Preliminary Discussion on Energy Saving Technology of Injection Molding Machine

Overview of injection molding machine:

At present, there are many types of injection molding machines in China, among which the general-purpose injection molding machine is the most widely used, and there are two types of general-purpose injection molding machines: plunger type and screw type. This paper mainly studies the screw-type injection molding machine. A general-purpose screw injection molding machine is mainly composed of three parts, injection system, mold clamping system, hydraulic transmission, and electrical control system. Figure 1 Schematic diagram of screw injection molding machine

 

Working principle and characteristics of injection molding machine:

Injection molding is to use the thermophysical properties of plastics to add plastics from the hopper into the barrel, and the outside of the barrel is heated by a heating device to melt the material. There is a screw installed in the barrel, which is driven by a hydraulic oil motor to rotate. Driven by the screw, the material is conveyed to the front along the groove of the screw and compacted. The material in the barrel is plasticized and melted under the double action of external heating and screw shearing. When the screw rotates, the material pushes the melted material to the head of the screw under the action of the shear force and friction of the screw groove. At the same time, The screw moves backward under the reaction force of the material, at this time, the head of the screw becomes the material storage space, and the plasticizing process is completed. Finally, under the thrust of the piston rod of the injection cylinder, the screw injects the molten plastic in the storage chamber into the cavity of the mold through the nozzle at high pressure and high speed. After the molten material in the cavity is kept under pressure, solidified and shaped, and cooled, the mold is opened under the action of the clamping device, and the shaped part is ejected and dropped from the mold by the ejector mechanism. From the perspective of the working principle of the injection molding machine, injection molding has the following characteristics: high mechanical production efficiency, easy-to-realize fully automated production; strong adaptability to the processing of various plastics; one-time molding with precise size, complex shape or inserts of plastic products.

 

Classification of injection molding machine screws:

According to the processing requirements of polymers with different properties, injection molding screws are divided into many structural forms. The injection molding machine screws we use generally have three types: general-purpose screws, gradual change screws, and sudden change screws. One is the general-purpose screw: During the use of the injection molding machine, according to the requirements of the parts, it is often necessary to change the type of plastic, so the number of times to replace the screw is relatively large. Stopping the machine to replace the screw not only causes high labor intensity but also affects the production efficiency of the injection molding machine. 

 

Therefore, although the injection molding machine is equipped with many kinds of screws, it is not often exchanged under normal circumstances. Therefore, we use a general-purpose screw with strong adaptability, and simply adjust the process conditions such as screw speed, Temperature, back pressure, etc., to meet the processing requirements of different materials and products, avoid frequent screw replacement and reduce the production cost of the injection molding machine. Replace the screw and reduce the production cost of the injection molding machine. The second is the gradual change screw: also known as the screw with a long compression section, it is characterized in that the energy conversion is relatively gentle when plasticizing materials, and it is mainly used for processing polyvinyl chloride and the like. The third is the mutant screw: also known as the short compression section screw, which is characterized by a relatively severe energy conversion when plasticizing materials, and is mainly used for processing crystalline plastics such as polyolefins and polyamides. The practice has proved that the effect of using the mutant screw is not ideal, and there are not many manufacturers of injection molding machines used.

 

The role of the barrel:

The role of the barrel is to heat and pressurize the plastic, so the barrel is required to be heat-resistant, pressure-resistant, corrosion-resistant, fatigue-resistant, and heat-transferable. Compared with the cylinder of the plunger injection molding machine, the screw-type injection molding machine has a screw to stir the material in the cylinder, so the thickness of the material layer is thinner and the heat transfer rate is relatively high.

 

Analysis of energy-saving technology for injection molding machines:

The energy consumption of the injection molding machine generally consists of three parts: the energy consumption of the hydraulic system oil pump, the energy consumption of the heating device, and the energy consumption of the cooling device. The energy consumption of the oil pump of the hydraulic system accounts for more than 80% of the total energy consumption of the injection molding machine. Reducing the energy consumption of the hydraulic oil pump is the key to energy saving for the injection molding machine. Therefore, it is of great significance to study the new energy-saving hydraulic system of the injection molding machine.

 

Frequency conversion adjustment technology:

Frequency conversion adjustment technology is to adjust the speed of the hydraulic oil pump to make the performance of the power mechanism similar to that of the proportional pump energy-saving system, to match the output flow of the power mechanism with the flow consumed by the actuator, and to minimize the flow loss. The frequency conversion adjustment technology uses the electrical control system and synchronous signal of the injection molding machine, and simulates the electro-hydraulic proportional control system as a load tracking control system according to the requirements of the injection molding process, so that the motor speed and the hydraulic flow and pressure required for the injection molding machine to work The product of is proportional to the relationship, and the traditional quantitative pump is transformed into a variable frequency control variable drive system, thereby minimizing the oil return volume of the relief valve without high-pressure throttling energy loss.

 

Proportional variable pump system: 

The most effective measure to save energy for injection molding machines is to achieve adaptive adjustment and matching of the flow rates of the power mechanism and the actuator. The load-sensing pump hydraulic system combines a variable pump and an electro-hydraulic proportional valve, its control mechanism is composed of a proportional variable pump, a high-speed proportional valve (including position closed loop), a pressure sensor and displacement sensor, etc. It is better to realize the adaptive adjustment and matching of the flow of the power mechanism and the actuator. The use of the variable pump makes the hydraulic system of the injection molding machine change from valve control to pump control and changes the traditional throttling speed regulation into volumetric speed regulation. The load-sensitive variable pump hydraulic system can make the hydraulic power output change with the change of the load. Compared with the hydraulic system combined with a fixed-quantity pump + PQ valve, its energy-saving efficiency can reach 20%-40%. The main disadvantages are that the response speed of the pump control system is slower than that of the hydraulic system combined with the pump + energy adjustment unit, and the proportional variable pump has high requirements for oil cleanliness and high working noise.

 

Quantitative pump & proportional pressure flow control valve system:

The control system of the injection molding machine uses a quantitative pump to supply oil, and through the proportional pressure flow control valve (ie PQ valve), excess oil is returned to the oil tank through the bypass in the way of overflow regulation. The pressure difference between the front and back of the proportional throttle valve is kept constant by the proportional relief valve and the main relief valve. The opening size of the throttle valve is controlled by the proportional electromagnet signal input to the throttle valve. It can be seen from the flow formula that the output flow is only related to the input signal of the proportional throttle valve is related. During the working process of the injection molding machine, the motor speed remains constant, and the oil supply volume is constant. The PQ valve receives an analog control signal from the control system and adjusts the PQ valve to realize continuous proportional control of the pressure and flow of the injection molding machine system. Quantitative pump ten PQ valve energy-saving hydraulic system meets the pressure and flow control requirements of the injection molding machine for mold opening and closing, pre-molding, injection, pressure maintaining, etc., and also ensures the repeatability and stability of the injection molding process. Because the quantitative pump ten PQ valve hydraulic system uses a small pressure difference to track the load pressure to control the pump pressure, it is improved in terms of energy saving compared with the traditional hydraulic transmission system of quantitative pump + overflow valve + throttle valve, and the control execution The speed of the mechanism is not affected by the hydraulic oil temperature, but there are still more throttling and overflow losses in the actual working process, especially for the molding of parts that require high pressure holding pressure and long holding time. Power loss is considerable.

 

Conclusion:

The application of energy-saving technology for injection molding machines can greatly improve the energy-saving effect of injection molding machines. It not only saves energy but also promotes the innovation and development of injection molding machines to a large extent, which is of great importance to its future development. Practical significance.

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