Plastics Technology

SEP 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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viscosity. As a result, these are approxi- mate values, adequate for equipment selection, and they show the effect of the important variables to give the processor the knowledge for analysis without benefit of a more rigorous computer analysis. As you can see from the table, for a round orifice, the pressure is increased by eight times the passage length but decreases by the fourth power of the radius. For a slit orifice and an annular passage, the pressure is increased by 12 times the length and reduced by the first power of the width and the third power of the height or difference in the radii. So, to minimize head pressure, flow passages should be as short as possible and as large as possible. However, "as large as possible" has a definite restriction. To clean the walls of the passage (not get scale or a static layer on the walls), the shear stress at the wall needs to be great enough to provide continuous renewal of the material near the wall (see Fig. 2). Different levels of stress are preferred by different tooling designers, but 10 psi is typical. Too low a shear stress at the wall builds temperature differentials into the extrudate from large differences in resi- dence time and will cause unstable flow. Static Layer FIG 2 Complete Blown Film THE X -DIE & High Performance V -RING ALPINE AMERICAN T H E B L O W N F I L M E X P E R T S www.halpine.com T: (508) 655-1123 F: (508) 655-9337 The shear stress at the wall needs to be great enough to provide continuous renewal of the material near the wall to avoid getting a scale or static layer on the walls. Tooling designers have their preferences, but 10 psi is typical. There is often no consideration of the effects of head pressure in the selection of the downstream components and its effect on the overall process. @plastechmag 41 Plastics Technology E X T R U S I O N

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