Blown film systems

Blown film extrusion is perhaps the most widely used extrusion technique, by production volume. Billions of pounds of polyethylene are processed annually by this method to make products such as grocery sacks and trash can liners. In a blown film system (Figure 14-30), the melt is generally extruded vertically upward through an annular die. The thin tube is filled with air as it travels up to a collapsing frame that flattens it before it enters the nip rollers, which pull the film away from the die. The flattened tube then travels over a series of idle rollers to a slitter,... 

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. 

Sophisticated control systems have been developed in response to the high interdependence of several process variables coupled with the demands of very high output rates [15]. While many lines still utilize primarily manual controls, a growing number of blown film systems depend on computer-based measurement and control of all key process variables. These computer-based systems can make an important contribution to increasing efficiency, reducing costs, and increasing profits on high output lines. 

Satellite HDPE blown film system Filmaster Inc. 

Cakmak M. and Wang M.D., Structure development in the tubular blown film of PP/EPDM thermoplastic elastomer, Antec 89, 47th Annual Tech. Conference of SPE, New York, May 1, 1989, 1756. Hashimoto T., Todo A., Itoi H., and Kawai H. Domain boundary structure of styrene-isoprene block copolymer films cast from solution. 2. Quantitative estimation of the interfacial thickness of lamellar microphase systems. Macromolecules, 10, 377, 1977. 

The mechanisms described above tell us how heat travels in systems, but we are also interested in its rate of transfer. The most common way to describe the heat transfer rate is through the use of thermal conductivity coefficients, which define how quickly heat will travel per unit length (or area for convection processes). Every material has a characteristic thermal conductivity coefficient. Metals have high thermal conductivities, while polymers generally exhibit low thermal conductivities. One interesting application of thermal conductivity is the utilization of calcium carbonate in blown film processing. Calcium carbonate is added to a polyethylene resin to increase the heat transfer rate from the melt to the air surrounding the bubble. Without the calcium carbonate, the resin cools much more slowly and production rates are decreased. 

Monolayer blown-film extrusion, VDC copolymers in, 25 725, 728-729 Monolayers, self-assembled, 77 57 Monolayer self-assembled systems, 16 800 Monolignols, 27 10—11, 13, 14 Monolithic drug delivery systems, 9 11 Monomagnesium phosphate, 78 839 Monomer addition, in PVC polymerization, 25 665-666... 

Throughputs of winders can be over 2,200 lb/h (1,000 kg/h). Transfers from one roll to another can take less then a second. Material speeds are up to at least 2,200 ft/min in cast film lines at least 999 ft/min in blown film lines. Blown film lines may want to use reverse winding systems to allow coextruded films to be wound with a particular material as the inside or outside layer. 

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). 

The blown film process has been studied analytically since the early 1970s (Table 24.1). The first analysis was proposed by Pearson and Petrie [3, 4], who followed a fluid mechanics approach. However, this model is restricted to Newtonian fluids under isothermal conditions. This first model has been modified several times to consider different aspects of the process, such as temperature variation and rheological behavior of the system. 

In the case of blown film simulations, a linear temperature profile can be used to obtain a greater stability in the solution of the system. The set of boundary conditions imposed on the system is given as shown in the following equation ... 

Each type of resin is melted in an individual extruder, and the melts are carefully brought together prior to or in the die, in a manner that keeps them in homogeneous layers, without mixing. The process used to combine the polymers is usually different in the cast and blown film processes. In cast processes, as illustrated in Fig. 7.12, the polymers are typically combined in an adapter, called a feed block, before they enter the coat hanger die. This permits a simpler design for the die itself. Multimanifold dies are used when plastics with widely different flow properties are to be combined, as such systems provide a shorter multilayer flow path before solidification, and thus minimize distortion of the interface. 

For blown film operations, the most common approach is to use a multilayer die in which cavities, including manifolds, have been cut from each polymer. The reason for this is that the spiral channel design normally used in the die cannot accommodate precombined flows and result in a layered structure. Figure 7.13 shows a schematic of a multichannel die. In such systems, the layers are combined just before they exit the die, or immediately after they exit. 

The rapid-water-quenching system produces good optical properties, has low equipment cost, but can be difficult to use to get precise control over the water temperature. Vibrations and currents can cause little marks on the film. A critical point is the necessity for maintaining a smooth surface in the water quench tank where the melt first enters. Different devices are used to control the flow of water such as baffles with openings. It has serious limitations when high production speeds are attempted the water must be kept from carrying over into any on-line pretreatment and the finished roll. However, these problems can be controlled. This liquid bath system has been used for blown tubular film inside the blown film to improve... 

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