Cooling bubble, film blowing

V. Sidiropoulos and J. Vlachopoulos, Numerical Study of Internal Bubble Cooling in Film Blowing, Int. Polym. Process., 16, 48-53, (2001). 

In some film blowing installations, the tube, after blowing and cooling, is nipped between a pair of rolls before being slit. This operation confines the air as a bubble between the extruder and the rolls, making continuous additions of air unnecessary. 

In film blowing, a tubular film is extruded upwards. It is blown upwards, with air introduced below the die, into a larger tubular film which is then picked up by a pair of nip rolls that seals the bubble (Fig. 3.11 Han, 2007). An external stream of chilled air cools and solidifies the film at a certain point called the freeze line, where the temperature of the film is equal to the melting temperature. A feature of this process is that the film is stretched biaxially, improving mechanical properties. Tangential circumferential stretching depends on blow-np ratio, i.e. the ratio between the tubular film diameter after air introduction and the initial tabular film diameter. This parameter is determined by the pressure level within the bubble. Axial stretching depends... 

The properties of the film are determined by the blow-up ratio and the speed. The blow-up ratio is the ratio between the diameter of the final tube of film and that of the die. The internal air pressure that expands the tube into the bubble is typically supplied through a port into the mandrel, the interior part of the die. Once the process is running steadily, little air is usually lost, so make-up requirements are small. When internal bubble cooling is used, air is constantly being exchanged inside the bubble. 

The double-bubble process involves the extrusion and blowing of a tube of molten plastic in a downward direction. The tube is then cooled, most often using a water bath, reheated to just below the melt temperature, and reinflated. The reinflation along with the increase in haul-off speed provides biaxial orientation. Typically the next step is annealing to relieve thermal stresses and stabilize the film. The double-bubble process is most often applied to PP film, but is also used with multilayer PP or PE-based films. One of the major advantages is that this process can deliver a high-clarity film with precise shrink characteristics and very uniform flatness. 

The film bubble is cooled below the softening point of the polymer by blowing air on it from a cooling ring placed round the die. When the polymer, such as polyethylene, cools below the softening point, the crystalline material is cloudy compared with the clear amorphous melt. The transition fine which coincides with this transformation is therefore called the frost line. 

Sheet material, i.e. material thicker than about 0.25 mm, is usually produced by using a slit-shaped die, whereas thinner material is often produced by a blown film extrusion process in which an annular die is used and air is blown into the centre of the tubular extrudate to blow it into a sort of bubble. At a certain distance from the die the polymer is sufficiently cool to solidify into a film, which is then flattened and collected on rollers. Figure 1.9 illustrates a blown-film system. 

Bubble cooling is generally accomplished by blowing a large volume of air on the film as it exits the die (Fig. 3.10). This may take place on only the outside of the bubble or on both the inside and the outside. Additionally, the bubble is kept inflated to remove more heat from the film as it travels up through ambient air in the cooling tower. 

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