Solution to Design Problem VIII

FIGURE 9.28 Stability and instability regions in the space Wf and Br, for various X, and for LLDPE. (Reprinted by permission of the publisher from Kanai and White, 1984.) 

In this design problem we need to analyze the process in two sections (a) from the die to the die swell level, and (b) from the die swell level to the nip rolls. In the first section, the critical wall shear rate for melt fracture, taking into consideration the safety factor, will dictate the dimensions of the die. The volume of material in each bag is calculated as 2 X (61 X 69.6 x 0.002541) = 21.6 cm. The mass flow 

Assume that Ro is the die outer radius, and that kRo is the die inner radius. As the final thickness of the film is 25.41 pm, it is reasonable to expect that the die gap is small, and consequently that the annular die can be approximated by a sUt of thickness, Hq, and width, Wq, where 

Die swell is present in this problem, and the radius and thickness at the maximum die swell level will be noted as Rp and Hp, respectively (see Fig. 3.1). Note that the dimensions at this die swell level will be considered as the initial dimensions for the film blowing process. From Table 1.1 we get that Hp = 0.018 in. = 0.0457 cm for a die land length of 3.81 cm. Thus, the reduction in film thickness becomes 

The dimensions of the die are determined by means of a mass balance and the thickness increase in the film due fo die swell as follows. Equafion 3.89 is used as 

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