On the control of plastic injection molding machine
Modern plastic molding machine is a set of mechanical, electrical and fluid in one typical system, as this device has forming complex products, less post-processing, processing characteristics of the plastic variety, since its inception has grown rapidly, the current full Over 80% of the world’s products are used plastic injection molding machine for processing.
Conventional injection molding machines generally use a simple open-loop control, that is, in accordance with a pre-set value of control. In the device manufacturing process, pre-configured parameter values, such as the clamping force, cycle time, temperature, etc., from the machine in the production process be maintained. For example, by controlling the die temperature can be maintained the temperature of heating fluid, plastic temperature can be outside the heating device by controlling the power to maintain. This control method of simple structure, however, poor anti-interference ability to control the temperature is relatively low.
At present, more plastic molding machine uses a closed-loop control, that is, according to on-line measurement value and set the value of the deviation control. Closed-loop control system uses a negative feedback loop, anti-jamming ability, when the injection speed, injection pressure, mold cavity temperature, cavity pressure, melt temperature and oil pressure due to interference in the production process, deviation of the machine through the adaptive control system for automatic correction of interference. This control method of anti-interference ability, high control accuracy. More advanced control method is to apply computer controlled, that is, to build a closed-loop real-time computer control system. It includes direct digital control system (DDC), supervisory computer control system (SCC), distributed control system (DCS) and multi-level control of several types.
Commonly used injection molding machine control system has three kinds, namely, the traditional relay-based, programmable controllers and computer-based control type. In recent years, programmable logic controller (referred to as PLC) with its high reliability, high-performance characteristics in injection molding machine control system has been widely used. In order to improve the level of injection molding machine control system and quality, a higher level than the PLC, control system design specifically for small and medium programmable computer controller (PCC) came into being. PCC integrates a standard PLC and industrial control computer features, multi-task time-sharing operating systems, data computing and processing power more powerful than the PLC. PCC formed with injection molding machine control system in order to achieve, including position control, speed control, temperature control, fault control and real-time display the whole process such as injection molding a variety of control can greatly improve the quality of plastic products will help enhance economic benefits.
Control of the contents of the injection molding machine
Plastic molding machine process control system mainly consists of two parts: First, the temperature control system in order to right barrel, melt and mold temperature control; second motion control system to the process of injection pressure, velocity and displacement multiple class switching.
In the temperature control, its control accuracy has reached a ± 1 ℃. Precise temperature control in precision injection molding that help improve product quality and availability of raw materials is a very important indicator. In the plastic processing, temperature control include barrel, nozzle and mold temperature control. Barrel temperature of the barrel surface heating temperature, due to relatively thick wall barrel, so the choice of thermocouple detection point is critical, and different test points on the temperature curve is a large difference. Therefore, parallel detection of double-point, namely, surface and deep in the barrel at the same time set the thermocouples, will be relatively stable temperature curve is conducive to temperature control accuracy. Nozzle melt temperature directly affects the passage of the shear flow, the quality of the products have a major impact, so the nozzle temperature control precision demanding. Mold temperature is in contact with the product mold cavity surface temperature, it will significantly affect the charge membrane, cooling and packing process. For the mold temperature control method can be used control the heat carrier temperature method can also directly control the mold temperature. In the former approach, in order to heat the temperature for the control of the export vector objects, is relatively simple and can meet the general temperature control requirements. When the temperature control precision is high generally use the second method of temperature control.
Injection speed, packing pressure, back pressure is the injection of part of the sol to control the first of the three variables directly affect the accuracy of its control product quality. Modern more advanced injection molding machine equipped with 5-10 multi-stage injection speed and packing, as well as melt glue back pressure control. Generally through displacement / velocity sensors, pressure sensors, closed-loop injection control, and high response servo valves used in conjunction to achieve the process of injection molding melt glue back pressure, injection speed and packing conditions of precise control. Another relatively simple approach is to use closed-loop proportional valve, through the proportional valve spool position of their own closed-loop control to improve control precision. However, spool position is an intermediate variable, so the control accuracy somewhat less.
