Processes water quench cast film process

Cast film is produced by extruding the melt from a slit die and cooling it either by contact with a chill roll or by quenching in a water bath. The most popular process used to produce the flat film is with the chill rolls. Chill roll lines can be arranged in different layouts to meet different requirements. Example is shown in Figure 5.7. Water chill tank or quench film is also a popular process. 

Isotactic PP has extremely good flow properties at a wide range of flow rates, and therefore good processing behavior. The melt flow index typically ranges from 0.5 to 50 g/10 min. Films, which can be produced by both blown and cast methods, can be oriented to provide improved optical characteristics and better strength. Because of the rapid crystallization of PP, blown films must be produced by either water quench or mandrel quench processes, unlike PE, which is cooled by air. 

Strapping tapes, film tapes, monofilaments, fibres and nonwovens are usually stretched immediately after extrusion to achieve considerable increase in ultimate tensile strength in the stretching direction as a result of molecular orientation. The elongation at break correspondingly decreases. The film for stretched tapes can be produced by a cast film technique using either the water quench or chill roll process. 

When the extrudate exits the die, it has to be quenched and possibly sized (drawn through a fixture) to maintain its final shape. Depending on the extrusion process, different methods are available to quench the final product. Cast film and sheet are quenched on rolls and in water baths, blown film is quenched by air in a blown film tower. Solid profiles, pipe, and tubing are quenched in calibration tanks filled with water and in some cases connected to a vacuum system. Polymer strands and monofilaments are quenched in air or water baths while wire coating is done horizontally in air or water. In small and large part blow molding, the melt is quenched in molds as the extruded tubular parison is inflated. 

Water Quench. The water queneh east film process (Fig. 8.9) is similar in concept to the chill roll process and uses similar downstream equipment. A water bath takes the place of the chill roll for film cooling, and by cooling both sides of the film equally, it produces a film with slightly different properties compared to chill roll cast film. The extruder s slit-die is arranged vertically and extrudes a melt web directly into the water bath at elose range. The film passes under a pair of idler rollers in the bath and, for any given rate of extrusion, it is the rate of downstream haul-off that regulates film drawdown and finished thickness. 

The majority of plastics films made today are produced by the tubular process, although for certain applications the flat film casting process is used. The so-called water quench system in which film is extruded directly into water is now not in general use. 

During the quenching process the homogeneous polymer solution separates into two phases a polymer-rich solid phase, which forms the membrane structure, and a solvent-rich liquid phase, which forms the liquid-filled membrane pores. Generally, the pores at the film surface, where precipitation occurs first and most rapidly, are much smaller than those in the interior or the bottom side of the film, which leads to the asymmetric membrane structure. There are different variations to this general preparation procedure described in the literature e.g., Loeb and Sourirajan used an evaporation step to increase the polymer concentration in the surface of the cast polymer solution and an annealing step during which the precipitated polymer film is exposed for a certain time period to hot water of 70° to 80°C.28... 

Data in this report are generated from both commercial and developmental flat-sheet CA membranes. CA manbranes are prepared by dissolving commercial grades of CA polymers into a solvenl/non-solvent mixture to give a highly viscous dope solution. After microfiltration a knife blade is used to spread the dope onto a woven nylon substrate. The commercial equipment utilized allows for a 1-m width to be cast. The thin dope film is quenched into a water bath to form the microporous structure by the phase inversion process. Membrane is washed with water and post-treated to give finished product in dry state as roll stock. 

Simultaneous Biaxial Orientation. There are two predominate systems available to do this, tubular and flat film. In the tubular process (see Fig. 9), also referred to as the double bubble process, a continuous tube is extruded and quenched. Typically, an interior cooled mandrel is hung from the die inside the tube. The surface of the mandrel may greatly influence the interior surface of the tube. Care must be taken not to impart scratch lines in the melt as it is pulled down over the mandrel. Air pressure in this primary tube is very critical. The melt needs to be held out over the mandrel but not too far away. A water bath on the external side of the tube helps quench the tube rapidly. A nip pulls the tube from the die and acts to isolate the casting bubble from the air pressime in the stretching bubble (27). 

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