Hottest analysis injection molding machine control

2022-08-21
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Analysis: injection molding machine control technology and development trend

plastic injection molding machine, referred to as injection molding machine, is the plastic machine with the largest output and application in China. China's first self-produced injection molding machine was born in the late 1950s. After more than 50 years of development, it has been able to produce most types of machines. Nevertheless, there is still a big gap between our technological level and that of advanced countries in the world. As Wang Xingtian, a famous plastic machine expert in China (Professor of Beijing University of Chemical Technology), said, plastic machine is the field that can best reflect the core technology value in the whole plastic industry, but in this field, we hardly have important core technology

the control technology of injection molding machine is one of the key technologies to improve the overall technical level of injection molding machine. The control technology of injection molding machine has experienced the development process of relay, contactor control, programmable controller control and computer control. The injection molding machine uses computer control technology, which makes it possible to intelligently control the parameters of the injection molding process to achieve the best. At the same time, the control mode has also changed from open-loop to closed-loop

I. control variables of injection molding process

from the current level of technological development, the process control of injection molding machine is mainly through the control of controllable process parameters of injection molding process

there are two controllable variables of the injection molding machine. One is the input variables in the form of analog quantity, such as the temperature of each section of the barrel, polymer melt temperature, screw temperature, mold temperature, injection system pressure, screw speed, etc; The other is the input variables in the form of digital quantity, such as screw backward stop position, injection seat front and rear position, template forward and backward movement, overtravel, etc. According to these input variables, we can control the injection parameters and temperature

1. Control of injection parameters

the control of injection parameters mainly includes the control of injection speed and injection pressure, PVT pressure maintaining process control, buffer volume control, back pressure control, etc. Injection speed refers to the speed at which the screw advances. Its speed will affect the injection pressure, melt flow rate and other parameters, and directly affect the quality of products. The control of injection speed includes two meanings: one is the open-loop or closed-loop control of the speed at which the screw (or plunger) pushes the material. The other is the multi-level switching of the position and speed value of the screw propulsion speed at the same time, which is called multi-level injection speed switching or control

for the control of injection pressure, the traditional injection molding machine generally switches the pressure based on the stroke and time, and its control also includes two meanings: one is to carry out open-loop or closed-loop control on the pressure of screw driven materials. The other is multi-level switching of position and pressure value of screw propulsion pressure at the same time, which is called multi-level injection pressure switching or control. The traditional injection molding machine adopts position switching from injection pressure to holding pressure, and the control method of switching from position switching to multi-level holding pressure with time at the holding stage is poor in accuracy and stability. The injection pressure control of advanced injection molding machine depends on the detection of mold cavity pressure to achieve pressure switching, that is, through a preset program to control. That is, the pressure value at the switching point is first stored in the upper control system in order not to cause too much time for the user, and then converted into a logic control signal. If the pressure in the mold cavity is the same as the set value, it is immediately switched to pressure maintaining control

PVT pressure maintaining control with high accuracy and stability is adopted in the pressure maintaining control of modern injection molding machines, that is, the pressure and temperature sensors are used to detect the mold cavity pressure, mold cavity temperature and material temperature at the nozzle, which are input to the control device as feedback signals for "PVT" (t-temperature of material in the mold cavity, V-specific volume, p-mold cavity pressure) logic operation, and then the command is issued to adjust the proportion of the proportional pressure solenoid valve, Thus, the system oil pressure during injection and pressure maintaining can change according to the instructions of PVT characteristic curve. The temperature of the melt in the mold is a function of time, which is obtained by continuous detection of the sensor

buffer volume control is the control of the remaining material at the screw head, which plays a role in buffering and controlling the accuracy of injection molding volume. It realizes the control by detecting the screw position through the displacement sensor and obtaining the set value of the next screw position after calculating according to the memory signal of the previous screw position detection

when the screw is pre molded, the pressure of the melt in the metering chamber is called the back pressure. Back pressure control is to reduce the axial temperature difference and facilitate melt compaction. It is realized by multi-stage control of preselection oil pressure. When the screw is pre molded, it will retreat under the action of materials, which will make the return oil in the injection cylinder cavity flow back to the oil tank through the back pressure valve. Therefore, by adjusting the oil discharge pressure of the back pressure valve at different positions of the screw, the pressure (shear heat) of the melt at the screw head can be adjusted, so that the axial temperature difference can be adjusted

