Interpretation: control system of modern injection molding machine

Time:2022-11-14 09:09:27 / Popularity: / Source:

Modern plastic injection molding machine is a typical system integrating machine, electricity and liquid. Due to characteristics of this equipment, such as molding complex products, less post-processing, and various types of plastics to be processed, it has developed extremely rapidly since its inception. More than 80% of engineering plastic products in the world are processed by injection molding machines.
Modern plastic injection molding machine 
Traditional injection molding machines generally use simple open-loop control, that is, control according to preset values. During manufacturing process of equipment, parameter values, such as clamping force, cycle time, temperature, etc., are preset and maintained by machine during production process. For example, mold temperature can be maintained by controlling temperature of heating fluid, and plasticizing temperature can be maintained by controlling power of external heating device. This control method has a simple structure, but anti-interference ability is poor, and control temperature is relatively low.
At present, more injection molding machines use closed-loop control, that is, control according to deviation between online measured value and set value. Closed-loop control system adopts a negative feedback loop, which has strong anti-interference ability. When injection speed, injection pressure, cavity temperature, cavity pressure, melt temperature and oil pressure deviate due to interference during production process, machine automatically corrects interference through adaptive control system.
This control method has strong anti-interference ability and high control precision. A more advanced control method is to use a computer to control, that is, to build a closed-loop real-time computer control system. It includes several types such as direct digital control system (DDC), supervisory computer control system (SCC), distributed control system (DCS) and multi-level control.
At present, there are three commonly used injection molding machine control systems, namely traditional relay type, programmable controller type and microcomputer control type. In recent years, programmable logic controller (PLC for short) has been widely used in injection molding machine control systems due to its high reliability and high performance. In order to improve level and quality of injection molding machine control system, a programmable computer controller (PCC), which is higher level than PLC and specially designed for small and medium-sized control systems, came into being.
PCC integrates characteristics of standard PLC and industrial control computer, has multi-task time-sharing operating system, has more powerful data operation and processing capabilities than PLC. PCC is used to form control system of injection molding machine to realize various control of the whole injection molding process including position control, speed control, temperature control, fault control and real-time display, which can greatly improve quality of plastic products and help improve economic benefits.
Content of injection molding machine control
At present, process control system of injection molding machine mainly includes two parts: one is temperature control system, which controls temperature of barrel, melt and mold; the other is motion control system, which performs multi-level switching of pressure, speed and displacement of injection molding process.
Modern plastic injection molding machine 
In temperature control, its control accuracy has reached ±1℃. Accurate temperature control is a very important indicator in precision injection molding, which is conducive to improving product quality and utilization of raw materials. In process of plastic processing, temperature control mainly includes temperature control of barrel, nozzle and mold.
Barrel temperature is heating temperature of barrel surface. Because wall of barrel is relatively thick, selection of thermocouple detection point is very critical, and temperature curves of different detection points are quite different. Therefore, double-point parallel detection, that is, setting thermocouples on the surface and depth of barrel at the same time, will obtain a relatively stable temperature curve, which is beneficial to accuracy of temperature control. Nozzle temperature directly affects shear flow when melt passes through, and has a great impact on quality of product, so control accuracy of nozzle temperature is higher. Mold temperature refers to surface temperature of mold cavity in contact with product, which will significantly affect film filling, cooling and pressure holding processes.
For control method of mold temperature, method of controlling temperature of heating carrier can be used, or mold temperature can be directly controlled. In former method, control object is outlet temperature of heating carrier, which is relatively simple and can meet general temperature control requirements. When temperature control accuracy is required to be high, second method is generally used for temperature control.
Injection speed, holding pressure, and sol back pressure are the first three variables to be controlled in injection part, and their control precision directly affects quality of product. Modern more advanced injection molding machines have 5 to 10 levels of injection speed and multi-stage holding pressure and sol back pressure control. Generally, through combination of displacement/velocity sensor, pressure sensor, closed-loop injection controller and high-response servo valve, precise control of sol back pressure, injection speed and pressure holding conditions during injection molding is realized. Another relatively simple method is to use a closed-loop proportional valve to improve control accuracy through closed-loop control of spool position of proportional valve itself. However, spool position is an intermediate variable, so control accuracy is slightly less.
The most important variable in mold transfer process control is clamping force, and repeated reproduction of clamping force is a necessary condition for a stable molding cycle. Another important variable that should be controlled during die transfer is position. On the one hand, advanced injection molding machines continue to pursue improvement of efficiency, and speed of mold transfer directly affects molding cycle, and rapid mold transfer itself puts forward higher requirements for deceleration control. On the other hand, due to continuous development and promotion of special processes, control accuracy of mold position is getting higher and higher.

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