Implementing wall thickness control in extrusion blow molding machines

There are many molding methods for manufacturing plastic containers, such as hollow blow molding, injection molding, rotational molding, compression molding, hot extrusion cold forming, and the like. 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 data, 80% to 90% of the hollow containers in the world are formed by hollow blow molding. The hollow 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 in which a plastic melted plastic melt is extruded from various core gaps to form cylindrical parisons of various shapes. The extruded parison is vertically suspended by its own weight. Hollow container products have a minimum wall thickness due to their strength requirements, while early hollow blow molding machines lacked 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. Causes 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, 501 wide mouth barrel, 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.
1. Technical requirements for wall thickness control system
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 accurately track the set value, and the response process is also required to be as fast as possible. Taking a simple 10-point wall thickness controller as an example, in the continuous extrusion mode, a parison is extruded from the die for a minimum time of about 5 s, and a finite length function curve is realized in 5 s, which needs to be Divided into 10 paragraphs, on the time axis, each paragraph only has a time of about 0.5s. According to the tracking theory, the unit pulse transition time of the wall thickness controller should be 1/5 to 1/10 of the passage time, that is, 0.05 to 0.1. s, this requires the system to respond very quickly. In the case of such a rapid response, it is also necessary to ensure that the amount of position controlled can accurately 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 design of the wall thickness control system 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.
2. Control principle of 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 curve of the parison wall thickness on the panel of the wall thickness controller, and the controller outputs the voltage or current signal according to the curve output size to the electro-hydraulic servo valve, by electric blow molding machine, blown film machine, plastic The machine, the plastic packaging machine, and the bag machine fluid servo valve drive actuator control the up and down movement of the core, thereby causing a change in the core gap. The electronic ruler returns the corresponding voltage signal to the wall thickness controller by measuring the size of the gap. This constitutes a closed loop wall thickness control system.
3. Status of wall thickness controller
The level of plastic processing machinery depends to a large extent on the level of measurement and control.
Foreign 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 tank 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 molding technology of hollow blow molding machines, the trend of foreign countries is controlled by the whole machine control and the upper machine and the lower machine, while the wall thickness control is included in the whole machine control. Still taking M00G as an example, it provides a complete machine control (TMC) solution. The wall thickness controllers used in China are products of 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, China’s production of hollow blow molding machines reached 3,405 units. Of the more than 3,000 machines, most of the machines were assembled by Chinese manufacturers and assembled with imported control systems. This results in high cost, low added value, and insignificant 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.