Double bubble

The double-bubble process may be used to produce biaxiaHy oriented film, primarily polypropylene. In this process the first bubble formation is similar to the conventional blown film, except that the bubble is not coUapsed. Rather it is reheated to the orientation temperature and blown and drawn further in a second stage. It is then coUapsed, sUt, and wound. This process is generally limited to a final film thickness of less than 24 p.m. 

Unlike other gem materials, it is common to see swirls and bubbles in amber and copal as the material does not form in the same way as mineral crystals. It is occasionally possible, though rare, to see double bubbles of gas and liquid inside these resins. 

Double/Bubble. [Hardman] Two-part epoxy pkgs. 

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. 

Fig. 1.3. Complete duodenal obstruction caused by a congenital duodenal web. The stomach (st) and duodenum (d) are dilated and with absence of air more distally in the gastrointestinal tract, producing the classical double bubble image...

The main causes for a double bubble are duodenal atresia, annular pancreas, and midgut volvulus. Less frequently it may be secondary to duodenal web, Ladd s band, or preduodenal portal vein (Crowe and Sumner 1978). 

Fig. 1.4a,b. Duodenal atresia, a Plain radiograph obtained 6 h after birth shows absence of air in the gastrointestinal tract of this neonate with severe lung disease, b Radiograph made after inflation ofthe stomach through a nasogastric tube demonstrates complete duodenal obstruction ( double bubble sign)... 

If the stomach is too big in the third trimester on two consecutive studies, the main cause is duodenal atresia with the typical double-bubble appearance. A total of 30% of these patients will have trisomy 21 (Buonomo et al. 1998). These patients also have an increased risk of an esophageal atresia, in which case they will not always present with a double-bubble appearance due to the obstruction of the GI tract more proximally, unless they have an associated... 

H-type tradieo-esophageal fistula. The differential diagnosis for the double-bubble should include duodenal stenosis and annular pancreas (Rathaus et al. 1992 Poki et al. 2005). 

In the case of atresia, there will be no air in other parts of the intestinal tract, the so-called double bubble sign (Fig. 5.3). No other imaging modality is then needed. However, the double bubble sign may be absent if the patient has vomited, has a feeding tube or if there is intermittent passage of air. 

Alternatively, in the double-bubble process, a cast tube is blown into a large bubble with simultaneous stretching in both directions to produce a balanced film. This process is used to make heat-shrink films as well as standard BOPP films. 

Fig. 4.6 Scheme of double-bubble blown film process [50, 51]... 

US 5674607 A, Double bubble process for making strong, thin films... 

Commercial biaxially oriented films are produced by the tenter-frame (Fig. 10), double-bubble (Fig. 11), or blown-film process (Fig. 12). In the tenter-frame and double-bubble processes, the polymer is stretched in the solid state below the crystalline melting point. In the blown-film process, the polymer is oriented in the melt, followed by rapid quenching to immobilize the orientation. 

Double-Bubble Method. In the double-bubble method (Fig. 11), the resin is extruded as a tube that is quenched and gauged in cold water. The tube is then reheated and oriented by blowing it into a bubble. Simultaneously, the speed of the takeoff roll is increased for MD orientation radiant electrical heaters control temperature. The bubble is collapsed, passed through nip rolls, reinflated, annealed in a controlled-temperature chamber, and collapsed again. After trimming the edges, it is separated into two webs, which are then treated by electric discharge for printability and wound onto rolls. 

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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