How to Select Power System of Injection Molding Machine (Part 1)

Electric charge is the largest cost of injection molding product processing except for raw materials. For most injection molding machines, electric energy is first converted into kinetic energy through the motor, and then into hydraulic energy through the oil pump, which drives each action in the injection molding cycle to complete the injection molding process. We all know the law of "conservation of energy". Energy will not increase or decrease in the process of changing from one form to another, but in fact, there is a loss of energy in the conversion process, which is because the conversion process produces "useless" output energy, such as heat energy. The essence of energy conservation is to reduce "useless" output energy.

Asynchronous motor constant displacement pump

Traditional injection molding machines are powered by asynchronous motors (squirrel cage type). The constant speed asynchronous motor converts electrical energy into kinetic energy. The "useful" kinetic energy output is only about 90% of the input electrical energy (when fully loaded), and the rest becomes thermal energy. Therefore, the motor will have its own fan to take away the heat energy.

On the one hand, the asynchronous motor drives the fixed displacement pump to output a constant flow; On the other hand, each action in the injection molding cycle, such as mold opening and closing, ejection, etc., has different requirements for flow, and the flow that is not used will flow back to the oil tank under the current set pressure. The slower the action is, or the higher the set pressure is, the more energy will flow back to the oil tank, and the more energy will be wasted. The wasted energy will become heat energy, which will increase the oil temperature. Therefore, generally speaking, when the action speed is farther from the full speed, the time is longer, and the pressure is greater, the potential energy saving will be greater.

Variable displacement pump of asynchronous motor

It can be seen from the above that the key to energy conservation is to change the flow. Variable displacement pump can provide flow from zero to maximum, and it is provided under constant speed rotation of asynchronous motor. The most commonly used variable displacement pump adopts swashplate axial plunger design. It is a good habit to turn off the lights when going out. When the light dark switch is widely used, we can work and rest in a dark but sufficient light, so as to save energy. The variable displacement pump is like a light dark switch. There are also two devices that can reduce the flow: frequency converter and servo motor.

Asynchronous motor frequency converter oil pump

The frequency converter changes the AC frequency, so that the speed of the asynchronous motor changes in 10-100%. With a constant displacement pump, the oil volume changes in 10-100%.

However, frequency converter is a high current electronic equipment, which also consumes electricity, and its energy-saving effect is inferior to that of variable displacement pump. In addition, the asynchronous motor is designed at constant speed, without considering the optimization of rotor inertia. If it takes 0.1 seconds for each acceleration and deceleration of the rotor, it takes 2 seconds for no less than 20 speed changes in a cycle. Generally, users will find that the use of frequency converter slows down the productivity. Finally, most constant displacement pumps are vane pumps. When the speed decreases, the centrifugal force also decreases. Therefore, at low flow, the internal leakage increases and the efficiency of the oil pump decreases.

Therefore, when the injection molding machine is renovated and improved, it is more suitable to use a frequency converter and connect it to an asynchronous motor. Because it only involves the change of the electric wire, it takes less time and is much simpler than changing a constant displacement pump to a variable displacement pump. In addition, it is a successful application to change the rotational speed of the extruder screw with a frequency converter. Because the extruder does not operate periodically, it does not need to accelerate or decelerate frequently, and its screw is driven by the gearbox, so it does not involve the oil pump.

Servo motor oil pump

Servo motor can optimize acceleration and deceleration. Its rotor uses a small diameter to reduce inertia, and then uses a long rotor to recover lost torque. Of course, the inertia also increases with the rotor length, but only linearly. It is also obvious from the appearance of the servo motor that its diameter is small but its length is large. It only takes 0.05 seconds for the servo motor to change from 0-2000 rpm. Therefore, the variable speed servo motor is used to drive the oil pump, which slows down the productivity only when the cycle is less than 5 seconds. The permanent magnet is used to generate the magnetic field of the rotor, which also avoids the copper loss and iron loss of the rotor.

The variable speed of the servo motor is realized by the controller with the principle of frequency conversion. 50Hz or 60Hz alternating current will be converted into alternating current of the required frequency after rectification to drive the servo motor. Therefore, as long as the voltage is appropriate in areas with different frequencies, it will not affect. This is different from asynchronous motor. The rotation speed of asynchronous motor at 60Hz is 20% faster than that at 50Hz. The controller also has feedback control of pressure and speed, and PID control of pressure and flow rise and fall.

The servo motor is no different from the generator. Which function to play depends on how to drive it. If three-phase power is supplied, it is a motor; If power is supplied to rotate its main shaft, it is a generator. When the servo motor is braked (such as when the mold is opened and stopped), it becomes a generator. The inertia (kinetic energy) of the moving template and the moving template drives the mold locking cylinder, and then drives the oil pump (at this time, the oil pump becomes an oil motor), which drives the main shaft of the "generator" to generate voltage. After passing through the brake resistance, it becomes current, and then becomes heat energy, which is distributed in the atmosphere. Although this energy is not recovered, it also serves the purpose of fast and accurate braking.

Gear pump or plunger pump

There are two types of oil pumps used with servo motors, namely, quantitative gear pump and variable plunger pump. The constant displacement pump for injection molding machine is mainly vane pump. The centrifugal force of vane pump decreases at low speed, and the internal leakage increases, so it is not suitable for variable speed servo motor.

The volumetric efficiency of the gear pump is below 90%, the structure is relatively simple, the cost is not high, the noise is not big, and the tolerance of oil pollution is large. The volumetric efficiency of the plunger pump is about 95%, the structure is precise, the tolerance to oil contamination is not high, and the noise is large. However, its variable characteristics can be used to reduce the torque load on the servo motor, reduce the current and heat, so that the pressure holding time can be longer. This has been brought into play in the dual displacement design of the oil research company.

The flow required for pressure maintaining and high-pressure mold closing is low but the pressure is high, which can convert the variable displacement pump into a small displacement, reducing the heat generated by the large current of the servo motor at low speed. This function is not even available for all motors. If double displacement is not used, the pressure maintaining and high-pressure mold closing will exceed the pressure flow range of the original displacement, but still within the overload range, and can be used for temporary pressure maintaining and high-pressure mold closing. If the mold locking method adopts machine hinge, the high-pressure mold closing is naturally short-lived. If the direct pressure mold locking force is not locked by the one-way valve, the motor oil pump must work continuously to maintain the mold locking force. The dual displacement oil pump can be seen as a dual gear transmission of a car. The low gear is used when driving slowly when climbing. Although the engine speed has not decreased much, the driving force of the car has improved.