Discussion on energy saving technology of the hott

2022-08-18
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Discussion on energy-saving technology of precision injection molding machine

the injection molding industry is facing an opportunity of rapid development. However, in the composition of finished products of injection molding products, electricity charges account for a considerable proportion. According to the needs of injection molding machine equipment and technology, the power consumption of injection molding machine oil pump motor accounts for 50%-65% of the power consumption of the whole equipment, so it has great energy-saving potential. Design and manufacture a new generation of "energy-saving" injection molding machine, It has become an urgent problem to be concerned and solved

the process of injection molding machine is generally divided into several stages, such as mold locking, glue injection, glue melting, pressure maintaining, cooling, mold opening, etc., each stage requires different pressure and flow. For the oil pump motor, the injection molding process is in a changing load state. In the hydraulic system of the quantitative pump, the oil pump motor provides a constant flow at a constant speed, and the excess hydraulic oil flows back through the overflow valve. This process is called high-pressure throttling. According to statistics, the energy loss caused by high-pressure throttling is as high as 36%-68%

with the increasingly stringent requirements of countries around the world in environmental protection, such as energy consumption, noise control in the next five years, leakage and other aspects, energy conservation has been completed as the research focus of injection molding electro-hydraulic system. Aiming at the shortage of large energy loss in valve controlled electro-hydraulic system, Germany, Japan and other countries have developed a load sensing electro-hydraulic control system for injection molding, which combines variable displacement pump and electro-hydraulic proportional valve. In order to further reduce energy consumption and noise, plastic machinery manufacturers have conducted in-depth and extensive research and Practice on various energy-saving control technologies in recent years to meet the requirements of customers to reduce production costs through energy conservation. At present, there are mainly two energy-saving control systems, proportional variable pump and variable-frequency quantitative pump, which should be selected according to the speed required by the relevant regulations of the tested material. The working principles of the two energy-saving technologies are analyzed below to explore their performance characteristics and applications

composition and working principle

◆ the proportional variable pump control system

is mainly composed of oil pump motor, proportional variable pump and conventional hydraulic valve. Users set various action pressures and speeds according to the requirements of injection molding process. These set values are converted into corresponding analog signals (current or voltage signals) by microprocessor and output to the proportional pressure valve and proportional flow valve of proportional variable pump, At the same time, the proportional pressure valve and proportional flow valve in the variable displacement pump output the pressure and flow to the system according to the set value. The pressure, flow difference and load changes generated by the pressure and flow quickly promote (adjust) the tilt angle of the variable displacement pump to change, so that the energy output of the hydraulic system changes with the changing load condition of the whole machine in the injection molding process. The proportional valve outputs the precise pressure and flow required by the whole machine to the system according to the set value, avoiding the loss of energy to the greatest extent

◆ frequency conversion quantitative pump control system

is mainly composed of frequency converter, oil pump motor, quantitative pump, proportional valve (pressure/flow) and other conventional valves (directional valve and overflow valve). When the system works, users set various action pressures and speeds according to the requirements of injection molding process, These set values are converted into corresponding analog signals (current or voltage signals) by the microprocessor and output to the proportional pressure valve and proportional flow valve of the system. At the same time, the analog signals corresponding to the speed setting are input to the control board of the frequency converter to control the speed of the AC motor to change according to the set value. The change of the motor speed leads to the change of the associated quantitative pump speed, In order to make the energy output of the hydraulic system change with the changing load condition of the whole machine in the injection molding process, it is necessary to maintain a very low speed of the subversive test equipment under the conditions of pressure maintaining, cooling and idling, so as to achieve the purpose of energy saving. Using the synchronous signal and electrical control system of the injection molding machine, according to the process requirements of injection molding, the electro-hydraulic proportional control system is simulated as a load tracking control system, so that the speed of the oil pump motor is proportional to the hydraulic flow and pressure product required by the operation of the injection molding machine, and the traditional quantitative pump is transformed into a variable frequency variable pump, so that the return oil flow of the overflow valve is minimized, without high-pressure throttling energy loss, Thus, the traditional "energy consuming" injection molding machine with high-pressure throttling is upgraded to an "energy-saving" injection molding machine without high-pressure throttling

performance characteristics

◆ response speed of the system

variable pump: the proportional pressure and flow are integrated with the pump, the control oil circuit stroke is short, the response is fast, the AC motor speed is constant, there is no hysteresis effect and the influence of moment of inertia, and the response time is fast (within 100ms)

frequency conversion system: the speed of AC motor changes with the load under the control of frequency converter, and the response speed is slow (MS) due to the influence of hysteresis effect and moment of inertia

◆ product forming cycle and work efficiency

variable displacement pump: the AC motor has constant speed, no hysteresis effect and the influence of moment of inertia, fast response time, can meet the requirements of any product, short forming cycle and high work efficiency

frequency conversion system: under the control of the frequency converter, the speed of the AC motor changes with the load. Affected by the hysteresis effect and moment of inertia, the response speed is slow, and it is difficult to form products with high requirements. The forming cycle is long and the work efficiency is low

◆ service life of oil pump and motor

variable pump: the speed of AC motor is constant, and the change of speed only changes the inclined angle of the oil pump, which has no impact on the service life of the oil pump

frequency conversion system: the speed of AC motor changes constantly, and the temperature should be higher than the temperature rise of constant speed. When the speed of oil pump is low, the output pressure and flow are unstable, the cooling effect is reduced, and the service life of oil pump and motor is reduced

◆ overload bearing capacity

variable displacement pump control system has good overload capacity; The frequency conversion control system has low overload capacity and has overload, overcurrent and phase loss protection for the motor

application of two control systems

according to the energy-saving performance characteristics of the two control systems, due to the relatively mature energy-saving control technology of proportional variable pump, at present, injection molding machine manufacturers generally adopt proportional variable pump energy-saving control technology, and energy-saving through proportional variable pump is still in the leading position; The application of frequency converter speed control technology in the plastic machine control system to achieve energy saving is undoubtedly a useful attempt. Domestic plastic machine manufacturers have also produced and put into the market. Due to the introduction of frequency conversion control technology software: technology still has its inherent technical problems to be optimized and solved, and at present, more are energy-saving technology transformation for old machines. According to the energy-saving working principle of the control system, the energy-saving of the injection molding machine also has additional advantages:

◆ reduce the impact of mold opening and locking, and prolong the service life of the machine and mold

◆ extend the service life of the oil circuit system (sealing components, etc.), reduce the number of repairs, and save maintenance costs

◆ reduce noise and improve working environment

◆ the system oil temperature is greatly reduced, and the cooling water consumption can be saved by more than 30%

◆ the frequency converter has a variety of protections for the motor, such as overvoltage, overcurrent, loss of voltage, etc

◆ the original control mode and oil circuit of the injection molding machine are unchanged

the investment in upgrading the injection molding machine to an "energy-saving" injection molding machine should be recovered within one year through the saved electricity or oil costs. In short, the theory of developing "energy-saving" injection molding machine is feasible, with small investment and obvious benefits. Energy saving injection molding machine has become a new selling point of injection molding machine manufacturing industry. (text/liuzhaoming, Wang Cunliang, Shenzhen Lingwei technology company of Lijin group)

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