Shear rate limitations screw speed

Shear Rate Limitations. Extruders are run at relatively low screw speeds. Small extruders have variable drives to adjust screw speeds from about 20 to 150 rpra for extruders up to about 2 in. in diameter. As the screw diameter increases the screw speed must be reduced. The peripheral screw speed is given by Equation k... 

High stock temperatures are likely to be a problem in extrusion operations where the extruder is run at high screw speed and where the polymer melt viscosity is high. The main screw design variable that affects viscous heating is the channel depth. Increasing the channel depth will reduce the shear rate and thus the viscous heating. There are limits to how deep the screw can be cut. One limit is the physical... 

A second consideration is the screw design. Most of the commercial lines have extruder screws that have been optimized to run olefin resins at the maximum output for the diameter of the screw. To do this, they need to design for the specific melt flow properties (rheology) of the resin. PLA has a significantly different rheological behavior from olefin materials. It does not shear thin as readily and therefore typically requires additional power input from the drive. This additional power input manifests itself as an excessive increase in melt temperature. Quite often, if the drive has enough power to process PLA, it will be screw speed limited (and hence rate limited) by excessive melt temperature. 

The diameter of the screw is largely determined by torque limitations. Peripheral screw speeds are usually limited to about 110 ft/min or less to avoid shear degradation of the polymer. The higher power required for higher output will therefore call for larger diameter screws to prevent breakage. (The torque rating of a screw is proportional to the cube of its diameter.) With a 2.5-inch diameter screw, for example, the maximum permissible... 

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