Automation, energy-saving, intelligent and unmanned are the inevitable directions for the development of machine tools. At present, control technology of plastic machine injection molding machines in the developed countries of the world is relatively advanced and mature. It is characterized by the commonly used real-time closed-loop control box CRT or LCD Screen process monitoring, and through the multi-processing will be as much as possible the distribution of processing to the peripheral device to complete, to improve the communications capabilities within the injection molding machine, reduce the main control part of compliance.
To a certain extent, this represents the future trend of injection molding machine control technology, which is also the future trend of injection molding machine control technology in China.
In the late 1950s, the first injection molding machine produced by our country, after more than 50 years of development, has been able to produce most of the models at present.Pressuring machine is the most embodying the core technical value in the whole plastics industry.
Since the appearance of injection molding machines, the control system has also been developed with the development of industrial technology.At present, the injection molding machine control system a wide range of different performance, but in general, its basic structure can be described by the following formula: Injection molding machine control system = drive system + electronic controller drive system drives all the movements of the injection molding machine, such as mold clamping mechanism, plasticizing mechanism, injection mechanism, ejection mechanism and so on.
Injection molding machine control technology is to enhance the overall technical level of injection molding machine is one of the key technologies. Injection molding machine control technology after the relay, contactor control and programmable logic controller and computer control of the development process.
As the use of computer-controlled injection molding machine control technology, the intelligent control of injection molding process parameters to achieve the best possible, while the control also took place from open-loop to closed-loop changes.
First, the control of injection molding process variables
From the current level of technological development point of view, the injection molding machine process control is mainly through the injection molding process controllable process parameters.
There are two kinds of controllable variables in the injection molding machine. One is the basic input variables in analog form, such as the temperature of each section of the barrel, the temperature of the polymer melt, the temperature of the screw, the temperature of the mold, the pressure of the injection system, the screw speed, etc .; One type is the input variables in digital form, such as screw back stop position, injection seat before and after the position, the template forward and backward movement and overtravel.
Based on these input variables, we can control injection parameters and temperature.
1, injection molding parameters control
Control of injection parameters are injection speed and injection pressure control, PVT packing process control, buffer control, back pressure control, etc. Injection speed refers to the speed of the screw forward, the speed will affect the injection pressure, Melt flow rate and other parameters, and directly affect the product quality.
The control of injection speed includes two meanings: one is open-loop or closed-loop control of the screw (or plunger) to promote the material speed and the other is to switch the screw advance speed at the same time multi-level position and velocity values, called Multi-stage injection speed switching or control.
For the injection pressure control, the traditional injection molding machine is generally based on the stroke and time to switch the pressure, the control also contains two meanings: one is the pressure on the screw to promote the material open-loop or closed-loop control the other is to push the screw pressure For multi-level switching position and pressure values, known as multi-stage injection pressure switch or control.This traditional injection molding machine from injection pressure to the holding pressure using the position switch, and to the holding stage from the position switch to the use of time The control method of switching multi-level packing pressure is poor in precision and stability.The injection pressure control of advanced injection molding machine is to control the pressure by detecting the pressure in the mold cavity, that is, through a preset program to control. Is the switching point pressure value is stored in the control system, then converted to logic control signal, if the pressure in the cavity and set the same value, then immediately switch to packing control.
The pressure control of modern injection molding machine uses the PVT packing control with high accuracy and stability. That is to say, the pressure and temperature sensor are used to detect the cavity pressure, the temperature of the cavity and the temperature of the material at the nozzle, and the input is used as a feedback signal The control device performs the logical operation of PVT (the temperature of the material in the T-die, V-specific volume and P-cavity pressure) and then sends a command to the proportional pressure solenoid valve for proportional adjustment so that the injection The system hydraulic pressure can be varied as instructed by the PVT characteristic curve.The temperature of the in-mold melt is a function of time, which is obtained by continuously detecting the temperature by the sensor.
The buffer volume control is the control of excess material on the head of the screw, which plays the role of buffer and control the precision of the injection volume.It is through the displacement sensor to detect the position of the screw, and according to the memory signal detected by the position of the previous screw Get the next screw position settings to achieve control.
