The blown film is made by an annular die, so the extruded hollow plastic tube film is often referred to as "bubble". The expansion of air pressure makes the hollow tube membrane form the ideal size. Although the film can be extruded horizontally or downward, it is usually extruded vertically. Cooling is accomplished by blowing cold air on the film through a cooling ring directly installed at the die outlet. Some also have internal and external cooling methods. Sometimes the tube membrane is quickly cooled and reheated before being stretched with air pressure.
The size of the film depends on the output of the extruder and the compressed air pressure. "Blow-up ratio" is the ratio of the diameter of the blown film to the diameter of the mold. Although compressed air can enter through the wall of the bubble, the wall can be automatically sealed. Once sealed, it runs steadily, with a slight leak, and the resulting bubble becomes smaller.
The channel through the top of the film is a variety of guides. Including air ring and guide roller. When the film is sufficiently cooled, the bubble is clamped by the plywood and rollers, rolled up, cut or not cut, folded or otherwise processed. Therefore, this process can be used to produce both bubble tubes and flat films.
Once the blown film process is running, almost no scrap is produced because there are no edge effects similar to flat-extruded films. However, it is quite difficult to get the production line up and running. Therefore, when starting up, more waste is generated. Compared with the flat extrusion film production line, the blown film production line has higher output and is more economical when it is started. Therefore, blown film is famous for its high output. It is estimated that about 90% of the polyethylene film is produced by this method.
In terms of film transparency and uniformity, blown film is not as good as flat-extruded film. Mainly due to the slow cooling rate of the film blowing process. In terms of transparency, slow cooling causes a large degree of crystallization and the formation of large crystals, leading to blurring of the film. Uneven cooling leads to huge changes in film thickness. Because even a little eccentricity and a little defect at the die opening will cause a significant unevenness in the roll film shape, the common method is to rotate the die during extrusion, so that the thickness change is instantaneous and produces a uniform thickness Roll film. In some cases, the thickness of the film varies by ± 15%, although the range of variation is ± 7% is more common. Of course, it is not necessary to rotate the mold, but to rotate the film during winding.
In blown film, the film can usually be stretched between the die and the nip roll to form a machine direction orientation. The film is also stretched laterally to obtain orientation as the bubble expands. Therefore, the film is oriented in two directions, and the orientation is balanced by two-way stretching. Through further processing of the downflow film blowing device, auxiliary orientation can be produced. Some blown film production lines simultaneously use two die heads for extrusion and feeding, each using its own blown film equipment.
Oriented polyethylene film is usually extruded downward, the extruded tube film is cooled, and then heated to below the melting temperature before inflation. If the film is thermally stable to achieve the desired effect, the film can be passed through a series of heated rollers, or inflated again in the "secondary inflation" process. In either case, the film is heated and then cooled, and when the cooling is completed, the film again gains profits due to shrinkage.