1. The Development Trend of Variable Frequency Drive (VFD) Retrofitting in the Injection Molding Industry For the plastics industry, electricity consumption is a major component of production costs, and injection molding machines are among the main energy-consuming equipment in plastics factories. Currently, the vast majority of injection molding machines are hydraulically driven, with power supplied by an electric motor driving an oil pump. During the injection molding production cycle, the flow rate and pressure required by the injection molding machine vary at different stages, necessitating the adjustment of these requirements using flow and pressure valves. The load on the hydraulic system varies significantly. Because the fixed displacement pump's output power is not adjustable, excess energy is consumed through baffles, oil line leaks, and oil temperature rise. This exacerbates wear on various valves, causes excessively high oil temperatures, excessive motor noise, and shortens the machine's lifespan. Furthermore, users often design oil pump motors with a capacity far exceeding actual needs, resulting in a "large horse pulling a small cart" phenomenon and significant energy waste. Therefore, promoting the application of AC variable frequency speed control devices in injection molding machines is of great significance for reducing energy waste and lowering production costs. 2. Feasibility Analysis of Frequency Conversion Retrofit for Injection Molding Machines 2.1 Production Process of Injection Molding Machines Injection molding machines typically employ hydraulic transmission. Their structure includes an injection unit, a mold opening and closing unit, a hydraulic transmission unit, and an electrical control unit. The latter's function is to ensure the accurate and efficient operation of the injection molding machine's predetermined process requirements (pressure, speed, temperature, time, position) and action sequence. In traditional injection molding machines, the hydraulic transmission unit mainly consists of an oil pump, hydraulic control valves, pressure solenoid proportional valves, flow solenoid proportional valves, various actuating cylinders, an oil pump motor, and other hydraulic accessories and pipelines. The power in the hydraulic transmission system is provided by the oil pump driven by the electric motor. Most oil pumps are fixed-displacement pumps. The electric motor typically provides rated power and speed. The oil pump converts the mechanical energy input by the electric motor into pressure energy, and then delivers hydraulic oil with a certain pressure and flow rate to the hydraulic components of the hydraulic system, meeting the energy requirements for the hydraulic actuators to drive the load. The injection molding process is a predetermined periodic operation process, namely, mold closing, nozzle advancement, injection, pressure holding, melting, cooling, mold opening, product removal, placing the injection molded part into the mold, and ejection after mold closing, etc., to achieve the molding of a certain product. As shown in Figure 1. 2.2 Hydraulic loss in the injection molding process The hydraulic loss in the injection molding process mainly consists of the following three parts: (1) Overflow loss The hydraulic pressure, flow rate, and pressure required for each process are different depending on the product and processing steps. For the oil pump motor, the load of the injection molding machine in the injection process is in a changing state. The flow rate of the oil pump is designed according to the required maximum flow rate. The hydraulic flow rate provided by the original injection molding oil pump motor at a constant speed is lost when the flow rate required by the injection molding machine is less than the maximum flow rate. The excess hydraulic pressure will flow back through the overflow valve, and this part of the energy is lost. (2) Throttling loss When the hydraulic oil flows through the throttling port of the valve, there will be a certain pressure drop, which is the throttling loss. Since the throttling area of ​​the directional valve is relatively large, most of the throttling loss occurs on the proportional valve. At the same time, due to the long-term full-speed circulation of hydraulic fluid and the severe mechanical friction of hydraulic components, the oil temperature is too high, the noise is too loud, and the mechanical life is shortened. (3) Design margin loss In the design, commonality is usually considered. The design is based on the maximum capacity. Therefore, the capacity of the user's oil pump motor is much higher than the actual need. There is a phenomenon of "big horse pulling small cart", which causes a lot of waste of electrical energy. 