Screw in the pre-molding, the pressure measurement chamber melt called back pressure back pressure control is to reduce the axial temperature difference, is conducive to melt compaction, it is pre-selected hydraulic multi-level control to achieve when When the screw is pre-molded, it should be retreated under the action of the material. This will cause the oil in the injection cylinder cavity to flow back to the tank through the back pressure valve. Therefore, by adjusting the drain pressure of the back pressure valve in different positions of the screw, the screw Head melt pressure (shear heat), so that the axial temperature difference is adjusted.
2, temperature control
Temperature control mainly includes the cylinder temperature, nozzle temperature, nozzle temperature, mold temperature, oil temperature control. Cylinder temperature is the barrel surface heating temperature.
The temperature control of the barrel is an important parameter of the injection molding process, the temperature control is not good will directly affect the quality of the product. The heating section of the injection barrel is generally divided into 3 to 4 sections, through a meter (thermocouple) and secondary meter heating Resistor ring control and regulation to form a closed-loop control loop.
The temperature control at the material inlet directly affects the friction coefficient and conveying efficiency of the solid material, and good temperature control can lead to a more stable control of the temperature of the material of other parts of the material tube. The temperature is detected by a thermocouple and controlled by PDPI Water flow to achieve control.
The nozzle temperature control will not only affect the stability of the injection molding process, but also directly affect the high-temperature melt through the nozzle when the shear heat and shear flow nozzle temperature control is done through the thermocouple.
Mold temperature is the temperature of the surface of the cavity in contact with the product, which affects the mold filling, cooling and holding pressure processes. The mold temperature control is also done via a thermocouple.
Injection molding machine oil temperature refers to the working fluid temperature of the hydraulic system, which has an important impact on the stability of the hydraulic system and injection molding product quality, injection molding machine system must be equipped with oil temperature control device, the temperature sensor to detect System to achieve the heating and cooling oil temperature control.
Second, China's injection molding machine control technology progress
1, the traditional control methods
The traditional injection molding machine generally adopts a simple relay and contactor control. Most of the control methods are open-loop control, that is, control is performed according to a preset set value (a preset value of each parameter is maintained by the machine in the production process). This control method has the advantages of simple structure, poor anti-interference ability and low control accuracy.
2, programmable controller control mode
The programmable logic controller (PLC) developed in the late 1960s is a good control device consisting of a central processing unit (CPU), input / output (I / O), memory, programmer, etc. The control program is entered into the memory by the user, CPU to scan the work in accordance with the program to determine the output state, to implement the control of the field devices.
Most of the application of PLC control device to plastic injection molding machine in China started from the reform of traditional control system, most of the domestic injection molding machine adopts PLC control system.Using PLC control can realize the automatic control and adjustment of injection molding machine, injection Process parameters of the molding process can also be basically guaranteed control.PLC control is mainly the use of displacement sensors and other procedures for the switching process and procedures, and to achieve proportional pressure and proportional flow control. At the same time can switch logic control can be both Closed-loop control with analogue (eg temperature, pressure, flow, speed, etc.) With keypad and LCD or digital display, process parameters can be set or modified with warning system and fault indication indicator to monitor production process, Thus greatly improving the overall performance.
Because the PLC control mode can make the injection molding machine realize the closed-loop control, that is, the deviation between the on-line measurement value and the set value is controlled by the negative feedback loop. Therefore, when the injection speed, injection pressure, cavity temperature, cavity pressure, When the parameters such as hydraulic pressure deviate due to interference in the production process, the machine can automatically correct the interference by the adaptive control system, so that the anti-interference ability and control accuracy of the PLC control mode are greatly improved compared with the traditional mode. However, PLC control methods also have their shortcomings, such as its anti-interference, reliability and temperature control accuracy is sometimes not ideal.
3, computer control
The use of computer control is a more advanced control method, which is the development trend of injection molding machine control system.
At present, there are two main types of microcomputer control system: stand-alone system and multi-machine system.The stand-alone system integrates information acquisition, conversion and processing with a single board structure, and its structure is composed of A / D, D / A, As well as a proportional amplifier or an external proportional amplifier.
Third, injection molding machine control technology trends
At present, plastic machinery injection molding machine control technology of the developed countries in the world is relatively advanced and mature, which is characterized by real-time closed-loop control and process monitoring of the CRT or LCD display and distributes as much processing as possible to the peripheral device through multiple processes , In order to improve the communication ability in the injection molding machine, reduce the main control part load.This to a certain extent, represent the injection molding machine control technology in the future.