Bubble Geometry

Although bubble geometry is not specifically hardware, it is included in this section because the hardware directly affects the bubble s geometry. The following paragraphs detail the geometric considerations of bubble shape. 

The specific shape of the bubble (Fig. 3.8) depends on the combined influence of several process parameters. In general, the bubble usually has a small diameter and large thickness at the die exit and transitions to a large diameter and small thickness as it moves upward toward solidification. Above some point, the geometry is frozen-in and remains virtually constant. 

There are several parameters used to describe the geometry of the bubble  

The die diameter represents the initial bubble diameter as it leaves the die, and the die gap determines the initial bubble wall thickness. As the bubble travels upward from the 

Several process variables work together to determine the bubble geometry  

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Figure 8.3 A more precise view of bubble geometry. [After D. Kunii and O. Levenspiel, Fluidization Engineering, with permission of Butterworth-Heinemann, Boston, MA, (1991).]...

As the bubble approaches the nip rolls, the bubble shape must be transformed from a round tube to a fiat material. While the bubble looks symmetrical, a thorough examination of bubble geometry reveals that the path traveled by a particle in the bubble Is not the same at all points in the circumference. Knittel [1] showed that the shortest path up the bubble Is at a location 15.5° from the crease (Fig. 7.11). The path differences are not much of a problem for polymer films that are flexible, such as LDPE or LLDPE. For HDPE and some stiff coextrusions, however, this path difference is noticeable and results In wrinkling of the film. 

The model provided excellent agreement with computational fluid dynamic simulations for capillaries with 1.5, 2, and 3 mm diameters and idealized bubble geometry as shown in Figure 7.12 [47]. The major contribution to mass transfer is in the film k fiim fUm) concentration of the solute is low in the... 

Polymer chemistry and molecular structure are vital in establishing film properties, but bubble geometry resulting from processing conditions is also significant. Molecular orientation and crystalline structure - controlled by bubble dimensions - affect properties such as tensile strength, impact toughness, and clarity. 

The strong interdependence of process variables is another aspect of the process that requires a high level of operator skill and has led to extensive advancements in measurement and control techniques. There are many process variables - screw speed, nip speed, internal bubble air volume, and cooling rate (frost-line height) - that influence bubble geometry and, as a result, film properties. An adjustment to any one... 

Table 4.1 The Effect of Each Main Process Variable on Bubble Geometry...

To properly describe and control the bubble-forming process, certain quantities have been developed to characterize the process conditions that influence bubble geometry. These quantities are the take-up ratio (TUR), the blow-up ratio (BUR), and the forming ratio (FR). 

Molecular structure, as imparted by processing, has a significant influence on the physical properties of extruded products. This is known as a process/structure/property relationship. In the case of blown film, the extruder conditions and the bubble geometry influence the molecular structure, which then affects film performance. 

Figure A.11 The measurements panel shows values for the key bubble geometry characteristics...

The biaxial deformation or strain Su =822 at the pole of the bubble is related to the bubble geometry by the following expression [6] ... 

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