There are many methods for manufacturing plastic containers, such as extrusion blow molding, injection molding, rotary molding, compression molding, hot extrusion cold press molding.
Among the molding methods of many plastic containers, hollow blow molding and injection molding are the two most widely used molding methods.
According to the introduction of blow molding machines, 80% to 90% of the hollow containers in the world are formed by hollow blow molding.
Extrusion blow molding technology is to shape the plasticized parison against the inner wall of the mold by air pressure.
The parison is produced in an extrusion type, that is a plastic melt which is heated and plasticized is extruded from a core gap of various shapes to form a cylindrical parison of various shapes.
The extruded parison is vertically suspended by its own weight.
Hollow container products due to their strength requirements.
The minimum wall thickness is specified, and the early hollow blow molding equipment lacks the parison wall thickness control system.
In order to make the thinnest part of the product reach the minimum wall thickness requirement, the other parts of the product should be correspondingly thickened, resulting in waste of materials.
In order to save costs, shorten the cooling time of products, and speed up the production cycle of products, it is more economical to control the wall thickness of the parison.
For example, 50 liter mouth bucket, the net weight of the product produced by ordinary hollow blow molding machine is 3.5kg to reach the minimum wall thickness requirement, and the use of the parison controller, the net weight of the product 2.4kg can fully meet the minimum wall thickness requirement, saving raw materials 20 % to 30%, and the products produced are even and thin.
Technical requirements for parison control systems
The wall thickness control system is a system that controls the opening and closing of the core slot, that is the position servo system.
In the production process of the hollow container, in order to ensure the quality of the product, the controlled quantity is required to correctly track the set value, and the response process is also required to be as fast as possible.
According to the blow molding machine, a simple 10-point wall thickness controller is used. In the continuous extrusion mode, a parison is extruded from the die for a minimum time of about 5 s, and a limited time is realized within 5 s.
The function curve of length needs to be divided into 10 paragraphs. On the time axis, each paragraph has only about 0.5s.
According to the tracking theory, the unit pulse transition time of the wall thickness controller should be 1/5 of the passage time. 1/10 which is 0.05 to 0.1 s, requires the system to respond very quickly.
In the case of such a rapid response, it is also necessary to ensure that the controlled position amount can correctly track the set value, otherwise, the wall thickness control will lose its meaning.
To achieve the control effect of the above two requirements, it is the difficulty of the wall thickness control system design for the heavy load system.
In addition, since the curve is drawn by taking 10 points on the parison contour curve, interpolation must be performed between the points to make the contour curve as smooth as possible.
Control principle of parison control system
The parison control system adopts a closed-loop feedback design, and its components include a parison controller, an electro-hydraulic servo valve, an action actuator, and a displacement sensor as a signal feedback device.
The user sets the parison wall thickness axial curve on the panel of the parison controller, and the controller outputs a voltage or current signal according to the curve output size to the electro-hydraulic servo valve, which is blown by a blow molding machine, a blown film machine, a blister machine.
The plastic packaging machine and the bag-making machine liquid servo valve drive actuator control the up-and-down movement of the core, thereby causing the change of the core gap.
The electronic ruler draws the corresponding voltage signal back to the parison controller by measuring the size of the gap.
This constitutes a closed loop parison control system.
Current status of parison controller
The level of production of plastic processing machinery depends to a large extent on the level of measurement and control.
The world’s advanced plastic machinery control systems generally use programmable logic controller (PLC) as the core of the programmable system, and on some high-precision plastic machinery using fuzzy control, statistical process control (SPC), and network-based Remote monitoring, troubleshooting and control systems.
The US 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, a 30-point, 100-point wall thickness controller (DIGIPACK) is provided in its independent wall thickness control system to control the parison wall thickness of the storage tank 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 storage cylinder.
The vertical coordinate on the liquid crystal display (LCD) shows the position of the storage cylinder, and the abscissa shows the opening of the die.
Continuous machines usually have multiple dies.
DIGIPACK controls the opening of the die according to the duty cycle.
At this time, the ordinate on the LCD shows the cycle time, usually with the cutter action as the beginning of a complete cycle.
At present, in the field of hollow blow molding technology, the trend of the world is controlled by the whole machine control and the upper machine and the lower machine, and the wall thickness control is included in the whole machine control.
Still taking GEFRAN as an example, it provides a complete machine control (TMC) solution.
The famous wall thickness controllers are products of Moog (M00G), GEFRAN, B&R, SIEMENS, etc.
Our PANTONE MACHINERY blow molding machine uses GEFRAN 300 points or MOOG100 points. Parison control system.
The future of the parison controller
Looking at the world’s plastic machinery industry technology, the future will focus on the following developments.
- Miniaturization and large-scale miniaturization are important development directions for various products in the future. There are more and more market demands, and they have shown obvious development in electronics, information, electrical appliances, medical care, and biology. At present, there are already extrusion blow molding machines for producing 3mL plastic bottles, but the production of smaller containers for medical and biological hollow container equipment has been developed in some countries. Large-scale is also one of the directions for future development. Commercial production equipment for producing 5000L hollow containers has appeared. The demand for various large hollow containers for the industry is obvious, and plastic storage containers of 10000L or even larger volumes are also in demand.
- For a long time, the uniformity and fixedness of the models, functions and specifications of the extrusion blow molding machine have not been able to meet the market demand. Hollow container manufacturers need to adapt flexibly to adapt to the ever-changing market demand, and promote the modular design, technology integration, professional production, international procurement capability and level of hollow blow molding machines, which requires plastic machinery enterprises to be technical talents. , technical innovation has a strong strength, but also requires enterprises to correctly grasp the individual needs of customers in the first time.
- The application of intelligent automatic control technology in the extrusion blow molding machine industry has developed to a relatively high level. Automatic control of equipment units, infinite loop control of parameters, process linkage, online feedback control, etc. are widely used in extrusion blow molding machines by means of electronic technology and computer technology. Simply put, the intelligent control system mainly refers to having the ability to understand the concept and intention of the staff, to identify and detect work errors, to answer questions raised by the staff, to propose solutions and measures, and to issue instructions to implement the corresponding production. Functional control system. The development of intelligent extrusion blow molding machine will significantly improve the operation stability and reliability of the extrusion blow molding machine, and earnestly improve the high-quality, high-efficiency, low-loss production capacity of the extrusion blow molding machine, and achieve The human workshop and the unmanned factory provide a solid technical foundation.
- Networking and Virtualization This is a brand new technology in terms of concept and mode. It will make the extrusion blow molding machine manufacturers’ competitiveness in terms of quality, efficiency, cost, service and sales. As a result, the economic benefits of enterprises have increased significantly. Although the development of virtual technology is in the initial stage of development in various industries, however, due to the potential of virtual technology to bring huge economic benefits, it will provide a powerful impetus for the future development of virtual technology.
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