Blown film

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Blown films are plastic films that are produced by an extruder . To produce a blown film, the granulate of the plastic to be used (PE, PP, PET) is fed into the extruder via a funnel. The extruder usually consists of an extruder screw that is driven by a motor with a corresponding reduction gear. The snail is in a heated tube. The screw conveys the plastic granulate, which by gravity gets from the funnel into the extruder pipe from above, continuously in the pipe forwards in the direction of the extruder nozzle. The extruder is heated from the outside with electrical heating elements. This is particularly necessary when starting up. In addition, the kneading and conveying work of the screw heats the granulate to such an extent that the granulate melts. Depending on the screw shape and length, the heating and melting of the granulate can even reach such high temperatures that external heating can be dispensed with or cooling is even necessary. As a rule, extruders are equipped with temperature controls that ensure that the melt has a temperature that is as constant as possible in the entire area of ​​the screw. The plastic is melted in the extruder and conveyed against a nozzle with pressure. A heated nozzle from which the melt emerges is therefore arranged at the exit of the extruder . In addition to the slot nozzles for the production of flat films, there are also round nozzles that create a hose. Air is blown into this hose, which causes a bubble to form due to the hose being laid flat further on. The air has several functions, it heats and cools the plastic film. The air pressure creates a force with which the film is stretched. The stretching of the film increases transparency, stability and strength. In this context, one speaks of a degree of stretching in the MD direction (machine direction) and in the TD direction (transverse direction). The degrees of stretching can be different and can influence the behavior of the film.

After the cooled and stretched film has been laid flat, the edges of the film are cut open and the top and bottom of the film are then wound up separately on separate winders. Such a film is usually called a blown film or a blown film. In particular, a PE film is produced in this way.

The blown film process is also used successfully for multilayer films. Among other things, there are 5 or 7-layer blown films that can have different material combinations. Each material is melted by its own extruder. The disadvantage of blown films is often the imperfect thickness profile, which is caused by temperature fluctuations during stretching. There are also blowing processes in which the air is replaced by a water bubble. This process is used in particular with polypropylene , because here much greater stretching forces are required.

In addition to blown films, there are also flat films. In the case of flat films, a so-called stretching frame is used for stretching. The stretching frame generates the stretching forces that are generated in the blown film by the internal air pressure of the bubble. A flat film line usually also consists of an extruder, a slot nozzle, some cooling rollers known as “chill rolls”, the stretching frame and a double-turning winder arranged behind it for the continuous winding of the film. Different processes are used to stretch the film. In this context, one speaks of “monoaxially” and “biaxially” stretched films. A special process is the “simultaneous biaxial” stretching of the film. The film is stretched simultaneously in the MD and TD directions.

The monoaxial stretching takes place in a monoaxial stretching system. Such a stretching system consists of several heated rollers that rotate faster from roller to roller and thereby produce stretching and lengthening of the film in the MD (machine direction) direction. At the outlet of the monoaxial stretching system, the length of the film has increased, which is evident from the fact that the web speed is several times higher.

In order to achieve biaxial stretching, a transverse stretching system consisting of 2 opposing stretching chains is located behind the monoaxial stretching system. The stretching chains consist of many interconnected chain clips that grip the film on the left and right edge and continuously pull it into the stretching furnace. First of all, the right and left chains run parallel as they enter the furnace. The film is brought to the stretching temperature and held in this preheating zone. This is followed by the actual stretching zone, in which the two chain runs diverge. This means that the distance between the two chains increases continuously until the desired transverse stretching ratio is reached. Behind the stretching zone there is another zone in which the chains run parallel again. This zone is the conditioning zone, in which the properties achieved by the film are fixed by cooling the film. At the exit from the oven, the film has almost reached room temperature again and the clips of the stretching chains open automatically to release the film from tension. While the film continues to run straight to the rewinder after trimming on both sides, the stretching clips of the two stretching chains return to the beginning of the oven, where they open the clip jaws again to pick up the right and left side of the monoaxially pre-stretched film.