December 26, 2022
How to Select Power System of Injection Molding Machine (Part 2)
Servo motor limitation
The back EMF of the servo motor at "low speed" is lower than that at "high speed", so the increase of current will cause excessive heating of the coil, which will occur when maintaining the voltage for a long time. The oil research company uses variable displacement pump to reduce the displacement and maintain the high speed of the motor to alleviate this problem. When purchasing the injection molding machine, pay attention to whether the servo motor power is too small, or it will be overloaded.
Large injection molding machines need large motors/oil pumps to drive them, but the rotors of large servo motors have large inertia, which cannot meet the requirements of 0 to 2000 rpm in 0.05 seconds. Take two Japanese suppliers of servo motor with oil pump as an example, the largest motor is only 35kW. Larger injection molding machines can only combine two or more servo motors/oil pumps to drive, which can meet the requirements of both oil flow and reaction time. One 3500 ton injection molding machine is driven by 10 sets of servo motors/oil pumps.
Energy saving effect comparison
The variable displacement pump driven by the servo motor does not rotate in the standby state, and only the energy of the servo motor driver is wasted. Taking the 11kW servo motor as an example, the current is less than 1A. The variable displacement pump driven by asynchronous motor rotates at a constant speed in standby state, but there is no flow or pressure. The current measured on the 11kW injection molding machine in standby state is 7A, which is 29% of the rated current 24A of the 11kW motor. This is because the efficiency of the asynchronous motor is about 90% at rated load, but it drops sharply when the load is less than 50%. The British company provides "Energy Saving Treasure" to reduce the supply voltage when the asynchronous motor is under low load, so as to reduce the excessive magnetic flux generated by the copper coil, reduce the loss and achieve the effect of energy conservation.
The oil temperature of a single cavity aviation cup with a 50t injection molding machine is only 37 ℃ in the summer of South China and without pressure oil cooling. For example, the temperature rise of pressure oil is an indication of energy conservation, which is beyond the reach of even asynchronous motor driven variable displacement pump.
In addition, servo motor driven oil pump can provide 3/4 closed loop control. Accurate speed and pressure control is a prerequisite for stable productivity. Because the oil temperature is not high, the oil temperature fluctuation is also low, which improves the stability. Low oil temperature can also save or even eliminate the need for pressure oil cooling.
The power saving effect of all motors is well known. The whole motor is driven by a servo motor, but it uses at least four servo motors to directly drive injection, plasticizing, mold opening and closing, and ejection. Other actions such as core pulling/spinning, shooting table and mold adjustment can be driven by servo motor or cheaper motor. For direct drive, the lead screw or curved arm is used to change the rotating kinetic energy into linear kinetic energy, or the belt or gear is used to change the high-speed rotating kinetic energy into low-speed rotating kinetic energy.
Compared with servo hybrid power, direct drive saves two conversion processes and 10% energy. It can save energy without pressure oil and elastic loss of pipeline.
Recommendations for powertrain selection
Since its invention, the injection molding machine has experienced several technological innovations. The use of motor and oil pressure is a progress, they replace human power; Reciprocating screw is an early innovation, which combines the plasticizing and injection functions to reduce the cost of the injection molding machine; Another innovation is the proportional pressure and flow valve, which is closely related to another innovation - the widespread use of microprocessors. The action pressure and flow no longer need to be manually set on the valve, and the injection molding parameters can be remembered for easy reuse next time; With the wide use of variable displacement pumps, this has something to do with the rising electricity bills. The introduction of full motor should be regarded as a revolution. Since there is no effect of oil temperature, it will greatly improve the stability, accuracy, parallel action, power saving, noise reduction, no oil leakage pollution, etc. The application of servo motor driven oil pump in injection molding machine will have the advantages of energy saving, 3/4 closed loop and noise reduction, which are almost all motors, but the cost is much lower.
When the injection molding machine does not operate at full speed, reducing the flow can reduce waste. Variable displacement pump and servo motor can reduce the flow, and the latter can provide zero flow with near zero energy consumption. However, not all products are suitable for production with injection molding machines with servo motors and oil pumps, just as not all products are produced with full motors. The constant displacement pump and variable displacement pump have their own value. Whether to adopt this technology requires the calculation of investment return to the current period to make a decision.
The saving effect of servo motor can not be generalized, which is related to the wall thickness of the product, whether cold runner is used for the mold, pressure holding time and standby time. Generally speaking, the greater the wall thickness is, the more energy saving of the servo motor will be. The injection molding of the preform belongs to this category. If the diameter of the cold runner is greater than the wall thickness, the cooling time is controlled by the diameter of the runner. On the contrary, the pressure holding time of thin-walled products (hot runner) is short, even 0, and the cooling time is also 0, so the energy saving of servo motor is limited.
To sum up, in addition to the full motor, Daming recommends that thin-walled lunch boxes (with a wall thickness of 0.5mm) with a cycle of 5 seconds or less be produced by injection molding machines with constant displacement pumps driven by asynchronous motors. Because most cycles are conducted at full speed and full pressure, the waste saved is limited, and even the acceleration and deceleration of the servo motor will prolong the injection molding cycle; Products with a cycle of 5-8 seconds can be produced by injection molding machines with variable displacement pumps driven by asynchronous motors; For products with a cycle of more than 8 seconds, the cooling time is longer than the plasticizing time, the injection time is more than 3 seconds, the screw speed is less than 70%, the mechanical arm is used to take them out, and semi-automatic operation is used, "Energy Saving Treasure" or servo motor injection molding machine is recommended for production. In addition, the cost of servo motor is high. If the additional investment returns to the current period within two years, it can be considered.
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