Application of frequency conversion speed regulati

Application of variable frequency speed regulation and energy saving of injection molding machine

I. working principle of injection molding machine

screw plastic injection molding machine has been launched since the 1950s, and it has a history of more than 50 years. At present, 80% of the engineering plastics processing industry adopts injection molding. Plastic particles (ABS, polyethylene, modified polystyrene, etc.) are heated and eroded by multi-stage heaters in the barrel of the injection molding machine. After being stirred and pressurized by the screw, they are injected into the mold cavity and formed by pressure maintaining cooling to complete the processing process of a workpiece. For plastic processing, the complete process flow of the injection molding machine is: mold closing - mold locking - injection - pressure maintaining - cooling - demoulding - mold opening. Among them, pressure maintaining and cooling, demoulding and mold opening are carried out at the same time, that is, during the pressure maintaining process, the mold is cooled by water; In the process of mold opening, the ejector pin in the mold gradually protrudes from the hidden place, so that the workpiece on the mold falls off. After the mold is opened in place, a processing process ends. No matter large, medium or small injection molding machines, their process flow is the same. At present, the vast majority of injection molding machines are hydraulic transmission injection molding machines. The power required for the above process action process is provided by the oil pump in the hydraulic system, which can be divided into variable pump and quantitative pump. When the injection molding machine is working, the load of each process changes greatly in a working cycle. The flow and pressure required by the hydraulic system are different. With the development trend of lightweight and electric vehicles, this change has been considered in the production of oil pumps. When the flow and pressure required by the hydraulic system change, the oil supply of the oil pump automatically increases or decreases to adapt. This oil pump is a variable displacement pump, There is no need to use frequency converter for speed control. Another widely used oil pump is the quantitative pump, whose oil supply is constant. The changes of flow and pressure during the operation of the injection molding machine are adjusted by the flow proportional valve and pressure valve, and the excess oil flows back to the oil tank through the overflow valve. In this way, the wear of valves and oil pumps is aggravated, resulting in the rise of oil temperature and excessive motor noise. In addition, from the perspective of the design of injection molding machine, the oil pump usually needs to leave a margin in the design, generally considering 10% - 15%, but the series of oil pumps is limited, and often lean up when the appropriate oil pump model cannot be selected, resulting in a serious phenomenon of "big horse pulling small car", resulting in a large waste of electric energy. Therefore, it is of great significance to carry out frequency conversion and speed regulation transformation for the injection molding machine of quantitative pump, save electric energy and improve economic efficiency

II. Energy saving analysis of injection molding machine

according to the process of injection molding machine, draw the relationship between system oil pressure P and time t, as shown in Figure 1

from the figure, it can be seen that the oil pressure required for mold closing and demoulding, mold opening system is low, and the time is short; The oil pressure required by the injection, pressure maintaining and cooling system is high and takes a long time, which is generally 40% - 60% of a working cycle. The length of time is related to the processing of workpieces; The interval is shorter, which is also related to the processing of workpieces. Sometimes the interval can be omitted. The above figure is only a simple approximation. In fact, if the injection screw is driven by an oil motor, the system oil pressure during injection will be higher. The weight of the workpiece processed by the injection molding machine ranges from tens of grams to tens of thousands of grams, and the largest injection molding machine has reached 92000 grams. Therefore, injection molding machines can be divided into medium-sized, small-sized and large-sized ones. The time of one cycle for processing small workpieces of tens of grams and large workpieces of several kilograms is also different; For the same injection molding machine, the raw materials for processing the workpiece are different, and the pressure and time required in each process flow are also different. The setting of these process parameters is formulated by the on-site technician based on empirical data and test conditions

it can be seen from Figure 1 that in a periodic workflow, the change of load leads to a relatively large change in system pressure, but the oil pump is still running at 50Hz, and its oil supply is constant. The excess hydraulic oil flows back to the oil tank through the overflow valve, doing useless work and wasting electric energy in vain. The frequency conversion and speed regulation experimental machine for oil pump is mainly used to measure the physical properties of materials or products, and change the quantitative pump into a characteristic similar to variable displacement pump. When the pressure required by the system is high, the oil pump motor operates at 50Hz. When the pressure required is small, the frequency converter operates at a reduced frequency. The output shaft power of the motor is proportional to the product of the outlet pressure and flow of the oil pump. When the speed of the oil pump motor decreases, the output shaft power decreases, which can achieve effective energy saving. Generally, the power saving rate is 20% - 50%

III. transformation scheme of frequency conversion, energy saving and speed regulation of injection molding machine

injection molding machines using hydraulic system can be divided into vertical and horizontal ones. Dozens of grams of vertical injection molding machine, the oil pump adopts a gear pump, the motor capacity is also small, and the electrical control circuit is also relatively simple. During the transformation, connect the frequency converter to the power supply circuit of the motor, and then convert the signal (0 ~ 1a) of the flow proportional valve into 4 ~ 20mA or 0 ~ 10V signal and send it to the corresponding port of the frequency converter. In this way, with the change of the processing process, the flow of hydraulic oil is also changing. Generally speaking, it is better to take the flow signal with large change in relative value as the control signal, and the change of control signal has a larger range of frequency regulation of the frequency converter; The relative value of the pressure signal changes less, and the range of frequency regulation of the frequency converter is smaller. If the frequency adjustment range of the frequency converter cannot meet the process requirements, it can be adjusted by the function "frequency gain" of the frequency converter. If Conway ZC or ZS series injection molding machine special frequency converter is used, the adjustment range of frequency gain is 50% - 200%. The special frequency converter for injection molding machine adds 0 ~ 1A signal conversion link on the basis of general frequency converter, which is more convenient to use

