The process flow of polyethylene blown film is as follows: the material is plasticized and extruded to form a tube, which is then blown and molded; cooled, pulled, and coiled. In the process of blown film molding, according to the different directions of extrusion and pulling, it can be divided into three types: flat blowing, top blowing, and bottom blowing. This is the main molding process. There are also special blow molding methods, such as top extrusion and top blowing.

Flat extrusion and top blowing

This method uses a right-angle die, that is, the die discharge direction is perpendicular to the extruder, and the extruded tube is upward. After being pulled to a certain distance, it is clamped by a herringbone plate. The compressed air introduced from the bottom of the extruded tube is blown into a bubble tube, and its lateral size is controlled by the amount of compressed air, and its longitudinal size is controlled by the pulling speed. After the bubble tube is cooled and shaped, a blown film can be obtained. The main plastic varieties suitable for the top blowing method are PVC, PE, PS, and HDPE.

Flat extrusion blowing method

This method uses a right-angle die head, and the process in which the bubble tube is led out from the bottom of the die head is called flat extrusion blowing method. This method is particularly suitable for raw materials with low viscosity and plastic films that require high transparency. Such as PP, PA, PVDC (vinylidene chloride).

Flat extrusion blowing method

This method uses a flat die head that is concentric with the extruder screw, and the process in which the bubble tube and the center line of the die head are on the same horizontal plane is called flat extrusion blowing method. This method is only suitable for blowing small-diameter film products, such as LDPE, PVC, PS film. Flat blowing method is also suitable for the production of blown heat shrinkable film.

The above three process flows have their own advantages and disadvantages, which are now compared in the table. Process flow advantages and disadvantages Flat extrusion upper blowing tube is hung on the cooling tube, and the traction is stable. It occupies a small area and is easy to operate. It is easy to produce films with large fold diameter and thick thickness. It requires a high factory building and high cost. It is not suitable for processing plastics with high fluidity. It is not conducive to film cooling and has low production efficiency. Flat extrusion lower blowing is conducive to film cooling and has high production efficiency. It can process plastics with high fluidity. The extruder is high from the ground and is inconvenient to operate. It is not suitable for producing thinner films. The flat extrusion flat blowing machine head is central, with a simple structure and a relatively uniform film thickness. It is easy to operate and easy to guide the film. The blowing ratio can be larger. It is not suitable for processing films with high relative density and large fold diameter. It occupies a large area. The bubble tube cools slowly and is not suitable for processing plastics with high fluidity.

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