There are many molding methods for manufacturing plastic containers, such as hollow blow molding, injection molding, rotational molding, compression molding, hot extrusion and cold pressing. Among the many molding methods for plastic containers, hollow blow molding and injection molding are the two most widely used and most productive molding methods. According to the introduction of the blow molding machine, 80% to 90% of the hollow containers in the world are formed by the hollow blow molding method. The hollow blow molding technology uses air pressure to make the plasticized parison close to the inner wall of the mold. The production method of the parison is extrusion type, that is, the plastic melt that has been heated and plasticized is extruded from the gaps of the mold cores of various shapes to form cylindrical parisons of various shapes. The extruded parison is suspended vertically by its own weight. The hollow container product has a minimum wall thickness due to its strength requirements, but the early hollow blow molding equipment lacks a parison wall thickness control system. In order to make the thinnest part of the product meet the minimum wall thickness requirement, the other parts of the product must be thickened accordingly , Resulting in waste of materials. In order to save costs, shorten the cooling time of products, and speed up the production cycle of products, a more economical approach is to control the wall thickness of the parison. For example, 501 wide-mouth barrels, the net weight of products produced by ordinary hollow blow molding machines is 3.5kg to meet the minimum wall thickness requirements, while using the parison controller, the net weight of products 2.4kg can fully meet the minimum wall thickness requirements and save raw materials 20 %-30%, and the thickness of the products produced is uniform.
1. Technical requirements for wall thickness control system
The wall thickness control system is a system that controls the opening and closing of the mold core gap, that is, the position servo system. In the production process of hollow containers, in order to ensure the quality of the product, the controlled quantity is required to track the set value correctly, and the response process is also required to be as fast as possible. According to the hollow blow molding machine, taking a simple 10-point wall thickness controller as an example, in the continuous extrusion mode, the shortest time for extruding a parison from the die is about 5s, and it needs to achieve a limited time within 5s. The length function curve needs to be divided into 10 sections. On the time axis, each section only has about 0.5s of time. According to the tracking theory, the unit pulse transition time of the wall thickness controller should be 15～110 of the section time. That is, 0.05 to 0.1s, which requires the system to respond very quickly. While responding so quickly, it is also necessary to ensure that the controlled position quantity can correctly track the set value, otherwise, the wall thickness control will lose its meaning. To achieve the above two required control effects, for heavy-load systems, it is precisely the difficulty of wall thickness control system design. In addition, since l0 points are taken on the contour curve of the parison to describe the curve, interpolation must be performed between points to make the contour curve as smooth as possible.
2. The control principle of the wall thickness control system
The wall thickness control system adopts a closed-loop feedback design, and its components include a wall thickness controller, an electro-hydraulic servo valve, an action actuator and an electronic ruler as a signal feedback device. The user sets the axial change curve of the parison wall thickness on the panel of the wall thickness controller. The controller outputs the voltage or current signal of the size change to the electro-hydraulic servo valve according to the curve. Machines, plastic packaging machinery, and bag-making machine-liquid servo valves drive the actuator to control the up and down movement of the mold core, resulting in changes in the gap between the mold cores. The electronic ruler obtains the corresponding voltage signal by measuring the size of the gap and feeds it back to the wall thickness controller. This constitutes a closed-loop wall thickness control system, as shown in Figure 1.
3. Current status of wall thickness controller
The level of plastic processing machinery depends to a large extent on the level of measurement and control.
The control system of foreign advanced plastic machinery generally adopts programmable system with programmable logic controller (PLC) as the core, and uses fuzzy control, statistical process control (SPC), and network-based on some high-precision plastic machinery. Remote monitoring, fault diagnosis and control system. The American company Moog (M00G) is a global leader in the design and manufacture of electro-hydraulic servo components and servo systems. In the field of blow molding control, its independent wall thickness control system provides 30-point and 100-point wall thickness controllers (DIGIPACK), which can be used to control the wall thickness of parisons in accumulator and continuous blow molding machines. Storage cylinder machines are often used to make larger containers. DIGIPACK controls the opening of the die according to the feedback of the electronic ruler of the accumulator. The ordinate on the liquid crystal display (LCD) displays the position of the accumulator, and the abscissa shows the opening of the die. Continuous machines usually have multiple die heads. DIGIPACK controls the die opening according to the working cycle time. At this time, the ordinate on the LCD displays the cycle time, usually with the cutter action as the beginning of a complete cycle. At present, in terms of hollow blow molding technology, the foreign trend is to use the whole machine control and the control of the upper and lower computers, and the wall thickness control is included in the control of the whole machine. Still taking M00G company as an example, it provides a complete machine control (TMC) solution. Wall thickness controllers used in China include products from companies such as Moog (M00G), B&R (B&R), and Siemens (SIEMENS), and foreign brands firmly occupy the domestic market. This situation is directly related to the low level of plastic processing in China. In 2003, the output of China’s hollow blow molding machines reached 3,405. Among the more than 3,000 machines, most of the machines were made by Chinese manufacturers and assembled with imported control systems. This results in high product costs, low added value, and unobvious benefits. If it can be equipped with a domestic wall thickness controller, the domestic hollow blow molding machine industry will naturally leap to a new level.
