Axial Mixing and Residence Time Distribution

Axial mixing correlates directly with the residence time distribution in an extruder. The width of the residence time distribution is determined by the flow profiles in the screw 

The time interval b is a measure of the self-cleaning ability of the machine. Clearly, b represents the length of time that must be waited in order to ensure that, in the event of a color or product change, the content of the extruder has been exchanged completely at least once. 

Ideal cross mixing Narrow residence time distribution 

Ideai cross mixing Wide residence time distribution = ideal axial mixing 

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This section will analyze how the velocity profiles, axial mixing, and residence time distribution are related. It will be shown why simple conveying screws have poor axial mixing capability. New mixer geometries that are specifically designed to improve backmixing will be discussed. 

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