2.3 Feasibility analysis of frequency conversion transformation of injection molding machine The process of injection molding machine is basically the same. It can be roughly divided into 7 process processes: mold closing, injection, pressure holding, melting, cooling, mold opening, and ejection. Each process requires different pressure and flow. That is to say, the workpieces being processed are not all made under the maximum pressure or flow. The pressure and flow are adjusted by the pressure proportional valve and the flow proportional valve. The pressure and flow are controlled by adjusting the opening degree of the pressure or flow proportional valve. However, during constant speed operation, the input power of the oil pump in each process does not change much. If the speed of the oil pump motor is automatically adjusted synchronously and proportionally according to the pressure or flow parameters required during operation by the frequency converter, the pressure and flow can be regulated. In this way, all the energy wasted on the proportional valve of the injection molding machine can be saved, achieving an economical effect that is both energy-saving and practical. Its specific effects are reflected in the following three aspects: (1) Speed ​​regulation and energy saving According to the process requirements of the injection molding machine, the switching signals of the main pressure valve, low pressure valve, first pressure valve, and second pressure valve are converted by the computer board of the injection molding machine and added to the input end of the frequency converter as the frequency given signal of the frequency converter. The output frequency of the frequency converter changes linearly with the analog signal of the proportional valve. In the process where the pressure and flow are small, the speed of the motor is reduced, thereby reducing the output power of the motor. During the cooling and semi-finished product placement process, the motor can be stopped, so that the energy loss of the motor in the entire load range is minimized. (2) Improve power factor and save energy Reactive power not only increases line loss and equipment heating, but more importantly, the reduction of the power factor leads to the reduction of the active power of the power grid. The power factor of a conventional quantitative pump injection molding machine is between 0.6 and 0.8. However, after using a variable frequency speed control device, the power factor is increased to over 0.96 due to the compensation effect of the filter capacitor in the frequency converter, thereby reducing reactive power loss and increasing the active power of the power grid. (3) Soft start energy saving: Since the original motor is directly started or Y-Δ reduced voltage started, the starting current is equal to (4-7) times the rated current. This will cause a serious impact on the electromechanical equipment and the power grid, and will also increase the grid capacity requirements. The large current and vibration generated during startup are extremely detrimental to the service life of the equipment. However, after using a variable frequency energy saving device, the soft start function of the frequency converter will make the starting current start from zero. The maximum value is limited to the current limit level set by the frequency converter during acceleration, generally not exceeding 1.7 times the rated current, which reduces the impact on the power grid and the grid capacity requirements, and extends the service life of the equipment and mold. 3 Control Circuit Scheme for Injection Molding Machine Frequency Conversion Retrofit 3.1 Control Circuit Diagram of Injection Molding Machine Frequency Conversion Retrofit (as shown in Figure 2) When the injection molding machine is converted to frequency conversion, it adopts frequency conversion + power frequency operation mode. The control circuit is shown in the figure below. The selection of frequency conversion operation mode, power frequency operation mode and stop operation is achieved by the multi-position switch WK. The purpose of using power frequency bypass is to be able to directly switch to power frequency operation when the frequency converter fails, without affecting production. 3.2 Frequency Conversion Operation Mode (1) Connect the frequency conversion power supply. When the multi-position switch WK is closed to the frequency conversion power supply, the contactor KMI coil is energized and the KMI auxiliary normally closed contact is opened, and the power frequency operation circuit cannot be energized, thereby realizing the interlocking of the frequency conversion operation circuit and the power frequency operation circuit; at the same time, the contactor KM4 coil is energized and the KM4 main contact is closed, and the motor is connected in a Δ configuration. When the KM1 main contact is closed, three-phase AC power is supplied to the frequency converter. When the normally open contact of the KMI auxiliary circuit closes, the coil of the KM2 contactor is energized and the main contact of the KM2 circuit closes, connecting the inverter to the three-phase AC motor circuit and waiting for the work command; at the same time, the inverter working indicator HL2 lights up. (2) Inverter speed regulation process. Press the inverter start switch SB, the inverter starts running, and the injection molding machine starts working. The injection molding machine computer board processes the pressure and flow signals of the hydraulic transmission of the injection molding machine into a 4-20mA current analog signal and sends it to ports 4 and 5 of the inverter. The inverter uses the 4-20mA current analog signal transmitted from the injection molding machine computer board to adjust the speed of the motor through the built-in PID regulation function, thereby realizing the control of the motor rotation by the pressure and flow signals of the hydraulic transmission of the injection molding machine, turning the constant pressure pump into a variable pressure pump, and meeting the different hydraulic and flow requirements of the hydraulic transmission of the injection molding machine. This greatly reduces the vibration during mold closing and