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    Causes and solutions to the instability of bubbles in blowing film

    release time:2020-03-03  viewed:684次
     
    The instability of blown film results in the fluctuation of film thickness and width, scratches and tears. These instabilities include tensile resonance, longitudinal spiral lines, oscillating frost lines, sagging, tearing, shaking and flapping of vesicles, etc.
     
     
     
     
     
    The method to correct the instability of extrusion bubble is to adjust the position of gas ring, stabilize the bubble from the bottom, and then take measures to solve it. There are three types of double lip gas rings for low melting strength foam: diaphragm type, porous stack type and stable ring type. The instability of membrane bubble should be corrected according to different steps, specific membrane bubble problems and different gas ring types until the problem is solved. In some cases, resin replacement or equipment modification may also help, but these methods take a long time
     
     
     
     
     
    Also known as the "hourglass", it is a common problem caused by the hardening of the molten film due to its rapid stretching at the beginning of blowing. Strain hardening increases and decreases the pressure in the cell every 2 to 10 seconds, resulting in the change of cell width.
     
     
     
     
     
    The suggested solution is to increase the output of the extruder (increase the screw speed and snap in speed) or reduce the cooling of the foam to improve the frost line. Adjust the gas ring to reduce the funnel effect and push the bubble away from the guide ring. The high-speed air flowing out of the gas ring reduces the pressure between the bubble and the gas ring, resulting in vacuum and funnel effect. The pressure in the bubble forces the molten film to adhere to the metal surface of the gas ring. Zhimeng content management system
     
     
     
    To reduce the funnel effect, for diaphragm type gas rings, close the diaphragm or increase the adjustable deflector ring. For the chimney pipe type air ring, open the air hole on the chimney pipe, release the air and reduce the cooling. Increasing the upper lip and lowering the stack can also reduce the air velocity between the bubble and the stack. Increasing the forming cone can reduce the air flow velocity of the lower lip, but most of the air rings can not complete this step in operation. For the stable ring type gas ring, reduce the stable ring to let the air leak out, leaving a small amount of air cooling bubble. The air flow of the lower lip can also be stopped and the funnel effect can be reduced by reducing the forming cone.
     
     
     
     
     
    Also known as "mayvis" or "serpentine", the common cause is too low frost line, or the air flow from the lip edge of the gas ring is spiral, which makes the bubble separate from the deflector ring, causing a fluctuation of 5 to 10 seconds. The suggested solution is to increase the output of extruder (increase screw speed and bite speed) to improve the frost line.
     
     
     
    If the above method is not effective, the static pressure can be reduced. For diaphragm gas rings, close the diaphragm and / or raise the adjustable deflector ring. For the adjustable stack pipe type air ring, open the air hole on the stack pipe, raise the upper lip, lower the adjustable stack pipe, and / or raise the forming cone. For a stationary ring, lower the forming cone and / or the stationary ring.
     
     
     
     
     
    Other correction methods include reducing the melting temperature, or changing to a resin with low melting index and high melting strength. A wider mode gap is also used so that the bubble can withstand more turbulence in the gas ring.
     
     
     
    High oscillations every 30 seconds to 5 minutes may be caused by sudden rise in ambient temperature, strong winds or relatively slow switching. The sudden rise of temperature causes the rise and fall of motor current, back pressure and frost line height. The simplest solution is to reduce the temperature of the extruder material and the second stage barrel. If the strong wind cannot be eliminated, cover the bubble between the air ring and frost line. If IBC is used, check the sensor height. When the die diameter ratio is greater than 2:1, the sensor should be 2 to 3 inches above the frost line. When the module film diameter ratio is less than 1.5:1, the sensor should be located at or below the frost line. The temperature curve of extruder should also be adjusted to enhance mixing.
     
     
     
     
     
    Other solutions are to avoid mixing resins with huge difference in melt index and poor mixing performance; check whether there are problems in the air ring or IBC blower of the equipment, whether the traction speed is stable, and whether the screw is damaged, which will affect the effective mixing of materials.
     
     
     
     
     
    The sagging of the bubble is caused by insufficient air cooling. The suggested solution is to reduce frost line or increase static pressure. For diaphragm type air rings, close the diaphragm and / or raise the adjustable deflection guide ring. For adjustable stack pipe type air rings, close the air holes on the stack pipe, lower the upper lip, lower the adjustable stack pipe, and / or raise the forming cone. For the stabilizing ring, lower the stabilizing ring and / or raise the forming cone. If there is no other option, it can reduce the output of the extruder (reduce screw speed and bite speed), reduce the melting temperature, or change to a resin with low melting index and high melting strength. For equipment, the use of a narrow die gap can speed up the cooling of the bubble.

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