Polymer films, film blowing

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

This example illustrates the simplified approach to film blowing. Unfortunately in practice the situation is more complex in that the film thickness is influenced by draw-down, relaxation of induced stresses/strains and melt flow phenomena such as die swell. In fact the situation is similar to that described for blow moulding (see below) and the type of analysis outlined in that section could be used to allow for the effects of die swell. However, since the most practical problems in film blowing require iterative type solutions involving melt flow characteristics, volume flow rates, swell ratios, etc the study of these is delayed until Chapter 5 where a more rigorous approach to polymer flow has been adopted. 

Figure 11.6 illustrates the general configuration of a film blowing operation. Molten polymer from the extruder is pumped into an annular die, where it is distributed around a tubular melt channel before emerging vertically as a relatively thick-walled molten tube. The top of... 

Sometimes the hazy optical properties of polypropylene are a detriment to its use in thin films. To address this problem, as well as to improve physical properties and reduce material costs, we can orient the molecular chains by stretching the polymer film after it has left the extrusion die. There are two methods of orienting films, film blowing, shown in Fig. 11.6, and tentering, shown in Fig, 19.6. 

Polymer scientists and engineers have developed a wide variety of processes by which to convert polypropylene into useable articles. These processes include fiber-spinning, production of melt blown and spun bond non-woven fabrics, film production, blow molding, and injection molding. The following sections describe these processes and the resulting properties of the product as a function of the process. 

Kaiser H. Scale-up of extrusion dies for tubular film blowing. Polym Proc Eng 1987 5(1) 1-21. 

A polymer composition and a polymer film, which protects thermoplastics from chemical attacks by various polymer foam blowing agents is made of a blend or mixture of polymers and copolymers comprising (29) ... 

C. Gutfinger, E. Broyer, and Z. Tadmor, Analysis of a Cross Head Film Blowing Die with the Flow Analysis Network (FAN) Method, Polym. Eng. Sci., 15, 385-386 (1975). 

H. W. Jung and J. C. Hyun, Fiber Spinning and Film Blowing Instabilities, in Polymer Processing Instabilities - Control and Understanding, S. G. Hatzikiriakos and K. B. Migler, Eds., Marcel Dekker, New York, 2004, Chapter 11. 

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

Multi-layer films can be produced, composed of different polymers. For each polymer a separate extruder is applied the extruders discharge into an accumulator, in which the separate streams are guided to their own position in the film. A laminated film can thus be made, in which one of the layers provides the strength, another one acts as a gas barrier, other layers promote adhesion and weldability, etc. This laminating process is applied with flat film extrusion as well as with film blowing. 

Hong, Y., Coombs, S. J., Cooper-White, J. J., Mackay, M. E., Hawker, C. J., Malmstrom, E., and Rehnberg, N. 2000. Film blowing of linear low density polyethylene blended with a novel hyperbranched polymer processing aid. Polymer, 41, 7705-7713. 

Starch can be destructured in the presence of more hydrophobic polymers such as aliphatic polyesters. Aliphatic polyesters with low melting points are difficult to process by conventional techniques such as film blowing and blow moulding. Films such as polycaprolactones (PCL) are tacky as extruded and have a low melt strength (over 130 °C). Also, the slow crystallisation of the polymer causes the properties to change with time. Blending starch with aliphatic polyesters improves processability and biodegradability. 

Products Polymer density is easily controlled from 0.915 to 0.970 g/cm. Depending on catalyst type, molecular weight distribution is either narrow or broad. Melt index maybe varied from less than 0.1 to greater than 200. Grades suitable for film, blow-molding, pipe, roto-molding and extrusion applications are produced. 

During stretching flow, material is drawn from one cross-sectional area to another. This type of flow dominates fibre, film, blow molding, and vacuum forming processes (57). Let us assume that a filament of a molten polymer is hauled off under a force F (see Fig. 13.40). Taking the die of the extruder as the origin of the reference frame, the cross-sectional area A of the filament... 

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. 

---