Film blowing process

Andre, J. M. ct al., 1998. Numerical modelling of the polymer film blowing process. Int. J. Forming Processes 1, 187-210. 

Although plastic sheet and film may be produced using a slit die, by far the most common method nowadays is the film blowing process illustrated in Fig. 4.20. The molten plastic from the extruder passes through an annular die and emerges as a thin tube. A supply of air to the inside of the tube prevents it from collapsing and indeed may be used to inflate it to a larger diameter. 

Despite the non-isothermal nature of the film blowing process we will develop here an isothermal model to show general effects and interactions during the process. In the derivation we follow Pearson and Petrie s approach [20], [19] and [21]. Even this Newtonian isothermal model requires an iterative solution and numerical integration. Figure 6.21 presents the notation used when deriving the model. 

Figure 6.21 Schematic diagram of the film blowing process.

Figure 6.23 Predicted film blowing process using an isothermal Newtonian model for a...

J.R. Pearson and C.J.S. Petrie. A fluid-mechanical analysis of the film-blowing process. Plast. 

Figure 8-7. Schematic diagram for film blowing process.

In the film blowing process where a continuous stable parison is blown from an annular die, it is crucial that the molten polymer exhibits certain elastic extensional properties and it is here that the viscoelastic nature of the polymer is beneficial.If, however, the manufacturer is concerned with profile and surface finish of an extrudate, viscoelastic effects of the polymer may well present difficulties. Both die swell and most polymer extrusion instabilities are linked to viscoelastic effects and as such different levels of viscoelasticity give rise to different extrusion characteristics. 

Figure 6.28 Schematic diagram of film blowing process...

Cellulose derivatives were the first thermoplastics formed in the film blowing process in the beginning of the 20th century. By the 1930 s, biaxially oriented films of cellulose acetate were commercially available. Forming of synthetic high polymers, especially LDPE, by the film blowing process began only in the 1940 s. 

The stretching force is a direct consequence of the distance of polymer solidification (DPS) in both processes (i.e., FLH in film blowing processes or cooling length in ribbon extrusion). In this sense, controlling DPS offers the possibility to control the final properties of films and ribbons. Table 24.3 lists typical values of stretching force as function of FLH and DR for blown films of LDPE. 

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. 

J. M. Andre et al., Numerical Modelling of the Film Blowing Process, in J. Huetink and F. P. T. Baaijens, eds.. Simulation of Materials Processing Theory, Methods and Applications, A. A. Balkema, Rotterdam, 1998. 

A schematic of the film blowing process is given in Figure 6.18. The plastic material is fed through an extruder to an aimular die opening. 

Figure 6.18 The film blowing process Courtesy of Alpha Marathon Technologies Inc.)...

The film-blowing process is used industrially to manufacture plastic films that are biaxially oriented. Many attempts have been made to predict and model this complex but important process, which continues to mystify rheologists and polymer processing engineers worldwide. A constitutive equation, able to predict well the polymer melt in all forms of deformation, is required to model the process, together with the standard conservation equations of continuity, momentum, and energy. Pearson and Petrie [125,126] were the first to predict the forces within the blown film by the use of the thin-shell approximation, force balances, and the Newtonian constitutive equation. The use of the thin-shell approximation and force balances is standard in any attempt to model the film-blowing process, and it has been used in the vast majority of subsequent studies. 

There have been numerous studies on the film-blowing process. Since the initial thin-shell approximation proposed by Pearson and Petrie [125, 126] with the Newtonian model assumed for deformation, various rheological models have been incorporated in simulations, such as the power-law model [127,128], a crystallization model [129], the Maxwell model [130-133], the Leonov model [133], a viscoplasti-c-elastic model [134], the K-BKZ/PSM model [135-137], and a nonisothermal viscosity model [138]. A complete set of experimental data was reported by Gupta [139] for the Styron 666 polystyrene and by Tas [140] for three different grades of LDPE. 

Beaulne, M. and Mitsoulis, E. (1998) Numerical simulation of the film-blowing process. Int.J. Form. Proc., 1, 451-484. 

Gupta, R.K. (1980) A New Non-Isothermal Rheological Constitutive Equation and its Application to Industrial Film Blowing Processes. PhD Thesis, Department of Chemical Engineering, University of Delawar e, Newark, DE. 

Another very important rheological property that is significant for the processing of LDPEs (for example, using the tubular film blowing process) is elongational viscosity... 

FIGURE 1.6 Film blowing process. (Reprinted by permission of the publisher from Richardson, 1974.)... 

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