Abstract: This paper mainly describes the working principle and main components of an extrusion blow molding machine, and the application of frequency conversion energy-saving technology in the transformation of a dual-station extrusion blow molding machine. It also elaborates on the necessity and feasibility of promoting this energy-saving technology. 1 Introduction In today's society, while the economy is developing rapidly, energy is becoming increasingly scarce. The contradiction between development and energy is becoming more and more prominent, and energy shortage has become a major issue in social development. Governments at all levels have put forward more specific and stricter quantitative targets for reducing energy consumption and saving energy. For enterprises, saving energy is an inescapable responsibility. Therefore, in recent years, energy-saving technologies and products in various industries have been continuously promoted and applied. Among them, frequency conversion energy-saving technology is one of the main energy-saving technologies and has been widely used in plastics, cement, chemicals, central air conditioning, and other fields. The plastics industry is one of the major industries, and its electricity consumption accounts for a large proportion of industrial electricity consumption. With increasingly fierce market competition, how to reduce production costs and improve product market competitiveness has become a concern for all plastic companies. Among the production costs of various companies, the electricity consumption of plastic equipment accounts for a large part, and there is a lot of energy waste in existing equipment. This article takes the Guozhao GZS series energy-saving device designed by Shenzhen Guozhao Energy Saving Technology Co., Ltd. as an example to illustrate the application of frequency conversion energy-saving technology in the energy-saving transformation of plastic equipment—a dual-station extrusion blow molding machine, and elaborates on the necessity and feasibility of promoting this energy-saving technology. 2. Working Principle and Composition of Extrusion Blow Molding Machine 2.1 Working Principle The blow molding machine is one of the main molding equipment for hollow products. It mainly includes extrusion blow molding machines, injection blow molding machines, stretch blow molding machines, and asymmetric blow molding machines. The extrusion blow molding machine uses the barrel heating of the extruder unit and the shearing action of the screw to plasticize the plastic. The rotating screw extrudes the plasticized and homogenized plastic through the die head to form a preform. The mold clamping device clamps the preform, and the cutting device cuts it. Before cutting, pre-blowing is performed, mainly to prevent the preform wall from sticking. The mold clamping air blowing device seals the preform cut and blows in air at a certain pressure, making the preform adhere tightly to the inner wall of the mold cavity. After cooling, the waste edges are removed, and the mold clamping device separates, thus obtaining the desired hollow product. The flowchart is shown in Figure 1. Figure 1. Process flow of extrusion blow molding machine. 2.2 Main components. The extrusion blow molding machine mainly consists of an extruder, die head, mold closing device, air blowing device, hydraulic transmission device, heating and cooling control system, and electrical control system. The external view of the equipment is shown in Figure 2. Figure 2. External view of extrusion blow molding machine. The mechanical devices mainly include the extrusion device (screw extruder), die head, mold moving mechanism, mold opening and closing mechanism, lifting mechanism, air blowing device, billet electric heating cutter device, and die lifting head device. The pneumatic device is mainly used for cylinder operation and product blow molding. The cooling device mainly uses cooling water for cooling, which is mainly used for cooling the mold, barrel, blow needle, etc. The electrical control system mainly consists of the extruder electrical drive control system, oil pump motor drive system control, die head and extruder heating and temperature control system, action program control system (PLC), human-machine interface, etc. The schematic diagram is shown in Figure 3. Figure 3. Electrical control schematic diagram of extrusion blow molding machine. 2.3 Dual-station extrusion blow molding machine. As mentioned above, blow molding machines mainly include extrusion blow molding machines, injection blow molding machines, stretch blow molding machines, and asymmetric blow molding machines. Among them, extrusion blow molding machines are further divided into single-station extrusion blow molding machines, dual-station extrusion blow molding machines, and multi-layer extrusion blow molding machines. Dual-station extrusion blow molding machines are mainly used for hollow products with long cooling times. They have two sets of mold closing and mold moving devices. While the product is cooling, another set of molds performs mold closing and molding actions. Compared with single-station machines, dual-station machines make full use of the cooling time when producing products, which increases output and ensures quality. This article takes dual-station extrusion blow molding as an example to introduce the application of frequency conversion energy-saving technology in this equipment. 3. Frequency Conversion Energy-Saving Retrofit of Dual-Station Extrusion Blow Molding Machine 3.1 Frequency Conversion Energy-Saving Principle In the original system of the dual-station extrusion blow molding machine, the oil pump motor always runs at a constant speed. When the pressure required by the process changes, it is adjusted by the proportional flow rate of the oil circuit system. Excess hydraulic oil is discharged back to the oil cylinder through the overflow valve, which causes a large amount of energy waste. According to the working characteristics of pump load, the pump flow rate is proportional to the speed, the head is squared to the speed, and the motor shaft power is cubed to the speed, as shown in the following formula: Q2/Q1=N2/N1 H2/H1=（N2/N1）2 P2/P1=（N2/N1）3 Where: Q—flow rate; N—speed; H—head. As shown in Figure 4, when the flow rate decreases from QA to QB, due to the pipe resistance characteristics, the operating point shifts from point A to point B. The power consumed is proportional to the area