With the development of industrial technology and the improvement of people’s living standards, people’s requirements for the types and quality of plastic products are increasingly increasing. In-depth research on plastic molding equipment and related control technologies to overcome defects in products and improve product quality for plastic molding The improvement of technology is of great significance. In the plastic molding equipment, extrusion molding and injection molding are the most widely used. Therefore, the control technologies of these two kinds of equipment are introduced in detail.
Extrusion equipment control
1, the need for extrusion molding control system
(1) Energy saving. In the extrusion molding process, the energy consumption is mostly used for the molten resin and the drive motor. Thermal energy includes the friction heat generated by the screw rotation and the heat energy of the barrel heater. With the increasingly tense energy supply , It is very necessary to research and develop extrusion molding equipment with high efficiency and energy saving.
(2) Stability. The stability of the extrusion process includes the quality stability and dimensional stability of the product. For the dimensional measurement of simple products such as sheets, films, and tubes, direct measurement and control can be performed by excellent sensors and computers. For complex shaped products with complex shapes, it is difficult to directly measure the dimensions. Therefore, the production process itself must be stable. Therefore, it is necessary to research and develop high-precision and high-stability control systems for extrusion molding equipment.
2, the design of precision extrusion control system
Extrusion control system requires process parameters for the entire extrusion process, such as melt pressure and temperature, temperature of each section of the fuselage, rotation speed of the main screw and feeding screw, feeding amount, proportion of various raw materials, and current and voltage of the motor. Parameters such as online detection and closed-loop control technology.
The design of extrusion control system includes: selection of main controller, interface circuit design, drive and amplifier circuit design, human-machine interface design (HMI) and control algorithm software design, etc. According to the requirements and characteristics of precision extrusion control, the main The controller can use PCC2003 programmable computer control system. The programmable computer controller integrates the standard functions of the programmable logic controller and the time-sharing multitasking operating system function of the industrial computer. It can easily handle the switch quantity, analog quantity and loop Adjustment. Its hardware has a unique and novel plug-in structure, which allows the system to be flexibly and variously expanded and configured. The software also has a module structure. The system expansion only needs to superimpose the application software module on the original basis and also has high-level language programming function. , And can be mixed with multiple language programming as needed. Precision extrusion process control system block diagram shown in Figure 1.
Figure 1. Block diagram of a precision extrusion process control system
In Fig. 1, the CPU module is CP476, with an RS232 interface and a CAN (Control Area Net) interface. Its instruction cycle is 0.5us, built-in hardware dog function. Interface components include analog input module, analog output Module, digital input and output mixing module, temperature module and communication interface module.
Injection molding machine control
1, Closed-loop control technology of precision injection molding machine
(1) Features of precision injection control Compared with conventional injection molding, precision injection molding requires higher repetitive accuracy of molding parameters. Therefore, precision injection molding machines should adopt multi-stage feedback control. This multi-stage control includes: Inductive displacement Control or time-controlled 10-shot closed-loop injection system, multi-stage holding time, pressure and speed control, and screw temperature intelligent PID control.
In addition, the screw and nozzle temperature control is more precise. That is, when overheating, the overshoot is small, so that the temperature fluctuation is small. For the conventional control, the temperature fluctuation caused by screw metering is 25 to 30°C. Above 4°C, precision temperature control precision injection molding machine is within ±0.5°C under the same conditions.
Hydraulic oil temperature control accuracy is higher. The change of working oil temperature will cause viscosity change, make the flow into each actuator fluctuate, cause open and close mold speed, injection speed and screw speed instability, but also lead to pressure fluctuations. For a typical small injection molding machine, if the oil temperature is not adjusted for 5 hours, the oil temperature will increase by 28°C, and the system pressure will increase by 0.19 MPa, which translates into about a holding pressure of the polymer melt in the flow channel. 1.9~2.9MPa, which will inevitably affect the dimensional deviation of the products. In order to prevent this from happening, the precision injection molding machine adopts a closed loop device with heating and cooling, so that the oil temperature of the working oil can be stabilized at 50~55°C.
The mold temperature control is stricter, and the dimensional accuracy of the product is greatly affected by the mold temperature. The thickness of different material products is not only affected by the cooling time, but also affected by the mold temperature. If the cooling time is the same, the mold cavity temperature is low, then the product Relatively large thickness.
Based on the above characteristics of precision injection molding control, the control system of precision injection molding machines is much more complex than conventional injection molding machines. It needs to achieve full closed-loop control of parameters such as pressure and speed.
(2) Closed-loop control principle and characteristics. The principle of closed-loop control is shown in Fig. 2. It can be seen from the figure that the starting point of closed-loop control is to measure the target variable, and a sensor corresponding to the target variable is used to detect the target variable. Generates an output proportional to the target variable, typically a voltage, current signal or analog signal. This signal is fed back to the closed-loop controller and compared with the setpoint. When the target variable deviates from the setpoint, the closed-loop controller generates With additional control signals, the control object is adjusted by the actuator so that the target variable tends to be consistent with the set requirements. In the above process, the closed-loop control can also eliminate the influence of different disturbances on the control object, thereby further improving the control. Accuracy.
