Film blowing illustration

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. 

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... 

A second type of anisotropic system is the biaxially oriented or planar random anisotropic system. This type of material is illustrated schematically in Figure 2A. Four of the five independent elastic moduli are illustrated in Figure 2B in addition there are two Poisson s ratios. Typical biaxially oriented materials are films that have been stretched in two directions by either blowing or tentering operations, rolled materials, and fiber-filled composites in which the fibers are randomly oriented in a plane. The mechanical properties of anisotropic materials arc discussed in detail in following chapters on composite materials and in sections on molecularly oriented polymers. 

The spherical foam films can be obtained by blowing a bubble from a vertical capillary tube. The principle of formation of such a bubble is illustrated in Fig. 2.22. A vertical capillary tube is placed in a vessel with the surfactant solution so that its upper orifice is close to the solution surface. When a gas (air) with a definite pressure is introduced into the tube over the solution surface a foam film is formed acquiring the shape of a hemisphere. 

The experiments performed in [49] reveal that the foam lifetime depends strongly on the humidity of the blowing air. This is illustrated in Fig. 6.8. However, a quantitative verification of Eq. (6.31) is not possible for the lack of data about film thickness, foam dispersity and rate of evaporation. 

As for all additives, interactions with other additives in solution, Fig. 3.16, and competition for surface reaction sites together with the effect of environmental factors such as temperature, blow-by gases, water and fuel dilution have variable effects on the formation of the film. Because ZDDPs are much more widely used as antiwear performance additives compared to other classes of compounds, these additive effects will now be discussed in greater detail than has been the case for other classes of anti-wear/friction additives. In particular the influence of structure, concentration, dispersant, detergent, antioxidancy and friction modifier on friction and wear will be discussed. In addition the influence of NO c and H2O will be briefly illustrated. 

Sheet material, i.e. material thicker than about 0.25 mm, is usually produced by using a slit-shaped die, whereas thinner material is often produced by a blown film extrusion process in which an annular die is used and air is blown into the centre of the tubular extrudate to blow it into a sort of bubble. At a certain distance from the die the polymer is sufficiently cool to solidify into a film, which is then flattened and collected on rollers. Figure 1.9 illustrates a blown-film system. 

Polyesters enter our lives in a most ubiquitous manner as textiles, carpets, tire cords, medical accessories, seat belts, automotive and electronic items, photographic film, magnetic tape for audio and video recording, packaging materials, bottles, and so on. Their utility is illustrated by the vast range of their applications. This article describes the properties, synthesis, manufacture, and raw materials for the two most widely used thermoplastic polyesters polyCethylene terephtha-late) (PET) [25038-59-9] and poly(butylene terephthalate) (PBT) [26062-94-2]. In order of volume, PET comes first by virtue of its enormous market tonnage in polyester fibers and films, as well as the resin for blow-molded bottles, containers, and food packaging. 

An unusual feature of fabricating HMW-HDPE into film is the length the molten polyethylene travels during the film-forming process before the bubble is inflated. The bubble shape is illustrated in Figure 6.17 and is referred to as high-stalk extrusion. This extrusion shape is used in order to produce film with more balanced orientation (more balanced properties in the MD and TD direction). After the melt exits the die, the melt is drawn down by about a factor of 20 in the machine direction before the bubble is inflated with a blow-up ratio of 4, i.e., the diameter of the bubble is four times the diameter of the die. Hence, the melt is drawn down by a factor of 80. Consequently, a final film thickness of 0.5 mils is obtained with a die... 

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