by the hydraulic system in one working cycle of a domestic XS-ZY-50 injection molding machine, and Figure 3 is a schematic diagram of the change in the power consumed by the cylinder. As can be seen from Figures 2 and 3, the working flow and power consumption of the hydraulic system vary greatly at different stages of the injection cycle, with the stage requiring the maximum working flow and power consumption being the mold closing injection stage. The hydraulic pump is the power source of the hydraulic system of the injection molding machine, providing the pressure oil required to complete the actions of the cylinder and hydraulic motor. Most hydraulic pumps in injection molding machines are fixed displacement pumps. The working flow and pressure of the hydraulic oil required for different actions in the injection process are regulated through a series of valves and related circuits. Since the pump's flow rate is constant, it means that the flow rate is at its maximum working flow rate at every stage of the injection molding cycle. In processes where the maximum working flow rate is not required, excess pressurized oil flows back to the oil tank through the relief valve circuit. The motor driving the hydraulic pump always maintains the speed required to maintain the maximum working flow rate, thus the power consumed by the motor remains at its maximum power during the injection molding cycle, resulting in a significant waste of electrical energy. Adding a frequency converter circuit to the hydraulic circuit of the injection molding machine allows the frequency converter to change the power frequency of the drive motor according to the load changes during the injection molding cycle. This ensures that each specific hydraulic working flow rate corresponds to a different motor speed (frequency), achieving real-time control of the hydraulic system's working flow rate and pressure. The motor speed changes in real-time according to the workload, thus saving electrical energy. 3. Variable Frequency Speed ​​Regulation 3.1 Working Principle Hydraulic system flow rate adjustment is usually achieved by adjusting the opening degree of the hydraulic valve, i.e., increasing the working pressure of the hydraulic system by increasing the resistance of the hydraulic circuit. However, the hydraulic pump speed does not change, resulting in a significant waste of electrical energy in overcoming circuit resistance. Using a frequency converter to control the working flow of the hydraulic system of an injection molding machine in real time means adjusting the speed of the motor according to the flow value required by the injection molding process, thereby replacing the adjustment of various hydraulic valves, realizing stepless speed regulation of the hydraulic pump output flow, and greatly reducing the use of various valves. Since there is a corresponding relationship between the working flow and working pressure of hydraulic oil, using a frequency converter to control the working flow of the hydraulic pump can achieve the regulation of working pressure by adjusting the flow without changing the hydraulic circuit resistance. The speed of the AC motor can be expressed by equation (1): As can be seen from equation (3), when the values ​​of 5, q, and p are constant, the working flow of the hydraulic pump is linearly related to the frequency of the drive motor, so that the flow can change smoothly and achieve the purpose of stepless speed regulation. 3.2 System Composition Figure 4 is a schematic diagram of the working process of adjusting the flow of the hydraulic pump of the injection molding machine using a frequency converter. The signal acquisition device (sensor) collects the required hydraulic oil flow rate at different stages of the injection molding process and transmits the flow rate setpoint as a current value to the PLC (Programmable Logic Controller). Simultaneously, an output flow detection device detects the actual hydraulic oil output flow rate of the pump. The obtained detection signal is converted into a voltage value by a current/voltage converter and transmitted to the PLC. The PLC compares the actual flow rate with the set flow rate. Based on the comparison result, the PLC outputs the set rotation speed of the drive motor to the frequency converter. The frequency converter adjusts the rotation speed of the drive motor/pump, thereby regulating the output flow rate of the hydraulic pump to meet the different flow requirements at each stage of the injection molding cycle, achieving energy saving. [align=center]Figure 4 Schematic diagram of flow regulation of the hydraulic system of injection molding machine by frequency converter[/align] 3.3 Characteristics and selection of frequency converters Frequency converters used for speed regulation of drive motors are required to have the following characteristics: ① Operate at a very small slip rate, with low loss and high power factor; ② Wide speed regulation range, with frequencies from a few hertz to several hundred hertz, and high speed regulation accuracy, which can reach 0.5% to 1%; ③ Can automatically and smoothly accelerate and decelerate; ④ Significant energy saving effect, generally reaching more than 30%; ⑤ Complete protection functions and very little maintenance workload. The selection of a frequency converter depends on the actual operating conditions of the injection molding machine. For injection molding machines with low speed control requirements and small load variations, the relatively inexpensive SANKEN SVF series frequency converter can be selected. For injection molding machines with high speed control requirements and small speed errors, ABB series or FUJI GT series frequency converters can be selected. For injection molding machines with large load variations and high reliability requirements, FUJI G7 series frequency converters can be selected, which can limit the output torque by controlling the slip rate of the drive motor. The selection of frequency converter capacity should follow these guidelines: ① The rated current of the frequency converter is greater than or equal to the rated current of the drive motor; ② The rated voltage of the frequency converter is greater than or equal to the rated voltage of the drive motor; ③ The rated power of the frequency converter - the power of the matched motor is greater than the rated power of the drive motor. 4. Example In practical applications, a frequency converter was used to modify the hydraulic system of a certain type of injection molding machine. The rated power of the hydraulic pump drive motor of this injection molding machine is 1kW. Before the modification, the useful power when using a power frequency supply was 7.5kW. After the frequency conversion modification, the useful power is approximately 4.5kW, a reduction of about 3kW. Assuming the injection molding machine operates for 5000 hours per year, the annual energy saving is: 3kW x 5000h = 15000kW·h = 5400MJ. Taking the industrial electricity price of 1.2 yuan/(kW·h) in the Pearl River Delta region as an example, the annual electricity cost saving is: 15000kW·h x 1.2 yuan/kW·h = 18000 yuan. Therefore, the economic benefits of frequency conversion modification are considerable, and the larger the scale of injection molding production, the more substantial the energy savings and the more obvious the economic benefits. The initial investment in modifying the injection molding machine can also be recovered in a relatively short period. 5. Conclusion The advantages of using frequency converter technology to retrofit the hydraulic system of injection molding machines are significant. It not only saves energy dramatically, but also reduces motor temperature rise, decreases mechanical wear, lowers operating noise, and greatly facilitates maintenance, thus extending the service life of the injection molding machine and molds. This AC frequency conversion speed control technology can be used not only in injection molding machines but also in extruders to adjust the speed of the extrusion screw. Therefore, it is a highly practical production technology that will undoubtedly be widely applied in the plastics processing industry. Further research suggests that frequency converter technology can be introduced into the manufacturing process of injection molding machines and extruders to improve their performance and efficiency, enabling them to achieve energy-saving technology in one step and increasing their market competitiveness. 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