Blown films freeze line

Film blowing. A tubular 50 pm thick low density polyethylene film is blown with a draw ratio of 5 at a flow rate of 50 g/s. The annular die has a diameter of 15 mm and a die gap of 1 mm. Calculate the required pressure inside the bubble and draw force to pull the bubble. Assume a Newtonian viscosity of 800 Pa-s, a density of 920 kg/m3 and a freeze line at 300 mm. 

See also Kuijk et al [46], who showed that the stress at the freeze line is the key morphological parameter in determining the mechanical properties of blown polyethylene films. 

From numerical solution of the set of equations that represent both processes, it is possible to predict the ribbon and film dimensions and temperature profiles from the die to the freeze line. Figure 24.3 shows two-dimensional simulations of a PA6/LDPE blown film (extruded at 250 °C and DR = 20) and a PS/FIDPE ribbon (extruded at... 

Frost line (freeze line) n. In the extrusion of blown film, a ring-shaped transition zone of frosty appearance located at the level at which the film reaches its final diameter and is changing from melt to solid. 

The principal controls for a blown film line are barrel (cylinder) and die temperatures, die gap, extrusion rate, internal air pressure, bubble diameter, cooling air flow or cooling rate, and line speed (take-off speed). These controls influence the film dimensions and properties. The frost (freeze) line height, which is a ring-shaped zone where the bubble frequently begins to appear frost because the film temperature falls below the softening range of the resin and crystallization occurs, is an indicator for many of these variables. The frost line may not be visible at times. When it is not, the zone where the bubble reaches its final diameter is considered to be the frost line. 

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... 

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