Transfer mold process control variable is the main clamping force, clamping force and time again is stable and the necessary conditions for forming the cycle. Transfer mode control of the process should be another important variable is location. On the one hand, advanced plastic molding machine constant pursuit of efficiency, directly affect the speed of transfer mold molding cycle, and quickly transfer to the deceleration control mode in itself has put forward higher requirements. On the other hand, due to special process of continuous development and promotion of position control precision molds have become increasingly demanding.
PLC and PCC
Since the late 20th century, 60 the United States the first programmable logic controller PLC inception, PLC control technology has gone through a 30-year development process, especially with modern computer technology and microelectronics technology, it has been in the soft hardware technology is far out of the original “sequence control” of the prototype stage. Programmable Computer Controller (PCC) is to represent the development trend of this new generation of programmable controllers.
PLC compared with conventional, PCC is a for motion control, process control and network control of dedicated control system, is a standard PLC, numerical control systems and industrial performance characteristics of the computer in one of the intelligent controller. PCC is its biggest feature is similar to a large computer time-sharing multi-tasking operating systems and diverse software applications design. Most of the conventional single-task PLC clock scanning or monitoring procedures to deal with the process itself of the logic operation instructions and external I / O channel status of collection and refresh. This treatment led directly to “control the speed,” depends on the size of the application, this result is no doubt that I / O channels in real-time control of the high demand religions. PCC’s system software the perfect solution to this problem, it uses time-sharing mechanism to build their applications multi-tasking software operating platform, so that the application program running the length of cycle and has nothing to do, but by the decision cycle of the operating system. As a result, it will scan cycle of the application with the real distinction between the external control cycle to meet the real requirements of real-time control. Of course, this control cycle in the CPU computing power to allow the premise, according to the user’s actual requirements, arbitrary changes.
PCC application modules from the multi-task structure, the development of application software to the project has brought great convenience. Because it can easily control the project in accordance with the various parts of the different functional requirements, such as motion control, data acquisition, alarm, PID regulator computing, communications, control, namely the preparation of a control program module (task), these modules not only operate independently, the data between one another to maintain a certain interrelated, these modules through the step by step after the independence of the development and debugging can be downloaded to the PCC along with the CPU, in the regulation and management of multi-tasking operating systems run in parallel under the joint realization of the project control requirements.
PCC a single task in the preparation of the different modules with a flexible choice of the characteristics of different programming languages, which means that not only in conventional PLC has been to people familiar ladder, instruction list language can continue to be used on the PCC, but users can be used more efficient and intuitive high-level language (PL2000), which is a complete text for the control of language, BASIC technical personnel familiar with the syntax it will feel a kind of deja vu, which control requirements for the description of a very simple, intuitive. In addition, PCC’s application software development also has an integrated “C” language program capacity, thus providing powerful data computing and processing power.
On the hardware structure, PCC is characterized by very significant. In its core computing module inside, PCC for its CPU is equipped with multiple conventional PLC, mass storage unit (100K/16M), this is undoubtedly a powerful system and application software provides a hardware-based monitoring. PCC on the hardware features, but also reflected in its various signals for industrial site designed many dedicated interface module, such as the high-frequency pulses, incremental encoder, temperature, weighing signal and ultrasonic signal interface module. They will all forms of on-site signal is very convenient to PCC joint into the core of the digital control system, the user may need to use the system’s hardware I / O channels in a single road, road, or dozens of way more than a decade as a unit module carried out dozens of point-to-hundreds of thousands of point-point expansion and networking.
PCC in the industrial control powerful advantages, and demonstrate the programmable controllers and industrial control computers and DCS (Distributed Industrial Control System) technology, the development trend of convergence, although this is still a relatively young technology, but in its more come more and more application areas, it is increasingly demonstrating its potential for development can not be underestimated. For the injection molding machine, the current widespread application of the PLC control system, but as technology advances, personalized, customized products, growing demand for injection molding machine control requirements more complex, higher and higher precision, PCC as a more advanced control system, will gradually occupy a certain market share. On the other hand, for the future network-based production requirements, obviously, PCC control system more competitive edge, so we have reason to believe, PCC control system will be in the future take place in the field of injection molding machines.