4. The future of wall thickness controller
Looking at the technology of the foreign plastic machinery industry, in the future, it will mainly develop in the following directions.
(1) Miniaturization and large-scale miniaturization is an important development direction for all kinds of products in the future. There are more and more market demands, and it has shown obvious development in electronics, information, electrical appliances, medical, biological and other sectors. Although there are already hollow blow molding machines for producing 3mL plastic bottles, some countries are already developing smaller-volume hollow container equipment for medical and biological applications. Large-scale is also one of the directions of future development. At present, commercial production equipment for producing 5000L hollow containers has appeared. The demand for various large-scale hollow containers for industrial use is obvious, and there is also a demand for plastic storage containers with a volume of 10,000L or more.
(2) Individualization. For a long time, the model, function, and specification of the hollow blow molding machine can not meet the market demand. Hollow container manufacturers need to adapt flexibly to adapt to the ever-changing market needs, and promote the modular design, technology integration, professional production, international procurement capabilities and level of the hollow blow molding machine. This requires both plastic machinery companies in technical personnel , With strong strength in technological innovation, it also requires companies to correctly grasp the individual needs of customers in the first time.
(3) The application of intelligent automatic control technology in the plastic machinery industry has developed to a very high level. Automatic control of equipment units, infinite loop control of parameters, process linkage, online feedback control, etc. have been widely used in plastic machinery with the help of electronic technology and computer technology. Simply put, the intelligent control system mainly refers to the ability to understand the ideas and intentions of the staff, to identify and detect work errors, to answer the questions raised by the staff, to propose solutions and measures to solve the problems, and to issue instructions to implement the corresponding production, etc. Functional control system. The development of intelligent plastic machinery will significantly improve the operating stability and reliability of plastic machinery, effectively improve the production capacity of plastic machinery with high quality, high efficiency and low loss, and provide a solid foundation for the realization of unmanned workshops and unmanned factories. technical foundation.
(4) Networking and virtualization are brand-new technologies in terms of concepts and models. They will greatly improve the competitiveness of plastic machinery manufacturers in terms of quality, efficiency, cost, service, and sales. The economic benefits of the company have increased significantly. Although the development of virtual technology is in the initial stage of development in various industrial sectors, due to the potential of virtual technology to bring huge economic benefits, it will provide a strong impetus for the future development of virtual technology.
The opening of the die is controlled according to the working cycle time. At this time, the ordinate on the LCD displays the cycle time, usually with the cutter action as the beginning of a complete cycle. At present, in terms of hollow blow molding technology, the foreign trend is to use the whole machine control and the control of the upper and lower computers, and the wall thickness control is included in the control of the whole machine. Still taking M00G company as an example, it provides a complete machine control (TMC) solution. Wall thickness controllers used in China include products from companies such as Moog (M00G), B&R (B&R), and Siemens (SIEMENS), and foreign brands firmly occupy the domestic market. This situation is directly related to the low level of plastic processing in China. In 2003, the output of China’s hollow blow molding machines reached 3,405. Among the more than 3,000 machines, most of the machines were made by Chinese manufacturers and assembled with imported control systems. This results in high product costs, low added value, and unobvious benefits. If it can be equipped with a domestic wall thickness controller, the domestic hollow blow molding machine industry will naturally leap to a new level.
5. Development countermeasures for wall thickness controllers in China
For a practical control system, advanced control theory is of course important, but more important is the experience in solving practical problems in practical applications. In view of the current production status of domestic hollow blow molding machines, the road to the development of wall thickness control systems in the future is mainly to start from vacancies, from scratch, from simple to complex. You can start with the 10 o’clock, applied to some machines that do not require high parison wall thickness accuracy, and develop a suitable wall thickness controller, first occupy the domestic low-end market, and then improve to develop a high-precision control Device. Increase the number of points to 30 or more, use advanced control methods, participate in international competition, and develop simultaneously with the world.
Post time: Nov-22-2021