Abstract: This paper first introduces the drive system of a traditional injection molding machine, and proposes three improvement schemes by analyzing the energy consumption of the system. The advantages and disadvantages of each scheme are analyzed in detail, and corresponding guidance is given. Finally, the paper introduces the Delta VJ injection molding machine dedicated servo driver and provides the corresponding system solution.
Keywords: Injection molding machine, servo drive, fixed displacement pump, frequency converter, PQ drive
1. Introduction
According to market analysis of plastic products, the current annual output value of China's plastic machinery industry is about 12 billion yuan, while the domestic market demand is nearly 22 billion yuan, leaving a large demand gap. The demand for injection molding machines is gradually spreading from the south to the north and from the east to the west, which also drives the rapid development of related industries such as the plastic industry.
The plastics industry has developed rapidly in recent years, with the injection molding sector experiencing a period of rapid growth. However, competition within the industry is also intensifying, and manufacturers are increasingly focusing on controlling production costs in addition to product quality and brand reputation. As can be seen from the injection molding process, electricity consumption accounts for a significant proportion of the cost of injection-molded products. Therefore, effectively reducing electricity consumption has become a major concern for both injection molding machine manufacturers and users.
With the deepening of national energy conservation and emission reduction policies, enterprises are considering how to save energy to respond to the national call and reduce production costs, especially in the injection molding machine industry. Achieving significant energy-saving retrofits has become a primary issue for improvement in the injection molding industry. This article introduces the traditional injection molding machine drive system, proposes three improvement schemes based on the energy-saving development trend of injection molding machines, analyzes their respective advantages and disadvantages, and finally presents an energy-saving solution based on Delta's dedicated servo drive for injection molding machines, which is worthy of industry reference.
2. Analysis of Traditional Injection Molding Machine Process and Energy Consumption
Ordinary injection molding machines typically use 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 that the injection molding machine accurately and effectively meets the requirements of the predetermined process (pressure, speed, temperature, time, position) and the action sequence.
In traditional injection molding machines, the hydraulic transmission system 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 motor. Most oil pumps are fixed-displacement pumps. The electric motor typically provides the rated power and speed. The oil pump converts the mechanical energy input from 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.
Injection molding is a process that follows a predetermined cycle of actions, namely, mold closing, nozzle advancement, injection, pressure holding, melting and cooling, mold opening, product removal, placing the injection molded part into the mold, and mold closing, to achieve the molding of a product.
The speed and pressure required for each process vary depending on the specific process, meaning the required hydraulic oil flow rate differs. Therefore, the entire operation of an injection molding machine is a variable load process for the hydraulic pump motor. In the hydraulic system of a fixed displacement pump injection molding machine, the hydraulic pump motor always provides a constant flow of hydraulic oil at a constant speed. Excess hydraulic oil during each operation flows back through the relief valve, resulting in a waste of electrical energy.
Injection molding machines are classified into vertical injection molding machines and horizontal injection molding machines according to their mold mounting structure, as shown in Figure 1:
Figure 1. Schematic diagram of vertical injection molding machine and horizontal injection molding machine.
Tests have shown that the energy consumption and number of stations per unit of a typical traditional injection molding machine are as follows:
Energy consumption of hydraulic systems: 75-80%
Energy consumption of the heating unit: 10-15%
Cooling system energy consumption: 5-10%
Energy consumption of control components: 1-5%
The largest energy consumption component is the hydraulic system, which accounts for over 75% of the electricity used in injection molding machines. Furthermore, processes such as mold clamping, injection, pressure holding, cooling, and mold opening require varying pressures and flow rates, placing the hydraulic pump motor under constant load fluctuations. When system demand exceeds the set flow rate and pressure, an overflow valve or proportional valve adjusts the pressure and flow rate; this process, known as high-pressure throttling, results in energy losses as high as 40%-75%.
3. Traditional Injection Molding Machine Drive System
The drive system of a traditional injection molding machine typically consists of three parts: a PQ controller, a fixed displacement pump, and an induction motor. The architecture diagram is shown in Figure 2.
Figure 2. Traditional injection molding machine drive system architecture
In traditional injection molding machines, the motor continues to drive the oil pump at high speed and full load during the pressure holding phase. Since the system is in pressure holding mode and no oil needs to be supplied, the oil pumped out returns to the oil tank via a safety valve. This results in wasted electrical energy, which is converted into heat, raising the oil temperature in the tank and reducing the lifespan of seals. In short, with traditional injection molding machine drive methods, the longer the pressure holding and cooling time, the more energy the system consumes.
4. Three Improved Driving Methods
4.1 Variable Pump Drive
The traditional injection molding machine drive method described above leads to increased energy waste. Therefore, the industry has introduced injection molding machines driven by variable pumps. This system typically consists of three parts: a PQ controller, a variable pump, and an induction motor, as shown in Figure 3.
