Residence and Relaxation Times

8 Residence and Relaxation Times (a) Residence (or Dwell) Time 

This refers to the time taken for the polymer melt to pass through the die or channel section. Mathematically it is given by the ratio 

For the uniform channels analysed earlier, it is a simple matter to show that the residence times are  

In Chapter 2 when the Maxwell and Kelvin models were analysed, it was found that the time constant for the deformations was given by the ratio of viscosity to modulus. This ratio is sometimes referred to as the Relaxation or Natural time and is used to give an indication of whether the elastic or the viscous response dominates the flow of the melt. 

To do this, a Deborah Number Adeb. has been deflned as 

If A deb 1 then the process is predominantly elastic whereas if Nm 1 then viscous effects dominate the flow. 

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