Extruder-blown film orientation

Figure 5.19 Example of upward extruded blown film process for biaxially orienting film...

Very few studies have been performed investigating the effect of branching on the extensional rheological properties of polystyrenes. Such investigations can be valuable because many of the fabrication operations associated with commercial applications of polystyrene include operations in which the polystyrene melt undergoes an extensional deformation. Some examples are extruded foam sheet, blown film, oriented (tentered) sheet, and thermoforming. The types of deformations associated with these processing operations are best described as... 

Thermoplasticity. High molecular weight poly(ethylene oxide) can be molded, extruded, or calendered by means of conventional thermoplastic processing equipment (13). Films of poly (ethylene oxide) can be produced by the blown-film extrusion process and, in addition to complete water solubility, have the typical physical properties shown in Table 3. Films of poly(ethylene oxide) tend to orient under stress, resulting in high strength in the draw direction. The physical properties, melting behavior, and crystallinity of drawn films have been studied by several researchers (14—17). 

Blown film is usually extruded vertically upward through a circular die. This forms a tube that is then blown into a bubble that thins or draws down to the required final gauge. Orientation takes place in two directions horizontally (transverse direction/TV) as the bubble is formed, and in the machine direction (MD) as controlled by adjustable-speed haul-off nip rolls. 

Water quenching can be used for wire insulation, tubing, and pipe. Sheet and film extruded from slit dies are frequently crystallized on polished steel rolls operating at 65°C-145°C. Extruded, blown, or flat film can be uniaxially or biaxially oriented to submil thickness. Monofilament is extrusion spun into a water bath, and then oriented and heat set at an elevated temperature. 

In the blown-film process, the resin is extruded through an annular die and internal air pressure used to expand the bubble. After the resin solidifies, the bubble is collapsed and may be wound as a tube or slit and wound as rolls of flat film. Since the film is stretched by the take-off rollers as well as by the bubble expansion, it has biaxial orientation. 

When extruding a tube through an annular die there are two kinds of orientation processes available, blown film process and tubular film process. Blown film conducts the orientation in the melt state. The tube is rapidly pulled away from the die by a nip at the top of a tower. Air is pumped through the annular die to infiate the tube and to provide additional cooling. The molecular orientation produced in blown film is quite low compared to solid-state orientation. The molecules are above their melt and have very fast relaxation times. One often refers to the frost line in a blown film process. This is the point where the melt crystallizes. The polymeric web goes through a clear to hazy transition at this point. Further molecular orientation in the web in this stage of the process typically does not occur. 

An early description of the cast film process was published in Modem Plastics in 1952 [26] which was very similar to the cast-film scheme shown in Figure 6.19 which represents the process used today. The cast film process involves the extrusion of polyethylene through a die to form a thin molten layer of material that is drawn down to a thinner gauge based on the difference between the rate the molten polymer exits the extruder and the rate that the molten polymer is cooled onto a chill roll. The film cools very rapidly on the chill roll and film orientation is only in the machine direction. Optical properties are usually better with cast film as compared to blown film and line rates may be higher with the cast film process. A cast film line may also involve coextrusion of several layers of different types of thermoplastics to fabricate specialty films with improved properties. A... 

The value of the approach using dichroic ratios coupled with a knowledge of the assignments of peaks is further illustrated by measurements on plain polyethylene films [2]. The production process leads to a rather complex orientation pattern. The molten polymer is extruded as a thin, hollow cylinder, in what may be termed the machine direction. It is simultaneously expanded in a plane perpendicular to the machine direction by the application of internal pressure. Additional variables are the extrusion temperature and the rate at which the blown film has been cooled. The resulting orientation behaviour is best studied by x-ray diffraction pole figure measurements [3,4,5] but the infrared approach provides a relatively simple means for obtaining a useful amount of information, particularly for the behaviour of chains in amorphous regions. 

With a given die cross-sectional area, there is only 1 ratio of puller speed to extruder throughput rate that produces a product with the correct cross-sectional dimensions. If the extruder throughput is increased, the puller speed must be increased proportionally to maintain the same finished product dimensions. Likewise if the throughput is decreased the puller speed must be decreased proportionally to maintain the same finished product cross-sectional area. The draw ratio and molecular orientation can only be increased or decreased by changing the die cross-sectional area relative to the puller speed assuming the final product dimensions are kept constant. This is easily done with sheet dies, cast film dies, or blown film dies that have adjustable die lips. Profile and cross head dies... 

In making multilayer structures, two or three melt streams from separate extruders can be combined and cast to form a multilayer structure. Alternatively, a melt stream, and a premade cast or blown film web or a quenched and oriented film web, may combined on a chilled drum providing the web with new surface functionality (Fig. 12,58). This method is known as the extrusion coating process [2],... 

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