I. Overview of Injection Molding Machines
For half a century, plastics have been one of the main materials used in industry, agriculture, and civilian applications. Injection molding machines are specialized plastic molding machines. They utilize the thermoplasticity of plastics, melting them through appropriate heating, then rapidly pumping them into a mold cavity under high pressure. After a certain period of pressure holding and cooling, they are formed into various profiles and plastic products. The widespread application of frequency converters in injection molding machines can achieve advantages such as energy saving, improved productivity and product quality, reduced oil and noise pollution, and extended machine lifespan. Therefore, their use will be gradually promoted.
Injection molding machines are the main equipment for molding and processing various plastics (such as polyethylene and polypropylene). Their working principle is: plastic granules are heated at multiple temperatures in a twin-screw injection molding machine, and after melting, stirring and pressurizing, the fluid material is injected into the mold cavity to complete the molding process.
II. Current Status of the Production Process of a Quantitative Dispenser Injection Molding Machine
Injection molding machines and extruders are the main production equipment in the plastics industry, and also the main power-consuming equipment, often referred to as "electricity guzzlers," accounting for about 80% of the electricity used in enterprise production. The high power consumption of injection molding machines not only results in a huge waste of electricity for society as a whole, but also leads to high production costs for enterprises, a lack of product competitiveness, and seriously affects their economic benefits.
The injection molding process generally consists of several stages: mold clamping, injection, melting, pressure holding, cooling, and mold opening. Each stage requires different working pressures and flow rates. For the hydraulic pump motor, the injection process is under a changing load. In a fixed-displacement pump hydraulic system, the pump motor always provides a constant flow rate at a constant speed. Excess hydraulic oil is returned through a relief valve; this process is called high-pressure throttling. Statistics show that high-pressure throttling causes energy losses as high as 25-50%. Simultaneously, due to the long-term full-speed circulation of hydraulic fluid and the intense friction with hydraulic and mechanical components, excessive oil temperature, excessive noise, and shortened mechanical lifespan occur.
In a complete injection molding process, all types of injection molding machines follow the same process, generally consisting of five steps. The pressure and time distribution in these different steps can be observed as follows:
(1) Mold delivery process: low pressure and short time.
(2) Mold closing process: The left and right molds are connected until they are completely closed. The pressure is slightly higher and the time is short.
(3) Pressure holding process: The material is fed into the mold cavity until it is formed and cured. The pressure is high and the time is long, accounting for about 40-60% of the manufacturing time of a product.
(4) Demolding process: After processing and shaping, open the mold, demold, and take out the processed part. The pressure is slightly higher and the time is not long.
(5) Mold removal process: The workpiece is removed, the mold is retracted to its original position, auxiliary work is performed, and it is ready for processing again. The pressure is low and the time is short.
For the same injection molding machine, the pressure (P) and time (t) required to process different plastic parts will vary. This is due to factors such as the complexity of the mold, the type of plastic used, the total material usage for a single part, and the presence or absence of inserts. Specific parameter settings are generally determined by engineering technicians after testing.
III. Principles and Feasibility Analysis of Energy Saving in Injection Molding Machines
1. Theoretical Principles
Output power of oil pump: pt=p＊Qt=p＊v＊n (1)
Theoretical torque of the oil pump: Tt=1/2π×p×V （2）
P is pressure, Qt is flow rate, V is pump displacement, and n is pump speed. Substituting equation (2) into equation (1) yields:
Pt=2π×Tt×n (3)
If we ignore the energy loss during the conversion from mechanical energy to hydraulic energy, we can approximate the output power of the oil pump as the product of the motor's output torque and its speed. Therefore, it can be seen that when the system requires a low flow rate, the power required by the system is actually very low. However, in reality, because the motor always operates at a 50Hz power frequency, it cannot reduce its speed according to actual needs, thus preventing a reduction in flow rate. Therefore, excess hydraulic oil can only flow back to the tank through the proportional flow valve, resulting in a waste of energy.
2. Oil pump variable frequency speed regulation for energy saving operation
As shown in Figure 1, during the P=f(t) process of the injection molding machine, the pressure of its main oil pump varies significantly across different time periods, indicating a potential for energy savings. Statistics show that injection molding machines can typically achieve energy savings of 25%-50%. It is well known that without a frequency converter, the main pump motor of an injection molding machine operates at a constant speed, which is extremely uneconomical.
An energy-saving controller for injection molding machines uses a 0-1A current signal controlling the proportional flow valve. Based on the flow ratio set for each production stage, a 0-1A proportional current meter is used to open the solenoid valve proportionally, providing the required system pressure. Excess flow is discharged through an overflow valve. The proportional flow signal is connected to the energy-saving controller as a drive command to control the speed of the hydraulic drive motor. The oil pump only replenishes the portion consumed in the oil circuit. The oil pump motor operates at the corresponding speed according to the proportional command, outputting the corresponding flow to provide the required system pressure. In other words, only the required amount is provided, preventing waste at the source. Therefore, the variable frequency energy-saving system has a significant energy-saving effect throughout the entire injection molding process, resulting in substantial energy savings and considerable economic benefits for users.
IV. Current Status of Hefei Xingtong Rubber & Plastics Co., Ltd.
Based on our understanding and research of Hefei Xingtong Rubber & Plastics Co., Ltd., your company possesses a series of computer-controlled injection molding machines, including 2800T, 2000T, 1450T, 1200T, 800T, 500T, 360T, 250T, and 80T models, with a total motor power of approximately 1000 kilowatts. Based on a theoretical energy saving rate of approximately 30%, and assuming all machines operate at full load, the overall energy saving potential is approximately 200 to 300 kilowatt-hours per hour, resulting in considerable economic benefits.
To verify the energy-saving potential of injection molding machines, we suggest that your company select an 80T injection molding machine and calculate its average hourly power consumption under full load operation at the power frequency by connecting a power meter. Then, perform frequency conversion energy-saving retrofit on this machine and calculate the power consumption under frequency conversion energy-saving mode. By comparing the data, your company can see the energy-saving potential of the retrofit. Once the effect is significant, you can gradually implement energy-saving retrofits on other machines.
