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    Some technical terms we often use in film blowing - Mao Xinjie

    release time:2020-03-04  viewed:1039次
     Some related technical terms we often use in film blowing:
    Traction ratio refers to the ratio between the traction speed of the film and the extrusion speed of the tube ring. The pull ratio is a longitudinal stretch multiple, which makes the film have a directional effect in the pull direction. If the traction ratio increases, the longitudinal strength will also increase, and the thickness of the film will become thinner. However, if the traction ratio is too large, the thickness of the film is difficult to control, and even the film may be pulled off, resulting in film breakage. The traction ratio of LDPE film is generally controlled between 4 and 6.
    Blowing ratio is one of the key points in the production process of blown film. It refers to the ratio between the bubble diameter after blowing and the diameter of the pipe ring without blowing. The expansion ratio is the lateral expansion multiple of the film. In fact, it is the lateral stretching of the film. The stretching will have a certain degree of orientation effect on the plastic molecules, and the expansion ratio will increase, so as to improve the lateral strength of the film. However, the blowing ratio should not be too large, otherwise it is easy to cause the film bubble instability, and the film is prone to wrinkle. Therefore, the inflation ratio should be properly matched with the traction ratio. Generally speaking, the inflation ratio of LDPE film should be controlled at 2.5 ~ 3.0.
    Dew point, also known as frost line, refers to the boundary line of plastic from viscous state to high elastic state. In the process of film blowing, LDPE is in a melting state when it is extruded from the die mouth, with good transparency. After leaving the die, the expansion area of the film bubble shall be cooled by the cooling air ring. When the cooling air blows to the plastic film bubble just extruded from the die head at a certain angle and speed, the high-temperature film bubble contacts with the cooling air, the heat of the film bubble will be taken away by the cold air, and its temperature will obviously drop below the viscosity flow temperature of low-density polyethylene (LDPE), so that it can be cooled and solidified It's blurry. On the blown film bubble, we can see a line between transparency and blur, which is the dew point (or frost line).
    In the film blowing process, the dew point has a certain influence on the film properties. If the dew point is high and located above the expanded bubble, the film is expanded in the liquid state. The expansion only makes the film thinner, but the molecules are not subject to tensile orientation. At this time, the performance of the expanded film is close to that of the cast film. On the contrary, if the dew point is relatively low, the inflation is carried out in the solid state. At this time, when the plastic is in the high elastic state, the inflation is just like the transverse stretching, which makes the molecular orientation, so that the performance of the blown film of the blown film machine is close to the directional film.

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