Extrusion blowing of common plastics such as polyethylenes into film is one of the oldest processing techniques (dating back to the 1930s in the United
States). The basic process is simple and is based on a special annular die that is connected to one or more extruders. In the simple case with a single extruder, the molten plastic material is extruded vertically upwards through the die into a thin-walled plastic tube. Blowing air into the tube expands the soft molten polymer, deforming it circumferentially into a tube with a wider diameter, while the pickup and winding up of the collapsed tube elongates the tube in the machine direction. The ratio of the pickup or haul-off rate to that of extrusion is called the draw-down ratio. The tubular film can be blown up by air only while it is soft and soon forms a "freeze line" at a maximum diameter (the ratio of the diameter at the freeze line to that of the annular die is the blow-up ratio for the film). To obtain a uniform film, it is crucial to maintain constant extrusion rates and a symmetric stable "bubble" or the inflated cylinder of polymer at all times during processing. Typically the bubble can be 15-30 ft tall and up to several feet in diameter. The processing variables as well as the grade of resin used for film blowing determines the quality and uniformity of the film product. Figure 2.16 shows a photograph of an annular die used in blowing films, and Figure 2.17 shows a diagram of film blowing equipment.
The same process can also be used to produce a multilayered film using several extruders, one for each type of resin used, and a feed block to direct the resin
into different layers. The layers need to be selected carefully for their processing characteristics as well as their performance in the final product. For instance, in coextrusion of a barrier film for packaging applications, different layers of the film might be selected for different functionality needed in the product.
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