How to save energy for servo motor of injection molding machine

In addition to plastics, electricity is a big cost of injection molded products, which needs to be saved.

Essence of energy saving: the injection molding machine needs to produce actions such as mold locking and injection molding, and also needs to generate heat to heat/plasticize the plastic in the barrel. The only way to save kinetic energy and heat energy is to reduce waste. Let's review the two laws of thermodynamics first.

The di definite law of thermodynamics: the di definite law of thermodynamics is energy conservation. In the conversion of energy, energy only changes from one type to another, but the energy before and after the conversion is equal, neither increasing nor decreasing.

The second law of thermodynamics: The second law of thermodynamics believes that in the process of conversion, the entropy will only increase, not decrease. The concept of heat entropy is difficult to explain in a few words. For this discussion, the second law can be simplified as "useless" output energy is heat energy.

Asynchronous motor: The traditional injection molding machine uses asynchronous (squirrel cage) motor to drive the oil pump. The stator of asynchronous motor generates a rotating magnetic field driven by three-phase electricity. Taking the four pole stator and 50 Hz power supply as an example, the magnetic field rotates at 1500 rpm.

There are several oblique copper branches on the rotor, which are short circuited at the end. The rat cage got its name from this. Under the rotating magnetic field, current is induced on the copper branch. The interaction of current and magnetic field generates torque to rotate the rotor. The rotor speed is 20~60 rpm lower than the rotating magnetic field, so it is called asynchronous (asynchronous) motor.

Asynchronous motors convert electrical energy into kinetic energy. In the conversion process, due to the resistance of the coil, the generation of eddy current in the silicon iron sheet and the friction of the bearing, the "useful" kinetic energy output is only about 90% of the input electrical energy (at full load), and the rest is converted into heat energy. Therefore, the motor will have its own fan to take away the heat energy, so as to prevent the motor from overheating.

Asynchronous motors can be overloaded twice for a short time. In case of overload, the current increases, so the heating also increases. Without overload protection, the coil will burn out.

Oil pump: the oil pump converts the kinetic energy of rotation into liquid energy (pressure and flow). Friction in the oil pump reduces the output pressure. The internal leakage of the oil pump will reduce the output flow. Therefore, the output (useful) liquid energy is lower than the input rotational kinetic energy, and the useless heat energy will run to the pressure oil to raise its temperature, which is one of the reasons why the injection molding machine needs to install the pressure oil cooler.

Oil motor and oil cylinder: the oil motor and oil cylinder respectively turn the liquid energy back to the rotational kinetic energy and linear kinetic energy to drive the plasticizing and injection action such as the screw. Like the oil pump, heat is generated during the conversion process, which improves the oil temperature.

Oil pipe: the pressure oil flows in the oil pipe and turns at the pipe joint, which will rub with the pipe wall and self phase to reduce the pressure and generate heat energy. Cheap injection molding machines will use small pipe diameters to reduce costs, but they will inevitably increase the flow rate of oil in the pipe, increase the friction loss, increase the oil temperature, and waste more energy.

Why does the constant displacement pump consume energy?

The constant speed asynchronous motor is used to drive the constant displacement pump. The oil pump outputs a constant flow, but each action in the injection molding cycle, such as mold opening and closing, ejection, plasticizing, injection and pressure maintaining, and even the standby state has different requirements on the flow. The flow that is not needed for action will flow back to the oil tank under the current set pressure. The slower the action required, the more energy will flow back to the tank, and the more energy will be wasted. Similarly, the greater the set pressure, the greater the waste flowing back to the tank. The wasted energy is turned into heat energy, which increases the oil temperature.

During the injection molding cycle, the amount of oil required for pressure maintaining is very low. Because the forward speed of the screw is only enough to fill the shrinkage of the finished product during cooling, it is estimated that it will not exceed 5% of the oil pump flow, and more than 95% of the oil pump flow will flow back to the oil tank under the pressure maintaining pressure. The greater the wall thickness of the finished product is, the longer the holding time will be, and the more energy will be wasted. On the other hand, the greater the extent of energy conservation, or the shorter the payback period of additional investment in energy conservation.

Generally speaking, when the action speed is farther from the full speed, the action time is longer, and the pressure is greater, the potential energy saving will be greater.

How does the variable displacement pump variable?

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 large, and it can provide flow under constant speed rotation of asynchronous motor.

The swashplate axial piston design is adopted for the commonly used variable displacement pump. When the swashplate angle is zero (Zui is large), the displacement of the plunger is zero. Change the angle of the swashplate according to the flow demand, so as to achieve the flow adjustment.

How is the frequency converter variable?

The frequency converter changes the frequency of AC power, changing 50 Hz of mains power to 5~50 Hz, so that the speed of asynchronous motor changes in 10%~100%. With a constant displacement pump, the oil flow will change from 10% to 100%.

As the frequency converter is a high current electronic equipment, it also consumes electric energy, so the energy-saving effect is inferior to that of the variable displacement pump.

The design of asynchronous motor is for constant speed, so the optimization of rotor inertia is not considered. 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 and reduces its attractiveness.

Constant displacement pumps are mostly vane pumps. The vane pump uses centrifugal force to press the vane against the pump casing to make a seal, and then the pump can come out. When the speed decreases, the centrifugal force also decreases, so at low flow, the internal leakage increases and the efficiency of the oil pump decreases.

In fact, the frequency converter is only added when the injection molding machine is renovated and improved. Because it only involves the change of wiring, it takes less time and is much simpler than changing a constant displacement pump into a variable displacement pump. When a new injection molding machine is purchased, a constant displacement pump will not be used with a frequency converter.

Servo motor:

The servo motor is optimized for acceleration and deceleration. How can servo motors reduce inertia while maintaining torque? The following physical relationships are used. (｜ is a mathematical symbol representing "proportional to".)

Torque ｜ rotor diameter (linear proportion)

Inertia ｜ (rotor diameter) 2 (square ratio)

torque