60 grams and above are horizontal injection molding machines. Injection molding machines of 60 to 500 grams have either one oil pump or two oil pumps. The transformation of an oil pump injection molding machine is the same as that of a vertical injection molding machine. The signal of 0 ~ 1a is still taken from the flow proportional valve as the speed regulation signal of the frequency converter. Although the speed regulation signal is fed back to the frequency converter by the hydraulic circuit elements, there is no given signal in the regulation circuit, so the control is still an open-loop control mode. Because of energy saving, there may be more than one oil pump for large and medium-sized injection molding machines, such as Mitsubishi 850-mm, 1300 wood plastic composite mm, 1800-mm and 2000-mm injection molding machines. Corresponding to the injection molding process, in the mold closing stage, the required system pressure is low, at this time, only 1# oil pump works. When the system pressure required in the mold locking stage is high, 2# oil pump is put into operation again. In the injection stage, the required pressure is the highest, three oil pumps are put into operation at the same time, and the pressure required for mold stripping and mold opening is low, and then stop 3# and 2# oil pumps respectively. As long as the machine is started, the 1# oil pump will run all the time. Using three small oil pumps to work intermittently according to different process stages is more energy-saving than using one large pump to operate all the time. How to reform the injection molding machine with more than two oil pumps? Here, the transformation of Mitsubishi 1800-mm injection molding machine is taken as an example to illustrate. Mitsubishi 1800-mm injection molding machine has three 45KW oil pump motors, which are driven by a frequency converter. The regulating signal of the frequency converter is taken from the flow proportional valve of the injection molding machine. In this way, the frequency of the frequency converter changes with the flow of the hydraulic oil of the injection molding machine. The other two oil pump motors can be driven by two frequency converters respectively. However, these two inverters do not regulate the speed of the motor, but only perform two position control, that is, start and stop. The start and stop signals of the control frequency converter are taken from the original start and stop signals of the oil pump motor. The upper limit frequency of the frequency converter is set below 50Hz, and the specific setting value is related to the workpiece size, material, barrel temperature and other factors. If the frequency converter operates at a frequency lower than 50Hz, it can save energy. In fact, there is allowance in the design of injection molding machine, and the oil pressure required for processing workpiece size and material changes will also change. If the injection pressure is too high and the clamping force is insufficient, the workpiece will have flash; If the injection force is insufficient, the plastic in the mold cavity will be insufficient and the workpiece will be scrapped; When the holding pressure is insufficient, the part with thick plastic in the workpiece will shrink. In this example, the operating frequency of the two frequency converters is 37hz, and the energy-saving rate of the injection molding machine is 23%

IV. precautions

1 Type selection of frequency converter

vane pump and plunger pump are widely used in the oil pump of injection molding machine. The mechanical characteristics are constant torque. Choose the frequency converter with constant torque characteristics, v/f control or vector control frequency converter, or choose the special type of injection molding machine. Considering that there are certain requirements for the time of each stage of the injection molding process, the acceleration and deceleration time of the frequency converter should be short, generally 1 second, and the capacity of the frequency converter should be appropriately increased. According to experience, the vector control frequency converter selects a larger gear, and the v/f control frequency converter selects a larger gear. In order to prevent overload from burning the motor, pay attention to setting the function of electronic thermal relay when debugging the frequency converter

note: if you use a special frequency converter for ZC or ZS injection molding machine, the frequency converter has reserved allowance and sufficient overload capacity when it is designed. You only need to choose a frequency converter with the same power

2. When the standby system

injection molding machine is transformed with frequency conversion and energy saving, the original power frequency starting circuit is retained as a standby, so that once the frequency converter fails, the oil pump motor can be started with power frequency to continue to operate

3. The interference of frequency converter on the digital instruments of injection molding machine

now the AC-DC-AC frequency converter is widely used in injection molding machine. Its output current contains harmonic components, which may interfere with the injection molding machine. The most vulnerable to interference is the temperature control instrument. Therefore, when installing the frequency converter, we should pay attention to:

(1) the frequency converter needs to be equipped with input and output reactors

(2) in order to avoid mutual interference, The control line leading in frequency conversion shall be shielded. 4 ~ 20mA current signal is preferred

(3) the inverter housing should be reliably grounded

(4) when the frequency converter is installed inside the injection molding machine, pay special attention to ventilation and heat dissipation

4. The temperature rise after the motor speed is reduced

after the motor is driven by the frequency converter, the temperature rise will increase by 10%. After the motor speed is reduced, the speed of the built-in fan will slow down, the heat dissipation efficiency will be reduced, and the temperature rise will increase; However, considering that the oil pump works intermittently, the temperature rise will not increase much. In the transformation of the injection molding machine we have done, some have added constant speed fans to the motor, and some have not. Whether to add constant speed fans or not depends on the temperature of the motor when it works

v. conclusion

the main purpose of frequency conversion speed regulation of injection molding machine is to save energy. From small, medium and large injection molding machines, frequency converters can be installed for energy-saving transformation. At the beginning of the transformation, it involves the estimation of energy-saving efficiency. The energy saving of injection molding machine is mainly related to the injection molding process. It is difficult to make an accurate calculation. Generally speaking, after transformation, the energy saving of injection molding machine with one oil pump motor is between 30% and 50%, and that of injection molding machine with multiple oil pump motors is between 15% and 30%. In addition, the reduction of oil pump speed reduces mechanical wear, and the indirect economic benefits cannot be underestimated

common troubleshooting of energy saving of injection molding machine

1. Why can the frequency converter of injection molding machine save more than 30% ~ 60%, and how does it achieve power saving

answer: because the injection molding machine is a typical periodic variable load equipment, a complete production cycle includes mold locking, glue injection, glue melting, cooling, mold opening and other stages. For its hydraulic system, the matching of pressure and flow in each stage is different, and the power of oil pump motor