Application of frequency converter in injection molding machine
Introduction of constant pressure pump and variable pressure pump
Constant pressure pump injection molding machine
At present, most reconstructed injection molding machines are fluid driven injection molding machines, and their fluid drive system is powered by motor-driven oil pump. The hydraulic oil pump is a vane pump, indicating that its fuel delivery is directly proportional to its speed. When the motor of the pump operates at a constant speed (50Hz), the fuel delivery will remain unchanged. Please refer to the following figure: A1: piston area other than rod cavity in oil cylinder F: working load Q1: work flow P1: working pressure △ P: differential pressure on both sides of proportional speed valve QY: overflow valve Pb flow: oil pump pressure QB: oil pump flow VB: displacement of oil pump n: motor speed
As shown above, the system uses a constant pressure pump for oil supply, and the motor speed n and the displacement of oil pump VB are constant.
QB = VB * n (QB is a constant) Pb is set by the overflow valve Q1 is set by the constant speed valve P1 = f / A1 (P1 is determined by F)
The production process of injection molding machine usually includes several parts, and the flow and pressure of different parts are different. Previously, plastic manufacturers used flow valves and pressure valves to generate the required flow and pressure. However, this not only caused a lot of energy waste, but also increased the risk of equipment wear. In contrast, if VFD is installed, it can meet the production requirements by adjusting the motor speed of the injection molding machine to produce appropriate flow, and greatly reduce overflow, so as to save a lot of energy.
2) Variable displacement pump injection molding machine
Variable displacement pump injection molding machine usually uses proportional variable displacement pump as its driving component. Please refer to the following figure: the system consists of load sensing proportional variable piston pump, proportional overflow valve, proportional speed valve, pressure feedback valve and flow feedback valve. A1: piston area other than rod cavity in oil cylinder F: working load Q1: work flow P1: working pressure △ P: differential pressure on both sides of proportional speed valve Pb: oil pump pressure QB: oil pump flow n: motor speed
(1) When the system is in flow control mode, Pb is greater than P1 but less than the rated pressure of the injection molding machine. The proportional relief valve will close. When the load changes, △ P will change, so Q1 will change. These changes will be transmitted to the regulating piston through the flow and pressure feedback valve. The regulating piston will drive the inclined plate of the variable displacement pump accordingly to change the displacement of the oil pump and finally determine QB.
(2) When the system is in pressure control mode, the inclination of the variable displacement pump swash plate is very small, so only a small amount of hydraulic fluid passes through the proportional relief valve to ensure appropriate pressure. The corresponding output pressure can be obtained by changing the input signal of the proportional relief valve.
Energy saving principle of variable displacement pump injection molding machine
From the above introduction, we can draw a conclusion that compared with the constant pump injection molding machine, the variable pump injection molding machine can reduce a lot of overflow waste and keep the motor under light load. However, since the motor always runs at full speed and cannot adjust the speed, a lot of energy will still be wasted. In addition, the starting mode of the machine is star delta slow start, which leads to a very low power factor.
