Bubbles, film blowing

The major advantage of film blowing is the ease with which biaxial orientation can be introduced into the film. The pressure of the air in the bubble determines the blow-up and this controls the circumferential orientation. In addition, axial orientation may be introduced by increasing the nip roll speed relative to the linear velocity of the bubble. This is referred to as draw-down. 

Bubbles are blown into the inverted funnel. Inside the funnel, the bubble film is transported away and collected. Since the bubble film consists of a surface-active substance and water, it is seen that even very minute amounts (less than milligram per liter) of surface-active substances will accumulate at the bubble surface. As shown previously, it would require a large number of bubbles to remove a gram of substance. However, since one can blow thousands of bubbles in a very short time, the method is found to be very feasible. 

A common form of analyzing film blowing is by setting-up a coordinate system, , that moves with the moving melt on the inner surface of the bubble, and that is oriented with the film as shown in Fig. 6.21. Using the moving coordinates, we can define the three non-zero terms of the local rate of deformation tensor as... 

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. 

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

The SH plays a positive role in film blowing (15, 44) or in wire coating ( ). LPX-30 does not show this effect and Its processability Index Is low. On the other hand, SH of LDPE Is quite high and so Is the processability Index. The accepted mechanism for Improvement of fllm bubble stability Is self healing of a potentially weak spot by a local Increase of extenslonal viscosity caused by SH. It Is Important to note that adding to LPX-30 either 10 wtZ LDPE, 202 LLDPE-10 or lOZ LPX-24 was sufficient for generation of SH. The subsequent trial on a production line confirmed these results Increased bubble stability allowed for higher production rates of the blends. 

Nonlinear dynamic behavior, nonisothermal film blowing with constant bubble pressure... 

Micic, R, S. N. Bhattacharya, and G. Field. 1998. Transient elongational viscosity of LLDPE/LDPE blends and its relevance to bubble stability in the film blowing process. Polymer Engineering and Science 38 1685-1693. 

Die, bubble, and take-off equipment in a film blowing unit. 

The percolation threshold near 0p 0.2 is evident in several plots of t/o versus (j), of LDPE/branched PE (Pcot et al. 2003), LDPE/HDPE (Sarkhel et al. 2006), or melt strength of LDPE/LLDPE (Field et al. 1999 Ho et al. 2002). The latter authors noted that for film-blowing bubble stability, miscibility and melt strength in elongatimi are important For good process ability of LLDPE, >20 wt% of LDPE was required. 

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... 

Ghijsels, A., Ente, J. and Raadsen, J. (1990) Melt strength behavior of polyethylene and its relation to bubble stability in film blowing, Int. Polymer Processing 5, 284-286. 

The blown film process is known to be difficult to operate, and a variety of instabilities have been observed on experimental and production film lines. We showed in the previous chapter (Figure 10.10) that even a simple viscoelastic model of film blowing can lead to multiple steady states that have very different bubble shapes for the same operating parameters. The dynamical response, both experimental and from blown film models, is even richer. The dynamics of solidification are undoubtedly an important factor in the transient response of the process, but the operating space exhibits a variety of response modes even with the conventional approach of fixing the location of solidification and requiring that the rate of change of the bubble radius vanish at that point. 

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