opening, stabilizes the production process, improves the grade and quality of products, reduces mechanical failures, extends the service life of machinery, and also saves electricity, generating good economic benefits. (3) Protection and fault handling of the frequency converter. The frequency converter itself has multiple protection functions such as overcurrent, overload, overvoltage, undervoltage, and short circuit. It can not only protect the frequency converter itself, but also protect the motor, and further protect the hydraulic system and mechanical system of the injection molding machine, preventing the harm that may be caused by improper operation. Therefore, there is no need to set up a special protection circuit. If the pressure and speed required to perform the action exceed the pressure and speed of the injection molding machine system, the frequency converter will enter the protection state. For example, if the material is stored at low temperature, the frequency converter will have overcurrent or overload protection due to the overload operation of the injection molding machine, cutting off the power supply of the motor and protecting the machine from damage. If it is in the power frequency state, the injection molding machine can work overload with the help of powerful power, which may cause accidents such as twisting the barrel, screw or damaging the hydraulic motor. When the frequency converter malfunctions, the normally closed switches at terminals B and C open, the contactor KMI coil de-energizes and resets, the frequency converter's power supply is disconnected, and it stops working; the normally open switches at terminals A and C close, and the fault indicator HL1 illuminates. After the fault is cleared, the frequency converter restarts. 3.3 Power Frequency Working Mode When the multi-position switch WK is connected to the power frequency working power supply, the contactor KM4 coil is energized and operates, the KM4 auxiliary normally closed contact opens, and the frequency converter working circuit cannot be energized, thus achieving interlocking between the power frequency working circuit and the frequency converter working circuit; simultaneously, the motor starts and runs using the Y-Δ voltage reduction method, the injection molding machine starts working, and the power frequency working indicator HL3 illuminates. 4. Advantages of Convenient Wiring for Injection Molding Machine Frequency Conversion Retrofit When retrofitting an injection molding machine with frequency conversion, the original computer control system, hydraulic transmission system oil circuit, and motor Y-Δ reduced voltage starting and running circuit of the injection molding machine are retained unchanged. A frequency conversion energy-saving/power frequency operation switching control mode is adopted, avoiding disruption to normal production in case of malfunction. When changing molds, only the working program of the injection molding machine's computer board needs to be changed; no adjustment to the frequency converter is required. 5. Benefits of Injection Molding Machine Frequency Conversion Retrofit 5.1 Significantly Reduced Production Costs and Energy Savings After adopting frequency conversion retrofit, the energy saving rate of injection molding machines is as high as 25% to 60%; the power factor increases from 0.6 to 0.8 before the retrofit to over 0.96, significantly improving the power factor of the power grid, reducing reactive current, and thus reducing line losses, achieving the goal of reducing production costs and saving energy. 5.2 Improved Reliability of Injection Molding Machine System Operation: After the injection molding machine is retrofitted with frequency converters, the frequency converters themselves have multiple protection functions such as overcurrent, overload, overvoltage, undervoltage, and short circuit protection. This not only protects the frequency converter itself but also the motor, hydraulic system, and mechanical system of the injection molding machine, preventing potential hazards from improper operation and providing timely fault detection and alarms. 5.3 Extended Equipment and Mold Lifespan, Improved Product Grade and Quality: Utilizing the soft-start function of the frequency converter allows the starting current to start from zero, reducing the impact of the starting current on the power grid and the requirements for grid capacity. It also reduces mold opening and closing vibration, extending the lifespan of the equipment and molds, reducing noise, and improving the working environment. Simultaneously, the smooth speed regulation of the frequency converter significantly improves the grade and quality of the injection molding machine products. The adoption of frequency conversion energy-saving technology in the injection molding machine not only achieves significant effects in terms of high energy saving, high reliability, and extended equipment and mold lifespan but also greatly improves the grade and quality of the injection molding machine products. 6. Conclusion This paper addresses the issue of significant energy loss in the hydraulic transmission system of injection molding machines. A specific circuit scheme using frequency converter technology was designed for its modification. Practical application has shown that this approach improves the product quality and grade of the injection molding machine, extends the service life of the equipment and molds, reduces production costs, and saves electricity.