of ​​0QBBHB. If frequency conversion control is used, the pipe resistance characteristics remain unchanged, and the operating point shifts from point A to point C. The power consumed is proportional to the area of ​​0QBCHC. From Figure 4, it can be seen that frequency conversion speed regulation is more energy-efficient than valve control. Moreover, as the speed decreases, the motor power decreases cubically. If the oil pump motor speed can be adjusted in a timely manner according to the injection molding process, energy saving can be achieved. Currently, most three-phase asynchronous motors use frequency conversion speed regulation. The synchronous speed formula for the motor is: n=60(1-S)f/p; where: S—slip rate; f—power supply frequency; p—number of pole pairs. As shown in the above formula, the motor speed can be changed by altering the power supply frequency. Adjusting the oil pump motor speed according to the process of the hollow blow molding machine minimizes the backflow of the overflow valve, ensuring the hydraulic system output power matches the required production power. This achieves energy saving; according to incomplete statistics, the motor's energy saving rate can reach 25%–65% after the modification. 3.2 Electrical Control Scheme The electrical system modification of the dual-station extrusion blow molding machine mainly involves collecting action signals such as mold closing, mold head rising, mold head falling, mold frame left movement, mold frame right movement, and mold opening. These signals are first sent to a dedicated controller for processing. The controller outputs signals to control the oil pump motor frequency converter, adjusting the oil pump motor speed according to process parameters to match the oil pump motor output power with the required process power, thus achieving energy saving. A schematic diagram of the control system is shown in Figure 5. Extrusion blow molding equipment is typically designed with a large margin for the maximum speed and pressure required by the process. However, actual production often doesn't reach the maximum design value. Based on actual working conditions and product process requirements, appropriately reducing the oil pump speed and oil supply ensures efficient utilization of the oil pump motor's output power without affecting the production process, reducing energy waste. Multi-stage speed control of the oil pump motor is implemented according to the blow molding process. For example, when higher pressure and speed are required during mold closing, the pump speed can be increased; conversely, when lower pressure and speed are required during mold opening, mold frame left and right movements, the pump speed can be reduced. However, the pump speed remains constant throughout each process stage. This control scheme ensures stable pump output when process parameters are adjusted, preventing any impact on product quality and stability. 3.3 Guozhao Energy Saving Device Introduction 3.3.1 Product Introduction: The Guozhao GZS series energy-saving device is a professionally designed energy-saving device by Shenzhen Guozhao Energy Saving Technology Co., Ltd., based on the process characteristics of blow molding equipment. It comes in three models: single-motor frequency converter GZS-Z1 series, dual-motor frequency converter GZS-Z2 series, and multi-motor frequency converter GZS-Z3 series, suitable for various blow molding equipment. This product adopts national patented technology and mainly consists of signal acquisition, signal processing controller, dedicated frequency converter, and power saving and mains power conversion circuit. Compared with existing energy-saving devices on the market, the Guozhao GZS series energy-saving device has the following characteristics: It adopts a dedicated signal controller, and corresponding control software can be written according to different types of blow molding equipment; it adopts a multi-speed control mode, and the frequency of the inverter can be adjusted in segments according to process requirements; it adopts a dedicated inverter with strong overload capacity and fast response speed, with acceleration and deceleration time down to 0.3s; it has multiple automatic protection functions such as motor overcurrent, overvoltage, undervoltage, power failure, overload, and output short circuit; it has a voltage stabilization function, and can automatically adjust the output frequency to achieve voltage stabilization when the grid voltage fluctuates; it adopts patented control technology, and the energy saving effect is more significant than that of general analog control and variable pump control schemes. 3.3.2 Design Features In terms of circuit design, the main circuit adopts an independent design for the mains circuit and the energy-saving conversion circuit (as shown in Figure 6). If the energy-saving device malfunctions during operation, it can be directly switched to the mains circuit through the conversion switch without affecting production. Figure 6 Schematic diagram of Guozhao energy-saving device circuit In terms of control circuit, its control signal is taken from the switching signals (solenoid valve control signals) of each process of the original equipment. After isolation processing by the energy-saving device signal acquisition circuit, it is sent to a dedicated processor for processing, so that the control signal is not easily affected by external interference. 4. Application of Energy Savers 4.1 Equipment Selection A container company in Dongguan mainly produces various plastic containers. It has 15 dual-oil-pump, dual-station extrusion blow molding machines (model DHD-2L) with a production capacity of 2L containers. The original extruder unit already has frequency conversion speed regulation. The oil pump motors are being retrofitted with frequency conversion. The main oil pump motor is a 22kW three-phase asynchronous motor, and the auxiliary oil pump motor is a 7.5kW three-phase asynchronous motor. A Guozhao GZS-Z2-4T-0220kW (dual motors, main motor frequency converter 22kW, auxiliary motor frequency converter 7.5kW) energy saver is selected for the retrofit. 