Film blowing parameters

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... 

Figure 10.8. Operating parameter space for isothermal film blowing of a Newtonian flnid, Z// o = 5. Reprinted from Cain and Denn, Polym. Eng. ScL, 28, 1528 (1988).

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. 

FS and film blowing, properties can be controlled by varying the process parameters and thus the structure created in the processing step. This shows the importance of such processes. 

It is clear from all this that the film blowing process stiU remains difficult to analyze theoretically. In practice, the operator of the film blowing tower adjusts the various parameters by trial and error. Consequently, one major consideration is the scaleup (Section 9.3.6) of a small experimental setup to an industrial scale film blowing tower, especially for polymers tested for the first time in the experimental setup. 

Yeow (1976) theoretically analyzed the instabilities due to axisymmetfic disturbances in an isothermal Newtonian fluid and presented neutral stability curves in the space Wf (= H(/Ro) and Br and for various values of the parameter X (X = Z/Ro, which is the dimensionless freeze line). Kanai and White (1984) experimentally studied the stability of nonisothermal film blowing of viscoelastic melts, such as LLDPE, LDPE, and HDPE, and their results are shown in Figures 9.27 and 9.28. LDPE is more stable than LLDPE and HDPE, which is in accord with LDPE s strain hardening... 

To form a surface layer, the object to be coated is dipped into the sol before this has fully turned into a solid. The object is then withdrawn and the excess of the solution is removed either by gravity or by blowing with compressed air. The thickness of the film, its porosity, and its adhesion to the substrate can be modified by changing the nature of the starting solution or by changing the procedure. Adjustable parameters with respect to the starting solution are the concentrations, the pH (which may be buffered), or the surface tension. Important procedural... 

There are some characteristic parameters in the blown film process (see Fig. 24.1) the blow-up ratio (BUR), which is the ratio between the final radius (Of) and the radius at the die exit (Uq) the thickness ratio (TR) calculated as the ratio of thickness at the die exit (//q) and the final film thickness (//f) and the draw ratio (DR) defined as the ratio of take-up roller velocity (Vf) to the extrusion velocity (Vq). The stretching force (F ) is the force needed to take up the bubble by the roller system (Fig. 24.1). 

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