2. Temperature control

temperature control mainly includes the control of barrel temperature, material mouth temperature, nozzle temperature, mold temperature and oil temperature

the barrel temperature is the heating temperature of the barrel surface. The temperature control of the barrel is an important parameter of the injection molding process. Poor temperature control will directly affect the quality of products. The heating section of the injection cylinder is generally divided into 3~4 sections, and a closed-loop control loop is formed by the control and adjustment of the heating resistance coil by the primary instrument (thermocouple) and the secondary instrument

the temperature control at the feeding port directly affects the friction coefficient and conveying efficiency of solid materials. Good temperature control can make the temperature of other materials in the barrel more stable. This temperature is detected by thermocouple and controlled by circulating cooling water flow controlled by PDPI

the quality of nozzle temperature control will not only affect the stability of the injection molding process, but also directly affect the shear heat and shear flow of high-temperature melt when passing through the nozzle. The nozzle temperature is controlled by thermocouples

mold temperature refers to the surface temperature of the mold cavity in contact with the product, which will affect the process of mold filling, cooling and pressure maintaining. The mold temperature is also controlled through the thermocouple with moderate particle size

the oil temperature of the injection molding machine refers to the temperature of the working oil of the hydraulic system, which has an important impact on the stability of the hydraulic system and the quality of injection molded products. Therefore, the injection molding machine system must be equipped with an oil temperature control device to realize the heating and cooling control of the oil temperature by the system through the detection of the temperature sensor

II. Progress of injection molding machine control technology in China

1. Traditional control mode

traditional injection molding machines generally adopt simple relay and contactor control, and the control mode is mostly open-loop control, that is, control according to the pre-set values (pre-set parameter values, which are maintained by the machine in the production process). This control method has simple structure, but poor anti-interference ability and low control accuracy

2. Programmable controller control mode

programmable controller (PLC) developed in the late 1960s is a better control device, which is composed of central processing unit (CPU), input and output (i/o), memory, programmer, etc. The control program is input into the memory by the user, and the CPU judges the output state according to the program in the scanning mode to control the field equipment

the application of PLC control device in injection molding machine in China mostly starts from the transformation of traditional control system. At present, most domestic injection molding machines use PLC control system. Using PLC control can realize the automatic control and adjustment of injection molding machine, and the control of process parameters in the injection molding process can also be basically guaranteed. PLC control mainly uses displacement sensors to switch action programs and process programs, and can realize proportional pressure and proportional flow control. At the same time, it can carry out on-off logic control, as well as closed-loop control of analog quantities (such as temperature, pressure, flow, speed, etc.). With the help of keyboard and LCD or digital display, the process parameters can be set or modified, and an early warning system and fault display indicator light are set to monitor the production process, which greatly improves the performance of the whole machine

because the PLC control mode can realize the closed-loop control of the injection molding machine, that is, it can be controlled through the negative feedback loop according to the deviation between the measured value and the set value. Therefore, when the injection speed, injection pressure, mold cavity temperature, mold cavity pressure, melt temperature, oil pressure and other parameters deviate due to interference in the production process, the machine can automatically correct the interference through the adaptive control local falling off or deformation control system. Thus, the anti-interference ability and control accuracy of PLC control mode have been greatly improved compared with the traditional mode

however, PLC control mode also has its shortcomings, such as its anti-interference, reliability and temperature control accuracy are not ideal at some times. In this regard, Professor Qu Jinping of the National Engineering Research Center for new polymer molding equipment of South China University of technology and others have adopted a new PLC device, namely bekale programmable computer controller (PCC), which has successfully upgraded the hydraulic mechanical large-scale injection molding machine. This is a new idea for the development of injection molding machine control system in China

3. Computer control mode

computer control is a more advanced control method, which is the development trend of injection molding machine control system

at present, microcomputer control systems mainly include single machine system and multi machine system. The single board structure of the single machine system can integrate the collection, conversion and processing of information. Its structure is composed of a/d, d/a, digital 1/0 and proportional amplifier or additional proportional amplifier. The system developed by Pan Cheng of Beijing University of science and technology and others, which combines the closed-loop control card to realize the oil pressure system of injection molding machine and the supporting upper computer monitoring software, belongs to this single machine system. The system has been put into trial in Zhenxiong group (Shenzhen) Co., Ltd., and the effect is good; The multiprocessor system adopts multiprocessor structure and large board design, which can allocate functions, and several CPUs independently complete the corresponding work. For example, one CPU is specially used to coordinate and transmit relevant instructions, and transmit data through parallel port and serial port. The structures of these two kinds of controllers have their own characteristics, and can basically meet the needs of different levels

it is worth mentioning that xuyuanchang of Shaanxi University of science and technology and others have developed a group control system scheme for the control of multiple injection molding machines. PC is used as the upper computer and its serial communication port is used to communicate with the lower computer 8098 to control the injection molding machine, thus realizing the function of host command and supervision slave computer. For the convenience of operation, the system also uses the PC operation interface as the man-machine interface, so as to realize the sequential action of the injection molding machine and the automatic control of process parameters such as temperature, speed and pressure

III. Development Trend of injection molding machine control technology

at present, the injection molding machine control technology in the developed countries in the world is relatively advanced and mature, which is characterized by the widespread use of real-time closed-loop control and process monitoring of CRT or LCD display foam particle machine screen, and the distribution of as much processing work as possible to peripheral devices through multiple processing, so as to improve the communication ability in the injection molding machine and reduce the load of the main control part. This represents the future trend of injection molding machine control technology to a certain extent

looking forward to the future, I believe that with the continuous development and improvement of control technology, the injection molding machine will develop in the direction of automation, energy saving, intelligence, unmanned and so on. If computer digital control is adopted

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