Technical requirements for wall thickness control of blow molding machines

Technical requirements for wall thickness control of blow molding machines

Many people may know that the molding technology of hollow extrusion blow molding machine is to make the plasticized billet close to the inner wall of the mold by air pressure. The production mode of mold billet is extrusion, that is, the heated plasticized plastic melt is extruded from the mold core gaps of various shapes to form various shapes of cylindrical billet. The extruded billet hangs vertically by dead weight. The minimum wall thickness of hollow container products is specified for its strength requirements, but the early hollow blowing molding machine lacks the billet wall thickness control system. In order to make the thinnest part of the product meet the minimum wall thickness requirements, the other parts of the product should be thickened accordingly, resulting in the waste of materials. In order to save the cost, shorten the cooling time and speed up the production cycle, a more economical method is to control the thickness of billet wall.

1. Technical requirements of wall thickness control system

The wall thickness control system is the system that controls the opening and closing degree of the mold core gap, namely the position servo system. In the process of hollow container production, in order to ensure the quality of products, the controlled quantity is required to be able to accurately track the setting value, and the response process is required to be as fast as possible. Taking the simple 10-point wall thickness controller as an example, in the continuous extrusion mode, the shortest time to extrude a billet from the die head is about 5s. In 5s, to realize a function curve of limited length, it needs to be divided into 10 paragraphs. On the time axis, each paragraph only takes about 0.5s. The unit pulse transition time of the wall thickness controller should be 1/5 ~ 1/10 of the paragraph time, that is, 0.05 ~ 0.1s, which requires the system to respond very quickly. While responding so quickly, it is necessary to ensure that the controlled position quantity can accurately track the set value, otherwise, the wall thickness control will be meaningless. In order to achieve the control effect of the above two requirements, it is difficult to design the wall thickness control system for the heavy load system. In addition, as l0 points on the contour curve of the billet are used to describe the curve, interpolation must be carried out 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 closed-loop feedback design, and its components include wall thickness controller, electro-hydraulic servo valve, action actuator and electronic ruler as signal feedback device. The user sets the final billet wall thickness axial change curve on the panel of the wall thickness controller, and the controller outputs the voltage or current signal with the change of the output size to the electro-hydraulic servo valve according to the curve. The electric bottle blowing machine, film blowing machine, plastic molding machine, plastic packaging machinery, bag making machine hydraulic servo valve drives the actuator to control the up and down movement of the mold core, thus causing the change of the mold core gap. By measuring the size of the gap, the electronic ruler gets the corresponding voltage signal and feeds it back to the wall thickness controller. This constitutes a closed-loop wall thickness control system.