Application of Sunwoele VFD (Variable-frequency Drive) in injection molding machines

Injection molding machine is a device that heats, melts, stirs, and pressurizes various plastics, and then injects plastic fluid into the mold control to complete the one-time injection molding of the workpiece. The process is basically the same. Each process requires different pressure and flow, which means that the workpieces to be processed do not all work at the maximum pressure or flow. In the traditional way, the pressure and flow are controlled by a pressure proportional valve. and flow proportional valve to adjust. The oil pump motor runs at a constant speed, causing a waste of energy. If VFD (Variable-frequency Drive) is used to adjust the speed of the motor (oil pump) to meet the pressure and flow requirements in real time, it is both economical and practical. Relevant units have successfully used our company's Sunwoele VFD (Variable-frequency Drive) modified injection molding machines and achieved significant economic benefits. Therefore, using VFD (Variable-frequency Drive) to modify injection molding machines to save energy is worth promoting to the majority of users. . According to actual experience, the power saving rate of medium and small injection molding machines after modification with VFD (Variable-frequency Drive) is generally 20% to 60%.

1. Overview of injection molding machine

As we all know, the injection molding machine is the main equipment for the molding and processing of various plastics (such as polyethylene and polypropylene). The plastic particles are heated at multi-stage temperatures in the twin-screw injection molding machine, and the fluid material is injected into the mold cavity after melting, stirring and pressurization. Within, the molding process is completed. Zhejiang Province is one of the provinces in China that is severely short of electricity, especially in summer. Injection molding machines are spread all over the province, so it is imperative to save electricity. Our company cooperated with Ningbo agents to inspect the injection molding machine site of a company in Ningbo. Frequency conversion transformation has been carried out, with a total installed capacity of more than 400 KW and more than 20 injection molding machines. Among them, the maximum capacity of a single machine is 37KW. It has been put into production for one year and is working normally with an average power saving rate of more than 30%. At the same time, the noise in the working environment is significantly reduced, and the system oil temperature drops significantly; smooth soft start protects the motor and extends the service life; reduces the impact of mold opening and extends the service life of the machinery and molds; extends the service life of the oil system, reduces the number of repairs, and saves money. A lot of maintenance costs; it has various protections for the motor such as overvoltage, overcurrent, phase loss, etc.; the frequency conversion and industrial frequency can be switched at will to ensure that production will not be affected when the frequency conversion failure occurs; the system installation is simple and fast. Well received by users. In view of the above advantages, it shows that the application of VFD (Variable-frequency Drive) to implement energy-saving technological transformation of injection molding machines is correct and effective, and it is worth promoting to everyone.

2. Injection molding machine process

No matter whether it is a large, medium or small injection molding machine, the process analysis is the same, which is roughly five processes, as shown in Figure 1.

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

①The process of sending the mold requires low pressure and a short time.

② During the mold closing process, the left and right molds are connected until they are completely closed, which requires slightly higher pressure and does not take long.

③In the process of glue injection, pressure holding, and cooling, the material is fed to the mold cavity until molding. After solidification, higher pressure is required, which takes about 40-60% of the time.

④ During the demoulding process, the mold can be opened during processing and molding. When the demoulding is completed, the processed parts can be taken out. The time is not long and the pressure is slightly high.

⑤ During the mold withdrawal process, the workpiece is taken out, the mold is moved back to its original position, and after auxiliary work is performed, it is processed again. At this time, low pressure is required and the time is short.

For the same injection molding machine, the pressure (p) and time required to process different plastic parts are different. It is related to the complexity of the shape of the workpiece, what kind of plastic is used, the total amount of material of a workpiece, whether there are inserts, etc. The specific parameter settings are generally given by the process technicians and after trial processing, finalized.

3. Possibility analysis of energy saving of injection molding machine

From the P=f(t) process of the injection molding machine in Figure 1, it can be seen that with different time periods, its main oil pump pressure changes and fluctuates greatly, which leads to the possibility of energy saving and great potential. , so the power saving rate of the injection molding machine can reach 20%-50%, or even greater. In injection molding machines that do not use VFD (Variable-frequency Drive), the pressure adjustment of the main pump relies on pressure unloading to adjust the opening of the bypass oil line to adjust the pressure. This method of adjusting the pressure of the main pump, The motor speed of the main pump is kept constant and the pressure is adjusted by the opening of the bypass valve. Practice has proved that the energy saving is very small. There is also an injection molding machine using a variable pump, which has an energy saving effect of more than ten percent, but the equipment maintenance cost 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, thereby changing the flow rate and pressure (at this time the valve opening Fully open 100%), which is why the main pump of the injection molding machine uses VFD (Variable-frequency Drive), which can save electricity significantly. As for the power source for mold movement, it is also hydraulic. There are two methods. :

① Equipped with a small-power low-pressure oil pump.

② The main pump is branched through the pressure reducing valve as the power source.

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

① Rely on the sliding rod of the moving platform and use the stroke to directly click the switch (small size)

② Rely on time relay timing control (medium size)

③ Relying on microcontroller or PLC control (large scale), this is our company's solution.

④ Use VFD (Variable-frequency Drive)’s own multi-stage speed and time control (small and medium-sized).

4. Characteristics of our company’s frequency conversion transformation plan for injection molding machines

(1) Select Sunwoele HSF600 series dedicated VFD (Variable-frequency Drive), which has strong overload capability and dual feedback signal interfaces for flow and pressure. According to the injection molding process requirements, dual signals and superposition are used to control the main oil pump motor speed. After one year of use, it has been proved that the VFD (Variable-frequency Drive) has good performance, reliable operation and high power saving rate. The main circuit adopts two power supply modes of industrial frequency and variable frequency to ensure uninterrupted operation.