The closed-loop control system has the following characteristics: The output signal of the system has a direct influence on the control function; there is a feedback link, and the feedback function is used to reduce the system error so that the output of the system tends to a predetermined value; when the interference occurs, the internal disturbance includes External interference can weaken its impact; the system may be unstable, so there is temperature checking problem (stability criterion).
2, Ultra-high speed injection molding machine control technology
Ultra-high-speed injection molding machines have higher injection speeds than conventional injection molding machines and can generally reach 1000mm/s or more. Such high injection speeds can bring the following benefits to injection molding and injection molding products:
(1) The extremely high shear rate during processing reduces the viscosity of the plastic, making it easier to achieve ultra-thin molding and reduce distortion and warpage of the product.
(2) It can reduce the surface rhyolite and fusion line, thereby improving the surface gloss of the product and the strength of the welding part, and prevent cooling deformation.
(3) shorten the injection cycle, improve injection efficiency and energy-saving effect.
In order to achieve ultra-high speed injection, special methods are required. For example, high-performance linear motors can be used in all-electric injection molding machines to achieve ultra-high injection speeds. Or, ultra-high-speed response in hydraulic injection molding machines can be used. High-precision large-capacity accumulators, and fast-response servo valves or proportional valves, can meet ultra-high injection speed requirements.
At present, the latter method has been industrially applied, such as the SW2HSB series from INEWELL MACH INERY, the SE2HY series from SUMITOMO, and the FT2260BMC type BF2260BMC type injection molding machine from BMC, all adopting this method. Figure 2 shows this method. Ultra-high speed injection molding machine hydraulic system structure.
Figure 2. Schematic diagram of hydraulic system for ultra-high speed injection molding machine
3 injection molding machine energy-saving control technology
Since the beginning of the 21st century, with the development of the plastics industry, injection molding processors have increasingly demanded the energy saving and environmental protection of injection molding machines. This has led to widespread attention on the energy-saving control technology of injection molding machines and the rapid development of energy-saving control technologies. The development. It can be said that energy saving has become an important direction for the development of the current injection molding machine.
(1) Proportional variable pump system. The principle of load-sensing control combined with the valve pump, although it can eliminate all flow-related energy losses caused by the proportional pump and three-way pressure flow proportional valve, but the proportional throttle valve is fixed. The operating pressure difference leads to large throttling losses, especially at high speeds. This loss is even more pronounced.
In order to solve this problem, a high-response variable displacement pump with direct flow closed loop control is used as a power source, and the conventional throttle speed control system is converted into a proportional variable speed control system, thereby realizing the hydraulic system of the injection molding machine from the valve-controlled pump. Control change.
The simplified circuit principle of the proportional variable pump system is shown in Fig. 3. This system changes the conventional proportional control method. It is changed from the proportional pump + PQ proportional valve system to the proportional variable pump system. The core control element adopts a proportional control system. Load-sensitive proportional variable displacement plunger pumps with multiple compound control functions such as pressure, proportional flow and load pressure feedback.
Figure 3. Scaling variable pump system schematic
The proportional variable pump system is compared with the conventional fixed pump + PQ proportional valve system. The energy saving effect is obvious. At the same time, the injection rate of the same power machine can be increased, and the system heat is reduced. However, the variable pump requires higher cleanliness of the oil. The noise is also higher.
(2) Variable frequency hydraulic control system. The energy-saving principle of variable frequency hydraulic control technology is that the inverter regulates the speed of the fixed-displacement pump motor to achieve real-time control of the working flow rate of the hydraulic system of the injection molding machine, so that the flow rate of the quantitative pump just meets the injection molding. The flow required by the process, so as to basically achieve the purpose of no overflow loss.
At present, the frequency conversion hydraulic injection molding machine power source mainly uses two types of variable frequency drive methods: one is that ordinary asynchronous motors form a power source through variable frequency speed regulation and quantitative pumps; the other is that with high-response AC (AC) servo motor Driving a quantitative pump as a power source.
The control system of the injection molding machine adopting the first energy-saving control scheme is shown in Fig. 4. In this scheme, the inverter for injection molding machine controls the motor by acquiring the proportional flow valve, the current control signal of the proportional pressure valve and the process flow signal of the injection molding machine. The speed of rotation.
Figure 4. Variable frequency hydraulic speed control system
Conclusion
After more than 20 years of development, China’s plastics machinery industry has made considerable progress. Large and small plastics machinery manufacturers have reached several thousand. Some domestic plastics machinery enterprises have seized the golden opportunity in recent years and achieved a leapfrog development. Occupy a large domestic market share, and there are a considerable number of products exported abroad.
However, as far as the overall level of technology is concerned, the gap between China's plastics machinery and foreign developed countries is still very large. The level of plastics processing equipment depends to a great extent on the level of measurement and control. The control systems of foreign advanced plastics machinery are widely used. Programmable logic controllers (PLCs) are the core control systems, and they use fuzzy control, statistical process control (SPC), and remote monitoring and fault diagnosis based on the network in some high-precision production equipment. There is still a large gap in this area. Therefore, in China, the development of a new generation of plastic molding equipment based on intelligent control technology will bring high economic and social benefits.