Cooling double bubble

Most blown film operations extrude the resin in an upward direction. However, blown polypropylene film is generally extruded downwards and water or mandrel quenched. The extruded tube is then reheated, to a point still below its melt temperature, before it is blown. The collapsed bubble can be fed over a series of heated rollers to reheat it and relieve thermal stresses if a heat-stabilized film is wanted or it can be heated and reinflated in what is known as the double bubble process, which will be discussed in Section 7.3.7. In either case, the film is restrained until cooling is complete, to keep it from shrinking. 

The double-bubble process involves the extrusion and blowing of a tube of molten plastic in a downward direction. The tube is then cooled, most often using a water bath, reheated to just below the melt temperature, and reinflated. The reinflation along with the increase in haul-off speed provides biaxial orientation. Typically the next step is annealing to relieve thermal stresses and stabilize the film. The double-bubble process is most often applied to PP film, but is also used with multilayer PP or PE-based films. One of the major advantages is that this process can deliver a high-clarity film with precise shrink characteristics and very uniform flatness. 

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). 

Introduce about 0.7 mL of freshly boiled and cooled double-distilled water into the sample tube from the bottom opening using a suitable syringe. The water must be free of even the smallest air or gas bubbles. The sample tube shall be completely full. Allow the water to reach thermal equilibrium at the test temperature and record the T-value for water and the test temperature. 

Gaseous ammonia was bubbled into 45 ml methyl alcohol, cooled to —70°C until the volume doubled, then treated with the Step 5 product (0.737 mmol), then stirred 48 hours at ambient temperature. The mixture was concentrated, the residue recrystallized from CHC13/hexane, and the product isolated in 66% yield as light brown crystals, mp = 145-148°C. 

The bubble chamber-magnet system being designed is expected to be a very complicated and costly apparatus to cool down, considering costs of both operating the bubble chamber and accelerators involved. Since this seal is only part of the large and complex cryogenic system, a double seal was desired to maximize its reliability. 

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