Sunwoele VFD (Variable-frequency Drive) in energy-saving transformation of rubber and plastic equipment

1. Overview of injection molding machine

Over the past half century, plastic has become one of the main materials in industry, agriculture and civilian use. An injection molding machine is a specialized plastic molding machine. It utilizes the thermoplasticity of plastic, and after being properly heated and melted, it quickly flows into the mold cavity under high pressure, and after a certain period of pressure holding and cooling, it becomes various profiles and plastic products. The promotion and application of VFD (Variable-frequency Drive) on injection molding machines can save electricity, improve productivity and product quality, reduce oil pollution and noise pollution, and extend the service life of the machine. Therefore, its use will be gradually promoted.

The injection molding machine is the main equipment for the molding and processing of various plastics (such as polyethylene and polypropylene). Its working principle is: by heating the plastic particles at multiple temperatures in the twin-screw injection molding machine, the fluid material is melted, stirred and pressurized. Injected into the mold cavity to complete the molding process.

2. Current status of the production process of quantitative pump injection molding machines

Injection molding machines and extruders are the main production equipment in the plastic industry, and they are also the main power-consuming equipment. They are called "electric tigers" and their electricity consumption accounts for about 80% of the electricity used by enterprises for production. Injection molding machines consume a lot of power. On the one hand, they cause a huge waste of electric energy in the whole society. On the other hand, the production costs of enterprises remain high, and the products lack competitive advantages, which seriously affects the economic benefits of enterprises.

The process of an injection molding machine is generally divided into several stages such as mold clamping, glue injection, glue melting, pressure holding, cooling, and mold opening. Each stage requires different working pressure and flow rate. For oil pump motors, the injection molding process is under changing load conditions. In the hydraulic system of a quantitative pump, the oil pump motor always provides a constant flow rate at a constant speed, and the excess hydraulic oil flows back through the relief valve. This process is called high-pressure throttling. According to statistics, the energy loss caused by high-pressure throttling is as high as 25-50%. At the same time, due to the long-term full-speed circulating flow of hydraulic pressure and severe friction with hydraulic and mechanical parts, the oil temperature is too high, the noise is too loud, and the mechanical life is shortened.

In the complete process of the injection molding machine, the process of various types of injection molding machines is the same, which is roughly five processes. It can be seen that the pressure and time in the different processes of the injection molding machine are distributed like this:

(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 not long.

(3) Pressure holding process: Feed the material to the mold cavity until it is formed and solidified. The pressure is higher and the time is longer, accounting for about 40-60% of the manufacturing time of a product.

(4) Mold demoulding process: processing and shaping, opening the mold, demoulding, and taking out the processed parts. The pressure is slightly higher and the time is not long.

(5) Mold withdrawal process: The workpiece is taken out, the mold is moved back to its original position, and after auxiliary work is performed, it will be processed again. Low pressure, shorter time.

For the same injection molding machine, when processing different plastic parts, the pressure (P) and time (t) are different. Due to the complexity of the workpiece mold, different plastics are used, the total amount of material used for a workpiece, whether there are inserts, etc. Specific parameter settings can generally be determined by engineering and technical personnel after testing.

3. Principles and feasibility analysis of energy saving of injection molding machines

1. Theoretical principles

The output power of the oil pump: pt=p*Qt=p*v*n (1)

Theoretical torque of oil pump: Tt=1/2π×p×V (2)

P is the pressure, Qt is the flow rate, V is the oil pump displacement, n is the oil pump speed, substitute equation (2) into equation (1) to get:

Pt=2π×Tt×n　　　　　　　　（3）

If the energy loss in the conversion process of mechanical energy to hydraulic energy is ignored, the output power of the oil pump can be approximately considered to be equal to the product of the output torque of the motor and the motor speed. Therefore, it can be seen that when the system requires low flow, the power required by the system is actually very low. However, the actual situation is that since the motor always runs at the power frequency of 50HZ, it cannot reduce its speed according to actual needs, thereby reducing the flow rate. Therefore, excess hydraulic oil can only flow back to the tank through the proportional flow valve, causing a waste of energy.

2. Oil pump frequency conversion speed regulation and energy-saving operation

From the P=f(t) process of the injection molding machine in Figure 1, it can be seen that in different time periods, its main oil pump pressure changes and fluctuates greatly, which leads to the possibility of energy saving and great potential. According to statistics, the power saving rate of injection molding machines can generally reach 25%-50%. As we all know, when the injection molding machine does not use VFD (Variable-frequency Drive), the motor of the main pump always runs at a constant speed, which is an extremely uneconomical operation method.

The injection molding machine power-saving controller is used to control the current signal (0-1A) of the proportional flow valve. According to the flow ratio set in each production process stage, a 0-1A ratio signal is obtained by the electric meter to open the proportional opening of the solenoid valve. , to provide the pressure required by the system. Excess flow is overflowed through the relief valve. Connect the proportional flow signal to the power-saving controller of the injection molding machine as a driving command to control the speed of the hydraulic drive motor. The oil pump only replenishes the part consumed in the oil circuit. The oil pump motor runs at the corresponding speed according to the proportional command and outputs the corresponding flow rate. to provide the pressure required by the system. That is, we provide as much as we need, so that no waste is produced at the source. Therefore, the frequency conversion power-saving system has obvious power-saving effects during the entire production process of the injection molding machine, thereby saving energy to a large extent and bringing considerable economic benefits to users.