Figure 3 Variable Pump Drive System Architecture
The biggest advantage of using a variable displacement pump is that, although the motor runs at high speed when the system is under pressure, the variable displacement pump will adjust its displacement according to the system pressure. That is, when the system pressure reaches the set value, the variable displacement pump will switch to a small displacement to reduce the pump's output flow during pressure holding, thereby reducing the motor's output torque, which in turn reduces the motor's power output and ultimately achieves energy saving.
In summary, the advantages of this solution are that it has a certain energy-saving effect, while the disadvantages are that it requires a PQ controller and the motor is still running at high speed, so the energy saving is not optimal.
4.2 Inverter Drive
The above-mentioned driving method is not very energy-efficient because the motor is still running at high speed. Therefore, the industry has also researched using frequency converters for driving. This system usually consists of four parts: PQ controller + frequency converter + variable frequency motor + fixed displacement pump, as shown in Figure 4.
Figure 4 Inverter drive system architecture
The biggest advantage of using a frequency converter to drive the motor is that when the system is in the holding pressure stage, the injection molding machine controller sends a proportional flow signal to the frequency converter, telling it to operate at low speed. The frequency converter then drives the motor to run at a very low speed, thus achieving energy savings. However, because the frequency converter has acceleration and deceleration times during startup and shutdown, the injection molding machine's operating cycle is slower than without a frequency converter. Therefore, frequency converters are not suitable for products with short holding pressure times and short operating times. Only injection molding machines with long holding pressure times and large tonnage can be considered for frequency converter drive. Furthermore, energy-saving retrofitting with frequency converters is the simplest, requiring virtually no changes to the injection molding machine's mechanical structure; simply adding a frequency converter to the three-phase input line is sufficient. Currently, Delta FG injection molding machine-specific frequency converters have successful applications in the injection molding machine industry.
In summary, the advantages of this solution are its simplicity and good energy-saving effect; the disadvantages are that when production capacity is being increased, it is necessary to switch back to mains power and still require a PQ controller.
4.3 Servo Drivers and Servo Motor Drives
In order to ensure that the injection molding machine's drive not only does not affect production capacity but can even increase it, the industry has begun to use servo drives and servo motors. This system typically consists of three parts: a built-in PQC servo drive, a servo motor, and a fixed displacement pump, as shown in Figure 5.
Figure 5 Servo driver and servo motor system architecture
The biggest advantages of using a servo drive are, firstly, energy saving. When the system is holding pressure, the injection molding machine controller simultaneously sends pressure and flow signals to the drive. The drive then uses PID control to adjust the servo motor at low speed, achieving energy savings. Secondly, production capacity is significantly improved. The servo drive responds very quickly to the servo motor, which can typically operate at speeds exceeding 2000 RPM. Therefore, when the injection molding machine is in operation except during the holding pressure phase, it is essentially running in speed mode, and the high motor speed accelerates the machine's cycle time, increasing production capacity. Thirdly, the system maintains excellent pressure stability during the holding pressure phase. This is because the drive system is a closed-loop control system. The injection molding machine system pressure is transmitted to the drive via a pressure sensor, and the drive's PID control quickly adjusts the response, ensuring system pressure stability.
In summary, the advantages of this solution are: energy saving, speed, space saving (no need to purchase an external PQ controller), low oil temperature, and small oil tank; the disadvantages are: more parts need to be replaced when retrofitting an old machine.
5 BWS Dedicated Servo Driver Solution
Currently, many domestic and international driver companies have launched servo drivers specifically for injection molding machines. Guangzhou Bowei Servo Technology has also launched its own BWS-BBZ servo driver specifically for injection molding machines, and the system diagram is shown in Figure 6:
Figure 6 BWS-BBZ Dedicated Servo Driver Solution
This solution has the following characteristics:
(1) Super energy saving: Under certain conditions, it saves 60% more electricity than the traditional fixed displacement pump hydraulic system.
(2) Low system oil temperature: The oil temperature can be 5-10 degrees lower than that of the traditional system.
(3) High repeatability: It achieves precise flow and pressure control.
(4) Long pressure holding time: better for large-sized plastic products.
(5) Fast response time: The speed response time is within 40ms
(6) Resistant to harsh environments: It adopts an oil-proof, vibration-proof, and dust-proof resolver encoder.
In summary, this solution can effectively implement energy-saving retrofits for injection molding machines, improve system operating efficiency, and is safe, reliable, and suitable for use in various industrial environments.
5 Conclusion
With the maturity of servo drive technology in the industry, dedicated servo drives are increasingly being used in the retrofitting of injection molding machines. Practice has proven that the servo drive solution based on the Guangzhou Bowei Servo Technology BWS-BBZ injection molding machine dedicated servo drive offers significant energy savings, high precision, and strong adaptability, making it worthy of promotion and application.
References
[1] Zhang Jiamin et al., Modification of Control and Power Components of Injection Molding Machine. Synthetic Resin & Plastics Industry [Journal] 2006
[2] Guo Zongren et al. Programmable Logic Controller Application System Design and Communication Network Technology. Posts & Telecom Press, 2003.
[3] BWS Servo User Manual, Guangzhou Bowei Servo Technology Internal Document, 2010