4.2 Circuit Implementation The circuit control for the oil pump motor retrofit is relatively simple. The main circuit retains the original star-delta reduced voltage starting circuit (for the 22kW oil pump motor), which is equivalent to connecting the energy saver in series between the original power supply and the motor starting circuit. The main circuit is shown in Figure 7. Figure 7 shows the main circuit diagram of the energy-saving retrofit of the dual-station extrusion blow molding machine. Its control circuit mainly realizes functions such as signal acquisition, signal processing, status indication, and switching between mains power and energy saving. A schematic diagram is shown in Figure 8. Figure 8: Schematic diagram of the control circuit for the energy-saving retrofit of the dual-station extrusion blow molding machine. 4.3 Parameter Settings Based on the working characteristics of the oil pump motor, multi-speed control is implemented for the main oil pump motor, while single-speed control is used for the auxiliary oil pump motor. The inverter parameter settings for the main oil pump motor are shown in the attached table. The blow molding process of a product is relatively complex; in the original system, the oil pump motor operated at a constant speed. 4.4 Equipment Debugging and Precautions The oil pump provides the rated oil volume. The actual oil volume required for each process is adjusted by proportional valves and overflow valves. The process parameters corresponding to each process of each different product are generally not significantly adjusted during production. After system modification, the oil volume is adjusted by changing the oil pump speed. Therefore, during debugging, the energy-saving device and the blow molding machine should be debugged together to achieve the best energy-saving effect. The following aspects should be noted during system debugging: 4.4.1 Preparations before debugging After the energy-saving device is installed, before powering on, check whether the circuit is correct, especially the main circuit. Pay attention to whether the input and output lines are reversed. Otherwise, the energy-saving device may be damaged. The motor reduced voltage starting circuit (often using Y-Δ starting) in the original equipment should remain unchanged. If other circuit power supplies are connected to the front end of the motor Y-Δ starting circuit contactor, they should be moved to the power supply end of the energy-saving device during modification, and attention should be paid to the voltage level. Otherwise, the equipment will not be able to operate normally. In addition, the control signal circuit should be checked, and attention should be paid to whether the polarity and voltage value of the control signal are correct. After confirming that everything is correct, power can be turned on and the power indicator light of the energy-saving device should be observed to see if it is normal. 4.4.2 Mains Power Debugging: After powering on the energy-saving device, first perform "Main Power" debugging. Switch the energy-saving device selector switch to the "Main Power" position, start the equipment oil pump motor, and perform manual debugging. Manually execute each action, such as mold opening, mold closing, mold left movement, and mold right movement, while observing whether the energy-saving device signal acquisition indicator light is corresponding. At this time, each action of the blow molding machine will be executed according to the manual command. If it is found that the blow molding machine does not perform any actions such as mold closing or mold opening during debugging, it may be that the oil pump motor is reversing. You can determine whether the oil pump motor is reversing. If it is reversing, first disconnect the power supply and then adjust the mains contactor output line. After the manual debugging is normal, trial production can be carried out with the product. When the quality and performance of the produced products meet the process requirements, the energy-saving state debugging can be performed. First, stop the oil pump motor and switch the energy-saving device selector switch to the "Energy Saving" position. 4.4.3 Energy Saving Mode Debugging: When debugging the energy saver in "energy saving" mode, adjustments should be made according to the process parameters. The general principle is to first set the operating frequency of the energy saver based on experience (as shown in the attached table), and then adjust the corresponding process parameters such as mold closing, mold opening, and mold frame left or right movement according to the type of product being produced. In energy saving mode, the parameter values ​​should be slightly larger than the requirements set in mains power mode, preferably not exceeding 10%. If adjusting the process parameters still cannot meet the requirements, the operating frequency of the corresponding section of the energy saver can be adjusted appropriately until all phase indicators of the products produced in energy saving mode meet the requirements. 5. Energy Saving Effect Analysis According to measured data, before the modification, the oil pump motors (two motors with rated power of 22kW and 7.5kW respectively) consumed a total of 17.5 kWh per hour. After the modification, the total power consumption was 11.5 kWh per hour, resulting in a total energy saving rate of 34.2%. The factory's equipment operates an average of 26 days per month, 22 hours per day. Therefore: Monthly electricity savings: = (17.5 - 11.5) × 22 × 26 = 3432 kWh; Annual electricity savings: = 3432 × 12 = 41184 kWh; Calculated at an average price of 0.9 yuan per kWh, each piece of equipment saves approximately 40,000 yuan annually. The company's 15 hollow blow molding machines have been operating stably since the modification, with no malfunctions. The quality and performance of the products produced after the modification meet the process requirements. The total investment in the equipment is approximately 500,000 yuan, with monthly savings of 50,000 yuan, meaning the investment is recovered in approximately 10 months. 6. Conclusion Currently, most equipment in the plastics industry still uses hydraulic transmission, with oil pump motors consuming a large proportion of electricity. Practical applications have proven that using frequency conversion energy-saving retrofit technology to reduce energy consumption and save costs is feasible and necessary. On the other hand, frequency conversion energy-saving technology is constantly maturing and improving, and its application in various industries is becoming increasingly widespread. The energy-saving technologies described above in blow molding equipment are provided for exchange and reference by relevant industry professionals.