Simple Overview

In the production process of blown film, the non-uniformity of layer thickness is a very important indicator, where the longitudinal thickness uniformity can be controlled by the stability of extrusion and stretching speed, while the transverse thickness non-uniformity of the film is usually affected by the manufacturing of the tool head and changes with production process parameters. In order to improve the lateral thickness non-uniformity of the film, an automatic lateral thickness control system must be introduced. The general control methods include automatic nozzle head (thermal expansion screw control) and automatic air ring. This article mainly introduces the principle and application of automatic gas ring.

Basics

The automatic air ring structure adopts a dual air outlet mode, with the lower air outlet maintaining a constant air volume, and the upper air outlet divided into multiple air ducts on the perimeter. Each air duct is composed of an air chamber, valves, motors, etc. The motor drives the valve to adjust the opening of the air duct and control the air volume of each duct. The computer compares the thickness signal with the current set average thickness, calculates the thickness deviation and curve change trend, and controls the motor to move the valve. When the film thins, the motor moves forward and the air outlet closes; On the contrary, the engine moves in the opposite direction and the air outlet increases. By changing the air volume at each point on the circumference of the air ring and adjusting the cooling speed at each point, the lateral thickness deviation of the film in the target area is controlled.

Control system

Automatic winding is an online real-time control system. The control object of this system is multiple motors distributed on the wind ring. The cooling air flow from the fan is distributed to each air channel based on the constant pressure of the air ring chamber. The motor drives the valve to adjust the size and airflow of the air outlet, change the cooling effect of the thin film blank at the outlet of the nozzle head, and control the thickness of the thin film. From the perspective of the control process, there is no clear relationship between the changes in film thickness and the motor control quantity. The thickness changes at different positions of films and valves with different thicknesses, as well as the nonlinear and irregular changes in control variables, are not clear. Every time a valve is set, it has a significant impact on adjacent points, and there is a delay set that connects different times together. For this highly nonlinear, strongly coupled, time and control uncertainty system, it is difficult to determine its precise mathematical model. Even if it can establish a mathematical model, it is still very complex. Although traditional control models have better control effects than deterministic control models, the control effect of highly nonlinear, unsafe, and complex feedback information is poor or even powerless. That's why we chose the fuzzy control algorithm. At the same time, the method of changing the fuzzy quantification factor is adopted to better adapt to changes in system parameters.

Extrusion blow molding is a process for producing thermoplastic hollow parts. Known blow molded objects are bottles, barrels, cans, boxes, and all containers used for packaging food, beverages, cosmetics, drugs, and daily necessities. Large blow molded containers are typically used for packaging chemical products, lubricants, and bulk materials. Other blow molded products include balls, bellows, and toys. For the automotive industry, the fuel tank, shock absorbers, backrest, center console, armrests, and headrests have all been blown off. Blow molded parts used for machine and furniture structures are pallets, door frames, frames, pottery jars, or boxes with open surfaces.

polymer

The most common raw material for blow molding and extrusion is high-density polyethylene, and most milk bottles are made of this polymer. Other polyolefins are often processed through blow molding. According to the application situation, styrene polymers, polyvinyl chloride, polyester, polyurethane, polycarbonate, and other thermoplastic plastics can also be used for blow molding. Recently, engineering plastics have been widely used in the automotive industry. Material selection is based on mechanical strength, weather resistance, electrical performance, optical performance, and other characteristics.

handle

Three quarters of blow molded products are made by extrusion blow molding. During the extrusion process, the material is forced to produce products through holes or molds.

The extrusion blow molding process includes five steps: 1. Plastic mold embryo (extrusion of hollow plastic pipes); 2. Close the flap mold on the mold, clamp the mold, and cut the mold; 3. Blow the mold onto the cold wall of the mold cavity, adjust the opening, and maintain a certain pressure during cooling; 4. Open the mold and remove the blown parts; 5. Trim the rough edges to obtain the finished product.

extrusion

Polymer mixtures are defined as methods of improving the degree of polymer or polymer system by melting the mixture. The mixing process ranges from adding a single additive to handling multiple additives, polymer alloys, and reactive mixing. It is estimated that one-third of polymer production in the United States is mixed. The mixed ingredients can be adjusted according to the performance requirements of the final application. Mixed products have mixing characteristics, such as high gloss and excellent impact strength or precision molds and good rigidity.

Mixed polymers are usually granulated for further processing. However, the industry is increasingly interested in combining mixing with the next process, such as profile extrusion, to avoid heating the polymer.

mixture

Different types of melt mixing equipment were used, from roller mills and batch mixers to single screw and twin screw extruders. Continuous mixing (extruder) is the most commonly used equipment as it can provide consistent quality products and reduce operating costs. There are two types of mixing: a distributed mixture can be uniformly distributed within the mixture without the use of high shear stress. This type of mixture is called a stretchable mixture or laminar flow mixture. Dispersive mixing, also known as strong mixing, involves applying high shear stress to destroy viscous solids. For example, if the additive particles break, the actual particle size decreases. Mixing operations typically require two types of mixing in one process.