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Knowledge of injection molding machine

Injection molding machine is a special plastic molding machine. It uses the thermoplastic of plastic, after heating and melting, it is made into the mold cavity quickly by high pressure. After a period of pressure preservation and cooling, it becomes a variety of plastic products. The power consumption of injection molding machine is mainly shown in the following parts:

① The electric energy consumption of the oil pump in the hydraulic system;② Electric energy consumption of heater;
③ The power consumption of circulating cooling water pump (in the injection molding workshop, many injection molding machines share one cooling water pump), among which the electric power consumption of hydraulic pump motor accounts for more than 80% of the total power consumption of the injection molding machine, so reducing the power consumption is the key to energy saving of injection molding machine.

一、 Working cycle of injection molding machine

1. Locking mode: the die plate is fast approaching the fixed die (including slow fast slow), and if no foreign matter exists, the system turns to high pressure and locks the formwork (keep the pressure in the cylinder).

2. The injection table moves forward to the position: the injection table moves forward to the specified position (the nozzle is close to the mold).

3. Injection molding: screw can be set at multiple speed, pressure and stroke to inject the solution in the front of the barrel into the mold cavity.

4. Cooling and pressure maintaining: according to the set pressure and time period, keep the pressure of the material cylinder, and form the mold cavity cooling.

5. Cooling and pre molding: the products in the mold cavity continue to cool, while the screw driven by hydraulic motor pushes the plastic particles forward, the screw is backward under the set back pressure control. When the screw is backward to the predetermined position, the screw stops rotating, and the injection cylinder will be released according to the setting, and it is expected to end.

6. Ejection back: after the pre molding is finished, the ejection table will be back to the specified position.

7. Open mode: pull back to the original position (including slow fast slow)

2、 Energy waste of traditional hydraulic driven injection molding machineThe pressure and flow of hydraulic driven injection molding machine are all changed in the above stages. When the demand for oil quantity of injection molding machine changes, the proportional flow valve and proportional pressure overflow valve set at the outlet of oil pump are used to adjust the load pressure and flow, and provide the thrust, pressure direction and moving speed required by each cylinder and hydraulic motor. The output power of the oil pump is equal to the product of the output torque of the motor and the motor speed. When the system requires low flow, the output power of the pump motor is constant, and the excess hydraulic oil flows back to the tank through the pressure proportional valve and overflow valve, even if it is empty (such as cooling). In this case, the throttling power loss is very large and the efficiency is very low, generally only 60% – 70%, Therefore, it causes energy waste. At the same time, due to the long-term full speed circulation of hydraulic oil, and the severe friction with hydraulic parts and mechanical parts, the oil temperature is too high, and the noise is too large, and the service life of the machine is shortened.
3、 Control oil pump motor with energy saving inverterThe motor controlled by frequency converter fundamentally overcomes the disadvantages of energy waste of traditional injection molding machine. When the flow required by the system changes, the motor speed changes accordingly, which makes the flow rate of oil discharged by quantitative oil pump change, that is, “how much supply is needed, thus saving a lot of electric energy.”. According to the different injection molding products, the power saving rate is 20% – 70%.
4、 Selection of control signal of frequency converterThe control circuit, hydraulic circuit and working principle of the modified injection molding machine should be fully understood, and the control signal should be obtained ingeniously to realize the combination of the existing injection molding machine and the inverter. The signal of hydraulic oil demand of injection molding machine is the current signal (0-1a) connecting pressure and flow proportional valve. After integral processing and converted into 4-20mA standard signal, it is directly connected to the input end of frequency conversion, so as to control the frequency converter to change the output frequency, and then change the speed of the oil pump motor, thus achieving the double function of voltage regulation and power saving. In view of the characteristics of high load overload, high frequency response, high torque at low speed and long time of motor acceleration and deceleration, PLC controller is applied in the control process, and the signal of each electro-hydraulic reversing valve is introduced. According to these signals, PLC identifies the process flow of injection molding machine, and gives acceleration and deceleration instructions to frequency converter in advance.
5、 The selection of frequency converter of injection molding machine1. In the common VVVF speed regulation mode, when the ratio of frequency to voltage is constant, the magnetic flux of the motor is unchanged. Because of no speed feedback, the motor speed cannot be accurately controlled (the general error is 15%), the rotor actual speed of three-phase asynchronous motor and the stator rotating magnetic field have a rotation difference. When the external load changes, the slip rate will change accordingly, But the inverter can not compensate for this, which causes the change of the oil pump speed, resulting in flow error and high defective rate. In the system of pressure closed-loop control, the injection molding machine will be in a state of instantaneous pressure imbalance, which can not control the quality of the product.2. Sensorless vector control converter adopts the control mode of rotor magnetic field direction, that is, coordinate transformation of asynchronous motor is equivalent to that of DC motor, which is equivalent to armature current proportional to torque in DC motor. Regardless of the state of motor, the excitation current is maintained constant, and the torque size only adjusts armature current independently, Therefore, the asynchronous motor can always be in a high dynamic state, which can accelerate from 0RPM to 1000rpm for 30ms, which improves the dynamic performance of the frequency converter speed regulating system. The adhesion of the whole injection molding cycle is greatly improved, the rotation difference is within 3% – 5%, which improves the stability of the motor in the face of load change, and the motor has more than twice the rated torque when running at low speed, It has good acceleration and deceleration performance, and the response is fast when the torque changes.In conclusion, the open-loop universal frequency converter is selected. Because of its slow response, it is not suitable for the speed regulation of injection pump. Compared with the inverter without sense vector control, it is more suitable for the use of injection molding machine because of its high dynamic and good stability.
6、 The heat dissipation mode of frequency converterThe special frequency converter of injection molding machine adopts water cooling cooling mode, which has high heat dissipation efficiency and the safety protection level can reach IP54. Because the frequency converter adopts water cooling mode, the main body of the inverter is actually closed in the electric control box, reducing the possibility of damage caused by dust and oil pollution invading the electronic equipment, ensuring the safe use of the frequency converter, and using the water cooling and cooling method can make the power single IGBT of the converter always work in a certain temperature range, effectively prolonging its service life, The average life of frequency converter can be prolonged by more than 10 years by using water cooling and cooling method.
7、 Other functions after using the inverterThe most remarkable feature is that the process characteristics of the machine can be improved and its efficiency can be improved. Some products require high injection speed. Ordinary injection molding machines may need to replace the screw, barrel, motor and oil pump to achieve the required speed. However, after using frequency converter, the motor speed can be increased by 10% (motor frequency reaches 55Hz) within the allowable range of motor oil pump, so that the required speed can be achieved without replacing any parts, Thus, the production efficiency is improved.

8、 Problems in useAfter the measurement adopts frequency conversion operation, the surface temperature of the motor has been improved, for example, the temperature of the motor in power frequency operation is 35 degrees, while the motor is 40 ℃ in frequency conversion operation, and it does not continue to rise (within the safety range of the motor). This is due to the decrease of wind force of the cooling fan of the motor due to the different speed operation of the motor during the whole cycle, It is recommended to keep the cooling fan and motor clean during use.

Post time: Jun-02-2021