Application of Sunwoele VFD (Variable-frequency Drive) on injection molding machine

The injection molding machine is a device that heats, melts, stirs, and pressurizes various plastics, injects the plastic fluid into the mold control, and completes the one-time injection molding of the workpiece. Its process is basically the same, and each process requires different pressure and flow, that is to say, not all the workpieces to be processed work under the maximum pressure or flow, and the pressure and flow in the traditional way rely on the pressure proportional valve. And flow proportional valve to adjust. However, the oil pump motor runs at a constant speed, resulting in a waste of energy. If VFD (Variable-frequency Drive) is used to adjust the speed of motor (oil pump) to meet the demand of pressure and flow in real time, it is economical and practical. Relevant units have successfully used our company's Sunwoele VFD (Variable-frequency Drive) to transform the injection molding machine, and achieved significant economic benefits. Therefore, using VFD (Variable-frequency Drive) to transform the injection molding machine for energy saving is worth promoting to the majority of users. . According to practical experience, the power saving rate after transformation of VFD (Variable-frequency Drive) for medium and small injection molding machines is generally 20% to 60%.

1. Overview of injection molding machine

As we all know, the injection molding machine is the main equipment for molding and processing various plastics (such as polyethylene and polypropylene). The plastic particles are heated in multiple stages in the twin-screw injection molding machine, and the fluid material is injected into the mold cavity after melting, stirring and pressurizing. Inside, complete the molding process. Zhejiang Province is one of the provinces with serious shortage of electricity in China, especially in summer, and injection molding machines are spread all over the province, so power saving is imperative. Our company cooperates with Ningbo agents to inspect the injection molding machine site of a company in Ningbo The frequency conversion transformation has been carried out, the total installed capacity is more than 400 KW, and more than 20 injection molding machines. Among them, the maximum capacity of a single machine is 37KW, which has been put into production for one year, works normally, and has an average power saving rate of more than 30%. At the same time, the noise in the working environment is significantly reduced, and the oil temperature of the system is greatly reduced; the smooth soft start protects the motor and prolongs the service life; A large amount of maintenance costs; the motor has multiple protections for overvoltage, overcurrent, and lack of phase; the frequency conversion and power frequency can be switched at will to ensure that the production will not be affected when the frequency conversion fails; the system is easy and fast to install. Well received by users. In view of the above advantages, it is correct and effective to apply VFD (Variable-frequency Drive) to implement energy-saving technical transformation of injection molding machines, and it is worth promoting to everyone.

2. Process of injection molding machine

Regardless of the large, medium or small injection molding machine, the process of analyzing it is the same, roughly five processes, as shown in Figure 1.

From Figure 1, we can see that the process of injection molding machine is:

①The mold feeding process requires low pressure and short time.

②In the mold closing process, the left and right molds are connected until they are completely closed. A slightly higher pressure is required and the time is not long.

③In the process of injection, pressure holding, and cooling, materials are fed to the mold cavity until it is formed. After curing, high pressure is required, and the long time accounts for about 40-60%.

④Demolding process, processing and forming, the mold can be opened, and the workpiece can be taken out after demoulding is completed, the time is not long, and the pressure is slightly high.

⑤ In the process of mold withdrawal, the workpiece is taken out, the mold is retreated to its original position, and after auxiliary work is performed, it needs to be processed again. At this time, low pressure is required and the time is short.

For the same injection molding machine, the pressure (p) and the length of time are different for processing different plastic parts. It is related to the complexity of the shape of the workpiece, what kind of plastic to use, the total material volume of a workpiece, whether there are inserts, etc. The specific parameter settings are generally given by the craftsman, and after the test processing, final.

3. Possibility Analysis of Energy Saving for Injection Molding Machines

From the P=f(t) process of the injection molding machine in Figure 1, it can be seen that the pressure of its main oil pump changes with different time periods, and the fluctuation is large, which has the possibility of saving energy and has a great potential , so the power saving rate of the injection molding machine can reach 20%-50%, or even greater. For injection molding machines that do not use VFD (Variable-frequency Drive), the pressure adjustment of the main pump is to adjust the opening of the bypass oil circuit by pressure unloading to achieve pressure adjustment. This way of adjusting the pressure of the main pump, The motor speed of the main pump is constant, and the pressure is adjusted depending on the opening of the road valve. Practice has proved that the energy saving is very small. There is also an injection molding machine that uses a variable pump, and the energy saving effect reaches more than ten percent, but the maintenance cost of the equipment is relatively high. High, the pump is easily damaged, so it is not an economical operation method. It is a more economical operation method to use VFD (Variable-frequency Drive) speed regulation to adjust the speed of the main pump to change the flow and pressure (at this time, the opening of the valve To fully open 100%), this is why the main pump of the injection molding machine uses VFD (Variable-frequency Drive), which has a relatively large power saving reason. As for the power source of the mold movement is also hydraulic, it has two methods :

① Equip another low-power low-pressure oil pump.

② The power source is divided by the main pump through the pressure reducing valve

As for the positioning control of each period of time, there are four common methods:

① Rely on the slide bar of the mobile platform, and use the stroke to directly point the switch (small)

② Time dependent relay timing control (medium-sized)

③ Rely on single-chip microcomputer or PLC control (large-scale), such as our company's solution.

④ Using VFD (Variable-frequency Drive) itself multi-stage speed